Potassium Gadolinium Phosphate with a Tunnel Structure: Synthesis, Structure, and Optical Properties of K₃Gd₅(PO₄)₆

A potassium gadolinium phosphate crystal, K3Gd5(PO4)6, has been synthesized by a high-temperature solution reaction and exhibits a new structural family of the alkali-metal−rare-earth phosphate system. Single-crystal X-ray diffraction analysis shows the structure to be monoclinic with space group C2/c and the following cell parameters: a = 17.470(3) Å, b = 6.9365(6) Å, c = 18.096(2) Å, β = 114.308(4)°, V = 1998.4(5) Å3, Z = 4. Its structure features a three-dimensional Gd5(PO4)63- anionic framework with interesting tunnels where the K atoms are located. The framework are constructed from Gd polyhedra and isolated PO4 tetrahedra. We have also investigated the optical properties of K3Gd5(PO4)6 in terms of the measured absorption and emission spectra.