High-Temperature Single-Crystal X-ray Diffraction Studies of Potassium and (Cesium, Potassium) Titanyl Arsenates

The crystal structures of the nonlinear optical materials KTiOAsO₄(KTA) and (Cs_0.6K_0.4)- TiOAsO₄ ((Cs,K)TA) have been studied at room temperature and up to 725 and 550 °C, respectively. With increasing temperature, the a and b cell dimensions and unit-cell volumes increase while c decreases. Although the unit-cell volume for (Cs,K)TA is larger than that for KTA, the c parameter is smaller. For KTA, the mean linear thermal expansion coefficients along a, b, and c are 1.29, 1.58, and −0.65 × 10⁻⁵K⁻¹, respectively, in the temperature range studied. The volumetric thermal expansion coefficient is 2.34 × 10₋₅ K⁻¹. The corresponding values for (Cs_0.6K_0.4)TiOAsO₄ are 1.20 × 10⁻⁵, 3.06 × 10⁻⁵, and −1.52 × 10⁻⁵K⁻¹ and 2.68 × 10⁻⁵ K⁻¹. In (Cs,K)TA the Cs is strongly partitioned into the larger of two extraframework cation sites below 500 °C but disorders above this temperature. Structure models refined at several intermediate temperatures showed significant changes in the potassium/cesium environment, and a reduction of distortion within the titanium octahedra. Polyhedral rotations within the corner-linked framework are related to a negative thermal expansion along c. Tilt angles θ and ϕ, defined for the AsO₄tetrahedra as inclination of an O-O edge to the a or b axis, increase as functions of both temperature and cation size. The structural trends are consistent with the onset of a phase transition from orthorhombic space group Pna2₁ to Pnan at higher temperatures.