CRYSTALLINE LEAD-CONTAINING OXYFLUORIDES

Abstract: A crystalline lead-containing oxyfluoride of the formula PbMO2F wherein M is selected from the group consisting of Ga, Sc, In, Lu, Yb, Tm, Er and Tl.

Description: TITLE CRYSTALLINE LEAD-CONTAINING OXYFLUORIDES Background of he Invention This invention relates to a group of new crystalline lead-containing oxyfluorides having useful dielectric properties. The use of dielectric materials to increase capacitance is well-known and long- used. Earlier capacitor dielectrics fell into two categories. The first category of dielectrics has a relatively temperature-independent, but low, e.g., 5-10, dielectric constant. Materials such as electrical porcelain and mica fall in this category. The second category of dielectrics has a very high, e.g., > 1000, but temperature- dependent dielectric constant. An example is barium titanate. Capacitance is proportional to the dielectric constant. Accordingly, materials with a high dielectric constant are desired. In order to perform acceptably in tuning or resonance circuits, the dielectric must have a dielectric constant that exhibits minimal temperature dependence; otherwise small changes in ambient temperature throw the circuit out of resonance. Other applications require a dielectric constant that exhibits minimal frequency dependence. It is also desirable to have the loss or dissipation factor be as small as possible. The need exists for materials with all ofthe above properties. Troyanchuk et al., Mater. Res. Bull. 30, 421 (1995) disclose the compositions PbFeθ2F and BaFeU2F. They report that PbFeθ 2 F is antiferromagnetic and has a dielectric constant of about 20 at 1 MHz and that the dielectric constant is practically the same in the temperature range of 77-293 K. They also report trying to synthesize other ABO2F compounds where A = Ba, Pb, Ca and B = In, Cr, Al. However, they could not obtain any of these, in spite of a wide variation of synthesis conditions. They hypothesize that the stability range of ABO 2 F compounds is narrow and probably connected with the use of Fe 3+ ions. Summary Of The Invention This invention provides novel crystalline lead-containing oxyfluorides ofthe formula PbMO2F wherein M is Ga, Sc, In, Lu, Yb, Tm, Er or Tl. These oxyfluorides are useful in dielectric applications. Detailed Description of the Invention The dielectric constants of the oxyfluorides of the formula PbMO2F. wherein M is Ga, Sc. In, Lu, Yb, Tm, Er, or Tl, are greater than 15 and are essentially independent of temperature and frequency. The dissipation factors are low. These oxyfluorides are useful in applications requiring dielectrics with such properties, e.g., to increase the capacitance of a capacitor. These PbMU2F compositions of this invention can be synthesized by first thoroughly mixing stoichiometric amounts ofthe precursors. Precursors such as PbO, PbF 2 , M 2 O 3 and MF 3 , wherein M is Ga, Sc, In, Lu, Yb, Tm, Er or Tl, are preferred. Especially preferred is the use of stoichiometric amounts of PbO, PbF 2 and M 2 O3. The intimately mixed starting material is loaded in a gold capsule and sealed. When M = Ga, Sc, In, Lu, Yb, Tm or Er, the capsule is placed in a tetrahedral anvil apparatus and subjected to a pressure of 50 x 10 8 Pa - 70 x 10 8 Pa (50-70 kbar) and heated to 700°C to 900°C. The temperature and pressure are maintained for 45 minutes to 75 minutes. In the case of M = Tl, the capsule should be heated to about 550°C and the pressure and temperature maintained for 30 minutes. The capsules are rapidly cooled, i.e., quenched, to room temperature and then the pressure is released. The capsule is removed and cut open and the dense pellet is recovered. The lead-containing oxyfluorides of this invention possess a perovskite-type crystal structure and X-ray diffraction results can be indexed on the basis of a simple cubic cell. Dielectric measurements reported in the Examples were carried out on a disk shaped sample, approximately 3 mm in diameter and approximately 1 mm thick, cut from the dense pellet of PbMO 2 F produced in each Example. The faces of the disk-shaped samples were polished with a fine-grit sand or emery paper. Silver paint electrodes were applied on the faces and dried at 200°C. The capacitance and the dielectric loss measurements were performed by the two-terminal method using a Hewlett-Packard 4285A LCR bridge in the frequency range 10 4 to 10 7 Hz and the capacitance, C, and the dissipation factor, DF, are read directly from the bridge. The dielectric constant (K) was calculated from the measured capacitance, C in picofarads, from the relationship, = (100 C t)/(8.854 A), where t is thickness ofthe disk shaped sample in cm and A is the area ofthe electrode in Examples Oxyfluorides of the formula PbMO 2 F, wherein M = Ga, Sc, In, Lu, Yb, Tm, Er and Tl were prepared by the following procedure: The precursors oxides and fluorides and amounts thereof (in stoichiometric ratios of PbO:PbF 2 :2M O3) as shown in Table I were mixed thoroughly and about 400 mg of the mixture was placed in a gold capsule (3 mm diameter and 6 mm length) and sealed. The capsule was placed in a tetrahedral anvil apparatus and was subjected to 58 x 10 8 Pa (58 kbar) pressure and heated to the temperature shown in Table I. Table I PbMO 2 F PbO PbF 2 M 2 O 3 Temp. Ex. Phases (grams) (grams) (grams) (°C) 1 PbGaO 2 F 1.1160 1.2260 0.9372 700 M=Ga 2 PbScO 2 F 1.1160 1.2260 0.6896 900 M=Sc 3 PbInO 2 F 1.1160 1.2260 1.3882 900 M=In 4 PbLuO 2 F 1.1160 1.2260 1.9897 800 M=Lu 5 PbYbO 2 F 1.1160 1.2260 1.9704 800 M=Yb 6 PbTmO 2 F 1.1160 1.2260 1.9294 800 M=Tm 7 PbErO 2 F 1.1160 1.2260 1.9126 800 M=Er 8 PbTlO 2 F 1.1160 1.2260 2.2837 550 M=TI For Examples 1 -7, the pressure and temperature were maintained for 1 hour. For Example 8 (M=T1), the pressure and temperature were maintained for 30 minutes. The power to the furnace was then turned off. The capsules were rapidly quenched to room temperature and then the pressure was released. The capsule was cut open and the dense pellet formed was recovered. A portion ofthe pellet was cut and ground to form a powder and X-ray powder diffraction measurements were performed. The X-ray diffraction patterns can be indexed on the basis of a cubic unit cell. The results for the PbScθ 2 F oxyfluoride of Example 2 are typical and are shown in Table 11. I he lattice parameters for all the product oxyfluorides of Examples I -8 are listed in Table III. The crystalline lead-containing oxyfluorides of this invention possess crystal structures that are closely related to that of the mineral perovskite. Table III PbMO 2 F Unit Cell Example Compound parameter (A) 1 PbGaO 2 F 3.974 2 PbScO 2 F 4.136 3 PbInO 2 F 4.289 4 PbLuO 2 F 3.289 5 PbYbO 2 F 4.309 6 PbTmO 2 F 4.332 7 PbErO 2 F 4.341 8 PbTlO 2 F 4.395 Dielectric measurements were carried out as described above on a disk-shaped sample approximately 3 mm in diameter and approximately 1 mm thick cut from the dense pellet of PbMO 2 F produced in each Example. The measurements ofthe dielectric constant (K) and the loss factor (DF) at 300 K and frequencies of 10 4 Hz to 10 7 Hz are given in Table IV. These data show that the dielectric constant is essentially independent of frequency. Measurements of the dielectric constant (K) and the loss factor (DF) at a frequency of 10 6 Hz and temperatures of 150 K and 400 K are given in Table V. These data show that the dielectric constant is essentially independent of temperature.

Claims: WHAT IS CLAIMED IS: 1. A crystalline lead-containing oxyfluoride ofthe formula PbMO 2 F, wherein M is selected from the group consisting of Ga, Sc, In, Lu, Yb, Tm, Er and Tl. 2. A process of making a crystalline lead-containing oxyfluoride of the formula PbMO 2 F, wherein M is selected from the group consisting of Ga, Sc, In, Lu, Yb, Tm, Er and Tl comprising the steps of: a) intimately mixing stoichiometric amounts of precursor oxides and fluorides; b) heating the mixture in a sealed container to a temperature of 550°C to 900°C and a pressure of 50 x 10 8 Pa to 70 x 10 8 Pa for 30 to 75 minutes; c) rapidly cooling the container to room temperature; and d) recovering the oxyfluoride.

Inventors: SUBRAMANIAN MUNIRPALLAM APPADO, SUBRAMANIAN, MUNIRPALLAM, APPADORAI
Applicants: DU PONT, E.I. DU PONT DE NEMOURS AND COMPANY
Application year: 1997
Main classification: C01G21
IPC codes: C01G21, H01G4