A Cationic Order‐Disorder Phase Transition in KPb 2 Cl 5

The phase transition occurring at T t = 528 K in the natural mineral and synthetic laser host compound KPb 2 Cl 5 has been characterized by means of both single‐crystal X‐ray diffraction (XRD) as a function of temperature and differential scanning calorimetry (DSC). The K + and Pb 2+ (2) cations order on passing from the high‐temperature orthorhombic phase (space group Pmcn ; a = 8.951, b = 8.015, c = 12.683 Å, Z = 4 at 623 K) to the low‐temperature monoclinic one (space group P 2 1 / c ; a = 8.849, b = 7.918, c = 12.472 Å, β = 90.11°, Z = 4, at room temperature), leading to a group‐subgroup first‐order phase transition displaying moderately fast kinetics and low entropy production upon thermal cycling around T t . The positional entropy calculated from our crystal structure models corresponds to the transition entropy measured by DSC within the experimental error, and amounts to about 0.34  R . The driving force for the phase transition, A ≤ 22 J mol –1 , remains negligible with respect to both the thermal energy at T t ( RT t ≈ 4.4 kJ mol –1 ) and the transition latent heat (Δ H ≈ 790 J mol –1 ), thereby suggesting that this phase transition is a close‐to‐equilibrium process. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)