The role of the Ca vacancy in the determination of the europium position in the energy gap, its valence state and spectroscopic properties in KCa(PO3)3

Abstract

A new very promising red phosphor KCa_1−xEu_x(PO₃)₃ (x = 1–5%) has been grown by the solid state method. Its luminescent quantum efficiency is close to 100% and the emission is stable over a wide temperature range i.e. 90% and 60% of the room temperature emission intensity remains at 200 °C and at 600 °C, respectively. The chromaticity coordinates were calculated as being x = 0.63, y = 0.37. The IR and Raman spectra were measured, and the maximum phonon energy of KCa_1−xEu_x(PO₃)₃ is 1276 cm⁻¹. In the measured emission and excitation spectra of all samples only Eu³⁺ ion emission was observed, emission of Eu²⁺ is not present. Quantum mechanical calculations showed that in a perfect crystal the 5d levels of Eu²⁺ are embedded in the conduction band. Consequently, neither absorption nor emission assigned to the Eu²⁺ ions could be observed. The presence of a calcium vacancy is crucial for the explanation of the observed spectrum. The existence of a Ca²⁺ vacancy compensates for the charge of Eu³⁺ and results in the creation of the magnetic moment which moves the 4f levels to the valence band. Thus, transitions to the Eu³⁺ excited levels located in the energy band gap are observed.