K<sub>0.9</sub>Rb<sub>2.1</sub>B<sub>8</sub>PO<sub>16</sub>: Design and Synthesis of a Deep-Ultraviolet Nonlinear Optical\nCrystal with Balanced Performance Derived from π‑Conjugated\nand Non-π-Conjugat

A new\ndeep-ultraviolet (DUV) nonlinear optical (NLO) material K<sub>0.9</sub>Rb<sub>2.1</sub>B<sub>8</sub>PO<sub>16</sub> (KRBPO) has\nbeen designed and synthesized by adjusting the B/P molar ratio to\n8:1. The KRBPO crystals were synthesized by a flux method and crystallized\nin noncentrosymmetric (NCS) and polar space group <i>Cc.</i> This compound exhibits a double-layer structure in which the A layer\nis composed of [B<sub>2</sub>O<sub>5</sub>] and [BO<sub>4</sub>] units\nand the B layer is formed by interconnected [B<sub>3</sub>O<sub>7</sub>] and [BO<sub>3</sub>] groups, and the two layers are connected by\n[PO<sub>4</sub>] tetrahedron. The theoretical calculations and experiments\nshow that the synergistic interaction of π-conjugated and non-π-conjugated\nunits leads to relatively well-balanced NLO properties of the title\ncompound, including moderated SHG (0.7 × KDP), short UV cutoff\nedge (λ<sub>cutoff</sub> < 190 nm), and a large band gap\nof 6.16 eV. Specifically, the coplanar arrangement of B–O groups\nin double-layer makes the KRBPO display a large birefringence (0.075@532\nnm) and enables the shortest phase-matched wavelength to reach an\nimportant laser output wavelength of 266 nm.