Pressure-induced order-disorder transitions in β-InS: an experimental and theoretical study of structural and vibrational properties.

This joint experimental and theoretical study of the structural and vibrational properties of β-InS upon compression shows that this tetragonal defect spinel undergoes two reversible pressure-induced order-disorder transitions up to 20 GPa. We propose that the first high-pressure phase above 5.0 GPa has the cubic defect spinel structure of α-InS and the second high-pressure phase (ϕ-InS) above 10.5 GPa has a defect α-NaFeO-type (3̄) structure. This phase, related to the NaCl structure, has not been previously observed in spinels under compression and is related to both the tetradymite structure of topological insulators and to the defect LiTiO phase observed at high pressure in other thiospinels. Structural characterization of the three phases shows that α-InS is softer than β-InS while ϕ-InS is harder than β-InS. Vibrational characterization of the three phases is also provided, and their Raman-active modes are tentatively assigned. Our work shows that the metastable α phase of InS can be accessed not only by high temperature or varying composition, but also by high pressure. On top of that, the pressure-induced β-α-ϕ sequence of phase transitions evidences that β-InS, a BIII2XV3 compound with an intriguing structure typical of ABIII2XVI4 compounds (intermediate between thiospinels and ordered-vacancy compounds) undergoes: (i) a first phase transition at ambient pressure to a disordered spinel-type structure (α-InS), isostructural with those found at high pressure and high temperature in other BIII2XV3 compounds; and (ii) a second phase transition to the defect α-NaFeO-type structure (ϕ-InS), a distorted NaCl-type structure that is related to the defect NaCl phase found at high pressure in ABIII2XVI4 ordered-vacancy compounds and to the defect LiTiO-type phase found at high pressure in ABIII2XVI4 thiospinels. This result shows that InS (with its intrinsic vacancies) has a similar pressure behaviour to thiospinels and ordered-vacancy compounds of the ABIII2XVI4 family, making β-InS the union link between such families of compounds and showing that group-13 thiospinels have more in common with ordered-vacancy compounds than with oxospinels and thiospinels with transition metals.