Formation of mineral-like phases in the system Sr₉In(PO₄)₇–Ca₉Ln(PO₄)₇

   Research subject. A series of mineral-like phases with strontiowhitlockite structure (1–x)Sr₉In(PO₄)₇–_xCa₉Ln(PO₄)₇ Ln = Eu³⁺, Yb³⁺.

   Aim: An analysis of phase formation patterns and crystal-chemical characteristics influencing the crystallization of mineral-like phases within the studied series.

   To achieve the aim, the following set of Materials and Methods is used: the powder X-ray diffraction (PXRD), the second harmonic generation (SHG), dielectric spectroscopy, the photoluminescence spectroscopy.

    Results. Doped phosphates with strontiowhitlockite structure were synthesized by a high temperature solid-state method. The structure was confirmed through X-ray diffraction method. The PXRD patterns of all samples were compared with strontiowhitlockite-type phosphate Sr₉In(PO₄)₇. There was an absence of SHG signals, conforming the non-polar structure. The λ-maximum is observed in the temperature dependence of the dielectric constant for all sensitized samples, while no anomalies were recorded on the loss tangent. The similar behaviour in dielectric curves may indicate isostructurality of studied samples, and crystallisation in non-polar space group. It was shown that samples demonstrate stable photoluminescence in red-orange region for Eu³⁺-doped phosphates, while Yb³⁺-doped ones shown IR-photoluminescence properties.

   Conclusions. A series (1–x)Sr₉In(PO₄)7–xCa₉Ln(PO₄)₇ was crystallised in Sr₉In(PO₄)₇ structure, where Sr²⁺ sites, with coordination number equals to 8, were substituted by Ca²⁺ and Eu³⁺. Such ions cannot occupy In³⁺ site, which is presented by small octahedral, due to high ionic radius difference between ions. However smallest ion as Yb³⁺ can occupy small octahedral site. Dielectric and photoluminescence properties were studied in (1–x)Sr₉In(PO₄)₇–_xCa₉Ln(PO₄)₇.

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