Spectroscopy and crystal-field analysis of low-symmetry Er3+ centres in K2YF5 microparticles

K${}_2$YF${}_5$ crystals doped with lanthanide ions have a variety of possible optical applications. Owing to the low symmetry of the system, the crystal structure cannot be unambiguously determined by x-ray diffraction. However, electron-paramagnetic resonance studies have demonstrated that lanthanide ions substitute for yttrium in sites of C${}_s$ local symmetry. In this work, we use high-resolution absorption and laser spectroscopy to determine electronic energy levels for Er${}^{3+}$ ions in K${}_2$YF${}_5$ microparticles. A total of 39 crystal-field energy levels, distributed among 7 multiplets of the Er${}^{3+}$ ion, have been assigned. This optical data is used for crystal-field modelling of the electronic structure of Er${}^{3+}$ in K${}_2$YF${}_5$. Our model is fitted not only to the electronic energy levels, but also to the ground-state g-tensor. This magnetic-splitting data defines the axis system of the calculation, avoiding ambiguities associated with low-symmetry crystal-field fits.