Growth And First-Principles Study Of Infrared Acousto-Optic Hg₂X₂(X=Cl,Br,I) Crystals

Hg₂X₂(X=Cl,Br,I),namely mercurous halides,possess the advantages of wide transmission range,large birefringence,low acoustic velocity and acoustic attenuation,and large acousto-optic figure of merit M2,becoming the excellent acousto-optic crystals applied in long wave infrared region.In past decades,studies on crystal growth,physical property,and device process of Hg₂X₂ have been carried out by researchers from America,Russia,Czech,etc.,while there is little research domestically.The exploration of its growth technique is of high values for property characterization and device design.Furthermore,the relationship between the wide transmission range,large birefringence and its linear X-Hg-Hg-X molecule is unclear,and the reports on its elastic and acoustic properties are rare.The first-principles study of optical and elastic properties of Hg₂X₂ is of scientific significance for understanding the relation between the property and structure,and the design and optimization of acousto-optic devices.This paper presents the research on crystal growth,origin of the wide transmission range and large birefringence,and elastic and acoustic properties of Hg₂X₂,which is classified into 5 chapters:Chapter 1 introduces the physical theory of acousto-optic phenomenon and the requirement of acousto-optic materials,summarizes the research progress of Hg₂X₂ crystals such as birefringence,thermal expansion,and device fabrication,and introduces the first-principles calculation method used in this paper.The topic basis,research aims,and main contents are presented in the end.Chapter 2 is about the exploration of the growth technique of Hg2Br2 crystal,and the design of polarizers based on Hg2Cl2 and Hg2Br2 crystals.The dense and suit-to-grow Hg2Br2 ingot was obtained through purification.The black-brown and orange impurities appeared in the purification procedure were identified to be Hg5Br2O4 and HgO,respectively.The Hg2Br2 crystals in size of ?35×40 mm3 with smooth surface,no streaks,high transparency,and high quality were obtained through the adjustment of the growth technique.The polarizer and beam splitter with different splitting angles based on Hg2Cl2 and Hg2Br2 crystals were designed to widen its application scope.Chapter 3 presents the analysis of the origin of wide transmission range and large birefringence of Hg₂X₂ through the first-principles calculation on band structure,birefringence,and phonon spectrum,etc.The corresponding wavelengths of the bandgaps of Hg₂X₂ are close to the experimental short-wave cut-offs and locate in visible region.This is caused by the ability of halogens to bind valence electron tightly.The short-wave cut-offs correspond to the electron transition between the Hg-6s and X-p orbits of valence band maximum and the Hg-6s orbital of conduction band minimum.The highest frequency of Hg₂X₂ at ? points is only 279.6 cm-1,205.4 cm-1 and 178.8 cm-1 for Hg2Cl2,Hg2Br2 and Hg2I2,respectively,which is caused by the high molecular weight and is beneficial to obtain long cut-off wavelength.According to our analysis,the cut-offs of Hg₂X₂ are caused by multiple phonon absorption.The large birefringence of Hg₂X₂ were reproduced by the calculation,with the average deviation from experimental values of about 0.05.Charge difference shows the bonds formed within molecules,which were confirmed by the electron localization function to be covalent Hg-X bond,metallic Hg-Hg bond along molecules and weak van der Waals interaction between molecules.Thus,the bond strength difference along different directions leads the large birefringence.The difference of electric polarization along different directions demonstrated by Born effective charges are in accordance with the bond strength.Chapter 4 focus on the polycrystalline elasticity,elastic anisotropy,and acoustic property of Hg₂X₂.Due to the strong covalent and metallic bonds along c-direction,C33 has relatively large value.The polycrystalline elasticity shows Hg₂X₂ is easier to deform than change volume under pressure,and the soft nature caused by the weak van der Waals interaction has been revealed.The average velocity and the Debye temperature of Hg₂X₂ decreases with the halide changing from Cl to I,reflecting the weakening of the lattice vibration.The universal elastic anisotropy index AU,compressibility anisotropy Acomp*and shear anisotropy Ashear*reflect the increasing elastic anisotropy from Hg2Cl2 to Hg2I2.According to the orientation-dependent Young's modulus E and linear compressibility ?,this anisotropy increase is caused by the increase of the bond strength along c-direction.The acoustic velocity calculation shows the acoustic anisotropy.The maximum and minimum velocities locate at[001]and[110]directions,respectively,and the ratio between them is as high as 15.Finally,the acoustic angles of Hg₂X₂ were calculated to provide information for the design of acousto-optic devices.Chapter 5 gives the main conclusions and innovations of this paper and puts forward the future works needed to be done.