Pressure-induced order-disorder transitions in beta-In2S3: an experimental and theoretical study of structural and vibrational properties

This joint experimental and theoretical study of the structural and vibrational properties of beta-In2S3 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 alpha-In2S3 and the second high-pressure phase (phi-In2S3) above 10.5 GPa has a defect alpha-NaFeO2-type (R3m) 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 LiTiO2 phase observed at high pressure in other thiospinels. Structural characterization of the three phases shows that alpha-In2S3 is softer than beta-In2S3 while phi-In2S3 is harder than beta-In2S3. Vibrational characterization of the three phases is also provided, and their Raman-active modes are tentatively assigned. Our work shows that the metastable alpha phase of In2S3 can be accessed not only by high temperature or varying composition, but also by high pressure. On top of that, the pressure-induced beta-alpha-phi sequence of phase transitions evidences that beta-In2S3, a BIII2XV3 compound with an intriguing structure typical of A(II)BIII(2)XVI(4) compounds (intermediate between thiospinels and ordered-vacancy compounds) undergoes: (i) a first phase transition at ambient pressure to a disordered spinel-type structure (alpha-In2S3), isostructural with those found at high pressure and high temperature in other BIII2XV3 compounds; and (ii) a second phase transition to the defect alpha-NaFeO2-type structure (phi-In2S3), a distorted NaCl-type structure that is related to the defect NaCl phase found at high pressure in A(II)BIII(2)XVI(4) ordered-vacancy compounds and to the defect LiTiO2-type phase found at high pressure in A(II)BIII(2)XVI(4) thiospinels. This result shows that In2S3 (with its intrinsic vacancies) has a similar pressure behaviour to thiospinels and ordered-vacancy compounds of the A(II)BIII(2)XVI(4) family, making beta-In2S3 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.