Ca2GeB2O7 and Ca1.78Cd0.22GeB2O7: two acentric borogermanates with a melilite-like structure and short ultraviolet cutoff edge

Abstract

Herein, two new acentric borogermanates, namely, Ca₂GeB₂O₇ and Ca_1.78Cd_0.22GeB₂O₇, were obtained by combining BO₄ with a GeO₄ tetrahedron. The structures of the isomeric Ca₂GeB₂O₇ and Ca_1.78Cd_0.22GeB₂O₇ feature two-dimensional (2D) [GeB₂O₇] layers parallel to the ab plane in which a GeO₄ tetrahedron corner is shared with four [B₂O₇] units; the divalent cations are filled in between layers. Ca₂GeB₂O₇ and Ca_1.78Cd_0.22GeB₂O₇ display second-harmonic generation (SHG) intensity around 0.4 and 0.6 times that of KH₂PO₄ (KDP), respectively. Ca₂GeB₂O₇ possesses a short UV absorption edge (about 194 nm). Thermal stability measurements show that Ca₂GeB₂O₇ melts congruently at high temperatures, whereas Ca_1.78Cd_0.22GeB₂O₇ melts incongruently and becomes amorphous at high temperatures. Moreover, measurements of the optical properties as well as theoretical calculations of these two compounds were conducted.