Structural Transition and Thermal Anomaly in LiTlSO4

A survey for discovering new ferroelectric or ferroelastic crystals has been worked in order to get more useful or functional materials or to understand the mechanism of ferroelectricity and ferroelasticity. In the last two decades, a family of crystals with a chemical formula A2BX4 have been widely investigated, because they have incommensurate phases.1) If two alkali metal ions A’s are not the same kind, then the crystal structure and the transition sequence become a little different from the A2BX4 family.1) Among such AA′BX4-type crystals, LiNH4SO4 and LiRbSO4 are ferroelectrics and take a general tridymite structure.4,5) At high temperature, both crystals belong to an orthorhombic Pmcn (Z = 4) system, where each SO4 tetrahedron occupies two configurations with equal probability.6,7) Ordering of the disordered tetrahedra takes place with lowering temperature. In LiRbSO4, incommensurately and commensurately modulated phases are realized above room temperature.8) The LiNH4SO4 crystal is ferroelectric in the room-temperature phase and takes a superstructure below room temperature.9) Recently Shimizu and Takashige synthesized LiTlSO4 by a Bridgman method.10) They performed differential thermal analyses and observed two anomalies on cooling and one anomaly on heating, which indicates structural transitions of the crystal. They also measured dielectric constants; the constant perpendicular to the cleavage plane showed three and two anomalies (stepwise change or hump) on cooling and heating runs, respectively. The crystal was considered not to be ferroelectric because no DE-hysteresis loop was observable. According to a preliminary study by X-ray scattering,11) the room-temperature phase of the crystal belongs to Pmcn. In the low temperature region, superstructure reflections have been found. The diffraction pattern in the (h k 0) zone seems to be a hexagonal one in the lowest temperature phase, although the degree of crystal symmetry usually decreases with decreasing