Substoichiometry in the Ba-Cu and Ba-Cu-O phase diagrams

TL;DR

This paper explores new phases in Ba-Cu and Ba-Cu-O materials, revealing unique electronic structures and properties.

Contribution

The study discovers new electride phases and intermetallic compounds with unique structural and electronic features.

Findings

Ba20Cu14O5 is a pseudo anti-Ruddlesden-Popper phase with square planar Cu− anions.

A novel electride phase ∼Ba7O3 contains Ba18O5 clusters with anti-Lindqvist geometry.

BaCu5 is a new intermetallic phase with Ba-Ba zigzag chains and isoelectronic similarity to SrCu5 and CaCu5.

Abstract

Electrides, which are a unique class of materials featuring anionic electrons (i.e., Farbe centers) localized at discrete crystallographic sites, provide an ideal platform for systematically investigating electron correlation. These compounds, often identified by unconventional stoichiometries such as [Ti2O]²+[(e−)2]²−, [Hf2S]²+[(e−)2]²−, [Ca2N]+(e−)−, and [Ca12Al14O32]²+[(e−)2]²−, exhibit intriguing electronic properties that remain largely unexplored. Here, we present our recent efforts to investigate the electrical behavior of cupride (Cu−) compounds within the Ba-Cu binary phase space. Our work has led to the discovery of several previously unknown phases and structure types, including: (1) Ba20Cu14O5, an pseudo anti-Ruddlesden-Popper phase where anionic Cu− is coordinated by Ba²+ in a square planar configuration; (2) a novel ∼Ba7O3 electride phase, featuring Ba18O5 clusters with an anti-Lindqvist (W5O18) geometry; and (3) BaCu5, a previously unidentified intermetallic phase that is isoelectronic to SrCu5 and CaCu5 but distinguished by infinite Ba-Ba zigzag chains. These findings expand our understanding of low-dimensional excess electron compounds and their potential applications in electronic and quantum materials.