Explorations of new UV nonlinear optical materials in the Na2CO3–CaCO3 system

Abstract

Three UV-transparent orthocarbonates, namely, the acentric Na₆Ca₅(CO₃)₈ (P63mc) and Na₂Ca₂(CO₃)₃ (Amm2) and the centric Na₂Ca(CO₃)₂ (Pbca), have been successfully synthesized by hydrothermal method and high-temperature solid-state method. The structure of Na₂Ca₂(CO₃)₃ consists of an alternate stacking of the [Na(CO₃)₂]_∞ and [NaCa₂CO₃]_∞ layers. The structures of Na₂Ca(CO₃)₂ and Na₆Ca₅(CO₃)₈ all present 2D [CaCO₃]_∞ layers, which are bridged via [CO₃] groups to construct a 3D framework. The Na and [Na_0.67Ca_0.33] atoms reside in the cavities of the 3D network. Thermal analysis of all compounds was performed. In addition, the results of UV-vis diffuse reflectance spectroscopy indicated that the three compounds all have wide transparent regions, among which the UV cutoff edge of Na₆Ca₅(CO₃)₈ is shorter than 200 nm, and the UV cutoff edges of Na₂Ca₂(CO₃)₃ and Na₂Ca(CO₃)₂ are about 205 nm and 247 nm, respectively. More importantly, the powder Na₆Ca₅(CO₃)₈ and Na₂Ca₂(CO₃)₃ samples exhibited moderate second harmonic generation (SHG) responses of 1 and 3 times that of reference KH₂PO₄ (KDP). Besides, for all compounds, their electronic structures and optical properties were analyzed by using density functional theory (DFT) calculation.