RELATIONSHIP BETWEEN THE STRUCTURES OF FERROELECTRIC PB5CR3F19 AND ANTIFERROELECTRIC PB5AL3F19 AT 295-K AND THE PHASE-III-PHASE-IV TRANSITION IN PB5AL3F19 ON COOLING TO ABOUT 110-K

Antiferroelectric phase III of lead aluminium fluoride, Pb5Al3F19, M(r) = 1477.9, tetragonal space group P4/n. At T = 295 K, a = 20.1738 (4) and c = 7.2205 (1) angstrom, V = 2939 (1) angstrom3, Z = 8, D(m) = 6.66 (5), D(x) = 6.681 Mg M-3. For lambda(Mo Kalpha) = 0.71069 ansgtrom, mu = 58.0 mm-1, F(000) = 4960. The structure was determined from 18 502 (1276 independent) F(m)2 greater-than-or-equal-to 3sigma(F(m)2) with (sin theta)/lambda less-than-or-equal-to 0.703 angstrom-1. Least-squares refinement on wF(m)2 resulted in R(F(m)) = 0.0579 with R(int)(F(m)) = 0.048. Pb5Al3F19 undergoes a first-order phase transition from antiferroelectric to ferroelectric at about 110 K, with a wide thermal hysteresis. Transformation of the atomic coordinates of ferroelectric Pb5Cr3F19 previously measured at 295 K and comparison with those of antiferroelectric Pb5Al3F19 at 295 K reveal differences between related atomic positions that range from 0.10 to 1.56 angstrom. The origin of the first-order transition from the antiferroelectric phase III to ferroelectric phase IV in Pb5Al3F19 is shown to be associated with the orientational change from an eclipsed arrangement of AlF6 octahedra along the inversion and rotation-tetrad axes in phase III to a staggered arrangement along the rotation-tetrad axes in phase IV.