COMPOSITION-DEPENDENT FERROELECTRIC CURIE-TEMPERATURE OF PB5(CR1-XALX)3F19 FROM 555-K TO 280-K AND OF PB5 (CR-1-XFEX)3F19 FROM 555-K TO 725-K FOR 0-LESS-THAN-OR-EQUAL-TO-X-LESS-THAN-OR-EQUAL-TO-1

Ferroelectric Pb5Cr3F19 forms a complete series of solid solutions both with Pb5Al3F 19 and Pb5Fe3F1 9. The end and intermediate compositions are isostructural with the ferroelectric A5M3F19 family previously reported [J. Appl. Phys. 62, 4299 (1987)]. The a and c axial lengths in Pb5(Cr0.2Al0.8)3F 19 undergo discontinuities of +0.5% and -0.8%, respectively on heating through the Curie temperature (Tc =325 K), and exhibit a hysteresis of about 40 K between the phase transition temperature on heating and on cooling, characteristic of a first-order phase change. The value of Tc may be varied continuously between 555 and 280 K for the solid solution Pb5(Cr1-xAlx) 3F 19, and between 555 and 725 K for Pb5(Cr 1-xFex) 3F19, by increasing x from 0 to 1. The entropy change at the phase transition in Pb 5Fe3F19, Pb5Cr 3F19, and Pb5(Cr 0.8Al0.2)3 F19 is high, at about Rln8. The maximum dielectric permittivity in Pb5Cr 3F19 is about 2000.