Growth,Structure And Properties Of Nonlinear Optical Crystals Based On Titanium/Bismuth-containing Phosphates

As a key component of solid-state laser,nonlinear optical crystals can effectively expand the wavelength range of laser light source and play a key role in the field of laser frequency conversion technology.The development of laser technology has put forward more requirements for NLO crystals,so it is still important to develop nonlinear optical crystals with good performance.Nonlinear optical crystals of phosphates have become a research hotspot in the field because of their good transmittance in ultraviolet range.However,the weak anisotropy of the non-π-conjugated[PO4]group and the small nonlinear optical polarizability result in the small birefringence and the weak frequency doubling effect.Therefore,the development of phosphate nonlinear optical crystals with large frequency doubling effect and moderate birefringelce is the focus of current research.In this paper,by introducing cations(Ti4+,Bi3+)with second-order John-Teller effect into the phosphate system,we expect to obtain phosphate nonlinear optical crystals with outstanding performance.The main research contents and achievements are as follows:1.A mixed anionic phosphate Rb3Ti3P5O20 was obtained by introducing the d0 electron configuration Ti4+ with second-order John-Teller effect into the phosphate system.Millimeter Rb3Ti3P5O20 crystals were grown by high temperature solution method for the first time,and its structure,thermal and optical properties were characterized.It is noteworthy that the crystal structure of Rb3Ti3P5O20 contains two anionic groups[PO4]and[P2O7],while the[Ti(2)O6]and[Ti(3)O6]octahedrons form a unique Ti-O-Ti bond bridges by sharing an O atom,resulting in greater octahedral distortion.Thermal analysis shows that the crystals have very high thermal stability,and no phase transition or mass change occurs at the temperature of up to 1000℃.The electronic structure and elemental composition of the crystal were studied by XPS and EPMA.The compound has a wide transmittance range(331-4500 nm)and a large optical band gap(3.54 eV).The frequency doubling intensity of the powder is 0.4xKDP,and the calculated birefringence of the crystal is 0.079@1064 nm.First-principles calculations show that the optical properties of the crystal are mainly derived from the distorted[TiO6]octahedron,with relatively small contributions from the[PO4]and[P2O7]groups.It is rare for a phosphate material to contain two different anionic groups.This work enriches the structure of phosphate and provides a new perspective for the design of a variety of anionic materials.2.Compounds with two non-centrosymmetric structural features often have synergistic effects and thus produce large frequency doubling effects.Two phosphate crystals,PbBi3PO8 and ZnBi6P2O15 containing lone pair electron Bi3+,were grown by spontaneous crystallization method.Thermal analysis showed that both compounds were congruent melting compounds and could be grown by melt methods.PbBi3PO8 crystallizes in I4mm space group with highly disordered structure.Its crystal structure is composed of Pb-O polyhedron,Bi-O polyhedron and[PO4]tetrahedron connected to form a rare complex three-dimensional structure through the way of common point or common edge.Optical measurements show that it has a wide optical transmittance range(381-5185 nm)and an optical band gap of 3.14 eV.The powder frequency doubling test shows that PbBi3PO8 has a large nonlinear optical response(2.3×KDP).According to the anionic group theory,the[PO4]groups in the crystal structure have the same orientation,which is beneficial to improve the macroscopic nonlinear optical effect of the crystal,and it is a potential nonlinear optical crystal.ZnBi6P2O15 belongs to the C2 space group,and its crystal structure is a staggered arrangement of layered structures formed by the connection of Bi and O atoms,which are connected by[ZnO4]and[PO4]to form a threedimensional structure.The optical measurements show that it has a large band gap of 3.58 eV and the ultraviolet cutoff reaches 308 nm.The theoretical calculated birefringence is 0.081@1064 nm,which is relatively large in phosphates.First principles calculations show that the electron transition near the Fermi level of the compound is mainly due to the interaction between the Bi and O atomic orbitals.3.Two kinds of nonlinear optical crystals Cs3MBiP4O14(M=Sr,Ca)have been successfully grown by introducing Bi3+with stereochemical active lone pair electrons into phosphates.Both compounds crystallize in polar space group P212121,whose crystal structure is formed by[P2O7]and[BiO6]groups connected to each other in a copoint way to form[BiP4O14]5-layered structure.Between the layers,the three-dimensional structure is formed by the polyhedron connected by the coordination of Sr/Ca and O atoms.Cs3MBiP4O14(M=Sr,Ca)show wide optical band gap of 4.14 eV and 4.20 eV,respectively.The UV cutoff of Cs3SrBiP4O14 and Cs3CaBiP4O14 is 298 nm and 290 nm respectively.The frequency doubling intensity of Cs3MBiP4O14(M=Sr,Ca)is 0.4 times and 0.7 times of KDP,respectively,and the theoretical calculated birefringence is 0.025 and 0.020@1064 nm,respectively.The first principles calculation shows the optical properties of the two are mainly determined by the electronic transition between Bi-O and alkali metals Cs and alkali earth metals Sr/Ca play an active role in expanding optical band gap.Cs3MBiP4O14(M=Sr,Ca)have good optical properties and are potential UV nonlinear optical crystals.