Role of the Eu³⁺ Distribution on the Properties of β-Ca₃(PO₄)₂ Phosphors: Structural, Luminescent, and ¹⁵¹Eu Mössbauer Spectroscopy Study of Ca_{9.5–1.5<i>x</i>}MgEu_<i>x</i>(PO₄)₇

Abstract

The series of β-Ca₃(PO₄)₂-type phosphors Ca_{9.5-1.5<i>x</i>}MgEu_<i>x</i>(PO₄)₇ were synthesized by a solid-state route. Observation of the proper Eu³⁺ ion distribution in the Ca_9.5Mg(PO₄)₇ host matrix was made by a direct method using ¹⁵¹Eu Mössbauer spectroscopy in combination with X-ray analysis and dielectric and luminescent spectroscopy. The photoluminescence properties were studied in detail. The samples exhibit an exceptionally narrow-band red emission according to the dominant ⁵D₀ → ⁷F₂ transition and fulfill the industrial requirements for high-energy-efficiency red phosphors. The contribution of Eu³⁺ ions in different crystal sites to the luminescent properties is discussed in detail. The difference of the excitation of Eu³⁺ in the M1 and M2 sites was revealed by photoluminescence excitation spectra in accordance with structure refinement. The temperature dependence of the luminescence intensity was studied. Different tendencies in the thermal behavior of emission lines allow one to consider the studied compounds as phosphors suitable for luminescence thermometry. The measured quantum yield for Ca_{9.5-1.5<i>x</i>}MgEu_<i>x</i>(PO₄)₇ shows excellent results and reaches 63%.