Cadmium-rare earth oxyborates Cd4ReO(BO3)3 (Re = Y, Gd, Lu): congruently melting compounds with large SHG responses

Abstract

A new series of cadmium–rare earth (Re) oxyborates Cd₄ReO(BO₃)₃ (Re = Y, Gd, Lu) have been synthesized through high-temperature solid-state reactions in platinum crucibles. In their crystal structures, ReO₆ (Re = Y, Gd, Lu) and CdO_n (n = 6, 8) are distorted polyhedra interconnected via shared edges and corners in a 3D framework with tunnels along the c-axis, where all B atoms are located. BO₃ planar triangles lie approximately parallel to the (001) plane. The combination of triangular BO₃ groups with π-conjugated systems and the d¹⁰ Cd²⁺ cation with polar displacement produces large SHG responses. Second harmonic generation (SHG) measurements indicate that Cd₄YO(BO₃)₃, Cd₄GdO(BO₃)₃, and Cd₄LuO(BO₃)₃ feature large SHG responses that are approximately 5.2, 5.0, and 5.3 × KH₂PO₄ (KDP), respectively. They are all phase matchable nonlinear optical materials that have wide transparent regions ranging from UV to near IR. These title compounds melt congruently, suggesting that the Cd₄ReO(BO₃)₃ (Re = Y, Gd, Lu) complexes are very promising NLO materials. Their electronic structures and optical properties were analyzed by using the Vienna ab initio theoretical method.