| Crystals have harmonious and regular structure.Beauty comes from harmony,usefulness comes from order.The atoms regular arrangements of the crystal make the crystal microstructure periodic.The microscopic structure and composition of crystal determine the macroscopic physical.Therefore,researchers could design and construct the corresponding microstructures according to the requirements of macroscopic physical properties to obtain targeted artificial crystals.With unique competitive advantages and great commercial potential,artificial crystals have greatly promoted the development of functional crystal materials.M2?M4O15(M=Mo,W)compounds in the huge yttrium and lanthanide molybdate(tungstate)family are important negative thermal expansion materials,which has important research value and application prospects in the fields of fluorescent and negative thermal expansion basic research.Based on the literature investigation of Y2Mo4O15,we suggest that Y2Mo4O15 crystal is an excellent Unixial NTE material with both laser matrix and stimulated Raman scattering.Firstly,as a laser matrix,the Y3+ ion lattice site can theoretically be doped with a variety of rare earth ions(Re:Y2Mo4O15 laser crystal),that is,can control the negative axial thermal expansion coefficient by controlling the concentration or type of rare earth ion doping.Thus they will be potential to obtain laser crystals with zero thermal expansion.Secondly,due to the existence of Mo-O bond in the structure,Y2Mo4O15 crystal has good Raman characteristics.However,no matter laser crystals,Raman crystals or self-Raman laser crystals,thermal deposition and thermal stress always occur in the operation processes,which may cause cracks in the crystals in operation.Therefore,the study of their thermal properties,especially the thermal expansion behavior,is helpful to solve the thermal problems encountered in the practical application of this kind of materials.In this paper,starting from the regulation of thermal expansion characteristics of crystals,Y2Mo4O15 crystals were selected as the research objects to conduct bulk crystal growth and physical properties characterization,and tried to reveal the mechanism of thermal expansion of crystals from the perspective of crystal structure.At the same time,high concentrations of Yb3+ion doping were carried out in the matrix crystal Y2Mo4O15 to explore the possibility of this material as a laser-host.This thesis is divided into five chapters,each chapter is introduced as follows In the first chapter,the development and research status of negative thermal expansion materials,molybdate crystals and yttrium molybdate materials are introduced,and the basis,purpose and main research contents of this thesis are put forwardIn the second chapter,the experimental reagents,solid phase synthesis method,crystal growth method(TSSG),growth equipment,phase analysis,performance characterization and first-principles calculations involved in the study of Y2Mo4O15 and Yb:Y2Mo4O15 crystals are introduced.In the third chapter,the Y2O3-MoO3-Li2O flux system was systematically explored,and an appropriate flux proportion for growing Y2Mo4O15 crystal was obtained.After optimizing the growth parameters,28 mm × 20 mm × 16 mm Y2Mo4O15 single crystal was grown.Laue test was carried on the(100)crystal plane of Y2Mo4O15 single crystal with double-sided fine polishing.The results show that the diffraction patterns at different positions of the(100)crystal plane are symmetrical,clear and consistent,which indicats that the overall quality of the crystal obtained by flux method is good The specific heat of Y2Mo4O15 crystal increases linearly from 0.780 J/(g·K)at 22? to 0.900 J/(g·K)at 250?.In the temperature range of 30?300?,the thermal diffusivity in the a*axis direction is reduced from 2.18 mm2/s to 1.36 mm2/s,and the thermal conductivity is reduced from 6.83 W/(m·K)to 4.27 W/(m·K),it is worth noting that the thermal conductivity and thermal diffusion coefficient of Y2Mo4O15 crystal is equivalent to that of lithium niobate and higher than that of ordinary molybdate crystals,indicating that it has good thermal properties.The average linear thermal expansion coefficient of Y2Mo4O15 crystal b-axis in the temperature range of 25?200? is calculated as ?b=5.3 ×10-5?-1,and the negative thermal expansion mechanism is explained from the crystal structure.The optical band gap of Y2Mo4O15 crystal is 3.50 eV from the diffuse reflection of ultraviolet-visible.The UV cut-off edge ofY2Mo4O15 crystal is at 345 nm and the infrared cut-off edge is at 5575 nm.It can maintain a high transmittance of more than 80%in the wavelength range of 360-5000 nm.The spontaneous Raman spectra of Y2Mo4O15 crystals under different configurations are measured and compared with YVO4 crystals under the same experimental conditions.When using 1064 nm laser pump,the strongest Raman frequency shift peak of Y2Mo4O15 crystal is 1.68 times of the strongest Raman frequency shift peak of YVO4 crystal,which indicats that Y2Mo4O15 crystal is a potential stimulated Raman matrix material.The theoretical Raman value and phonon spectrum were calculated in cooperation with Jia Yu's research group of Zhengzhou University,and the group vibration mode that produced the relevant Raman peak was identified.In the forth chapter,the influence of high doping of Yb3+ ion on the negative thermal expansion of the crystal is studied comprehensively.Yb:Y2Mo4O15 crystal with a size of 20 mm × 8 mm × 5 mm was successfully grown by the flux method.The half width of the rocking curve of the Yb:Y2Mo4O15 crystal(100)surface is 82 arcsec.The thermal and optical properties of Yb:Y2Mo4O15 crystal are studied.In the temperature range of 25?250?,the thermal diffusion coefficient in the a*axis direction is reduced from 0.66 mm2/s to 0.59 mm2/s,and the thermal conductivity coefficient is reduced from 2.02 W/(m·K)is reduced to 1.16 W/(m·K).The b-axis of Yb:Y2Mo4O15 crystal becomes positive thermal expansion.The reason of b-axis positive thermal expansion is explained from the crystal structure by measuring the variable temperature polarization Raman.The optical band gap of Yb:Y2Mo4O15 crystal is 3.49 eV from the ultraviolet-visible diffuse reflection test.The optical transmission range of Yb:Y2Mo4O15 crystal is 341?5100 nm,and the transmittance is as high as 70%in the entire transmission range.The absorption spectrum shows that the non-uniformly broadened absorption band at 975 nm is 78 nm,and the absorption cross section at the highest absorption peak at 975 nm is ?abs=1.8 ×10-20cm2,which is very suitable for LD as a pump source.The emission spectrum shows that the half-height width of the emission band is up to 48 nm,which indicates that the disordered crystal structure enlarges the spectral bandwidth nonuniformly,which can be used to achieve the output of the broad-spectrum tunable laser.In the fifth chapter,the main conclusions,innovations and the work to be further studied are summarized. |
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