Ge-based Clinopyroxene series: first principles and experimental local probe study

TL;DR

This study combines experimental PAC measurements with ab-initio DFT calculations to investigate the structural and electronic properties of Ge-based clinopyroxenes, revealing insights into site substitution and band-gap tuning.

Contribution

It is the first combined experimental and theoretical study of CaMnGe$_2$O$_6$ and SrMnGe$_2$O$_6$ clinopyroxenes, highlighting the role of Hubbard-$U$ and site substitution effects.

Findings

Hubbard-$U$ is necessary to describe Mn $3d$ states.

Band gaps are approximately 1.7-1.8 eV for the studied compounds.

Cd can substitute Ca, Sr, or Mn sites, affecting electronic properties.

Abstract

The structural and electronic properties of the CaMnGe$_2$O$_6$ and SrMnGe$_2$O$_6$ clinopyroxene systems have been investigated by means of perturbed angular correlation (PAC) measurements, performed at ISOLDE, combined with $ab-initio$ electronic structure calculations within the density functional theory (DFT) framework. The partial density of states (PDOS) of the CaMnGe$_2$O$_6$ and SrMnGe$_2$O$_6$ stable compounds has been determined, and it has been observed that the requirement of including an on-site Hubbard-$U$ potential was necessary in order to describe the highly correlated Mn $3d$-states. By considering $U_{eff}$=4 eV, we obtained a band gap width of 1.82 eV and 1.70 eV, for the CaMnGe$_2$O$_6$ and SrMnGe$_2$O$_6$, respectively. Combining electric field gradient (EFG) first principles calculations, using a supercell scheme, with experimental PAC results, we were able to infer that the Cd probe can replace either the $A$ (Ca, Sr) or the Mn sites in the crystalline structures. We also showed that Cd substitution is expected to lead to a reduction in the width of the band gap in these systems, evidencing opportunities for potential band-gap engineering.