Growth and characterization of Na3R(BO3)2 (R = La–Gd) borates: crystal structure, high-temperature behavior, and optical properties

Abstract

Na₃R(BO₃)₂ (R = La–Gd) borates, grown with Na₂O–B₂O₃–NaF flux, crystallize in the space group P2₁/n. Contrary to previously published data, the peaks in the 980(La)–950(Gd) °C range are attributed to phase transition process, while the melting process occurs at about 100° higher. Another feature of thermal behavior consists of the gradual decomposition of the compound into Na₃R₂(BO₃)₃. The end member in the decomposition series is Na₂R₂O(BO₃)₂, which originates under oxygen/water atmosphere. In addition, the components of the thermal expansion tensor and the dependings of the unit-cell parameters of Na₃Nd(BO₃)₂ on temperature were defined. A series of Pr³⁺, Nd³⁺, and Sm³⁺-doped Na₃Gd(BO₃)₂ (NGB) borates were prepared from mixtures of grown crystals by solid-state synthesis, and their photoluminescence properties were studied in detail. The excitation and emission spectra of NGB: Pr³⁺ and NGB: Nd³⁺ are comparable with similar compounds, and the relative intensity reaches the maximum at 1 mol% of Pr³⁺, Sm³⁺, and 10 mol.% for Nd³⁺ doping.