VUV spectroscopy of lanthanide doped fluoride crystals K2YF5

K$_2$Y$_0.99$Ln$_0.01$F$_5$ (Ln = Ce, Eu and Sm) single crystals have been synthesized by the hydrothermal technology. The optical properties of Ce$³⁺$ and Eu$³⁺$ ions doped into K$_2$YF$_5$ matrix have been studied under excitation in the deep UV and vacuum UV (VUV) region. The obtained data from excitation spectra and Dorenbos models have been used to predict the energies of the f-d and charge transfer transitions as well as the energy levels location of divalent (Ln$²⁺$) and trivalent (Ln$³⁺$) lanthanide ions within the band gap of K$_2$YF$_5$. In particular, the f-d and charge-transfer transitions of Sm$³⁺$ in the luminescence excitation spectrum of K$_2$YF$_5$:Sm$³⁺$ have been identified. The host referred binding energy (HRBE) and vacuum referred binding energy (VRBE) schemes have been constructed for Ln$²⁺$ and Ln$³⁺$ ions doped K$_2$YF$_5$ crystal which were used for the analysis of possible applications of K$_2$YF$_5$ doped with Ln$³⁺$ ions as thermoluminescence materials.