Phase transition at 555 K in nonlinear optic Pb 5 Cr 3 F 19

The a lattice constant of tetragonal Pb 5 Cr 3 F 19 , 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 i 295 [1 + a i ΔT + β i (ΔT ) 2 ] with α 1 = 1.2 × 10 −6 K −1 and β = 9.8 × 10 −8  K −2 ; the c lattice constant thermal variation is linear with α 3 = 10.3 × 10 –6 K −1 , for ΔT = T − 295 K. Above T C the thermal expansion of both axes becomes linear with α 1 = 2.5 and α 3 = 4.9 × 10 −6  K −1 for a 555 = 14.568(8) and c 555 = 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