An EPR study of trigonally symmetric Cr³⁺ centres in TlZnF₃ single crystals

EPR measurements have been made on the single crystals of TlZnF₃ doped with chromium and co-doped with chromium and lithium. For the Cr-only doped crystal, three kinds of Cr³⁺ spectra with trigonal symmetry were observed. These centres are ascribed respectively to a charge-uncompensated Cr³⁺ ion at the Zn₂F₉ unit (Zn site II), a Cr³⁺ ion associated with a nearest Zn²⁺ vacancy at the Zn₂F₉ unit, and a charge-uncompensated Cr³⁺ ion at the ZnF₆ unit (Zn site I). In the Cr, Li co-doped crystal, preferential formation of a Cr³⁺–Li⁺ pair at the Zn₂F₉ unit was found. The ligand octahedra are considered to be elongated in the Cr³⁺–Zn²⁺ and the Cr³⁺–Li⁺ centres at the Zn₂F₉ units and compressed in the Cr³⁺ centre at the ZnF₆ unit.