Optical recording medium

Abstract: An optical recording medium comprising a substrate having provided thereon a layer of a recording material whose optical properties reversibly change on application of light, heat, etc., to conduct recording, reproduction, or erasion of information is disclosed, wherein the recording material has a composition (i) to (vii) containing three or more kinds of elements specifined in the specification. The optical recording material of the invention is capable of conducting rewriting of information at an increased speed and stably maintaining the recorded information for an extended period of time.

Description: FIELD OF THE INVENTION This invention relates to a rewritable optical recording medium for recording, reproducing, and erasing information whose optical properties change on irradiation with a condensed light beam, such as a laser beam, to conduct recording, reproduction or erasion of information. More particularly, it relates to an improved optical recording medium with which rewriting of information can be carried out at an increased speed and the recorded information can be maintained stably for an extended period of time. BACKGROUND OF THE INVENTION Known rewritable optical recording media using a laser beam, etc. for recording information include magneto-optical recording media, and some of them have been put into practical use. An magneto-optical recording system comprises inverting magnetism of a recording layer by light energy and a magnetic field to conduct recording and detecting a difference of angle of Faraday rotation or angle of Kerr rotation due to the direction of magnetization to obtain reproduced signals. However, this system finds no practical method for conducting rewriting within one sector so that it has been of limited application. On the other hand, a so-called phase transition type optical recording medium which utilizes a phase transition between a crystal phase and an amorphous phase has been proposed and receiving studies as another type of rewritable optical recording media. According to this recording system, rewriting within one sector can be carried out by using two light beams (i.e., recorded information is erased by the preceding beam, and recording is then effected with the beam following). When, in particular, a recording material requiring a short time for crystallization is employed, it is possible to conduct overwriting (simultaneous erasing and writing) with one light beam. Thus, the phase transition type recording medium and phase change type recording medium, resp., is of wide application in many fields. So far proposed recording materials applicable to the phase transition type optical recording media include a binary compound, Sb₂Te₃, and a ternary compound, Ge-Sb-Te, as disclosed in JP-A-63-225934 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"). Recording is effected by irradiating a condensed light beam, such as a semi-conductor laser beam, on a recording layer comprising these materials to melt the irradiated part, followed by quenching to form an amorphous phase, while erasing is effected by irradiating the recorded part with a condensed light beam of a power lower than that used for recording to maintain the irradiated area at a crystallizing temperature for a given time to thereby return the amorphous phase to a crystalline phase. In view of simplification of an optical system or improvement of a rate of transition, a recording material for use in the phase transition type optical recording media preferably has a short crystallizing time. Further, in view of long-term maintenance of recorded information, it is desirable that the recording material exhibits a stable amorphous phase. In this connection, although the conventional recording material made of Sb₂Te₃ has a short crystallizing time, i.e., satisfies the first requirement, it has insufficient stability in its amorphous phase so that the recorded information cannot be maintained for a long time. On the other hand, Ge-Sb-Te disclosed in JP-A-63-225934 is synthesized by mixing Sb₂Te₃ with GeTe which exhibits high stability in its amorphous phase but requires a long time for crystallization. What Ge-Sb-Te aims at is to exhibit properties midway between Sb₂Te₃ and GeTe and thus to achieve both an increased rate of transition and improved preservation stability of recorded information. Nevertheless, since mixing of GeTe necessarily makes the crystallizing time longer than that of Sb₂Te₃ alone, a sufficient rate of transition cannot be obtained. EP-A-0 286 406 discloses an optical recording medium wherein the recording phase is composed of a compound of three or more elements. For example, the compound Bi₄PbTe₇ can be expressed as PbTe.(Bi₂Te₃)₂ which has to be regarded as a stoichiometric compound. Congruent compounds are, however, not disclosed. Further, EP-A-0 294 932 discloses optical recording media wherein the recording layer is composed of Sb, Te, Ge and at least one element selected from Pb, Bi, In, Sn and Tl. The materials forming the recording layer are, however, not congruent compounds so that there was still a need for improving the prior art. EP-A-0 362 852 is directed to information-recording thin films comprising as the material of the recording film a compound of at least 4 elements including Sb and Te. The materials of the recording film are not congruent compounds. EP-A-0 392 179 discloses an optical recording medium wherein the recording layer essentially consists of at least one element of the group As, Sb, Bi and at least one element of the group consisting of S, Se and Te, contains at least one element of Se and Te and has a composition corresponding to that of a mixture of at least two stoichiometric compounds represented by the chemical formula Vb₂ VIb₃, wherein Vb and VIb are an element of the fifth and sixth main group of the periodic table. Both quotations concern, however, not prepublished prior art. SUMMARY OF THE INVENTION An object of this invention is to provide a optical recording medium which exhibits stability in its amorphous phase and has a reduced crystallization time to thereby achieve rewriting of information at a high speed and to stably maintain recorded information for a prolonged period of time. The present invention relates to an optical recording medium comprising a substrate having provided thereon a layer of a recording material whose optical properties reversibly change on application of light or heat, to conduct recording, reproduction, or erasion of information, wherein said recording material is one of: (i) a recording material having a composition corresponding to a mixture of (i-a) a congruent compound having a composition corresponding to a mixture of (i-a-1) a IV b VI b type stoichiometric compound composed of the group IV b element selected from Sn and Pb and the group VI b element selected from S, Se, and Te and represented by chemical formula IV b VI b and (i-a-2) a V b2 VI b3 type stoichiometric compound composed of the group V b element selected from Sb, Bi, and As and the group VI b element selected from S, Se, and Te and represented by chemical formula V b2 VI b3 and (i-b) the above-described V b2 VI b3 type stoichiometric compound, said composition containing at least one of Se and Te, (ii) a recording material having a composition corresponding to a mixture of (ii-a) a I₂VI b type stoichiometric compound composed of the group I element selected from Li, Na, K, Rb, Cs, Cu, Ag, and Au and the group VI b element selected from S, Se, and Te and represented by chemical formula I₂VI b and (ii-b) a stoichiometric compound composed of elements selected from at least two groups of the group V b consisting of As, Sb, and Bi, and the group VI b consisting of S, Se, and Te, said composition containing at least one of Se and Te, wherein the crystal phase of said recording material (ii) is a single phase composed of a stoichiometric compound and said stoichiometric compound is a congruent compound; (iii) a recording material having a composition corresponding to a mixture of (iii-a) a III b VI b type stoichiometric compound composed of the group III b element selected from Ga, In, and Tℓ and the group VI b element selected from S, Se, and Te and represented by chemical formula III b VI b and (iii-b) a stoichiometric compound composed of elements selected from at least two groups of the group III b consisting of Ga, In, and Tℓ, the group IV b consisting of Ge, Sn, and Pb, the group V b consisting of As, Sb, and Bi, and the group VI b consisting of S, Se, and Te, said composition containing three or more kinds of elements and containing at least one of Se and Te, wherein the crystal phase of said recording material (iii) is a single phase composed of a stoichiometric compound and said stoichiometric compound is a congruent compound; (iv) a recording material having a composition corresponding to a mixture of (iv-a) a III b2 VI b3 type stoichiometric compound composed of the group III b element selected from Ga, In, and Tℓ and the group VI b element selected from S, Se, and Te and represented by chemical formula III b2 VI b3 and (iv-b) a stoichiometric compound composed of elements selected from at least two groups of the group III b consisting of Ga, In, and Tℓ, the group IV b consisting of Ge, Sn, and Pb, and the group VI b consisting of S, Se, and Te, said composition containing three or more kinds of elements and containing at least one of Se and Te, (v-1) a recording material made of a solid solution containing four or more kinds of elements at least one of which is Se or Te and having a crystal structure of a stoichiometric three-element compound having a composition corresponding to a mixture of (v-1-a) a IV b VI b type stoichiometric compound composed of the group IV b element selected from Sn and Pb and the group VI b element selected from S, Se, and Te and represented by chemical formula IV b VI b and (v-1-b) a V b2 VI b3 type stoichiometric compound composed of the group V b element selected from Sb, Bi, and As and the group VI b element selected from S, Se, and Te and represented by chemical formula V b2 VI b3 , in which at least one of the three constituent elements of said stoichiometric three-element compound is partly substituted by at least one element selected from the group consisting of the above-described elements of the groups IV b , V b , and VI b and Si and Ge of the group IV b , (v-2) a recording material made of a solid solution containing four or more kinds of elements at least one of which is Se or Te and having a crystal structure of a stoichiometric three-element compound having a composition corresponding to a mixture of (v-2-a) a I₂VI b type stoichiometric compound composed of the group I element selected from Li, Na, X, Rb, Cs, Cu, Ag, and Au and the group VI b element selected from S, Se, and Te and represented by chemical formula I₂VI b and (v-2-b) a V b2 VI b3 type stoichiometric compound composed of the group V b element selected from As, Sb, and Bi and the group VI b element selected from S, Se, and Te and represented by chemical formula V b2 VI b3 , in which at least one of the three constituent elements of said stoichiometric three-element compound is partly substituted by at least one element selected from the above-described elements of the groups I, V b , and VI b , (v-3) a recording material made of a solid solution containing four or more kinds of elements at least one of which is Se or Te and having a crystal structure of a stoichiometric three-element compound having a composition corresponding to a mixture of (v-3-a) a I₂VI b type stoichiometric compound composed of the group I element selected from Li, Na, K, Rb, Cs, Cu, Ag and Au and the group VI b element selected from S, Se, and Te and represented by chemical formula I₂VI b and (v-3-b) a IV b VI b2 type stoichiometric compound composed of the group IV b element selected from Ge, Sn, and Pb and the group VI b element selected from S, Se, and Te and represented by chemical formula IV b VI b2 , in which at least one of the three constituent elements of said three-element compound is partly substituted by at least one element selected from the group consisting of the above-described elements of the groups I, IV b , and VI b and Si of the group IV b , (v-4) a recording material made of a solid solution containing four or more kinds of elements at least one of which is Se or Te and having a crystal structure of a stoichiometric three-element compound having a composition corresponding to a mixture of (v-4-a) a III b VI b type stoichiometric compound composed of the group III b element selected from Ga, In, and Tℓ and the group VI b element selected from S, Se, and Te and represented by chemical formula III b VI b and (v-4-b) a IV b VI b type stoichiometric compound composed of the group IV b element selected from Ge, Sn, and Pb and the group VI b element selected from S, Se, and Te and represented by chemical formula IV b VI b , in which at least one of the three constituent elements of said three-element compound is partly substituted by at least one element selected from the group consisting of the above-described elements of the groups III b , IV b , and VI b , Aℓ of the group III b , and Si of the group IV b , (v-5) a recording material made of a solid solution containing four or more kinds of elements at least one of which is Se or Te and having a crystal structure of a stoichiometric three-element compound having a composition corresponding to a mixture of (v-5-a) a III b VI b type stoichiometric compound composed of the group III b element selected from Ga, In, and Tℓ and the group VI b element selected from S, Se, and Te and represented by chemical formula III b VI b and (v-5-b) a IV b VI b2 type stoichiometric compound composed of the group IV b element selected from Ge, Sn, and Pb and the group VI b element selected from S, Se, and Te and represented by chemical formula IV b VI b2 , in which at least one of the three constituent elements of said three-element compound is partly substituted by at least one element selected from the group consisting of the above-described elements of the groups III b , IV b and VI b , Aℓ of the group III b , and Si of the group IV b , (v-b) a recording material made of a solid solution containing four or more kinds of elements at least one of which is Se or Te and having a crystal structure of a stoichiometric three-element compound having a composition corresponding to a mixture of two kinds of a III b V Ib type stoichiometric compound composed of the group III b element selected from Ga, In, and Tℓ and the group VI b element selected from S, Se, and Te and represented by chemical formula III b VI b , in which at least one of the three constituent elements of said three-element compound is partly substituted by at least one element selected from the group consisting of the above-described elements of the groups III b and VI b and Aℓ of the group III b , (v-7) a recording material made of a solid solution containing four or more kinds of elements at least one of which is Se or Te and having a crystal structure of a stoichiometric three-element compound having a composition corresponding to a mixture of two kinds of a IIIb₂VI b3 type stoichiometric compound composed of the group III b element selected from Ga, In, and Tℓ and the group VI b element selected from S, Se, and Te and represented by chemical formula III b2 VI b3 , in which at least one of the three constituent elements of said three-element compound is partly substituted by at least one element selected from the group consisting of the above-described elements of the groups III b and VI b and Aℓ of the group III b , (v-8) a recording material made of a solid solution containing four or more kinds of elements at least one of which is Se or Te and having a crystal structure of a stoichiometric three-element compound having a composition corresponding to a mixture of (v-8-a) a III b2 VI b3 type stoichiometric compound composed of the group III b element selected from Ga, In, and Tℓ and the group VI b element selected from S, Se, and Te and represented by chemical formula III b2 VI b3 and (v-8-b) a IV b VI b type stoichiometric compound composed of the group IV b element selected from Ge, Sn, and Pb and the group VI b element selected from S, Se, and Te and represented by chemical formula IV b VI b , in which at least one of the three constituent elements of said three-element compound is partly substituted by at least one element selected from the group consisting of the above-described elements of the groups III b , IV b , and VI b , Aℓ of the group III b , and Si of the group IV b , and (vii) a recording material made of a solid solution composed of at least three elements selected from the group consisting of the group III b element selected from Ga, In and Tl, the group IV b element selected from Ge, Sn, and Pb, the group V b element selected from As, Sb, and Bi, and the group VI b element selected from S, Se, and Te and containing at least one of Se and Te, said solid solution being synthesized from plural elements or stoichiometric compounds having the same crystal system, wherein said solid solution is a congruent solid solution. The symbols "I, III b , IV b , V b , and VI b " as used herein mean group numbers of the periodic table. The symbol "I" represents the subgroups I a and I b . Further, as regards the denomination of the main group elements, reference is made to Encyclopedic Dictionary of Chemistry, page 1080, Tokyo Kagaku Dojin (Publ.), 1989. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a cross-section showing a layer structure of the optical recording medium according to the present invention. Figures 7 through 9 each illustrate a pseudo-binary phase diagram of the recording material used in Examples 1 to 3, respectively. Figures 14 through 23 each illustrates a pseudo-binary phase diagram of the recording material used in Examples 4 to 13, respectively. Figures 24, 25 and 27 through 32 each illustrates a pseudo-binary phase diagram of the stoichiometric three-element compound before substitution used as a recording material in Comparative Example 2 and Examples 16 to 18 and 20 to 23, respectively. Figures 34 through 41 each illustrates a pseudo-binary phase diagram of the recording material used in Examples 24 to 31, respectively. DETAILED DESCRIPTION OF THE INVENTION Recording Materials (i): The IV b VI b type stoichiometric compound in recording materials (i) includes SnS, SnSe, SnTe, PbS, PbSe and PbTe. The V b2 VI b3 type stoichiometric compound includes those composed of the same V b elements and the same VI b elements, e.g., Sb₂S₃, Sb₂Se₃, Sb₂Te₃, Bi₂S₃, Bi₂Se₃, Bi₂Te₃, As₂S₃, As₂Se₃, and As₂Te₃; and those containing different V b elements or different VI b elements, e.g., Sb₂SeTe₂, Bi₂SeTe₂, Bi₂Se₂Te, and Bi₂STe₂ . Recording materials (i) which have a composition corresponding to a mixture of the IV b VI b type stoichiometric compound and the V b2 VI b3 type stoichiometric compound may be any of materials composed of at least three kinds of elements selected from the group consisting of the group IV b elements (i.e., Sn and Pb), the group V b elements (i.e., Sb, Bi, and As), and the group VI b elements (i.e., S, Se, and Te), containing at least one of Se and Te, and having a composition corresponding to a mixture obtained by mixing the IV b VI b type stoichiometric compound and the V b2 VI b3 type stoichiometric compound at an arbitrary mixing ratio. The mixture preferably contains one of the two stoichiometric compounds in an amount of from 5 to 95 mol% based on the mixture, and the total content of Se and Te in the recording material is preferably not less than 30 atomic% based on the total atoms constituing the recording material. Examples of such a composition include a composition consisting of three elements containing Se and/or Te, a composition consisting of four elements containing Se and/or Te, and compositions consisting of more than four elements (e.g., a five-element composition and a six-element composition). Specific examples of recording materials of this type are mixtures composed of at least three elements containing Se and/or Te which is obtained by mixing the IV b VI b type stoichiometric compound selected from SnS, SnSe, SnTe, PbS, PbSe, PbTe, etc. with the V b2 VI b3 type stoichiometric compound selected from Sb₂S₃, Sb₂Se₃, Sb₂Te₃, Bi₂S₃, Bi₂Se₃, Bi₂Te₃, As₂S₃, As₂Se₃, As₂Te₃, Sb₂SeTe₂, Bi₂SeTe₂, Bi₂Se₂Te, Bi₂STe₂, etc., at an arbitrary mixing ratio; stoichiometric compounds made of these mixtures, e.g., Sn₂Sb₂Se₅, SnBi₂Te₄, PbBi₄Se₇, PbBi₂Se₄, and Pb₃Bi₄Se₉; congruent compounds made of these mixtures, e.g., Sn₂Sb₆Se₁₁, SnSb₂Te₄, Pb₂Sb₆Te₁₁, and PbBi₄Te₇; solid solutions made of these mixtures, composed of at least three elements containing Se and/or Te; and mixtures of the above-described stoichiometric compounds, solid solutions, etc. at an arbitrary ratio. Further included are mixtures of the above-described congruent compound and the V b2 VI b3 type stoichiometric compound, e.g., Sb₂S₃, Sb₂Se₃, Sb₂Te₃, Bi₂S₃, Bi₂Se₃, Bi₂Te₃, As₂S₃, As₂Se₃, As₂Te₃, Sb₂SeTe₂, Bi₂SeTe₂, Bi₂Se₂Te, and Bi₂STe₂, at an arbitrary mixing ratio. Use of recording material (i) makes it possible to increase stability of the amorphous phase of the recording layer and to reduce the crystallization time of the recording layer. In addition, there is formed no IV b VI b type stoichiometric compound having a high melting point at the time of phase transition between a crystal phase and an amorphous phase. Reasons, which the inventors believe, for the reduction of the crystallization time of the above-described recording material layer as compared with the conventionally employed Ge-Sb-Te recording material layer are as follows. The IV b VI b type stoichiometric compound in recording materials (i) has a melting point as shown in Table 1 below. The "melting point" as herein referred to means a temperature necessary for cutting all the atomic bonds forming the solid phase. With a crystal structure being the same, the melting point is considered to faithfully reflect the interatomic bond strength. Accordingly, as the melting point becomes higher, the crystal phase seems to have a higher interatomic bond strength. Because all the IV b VI b type stoichiometric compounds listed in Table 1 have an NaCℓ type crystal structure, the way of thinking stated above can be applied to these compounds. Although GeTe has a layered structure called a GeTe type or an As type at room temperature, it is transformed to the above-mentioned NaCℓ type at temperatures of 440°C or higher so that the melting point of GeTe shown in Table 1 is regarded to the one for the NaCℓ type. That is, seeing that the IV b VI b type stoichiometric compounds in recording materials (i) have higher melting points than the melting point of GeTe, it is believed that the IV b -VI b interatomic bond strength is higher than the Ge-Te interatomic bond strength in their crystalline phase. Thus, in the recording material having a composition corresponding to a mixture of the IV b VI b type stoichiometric compound and the V b2 VI b3 type stoichiometric compound, the IV b -VI b interatomic bond strength is strong, and the free energy in its crystal phase is low. It is therefore assumed that the above-described recording materials show a greater difference in free energy between the crystal phase and the amorphous phase, i.e., a greater transition energy, as compared with the conventional Ge-Sb-Te recording material and, as a result, the crystallization time can be reduced. On the other hand, the reason why the recording layer comprising recording material (i) exhibits higher stability in its amorphous phase than the conventional Sb₂Te₃ recording material is that recording material (i) is a three or more element material having stronger inhibitory effects on atomic movement due to a difference in atomic radius and thus possesses a greater activation energy than two-element materials. Recording Material (ii): The I₂VI b type stoichiometric compound in recording material (ii) includes those containing the same group I elements, e.g., Li₂S, Li₂Se, Li₂Te, Na₂S, Na₂Se, Na₂Te, K₂S, K₂Se, K₂Te, Rb₂S, Rb₂Se, Rb₂Te, Cs₂S, Cs₂Se, Cs₂Te, Cu₂S, Cu₂Se, Cu₂Te, Ag₂S, Ag₂Se, Ag₂Te, Au₂S, Au₂Se, and Au₂Te; and those containing different group I elements, e.g., CuAgS, CuAgSe, and CuAgTe. The multi-element stoichiometric compound is a stoichiometric compound composed of elements selected from at least two groups of the groups III b , IV b , V b , and VI b . Examples of such a multi-element stoichiometric compound include a V b2 VI b3 type stoichiometric compound composed of the group V b element selected from As, Sb, and Bi and the group VI b element selected from S, Se, and Te and represented by chemical formula V b2 VI b3 ; a III b2 VI b3 type stoichiometric compound composed of the group III b element selected from Ga, In, and Tℓ and the group VI b element selected from S, Se, and Te and represented by chemical formula III b2 VI b3 ; and a IV b VI b2 type stoichiometric compound composed of the group IV b element selected from Ge, Sn, and Pb and the group VI b element selected from S, Se, and Te and represented by chemical formula IV b VI b2 . The V b2 VI b3 type stoichiometric compound includes those composed of the same group V b elements and the same group VI b elements, e.g., Sb₂S₃, Sb₂Se₃, Sb₂Te₃, Bi₂S₃, Bi₂Se₃, Bi₂Te₃, As₂S₃, As₂Se₃, and As₂Te₃; and those containing different group V b elements or different group VI b elements, e.g., Sb₂SeTe₂, Bi₂SeTe₂, Bi₂Se₂Te, and Bi₂STe₂. The III b2 VI b3 type stoichiometric compound includes those containing the same group III b elements and the same group VI b elements, e.g., Ga₂s₃, Ga₂Se₃, Ga₂Te₃, In₂S₃, In₂Se₃, In₂Te₃, Tℓ₂S₃, Tℓ₂Se₃, and Tℓ₂Te₃; and those containing different III b elements or different VI b elements, e.g., GaInS₃ and In₂STe₂. The IV b VI b2 type stoichiometric compound includes GeS₂, GeSe₂, SnS₂, and SnSe₂. Recording material (ii) which has a composition corresponding to a mixture of the I₂VI b type stoichiometric compound and the multi-element stoichiometric compound may be any of materials composed of at least three elements selected from the group I elements (i.e., Li, Na, K, Rb, Cs, Cu, Ag, and Au), the group III b elements (i.e., Ga, In, and Tℓ), the IV b elements (i.e., Ge, Sn and Pb), the group V b elements (i.e., As, Sb, and Bi), and the group VI b elements (i.e., S, Se, and Te), containing at least one of Se and Te, and having a composition corresponding to a mixture obtained by mixing the I₂VI b type stoichiometric compound with a multi-element stoichiometric compound (e,g., a V b2 VI b3 type stoichiometric compound, a III b2 VI b3 type stoichiometric compound, and a IV b VI b2 type stoichiometric compound) at an arbitrary mixing ratio. The mixture preferably contains one of the two stoichiometric compounds in an amount of from 5 to 95 mol% based on the mixture, and the total content of Se and Te in the recording material is preferably not less than 30 atomic% based on the total atoms constituing the recording material. Examples of such a composition include a composition consisting of three elements containing Se and/or Te, a composition consisting of four elements containing Se and/or Te, and compositions consisting of more than four elements (e.g., a five-element composition and a six-element composition). Specific examples of recording material (ii) are mixtures composed of at least three elements containing Se and/or Te which are obtained by mixing the above-described I₂VI b type stoichiometric compound with a V b2 VI b3 type stoichiometric compound selected from Sb₂S₃, Sb₂Se₃, Sb₂Te₃, Bi₂S₃, Bi₂Se₃, Bi₂Te₃, As₂S₃, As₂Se₃, As₂Te₃, Sb₂SeTe₂, Bi₂SeTe₂, Bi₂Se₂Te, Bi₂STe₂, etc. at an arbitrary ratio; stoichiometric compounds made of these mixtures, e.g., Sn₂Sb₂Se₅, SnBi₂Te₄, PbBi₄Se₇, PbBi₂Se₄, Pb₃Bi₄Se₉, etc.; congruent compounds made of these mixtures, e.g., Sn₂Sb₆Se₁₁, SnSb₂Te₄, Pb₂Sb₆Te₁₁, PbBi₄Te₇, etc.; solid solutions made of these mixtures, which are composed of at least three elements containing Se and/or Te; and mixtures of the above-described stoichiometric compounds, solid solutions, etc. at an arbitrary ratio. Further included in recording material (ii) are mixtures composed of at least three elements containing Se and/or Te which are obtained by mixing the above-described I₂VI b type stoichiometric compound and a III b2 VI b3 type stoichiometric compound, e.g., Ga₂S₃, Ga₂Se₃, Ga₂Te₃, In₂S₃, In₂Se₃, In₂Te₃, Tℓ₂S₃, Tℓ₂Se₃, Tℓ₂Te₃, GaInS₃, In₂STe₂, etc. at an arbitrary mixing ratio; stoichiometric compounds made of these mixtures, e.g., CuGaSe₂, CuInSe₂, CuInTe₂, AgInTe₂, etc.; congruent compounds made of these mixtures, e.g., AgGaSe₂, AgIn₂Se₈, etc.; solid solutions made of these mixtures, which are composed of at least three elements containing Se and/or Te; and mixtures of the above-described stoichiometric compounds, solid solutions, etc. at an arbitrary ratio. Furthermore included in recording material (ii) are mixtures composed of at least three elements containing Se and/or Te which are obtained by mixing the above-described I₂VI b stoichiometric compound with a IV b VI b2 type stoichiometric compound selected from GeS₂, GeSe₂, SnS₂, SnSe₂, etc. at an arbitrary mixing ratio; stoichiometric compounds made of these mixtures, e.g., Cu₂SnSe₃; solid solutions made of these mixtures, which are composed of at least three elements containing Se and/or Te; and mixtures of the above-described stoichiometric compounds, solid solutions, etc. at an arbitrary mixing ratio. Use of recording material (ii) makes it possible to increase stability of the amorphous phase of the recording layer and to reduce the crystallization time of the recording layer. Reason, which the inventors believe, for the reduction of the crystallization time of the above-described recording material layer as compared with the conventionally employed Ge-Sb-Te recording material layer is as follows. Electronegativities of elements constituting the conventional recording materials and recording materials (ii) of the invention are shown in Table 2 below. The electronegativities herein referred to are values of Phillips' electronegativity which are obtained by adding the screening effect by valence electrons to the values of Pauling's electronegativity (see J.C. Phillips, Bonds and Bands in Semiconductors , Academic Press, New York and London (1973), with the exception that the values for K, Rb, and Cs are Pauling's electronegativities since Phillips' electronegativities for these elements have not yet been obtained. It is known that ionic character of an interatomic bond tends to increase as a difference in electronegativity between the atoms becomes greater. The Phillips' electronegativities herein used are calculated values obtained for regular tetrahedral structures. Since the electron orbit in the atoms constituting recording material (ii), inclusive of a non-bonding pair, is regular-tetrahedral, it would be safe to apply the values of Phillips' electronegativity to these atoms. Interatomic bonds in the conventional Ge-Sb-Te being considered, the Phillips' electronegativity values of Ge, Sb, and Te are 1.35, 1.31, and 1.47, respectively, as shown in Table 2, giving a difference of 0.04 between Ge and Sb, 0.16 between Sb and Te, and 0.12 between Ge and Te. Thus, it can be seen that ionic bond strength of Ge-Sb, Sb-Te and Ge-Te bonds constructing Ge-Sb-Te is weak. In the I₂VI b type stoichiometric compound constituting a part of recording material (ii), to the contrary, a difference in electronegativity between Te whose electronegativity is the lowest (1.47) of the group VI b elements and Li whose electronegativity is the highest (1.00) of the group I elements is 0.47. On the other hand, a difference between S whose electronegativity is the highest (1.87) of the group VI b elements and Ag whose electronegativity is the lowest (0.57) of the group I elements is 1.30. That is, the electronegativity difference between the group I elements and the group VI b elements widely ranges from 0.47 to 1.30, indicating strong ionic bond strength between the group I atom and the group VI b atom. From these considerations, it is believed that the group I atom-group VI b atom bond strength is higher than the strength of the interatomic bonds constituting Ge-Sb-Te (most of them are covalent bonds) by the strength corresponding to the above-described ionic bond strength. Therefore, when the I₂VI b type stoichiometric compound is mixed with a multi-element stoichiometric compound composed of elements selected from at least two groups of the group III b (i.e., Ga, In, and Tℓ), the group IV b (i.e., Ge, Sn, and Pb), the group V b (i.e., As, Sb, and Bi), and the VI b (i.e., S, Se, and Te), the mixture contains a group I atom-group VI b atom bond having high bond strength and thereby has a reduced free energy in the crystal phase thereof. As a result, a difference in free energy between the crystal phase and the amorphous phase, i.e., a transition energy, increases. This is assumed to be the reason why recording material (ii) has a reduced time of crystallization as compared with the conventional Ge-Sb-Te recording material. The fact that the conventional Sb₂Te₃ recording material has a short crystallization time, though it contains no bond exhibiting strong ionic bond strength, is not opposed to the consideration mentioned above because such a short crystallization time is believed to arise from its small activation energy but not from large transition energy. On the other hand, the reason, the inventors believe, for the improved stability of recording material (ii) in its amorphous phase as compared with the conventional Ge-Sb-Te or Sb₂Te₃ recording material is as follows. As discussed above, Ge-Sb-Te has weak character of ionic bond and strong character of covalent bond. Therefore, according as the number of elements increases, the amorphous phase easily becomes unstable due to an increase of free energy with the increasing strain. To the contrary, since recording material (ii) has strong character of ionic bond which is less limited by bond direction or bond distance, it is stabilized in spite of a somewhat strained structure and thus exhibits higher stability in its amorphous phase than Ge-Sb-Te. Further, since recording material (ii) is a multi-element material composed of three or more elements, the inhibitory effect on atomic movement due to differences in atomic radius is greater, and the activation energy is so greater than the two-element material Sb₂Te₃. The high stability of recording material (ii) in its amorphous phase would also be explained from this point of view. Recording Materials (iii) and (iv): The III b VI b type stoichiometric compound in recording material (iii) includes GaS, GaSe, GaTe, InS, InSe, InTe, TℓS, TℓSe, TℓTe, GaInS₂, GaInSe₂, Ga₂InSe₃, Ga₂SeTe, Ga₃Se₂Te, GaTℓSe₂, and InTℓSe₂. The multi-element stoichiometric compound in recording material (iii) includes stoichiometric compounds composed of elements arbitrarily selected from at least two groups of the groups III b , IV b , V b , and VI b . Examples of such multi-element stoichiometric compounds are IV b VI b type stoichiometric compounds composed of the group IV b element selected from Ge, Sn, and Pb and the group VI b element selected from S, Se, and Te and represented by chemical formula IV b VI b ; IV b VI b2 type stoichiometric compounds composed of the group IV b element selected from Ge, Sn, and Pb and the group VI b element selected from S, Se, and Te and represented by chemical formula IV b VI b2 ; and III b VI b type stoichiometric compounds composed of the group III b element selected from Ga, In, and Tℓ and the group VI b element selected from S, Se, and Te and represented by chemical formula III b VI b . Specific examples of the IV b VI b type stoichiometric compounds are GeS, GeSe, GeTe, SnS, SnSe, SnTe, PbS, PbSe, PbTe, GeSnTe₂, and Ge₂SeTe. Specific examples of the IV b VI b2 type stoichiometric compounds are GeS₂, GeSe₂, SnS₂, SnSe₂, and SnTe₂. Specific examples of the III b VI b type stoichiometric compounds are the same as above enumerated, i.e., GaS, GaSe, GaTe, InS, InSe, InTe, TℓS, TℓSe, TℓTe, GaInS₂, GaInSe₂, Ga₂InSe₃, Ga₂SeTe, Ga₃Se₂Te, GaTℓSe₂, and InTℓSe₂. Recording material (iii) which has a composition corresponding to a mixture of the III b VI b type stoichiometric compound and the multi-element stoichiometric compound may be any of materials composed of at least three elements selected from the group III b elements (i.e., Ga, In, and Tℓ), the IV b elements (i.e., Ge, Sn and Pb), the group V b elements (i.e., As, Sb, and Bi), and the group VI b elements (i.e., S, Se, and Te), containing at least one of Se and Te, and having a composition corresponding to a mixture obtained by mixing the III b VI b type stoichiometric compound with a multi-element stoichiometric compound (e,g., a IV b VI b type stoichiometric compound, a IV b VI b2 type stoichiometric compound, and a III b VI b type stoichiometric compound) at an arbitrary mixing ratio. The mixture preferably contains one of the two stoichiometric compounds in an amount of from 5 to 95 mol% based on the mixture, and the total content of Se and Te in the recording material is preferably not less than 30 atomic% based on the total atoms constituing the recording material. Examples of such a composition include a composition consisting of three elements containing Se and/or Te, a composition consisting of four elements containing Se and/or Te, and compositions consisting of more than four elements (e.g., a five-element composition and a six-element composition). Specific examples of recording material (iii) are mixtures composed of at least three elements containing Se and/or Te which are obtained by mixing the above-described III b VI b type stoichiometric compound with a IV b VI b type stoichiometric compound selected from GeS, GeSe, GeTe, SnS, SnSe, SnTe, PbS, PbSe, PbTe, GeSnTe₂, Ge₂SeTe, etc. at an arbitrary ratio; stoichiometric compounds made of these mixtures, e.g., Ga₂GeTe₃, In₂GeTe₃, In₃SnTe₄, InPbTe₂, TℓGeTe₂, and Tℓ₂GeTe₃; congruent compounds made of these mixtures, e.g., GaGeTe₂ and InGeTe₂; solid solutions made of these mixtures, which are composed of at least three elements containing Se and/or Te; and mixtures of the above-described stoichiometric compounds, solid solutions, etc. at an arbitrary ratio. Further included in recording material (iii) are mixtures composed of at least three elements containing Se and/or Te which are obtained by mixing the above-described III b VI b type stoichiometric compound and a IV b VI b2 type stoichiometric compound, e.g., GeS₂, GeSe₂, SnS₂, SnSe₂, and SnTe₂, at an arbitrary mixing ratio; stoichiometric compounds made of these mixtures, e.g., GaGeSe₃; solid solutions made of these mixtures, which are composed of at least three elements containing Se and/or Te; and mixtures of the above-described stoichiometric compounds, solid solutions, etc. at an arbitrary ratio. Furthermore included in recording material (iii) are mixtures composed of at least three elements containing Se and/or Te which are obtained by mixing the above-described III b VI b stoichiometric compound with a III b VI b type stoichiometric compound, e.g., GaS, GaSe, GaTe, InS, InSe, InTe, TℓS, TℓSe, TℓTe, GaInS₂, GaInSe₂, Ga₂InSe₃, Ga₂SeTe, Ga₃Se₂Te, GaTℓSe₂, and InTℓSe₂, at an arbitrary mixing ratio; stoichiometric compounds made of these mixtures, e.g., GaInSe₂, Ga₂InSe₃, and Ga₃SeTe₂; congruent compounds made of these mixtures, e.g., GaTℓSe₂, InTℓSe₂, and Ga₂SeTe; solid solutions made of these mixtures, which are composed of at least three elements containing Se and/or Te; and mixtures of the above-described stoichiometric compounds, solid solutions, etc. at an arbitrary mixing ratio. In recording material (iv), the III b2 VI b3 type stoichiometric compound includes those containing the same III b elements and the same VI b elements, e.g., Ga₂S₃, Ga₂Se₃, Ga₂Te₃, In₂S₃, In₂Se₃ In₂Te₃, Tℓ₂S₃, Tℓ₂Se₃, and Tℓ₂Te₃, and those containing different III b elements or different VI b elements, e.g., In₂STe₂. The multi-element stoichiometric compound in recording material (iv) is a stoichiometric compound composed of elements selected from at least two groups of the groups III b , IV b , and VI b , including III b2 VI b3 type stoichiometric compounds composed of the group III b element selected from Ga, In, and Tℓ and the group VI b element selected from S, Se, and Te and represented by chemical formula III b2 VI b3 ; and IV b VI b type stoichiometric compounds composed of the group IV b element selected from Ge, Sn, and Pb and the group VI b element selected from S, Se, and Te and represented by chemical formula IV b VI b . Specific examples of the III b2 VI b3 stoichiometric compounds are the same as enumerated above, i.e., Ga₂S₃, Ga₂Se₃, Ga₂Te₃, In₂S₃, In₂Se₃ In₂Te₃, Tℓ₂S₃, Tl₂Se₃, and In₂STe₂. Specific examples of the IV b VI b type stoichiometric compounds are the same as enumerated above, i.e., GeS, GeSe, GeTe, SnS, SnSe, SnTe, PbS, PbSe, PbTe, GeSnTe₂, and Ge₂SeTe. Recording material (iv) which has a composition corresponding to a mixture of the above-described III b2 VI b3 type stoichiometric compound and the multi-element stoichiometric compound may be any of materials composed of at least three elements selected from the group III b elements (i.e., Ga, In, and Tℓ), the group IV b elements (i.e., Ge, Sn and Pb), and the group VI b elements (i.e., S, Se, and Te), containing at least one of Se and Te, and having a composition corresponding to a mixture obtained by mixing the III b2 VI b3 type stoichiometric compound with a multi-element stoichiometric compound (e.g., a III b2 VI b ₃ type stoichiometric compound and a IV b VI b type stoichiometric compound) at an arbitrary mixing ratio. The mixture preferably contains one of the two stoichiometric compounds in an amount of from 5 to 95 mol% based on the mixture, and the total content of Se and Te in the recording material is preferably not less than 30 atomic% based on the total atoms constituing the recording material. Examples of such a composition include a composition consisting of three elements containing Se and/or Te, a composition consisting of four elements containing Se and/or Te, and compositions consisting of more than four elements (e.g., a five-element composition and a six-element composition). Specific examples of recording material (iv) are mixtures composed of at least three elements containing Se and/or Te which are obtained by mixing the above-described III b2 VI b3 type stoichiometric compounds, e.g., Ga₂S₃, Ga₂Se₃, Ga₂Te₃, In₂S₃, In₂Se₃ In₂Te₃, Tℓ₂S₃, Tℓ₂Se₃, Tℓ₂Te₃, and In₂STe₂, at an arbitrary ratio; congruent compounds made of these mixtures, e.g., In₂STe₂; solid solutions made of these mixtures, which are composed of at least three elements containing Se and/or Te; and mixtures of the above-described congruent compounds, solid solutions, etc. at an arbitrary ratio. Further included in recording material (iv) are mixtures composed of at least three elements containing Se and/or Te which are obtained by mixing the above-described III b2 VI b3 type stoichiometric compound and a IV b VI b type stoichiometric compound, e.g., GeS, GeSe, GeTe, SnS, SnSe, SnTe, PbS, PbSe, PbTe, GeSnTe₂, and Ge₂SeTe, at an arbitrary mixing ratio; stoichiometric compounds made of these mixtures, e.g., Ga₄SnTe₇; solid solutions made of these mixtures, which are composed of at least three elements containing Se and/or Te; and mixtures of the above-described stoichiometric compounds, solid solutions, etc. at an arbitrary ratio. Use of recording material (iii) or (iv) makes it possible to increase stability of the amorphous phase of the recording layer and to reduce the crystallization time of the recording layer. The reason for the reduction of the crystallization time of the above-described recording material layer as compared with the conventionally employed Ge-Sb-Te recording material layer is believed to be the same as set forth above with respect to recording material (ii). More specifically, in the III b VI b type stoichiometric compounds constituting a part of recording material (iii) and the III b2 VI b3 type stoichiometric compounds constituting a part of recording material (iv), a difference in electronegativity between Te whose electronegativity is the lowest (1.47) of the group VI b elements and Ga whose electronegativity is the highest (1.13) of the group III b elements is 0.34. On the other hand, a difference between S whose electronegativity is the highest (1.87) of the group VI b elements and Tℓ whose electronegativity is the lowest (0.94) of the group III b elements is 0.93. That is, the electronegativity difference between the group III b elements and the group VI b elements widely ranges from 0.34 to 0.93, indicating strong ionic bond strength between the group III b atom and the group VI b atom. From these considerations, it is believed that the group III b atom-group VI b atom bond strength is higher than the strength of the interatomic bonds constituting Ge-Sb-Te (most of them are covalent bonds) by the strength corresponding to the ionic bond strength. Therefore, when the III b VI b type stoichiometric compound is mixed with a multi-element stoichiometric compound composed of elements selected from at least two groups of the group III b (i.e., Ga, In, and Tℓ), the group IV b (i.e., Ge, Sn, and Pb), the group V b (i.e., As, Sb, and Bi), and the VI b (i.e., S, Se, and Te), or when the III b2 VI b3 type stoichiometric compound is mixed with a multi-element stoichiometric compound composed of elements selected from at least two groups of the group III b (i.e., Ga, In, and Tℓ), the group IV b (i.e., Ge, Sn, and Pb), and the group VI b (i.e., S, Se, and Te), the resulting mixture contains a group III b atom-group VI b atom having high bond strength. As a result, the mixture has reduced free energy in its crystal phase as compared with Ge-Sb-Te and thereby has a greater difference in free energy between the crystal phase and the amorphous phase, i.e., a greater transition energy. This seems to be the reason why recording materials (iii) and (iv) have a reduced time of crystallization as compared with the conventional Ge-Sb-Te recording material. It is considered that the higher stability of the amorphous phase of recording materials (iii) and (iv) as compared with Ge-Sb-Te can also be accounted for by the reason stated above with respect to recording material (ii). Recording Materials (v-1) to (v-8): Recording materials (v-1) to (v-8) achieve further improvement in stability in the amorphous phase while having a rate of crystallization equal to that of the above-described recording materials (i) to (iv). In recording material (v-1), the IV b VI b type stoichiometric compound includes SnS, SnSe, SnTe, PbS, PbSe, and PbTe, and the V b2 VI b3 type stoichiometric compound includes Sb₂S₃, Sb₂Se₃, Sb₂Te₃, Bi₂S₃, Bi₂Se₃, Bi₂Te₃, As₂S₃, As₂Se₃, and As₂Te₃. The stoichiometric three-element compound having a composition corresponding to a mixture of the IV b VI b type stoichiometric compound and the V b2 VI b3 type stoichiometric compound may be any of stoichiometric three-element compounds consisting of three elements selected from the IV b elements (i.e., Sn and Pb), the group V b elements (i.e., Sb, Bi, and As), and the group VI b elements (i.e., S, Se, and Te), containing at least one of Se and Te, and having a composition corresponding to a mixture of the IV b VI b type stoichiometric compound and the V b2 VI b3 type stoichiometric compound. The mixture preferably contains one of the two stoichiometric compounds in an amount of from 5 to 95 mol% based on the mixture. Specific examples of such stoichiometric three-element compounds are Sn₂Sb₂Se₅, Sn₂Sb₆Se₁₁, SnSb₂Te₄, Pb₂Sb₆Te₁₁, SnBi₂Te₄, PbBi₄Se₇, PbBi₂Se₄, Pb₃Bi₄Se₉, and PbBi₄Te₇. In recording material (v-1), at least one of the constituent elements of the above-described stoichiometric three-element compound is partly substituted by at least one of other elements selected from elements of the above-enumerated groups and Si and Ge belonging to the group IV b (i.e., the group IV b element selected from Sn, Pb, Si, and Ge, the group V b element selected from Sb, Bi, and As, and the group VI b element selected from S, Se and Te) to thereby form a solid solution containing 4 or more kinds of elements and containing at least one of Se and Te. The upper limit of the degree of substitution of the three elements constituting the stoichiometric three-element compound is the degree where the composition containing 4 or more kinds of elements resulted from the substitution no longer constitutes a solid solution possessing the crystal structure of the stoichiometric three-element compound. As long as the degree of substitution is below such a limit, the constituent element(s) may be substituted by other element(s) to an arbitrary extent. Where one constituent element is partly substituted by one other element, there is produced a four-element solid solution; and where it is partly substituted by two other elements, there is produced a five-element solid solution. Where each of two constituent elements is partly substituted by one other element, respectively, there is obtained a five-element solid solution. Where each of three constituent elements is partly substituted by one other element, respectively, there is obtained a six-element solid solution. In the cases where the constituent element to be substituted and the other element substituting for the constituent element are in the relationship that they can form a solid solution regardless the mixing ratio of the former element to the latter element (hereafter, referred to as a "full-range solid solution"), the strain of crystal structure attended by the substitution is relatively small so that structural changes of the stoichiometric three-element compound due to the substitution can be minimized. Therefore, a permissible degree of substitution can be broadened to advantage. Examples of combinations of elements having such a relationship are a combination of Ge and Si (the group IV b elements), a combination of Sb and Bi (the group V b elements), and a combination of Se and Te (the group VI b elements). The terminology "crystal structure of a stoichiometric three-element compound" as used herein means a structure of a certain stoichiometric three-element compound specified by atomic arrangement in a crystal lattice characteristic of the stoichiometric three-element compound. The terminology "solid solution possessing a crystal structure of a stoichiometric three-element compound" as used herein means a solid solution in which a part of atoms on lattice points of a crystal lattice characteristic of the stoichiometric three-element compound is substituted by a different atom(s) at random in such a manner that the different atom may be regarded as being uniformly dissolved in the crystal lattice. Recording materials (v-2) to (v-3) correspond to improved recording material (ii). In recording material (v-2), the I₂VI b type stoichiometric compound includes Li₂S, Li₂Se, Li₂Te, Na₂S, Na₂Se, Na₂Te, K₂S, K₂Se, K₂Te, Rb₂S, Rb₂Se, Rb₂Te, Cs₂S, Cs₂Se, Cs₂Te, Cu₂S, Cu₂Se, Cu₂Te, Ag₂S, Ag₂Se, Ag₂Te, Au₂S, Au₂Se, and Au₂Te. The V b2 VI b3 type stoichiometric compound includes Sb₂S₃, Sb₂Se₃, Sb₂Te₃, Bi₂S₃, Bi₂Se₃, Bi₂Te₃, As₂S₃, As₂Se₃, and As₂Te₃. The stoichiometric three-element compound which has a composition corresponding to a mixture of the I₂VI b type stoichiometric compound and the V b2 VI b3 type stoichiometric compound may be any of stoichiometric three-element compounds consisting of three elements selected from the group I elements (i.e., Li, Na, K, Rb, Cs, Cu, Ag, and Au), the group V b elements (i.e., As, Sb, and Bi), and the group VI b elements (i.e., S, Se, and Te), containing at least one of Se and Te, and having a composition corresponding to a mixture obtained by mixing the I₂VI b type stoichiometric compound with the V b2 VI b3 type stoichiometric compound. The mixture preferably contains one of the two stoichiometric compounds in an amount of from 5 to 95 mol% based on the mixture. Examples of such stoichiometric three-element compounds are Sn₂Sb₂Se₅, Sn₂Sb₆Se₁₁, SnSb₂Te₄, Pb₂Sb₆Te₁₁, SnBi₂Te₄, PbBi₄Se₇, PbBi₂Se₄, Pb₃Bi₄Se₉, and PbBi₄Te₇. Recording material (v-2) is characterized by comprising a solid solution containing 4 or more kinds of elements, obtained by partly substituting at least one of the constituent elements of the above-described stoichiometric three-element compound by at least one of other elements selected from elements of the above-enumerated groups, i.e., the group I element selected from Li, Na, K, Rb, Cs, Cu, Ag, and Au, the group V b element selected from As, Sb, and Bi, and the group VI b element selected from S, Se, and Te. In recording material (v-3), examples of the I₂VI b type stoichiometric compound are the same as those mentioned for recording material (v-2). Examples of the IV b VI b2 type stoichiometric compound are GeS₂, GeSe2₂, SnS₂, and SnSe₂. The stoichiometric three-element compound which has a composition corresponding to a mixture of the I₂VI b type stoichiometric compound and the IV b VI b2 type stoichiometric compound may be any of stoichiometric compounds consisting of three elements selected from the group I elements including Li, Na, K, Rb, Cs, Cu, Ag, and Au, the group IV b elements including Ge, Sn, and Pb, and the group VI b elements including S, Se, and Te, containing at least one of Se and Te, and having a composition corresponding to a mixture obtained by mixing the I₂VI b type stoichiometric compound with the IV b VI b2 type stoichiometric compound. The mixture preferably contains one of the two stoichiometric compounds in an amount of from 5 to 95 mol% based on the mixture. Examples of such stoichiometric three-element compounds include Cu₂SnSe₃. Recording material (v-3) is characterized by comprising a solid solution containing 4 or more kinds of elements, obtained by partly substituting at least one of the constituent elements of the above-described stoichiometric three-element compound by at least one of other elements selected from the group consisting of elements of the above-enumerated groups and Si belonging to the group IV b , i.e., the group I element selected from Li, Na, K, Rb, Cs, Cu, Ag, and Au, the group IV b element selected from Si, Ge, Sn, and Pb, and the group VI b element selected from S, Se, and Te. Recording materials (v-4), (v-5), and (v-6) correspond to improved recording material (iii). In recording material (v-4), the III b VI b type stoichiometric compound includes GaS, GaSe, GaTe, InS, InSe, InTe, TℓS, TℓSe, and TℓTe. The IV b VI b type stoichiometric compound includes GeS, GeSe, GeTe, SnS, SnSe, SnTe, PbS, PbSe, and PbTe. The stoichiometric three-element compound which has a composition corresponding to a mixture of the III b VI b type stoichiometric compound and the IV b VI b type stoichiometric compound may be any of stoichiometric compounds consisting of three elements selected from the group III b elements including Ga, In, and Tℓ, the group IV b elements including Ge, Sn, and Pb, and the group VI b elements including S, Se, and Te, containing at least one of Se and Te, and having a composition corresponding to a mixture obtained by mixing the III b VI b type stoichiometric compound with the IV b VI b type stoichiometric compound. The mixture preferably contains one of the two stoichiometric compounds in an amount of from 5 to 95 mol% based on the mixture. Examples of such stoichiometric three-element compounds are Ga₂GeTe₃, GaGeTe₂, In₂GeTe₃, In₃SnTe₄, InPbTe₂, InGeTe₂, TℓGeTe₂, and Tℓ₂GeTe₃. Recording material (v-4) is characterized by comprising a solid solution containing 4 or more kinds of elements, obtained by partly substituting at least one of the constituent elements of the above-described stoichiometric three-element compound by at least one of other elements selected from the group consisting of the above-enumerated elements, Al belonging to the group III b , and Si belonging to the group IV b , i.e., the group III b element selected from Aℓ, Ga, In, and Tℓ, the group IV b element selected from Si, Ge, Sn, and Pb, and the group VI b element selected from S, Se, and Te. In recording material (v-5), examples of the III b VI b type stoichiometric compound are the same as those enumerated with respect to recording material (v-4), and examples of the IV b VI b2 type stoichiometric compound are GeS₂, GeSe₂, SnS₂, SnSe₂, and SnTe₂. The stoichiometric three-element compound which has a composition corresponding to a mixture of the III b VI b type stoichiometric compound and the IV b VI b2 type stoichiometric compound may be any of stoichiometric compounds consisting of three elements selected from the group III b elements including Ga, In, and Tℓ, the group IV b elements including Ge, Sn, and Pb, and the group VI b elements including S, Se, and Te, containing at least one of Se and Te, and having a composition corresponding to a mixture obtained by mixing the III b VI b type stoichiometric compound with the IV b VI b2 type stoichiometric compound. The mixture preferably contains one of the two stoichiometric compounds in an amount of from 5 to 95 mol% based on the mixture. Examples of such stoichiometric three-element compounds are GaGeSe₃. Recording material (v-5) is characterized by comprising a solid solution containing 4 or more kinds of elements, obtained by partly substituting at least one of the constituent elements of the above-described stoichiometric three-element compound by at least one of other elements selected from the group consisting of the above-enumerated elements, Aℓ belonging to the group III b , and Si belonging to the group IV b , i.e., the group III b element selected from Aℓ, Ga, In, and Tℓ, the group IV b element selected from Si, Ge, Sn, and Pb, and the group VI b element selected from S, Se, and Te. In recording material (v-6), examples of the III b VI b type stoichiometric compound are the same as those enumerated with respect to recording material (v-4). The stoichiometric three-element compound which has a composition corresponding to a mixture of two III b VI b type stoichiometric compounds may be any of stoichiometric compounds consisting of three elements selected from the group III b elements including Ga, In, and Tℓ and the group VI b elements including S, Se, and Te, containing at least one of Se and Te, and having a composition corresponding to a mixture obtained by mixing two III b VI b type stoichiometric compounds. The mixture preferably contains one of the two stoichiometric compounds in an amount of from 5 to 95 mol% based on the mixture. Examples of such stoichiometric three-element compounds are GaInSe₂, Ga₂InSe₃, Ga₃SeTe₂, Ga₂SeTe, GaTℓSe₂, and InTℓSe₂. Recording material (v-6) is characterized by comprising a solid solution containing 4 or more kinds of elements, obtained by partly substituting at least one of the constituent elements of the above-described stoichiometric three-element compound by at least one of other elements selected from the group consisting of the above-enumerated elements and Aℓ belonging to the group III b , i.e., the group III b element selected from Aℓ, Ga, In, and Tℓ and the group VI b element selected from S, Se, and Te. Recording materials (v-7), (v-6), and (v-8) corresponde to improved recording material (iv). In recording material (v-7), the III b2 VI b3 type stoichiometric compound includes Ga₂S₃, Ga₂Se₃, Ga₂Te₃, In₂S₃, In₂Se₃, In₂Te₃, Tℓ₂S₃, Tℓ₂Se₃, and Tℓ₂Te₃. The stoichiometric three-element compound which has a composition corresponding to a mixture of two III b2 VI b3 type stoichiometric compounds may be any of stoichiometric compounds consisting of three elements selected from the group III b elements including Ga, In, and Tℓ, and the group VI b elements including S, Se, and Te, containing at least one of Se and Te, and having a composition corresponding to a mixture obtained by mixing two III b2 VI b3 type stoichiometric compounds. The mixture preferably contains one of the two stoichiometric compounds in an amount of from 5 to 95 mol% based on the mixture. Examples of such stoichiometric three-element compounds include In₂STe₂. Recording material (v-7) is characterized by comprising a solid solution containing 4 or more kinds of elements, obtained by partly substituting at least one of the constituent elements of the above-described stoichiometric three-element compound by at least one of other elements selected from the group consisting of the above-enumerated elements and Al belonging to the group III b , i.e., the group III b element selected from Aℓ, Ga, In, and Tℓ and the group VI b element selected from S, Se, and Te. In recording material (v-8), examples of the III b2 VI b3 type stoichiometric compound are the same as those enumerated with respect to recording material (v-7), and examples of the IV b VI b type stoichiometric compound are GeS, SeSe, SeTe, SnS, SnSe, SnTe, PbS, PbSe, and PbTe. The stoichiometric three-element compound which has a composition corresponding to a mixture of the III b2 VI b3 type stoichiometric compound and the IV b VI b type stoichiometric compound may be any of stoichiometric compounds consisting of three elements selected from the group III b element selected from Ga, In, and Tℓ, the group IV b element selected from Ge, Sn, and Pb, and the group VI b element selected from S, Se, and Te, containing at least one of Se and Te, and having a composition corresponding to a mixture obtained by mixing the III b2 VI b3 type stoichiometric compound with the IV b VI b type stoichiometric compound. The mixture preferably contains one of the two stoichiometric compounds in an amount of from 5 to 95 mol% based on the mixture. Examples of such stoichiometric three-element compounds include Ga₄SnTe₇. Recording material (v-8) is characterized by comprising a solid solution containing 4 or more kinds of elements, obtained by partly substituting at least one of the constituent elements of the above-described stoichiometric ternary compound by at least one of other elements selected from the group consisting of the above-enumerated elements, Aℓ belonging to the group III b , and Si belonging to the group IV b , i.e., the group III b element selected from Aℓ, Ga, In, and Tℓ, the group IV b elemen selected from Si, Ge, Sn, and Pb, and the group VI b element selected from S, Se, and Te. In recording materials (v-2) to (v-8), the upper limit of the degree of substitution of the three elements constituting the stoichiometric three-element compound is the degree where the composition containing 4 or more kinds of elements resulted from the substitution no longer constitutes a solid solution possessing the crystal structure of the stoichiometric three-element compound. As long as the degree of substitution is below such a limit, the constituent element(s) may be substituted by other element(s) to an arbitrary extent. In the cases where the constituent element to be substituted and the other element substituting for the constituent element those capable of forming a full-range solid solution, the strain of crystal structure attended by the substitution is relatively small so that structural changes of the stoichiometric three-element compound due to the substitution can be minimized. Therefore, a permissible degree of substitution can be broadened to advantage similarly to the case of recording material (v-1). To assure an increased rate of crystallization, it is advantageous to use Tℓ as the "other element" which substitutes for a part of the constituent elements of the stoichiometric three-element compound. In recording materials (v-1) to (v-8), the degree of substitution of the three elements constituting the stoichiometric three-element compounds is preferably not less than 0.5% of the element(s) a part of which is or are substituted, and the total content of Se and Te in recording material is preferably not less than 30 atomic% based on the total atoms constituting the recording mateiral. As mentioned above, since the recording material layer using each of recording materials (v-1) to (v-8) has a crystal structure of a stoichiometric three-element compound, it has an increased rate of crystallization and exhibits improved stability in its amorphous phase. Further, being comprised of a solid solution in which at least one of the three elements constituting the stoichiometric three-element compound is partly substituted with other element(s), the recording layer exhibits further enhanced stability in the amorphous phase. The increased rate of crystallization obtained by using recording material (v-1) seems to be based on the same reason as described with respect to recording material (i) ; and that obtained by using recording materials (v-2) to (v-9) is believed to be based on the same reason as described with respect to recording material (ii). More specifically, in the I₂VI b type stoichiometric compounds constituting a part of recording materials (v-2) to (v-3), a difference in electronegativity between Te whose electronegativity is the lowest (1.47) of the group VI b elements and Li whose electronegativity is the highest (1.00) of the group I elements is 0.47. On the other hand, a difference between S whose electronegativity is the highest (1.87) of the group VI b elements and Ag whose electronegativity is the lowest (0.57) of the group I elements is 1.30. That is, the electronegativity difference between the group I element and the group VI b element widely ranges from 0.47 to 1.30, indicating high ionic bond strength between the group I atom and the group VI b atom. Likewise, in the III b VI b type stoichiometric compounds constituting a part of recording materials (v-4) to (v-8), a difference in electronegativity between Te whose electronegativity is the lowest (1.47) of the group VI b elements and Ga whose electronegativity is the highest (1.13) of the group III b elements is 0.34. On the other hand, a difference between S whose electronegativity is the highest (1.87) of the group VI b elements and Tℓ whose electronegativity is the lowest (0.94) of the group III b elements is 0.93. That is, the electronegativity difference between the group III b element and the group VI b element widely ranges from 0.34 to 0.93, indicating high ionic bond strength between the group III b atom and the group VI b atom. From these considerations, it is believed that the group I atom-group VI b atom bond strength or the group IIIb b atom-group VI b atom bond strength is higher than the strength of the interatomic bonds constituting Ge-Sb-Te (most of them are covalent bonds) by the strength corresponding to the ionic bond strength. In recording materials (v-2) to (v-3), the stoichiometric three-element compound having a composition corresponding to a mixture of the I₂VI b type stoichiometric compound and the V b2 VI b3 type stoichiometric compound, the III b2 VI b3 type stoichiometric compound, or the IV b VI b2 type stoichiometric compound contains a bond of the group I atom and the group VI b atom exhibiting high bond strength. In recording materials (v-4) to (v-6), the stoichiometric compound having a composition corresponding to a mixture of the III b VI b type stoichiometric compound and the IV b VI b type stoichiometric compound, the IV b VI b2 type stoichiometric compound, or the III b VI b type stoichiometric compound contains a bond of the group III b atom and the group VI b atom exhibiting high bond strength. In recording materials (v-7) and (v-8), the stoichiometric compound having a composition corresponding to a mixture of the III b2 VI b3 type stoichiometric compound and the III b2 VI b3 type stoichiometric compound or the IV b VI b type stoichiometric compound contains a bond of the group III b atom and the group VI b atom exhibiting high bond strength. As a result, these materials have a reduced free energy in the crystal phase thereof as compared with Ge-Sb-Te and thereby have a greater difference in free energy between the crystal phase and the amorphous phase, i.e., a greater transition energy. This is assumed to be the reason why recording materials (v-2) to (v-8) have an increased rate of crystallization as compared with the conventional Ge-Sb-Te recording material. Further, it is considered that the higher stability of the amorphous phase of recording materials (v-1) to (v-8) as compared with the conventional Ge-Sb-Te or Sb₂Te₃ recording materials can also be accounted for by the reason stated above with respect to recording material (ii). In addition, recording materials (v-1) to (v-8) exert stronger inhibitory effects on atomic movement due to a difference in atomic radius from the nature of materials containing 4 or more kinds of elements and thus have higher activation energy as compared with the two-element material, Sb₂Te₃, or the three-element material, Ge-Sb-Te. From this viewpoint, too, these materials show further improved stability in the amorphous phase thereof. Recording Material (vi): Recording material (vi) is characterized by comprising a solid solution synthesized from plural elements or stoichiometric compounds having the same crystal system. The term "crystal system" as used herein is in accordance with a classification of unit space lattices based on angle between two axes and length of axes, including seven crystal systems, i.e., triclinic, cubic, tetragonal, rhombohedral, orthorhombic, hexagonal, and monoclinic systems. For example, Ga, In and Tℓ as the group III b elements; Ge, Sn, and Pb as the group IV b elements; As, Sb, and Bi as the group V b elements; S, Se, and Te as the group VI b elements; and stoichiometric compounds composed of these elements which constitute the solid solution of recording material (vi) can be classified based on the crystal system as shown in Table 3 below. While a classification by strukturbericht, which relates to crystal structures taking the bond state of compounds into consideration in addition to the above-described classification of crystal system, divides crystals into A type (element) of from A1 to A20, B type (compound XY) of from B1 to b37, C type (compound XY₂) of from C1 to C54, D type (X m Y n ) of from D 0₂ to D 8₁₁, H 1₁, L 1₀ to L 2₂, etc., it is preferred that the solid solution be synthesized from plural elements or stoichiometric compounds having the same crystal system based on the classification by strukturbericht. In Table 3, symbols in parentheses indicate crystal structure according to the classification by strukturbericht. The solid solution synthesized from plural elements or stoichiometric compounds having the same crystal system includes a solid solution containing three or more kinds of elements and containing Se and/or Te, which is synthesized by mixing at least three elements having the same crystal system; a solid solution containing three or more kinds of elements and containing Se and/or Te, which is synthesized by mixing at least one element and at least one two-element compound having the same crystal system; a solid solution containing three or more kinds of elements and containing Se and/or Te, which is synthesized by mixing at least one element and at least one three-element compound having the same crystal system; a solid solution containing three or more kinds of elements and containing Se and/or Te, which is synthesized by mixing at least two two-element compounds having the same crystal system; a solid solution containing three or more kinds of elements and containing Se and/or Te, which is obtained by mixing at least two three-element compounds having the same crystal system; and a solid solution containing three or more kinds of elements and containing Se and/or Te, which is synthesized by mixing a two-element compound and a three-element compound having the same crystal system. Referring to Table 3 above, specific examples of these solid solutions include a solid solution containing three or more kinds of elements synthesized by mixing elements selected from Tℓ, S, Se, and Te, all of which belong to the hexagonal system; a solid solution containing three or more kinds of elements synthesized by mixing at least two 2-element compounds selected from Sb₂Te₃, Bi₂Te₃, Bi₂Se₃, etc., all of which belong to the rhombohedral system; a three-element solid solution synthesized by mixing SnTe (B1), GeTe (B1), and PbTe (B1), all of which belong to the cubic system; a three-element solid solution synthesized by mixing GeTe (B1), InSb (B3), and Tℓ₇Sb₂ (B2), all of which belong to the cubic system; a solid solution containing three or more kinds of elements synthesized by mixing at least two three-element compounds selected from GeSb₂Te₄, Ge₂Bi₂Te₅, PbBi₄Se₇, etc., all of which belong to the rhombohedral system; a three-element solid solution synthesized by mixing an element, e.g., As, Bi, and Sb, and a two-element compound, e.g., Sb₂Te₃ and Bi₂Te₃, all of which belong to the rhombohedral system; and a solid solution containing three or more kinds of elements synthesized by mixing a two-element compound, e.g., Sb₂Te₃ and Bi₂Te₃, and a three-element compound, e.g., GeSb₄Te₇ and GeBi₂Te₄, all of which belong to the rhombohedral system. The terminology "solid solution" as used herein means a phase which is formed by substituting an atom at a lattice point of a crystal phase characteristic of a certain element or compound by a different atom at random or a phase in which a different atom is statistically distributed in interstices of the lattice of a crystal phase characteristic of a certain element or compound, that is, a phase in which a foreign substance is uniformly dissolved in a certain crystal phase. Of these solid solutions, congruent solid solution which has an inherent melting point and does not decompose up to this melting point are preferably used for recording material (vi). Use of recording material (vi) makes it possible to shorten the time of crystallization of a recording material layer and, at the same time, to obtain increased stability in its amorphous phase. The reason, the inventors believe, for the reduction in crystallization time achieved by recording material (vi) is as follows. Recording material (vi) is comprised of a solid solution whose crystal phase is a single phase. Moreover, since the solid solution is synthesized from plural elements or stoichiometric compounds having the same crystal system, it has satisfactory crystal alignment with small lattice strain and a very small free energy in its crystal phase. Accordingly, the difference in free energy between the crystal phase and the amorphous phase, i.e., the transition energy becomes great to thereby reduce the time of crystallization in the recording material layer. To the contrary, since the conventional Ge-Sb-Te recording material is synthesized by mixing cubic GeTe and rhombohedral Sb₂Te₃, it has extremely poor crystal alignment and contains large strain in the crystal lattice. Accordingly, the free energy in the crystal phase is large, and the difference in free energy between the crystal phase and the amorphous phase, i.e., the transition energy, is small. This appears to account for the slow crystallization of the conventional Ge-Sb-Te recording material layer. The inventors believed, on the other hand, that the improvement in amorphous phase stability of recording material (vi) over the conventional Sb₂Te₃ recording material arises from strong covalent bond character of interatomic bonds between constituent elements of recording material (vi), i.e., the group III b elements (Ga, In, and Tℓ), the group IV b elements (Ge, Sn, and Pb), the group V b elements (As, Sb, and Bi), and the group VI b elements (S, Se, and Te). Further, from the nature of a material containing three or more kinds of elements, recording material (vi) exerts strong inhibitory effects on atomic movement due to a difference of atomic radius thereby having larger activation energy than the two-element material, Sb₂Te₃. This is also a reason for the high stability of the amorphous phase. In addition, since recording materials (i) to (vi) according to the present invention contain at least one of Se and Te, they absorb visible light and near infrared light. The optical recording medium using the recording material according to the present invention basically comprises a light-transmitting substrate having thereon a recording material layer generally having a thickness of from 10 to 200 nm and preferably from 20 to 40 nm. For the purpose of preventing deformation of the recording material layer from being deformed in the period of from protecting the recording material layer from mechanical damage or oxidation, a protective layer may be provided on the recording material layer. Examples of suitable light-transmitting substrates include glass and resin materials, e.g., acrylic resins, polycarbonate resins, and epoxy resins. In using resin materials, an inorganic dielectric material layer comprised of, for example, SiO₂, ZrO₂, ZnS, Si₃N₄, Ta₂O₅, AℓN, Aℓ₂O₃, etc., or a mixture thereof, may be provided between the recording material layer and the substrate in order to prevent thermal damage of the resin material. The thickness of the inorganic dielectric material layer is generally from 50 to 200 nm. In optical recording media which perform recording, reproduction, and erasing by irradiation with a light beam from the side opposite to the substrate, the substrate may be, as a matter of course, comprised of a light non-transmitting material, such as aluminum. Materials of the protective layer include the same materials constituting the above-described inorganic dielectric material layer and, in addition, resin materials, e.g., ultraviolet curing resins, acrylic resins, polycarbonate resins, and epoxy resins, and glass. The protective layer may have either a single-layered structure or a multi-layered structure. The thickness of the protective layer is generally from 10 to 200 nm. Where a part of the protective layer in contact with the recording material layer is made of a resin material, an inorganic dielectric material layer may be inserted therebetween as in the case of the resinous substrate. Further, an adhesive layer of 1 to 10 µm in thickness may also be provided between the protective layer and the inorganic dielectric material layer. For the purpose, conventional adhesives such as a ultraviolet curing adhesive may be used. The recording material layer can be formed by sputtering, vacuum deposition, or the like technique. Sputtering can be carried out by co-sputtering whereby a plurality of targets are employed, and a controlled power is applied to each target to synthesize any desired composition and, at the same time, to deposit the composition onto a substrate. Sputtering can also be performed by using a single alloy target corresponding to the desired composition. For example, a plurality of targets which can be used for forming a recording layer of recording material (i) by simultaneous sputtering include alloy targets composed of the above-described IV b VI b type stoichiometric compound or V b2 VI b3 type stoichiometric compound, alloy targets prepared by arbitrarily mixing elements of the IV b , V b , and VI b groups, and a single target made up of an element selected from the groups IV b , V b , and VI b . A plurality of targets which can be used for forming a recording layer of recording material (ii) by simultaneous sputtering include alloy targets composed of the above-described I₂VI b type stoichiometric compound, V b2 VI b3 type stoichiometric compound, III b2 VI b3 type stoichiometric compound, etc., alloy targets prepared by arbitrarily mixing elements of the I, III b , IV b , V b , and VI b groups, and a single target made up of an element selected from the groups I, III b , IV b , V b , and VI b . A plurality of targets for forming a recording layer of recording material (iii) or (iv) by co-sputtering include alloy targets composed of the above-described III b VI b type stoichiometric compound, IV b VI b type stoichiometric compound, IV b VI b2 type stoichiometric compound, III b2 VI b3 type stoichiometric compound, etc., alloy targets prepared by arbitrarily mixing elements of the III b , IV b , V b , and VI b groups, and a single target made up of an element selected from the groups III b , IV b , V b , and VI b . The above-described vacuum deposition can be conducted by using a plurality of vapor sources and controlling the respective rate of vaporization to obtain a desired composition and simultaneously deposit the composition onto a substrate. In the optical recording medium of the invention, the amorphous phase of the recording material layer may correspond to a recorded state, with the crystal phase to an erased state, or vice versa. The present invention is now illustrated in greater detail by way of Examples and Comparative Examples, but it should be understood that the present invention is not deemed to be limited thereto. In Examples and Comparative Examples, recording and erasion characteristics of optical recording media were evaluated in terms of time of crystallization and stability of amorphous phase according to the following standards. Standards for Evaluation: 1. Time of Crystallization Good Crystallization completes in 100 ns. Poor Crystallization does not complete in 100 ns. 2. Stability Good Crystallization temperature is 120°C or higher. Poor Crystallization temperature is less than 120°C. COMPARATIVE EXAMPLE 1 Optical recording media having a structure shown in Fig. 1 were prepared, each comprising 1.2 mm-thick acrylic resin substrate (1) having thereon 100 nm-thick inorganic dielectric material layer (2) made of SiO₂, 100 nm-thick recording material layer (3), 100 nm thick inorganic dielectric material layer (4) made of SiO₂, ultraviolet curable acrylic resin adhesive layer (5), and 1.2 mm-thick acrylic resin protective layer (6) in this order. Recording material layer (3) was formed by sputtering using a radio frequency (RF) magnetron. In more detail, the alloy targets were mounted in the RF magnetron sputtering chamber, and the pressure in the chamber was reduced to 2 x 10⁻⁶ Torr, followed by introducing argon gas to adjust the pressure to 4 x 10⁻³ Torr. The RF power applied to the respective alloy targets was suitably controlled within the range of from 25 to 200 W to obtain recording material layers having compositions shown in Table 4 below. The resulting optical recording media were designated Samples 1 to 2. Comparative Sample 1 or 2 was prepared by using Sb-Te or Ge-Sb-Te, respectively, as a recording material, with the layer structure being as indicated above. The recording material layer of Samples 1 or 2 was formed by RF magnetron sputtering using a single alloy target of Sb₂Te₃ or two alloy targets of Sb₂Te₃ and GeTe, respectively. Samples 1 to 2 showed absorption in the wavelength region of from visible light to near infrared light and could be used as optical recording media at least in the wavelength region of from 400 nm to 860 nm. Recording and erasion were carried out on each of Samples 1 to 2 using a semi-conductor laser having a wavelength of 830 nm as a light source. Recording and erasing characteristics were examined and evaluated according to the above-described standards, and the results obtained are shown in Table 4. EXAMPLE 1 Optical recording media having a structure shown in Fig. 1 were prepared, each comprising 1.2 mm-thick acrylic resin substrate (1) having thereon 100 nm-thick inorganic dielectric material layer (2) made of SiO₂, 100 nm-thick recording material layer (3) made of Li-Sb-Se, 100 nm-thick inorganic dielectric material layer (4) made of SiO₂, ultraviolet curable acrylic resin adhesive layer (5), and 1.2 mm-thick acrylic resin protective layer (6) in this order. Recording material layer (3) was formed by sputtering using an RF magnetron and two alloy targets, Li₂Se and Sb₂Se₃ while appropriately controlling the RF power to the respective alloy target so as to synthesize a composition shown in Table 5 below. The resulting optical recording media were designated Samples 3 and 4. Samples 3 and 4 showed absorption in the wavelength region of from visible light to near infrared light and could be used as optical recording media at least in the wavelength region of from 400 nm to 860 nm. Recording and erasion characteristics when these recording media were used by using a semi-conductor laser having a wavelength of 830 nm as a light source are also shown in Table 5. As is apparent from Table 5, Samples 3 and 4 have a reduced time of crystallization, making it possible to conduct recording and erasion of information at a higher speed, as compared with Sample 2 of Comparative Example 1 and also have an increased crystallization temperature, an indication of improved stability of the amorphous phase, as compared with Sample 1 of Comparative Example 1. In particular, it was confirmed that the crystal phase of Li-Sb-Se in Sample 3 is a single phase composed of a congruent compound as expected from the pseudo-binary phase diagram of Fig. 7 , i.e., LiSbSe₂. Therefore, the recording material had a short crystallization time and exhibited satisfactory stability on repetition of rewriting. EXAMPLE 2 Optical recording media (Samples 5 and 6) were prepared in the same manner as in Example 1, except that the recording material layer was composed of Na-Sb-Se. The Na-Sb-Se recording material was formed by binary co-sputtering using two alloy targets of Na₂Se and Sb₂Se₃. Recording and erasion characteristics of the resulting optical recording media are shown in Table 6 below. EXAMPLE 3 Optical recording media were prepared in the same manner as in Example 1, except that the recording material layer was composed of Cu-Sb-Se. The Cu-Sb-Se recording material was formed by binary co-sputtering using a single target prepared by mixing Cu₂Se and Sb₂Se₃ at a ratio of 60:40 (Sample 7), 50:50 (Sample 8), or 40:60 (Sample 9). Recording and erasion characteristics of the resulting optical recording media are shown in Table 6 below. As is apparent from Table 6, Samples 5 to 9 have a reduced time of crystallization, making it possible to conduct recording and erasing of information at a higher speed, and also have a higher crystallization temperature, an indication of improved stability of the amorphous phase, as compared with Comparative Example 1, similarly to Samples 3 and 4 of Example 1. As expected from the pseudo-binary phase diagrams of Figs. 8 to 9 , it was confirmed that the crystal phase of Na-Sb-Se in Sample 5, and Cu-Sb-Se in Sample 8 is a single phase composed of a stoichiometric compound which is stable at room temperature, i.e., NaSbSe₂, and CuSbSe₂, respectively. It is apparent from Examples 1 to 3 that use of recording material (ii) according to the present invention reduces the time of crystallization of the recording material layer and increases stability of the recording material layer in its amorphous phase. The optical recording media using recording material (ii) are thus capable of rewriting at a high speed and maintaining recorded information for an extended period of time. Because recording material (ii) shows absorption of visible and near infrared light due to the presence of at least one of Se and Te, widespread light sources inclusive of semiconductor lasers can be applied to the optical recording media using recording material (ii). Where the crystal phase of recording material (ii) is a single phase composed of a stoichiometric compound, the free energy of the crystal phase is further reduced, which leads to further reduction of the crystallization time. In addition, because such a crystal phase does not suffer from phase separation, the recording characteristics remain unchanged even rewriting is conducted many times. Where the above-described stoichiometric compound is composed of a congruent compound, local crystallization does not take place during cooling so that unchangeability of the recording characteristics on repeated use is further ensured. EXAMPLE 4 Optical recording media having a structure shown in Fig. 1 were prepared, each comprising 1.2 mm-thick acrylic resin substrate (1) having thereon 100 nm-thick inorganic dielectric material layer (2) made of SiO₂, 100 nm-thick recording material layer (3) made of Ga-Ge-Te, 100 nm-thick inorganic dielectric material layer (4) made of SiO₂, ultraviolet curable acrylic resin adhesive layer (5), and 1.2 mm-thick acrylic resin protective layer (6) in this order. Recording material layer (3) was formed by sputtering using an RF magnetron and two alloy targets, GaTe and GeTe while appropriately controlling the RF power to the respective alloy target so as to synthesize a composition shown in Table 7 below. The resulting optical recording media were designated Samples 10 to 12. Samples 10 to 12 showed absorption in the wavelength region of from visible light to near infrared light and could be used as optical recording media at least in the wavelength region of from 400 nm to 860 nm. Recording and erasion characteristics when these recording media were used by using a semi-conductor laser having a wavelength of 830 nm as a light source are also shown in Table 7. As is apparent from Table 7, Samples 10 to 12 have a reduced time of crystallization as compared with Sample 2 of Comparative Example 1, making it possible to conduct recording and erasing of information at a higher speed, and also have a higher crystallization temperature as compared with Sample 1 of Comparative Example 1, indicating improved stability of the amorphous phase. As expected from the pseudo-binary phase diagrams of Fig. 14 , it was confirmed that the crystal phase of Ga-Ge-Te in Samples 10 and 12 is a single phase composed of a stoichiometric compound which is stable at room temperature, i.e., GaGeTe₂ and Ga₂GeTe₃, respectively. In particular, GaGeTe₂ of Sample 10 is a congruent compound and, therefore, the sample had a shorter crystallization time and exhibited higher stability on repetition of rewriting. EXAMPLE 5 Optical recording media (Samples 13 to 15) were prepared in the same manner as in Example 4, except that the recording material layer was composed of In-Ge-Te. The In-Ge-Te recording material was formed by binary co-sputtering using a single target obtained by mixing InTe and GeTe at a ratio of 50:50 (Sample 13), 60:40 (Sample 14) or 67:33 (Sample 15). Recording and erasing characteristics of the resulting optical recording media are shown in Table 8 below. EXAMPLE 6 A optical recording medium was prepared in the same manner as in Example 4, except that the recording material layer was composed of In-Sn-Te. The In-Sn-Te recording material was formed by binary co-sputtering using two alloy targets of InTe and SnTe. Recording and erasing characteristics of the resulting optical recording media are shown in Table 8 below. EXAMPLE 7 Optical recording media (Samples 17 to 18) were prepared in the same manner as in Example 4, except that the recording material layer was composed of In-Pb-Te. The In-Pb-Te recording material was formed by binary co-sputtering using two alloy targets of InTe and PbTe. Recording and erasing characteristics of the resulting optical recording media are shown in Table 8 below. EXAMPLE 8 Optical recording media (Samples 19 to 21) were prepared in the same manner as in Example 4, except that the recording material layer was composed of Tℓ-Ge-Te. The Tℓ-Ge-Te recording material was formed by binary co-sputtering using two alloy targets of TℓTe and GeTe. Recording and erasing characteristics of the resulting optical recording media are shown in Table 8 below. EXAMPLE 9 Optical recording media (Samples 22 to 24) were prepared in the same manner as in Example 4, except that the recording material layer was composed of Ga-Tℓ-Se. The Ga-Tℓ-Se recording material was formed by binary co-sputtering using a single target prepared by mixing GaSe and TeSe at a ratio of 40:60 (Sample 22), 50:50 (Sample 23), or 60:40 (Sample 24). Recording and erasing characteristics of the resulting optical recording media are shown in Table 8 below. EXAMPLE 10 Optical recording media (Samples 25 to 27) were prepared in the same manner as in Example 4, except that the recording material layer was composed of In-Tℓ-Se. The In-Tℓ-Se recording material was formed by binary co-sputtering using two alloy targets of InSe and TℓSe. Recording and erasing characteristics of the resulting optical recording media are shown in Table 8 below. EXAMPLE 11 Optical recording media (Samples 28 to 30) were prepared in the same manner as in Example 4, except that the recording material layer was composed of Ga-Se-Te. The Ga-Se-Te recording material was formed by binary co-sputtering using two alloy targets of GaSe and GaTe. Recording and erasing characteristics of the resulting optical recording media are shown in Table 8 below. EXAMPLE 12 Optical recording media (Samples 31 to 33) were prepared in the same manner as in Example 4, except that the recording material layer was composed of In-S-Te. The In-S-Te recording material was formed by binary co-sputtering using two alloy targets of In₂S₃ and In₂Te₃. Recording and erasing characteristics of the resulting optical recording media are shown in Table 8 below. EXAMPLE 13 Optical recording media (Samples 34 to 35) were prepared in the same manner as in Example 4, except that the recording material layer was composed of Ga-Sn-Te. The Ga-Sn-Te recording material was formed by binary co-sputtering using two alloy targets of Ga₂Te₃ and SnTe. Recording and erasing characteristics of the resulting optical recording media are shown in Table 8 below. EXAMPLE 14 A optical recording medium (Sample 36) was prepared in the same manner as in Example 4, except that the recording material layer was composed of Ga-Ge-Se. The Ga-Ge-Se recording material was formed by binary co-sputtering using two alloy targets of GaSe and GeSe₂. Recording and erasing characteristics of the resulting optical recording medium are shown in Table 8 below. As is apparent from Table 8, Samples 13 to 36 have a reduced time of crystallization, making it possible to conduct recording and erasing of information at a higher speed, and also have a higher crystallization temperature, an indication of improved stability of the amorphous phase, as compared with Comparative Example 1, similarly to Samples 10 to 12 of Example 4. As expected from the pseudo-binary phase diagrams of Figs. 15 to 23 , it was confirmed that the crystal phase of In-Ge-Te in Samples 13 and 15, In-Sn-Te in Sample 16, In-Pb-Te in Sample 17, Tℓ-Ge-Te in Samples 19 and 21, Ga-Tℓ-Se in Sample 23, In-Tℓ-Se in Sample 26, Ga-Se-Te in Sample 29, In-S-Te in Sample 32, and Ga-Sn-Te in Sample 35 is a single phase composed of a stoichiometric compound which is stable at room temperature, i.e., InGeTe₂, In₂GeTe₃, In₃SnTe₄, InPbTe₂, TℓGeTe₂, Tℓ₂GeTe₃, TℓGaSe₂, TℓInSe₂, Ga₂SeTe, In₂STe₂, and Ga₄SnTe₇, respectively. In particular, InGeTe₂ of Sample 13, TℓGaSe₂ of Sample 23, TℓInSe₂ of Sample 26, Ga₂SeTe of Sample 29, and In₂STe₂ of Sample 32 are congruent compounds and, therefore, these samples had a short crystallization time and exhibited satisfactory stability on repetition of rewriting. Further, the crystal phase of Ga-Ge-Se of Sample 36 is also composed of a stoichiometric compound stable at room temperature as represented by GaGeSe₃, and the sample had a short crystallization time and exhibited satisfactory stability on repetition of rewriting. It is apparent from Examples 4 to 14 that use of recording material (iii) or (iv) according to the present invention reduces the time of crystallization of the recording material layer and increases stability of the recording material layer in its amorphous phase. The optical recording media using these recording materials are thus capable of rewriting at a high speed and maintaining recorded information for an extended period of time. Because recording materials (iii) and (iv) show absorption of visible and near infrared light due to the presence of at least one of Se and Te, semi-conductor lasers can be applied to the optical recording media using these recording materials. Where the crystal phase of the recording material is a single phase composed of a stoichiometric compound, the free energy of the crystal phase is further reduced, which leads to further reduction of the crystallization time. In addition, because such a crystal phase does not suffer from phase separation, the recording characteristics remain unchanged even rewriting is conducted many times. Where the above-described stoichiometric compound is composed of a congruent compound, segregation does not take place during cooling so that unchangeability of the recording characteristics on repeated use is further ensured. EXAMPLE 15 AND COMPARATIVE EXAMPLE 2 Optical recording media (Samples 37 to 38) having a structure shown in Fig. 1 were prepared, each comprising 1.2 mm-thick acrylic resin substrate (1) having thereon 100 nm-thick inorganic dielectric material layer (2) made of SiO₂, 100 nm-thick recording material layer (3) made of Sn-Sb-Bi-Te (Sample 37) or Sn-Sb-Se-Te (Sample 38), 100 nm-thick inorganic dielectric material layer (4) made of SiO₂, ultraviolet curable acrylic resin adhesive layer (5), and 1.2 mm-thick acrylic resin protective layer (6) in this order. Recording material layer (3) was formed by sputtering using an RF magnetron and two alloy targets of (SnSb₂Te₄) and (SnBi₂Te₄) or two alloy targets of (SnSb₂Te₄) and (SnSe₂Te₄), respectively, while appropriately controlling the RF power to the respective alloy target so as to obtain a four-element solid solution having a composition shown in Table 9 below. In Sample 37, (SnSb₂Te₄) and (SnBi₂Te₄) were used at a ratio of 45:55, and in Sample 38, (SnSb₂Te₄) and (SnSe₂Te₄) were used at a ratio of 74:26. Comparative Samples 39 and 40 were prepared by using as a recording material Ge-Sb-Te or Sn-Sb-Te, respectively, with the layer structure being the same as Samples 37 and 38. (Comparative Example 2) The recording material layer of Samples 39 or 40 was formed by RF magnetron sputtering using two alloy targets of Sb₂Te₃ and GeTe at a ratio of 67:33 or two alloy targets of Sb₂Te₃ and SnTe at a ratio of 50:50, respectively. Samples 37 to 40 showed absorption in the wavelength region of from visible light to near infrared light and could be used as optical recording media at least in the wavelength region of from 400 nm to 860 nm. Recording and erasing characteristics when these recording media were used by using a semi-conductor laser having a wavelength of 830 nm as a light source are also shown in Table 9. In Table 9, stability was rated according to the following standards. Standards for Evaluation: Excellent Crystallization temperature is higher than 160°C. Good Crystallization temperature is from 120°C to 160°C. Poor Crystallization temperature is less than 120°C. As is apparent from Table 9, Samples 37 and 38 according to the present invention have a reduced time of crystallization as compared with Sample 39 of Comparative Example 2, making it possible to conduct recording and erasing of information at a higher speed, and also have a higher crystallization temperature, an indication of improved stability of the amorphous phase, as compared with Sample 40 of Comparative Example 2 and Sample 1 of Comparative Example 1 (see Table 4). The recording material in Samples 37 or 38 is a four-element solid solution having a composition corresponding to a structure obtained by partly substituting Sb and Te in SnSb₂Te₄ of Sample 40 whose pseudo-binary phase diagram is shown in Fig . 24 by Bi or Se, respectively (Bi or Se and the elements to be substituted can four a full-range solid solution. The results of Table 9 proved that the stability of the amorphous phase can be ensured by such partial substitution. EXAMPLE 16 A optical recording medium was prepared in the same manner as in Example 15, except that the recording material layer was composed of Li-Sb-Se-Te. The Li-Sb-Se-Te recording layer was formed by sputtering using a single target of a four-element solid solution having a composition of Li₂₅Sb₂₅Se₃₀Te₂₀. The four-element solid solution had a composition corresponding to a structure obtained by partly substituting Se, a constituent element of LiSbSe₂ whose pseudo-binary phase diagram is shown in Fig. 25 , by Te which can form a full-range solid solution with Se. Recording and erasing characteristics of the resulting optical recording medium are shown in Table 10 below. EXAMPLE 17 A optical recording medium was prepared in the same manner as in Example 15, except that the recording material layer was composed of Cu-Au-Sn-Se. The Cu-Au-Sn-Se recording layer was formed by sputtering using a single target of a four-element solid solution having a composition of Cu₂₇Au₂₅Sn₁₃Se₃₇. The four-element solid solution had a composition corresponding to a structure obtained by partly substituting Cu, a constituent element of Cu₂SnSe₃ whose pseudo-binary phase diagram is shown in Fig. 27 , by Au. Recording and erasion characteristics of the resulting optical recording medium are shown in Table 10 below. EXAMPLE 18 A optical recording medium was prepared in the same manner as in Example 15, except that the recording material layer was composed of In-Ge-Se-Te. The In-Ge-Se-Te recording layer was formed by sputtering using a single target of a four-element solid solution having a composition of In₂₅Ge₂₅Se₁₃Te₃₇. The four-element solid solution had a composition corresponding to a structure obtained by partly substituting Te, a constituent element of InGeTe₂ whose pseudo-binary phase diagram is shown in Fig. 28 , by Se which can form a full-range solid solution with Te. Recording and erasing characteristics of the resulting optical recording medium are shown in Table 10 below. EXAMPLE 19 A optical recording medium was prepared in the same manner as in Example 15, except that the recording material layer was composed of Ga-Tℓ-Ge-Se. The Ga-Tℓ-Ge-Se recording layer was formed by sputtering using a single target of a four-element solid solution having a composition of Ga₁₅Tℓ₅Ge₂₀Se₆₀. The four-element solid solution had a composition corresponding to a structure obtained by partly substituting Ga, a constituent element of GaGeSe₃, by Tℓ. Recording and erasing characteristics of the resulting optical recording medium are shown in Table 10 below. EXAMPLE 20 A optical recording medium was prepared in the same manner as in Example 15, except that the recording material layer was composed of In-Tℓ-Se-Te. The In-Tℓ-Se-Te recording layer was formed by sputtering using a single target of a four-element solid solution having a composition of In₂₅Tℓ₂₅Se₃₀Te₂₀. The four-element solid solution had a composition corresponding to a structure obtained by partly substituting Se, a constituent element of InTℓSe₂ whose pseudo-binary phase diagram is shown in Fig. 2 9, by Te which can form a full-range solid solution with Se. Recording and erasion characteristics of the resulting optical recording medium are shown in Table 10 below. EXAMPLE 21 An optical recording medium was prepared in the same manner as in Example 15, except that the recording material layer was composed of In-S-Se-Te. The In-S-Se-Te recording layer was formed by sputtering using a single target of a four-element solid solution having a composition of In₄₀S₂₀Se₁₃Te₂₇. The four-element solid solution had a composition corresponding to a structure obtained by partly substituting Te, a constituent element of In₂STe₂ whose pseudo-binary phase diagram is shown in Fig. 30, by Se which can form a full-range solid solution with Te. Recording and erasing characteristics of the resulting optical recording medium are shown in Table 10 below. EXAMPLE 22 A optical recording medium (Sample 47 or 48) was prepared in the same manner as in Example 15, except that the recording material layer was composed of Ga-Tℓ-Sn-Te or Ga-Sn-Se-Te, respectively. The Ga-Tℓ-Sn-Te recording material layer of Sample 47 was formed by sputtering using a single target of a four-element solid solution having a composition of Ga₂₉Tℓ₅Sn₈Te₅₈. The Ga-Sn-Se-Te recording material layer of Sample 48 was formed by sputtering using a single target of a four-element solid solution having a composition of Ga₃₄Sn₈Se₁₅Te₄₃. The four-element solid solution of Sample 47 had a structure obtained by partly substituting Ga, a constituent element of Ga₄SnTe₇ whose pseudo-binary phase diagram is shown in Fig. 31 , by Tℓ. On the other hand, the four-element solid solution of Sample 48 had a structure obtained by partly substituting Te, a constituent element of Ga₄SnTe₇ whose pseudo-binary phase diagram is shown in Fig. 31 , by Se which can form a full-range solid solution with Te. Recording and erasing characteristics of the resulting optical recording medium are shown in Table 10 below. EXAMPLE 2 3 A optical recording medium was prepared in the same manner as in Example 15, except that the recording material layer was composed of Pb-Bi-S-Se-Te. The Pb-Bi-S-Se-Te recording material layer was a five-element solid solution formed by simultaneous sputtering using three alloy targets of (PbSe), (Bi₂Se₃), and (Bi₂STe₂) at a ratio of 33:51:16. The five-element solid solution had a structure obtained by partly substituting Se, a constituent element of PbBi₄Se₇ whose pseudo-binary phase diagram is shown in Fig. 32, by S and Te. Recording and erasing characteristics of the resulting optical recording media are shown in Table 11 below. As is apparent from Table 10, Samples 41 to 49 according to the present invention have a reduced time of crystallization, making it possible to conduct recording and erasing of information at a higher speed, and also have a higher crystallization temperature, an indication of improved stability of the amorphous phase, as compared with Comparative Examples 1 and 2, similarly to Samples 37 and 38 of Example 15. The four-element solid solutions of Samples 44 and 47 possess a structure obtained by partly substituting Ga of GaGeSe₃ or Ga or Ga₄SnTe₇, respectively, by Tℓ and, hence, exhibited accelerated crystallization. As is obvious from Examples 15 to 23, recording materials (v-1) to (v-8) have a crystal structure of a stoichiometric ternary compound having the respectively specified composition so that they have an increased rate of crystallization and improved stability in the amorphous phase thereof. In recording materials (v-1) to (v-8), since at least one of the three constituent elements of the stoichiometric three-element compound is partly substituted by other element(s) to form a solid solution containing 4 or more kinds of elements, the stability of the amorphous phase of the recording material layer can further be improved. Accordingly, the optical recording media using these recording materials are capable of not only conducting rewriting at a high speed but also maintaining the recorded information for a prolonged period of time. Further, since recording materials (v-1) to (v-8) contain at least one of Se and Te and thus absorb visible to near infrared light, widespread light sources inclusive of semi-conductor lasers, can be utilized for recording. In the cases when the constituent element(s) of the stoichiometric three-element compound which is(are) to be substituted and the element(s) which substitute for the constituent element(s) are in the mutual relationship that they can form a full-range solid solution, the strain of crystal structure attended by the substitution is relatively small so that structural changes of the stoichiometric three-element compound due to the substitution can be minimized. Therefore, a permissible degree of substitution can be broadened to advantage. In particular, where constituent element(s) of the stoichiometric three-element compound is(are) partly substituted by Tℓ of the group III b , the resulting solid solution containing 4 or more kinds of elements has a furthermore increased rate of crystallization, thus increasing the recording speed. EXAMPLE 24 Optical recording media having a structure shown in Fig. 1 were prepared, each comprising 1.2 mm-thick acrylic resin substrate (1) having thereon 100 nm-thick inorganic dielectric material layer (2) made of SiO₂, 100 nm-thick recording material layer (3) made of Sb-Te-Bi-Se, 100 nm-thick inorganic dielectric material layer (4) made of SiO₂, ultraviolet curable acrylic resin adhesive layer (5), and 1.2 mm-thick acrylic resin protective layer (6) in this order. Recording material layer (3) was formed by sputtering using an RF magnetron and two alloy targets, Sb₂Te₃ (crystal structure: C33) and Bi₂Se₃ (crystal structure C33) both belonging to the rhombohedoral system, while appropriately controlling the RF power to the respective alloy target so as to synthesize a composition shown in Table 11 below. Samples 50 and 51 showed absorption in the wavelength region of from visible light to near infrared light and could be used as optical recording media at least in the wavelength region of from 400 nm to 860 nm. Recording and erasing characteristics when these recording media were used by using a semi-conductor laser having a wavelength of 830 nm as a light source are also shown in Table 11 below. As is apparent from Table 11, Samples 50 and 51 according to the present invention have a reduced time of crystallization as compared with Sample 1 of Comparative Example 1, making it possible to conduct recording and erasing of information at a higher speed, and also have a higher crystallization temperature as compared with Sample 2 of Comparative Example 1, indicating improved stability of the amorphous phase. The recording materials in these samples are composed of a mixture of Sb₂Te₃ and Bi₂Se₃ both of which have not only the same crystal system (i.e., rhombohedoral system) but the same crystal structure (i.e., C33). The strain in the crystal lattice was therefore extremely small with the free energy of the crystal phase being further reduced so that the time of crystallization required was especially short. In addition, as expected from the pseudo-binary phase diagram shown in Fig. 34, the recording material of Sample 50 is a congruent solid solution. As a result, the recording material suffers from no local crystallization during cooling while retaining the excellent characteristics even on repetition of rewriting. EXAMPLE 25 Optical recording media (Samples 52 and 53) were prepared in the same manner as in Example 24, except that the recording material layer was composed of Sb-Se-Bi-S. The Sb-Se-Bi-S recording material layer was formed by sputtering using a single target prepared by mixing Sb₂Se₃ (crystal structure: D58) and Bi₂S₃ (crystal structure: D58), both of which belong to the orthorhombic system, at a ratio of 84:16 (Sample 52) or 65:35 (Sample 53). Recording and erasion characteristics of the resulting optical recording media are shown in Table 12 below. EXAMPLE 26 Optical recording media (Samples 54 and 55) were prepared in the same manner as in Example 24, except that the recording material layer was composed of In-Te-Tℓ-Se. The In-Te-Tℓ-Se recording material layer was formed by sputtering using two alloy targets of InTe (crystal structure: B37) and TℓSe (crystal structure: B37) both of which belong to the tetragonal system. Recording and erasing characteristics of the resulting optical recording media are shown in Table 12 below. EXAMPLE 27 A optical recording medium (Sample 56) was prepared in the same manner as in Example 24, except that the recording material layer was composed of Pb-Sn-Te. The Pb-Sn-Te recording material layer was formed by simultaneous binary sputtering using two alloy targets of PbTe (crystal structure: B1) and SnTe (crystal structure: B1) both of which belong to the cubic system. Recording and erasing characteristics of the resulting optical recording media are shown in Table 12 below. EXAMPLE 28 A optical recording medium (Sample 57) was prepared in the same manner as in Example 24, except that the recording material layer was composed of In-Ga-Te. The In-Ga-Te recording material layer was formed by simultaneous binary sputtering using two alloy targets of InSb (crystal structure: B3) and GaSb (crystal structure: B3) both of which belong to the cubic system. Recording and erasing characteristics of the resulting optical recording media are shown in Table 12 below. EXAMPLE 29 A optical recording medium (Sample 58) was prepared in the same manner as in Example 24, except that the recording material layer was composed of In-Ge-Te. The In-Ge-Te recording material layer was formed by simultaneous binary sputtering using two alloy targets of In₂Te₃ (crystal structure: B3) and GeTe (crystal structure: B1) both of which belong to the cubic system. Recording and erasing characteristics of the resulting optical recording media are shown in Table 12 below. EXAMPLE 30 A optical recording medium (Sample 59) was prepared in the same manner as in Example 24, except that the recording material layer was composed of Sb-Ge-Se. The Sb-Ge-Se recording material layer was formed by simultaneous binary sputtering using two alloy targets of Sb₂Se₃ (crystal structure: D58) and GeSe (crystal structure: B29) both of which belong to the orthorhombic system. Recording and erasing characteristics of the resulting optical recording media are shown in Table 12 below. EXAMPLE 31 A optical recording medium (Sample 60) was prepared in the same manner as in Example 24, except that the recording material layer was composed of Bi-Te-S-Se. The Bi-Te-S-Se recording material layer was formed by simultaneous binary sputtering using two alloy targets of Bi₂Te₂S (crystal structure: C33) and Bi₂Se₃ (crystal structure: C33) both of which belong to the rhombohedral system. Recording and erasing characteristics of the resulting optical recording media are shown in Table 12 below. EXAMPLE 32 A optical recording medium (Sample 61) was prepared in the same manner as in Example 24, except that the recording material layer was composed of Sb-Te-Bi-Se. The Sb-Te-Bi-Se recording material layer was formed by simultaneous ternary sputtering using three alloy targets of Sb₂Te₃ (crystal structure: C33), Bi₂Te₃ (crystal structure: C33), and Bi₂Se₃ (crystal structure: C33) all of which belong to the rhombohedral system. Recording and erasing characteristics of the resulting optical recording media are shown in Table 12 below. As is apparent from Table 12, Samples 52 to 61 according to the present invention have a reduced time of crystallization, making it possible to conduct recording and erasing of information at a higher speed, and also have a higher crystallization temperature, an indication of improved stability of the amorphous phase, as compared with Comparative Example 1, similarly to Samples 50 to 51 of Example 24. In particular, the recording materials in Examples 24 to 28 and 31 to 32 were those synthesized from a mixture of stoichiometric compounds which are the same not only in crystal system but in crystal structure. As a result, the strain in the crystal lattice in these materials is extremely small to further reduce the free energy of the crystal phase, to thereby achieve marked reduction in crystallization time. Since each of the recording materials of Samples 52 and 54 is composed of a congruent solid solution as expected from the respective pseudo-binary phase diagram shown in Fig. 35 to 36, they do not suffer from segregation during cooling and retain satisfactory characteristics even on repetition of rewriting. It is apparent from Examples 24 to 32 that use of recording material (vi) according to the present invention increases stability of the recording material layer in its amorphous phase and reduces the time of crystallization of the recording material layer. The optical recording media using recording material (vi) are thus capable of rewriting at a high speed and maintaining recorded information for an extended period of time. Because recording material (vi) shows absorption of visible and near infrared light due to the presence of at least one of Se and Te, widespread light sources inclusive of semiconductor lasers can be applied to these materials. Where the above-described elements or stoichiometric compounds having the same crystal system also have the same crystal structure according to classification by strukturbericht, the recording material composed of such elements or stoichiometric compounds has an extremely small strain in its crystal lattice, and the free energy of the crystal phase is further reduced, which leads to further reduction of the crystallization time. In addition, where the solid solution constituting recording material (vi) is a congruent solid solution, segregation does not take place during cooling so that unchangeability of the recording and erasing characteristics on repeated use is further ensured.

Claims: An optical recording medium comprising a substrate having provided thereon a layer of a recording material whose optical properties reversibly change on application of light or heat, to conduct recording, reproduction, or erasing of information, wherein said recording material is one of: (i) a recording material having a composition corresponding to a mixture of (i-a) a congruent compound having a composition corresponding to a mixture of (i-a-1) a IV b VI b type stoichiometric compound composed of the group IV b element selected from Sn and Pb and the group VI b element selected from S, Se and Te and represented by chemical formula IV b VI b and (i-a-2) a V b2 VI b3 type stoichiometric compound composed of the group V b element selected from Sb, Bi, and As and the group VI b element selected from S, Se, and Te and represented by chemical formula V b2 VI b3 and (i-b) the above-described V b2 VI b3 type stoichiometric compound, said composition containing at least one of Se and Te, (ii) a recording material having a composition corresponding to a mixture of (ii-a) a I₂VI b type stoichiometric compound composed of the group I element selected from Li, Na, K, Rb, Cs, Cu, Ag and Au and the group VI b element selected from S, Se, and Te and represented by chemical formula I₂VI b and (ii-b) a stoichiometric compound composed of elements selected from at least two groups of the group V b consisting of As, Sb, and Bi, and the group VI b consisting of S, Se, and Te, said composition containing at least of of Se and Te, wherein the crystal phase of said recording material (ii) is a single phase composed of a stoichiometric compound and said stoichiometric compound is a congruent compound; (iii) a recording material having a composition corresponding to a mixture of (iii-a) a III b VI b type stoichiometric compound composed of the group III b element selected from Ga, In, and Tl and the group VI b element selected from S, Se, and Te and represented by chemical formula III b VI b and (iii-b) a stoichiometric compound composed of elements selected from at least two groups of the group III b consisting of Ga, In, and Tl, the group IV b consisting of Ge, Sn, and Pb, the group V b consisting of As, Sb, and Bi, and the group VI b consisting of S, Se, and Te, said composition containing three or more kinds of elements and containing at least one of Se and Te, wherein the crystal phase of said recording material (iii) is a single phase composed of a stoichiometric compound and said stoichiometric compound is a congruent compound; (iv) a recording material having a composition corresponding to a mixture of (iv-a) a III b2 VI b3 type stoichiometric compound composed of the group III b element selected from Ga, In, and Tl and the group VI b element selected from S, Se, and Te and represented by chemical formula III b2 VI b3 and (iv-b) a stoichiometric compound composed of elements selected from at least two groups of the group III b consisting of Ga, In, and Tl, the group IV b consisting of Ge, Sn, and Pb, and the group VI b consisting of S, Se, and Te, said composition containing three or more kinds of elements and containing at least one of Se and Te, (v-1) a recording material made of a solid solution containing four or more kinds of elements at least one of which is Se or Te and having a crystal structure of a stoichiometric three-element compound having a composition corresponding to a mixture of (v-1-a) a IV b VI b type stoichiometric compound composed of the group IV b element selected from Sn and Pb and the group VI b element selected from S, Se, and Te and represented by chemical formula IV b VI b and (v-1-b) a V b2 VI b3 type stoichiometric compound composed of the group V b element selected from Sb, Bi, and As and the group VI b element selected from S, Se and Te and represented by chemical formula V b2 VI b3 , in which at least one of the three constituent elements of said stoichiometric three-element compound is partly substituted by at least one element selected from the group consisting of the above-described elements of the groups IV b , V b , and VI b and Si and Ge of the group IV b , (v-2) a recording material made of a solid solution containing four or more kinds of elements at least one of which is Se or Te and having a crystal structure of a stoichiometric three-element compound having a composition corresponding to a mixture of (v-2-a) a I₂VI b type stoichiometric compound composed of the group I element selected from Li, Na, K, Rb, Cs, Cu, Ag, and Au and the group VI b element selected from S, Se, and Te and represented by chemical formula I₂VI b and (v-2-b) a V b2 VI b3 type stoichiometric compound composed of the group V b element selected from As, Sb, and Bi and the group VI b element selected from S, Se, and Te and represented by chemical formula V b2 VI b3 , in which at least one of the three constituent elements of said stoichiometric three-element compound is partly substituted by at least one element selected from the above-described elements of the groups I, V b , and VI b , (v-3) a recording material made of a solid solution containing four or more kinds of elements at least one of which is Se or Te and having a crystal structure of a stoichiometric three-element compound having a composition corresponding to a mixture of (v-3-a) a I₂VI b type stoichiometric compound composed of the group I element selected from Li, Na, K, Rb, Cs, Cu, Ag, and Au and the group VI b element selected from S, Se, and Te and represented by chemical formula I₂VI b and (v-3-b) a IV b VI b2 type stoichiometric compound composed of the group IV b element selected from Ge, Sn, and Pb and the group VI b element selected from S, Se, and Te and represented by chemical formula IV b VI b2 , in which at least one of the three constituent elements of said three-element compound is partly substituted by at least one element selected from the group consisting of the above described elements of the groups I, IV b , and VI b and Si of the group IV b , (v-4) a recording material made of a solid solution containing four or more kinds of elements at least one of which is Se or Te and having a crystal structure of a stoichiometric three-element compound having a composition corresponding to a mixture of (v-4-a) a III b VI b type stoichiometric compound composed of the group III b element selected from Ga, In, and Tl and the group VI b element selected from S, Se, and Te and represented by chemical formula III b VI b and (v-4-b) a IV b VI b type stoichiometric compound composed of the group IV b element selected from Ge, Sn, and Pb and the group VI b element selected from S, Se, and Te and represented by chemical formula IV b VI b , in which at least one of the three constituent elements of said three-element compound is partly substituted by at least one element selected from the group consisting of the above described elements of the groups III b , IV b , and VI b , Al of the group III b , and Si of the group IV b , (v-5) a recording material made of a solid solution containing four or more kinds of elements at least one of which is Se or Te and having a crystal structure of a stoichiometric three-element compound having a composition corresponding to a mixture of (v-5-a) a III b VI b type stoichiometric compound composed of the group III b element selected from Ga, In, and Tl and the group VI b element selected from S, Se, and Te and represented by chemical formula III b VI b and (v-5-b) a IV b VI b2 type stoichiometric compound composed of the group IV b element selected from Ge, Sn, and Pb and the group VI b element selected from S, Se, and Te and represented by chemical formula IV b VI b2 , in which at least one of the three constituent elements of said three-element compound is partly substituted by at least one element selected from the group consisting of the above described elements of the groups III b , IV b , and VI b , Al of the group III b , and Si of the group IV b , (v-6) a recording material made of a solid solution containing four or more kinds of elements at least one of which is Se or Te and having a crystal structure of a stoichiometric three-element compound having a composition corresponding to a mixture of two kinds of a III b VI b type stoichiometric compound composed of the group III b element selected from Ga, In, and Tl and the group VI b element selected from S, Se, and Te and represented by chemical formula III b VI b , in which at least one of the three constituent elements of said three-element compound is partly substituted by at least one element selected from the group consisting of the above described elements of the groups III b and VI b and Al of the group III b , (v-7) a recording material made of a solid solution containing four or more kinds of elements at least one of which is Se or Te and having a crystal structure of a stoichiometric three-element compound having a composition corresponding to a mixture of two kinds of a III b2 VI b3 type stoichiometric compound composed of the group III b element selected from Ga, In, and Tl and the group VI b element selected from S, Se, and Te and represented by chemical formula III b2 VI b3 , in which at least one of the three constituent elements of said three-element compound is partly substituted by at least one element selected from the group consisting of the above described elements of the groups III b and VI b and Al of the group III b , (v-8) a recording material made of a solid solution containing four or more kinds of elements at least one of which is Se or Te and having a crystal structure of a stoichiometric three-element compound having a composition corresponding to a mixture of (v-8-a) a III b2 VI b3 type stoichiometric compound composed of the group III b element selected from Ga, In, and Tl and the group VI b element selected from S, Se, and Te and represented by chemical formula III b2 VI b3 and (v-8-b) a IV b VI b type stoichiometric compound composed of the group IV b element selected from Ge, Sn, and Pb and the group VI b element selected from S, Se, and Te and represented by chemical formula IV b VI b , in which at least one of the three constituent elements of said three-element compound is partly substituted by at least one element selected from the group consisting of the above described elements of the groups III b , IV b , and VI b , Al of the group III b , and Si of the group IV b , and (vi) a recording material made of a solid solution composed of at least three elements selected from the group consisting of the group III b element selected from Ga, In, and Tl, the group IV b element selected from Ge, Sn, and Pb, the group V b element selected from As, Sb, and Bi, and the group VI b element selected from S, Se and Te and containing at least one of Se and Te, said solid solution being synthesized from plural elements or stoichiometric compounds having the same crystal system, wherein said solid solution is a congruent solid solution. An optical recording medium as claimed in claim 1, wherein said recording material is the recording material (i). An optical recording medium as claimed in claim 2, wherein the crystal phase of said recording material (i) is a single phase composed of a stoichiometric compound. An optical recording medium as claimed in claim 3, wherein said stoichiometric compound is a congruent compound. An optical recording medium as claimed in claim 1, wherein said stoichiometric compound (ii-b) is a V b2 VI b3 type stoichiometric compound. An optical recording medium as claimed in claim 1, wherein said multiple stoichiometric compound (ii-b) is a III b2 VI b3 type stoichiometric compound. An optical recording medium as claimed in claim 1, wherein said multiple stoichiometric compound (ii-b) is a IV b VI b2 type stoichiometric compound. An optical recording medium as claimed in claim 1, wherein said stoichiometric compound (iii-b) is a IV b VI b type stoichiometric compound. An optical recording medium as claimed in claim 1, wherein said stoichiometric compound (iii-b) is a IV b VI b2 type stoichiometric compound. An optical recording medium as claimed in claim 1, wherein said stoichiometric compound (iii-b) is a III b VI b type stoichiometric compound. An optical recording medium as claimed in claim 1, wherein said recording material is the recording material (iv). An optical recording medium as claimed in claim 11, wherein said stoichiometric compound (iv-b) is a III b2 VI b3 type stoichiometric compound. An optical recording medium as claimed in claim 11, wherein said stoichiometric compound (iv-b) is a IV b VI b type stoichiometric compound. An optical recording medium as claimed in claim 1, wherein the crystal phase of the recording material (iv) is a single phase composed of a stoichiometric compound. An optical recording medium as claimed in claim 14, wherein said stoichiometric compound is a congruent compound. An optical recording medium as claimed in claim 1, wherein said recording material is the recording material (v-1). An optical recording medium as claimed in claim 1, wherein said recording material is the recording material (v-2). An optical recording medium as claimed in claim 1, wherein said recording material is the recording material (v-3). An optical recording medium as claimed in claim 1, wherein said recording material is the recording material (v-4). An optical recording medium as claimed in claim 1, wherein said recording material is the recording material (v-5). An optical recording medium as claimed in claim 1, wherein said recording material is the recording material (v-6). An optical recording medium as claimed in claim 1, wherein said recording material is the recording material (v-7). An optical recording medium as claimed in claim 1, wherein said recording material is the recording material (v-8). An optical recording medium as claimed in claims 16 to 23, wherein at least one of the three constituent elements of said stoichiometric three-element compound which is partly substituted and at least one element which substitutes for said constituent element(s) are in the mutual relationship that they can form a solid solution regardless of the mixing ratio of the former element to the latter element. An optical recording medium as claimed in claims 19 to 23, wherein at least one element which substitutes for at least one of the three constituent elements of said stoichiometric three-element compound is Tl of the group III b . An optical recording medium as claimed in claim 1, wherein said elements or stoichiometric compounds having the same crystal system have the same crystal structure according to the classification by strukturbericht. An optical recording medium as claimed in claim 26, wherein said solid solution is a congruent solid solution.

Inventors: KOBAYASHI HIDEO, UENO OSAMU, UEYANAGI KIICHI, KOBAYASHI, HIDEO, UENO, OSAMU, UEYANAGI, KIICHI
Applicants: FUJI XEROX CO LTD, FUJI XEROX CO., LTD.
Application year: 1990
Main classification: G11B7
IPC codes: G11B7