Fresnoite: A new ferroelectric mineral

Fresnoite, Ba2TiOSi2O7, has been predicted structurally to be a new ferroelectric. Detection of both ac and dc dielectric hysteresis verifies the prediction. The spontaneous polarization P-s estimated from the hysteresis at 1.2 MV m(-1) ac is similar to 0.2C m(-2) at 295 K, comparable with the minimum P-s observed in one-dimensional ferroelectrics. A reproducible calorimetric anomaly with entropy change 0.19( 3) J mol(-1) K-1 at 433(2) K in polycrystalline material coincides with a small dielectric and pyroelectric anomaly previously reported in single crystals; an entropy change similar to 0.2 J mol(-1) K-1 at 810(5) K also accompanies a dielectric anomaly observable in ceramic samples at 805(5) K. Both calorimetric anomalies are similar to 60 K wide. Observation of dielectric hysteresis at 875 K shows that neither anomaly corresponds to the Curie temperature; both are likely associated with small changes in atomic position, not with symmetry changes. Melting onset in fresnoite is 1703(1) K with undercooling as deep as 435 K. (C) 1999 American Institute of Physics. [S0021-8979(99)01204-9].