Thermal expansion and low temperature phase transition of thallous azide

Abstract

Thallous azide, TIN3, is tetragonal at room temperature. It transforms at 248 ± 5 °K to a phase that can be indexed on the basis of an orthorhombic cell. Lattice parameters of the tetragonal phase have been determined by the Bond single crystal method at intervals of approximately 25 °K from 248 to 498 °K. Single crystals do not survive the transition, so the parameters of the orthorhombic phase were measured by powder diffraction at intervals of 25 °K down to 133 °K. Representative parameters, after corrections for the effects of radiation damage, are a = 6.2094 Å, c = 7.3583 Å at 298.2 °K for the tetragonal phase, and a = 8.718 Å, b = 8.766 Å, c = 7.395 Å at 238.2 °K for the orthorhombic. Thermal expansion parameters show anomalies that are believed to be the result of changes in the orientation of azide ions. The linear expansion coefficients, αa and αc, for the tetragonal phase are both approximately 5.2 × 10−5 °K−1 at the transition. By 486 °K, αc has increased to 21 × 10−5 °K−1 and αa has decreased to −1.2 × 10−5 °K−1. For the orthorhombic phase αb remains constant at 5.2 × 10−5 °K−1 while αa increases from 10 × 10−5 °K−1 to 22 × 10−5 °K−1 and αc decreases from −1.5 × 10−5 °K−1 to −11 × 10−5 °K−1 in the temperature range 150 to 225 °K. There does not appear to be a discontinuity in the volume at the transition. The volume expansion coefficient is approximately 17 × 10−5 °K−1 for the orthorhombic phase and 15 × 10−5 °K−1 for the tetragonal.