| Design and growth of new functional crystal materials is of great significance.Due to diversity of the structure and tunability of the property,KTP family crystals have received continuous attention from researchers.RbTiOPO4(RTP)belongs to KTP family crystals,and has excellent properties including high nonlinear optical coefficient,high electrical conductivity,high electro-optic coefficient and high laser damage threshold.It can be applied to the fields of nonlinear,electro-optic,ferroelectric and superionic conductor.Based on the unique advantages of structure and properties of RTP crystals,it is of great significance to study doping RTP crystals.At present,there are still many problems in the study of doping RTP crystals.For example,growth of high-quality doping RTP crystal has not been solved;the nonlinear origin of RTP-type crystal structure has not been determined;the self-frequency doubling application of rare-earth ions doped RTP crystals needs further study.In view of the above problems,based on the growth and property characterizations of RTP crystals,the study of Ta,Yb doped RTP optical crystals was carried out.On the one hand,Ta element can improve the optical properties of the crystal;on the other hand,the energy level structure of the Yb element is simple,which is suitable as an activated ion.Using high-temperature solution method,we have successfully grown a series of better quality doping RTP crystals without inclusions,including Ta:RbTiOP04(RTP:Ta)crystal and Ta:Yb:RbTiOPO4(RTP:Ta,Yb)crystal.Also,the langbeinite structure Rb2Ti0.80Yb1.20(PO4)3 crystal was obained.The microstructure and their relationship with properties of this series of doping RTP and langbeinite structure crystals were studied by X-ray techniques(X-ray single crystal diffraction,X-ray photoelectron spectroscopy,synchrotron radiation XAFS).The nonlinear optical properties of Ta:RbTiOPO4 crystal,the luminescence properties of Ta:Yb:RbTiOPO4 crystal and the up-conversion luminescence properties of Rb2Ti0.80Yb1.20(PO4)3 crystal were mainly studied,in order to explore the application value of this series of crystals in the fields of nonlinear optics,self-frequency doubling laser and up-conversion laser.The specific research work of this paper is as follows(1)RTP crystals were grown by high-temperature top seeded solution growth,by continuously optimizing crystal growth conditions,we successfully grown RTP crystals without macro-defects,the crystal size reached 19×19×19 mm3,and the crystal defects and impurities were less,and the internal components were uniform.At the same time,thermal and optical properties of RTP crystal are characterized.Thermal analysis results show that the RTP crystal has good thermal stability within 1090 ?.In the temperature range of 20-300 ?,the specific heat of the RTP crystal is 0.521-0.886 J·g-1·K-1,the thermal diffusivity is 0.5-1.4 mm2/s,and the thermal conductivity is 1.4-2.6 W·m-1·K-1,its thermal diffusivity and thermal conductivity properties exhibit significant anisotropy.The RTP crystal has a light transmission range of 0.35-45 ?m with transmittance of 85%.At 532 nm,the absorption coefficients of RTP crystals in three directions a,b,and c are 0.42 cm-1,0.48 cm-1,and 0.08 cm-1,respectively.At 1064 nm,the absorption coefficients of RTP crystals in three directions a,b,and c are 0.20 cm-1,0.41 cm-1,and 0.01 cm-1,respectively.The basic study of crystal properties shows that the grown RTP crystal is an optical crystal with good thermal properties and low absorption loss(2)Ta single-doped RTP crystals with different concentrations were grown by high temperature solution spontaneous nucleation and top seeded methods.The growth characteristics and morphology change were discussed.The effects of Ta dopants on the electronic structure,thermal properties and optical properties of RTP crystals were studied.Through optimizing crystal growth conditions,RTP:Ta crystals without inclusions were successfully grown.EPMA measurement shows that the Ta element has a large segregation coefficient,high-resolution XRD test shows that the crystal quality of RTP:Ta crystal is close to that of the RTP crystal.We used experimental observation and theoretical calculation to study the(100)and(201)crystal faces of Ta doped RTP crystal.From the view of growth mechanism and surface energy of crystal faces,the reasons for the growth rate decrease and a-direction size decrease of the crystal caused by Ta dopants are explained.In addition,the method of obtaining isometric RTP:Ta crystal is provided,and the size ofRTP:Ta crystal is successfully increased from 4×31×18 mm3 to 6x24x22 mm3.The electronic structure of RTP:Ta crystal was studied using high resolution XPS.With the increase of Ta dopants,the peaks of Rb 3d,Ti 2p and P 2p high resolution XPS are shifted by 0.2-0.4 eV toward the high binding energy,while the peak of O 1s high resolution XPS is shifted towards high binding energy by 0.4-0.5 eV,and the chemical shift is related to strong electronegativity of Ta2+.Thermal analysis shows that Ta dopants improve the thermal stability and specific heat performance of the RTP crystal.In particular,the specific heat of RTP:Ta crystal with 9 mol%doping concentration is 1.5 times that of RTP crystal at the temperature of 300?.In the temperature range of 25-350 ?,the thermal diffusivity is 0.4-1.6 mm2/s and the thermal conductivity is 1.2-2.2 W·m-1·K-1.The transmission range of RTP:Ta crystal is 0.35-4.5 ?m,the transmittance is 85%,and the UV cut-off edge is red-shifted by 6 nm compared to RTP crystal.At 532 nm,the absorption coefficients of RTP:Ta crystals in three directions a,b,and c are 0.42 cm-1,0.38 cm-1,and 0.10 cm-1,respectively.At 1064 nm,the absorption coefficients of RTP:Ta crystals in three directions a,b,and c are 0.19 cm-1,0.15 cm-1,and 0.06 cm-1,respectively.RTP:Ta crystal is an optical crystal with better thermal performance and lower absorption loss than RTP crystal.(3)Effects of Ta dopants on the nonlinear optical properties of RTP crystal was studied.The relationship between nonlinear optical properties and structure of RTP:Ta crystal was studied by X-ray diffraction and synchrotron radiation XAFS techniques,including doping ions' occupancy tendency and bond length,TiO6 octahedron distortion and so on.The frequency doubling performance test shows that Ta dopants effectively improve the nonlinear optical property of RTP crystal.Particularly,when Ta doping concentration reaches 9 mol%,the SHG intensity of RTP:Ta crystal is 1.59 times that of RTP crystal.Therefore,RTP:Ta crystal has excellent nonlinear optical properties,which is a potential nonlinear optical crystal.Single crystal structure ofRb0.94Ti0.94Ta0.06OPO4 and Rb0.90Ti0.90Ta0.10OPO4 crystals show that the volume difference between Ti(1)O6 octahedron and Ti(2)O6 octahedron increases with the increase of Ta dopants.Ta5+ cations tend to occupy the Ti(1)position in RTP crystal,because Ti(1)position is weakly affected by the electrostatic repulsion of Rb' cations,which cause Ta5+ cations preferentially occupy the Ti(1)position.Ta5+ heterovalent substitution of Ti4+ resulted in the generation of Rb vacancies in the RTP crystal,and the Rb vacancies were produced more at the Ti(2)position,resulting in larger expansion of Rb(2)09 cage than Rb(1)O9 cage.In RTP-type crystal(RTP:Ta,RTP:Nb,RTP),the degree of distortion of the Ti(1)O6 octahedron is greater than that of the Ti(2)O6 octahedron.The highly distorted TiO6 octahedron and short Ti-O bond length in the crystal structure make RTP:Ta crystals possess comparable nonlinear optical properties to those of RTP crystal.X-ray absorption near-edge structure analysis shows that Ta dopants increase the distortion of TiO6 octahedron in RTP crystal,which improves the frequency doubling performance of the crystal.(4)Growth conditions of Ta,Yb double-doped RTP crystals were investigated by high temperature solution method.Electronic structures and bond strengths of Ta and Yb double-doped RTP crystals were compared.The influence of Ta,Yb dopants on optical properties of RTP crystals were discussed,and the luminescence properties of different occupied Yb3+ were discussed.By optimizing the crystal growth conditions,RTP;Ta,Yb crystals without inclusions were successfully grown,and the size was 3×30×15 mm3.The XRD test shows that the RTP:Ta,Yb crystal has high phase purity and good crystallinity.High-resolution XRD tests show that RTP:Ta,Yb crystals have good uniformity and integrity,and their crystal quality is close to that of RTP crystals.EPMA analysis showed that Ta segregation coefficient is 0.97-1.28 and the Yb segregation coefficient is 0.13-0.37.Ta2+ ions can effectively increase the concentration of Yb3+ ions in RTP crystal,and the concentration of Yb3+ ion in RTP crystal reaches 1.26×1020 atom/cm3,which can meet the luminescence requirement.The electronic structure of RTP:Ta,Yb crystals was comprehensively studied using high resolution XPS.Ta,Yb dopants cause chemical shift of element binding energy in RTP crystal,Rb 3d5/2 increases by 0.4-0.6 eV,Ti 2p and P 2p increases by 0.3-0.4 eV,and O 1s increases by 0.4-0.5 eV.With the increase of Ta and Yb doping concentration,the strength of Ti-O ionic bond decreases and then increases,and the strength of P-O covalent bond increases and then decreases.The binding energy of Ta 4f7/2 in RTP:Ta,Yb crystal is 26.4-26.5 eV,and the energy splitting of spin-coupling is 1.9 eV.RTP:Ta,Yb crystals have transmission range of 0.35-4.5 ?m and transmittance of 83%.RTP:Ta,Yb crystal has good absorption characteristics and low absorption loss in the transmission range.The emission peak of Yb3+ is located at 972 nm(2F5/2(0')?2F7/2(0)),1002 nm(2F5/2(0')?2F7/2(1)),1023 nm(2F5/2(0')?2F7/2(2)),1050 nm(2F5/2(1')?2F7/2(3))and 1071 nm(2F5/2(0')?2F7/2(3)),the wavelength of 972 nm is the main emission peak of the crystal.Since Yb3+ occupies two positions Ti(1)and Ti(2)in the RTP crystal,its energy level is complicated in the crystal field of the RTP structure.Using the experiments of low-temperature emission spectroscopy,we calculated the ground state and excited state energy of Yb3+ in Ti(1)and Ti(2)positions.Combined the good nonlinear optical properties of RTP:Ta crystal and good luminescence properties of Yb3+ around 1 ?m,RTP:Ta,Yb crystal is a self-frequency doubling optical crystal with potential application value(5)The dark blue langbeinite-structure Rb2Ti2-xYbx(PO4)3 crystal was successfully grown by high temperature solution method.The phase,morphology and composition of the crystal were analyzed.X-ray single crystal diffraction and X-ray photoelectron spectroscopy were used to study the crystal structure including group connection and bond strength.Also,thermal and upconversion luminescence properties of this crystal are characterized.The phase purity was determined by XRD,and the chemical formula was determined to be Rb2Ti0.80Yb1.20(PO4)3 by EPMA quantitative analysis.The theoretical density of the Rb2Ti0.80Yb1.20(PO4)3 crystal is 4.378g/cm3,and the unit cell parameter is a=10.2114(9)A,V=1064.8(3)A3.The occupancy of Yb3+ ions at the Ti(1)position is 0.751,and the occupancy at the Ti(2)position is 0.449.The Ti(1)O6 octahedron is larger and can accommodate more Yb3+with a larger ionic radius,which is consistent with the tendency of Yb3+ ions to occupy the Ti(1)position.Ti(2)O6 octahedron is more distorted,and its degree of distortion is 1.48×10-4,and its O-Ti-O bond angle is 172.0(4)°.PO4 tetrahedron is distorted to some extent,and its degree of distortion is 3.76×10-4.The Rb(2)O12 cage is slightly larger than the Rb(1)O12 cage.In addition,the distance between Rb(2)ion and TiO6 octahedron,PO4 tetrahedron is larger than those of Rb(1)ion.The binding energies of Rb 3d5/2,Ti 2p3/2,P 2p,O 1s and Yb 4d5/2 in Rb2Ti0.80Yb1.20(PO4)3 crystal are 109.4 eV,459.4 eV,133.1 eV,530.9 eV,185.1 eV,respectively.Compared to RTP crystal,the binding energy of each element is increased by 0.1-0.7 eV.Compared with the Rb2Ti1.95Yb0.05(PO4)3 crystal,the binding energy of each element is increased by 0.1-0.5 eV.In terms of bond strength,Rb2Ti0.80Yb1.20(PO4)3 crystal has stronger Ti-O ionic bond and weaker P-O covalent bond than RTP and Rb2Ti1.95Yb0.05(PO4)3 crystals.The Rb2Ti0.80Yb1.20(PO4)3 crystal has a weaker Yb-O ionic bond than Yb2TiO5 and Yb2O3 crystals.The melting point and decomposition point of the crystal were determined by thermal analysis at around 1150 ? and 1180 ?,respectively.The specific heat is between 0.482 and 0.575 J·g-1·K-1 at the temperature of 25-300 ?Rb2Ti0.80Ybi.20(PO4)3 crystal exhibits strong up-conversion luminescence under 980 nm laser excitation.The wavelength of blue emission peak is 486 nm,the peak width is about 10 nm,and the fluorescence lifetime is 6.56 ?s.Our study indicates that Rb2Ti0.80Yb1.20(PO4)3 crystal is a promising up-conversion crystal with good thermal properties. |
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