A-site cation manipulation of exemplary second harmonic generation response and optical anisotropy in rare-earth borates

Abstract

Ultraviolet nonlinear optical (UV NLO) materials have garnered significant interest for their prospective applications in advanced laser technologies. However, tailoring the desired structure in these materials remains a formidable challenge. Here, we propose a simple yet effective strategy for synthesizing rare-earth borates, K_xNa_3−xLa₂B₃O₉ (x = 2–3), by manipulating the A-site cations to induce structural evolution. Notably, K_xNa_3−xLa₂B₃O₉ undergoes a phase transition from the Pnc2 to the Amm2 space group by adjusting the K⁺ content to reach x = 2.6. Moreover, the target compounds exhibit strong phase-matching second harmonic generation (SHG) efficiencies, ranging from 1.3 to 3.3 times that of KDP (KH₂PO₄), and feature short UV cutoff edges of around 204–208 nm. Additionally, the correlation between microscopic polarizability, optical anisotropy, and the structural evolution of these materials was characterized through structural and theoretical analyses. These findings highlight the potential applications of K_xNa_3−xLa₂B₃O₉ as UV NLO materials and underscore the viability of manipulating A-site cations to fabricate NLO crystals with desirable properties.