Unveiling the unique site preference of Ga in the defect wurtzite type In2Se3–Ga2Se3 system: an experimental and computational study†
Abstract
The pseudo-binary phases in the In2Se3–Ga2Se3 system have drawn attention due to their interesting structural features and exciting non-linear optical (NLO) properties. Although the phases have been known since 1977, detailed structural characterization has not been carried out in previous studies. In this research program, the pseudo-binary In2−xGaxSe3 (x = 0.6–1.4) (hexagonal, P65/P61) has been prepared using high-temperature synthesis and characterized using X-ray diffraction techniques and DFT calculations. For x ≤ 1, the structure is similar to the parent γ-In2Se3 and adopts the layered defect wurtzite type (γ1), where gallium preferentially substitutes the tetrahedrally coordinated indium site without affecting the indium in the trigonal bipyramidal (TBP) environment. At x = 1, the ordered γ1-GaInSe3 is formed. In the compositional range 1.2 < x ≤ 1.4, an ideal defect wurtzite structure (γ2) is observed, where both cationic sites are tetrahedrally coordinated by selenium. Ga occupies one of the two tetrahedrally coordinated cationic sites in this region, while the other site is statistically occupied by Ga and In. Interestingly, at x = 1.2, the structure is heavily disordered and can be interpreted as an incoherent intergrowth of γ1- and γ2-phases. A detailed theoretical calculation is performed on various compounds in the III2–VI3 family (e.g., α-Al2S3, α-Ga2S3, γ-In2Se3, and InGaSe3) to reveal the preference of Ga for tetrahedral co-ordination in the pseudo-binary In2−xGaxSe3.