Synergistic effect of 2[V2P2O14]∞ layers and hydrogen bonds inducing large birefringence in M(VO)2(PO4)2·4H2O (M = Ca, Sr, Ba) systems

Three non-centrosymmetric (NCS) alkaline earth vanadyl(IV) phosphate hydrates, M(VO)2(PO4)2·4H2O (M = Ca, Sr, Ba) 1–3, have been successfully synthesized by a mild hydrothermal technique. The birefringence of 1–3 was calculated to be 0.093@1064 nm, 0.070@1064 nm and 0.133@1064 nm by first principles calculations, respectively. The large birefringence mainly comes from the perfect parallel arrangement of 2[V2P2O14]∞ layers and the force of hydrogen bonds, which is conducive for the enhancement of polarization anisotropy. Furthermore, we also studied other optical and thermal properties for the title compounds as well as the relationship between structures and properties. The results provide a new perspective for exploring nonlinear optical (NLO) materials with large birefringence in vanadyl phosphate hydrates.