Low-temperature behavior of natural kalsilite with P31c symmetry: An in situ single-crystal X-ray diffraction study

The low-temperature behavior of a natural kalsilite (ideal formula KAISiO(4)) with P31c symmetry has been investigated by in situ single-crystal diffraction. A series of intensity data collections and structural refinements have been performed at 298, 250, 200, 150, and 100 K on decreasing temperature, and 175, 225, and 275 K on increasing T. The variations of the unit-cell parameters of kalsilite as a function of Tare continuous, and show no evidence of any phase transitions or thermo-elastic anomalies in this temperature range. An expansion is observed along [0001] with decreasing temperature. The axial and volume thermal expansion coefficients (alpha(j) = l(j)(-1.)partial derivative l(j/partial derivative)T, alpha(V) = V(-1).partial derivative V/partial derivative T) between 298 and 100 K, calculated by weighted linear regression through the data points, are alpha(a) = alpha(b) = 1.30(6).10(-5), alpha(c) = -1.5(1).10(-5), alpha(V) = 1.1(2).10(-5) K(-1). The main structural change on decreasing temperature is a cooperative anti-rotation of tetrahedra forming the six-membered rings lying parallel to (0001). This tetrahedral rotation is coupled with a change in the distances between the extra-framework cations and the framework 0 atoms. A small decrease in the tetrahedral tilts perpendicular to [0001] is responsible for the negative thermal expansion along [0001]; the implications of these mechanisms for thermal expansion in nephelines and kalsilites are discussed.