X-ray diffraction study of the phase transitions in the ferroelectric Pb₅(Cr_1-xAl_x)₃F₁₉system

Phase transitions in the Pb5(Cr1−xAlx)F19system

“…4 and 5 remains constant at about 10 K for 0 < x < 0.80. A comparable result was found by X-ray powder diffraction measurement (Andriamampianina, Ravez, Ihringer & Abrahams, 1991). In addition, AS remains approximately constant at about 16.7 J tool-1 K-…”

“…The phase transition for the composition with x = 0 undergoes a thermal hysteresis of about 10 K between heating and cooling, consistent with the first-order nature of the transition (Arquis-Canouet, Ravez & Abrahams, 1986). A comparable thermal hysteresis is exhibited by compositions throughout the range 0 < x < 0.80, increasing rapidly thereafter to about 170 K at x = 1 (Andriamampianina, Ravez, Ihringer & Abrahams, 1991). The origin of the increasing thermal hysteresis in Tc as x increases has been studied and the phase diagram for the Pbs(Cr~_xAlx)3F~9 system has been clarified by the use of X-ray diffraction, calorimetric and dielectric measurements made over the appropriate thermal regime on 15 compositions within the range 0 < x < 1.…”

“…In view of the general trend toward lower T~ with decreasing M3~ -ionic radius [r(M3+)], and the narrow range of radii from r(M3+)=0.615~ for chromium to 0.670/~, for titanium (Shannon, 1976) for these five M 3 + ions, it was of interest to determine T~ for much smaller r(M 3+) ions. Since aluminium, with r(M 3+) = 0.535 ~, could be readily substituted for chromium in the solid solution Pbs(Cr ~ _ xAlx)3 F 19 (Andriamampianina, Ravez, Ihringer, & Abrahams, 1991), the latter system appeared ideal for further study. The general trend of T~ with r(M 3 +) is clearly valid over the entire composition range 0 < x < 1 and includes the end member PbsA13F19 with T~ ~_ 265 K (see Fig.…”