A thorough investigation of the crystal structure of willemite-type Zn2GeO4

The thermodynamically stable phase of Zn2GeO4 contains tetrahedrally coordinated cations only and crystallizes isostructurally to Zn2SiO4 (willemite, space group R3? , no. 148). While this material is considered for a plethora of energy-related applications, such as transparent conducting oxide, battery material and photocatalyst, cation ordering in the crystal structure has not been investigated thoroughly. We have therefore re-determined the crystal structure of Zn2GeO4 using a combination of X-ray and neutron powder diffraction. The additional neutron diffraction study helps to distinguish between the isoelectronic Zn2+ and Ge4+ cations and yields valuable information about a partial or complete cation permutation in this material. The experimental study is supported by first-principles calculations on the structural properties of Zn2GeO4 utilizing a standard generalized gradient approximation, and the more accurate hybrid functional HSE06. In order to better understand cation permutations, additional calculations including defective Zn2GeO4 have been performed based on a supercell approach. Our results show that, with the preparation conditions applied, cation permutation is unlikely to occur in our samples.