Role of lone-pair electron localization in temperature-induced phase transitions in mimetite.

The crystal structure of mimetite Pb₅(AsO₄)₃Cl, a phosphate with apatite structure-type has been investigated in situ at 123, 173, 273, 288, 353 and 393 K. A careful inspection of the diffraction pattern and subsequent structure refinements indicated that mimetite transforms from the monoclinic to the hexagonal polymorph with increasing temperature. At 123 K, a monoclinic superstructure, mimetite-2M, with cell parameters a = 20.4487 (9),  b = 7.4362 (2), c = 20.4513 (9) Å, β = 119.953 (6)°, V = 2694.5 (2) ų and space group P2₁ was observed. From 173 to 353 K, the reflections of the supercell were evident only along one direction of the corresponding hexagonal apatite-cell and the structure transforms to the polymorph mimetite-M with space group P2₁/b and unit-cell parameters a = 10.2378 (3), b = 20.4573 (7), c = 7.4457 (2) Å, β = 120.039 (5)°, V = 1349.96 (9) ų. Only at higher temperature, i.e. 393 K, does mimetite adopt the hexagonal space group P6₃/m characteristic of apatite structure-types. The role of the electron lone pairs of Pb atoms in the phase transition was investigated through the analysis of the electron localization function (ELF) calculated based on the DFT-geometry optimized structures of the three polymorphs. The changes in spatial distribution of the 6s² electron density during the phase transitions were explored by means of the Wannier Function Centres (WFCs) derived from ab initio molecular dynamics trajectories. In the high-temperature hexagonal structure the 6s² electrons are spherically symmetric relative to the position of Pb atoms. At low temperature the maximum of 6s² electron density is displaced relative to the position of Pb atom contributing to the polar interaction in the monoclinic polymorphs.