Synthesis And Characterization Of Novel Bismuth Phosphates And Tellurite Phosphates With Lone Pair Electrons

As an important information functional material,nonlinear optical crystal must have non-centrosymmetric crystal structure.Rational design and synthesis of compounds with non-centrosymmetric crystal structure is still a hot and difficult topic.It is found that the coordination of cations,which have stereo chemical active lone pair electrons(Bi3+,Te4+,etc),with anions can easily form asymmetric structural units,which can significantly increase the possibility of the formation of noncentrosymmetric crystal structure.Therefore,in this paper,based on phosphate,we hope to design and synthesize phosphate compounds with non-centrosymmetric crystal structure by introducing Bi3+ and Te4+ cations into the phosphate compound,and study the relationship between their crystal structure and optical properties as well as explore their applications in the field of nonlinear optics.The results of this paper are as following:(1)A new strategy of rational design synthesis has been proposed.Based on BiPO4,two novel alkaline earth bismuth phosphates Bi0.92Ca1.08H1.08(PO4)2 and Bi0.92Sr1.08H1.08(PO4)2 with non-centrosymmetric crystal structure have been synthesized by hydrothermal method through partial substitution of alkaline earth metal ions.The results show that the crystal structure has a "sandwich-like" threedimensional framework structure formed by the planar layer composed of Ca(Sr)/BiO8 polyhedra,which is further interconnected by HPO4 groups.At the same time,Ca and Sr can replace Bi together to form a finite solid solution which is isomorphic to the two terminal members.It is found that the difference of ion radius greatly affects the crystal structure and optical properties.Both compounds revealed the type-Ⅱ phase-matching second harmonic generation(SHG)in nonlinear optics.The Ca compound has a SHG intensity of～0.43×KDP and the Sr compound has a strong SHG intensity(～3.27×KDP).The two alkaline earth metal bismuth phosphates also show red luminescence after doping with Eu3+.(2)Two transition metal tellurite phosphates,Ni2MTe2O2(PO4)2(OH)4(M=Ni,Zn)have been synthesized via hydrothermal method which crystallizes in centrosymmetric space group C2/m for the first time.The two compounds are isomorphic.The infinite linear chain along the direction of[010]formed by the NiO6 octahedra via edge-sharing is connected by PO4 tetrahedra to form NiPO layer,the MTeO single chain along the b axis is formed by MO2(OH)4 octahedra sharing the equatorial OH groups each with a TeO3(OH)2 tetragonal pyramid,the NiPO layers are interconnected by MTeO single chains via vertex-sharing to form the three-dimensional framework structure.The dipole moments of TeO3(OH)2 are antiparallel to each other and cancel each other,leading to the formation of the centrosymmetric crystal structure.