【作者】 刘永刚；

【导师】 程新路；

【作者基本信息】 四川大学 ， 原子与分子物理， 2007， 硕士

【摘要】 运用密度泛函理论可将多电子系统转化为单电子系统，由此对各类半导体材料和金属材料的弹性性质、晶格常数、体变模量做计算得到了与实验符合较好的结果，使之成为近年来电子理论中的一项重要的成就。弹性常数决定了晶体对于外力的响应，是决定材料的强度的重要参数。无论是在工程领域还是在基础理论研究中，得到弹性常数的准确数值具有重要意义。NaI晶体在冲击波作用下发光强度较高，发光均匀，常用作冲击波高温测量的标准材料，对它的高压物性参数的精确理论计算，有助于对材料高温、高压极端条件下性质的深入理解和对一些实验现象的理论解释。碘化钠晶体在较低压强(低于30 GPa)呈现稳定的面心立方结构，在高压(30～200 GPa)范围内，碘化钠晶体发生相变，由面心立方结构(B1)直接变成稳定的正交结构(B33)，这也是不同于类NaCl(B1—B2)晶体的主要特点。本文分两个章节对碘化钠不同晶系不同压强的弹性性质和热力学性质进行了模拟计算并与实验进行了比较。首先本文从第一性原理出发，利用广义梯度近似(GGA)对典型的碱金属卤化物碘化钠晶体进行了理论探讨，计算了它们的弹性模量以及声速等与物质本构密切相关的参量。在冷压条件下得到的弹性常数、晶格常数等参量取得了与实验符合较好的计算结果。证明了该方法的可靠性，同时用此方法在缺乏实验数据的区域也进行了预估研究，并同有关的理论结果进行了比较分析。由于材料的弹性常数主要与体积有关，因此温度对其的影响则主要从晶格的体积变化对弹性的影响上体现出来，因此可以从冷压结果推算材料的热弹性。同时通过拟和二阶Vinet方程并结合准谐德拜模型(quasi-harmonic Debye model)计算得到了B1结构和B33结构NaI晶体的热力学性质，计算了零压低温条件下B1结构的NaI晶体的比热容Cp，取得了与实验符合较好的结果。用此方法计算了高温条件下B1结构和B33结构NaI的等压比热容Cp、及等容比热容Cv，并由比容与德拜温度(?)的关系计算了格林参数。更多还原

【Abstract】 The multi-electron system can be converted to single-electron system by using density function theory (DFT). The lattice parameter, elastic constants and the bulk modulus of semiconductor and metal materials are calculated which are in agreement with that of experiment. It has become an important achievement of electron theory. The elastic constant is an important parameter to decide the intensity of materials, so obtaining the precise elastic constants has important significance in the engineer and studying of ground theory. NaI has the property of high intensive and uniform illumination, and therefore can be used as the high-temperature measuring standard materials of shocking wave. So the accurate theory calculation of properties of NaI under high pressure is necessary to understand the properties of the extreme conditions of high temperatures and pressures and theoretic explanation of the experiment phenomena. NaI has the stable face-centre-cubic (FCC) structure under lower pressures and has the phase transition under the domain (30-200 GPa) of high pressure. It transforms from the B1 structure to the stable orthogonal B33 structure which is different from NaCl (B1-B2). This paper studied the elastic and thermodynamic properties of NaI with different structures under different pressures which accords to the experimental results.The elastic modulus and sound velocity of NaI has been calculated by using the generalized gradient approximate (GGA) based on the first-principle calculation. The obtained elastic constants and lattice parameter accord with those of experimental results, demonstrating the validity of simulations. We also obtained the elastic constants at various temperature and pressure with the absence of experimental data, and compared them with simulation results of other methods. Due to the correlation of the elastic constants and the volume, the implication of temperature on that are exhibited by the influence of volume change on elasticity, so the finite- temperature elasticity can be obtained.By fitting the Vinet equation of second order associated with the quasi-harmonic Debye model, we obtained the thermodynamic properties of B1 and B33 structure of NaI and the heat capacity C_p of B1 structure which is according to that of experiment. Therefore we also calculated the iso-pressure heat capacity Cp and iso-volume heat capacity Cv of B1 and B33 structure of NaI under high temperature, the Gruneisen parameter was calcualted by the relation of the heat capacity and Debye temperature.更多还原