Phase transition at 555 K in nonlinear optic Pb₅Cr₃F₁₉

The a lattice constant of tetragonal Pb₅Cr₃F₁₉, 14.384(5) Å at 295 K, expands sharply by 0.6% on heating through the Curie temperature [T_C = 555(5) K] as the c lattice constant, 7.408(2) Å at 295 K, abruptly contracts by 0.6% and the uniaxial indicatrix of the crystal reverses sign from negative to positive. The phase transition is first order. The a lattice constant below T_C fits a second-order polynomial of the form a^T_i = a_i²⁹⁵ [1 + a_iΔT + β_i(ΔT)²] with α₁ = 1.2 × 10⁻⁶ K⁻¹ and β = 9.8 × 10⁻⁸ K⁻²; the c lattice constant thermal variation is linear with α₃ = 10.3 × 10^–6 K⁻¹, for ΔT = T− 295 K. Above T_C the thermal expansion of both axes becomes linear with α₁ = 2.5 and α₃ = 4.9 × 10⁻⁶ K⁻¹ for a⁵⁵⁵ = 14.568(8) and c⁵⁵⁵ = 7.388(5) Å. The birefringence at 295 K is 310 × 10⁻⁴ and at 640 K is −35 × 10⁻⁴, for λ = 543 nm. The entropy change at T_C is 13(3) J mol⁻¹ K⁻¹