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

Herein, two new acentric borogermanates, namely, Ca2GeB2O7 and Ca1.78Cd0.22GeB2O7, were obtained by combining BO4 with a GeO4 tetrahedron. The structures of the isomeric Ca2GeB2O7 and Ca1.78Cd0.22GeB2O7 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 Ca1.78Cd0.22GeB2O7 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 Ca1.78Cd0.22GeB2O7 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.