Crystal Growth, Structural Characterization, and Linear Thermal Evolution of KGd(PO₃)₄

We determined the crystallization region of KGd(PO 3 ) 4 (KGP) in the ternary system Gd 2 O 3 −K 2 O−P 2 O 5, the isotherms of saturation temperature and six neighboring phases: KPO 3, GdPO 4, KGdP 4 O 12, Gd(PO 3 ) 3 -orthorhombic, Gd(PO 3 ) 3 -monoclinic, and GdP 5 O 14 . We successfully grew KGP single crystals by the top-seeded solution growth (TSSG) slow cooling method. To optimize the growth process and obtain high-quality KGP crystals, solution composition and seed orientation were analyzed and a growth device was added. KGP is isostructural to KNd(PO 3 ) 4, crystallizes in the monoclinic system P 2 1 and the cell parameters determined are a = 7.2701(11), b = 8.3818(8), and c = 7.9608(10) Å, β = 91.776(12)°, Z = 2, and V = 484.87(11) Å 3 . We studied how they evolved as the temperature increased from room temperature to 773 K and observed an almost isotropic linear thermal expansion, the tensor of which we also calculated. In the room temperature to 1273 K range analyzed, we found that KGP irreversibly decomposed at 1142 K. The second harmonic efficiency, qualitatively measured by the Kurtz method, was at least similar to that of KH 2 PO 4 (KDP).