AMgPO4·6H2O (A = Rb, Cs): strong SHG responses originated from orderly PO4 groups

Abstract

Two UV NLO phosphates AMgPO₄·6H₂O (A = Rb, Cs) were synthesized by a soft hydrothermal method. RbMgPO₄·6H₂O originates from the struvite structure type and crystallizes into the orthorhombic space group Pmn2₁. CsMgPO₄·6H₂O derives from the nickeline structure type and crystallizes in the polar space group P6₃/mmc. The structure of CsMgPO₄·6H₂O features two-layer arrangements, while there is only one layer in the struvite RbMgPO₄·6H₂O. Without the spatial inhibition of corner-shared behaviour, the isolated PO₄ groups in CsMgPO₄·6H₂O align in parallel to make maximum contribution to achieving high-performance nonlinear properties. The d_eff coefficient for CsMgPO₄·6H₂O is 0.53 pm V⁻¹, which is one of the strongest SHG responses among reported UV NLO phosphates. In addition, CsMgPO₄·6H₂O shows a wide transparent region ranging from UV to near IR with a UV cutoff edge at approximately 258 nm.