A crystalline lead-containing oxyfluoride of the formula PbMO.sub.2 F wherein M is selected from the group consisting of Ga, Sc, In, Lu, Yb, Tm, Er and Tl.

This is a complete application based on US Provisional application Ser. No. 
60/023,934 filed on Aug. 9, 1996. 
BACKGROUND OF THE 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., .gtoreq.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 of the above properties. 
Troyanchuk et al., Mater. Res. Bull. 30, 421 (1995) disclose the 
compositions PbFeO.sub.2 F and BaFeO.sub.2 F. They report that PbFeO.sub.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 77-293K. They also report trying to synthesize other 
ABO.sub.2 F 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.sub.2 F 
compounds is narrow and probably connected with the use of Fe.sup.3+ 
ions. 
SUMMARY OF THE INVENTION 
This invention provides novel crystalline lead-containing oxyfluorides of 
the formula PbMO.sub.2 F 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 PbMO.sub.2 F, 
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 PbMO.sub.2 F compositions of this invention can be synthesized by 
first thoroughly mixing stoichiometric amounts of the precursors. 
Precursors such as PbO, PbF.sub.2, M.sub.2 O.sub.3 and MF.sub.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.sub.2 and M.sub.2 
O.sub.3. 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.times.10.sup.8 Pa-70.times.10.sup.8 Pa (50-70 kbar) and heated to 
700.degree. C. to 900.degree. 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.degree. 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.sub.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.degree. 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.sup.4 to 
10.sup.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, 
K=(100 C t)/(8.854 A), where t is thickness of the disk shaped sample in 
cm and A is the area of the electrode in cm.sup.2.