Study On The Physical Properties Of Rare Earth Sulfides Based On The First-Principles

In this thesis, First-principles Method was introduced. First of all, the crystal structures were optimized. Then, the band structure, density of state, phonon and optical properties of Y₂S₃ are calculated by Density Functional Theory. Energy band, phonon absorption, dielectric function, reflection, extinction coefficient and refractive index spectroscopy were obtained by analysis. Also the band structure, Elastic constants, phonon, optical and thermodynamic properties of La₂S₃ were calculated. And Band, phonon dispersion, dielectric function, reflection, Debye temperature and heat capacity spectroscopy were obtained.The band gap of La₂S₃ is 0.191 e V. The band gap of Y₂S₃ is 1.314 e V. Since there is no negative frequency in phonon spectrum of Y₂S₃ and La₂S₃, both crystals contained the lowest energy, which proves lattice dynamics is stable. The calculation of elastic constants of La₂S₃ shows that this crystal meet the mechanical stability convergence criteria. There is no peak in the reflection and dielectric spectrum with the wavelength higher than 1000 nm. So the material has smaller losses. Light through the material easily. The absorption and reflection spectrum peaks of La₂S₃ appeared with the wavelength below 300 nm. The peaks of dielectric function appeared with the wavelength below 1000 nm. When the wavelength is greater than 1000 nm, Light transmits more easily.The band gap of Y₂S₃:Eu is 0.128 e V. New bands appeared at the bottom of valence band around-40 e V and between-20 e V and-19 e V. Reflectivity of Y₂S₃:Eu reached the maximum value of 55% at 181 nm. The maximum value reduced 28% compared with the original structure. The value of reflectivity is more lower in the infrared region that greater than 750 nm. The maximum value of absorption reduced 30669 cm-1 than before. When the wavelength is greater than 500 nm, extinction coefficient is more smaller. In the 610 nm to 750 nm wavelength range, the refractive index reduced compared with the original structure, to less than 10%. The figures of refractive index and dielectric function of Y₂S₃:Eu move down under the 600 nm wavelength.This thesis provides theoretical basis for the application of Y₂S₃ and La₂S₃ by studying their properties. We study the rare earth sulfide more deeply. At the same time, this thesis also studied the changes, when Eu is adopted to Y₂S₃. This article provides basis to improve photoelectric properties.