High-resolution spectroscopy and optically detected nuclear magnetic resonance studies of Ho³⁺ ions in SrF₂ crystals containing Ba²⁺ and Ca²⁺ impurities

Optical-RF double-resonance techniques have been used to measure the superhyperfine transition frequencies of two Ho³⁺ centres of C_4v symmetry present in SrF₂:0.001%Ho³⁺ crystals containing about 0.2% of Ba²⁺ and Ga²⁺ as inadvertent impurities. One centre has a charge-compensating F^- ion located in the nearest-neighbour interstitial position along the (100) axis while the other has an additional Ba²⁺ ion also located on this axis. The superhyperfine transition frequencies are about 60% smaller than those reported for CaF₂:Ho³⁺ and these decreases are attributed principally to the decreased Ho³⁺ ground-state magnetic moment resulting from the increased energy separation between the ground and first excited state Ho³⁺ electronic energy levels. Another contribution arises from the larger host lattice dimension for SrF₂. The two sets of nearest-neighbour F^- ions have almost identical resonance frequencies for both SrF₂ centres while the interstitial F^- ion has a substantially reduced resonance frequency in the Ba²⁺ modified centre. These trends correlate with those in the relevant crystal-field parameters. Some qualitative conclusions are drawn about the positions of the Ho³⁺ and F^- ions in these centres.