Crystal structure, phase transition and properties of indium(III) sulfide

Experimental and Theoretical Study of Stable and Metastable Phases in Sputtered CuInS2

The chalcopyrite Cu(In,Ga)S₂ has gained renewed interest in recent years due to the potential application in tandem solar cells. In this contribution, a combined theoretical and experimental approach is applied to investigate stable and metastable phases forming in CuInS₂ (CIS) thin films. Ab initio calculations are performed to obtain formation energies, X-ray diffraction (XRD) patterns, and Raman spectra of CIS polytypes and related compounds. Multiple CIS structures with zinc-blende and wurtzite-derived lattices are identified and their XRD/Raman patterns are shown to contain overlapping features, which could lead to misidentification. Thin films with compositions from Cu-rich to Cu-poor are synthesized via a two-step approach based on sputtering from binary targets followed by high-temperature sulfurization. It is discovered that several CIS polymorphs are formed when growing the material with this approach. In the Cu-poor material, wurtzite CIS is observed for the first time in sputtered thin films along with chalcopyrite CIS and CuAu-ordered CIS. Once the wurtzite CIS phase has formed, it is difficult to convert into the stable chalcopyrite polymorph. CuIn₅ S₈ and NaInS₂ accommodating In-excess are found alongside the CIS polymorphs. It is argued that the metastable polymorphs are stabilized by off-stoichiometry of the precursors, hence tight composition control is required.

Crystal Structure and Thermoelectric Properties of Novel Quaternary Cu2MHf3S8 (M-Mn, Fe, Co, and Ni) Thiospinels with Low Thermal Conductivity

Uncovering of the origin of intrinsically low thermal conductivity in novel crystalline solids is among the main streams in modern thermoelectricity. Because of their earth-abundant nature and environmentally friendly content, Cu-based thiospinels are attractive functional semiconductors, including thermoelectric (TE) materials. Herein, we report the crystal structure, as well as electronic and TE properties of four new Cu₂MHf₃S₈ (M-Mn, Fe, Co, and Ni) thiospinels. The performed density functional theory calculations predicted the decrease of the band gap and transition from p- to n-type conductivity in the Mn-Fe-Co-Ni series, which was confirmed experimentally. The best TE performance in this work was observed for the Cu₂NiHf₃S₈ thiospinel due to its highest power factor and low thermal conductivity. Moreover, all the discovered compounds possess very low lattice thermal conductivity κ_lat over the investigated temperature range. The κ_lat for Cu₂CoHf₃S₈ has been found to be as low as 0.8 W m^-1 K^-1 at 298 K and 0.5 W m^-1 K^-1 at 673 K, which are significantly lower values compared to the other Cu-based thiospinels reported up to date. The strongly disturbed phonon transport of the investigated alloys mainly comes from the peculiar crystal structure where the large cubic unit cells contain many vacant octahedral voids. As it was evaluated from the Callaway approach and confirmed by the speed of sound measurements, such a crystal structure promotes the increase in lattice anharmonicity, which is the main reason for the low κ_lat. This work provides a guideline for the engineering of thermal transport in thiospinels and offers the discovered Cu₂MHf₃S₈ (M-Mn, Fe, Co, and Ni) compounds, as new promising functional materials with low lattice thermal conductivity.

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

This joint experimental and theoretical study of the structural and vibrational properties of β-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 α-In₂S₃ and the second high-pressure phase (ϕ-In₂S₃) above 10.5 GPa has a defect α-NaFeO₂-type (R3̄m) 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 α-In₂S₃ is softer than β-In₂S₃ while ϕ-In₂S₃ is harder than β-In₂S₃. 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 In₂S₃ 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-In₂S₃, a BIII2XV3 compound with an intriguing structure typical of A^IIBIII2XVI4 compounds (intermediate between thiospinels and ordered-vacancy compounds) undergoes: (i) a first phase transition at ambient pressure to a disordered spinel-type structure (α-In₂S₃), isostructural with those found at high pressure and high temperature in other BIII2XV3 compounds; and (ii) a second phase transition to the defect α-NaFeO₂-type structure (ϕ-In₂S₃), a distorted NaCl-type structure that is related to the defect NaCl phase found at high pressure in A^IIBIII2XVI4 ordered-vacancy compounds and to the defect LiTiO₂-type phase found at high pressure in A^IIBIII2XVI4 thiospinels. This result shows that In₂S₃ (with its intrinsic vacancies) has a similar pressure behaviour to thiospinels and ordered-vacancy compounds of the A^IIBIII2XVI4 family, making β-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.