A sodium gadolinium phosphate with two different types of tunnel structure: Synthesis, crystal structure, and optical properties of Na3GdP2O8

A sodium gadolinium phosphate crystal, Na₃GdP₂O₈, has been synthesized by a high-temperature solution reaction, and it exhibits a new structural family of the alkali-metal-rare-earth phosphate system. Although many compounds with formula M₃LnP₂O₈ have been reported, but they were shown to be orthorhombic [R. Salmon, C. Parent, M. Vlasse, G. LeFlem, Mater. Res. Bull. 13 (1978) 439] rather than monoclinic as shown in this paper. Single-crystal X-ray diffraction analysis shows the structure to be monoclinic with space group C2/c and the cell parameters: a=27.55 (25), b=5.312 (4), c=13.935(11) Å, β=91.30(1)°, and V=2038.80 Å³, Z=4. Its structure features a three-dimensional GdP₂O₈³⁻ anionic framework with two different types of interesting tunnels at where Na atoms are located by different manners. The framework is constructed by Gd polyhedra and isolated PO₄ tetrahedra. It is different from the structure of K₃NdP₂O₈ [R. Salmon, C. Parent, M. Vlasse, G. LeFlem, Mater. Res. Bull. 13 (1978) 439] with space group P2₁/m that shows only one type of tunnel. The emission spectrum and the absorption spectrum of the compound have been investigated. Additionally, the calculations of band structure, density of states, dielectric constants, and refractive indexes have been also performed with the density functional theory method. The obtained results tend to support the experimental data.