Theoretical Studies On The Microscopic Mechanism Of Heat Transfer In Chalcogenide Infrared Nonlinear Optical Crystals

The infrared laser with a wavelength of 2-20 ?m has a wide range of applications in military and civil fields.For example,infrared laser has great application prospects in infrared guidance,infrared laser radar,infrared laser communication,medical field and environmental monitoring and so on.Nonlinear frequency conversion technology can effectively convert the laser directly emitted by ions,amplify the laser frequency and realize miniaturized and fully cured devices.However,the technology needs high quality nonlinear crystals with strong nonlinear optical response,wide band gap,high thermal conductivity and appropriate birefractive index.Chalcogenide nonlinear optical crystals exhibit excellent nonlinear optical properties due to the presence of many nonlinear optical response groups,such as alkali metal or alkaline earth metal elements,lone-pair electrons and diamond-like structural units.First-principles prediction of the properties of nonlinear optical crystals has attracted researchers'extensive attention because it is difficult to grow large size chalcogenide nonlinear optical crystals,and first-principle calculation can avoid wasting time and experimental costs.At present,first-principles calculation has been used to study the structure-property relationship between the active group and nonlinear optical properties and the band gap,but there are few studies on the influence of different groups on the thermal conductivity.Low thermal conductivity of nonlinear optical crystals will cause the energy that is not converted into the output of the laser to be converted into heat and remain in the crystal,resulting in a variety of thermal effects in the crystal,such as thermal lens effect,thermal diffraction loss and even thermal fracture.The thermal effect of nonlinear optical crystals will not only decrease the output power and change the polarization of the laser,but also reduce the service life of the laser.Therefore,it is necessary to study the thermal conductivity of nonlinear optical crystals theoretically.In recent years,the anharmonic dynamics method based on density functional perturbation theory has gradually become the main method to study the heat transport mechanism of semiconductor materials.With this method,more accurate thermal conductivity can be obtained,and the parameters closely related to thermal conductivity,such as phonon relaxation time,phonon group velocity and heat capacity,can be analyzed from the microscopic level.However,this method only elucidated the scattering mechanism and the source of thermal conductivity from the phonon level,but did not go into the electronic properties and microstructure of materials.In this paper,we will explore the mechanism of the influence of electronic states and microscopic groups on the phonon states of crystals,especially the thermal transport properties.The main research contents and results are as follows:1.To study the effects of metal cations on heat transport properties,the thermal conductivity and microcosmic origin of commercial nonlinear optical crystals AgGaS2 and LiGaS2 are used for comparison.By calculating the thermal conductivity of the two,it is found that the thermal conductivity increases from 1.6 W/(m·K)to 5.8 W/(m-K)obviously when the alkali metal Li replaces Ag.Based on first-principle calculations and lattice dynamics,the reasons for the high thermal conductivity of LiGaS2 were investigated in details.The results show that the three acoustic modes of LiGaS2 have higher phonon group velocities,which is the main reason for the difference of thermal conductivity between LiGaS2 and AgGaS2.By analyzing the vibration modes of the frequency range where the acoustic modes are located,it's found that vibrations of AgGaS2 in this frequency range mainly involves Ag-S bonds,while the vibrations of LiGaS2 are mainly related to Ga-S because the Li is relatively lighter.Since Ga-S bond is a covalent bond while Ag-S is and ionic bond with weak covalence,LiGaS2 has a higher phonon group velocity in this frequency range,and thus has a higher thermal conductivity.2.To study the influence of the lone-pair electrons on the heat transport property,the internal microscopic mechanism of how the lone-pair electron of As3+ affected the LiAsS2 thermal conductivity were explored.On the one hand,the layered-structural LiAsS2 exhibits anisotropic thermal transport behaviors due to lone-pair electron.The different bond strength of Li-S and As-S bonds lead to diverse phonon group velocity along a direction and in be plane,which is the soxurce of anisotropic lattice thermal conductivity of LiAsS2.On the other hand,the existence of lone-pair electron dramatically modifies the phonon anharmonic behavior of LiAsS2.Lattice vibrational results demonstrate that the rigidity of pyramidal units AsS3 in LiAsS2 is weaker than tetrahedral units GaS4 in LiGaS2,which induces the positive Gruneisen parameters in low frequency domain of LiAsS2.As a result,the decrease of phonon lifetime and reduction of lattice thermal conductivity of LiAsS2 are generated.Nevertheless,high-power nonlinear optic devices can be fabricated along bc plane of LiAsS2 due to the remarkably high thermal conductivity in bc plane and the intrinsic high nonlinear optical coefficients.3.New quaternary lithium-sulfur compounds Li2CdSnS4 and Li2BaSnS4 were used as research objects.The influence of different polyhedral on the lattice thermal conductivity was studied.Because the ion radius of Ba2+ is larger than that of Cd2+,Li2CdSnS4 is composed of three types of tetrahedrons LiS4,SnS4 and CdS4,while Li2BaSnS4 is composed of two types of tetrahedrons LiS4 and SnS4 and BaSs dodecahedrons.From the calculated result,the lattice thermal conductivity of Li2CdSnS4 is higher than that of Li2BaSnS4.On the one hand,from the harmonic properties of two,it can be known that the phonon group velocity of Li2CdSnS4 is much higher than that of Li2BaSnS4 in low-frequency optical modes range.In this frequency region,the vibrations of Li2CdSnS4 involve Cd-S and Sn-S bonds,while for Li2BaSnS4 the vibrations are mainly related to Ba-S and Sn-S bonds.The bond strength of Cd-S is higher than Ba-S,resulting for the lager phonon group velocity of Li2CdSnS4 in low-frequency optical modes range.On the other hand,the phonon lifetime of Li2CdSnS4 is also larger than Li2BaSnS4 in this frequency range.It's found that covalent bonds Sn-S forms stable tetrahedrons and BaS8 are surrounds by SnS4 tetrahedrons.The mainly vibrations in this frequency range of Li2BaSnS4 are the rotation vibrations of SnS4 and bending vibrations of S-Ba-S bonds.For Li2CdSnS4,the mainly vibrations are rotation vibrations of SnS4 and CdS4 without the inherent vibrations of polyhedrons.The inherent vibrations in BaS4 dodecahedrons make the Grunensen of Li2BaSnS4 a large positive value,which increases the anharmonic effect and decreases the thermal conductivity of Li2BaSnS4.Therefore,Li2CdSnS4 composed of tetrahedrons has more application prospects as a nonlinear optical crystal and polyhedral with low coordination number should be selected when bringing in new elements.