Interplay of electron correlations, spin-orbit couplings, and structural effects for Cu centers in the quasi-two-dimensional magnet InCu\(_{2/3}\)V\(_{1/3}\)O\(_3\)

Less common ligand coordination of transition-metal centers is often associated with peculiar valence-shell electron configurations and outstanding physical properties. One example is the Fe\(^+\) ion with linear coordination, actively investigated in the research area of single-molecule magnetism. Here we address the nature of 3\(d^9\) states for Cu\(^{2+}\) ions sitting in the center of trigonal bipyramidal ligand cages in the quasi-two-dimensional honeycomb compound InCu\(_{2/3}\)V\(_{1/3}\)O\(_3\), whose unusual magnetic properties were intensively studied in the recent past. In particular, we discuss the interplay of structural effects, electron correlations, and spin-orbit couplings in this material. A relevant computational finding is a different sequence of the Cu (\(xz\), \(yz\)) and (\(xy\), \(x^2\!-\!y^2\)) levels as compared to existing electronic-structure models, which has implications for the interpretation of various excitation spectra. Spin-orbit interactions, both first- and second-order, turn out to be stronger than previously assumed, suggesting that rather rich single-ion magnetic properties can be in principle achieved also for the 3\(d^9\) configuration by properly adjusting the sequence of crystal-field states for such less usual ligand coordination.