POLYMORPHISM OF IN2SE3

The phase relationships of polymorphic In2Se3 were reinvestigated by high-temperature X-ray powder studies. The modification, which is (contrary to most references) stable at room temperature, is γ-In2Se3 (hP30) with fourfold and fivefold coordinated indium ions. Above 960 K γ-In2Se3 undergoes a phase transition to the more dense (CN=6?) β-In2Se3(H) (hP10), which changes to δIn2 Se3 (hP5) above 1080 K. On cooling the high-temperature polymorphs to ambient temperature the stable γIn2Se3 is not normally obtained, but metastable variants of β-In2 Se3, viz. α-In2Se3(H) (α(2)-In2Se3) (hP10) or α-In2Se3(R) (α(3)-In2Se3) (hR5), which transform on heating via the similarly metastable β′-In2Se3 (hP15) and β-In2Se3(R) (hR5), respectively, into γ-In2Se3. The only stable or metastable polymorphs which exist at ambient temperature are γ-In2Se3, α-In2Se3(H) and α-In2 Se3(R). The IR spectra and X-ray densities of the various polymorphs of In2Se3 as well as the phase transitions are discussed in terms of the possible structures.