Unveiling the unique site preference of Ga in the defect wurtzite type In₂Se₃–Ga₂Se₃ system: an experimental and computational study

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 In_2-<i>x</i>Ga_<i>x</i>Se₃ (<i>x</i> = 0.6-1.4) (hexagonal, <i>P</i>6₅/<i>P</i>6₁) has been prepared using high-temperature synthesis and characterized using X-ray diffraction techniques and DFT calculations. For <i>x</i> ≤ 1, the structure is similar to the parent <i>γ</i>-In₂Se₃ and adopts the layered defect wurtzite type (<i>γ</i>₁), where gallium preferentially substitutes the tetrahedrally coordinated indium site without affecting the indium in the trigonal bipyramidal (TBP) environment. At <i>x</i> = 1, the ordered <i>γ</i>₁-GaInSe₃ is formed. In the compositional range 1.2 < <i>x</i> ≤ 1.4, an ideal defect wurtzite structure (<i>γ</i>₂) 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 <i>x</i> = 1.2, the structure is heavily disordered and can be interpreted as an incoherent intergrowth of <i>γ</i>₁- and <i>γ</i>₂-phases. A detailed theoretical calculation is performed on various compounds in the III₂-VI₃ family (<i>e.g.</i>, <i>α</i>-Al₂S₃, <i>α</i>-Ga₂S₃, <i>γ</i>-In₂Se₃, and InGaSe₃) to reveal the preference of Ga for tetrahedral co-ordination in the pseudo-binary In_2-<i>x</i>Ga_<i>x</i>Se₃.