Temperature dependence of the Cr³⁺ site axial distortion in LiSrAlF₆ and LiSrGaF₆ single crystals

The electron spin resonance (ESR) of Cr³⁺ in LiSrAlF₆ (LiSAF:Cr) and LiSrGaF₆ (LiSGaF:Cr) single crystals has been measured in the temperature range between 270 and 470 K. The external magnetic field orientation dependence of the spectra is described by a spin Hamiltonian of axial symmetry for both compounds. The value of the zero-field splitting axial parameter (D) for LiSGaF:Cr decreases from 6.60 × 10⁻² cm⁻¹ to 5.60 × 10⁻² cm⁻¹, while for LiSAF:Cr it increases from 1.30 × 10⁻² cm⁻¹ to 1.60 × 10⁻² cm⁻¹ for increasing temperature. The larger values of D obtained for LiSGaF:Cr show that the distortion of the octahedral Cr site is greater in this compound, in agreement with x-ray diffraction results reported in the literature.