Crystal structure, theoretical studies and luminescent properties of a new borate Na₃GdB₈O₁₅ with one-dimensional...

Inorganic borate compounds exhibit significant diversity in their structure types, which are associated with interesting optical and magnetic properties. In our work, a new rare-earth polyborate Na₃GdB₈O₁₅ was prepared with an infinite one-dimensional (1D) broad-banded framework of [B₈O₁₅]_∞ running along the a-axis, in which large Gd³⁺ and Na⁺ ions reside to ensure cohesion and neutrality of the structure. The basic fundamental building block (FBB) of [B₈O₁₅]_∞ is B₁₆O₃₂, which is composed of five BO₃ and three BO₄ groups and can be written as 5Δ3□: 〈2Δ□〉-〈Δ2□〉〈2Δ□〉. First principle studies reveal that Na₃GdB₈O₁₅ is an indirect bandgap semiconductor and the optical absorption at 280 nm originates from the O^2-→ Gd³⁺ transition. Solid solutions of Na₃Gd_1-xCe_xB₈O₁₅ and Na₃Gd_0.98-yY_yCe_0.02B₈O₁₅ were prepared and exhibited a bluish violet emissive luminescence by near-UV excitation due to the 5d¹→ 4f¹ transition of Ce³⁺. Substitution of Gd³⁺ by Y³⁺ enhanced the luminescence efficiency and significantly improved the thermal stability. At 423 K, the luminescence intensity of Na₃Gd_0.58Y_0.4Ce_0.02B₈O₁₅ remained 77% of that at 298 K. We hypothesize that Na₃GdB₈O₁₅ is a potential inorganic luminescent host matrix.