“…Mixed-anion compounds, in which two or more anionic species are incorporated into one crystal structure, have provided a playground for solid-state chemists to explore unique properties. Differences in the essential characteristics of anions, i.e., ionic radius, electronegativity, oxidation state, and polarizability, often lead to short- or long-range ordering of the anions, resulting in the formation of new anion sublattices and building blocks. − Mixed-anion structures containing oxide ions and different period 3–5 anions, such as oxide-sulfide and oxide-arsenide combinations, exclusively exhibit long-range anion ordering, particularly anion segregation in two-dimensional (2D) layers. − These anion segregations can be explained by the hard–soft acid–base (HSAB) theory, in which highly electropositive/polarizable cations prefer to bond with highly electronegative/polarizable anions. However, exceptions to the HSAB theory, namely, higher dimensional anion distributions, exist when differences in electronegativity (or polarizability) are small or when the concentration of a given anion is high compared to that of the other.…”