Luminescent Cryothermometer Based on K₂YF₅:Er³⁺

The luminescence spectra of the K₂YF₅:Er³⁺ (0.5 at %) were recorded in the spectral regions of the intermultiplet transitions ⁴I_13/2 → ⁴I_15/2 (region of about 1.5 μm, falling within the transparency window of a standard optical fiber) and ⁴I_11/2 → ⁴I_15/2 (region of about 0.98 μm) with high spectral resolution at different temperatures. Pairs of spectral lines are proposed for the implementation of a Boltzmann luminescent ratiometric cryothermometer in the temperature ranges of about 20, 40, and 60 K with a relative sensitivity of 8 to 3% K^–1. It is shown that measuring the half-width of a luminescence line with a wavelength of 1.538 μm (6500 cm^–1) provides a simple and reliable method for recording temperatures in the range from 20 to 90 K with a relative sensitivity of 3 to 2% K^–1.