Abstract:
Ferromagnetic chromium dioxide is prepared by thermally decomposing, at a temperature of 250°-500°C, and a pressure of at least 80 atmospheres, a mixture of hydrated trivalent chromium chromate of the formula Cr 2  (CrO 4 ) 3 .nH 2  O, wherein n is 7-12, and antimony or a compound thereof, the amount of antimony in the mixture being between 0.2 and 1.8% by weight based on the weight of anhydrous Cr 2  (CrO 4 ) 3 . In another embodiment, other known modifiers for chromium dioxide, such as lanthanum, lithium, strontium, iron or lead, or a compound thereof may be added to the mixture of chromium chromate and antimony. The amount of said other modifiers is between 0.1 and 2% by weight of the element, based on the weight of anhydrous Cr.sub. 2 (CrO 4 ) 3 .

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is related to and incorporates by reference the contents of co-pending application Ser. No. 231,028 filed on Mar. 1, 1972, now U.S. Pat. No. 3,929,978 which application is owned by the unrecorded assignee hereof. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a process for preparing ferromagnetic chromium dioxide. 
     2. The Prior Art 
     Ferromagnetic chromium dioxide products are, of course, known. Such products are used in various aspects of magnetic recording, such as audio and video magnetic recording tapes, tapes and memories for electronic computers, magnetic discs and cards, etc. 
     In some of these uses, for example, video recording tapes, computer tapes and high fidelity audio recording tapes, it is necessary that the chromium dioxde possess particular magnetic characteristics, that is, a coercive force of at least 300 Oersteds, a magnetization saturation that is as high as possible and a ratio of residual mangetizaton to saturation magnetization of at least 0.4. Moreover the chromium dioxide must consist of elongated particles with as uniform a length as possible. 
     There are known ferromagnetic chromium dioxides that are obtained by thermally decomposing CrO 3  at high temperatures and under high pressures, the CrO 2  thereby obtained is a pure product, that is, it is free of other chromium oxides, but it possesses magnetic and granulometric characteristics which are insufficient to permit its use in the above mentioned areas. 
     Specifically, its coercive force is less than 200 Oersteds and its granulometric characteristics are rather poor. In fact the obtained particles are rather large, non-homogeneous and not very elongated. 
     A chromium dioxide of smaller and more elongated particle size and characterized by higher values of coercive force is, on the contrary, obtained by heating the chromium anhydride under pressure and in the presence of particular modifying elements or of compounds thereof, such as antimony or tin. In this chromium dioxide, the modifying elements are inserted into the crystalline lattice of the chromium dioxide. However, in such products it is difficult to balance the necessity of obtaining high coercive forces with the necessity of getting high values of magnetization and thus, the product is unsuitable for applications where both high coercive force and a high magnetization value are required. 
     Copending application Ser. No. 231,028, filed Mar. 1, 1972 and owned by unrecorded assignee hereof, describes a process for the preparation of chromium dioxide starting from a hydrated trivalent chromium chromate of the formula: 
     
         Cr.sub.2 (CrO.sub.4).sub.3.nH.sub.2 O 
    
     wherein n varies from 1 to 8. The chromium chromate hydrate is decomposed into CrO 2  at temperatures between 250° and 500°C. and under pressures between 30 and 1,000 atm. 
     The product thereby obtained has good magnetic and granulometric characteristics. The maximum coercive force of that product is about 350 Oersteds, while the particles are elongated and of very uniform dimensions. The best results according to that process are obtained when n is between 2 and 4. When n is greater than 4, the magnetic properties tend to become less good as the value of n rises. When n is greater than 8, the obtained product is in general impure due to the presence of CrOOH, which forms as a separate phase or coats the CrO 2  particles thereby reducing both the residual magnetization and the saturation magnetization. 
     U.S. Pat. No. 3,874,923, relates to ferromagnetic compositions based on CrO 2  modified with lanthanum, yttrium, barium or strontium. The addition of these modifiers, in elemental form or in the form of compounds thereof allows one to obtain, when using the same process as described in copending application Ser. No. 231,028 products with an even greater coercive force, (i.e., up to about 440 Oersteds). In this process as well, the optimum value of n in the formula Cr 2 (CrO 4 ) 3 .nH 2  O is between 2 and 4. When n is greater than 8, the product is generally impure due to the presence of CrOOH. 
     The products obtained according to U.S. Pat. No. 3,874,923, particularly when lanthanum is used as the modifier, are very well suited for use in high fidelity audio tapes and in video tapes employed in coventional equipment. However, in the case of video tapes used in equipment requiring particularly high performance (for example in &#34;helical scan&#34; recordings) even higher coercive forces of around 500 Oersteds and more are desirable. Materials having this property are provided by the present invention. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a new process for preparing ferromagnetic CrO 2  having a high coercive force, which if desired, may be equal to or greater than 500 Oersteds, without simultaneously depressing to any appreciable extent either the saturation magnetization or the residual magnetization of the product. 
     According to the process of this invention, ferromagnetic chromium dioxide is produced by thermally decomposing hydrated trivalent chromium chromate under high pressure, by proceeding as follows: 
     A mixture of Cr 2  (CrO 4 ) 3 .nH 2  O, wherein n is between 7 and 12, and antimony or a compound thereof is prepared in such a proportion that the antimony is present in an amount between 0.2 and 1.8% by weight based on the weight of the anhydrous chromium chromate. The mixture is then heated to a temperature between 250° and 500°C., preferably, 320°-400°C., and under a pressure of at least 80 atm., preferably 80-1000 and most preferably, 200-500 atm. 
     It has been found, according to the invention, that by using antimony as a modifier it is necessary, in order to obtain a CrO 2  of the desired characteristics, to use, as the starting material, a hydrated chromium chromate with a hydration degree different from that which, according to U.S. Pat. No. 3,874,923 and copending application, Ser. No. 231,028, is used for producing unmodified CrO 2  or CrO 2  modified with lanthanum, yttrium, strontium or barium. According to the above patent and patent application, the degree of hydration must, in fact, be between 1 and 8, preferably between 2 and 4, and it must not in any event exceed 80because in that case products are obtained which, in addition to having very poor magnetic characteristics, are also impure due to the presence of CrOOH. According to the invention, it has been discovered that, in the case of antimony, by using hydration degrees near or above 8, and more precisely, between 7  and 12, not only does CrOOH not form, but moreover, one obtains a CrO 2  having particularly good magnetic properties and in particular, coercive forces that may exceed 500 Oersteds, i.e., much higher than those obtained according to the above mentioned patent and patent application. 
     It has also been found that in order to obtain good results, it is necessary to limit the quantity of added antimony within very strict limits, that is, between 0.2 and 1.8% by weight based on the weight of anhydrous chromium chromate. The very best results, in terms of the highest coercive forces are obtained with surprisingly low quantities of antimony, that is, between about 0.3 and 0.7% by weight, based on the weight of anhydrous chromium chromate. 
     The antimony may be added in the form of the element, per se, an oxide thereof, or in the form of various trivalent or pentavalent antimony salts such as the chlorides, sulphides, nitrates or sulphates; antimonic acid, sodium and potassium antimonites and antimonates. 
     It has, however, been observed that when antimony salts are added, the anion of the salt may exert an influence on the characteristics of the obtained product which while modest, may nevertheless be undesirable. 
     Preferably the antimony is added in the form of Sb 2  O 3  or Sb 2  O 5 . 
     Regardless of which antimony compound is used, the quantity of such compound added is calculated so as to provide in the reaction mixture, between 0.2 and 1.8% by weight of elemental antimony with respect to the weight of anhydrous chromium chromate. The products with the best magnetic and granulometric characteristics are obtained when the quantity of antimony is between about 0.3 and about 0.7% by weight. 
     When operating with from about 0.3-0.7% by weight of antimony, the obtained products have a coercive force equal to or greater than 500 Oersteds, and at the same time have excellent saturation magnetization values (σ s ) usually equal to or greater than 85 electromagnetic units/g. (e.m.u./g.) and a ratio of residual magnetization (σ r ) to saturation magnetization (σ s ) i.e. (σr/σs), usually equal to or greater than 0.5. 
     Those products obtained starting from mixtures containing from 0.2 to 0.3% and from 0.7 to 1.8% by weight of antimony, also have excellent magnetic and granulometric properties which make them useful in those applications for which particularly high coercive forces are not required. 
     The degree of hydration of the chromium chromate, that is, the number of moles of H 2  O per mole of Cr 2  (CrO 4 ) 3  may be any value between 7 and 12 which corresponds to between 21.7 and 32.3% by weight of H 2  O, calculated with respect to the weight of anhydrous chromium chromate. 
     Since the hydrated chromium chromate is obtained by evaporating an aqueous solution of trivalent chromium chromate to form a paste and then drying the paste until reaching a preselected degree of residual hydration, the number of moles of H 2  O per mole of Cr 2  (CrO 4 ) 3  in the dry mass is obviously an average value which may or may not correspond to a whole number. Thus, the definition of n as being between 7 and 12, is intended to mean the whole numbers 7, 8, 9, 10, 11 and 12 as well as any intermediate value between any two consecutive whole numbers from 7 to 12. 
     Usually it is preferred to use a degree of hydration between 8 and 12. Particularly satisfactory results are obtained when the degree of hydration is between 9 and 12. It is in this latter range that products with the highest coercive forces are obtained. 
     The temperature at which the starting mixture is heated, is between 250° and 500°C., preferably, between 320° and 400°C. 
     The duration of the preliminary heating for the purpose of bringing the starting mixture up to the above indicated reaction temperatues is not critical, nor is the duration of the cooling off of the CrO 2  at the end of the reaction. The duration of both these operations is thus adjusted depending on the characteristics of the available equipment. 
     The minimum period of heating of the starting mixture at the reaction temperature which is necessary to complete the reaction decreases as the temperature increases. Lengthening of the heating period beyond the minimum does not, however, cause any problems. Generally, the reaction mixture is heated at the given reaction temperature for between 0.5 and 2 hours. 
     The pressure at which the reaction is conducted, as set forth above, is intended to mean the pressure that exists in the reaction environment at the end of the reaction. In considering this aspect of the process, it should be borne in mind that the decomposition of the chromium chromate to CrO 2  occurs with the evolution of O 2  which, in the absence of a venting device increases the pressure in the autoclave in which the reaction is preferably carried out. 
     These final reaction pressures, according to the invention, are in general, between 80 and 1,000 atm. Pressures above 1,000 atm. allow one to obtain just as good results, although the use of such high pressures is economically unwarranted. It is preferred to conduct the reaction at pressures between 200 and 500 atm. 
     When the autoclave in which the reaction is conducted is pressurized before heating, then in order to attain under heating the desired end pressure, one must take into account, not only the thermal expansion of the pressurized gas but also the O 2  that is evolved in the reaction, which O 2  will by itself create a pressure. 
     Account must also be taken of the pressure which forms in the autoclave due to the evaporation, under heating of the water in the reaction mixture derived from the water of hydration of the chromium chromate. 
     Pressurization of the autoclave before heating may be effected with a gas, for example, oxygen, air or nitrogen. 
     If desired, it is possible to introduce water into the hollow space between the test tube containing the reaction mixture and the autoclave walls. In this way the water pressure derives prevailingly from the evaporation of the thus added H 2  O instead of from the water of hydration of the chromium chromate. 
     The addition of water into the hollow space as thus described does not alter the course of the reaction nor does it alter the results which are obtained. 
     According to a further, and preferred embodiment of the invention, there may be introduced into the starting mixture, in addition to antimony, one or more elements (or their compounds) known in the art as modifying elements for chromium dioxide such as, for example, the alkali metals, particularly lithium, sodium and potassium, the alkaline earth metals, particularly strontium and barium, and various other elements such as, for example, lanthanum, yttrium, iron, lead, copper, zinc, titanium and aluminum. 
     The preferred amount of these other modifiers is 0.1-2% by weight of the element, based on the weight of the anhydrous Cr 2  (CrO 4 ) 3  ; the most preferred amount being 0.3-1% by weight. The use of these other modifiers is particularly preferred when it is desired to attain coercive forces of a particularly high value. In order to achieve this end, i.e., very high coercive forces, it is particularly preferred to use lanthanum, iron, or strontium as the additional modifier. In fact, when one or more of lanthanum, iron or strontium is used in combination with antimony, it is possible to obtain coercive forces definitely exceeding 500 Oersteds, and which in certain instances may even reach 600 Oersteds. 
     The hydrated trivalent chromium chromate which is used as the starting material according to the invention, is a saline compound, which may be obtained according to simple chemical reactions, long known and described in the technical literature, for example, Gmelins Handbuch der anorganischen Chemie, Verlag Chemie (1962), 8th edition Auflage-Chrom, Teil B, pages 104-105. 
     It is soluble in water and is amorphous to X-rays. Infrared spectroscopy shows a wide absorption band which begins at 9.5 microns, attains a maximum at 10.5 microns and extends up to 15 microns. 
     A convenient method for obtaining this material is by reducing CrO 3  in aqueous solution with methyl alcohol in a stoichiometric quantity, according to the reaction: 
     
         5 CrO.sub.3 + CH.sub.3 OH → Cr.sub.2 (CrO.sub.4).sub.3 +  2H.sub.2 O + CO.sub.2. 
    
     the resulting solution is then evaporated and the thus obtained paste is dried, for example, at 110°-160°C. under vacuum until the desired degree of hydration is achieved. 
     The antimony and the other optional modifying elements may be mixed together with the chromium chromate using common mixing techniques. For example, the hydrated chromium chromate may be ground in an agate mortar together with the modifiers. It has been found to be particularly advantageous to add the modifiers to the aqueous solution of chromium chromate, under stirring, before obtaining the solid chromium chromate from the solution. This procedure enables one to obtain products having higher coercive forces. 
     A description of apparatus suitable for carrying out the reaction according to the invention follows. 
     The apparatus comprises an autoclave, made of stainless steel or some other suitable material, provided with a valve which, when desired, allows one to discharge the oxygen that forms during the reaction, whereby the pressure may be kept at a constant value if and when desired; a second valve for creating a preselected initial pressure by means of an external source of gas before starting the reaction; and a pressure gauge for measuring the pressure. 
     A thermocouple which is inserted into the reaction mass permits the course of the internal temperature in relation to the time to be followed and recorded on a recording device. 
     The autoclave is heated in a muffle furnace of suitable size, a chamber with forced hot-gas circulation, or other heating means. 
     The determination of the characteristics of the obtained products is carried out: 
     by means of an X-ray diffractometer, since the CrO 2 , as is known, has a characteristic diffraction spectrum; 
     by means of an electron microscope, for example, at 50,000 enlargements which enables one to define the dimensions, shape and granulometric distribution of the obtained particles; 
     by evaluating the following magnetic characteristics: saturation magnetization (σ s ) and residual magnetization (σ r ) expressed in electromagnetic units/g. (e.m.u./g.); and intrinsic coercive force (Hci), expressed in Oersteds. 
     The products obtained according to the invention have the same X-ray diffraction diagram as unmodified CrO 2 . 
     The products consist of acicular particles prevailingly or totally having a single magnetic domain. The average length of the particles in general is between 0.15 and 0.5 micron; the mean axial ratio (i.e., the mean ratio of length/width) of the particles is generally between 5:1 and 10:1. 
     The coercive force of the products can be varied by varying the degree of hydration of the chromium chromate and the quantity of antimony as well as the other optional modifiers, and it may exceed, if desired, 500 Oersteds. 
     The saturation magnetization is in general greater than 75 e.m.u./g. and may reach 90 e.m.u./g. or more. The ratio σ r  /σ s  is generally greater than 0.4 and may reach and exceed 0.6. 
     The main advantages of this invention are: 
     It is possible to obtain products with a very high coercive force and, when desired, coercive forces that equal and even exceed 500 Oersteds. 
     These results may be attained with very small quantities of the modifier, as a result of which the saturation and residual magnetization of the product are not appreciably reduced in comparison with that of the unmodified CrO 2 . 
     The process is simple and economical. In comparison with the method of preparation of pure CrO 2  or CrO 2  modified with La, Y, Sr or Ba, disclosed in application Ser. No. 231,028 and U.S. Pat. No. 3,874,923, one obtains a further considerable advantage: the chromium chromate must only be dehydrated to a degree of hydration of about 10 (i.e., to a water content of about 28.5% by weight) instead of about 3 (i.e., to a water content of about 10.67%). The dehydration operation is thus considerably simplified. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The following examples are given in order to further illustrate the invention without, however, limiting same. 
     EXAMPLE 1 
     The trivalent chromium chromate used as the starting material in this example was prepared in the following manner: 
     2,000 g. of CrO 3  were dissolved in distilled water thereby obtaining 4 liters of solution. The solution was then placed in a four-necked 10 liter flask fitted with a stirrer, a reflux condenser and a thermometer, after which 128 g. of CH 3  OH were added dropwise and the solution was brought to boiling, which was continued for about 15 hours until complete reaction of the methyl alcohol occurred whereby the methyl alcohol was converted into CO 2 . From the flask there was then withdrawn a 10 cc sample of the solution. The Cr 6   +  /Cr 3   +   ratio of the sample was then determined by means of an iodometric titration of the hexavalent chromium and by determination of the total chromium after oxidation with Na 2  O 2 . 
     The thus determined ratio was found to be 1.5. Moreover, the concentration of anhydrous chromium chromate in the solution was also determined and found to be 478 g/l of Cr 2  (CrO 4 ) 3 . 
     104.6 cc of the solution, which contained 50 g. of anhydrous Cr 2  (CrO 4 ) 3  were then placed into a glass vessel. While vigorously stirring the solution, 0.178 g. of Sb 2  O 3  (equivalent to 0.3% by weight of Sb based on the anhydrous chromium chromate) was added thereto. The vessel which was continuously stirred was then heated in a water bath to evaporate the excess water and obtain a sufficiently thick paste which was then dried in an oven under vacuum at 160°C. to obtain trivalent chromium chromate in the solid state and having a degree of hydration of 10 moles of H 2  O. 
     After thoroughly grinding this material in an agate mortar, it was placed into a 130 ml. titanium test tube which was then placed in an autoclave of the above described type, made of stainless steel and having a holding capacity of 200 ml. 
     Into the hollow space between the test tube and the walls of the autoclave there were placed 10 cc of distilled water. The autoclave was then heated in a muffle oven set to a temperature of 380°C. At the beginning of the autoclaving, an internal pressure of 105 atm. of oxygen was established by means of oxygen supplied from an oxygen bottle. The pressure within the autoclave rose during the heating as a result of (1) the oxygen that was forming during the heating, (2), the water that evaporated and (3) the thermal expansion of the gases within the autoclave. After 3 hours the temperature inside the autoclave reached 375°C., a value which was maintained for 120 minutes. The final pressure amounted to 350 atm. 
     After cooling the autoclave, the pressure was released and the autoclave was opened. It was found that in the container there had formed a black powder which was ground in a ball mill, then washed with water until the wash water became clear, and finally dried in an oven. 
     An X-ray diffractogram of the obtained product indicated that it consisted of CrO 2 . Its Sb content as determined by X-ray fluorescence, amounted to 0.29% by weight. 
     The coercive force of the product was determined with an alternate current hysterisigraph operating with a field of about 1,000 Oersteds and was found to be 440 Oersteds. 
     The saturation magnetization and the residual magnetization of the product were determined by means of a Foner type vibrating sample magnetometer capable of supplying a maximum field of 18,000 Oersteds. The σ s  amounted to 88.0 e.m.u./g., while the ratio σ r  /σ s  was 0.51. 
     The mean ratio length/width of the particles was 8:1, and their mean length was 0.2 micron. 90% of the particles had a length between 0.08 and 0.4μ. 
     EXAMPLES 2-8 
     The procedure of Example 1 was repeated in the following examples, with varying amounts of Sb 2  O 3  being introduced into the chromium chromate solution, whereby to obtain, in the starting mixture, different values of percent by weight of antimony with respect to the anhydrous chromium chromate. 
     With the exception of Example 3, no distilled water was introduced into the hollow space between the test tube and the autoclave walls. 
     The results obtained in these Examples are recorded in Table 1. 
     
                                           TABLE 1__________________________________________________________________________               % by weight of               antimonyNumber of moles of water of               in the starting mix-Examplehydration      ture       Hci σ.sub.s                                  σ.sub.r /σ.sub.s__________________________________________________________________________2    10             0.2        320 88.4                                  0.423    10             0.4        530 88.2                                  0.564    10             0.5        500 87.6                                  0.535    10             0.6        470 87.1                                  0.526    10             0.7        450 87.2                                  0.497    10             1.0        400 85.3                                  0.478    10             1.6        340 83.6                                  0.41__________________________________________________________________________ 
    
     The product obtained in Example 4 contained 0.51% by weight of Sb and had the following granulometric characteristics: 
     
         ______________________________________Mean axial ratio    10:1Average length      0.4μLength of 90% of the particles               between 0.1 and 0.6μ______________________________________ 
    
     The product obtained in Example 7 contained 0.93% by weight of Sb and had the following granulometric characteristics: 
     
         ______________________________________Mean axial ratio    7:1Average length      0.17μLength of 90% of the particles               between 0.08 and 0.3μ______________________________________ 
    
     EXAMPLES 9-14 
     The procedures followed in these Examples were the same as in Example 1, except for the following differences: 
     a. The amount of Sb 2  O 3  introduced into the chromium chromate solution was 0.299 g. in Examples 9, 10, 11 and 12 and thus, the percent by weight of antimony amounted to 0.5 based on the anhydrous chromium chromate. 
     b. In examples 13 and 14, the percent by weight of antimony was 0.4 and 0.65, respectively. 
     c. The drying of the product in the oven was continued until the degree of hydration of the hydrated chromium chromate set forth in Table 2 was reached. 
     d. No distilled water was introduced into the hollow space between the test tube and the autoclave walls. 
     The results obtained in Examples 9-14 are recorded in Table 2. 
     
                                           TABLE 2__________________________________________________________________________Number of moles         % by weight ofof water of         antimonyExamplehydration         in the starting mixture                      Hci σ.sub.s                              σ.sub.r /σ.sub.s__________________________________________________________________________ 9    7       0.5          405 87.4                              0.4610    8       0.5          450 87.9                              0.51 11* 10       0.5          500 87.6                              0.5312   12       0.5          495 88.1                              0.5113    9       0.4          440 87.1                              0.4714   11        0.65        455 87.5                              0.51__________________________________________________________________________ *Example 11 is the same as Example 4 (Table 1). 
    
     The product obtained in Example 10 contained 0.49% by weight of Sb and had the following granulometric characteristics: 
     
         ______________________________________Mean axial ratio    8:1Average length      0.25μLength of 90% of particles               between 0.1 and 0.6μ______________________________________ 
    
     EXAMPLE 15 
     The procedure followed in this Example was the same as in Example 1 except that the amount of Sb 2  O 3  introduced into the chromium chromate solution was 0.239 g., and thus the percent by weight of antimony amounted to 0.4% with respect to the anhydrous chromium chromate. 
     No distilled water was introduced into the hollow space between the test tube and the autoclave walls. The autoclave itself was heated in a muffle oven thermostabilized at 350°C. At the beginning of the Example, an internal pressure of 46 atm. of oxygen was established. 
     After 3 hours, the temperature inside the autoclave was 340°C. and that temperature was maintained for 120 minutes. The final pressure amounted to 250 atm. The magnetic characteristics of the obtained product were as follows: Hci = 480 Oersteds; σ s  = 79.1 e.m.u./g.; σ r  /σ s  =  0.55. 
     EXAMPLE 16 
     The procedure followed in this Example was the same as in Example 1, except that the amount of Sb 2  O 3  introduced into the chromium chromate solution was 0.239 g., and thus the percent by weight of antimony amounted to 0.4% with respect to the anhydrous chromium chromate. 
     In the hollow space between the test tube and the autoclave walls no distilled water was introduced. The autoclave was heated in a muffle oven thermostabilized at a temperature of 380°C. At the beginning of the Example, an internal pressure of 85 atm. was established by means of nitrogen added from a nitrogen bottle. 
     After 3 hours, the temperature inside the autoclave reached 375°C., and was maintained thereat for 120 minutes. The final pressure amounted to 350 atm. 
     The magnetic characteristics of the obtained product were: Hci = 490 Oersted; σ s  = 87.3 e.m.u./g.; σ r  /σ s  = 0.54. 
     The following Examples are given to show the preparation of CrO 2  modified by the inclusion of a second element in addition to Sb. In Examples 17-24, this second element is La; in Examples 25-28, this second element is respectively Li, Sr, Fe and Pb. 
     EXAMPLES 17-24 
     The procedures followed in these Examples were the same as in Example 1, except that, before introducing the Sb 2  O 3  into the chromium chromate solution in varying quantities, there was also introduced into this same solution, varying quantities of La 2  O 3 . 
     In Examples 23 and 24 the number of moles of water of hydration of the chromium chromate was different than in Example 1. 
     The results obtained are set forth in Table 3, in which the percentages by weight of lanthanum are expressed in terms of elemental lanthanum and are calculated by weight with respect to the anhydrous chromium chromate. 
     
                                           TABLE 3__________________________________________________________________________Number of moles of water of               % by weight of                       % by weight ofExamplehydration      antimony                       lanthanum                               Hci σ.sub.s                                       σ.sub.r /σ.sub.s__________________________________________________________________________17   10             0.3     0.3     480 87.6                                       0.5118   10             0.4     0.4     550 87.1                                       0.5519   10             0.5     0.5     530 87.2                                       0.5520   10             0.6     0.6     495 87.0                                       0.5221   10             0.7     0.7     465 86.5                                       0.4922   10             1.0     1.0     410 85.3                                       0.4823    8             0.5     0.5     470 86.7                                       0.5024   12             0.5     0.5     525 87.0                                       0.54__________________________________________________________________________ 
    
     EXAMPLES 25-28 
     The procedures followed in these Examples were the same as in Example 1, except for the following variations: 
     a. Sb 2  O 3  was introduced in a quantity corresponding to 0.5% by weight of antimony. 
     b. The Li, Sr, Fe and Pb compounds were introduced in quantities corresponding to 0.5% by weight of the corresponding elements, based on the anhydrous chromium chromate. 
     c. No distilled water was introduced into the hollow space between the test tube and the autoclave walls. 
     d. In each Example, the compound of the second element, i.e., Li, Sr, Fe, or Pb, was added to the chromium chromate solution before adding the Sb 2  O 3 . 
     e. In Examples 25 and 26, the respective lithium and strontium carbonates were introduced in the solid state. 
     f. In Example 27, the hydrated Fe(OH) 3  was prepared by precipitation of ferric hydrate from an FeCl 3  solution with an excess of ammonia. The precipitate was filtered and washed until complete disappearance of Cl -  ions. 
     g. In Example 28, the hydrated Pb(OH) 2  was prepared by precipitation of lead hydrate at the boiling point from a solution of Pb(CH 3  COO) 2  with excess ammonia, with the precipitate being filtered and thoroughly washed. 
     The results of these Examples are set forth in Table 4. 
     
                                           TABLE 4__________________________________________________________________________                 Nature of the                 compound of                         % by weightNumber of moles of          % by weight                 the second                         of theExamplewater of hydration          of antimony                 element second element                                 Hci σ.sub.s                                         σ.sub.r /σ.sub.s__________________________________________________________________________25   10        0.5    Li.sub.2 CO.sub.3                         0.5     485 87.5                                         0.4626   10        0.5    SrCO.sub.3                         0.5     530 85.2                                         0.5327   10        0.5    Fe(OH).sub.3,                         0.5     585 84.1                                         0.58                 hydrated28   10        0.5    Pb(OH).sub.2,                         0.5     505 84.3                                         0.51                 hydrated__________________________________________________________________________ 
    
     Variations can, of course, be made without departing from the spirit and scope of the invention.