Phase transitions and ferroelectric attributes in A sub 5 M sub 3 F sub (19) compounds with A = Sr, Ba and M = Ti, V, Cr, Fe, Ga.

Phase transitions have been found in ten new nonlinear optic materials by calorimetry and dielectric permittivity measurement: Sr sub 5 Ti sub 3 F sub (19) at 755K, Sr sub 5 V sub 3 F sub (19) at 735K, Sr sub 5 Cr sub 3 F sub (19) at 953K, Sr sub 5 Fe sub 3 F sub (19) at 720k, Sr sub 5 Ga sub 3 F sub (19) at 893K. Ba sub 5 Ti sub 3 F sub (19) at 1005K, Ba sub 5 V sub 3 F sub (19) at 1020K, Ba sub 5 Cr sub 3 F sub (19) at 1090K, Ba sub 5 Fe sub 3 F sub (19) at 960K, and Ba sub 5 Ga sub 3 F sub (19) at 1070K. All ten materials are isomorphous with archetypal Pb sub 5 W sub 3 O sub 9 F sub (10), which has previously been shown to satisfy the structural criterial for crystals exhibiting ferroelectricity. The optical birefringence of Sr sub 5 Fe sub 3 F sub (19) undergoes an anomaly at the phase transition.