Identification of Cr³⁺ centers in Cs₂NaAlF₆ and Cs₂NaGaF₆ crystals by EPR and ENDOR paramagnetic resonance techniques

Chromium-doped Cs₂NaAlF₆ and Cs₂NaGaF₆ crystals have been investigated by using the techniques of electron paramagnetic resonance (EPR) and electron nuclear double resonance (ENDOR) at X-band (9.5 GHz) and Q-band (34 GHz) frequencies. In both crystals, which have the hexagonal elpasolite structure, two inequivalent Cr³⁺ centers are detected. From the angular dependence of the ENDOR spectra for the central ⁵³Cr nucleus and the nearest shells of ¹⁹F and ²³Na nuclei, the crystallographic sites at which the paramagnetic Cr³⁺ ions are incorporated in these crystals have been unambiguously determined for each type of center. The results of the EPR and ENDOR investigation are discussed in the context of their correlation with optical and crystallographic data for these crystals and give considerable information about local distortions around the paramagnetic impurity ions.