Chemical perspectives on heteroanionic compounds: a potential playground for multiferroics†

Heteroanionic compounds, which host two or more different anions, have emerged as a huge family of functional materials. Different from polyanionic compounds, there is no direct connection between anions within heteroanionic compounds. The connectivity between anions and central atoms constitutes various distorted basic building units (BBUs). The linkage between BBUs further promotes the structural flexibility of heteroanionic compounds. The diverse bonding modes of anion–metal interactions, which originate from the various physical and chemical properties of anions, explain the existence of many important applications of heteroanionic compounds. In this short review, we summarize the synthesis, structures, and physical applications of selected heteroanionic compounds. From a synthesis perspective, a deep understanding of crystal growth mechanisms and a better controlled growth process should be emphasized in future research. The interactions between distinct anions and other featured elements such as elements with lone electron pairs, d⁰ and d¹⁰ transition metals, etc., or other systems such as high entropy systems would further promote more interesting applications. Heteroanionic compounds that exhibit comparable structural features with known multiferroics might be new frameworks for discovering multiferroics. Machine learning and quickly developed calculation capabilities can also aid the study of heteroanionic compounds by understanding growth mechanisms, searching for new compounds, and targeting specific properties.