The Pb5(Cr1 −xAlx)3F19 system forms four phases. Ferroelectric phase IV, with point group 4mm, undergoes a first‐order transition on heating to paraelectric phase I, point group 4/m, in the composition range 0 ≤x≲ 0.80. The Curie temperature varies linearly with x over this range, from 555 K at x = 0 to 365 K at x = 0.80. Phase IV transforms to monoclinic phase II in the range 0.80 ≲x≲ 0.88 in a first‐order transition before undergoing a second‐order transition at higher temperatures to phase I. In the range 0.88 ≲x≲ 1, phase IV initially transforms on heating to phase III, point group 4/m, in a first‐order transition; at higher temperatures, this phase transforms to phase II in another first‐order transition; at still higher temperatures, it transforms to phase I in a second‐order transition. Multiple phase formation at x≲ 0.80 is a result of structural instabilities that are related to the small size of the Al3+ cation and to the decreased interaction strength between the 6(sp)2 electron lone pair of Pb2+ and the residual 3d electron spin moment at the Cr‐atom site as it is increasingly substituted by aluminum.
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