Three non-centrosymmetric bismuth phosphates, Li2ABi(PO4)2 (A = K, Rb, and Cs): effects of cations on the crystal structure and SHG response

Abstract

Three new bismuth phosphates, Li₂ABi(PO₄)₂ (A = K, Rb, and Cs), were synthesized through the high-temperature solution method. They all exhibit three-dimensional (3D) [Li₂P₂O₈]⁴⁻ frameworks built by isolated PO₄ tetrahedra and Li–O chains but crystallize in two different space groups, which can be attributed to the effects of alkali metal cations that control the connection modes of units. Li₂KBi(PO₄)₂, Li₂RbBi(PO₄)₂ and Li₂CsBi(PO₄)₂ show SHG intensities about 5.0, 3.1 and 2.5 times that of KDP, respectively, and are all phase-matchable. To the best of our knowledge, Li₂KBi(PO₄)₂ exhibits the largest SHG response among the bismuth phosphates. The effects of cations on the SHG responses have been discussed in detail. Our results indicate that introduction of the Bi³⁺ cation with large distortion and synclastic arrangement of the Bi–O polyhedra is crucial to produce the large SHG response for phosphate NLO materials.