A Wide Band Gap Germanate with the Largest Second Harmonic Generation Response Created by Hypoxic Strategy

Abstract

Nonlinear optical material is one of the core components for modern laser equipment. The second harmonic generation (SHG) intensity and optical band gap are two key indicators of such materials. Herein, a wide band gap germanate, Li₃(OH)PbGeO₄, with the largest SHG intensity has been created successfully by hypoxic strategy via traditional hydrothermal reactions. The cations in this structure all coordinate with oxygen anions at their lowest coordination number, forming polar PbO₃ triangular pyramid and noncentrosymmetric GeO₄, LiO₄ tetrahedrons. Li₃(OH)PbGeO₄ exhibits a remarkable SHG response of about 27×KH₂PO₄ (KDP), 54 % higher than the previous record. The band gap of Li₃(OH)PbGeO₄ can reach to 3.74 eV, exceeding most inorganic germanates with SHG intensity larger than 3×KDP. Structural analysis and PAWED calculations indicate that its strong SHG response is due to the synergistic effects from PbO₃, GeO₄ and LiO₄ groups. Furthermore, Li₃(OH)PbGeO₄ can also present compelling broadband white-light emission with a high color-rendering index of up to 93. Our work not only breaks the record of SHG intensity in germanates but also provides an effective strategy in exploring new inorganic optical functional crystals.