High-pressure anisotropic distortion of Pb3Bi2S6: a pressure-induced, reversible phase transition with migration of chemical bonds.

The compound Pb 3Bi 2S 6 is investigated by X-ray diffraction on single crystals in a diamond-anvil cell between 0.0001 and 10.5 GPa. It undergoes a first-order phase transition at hydrostatic pressure between 3.7 and 4.9 GPa. The space group symmetry changes from Bbmm to Pbnm, and the unit-cell volume decreases by 4%. The transition is strongly anisotropic, with a contraction along one of the crystal axes by 16% and expansion along another one by 14%. This is a piezoplastic phase transition, a displacive pressure-induced phase transition with systematic shearing of atomic planes and a migration of chemical bonds in the structure. In the case of Pb 3Bi 2S 6 the transition is achieved by the change of the archetypal architecture of the structure-building modules from a PbS-like to a SnS-like arrangement and a loss of mirror planes on the contact surfaces of modules. The phase transition is reversible with a preservation of the single crystal, which is a result of the stereochemical influence and migration of the s (2) lone electron pairs of Pb (II) and Bi (III).