Exploring quaternary vanadates containing alkali metals, alkaline earth metals, and [VO4] in the quest for nonlinear optical materials†

Abstract

VO₄ units are highly susceptible to distortion induced by crystal fields, and thus, they are favorable for generating second-order nonlinear optical properties in the visible-light frequency range. Among the quaternary compounds containing VO₄ units, vanadates with cations of alkali metals and/or alkaline earth metals exhibit a higher proportion of non-centrosymmetric structures, making them promising candidates for exploring nonlinear optical materials. However, to date, the optical properties of compounds within this category have rarely been characterized. In this work, we successfully synthesized two non-centrosymmetric compounds, NaMg₄(VO₄)₃ and LiMg₄(VO₄)₃. Experimental results showed that NaMg₄(VO₄)₃, which has a bandgap of 3.14 eV, showed a second-harmonic generation response of 1.1 × KDP and 0.4 × AGS at its maximum particle size. LiMg₄(VO₄)₃, with a bandgap of 3.10 eV, showed a second-harmonic generation response of 0.7 × KDP and 0.1 × AGS at its maximum particle size. NaMg₄(VO₄)₃ could achieve phase matching at the fundamental frequency of 2090 nm but not at 1064 nm. LiMg₄(VO₄)₃ was unable to achieve phase matching at both the fundamental frequencies of 1064 nm and 2090 nm. In summary, we conducted a systematic investigation of quaternary compounds containing alkali and alkaline earth metal cations and VO₄ anions and performed a comprehensive characterization of the nonlinear optical properties of NaMg₄(VO₄)₃ and LiMg₄(VO₄)₃. The findings give insights into designing new vanadate-based compounds for the application of nonlinear optical materials in the future.