Electronic transitions and genuine crystal-field parameters in copper metaborate CuB2O4

Abstract

We present and analyze high-resolution $\ensuremath{\alpha}$-, $\ensuremath{\sigma}$-, and $\ensuremath{\pi}$-polarized absorption spectra related to d-d electronic transitions in tetragonal metaborate CuB${}_{2}$O${}_{4}$ where copper Cu${}^{2+}$ ions occupy two crystallographically distinct 4$b$ and 8$d$ positions. The spectra are characterized by exceptionally rich fine structure in the spectral range of 1.4--2.4 eV. Six zero-phonon (ZP) lines originating from the electronic transitions within the Cu${}^{2+}$ ions in both positions are distinguished and identified. Symmetry analysis explains polarization properties of the ZP lines in the 8$d$ positions but only partially explains them in the 4$b$ positions. Reliable assignment of all six ZP lines to specific transitions allowed us to calculate genuine cubic $\mathit{Dq}$ and tetragonal $\mathit{Ds}$ and $\mathit{Dt}$ crystal-field parameters for both positions. We show that the ($3{r}^{2}\ensuremath{-}{z}^{2}$) state, the energy of which is the measure of the Jahn-Teller splitting, is the highest 3$d$ state for both types of Cu${}^{2+}$ ion positions. Using the obtained crystal-field parameters as the reference values, we estimated $\mathit{Dq}$, $\mathit{Ds}$, and $\mathit{Dt}$ for several other cuprates with different Cu-O bond lengths. In particular, the 3$d$ level splitting in La${}_{2}$CuO${}_{4}$, Nd${}_{2}$CuO${}_{4}$, CuGeO${}_{3}$, Sr${}_{2}$CuO${}_{2}$Cl${}_{2}$, and Cu${}_{3}$B${}_{7}$O${}_{13}$Cl was analyzed. Our estimates suggest that the Jahn-Teller splitting in some of these cuprates is larger than it was assumed previously.