磷硅镉晶体的非线性光学效应与热导率的第一性原理研究

CdSiP2 crystal with larger nonlinear optical coefficient and higher infrared transmittance merit, is being paid more and more attention, has become the research frontier and hot of high-tech new materials. The crystals which grow in the laboratory have many defects and present severe anomalous thermal expansion effect, these characteristics have a great influence on the crystal performance and their interactive mechanisms are complex, it is difficult to explain these experimental phenomena. Owning to the deficiency of the experiment method, this project aims at investigating the linear and nonlinear optical properties of CdSiP2 using the first principles method combined with the GW approximation as well as the density functional perturbation theory, giving out a method of calculating the nonlinear optical coefficient , which does not depend on the experimental band gap. The objective of the project is to study the influences of the structure, defects and impurities to the linear and nonlinear optical properties, thermal conductivity of CdSiP2 on atomic level, analysing the microscopic origin and mechanism of nonlinear optical properties and thermal conductivity of the crystal. We further explore the internal relationship of microstructure-performance-crystal defects of crystal, to make the theoretical results be applied to the experiment as early as possible. The research of this project can provide the theory reference for chalcopyrite nonlinear optical crystal materials experimental research and application development, having the vital significance.

磷硅镉晶体具有较大的非线性光学系数和中远红外透过率高等优点，受到人们的高度重视，已成为高科技研究的前沿和热点的新材料。实验上生长出的晶体具有一系列缺陷和严重反常热膨胀等对晶体性能影响较大且它们的作用机理很复杂，人们无法解释这些实验现象。本项目针对实验方法上的不足，拟采用第一性原理方法结合GW近似方法和密度泛函微扰理论知识为背景，以磷硅镉晶体为研究对象，研究磷硅镉的线性和非线性光学性质，给出一种不依赖实验带隙计算非线性光学系数的方法；从原子层次出发研究结构、缺陷和杂质等对磷硅镉的线性和非线性光学性质、热导率的影响，分析晶体非线性光学性质和热导率的微观起源及其机理。从而探索晶体的微观结构-性能-缺陷等的内在联系，及时将理论研究结果应用到晶体生长的实验中去。本项目的研究对黄铜矿非线性光学晶体材料的实验研究和应用开发提供理论参考，具有重要意义。

CdSiP2晶体是一种先进高功率中红外非线性光学新材料。在激光武器、红外制导、激光雷达、激光热核聚变和大气监测等领域有广泛而重大用途。本项目采用基于密度泛函理论的第一性原理方法，研究CdSiP2在不同压力和不同掺杂CdSixGe1-xP2（x = 0，0.25，0.5，0.75，1）的一系列描述晶体力学性能的物性参量，主要包括弹性常数、弹性模量、杨氏模量、剪切模量、泊松比、德拜温度、声速、各项异性等一系列描述晶体力学性能的物性参量，且利用相关模型得出晶体的热导率；接着研究CdSiP2在不同压力和不同掺杂CdSixGe1-xP2（x = 0，0.25，0.5，0.75，1）晶体的电子结构和光学性质；且采用杂化泛函近似方法研究CdSiP2晶体的能隙，在此基础上研究了晶体的光学性质；最后采用密度泛函微扰方法研究CdXP2 (X = Si，Ge)晶格动力学和热力学性质。计算结果表明CdXP2 (X = Si，Ge)声子频率没有虚频出现，证明了CdXP2 (X = Si，Ge)满足晶格动力学稳定性。项目通过基于声子态密度和准谐德拜模型研究晶体的热力学性质，系统地给出了晶体诸如自由能、内能、熵、热容量等热力学参量，详细讨论了这些物理量与温度等的关系。通过对热力学性质的系统计算，得到了系列热力学数据，对于在实验上改进单晶生长工艺具有重要的参考价值。除此以外，团队还对其他四方结构晶体的力学、热力学、动力学等性质进行了研究。