Spontaneous non-stoichiometry and ordering of metal vacancies in degenerate insulators

TL;DR

This paper reveals that in certain degenerate insulators, free carriers induce spontaneous formation of ordered cation vacancies, leading to non-stoichiometric compounds with unique atomic arrangements, challenging traditional views on compound stoichiometry.

Contribution

It introduces the concept of Fermi level-induced spontaneous non-stoichiometry and explains the formation of ordered vacancy compounds in degenerate insulators, a novel mechanism not previously recognized.

Findings

Carrier-induced vacancy formation explains unusual atomic sequences.

Ordered vacancy compounds (OVCs) can form spontaneously due to electronic effects.

This mechanism impacts the understanding of defect chemistry in degenerate insulators.

Abstract

We point to a class of materials representing an exception to the Daltonian view that compounds maintain integer stoichiometry at low temperatures, because forming stoichiometry-violating defects cost energy. We show that carriers in the conduction band (CB) of degenerate insulators in Ca-Al-O, Ag-Al-O, or Ba-Nb-O systems can cause a self-regulating instability, whereby cation vacancies form exothermically because a fraction of the free carriers in the CB decay into the hole states formed by such vacancies, and this negative electron-hole recombination energy offsets the positive energy associated with vacancy bond breaking. This Fermi level-induced spontaneous non-stoichiometry can lead to the formation of crystallographically ordered vacancy compounds (OVCs), explaining the previously peculiar occurrence of unusual atomic sequences such as BalNbmOn with l:m:n ratios of 1:2:6, 3:5:15, or 9:10:30. This work has general ramifications as the degenerate insulators have found growing interests in many fields ranging from transparent conductors to electrides that are electron donating promotors for catalyst.