First-principles study of high-pressure phases of materials

Abstract

Solid hydrogen under megabar pressure and a high pressure atomic phase of SnI4 have been investigated with the first-principle molecular dynamics method based on the density functional theory. As to the study on the solid hydrogen, three representative points in the P-T phase diagram of solid hydrogen around 150 GPa have been theoretically calculated by applying the first-principles path integral molecular dynamics method. The characteristic feature in each phase is summarized as follows: (1) in the phase at P = 120 GPa and T = 300 K, hydrogen molecules rotate almost freely at the time when their centers form an hcp (hexagonal close packed) lattice; (2) in the phase at P = 130 GPa and T = 80 K, the molecules were aligned and rotation stopped; and (3) in the phase at P = 180 GPa and T = 100 K, the centers of the molecules were shifted from the hcp lattice. The calculated results have been compared with the results obtained by ordinary Car-Parrinello method with classical treatment of atoms. The structure of the fcc (face centered cubic) SnI4 phase at ultra high pressure has been investigated by the first-principles molecular dynamics method with a constant pressure scheme. The optimized cell parameters of the interstitial solid solution model were 13 percent larger than the experimental data. On the other hand, the average cell parameters of the substitutional solid solution model agreed with the experimental data within one percent error.

メガバール圧力下の固体水素およびSnI4の高圧原子相を密度汎関数理論に基づいた第一原理分子動力学法を用いて検討した。固体水素の研究については、150GPa近傍の固体水素のP-T状態図中の代表的な3点を第一原理経路積分分子動力学法を適用して理論的に計算した。各相の特質は以下の通りであった。(1)P=120GPa、T=300Kにおける相では、水素分子の中心にhcp(6方最密)格子を形成する際には、水素分子はほぼ自由に回転する。(2)P=130GPa、T=80Kにおける相では、水素分子は整列し、回転を停止する。(3)P=180GPa、T=100Kにおける相では、分子の中心はhcp格子からずれる。計算結果を通常の古典的原子処理を用いたCar-Parrinello法により求めた結果と比較した。超高圧下におけるfcc(面心立方)SnI4相の構造を圧力スキームを一定にして第一原理分子動力学により検討した。侵入型固溶体模型の最適化セルパラメータは実験データよりも13%大きかった。他方、置換型固溶体模型の平均セルパラメータは誤差1%以内で実験データと一致した。