High-pressure single-crystal synchrotron X-ray diffraction study of lillianite

In this paper, high-pressure data from a synchrotron X-ray diffraction study on a lillianite (Pb₃Bi₂S₆) single crystal up to ~21 GPa are presented. A phase transition from lillianite (space group Bbmm, LP lillianite) to the high-pressure form β-Pb₃Bi₂S₆ (space group Pbnm, HP lillianite) was confirmed and bracketed between 4.90 and 4.92 GPa. The transition is reversible but of first-order with a hysteresis of ~2.8 GPa. It showed weak effects of pseudo-merohedral twinning that disappeared upon decompression, testifying to a full recovery of the single crystal of lillianite. This makes lillianite an interesting shape-memory material.

With a bulk modulus K_4.9 = 78(3) GPa and K′ = 5.1(4), β-Pb₃Bi₂S₆ is markedly less compressible than lillianite [K₀ = 44(2) GPa, K′ = 7(1)]. Compressional anisotropy increases markedly in β-Pb₃Bi₂S₆ with compressibility along the b axis [M_0b = 130(6) GPa and $Mb′$ = 19(3) in lillianite, M_4.9b = 145(4) GPa and $Mb′$ = 16.0(7) in β-Pb₃Bi₂S₆] significantly larger than that along the other two axes [M_0a = 118(5) GPa, $Ma′$ = 21(3), M_0c = 139(12) GPa, and $Mc′$ = 31(10) in lillianite, M_4.9a = 242(12) GPa, $Ma′$ = 8(1), M_4.9c = 242(5) GPa, and $Mc′$ = 29(1) in β-Pb₃Bi₂S₆].

The behavior of lillianite at high pressure is an interesting case study in relation to non-quenchable ultrahigh-pressure phases likely occurring in the inner Earth, like post-perovskite MgSiO₃, the oxide homologue N = 1 of the lillianite series. The β-Pb₃Bi₂S₆ structure, on the other hand, is the N = 3 homologue of the meneghinite series to which the higher-pressure modification of the post-perovskite structure also belongs (homologue N = 1). This makes the two forms of Pb₃Bi₂S₆ potential equivalents of high- and ultrahigh-pressure Mg silicates that could occur both in the deep earth and in other rocky extrasolar planetary bodies.