Pyroelectricity and related properties in the fresnoite pseudobinary system Ba2TiGe2O8-Ba2TiSi2O8

Complete solid solubility is found in the pseudobinary system Ba₂TiGe₂O₈-Ba₂TiSi₂O₈ by studies on ceramics and single crystals (Czochralski technique). The spontaneous birefringence perpendicular to the polar axis of the orthorhombic low temperature phase has been measured versus temperature and silicon contents: the temperature of the phase transition of species 4mmFmm2 is of second order and decreases from 1103 K at 0 at % Si to about 400 K at 40 at % Si. The pyroelectric coefficient in the orthorhombic mm2 and the tetragonal 4 mm phase is of the order of 10^–6 C m^–2 K^–1 in the entire system and has a positive temperature coefficient for all compositions studied except for Ba₂TiGe₂O₈, in which the pyroelectric coefficient changes sign at about 308 K. The spacegroup of the orthorhombic phase is proposed to be C _2v ¹¹ -Cmm2.