Luminescence centers generated by chromium ions in potassium-aluminum (indium) diphosphates

Crystals of potassium-aluminum (indium) diphosphates KMeP₂O₇ (Me = Al, In) doped by chromium ions are synthesized for the first time, and their main spectral luminescence properties are described. The luminescence of these crystals is studied in the temperature range 4.2–300 K under excitation by light with a wavelength of 270–650 nm. The luminescence spectrum has two wide bands in the ranges 450–650 nm (“green” photoluminescence) and 700–800 nm (“red” photoluminescence). The green photoluminescence band is assumed to be a superposition of the intrinsic radiation of the matrix and the radiation of molecular centers generated by chromium ions that are surrounded by four oxygen atoms. At low temperatures, the red photoluminescence band consists of a wide unstructured band and narrow lines on its short-wavelength wing and is associated with the radiation of Cr³⁺ ions that replace Al³⁺ matrix ions and are in an octahedral oxygen environment. Parameters of the electron energy level diagram of the Cr³⁺ ion in the matrices of the investigated crystals are determined.