| As an important photoelectric information functional material,the second-order nonlinear optical crystal materials have very important applications in frequency conversion,electro-optic modulation,communication and information processing,and have made tremendous contributions to the development and progress of society.It is one of the frontiers of material research.First of all,it must be non-centrosymmetric.How to efficiently obtain compounds with non-centrosymmetric structures has always been a problem since most of the compounds tend to crystallize in a centrosymmetric manner.In recent years,scientists have discovered that introducing structural unit with an asymmetric coordination environment can be greatly increase the possibility of obtaining compounds with non-centrosymmetric structures.For example,cations with second-order Jahn-Teller(SOJT)distortion include d0 transition metal cations with octahedral coordination environment(Nb5+,Mo6+,W6+,etc.)and cations with stereochemically active lone pairs(Te4+,I5+,Pb2+,etc.).Many of compounds formed by these structural units have excellent nonlinear optical properties and are potential second-order nonlinear optical crystal materials.Therefore,in this paper,we use the Te4+ with asymmetric coordination environment as the breakthrough point,combining the perfect component of PO4 tetrahedron,in order to obtain tellurite phosphate with non-centrosymmetric structure.(1)Three new alkaline earth tellurite phosphates were synthesized by hydrothermal method under different temperatures,namely,BaTeO(HPO4)2,BaTe2O2(PO4)2 and BaTe3O3(HPO4)(PO4)2.The synthesis temperatures of the three compounds were 190,210 and 230℃,respectively,showing a trend from low to high,and with the change of the Ba and Te molar ratio 1:1-1:2-1:3,the structure changes from simple to complex,showing changes from 0D-2D-3D.And BaTe3O3(HPO4)(PO4)2 is a non-centricsymmetric structure,the second harmonic intensity is close to KDP.At present,the nonlinear optical coefficient of the compound is the highest in tellurite phosphate.Its cut-off absorption edge is 421 nm,indicating that it has potential nonlinear optical applications in ultraviolet region.The first-principles calculation and the dipole moment calculation results show that the main contribution of the optical property comes from the Te4+ cation which with asymmetric coordination environment.It is the first non-centrosymmetric structure in the alkaline earth tellurite phosphate system,which provides a reference for the synthesis of other compounds with non-centrosymmetric crystal structures.In addition,a compound of SrTe(PO4)2 with 2D layered structure has been obtained by hydrothermal method.(2)Two alkali metal tellurite phosphates LiTe2O2(Ho.5PO4)2 and NaTeO(PO4)have been synthesized by hydrothermal method.Both of them are easy to grow into large single crystals,especially the size of LiTe2O2(HO.SPO4)2 can reach centimeter level.It is the first time to discovery compounds containing lithium or sodium in tellurite phosphate.They exhibit similar chemical structures with 2D layered structure composed of distorted TeO5 square pyramids and PO4 tetrahedra.Due to the different radius of Li+ and Na+,the alkali metal is located in the different location between the layers.(3)This is the first time to synthesis of alkaline earth and alkali tellurite phosphate by hydrothermal method.It breaks through the limitation of the traditional solid-state reaction.(4)A new tellurite phosphate β-Te3O3(PO4)2 has been successfully synthesized by hydrothermal method in TeO2-P2O5 system.It is polymorphism withα-Te3O3(PO4)2,which synthesised by solid-state reaction.The structure difference between α-and β-forms of Te3O3(PO4)2 lies on the polymerization of TenOm.The TenOm in β-Te3O3(PO4)2 is Te6O2220-hexanuclear clusters whereas 1∞{[Te3O11]10-}single chains in α-Te3O3(PO4)2.This is the first time to find polymorphism in the tellurite phosphate system.Note that Te6O2220-hexanuclear cluster is the first time to be found in tellurites. |
