Crystal Structure and Optical Characterization of Pure and Nd-Substituted Type III KGd(PO₃)₄

The crystal structure of type III KGd(PO 3 ) 4 has been solved, and the related data are presented in this paper. KGd(PO 3 ) 4 (KGP) is monoclinic and has P 2 1 as space group. Its cell parameters are a = 7.255(4) Å, b = 8.356(5) Å, c = 7.934(5) Å, β = 91.68(5)°, and Z = 2. KGP is characterized by infinite polyphosphate long chains, which run along the [100] direction and are linked through gadolinium polyhedra. We determined the crystallization region of KGd 0.47 Nd 0.53 (PO 3 ) 4, and compared it to those of KGP and KNd(PO 3 ) 4 (KNP). The isotherms of the saturation temperature and four neighboring phases were also determined. Small Nd-substituted KGP single crystals free of macroscopic defects were successfully grown under conditions similar to those used to grow KGP and KNP crystals. Nd substitution on the structure was efficient, and its effect was analyzed. We determined the transparency windows of both KGP and KNP crystals, and evaluated the optical tensors at room temperature and 632.5 nm. Finally, we used the Kurtz method to qualitatively measure the second harmonic efficiencies of Nd-substituted KGP crystals. The values obtained for all substitutions were similar to those showed by KGP and KNP and at least to that of H 2 PO 4 (KDP).