Crystal structure built from a GeO$_6$-GeO$_5$ polyhedra network with high thermal stability: $\beta$-SrGe$_2$O$_5$

TL;DR

This study reports a novel Ge-based crystal structure, $eta$-SrGe$_2$O$_5$, with a stable GeO$_6$-GeO$_5$ polyhedra network, high thermal stability, and promising electronic properties for transparent oxide semiconductors.

Contribution

The discovery and characterization of a new crystal structure, $eta$-SrGe$_2$O$_5$, with unique GeO$_6$ and GeO$_5$ polyhedra arrangement and high thermal stability.

Findings

New $eta$-SrGe$_2$O$_5$ crystal structure identified.

High thermal stability up to 700°C.

Band gap of 5.2 eV with low effective electron mass.

Abstract

By tackling the challenge of extending transparent oxide semiconductors to Ge based oxides, we have found a not-yet-reported crystal structure, named $\beta$-SrGe$_2$O$_5$, which is composed of edge-sharing GeO$_6$ octahedra interconnected by GeO$_5$ bipyramids. Single crystals were successfully grown by the high-pressure flux method. $\beta$-SrGe$_2$O$_5$ has a band gap of 5.2 eV and a dispersive conduction band with an effective mass as small as 0.34 times the electron rest mass, which originates from the edge-sharing GeO$_6$ octahedra network. Although known compounds with octahedral GeO$_6$ coordination are commonly unstable at atmospheric pressure and elevated temperatures, $\beta$-SrGe$_2$O$_5$ exhibits thermal stability up to 700 $^\circ$C.