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, Pb 5 Al 3 F 19 , M r =1477.9, tetragonal space group P4/n. At T=295 K, a=20.1738 (4) and c=7.2205 (1) A V=2939 (1) A 3 , Z=8, D m =6.66 (5), D x =6.681 Mg m -3 . For λ(Mo Kα)=0.71069 A, μ=58.0 mm -1 , F(000)=4960. The structure was determined from 18502 (1276 independent) F m 2 ≥3σ(F m 2 ) with (sin θ)/λ≤0.703 A -1 . Least-squares refinement on wF m 2 resulted in R(F m )=0.0579 with R int (F m )=0.048. Pb 5 Al 3 F 19 undergoes a first-order phase transition from antiferroelectric to ferroelectric at about 110 K, with a wide thermal hysteresis