Structural studies of phase transitions in crystalline and liquid Halides (ZnCl, AlCl) under pressure

高圧下での結晶及び液体ハライド(ZnCl, AlCl)の相転移の構造研究

Brazhkin, V. V.*; 片山 芳則; Lyapin, A. G.*; Popova, S. V.*; 稲村 泰弘*; 齋藤 寛之; 内海 渉

Brazhkin, V. V.*; Katayama, Yoshinori; Lyapin, A. G.*; Popova, S. V.*; Inamura, Yasuhiro*; Saito, Hiroyuki; Utsumi, Wataru

ZnClとAlClハライドの相図と対応する融体の近距離秩序を6.5GPaまでの圧力でエネルギー分散型X線回折法によって調べた結果を報告する。ZnCl結晶が圧縮されると、相(HgI構造)から相(不規則CdI構造,WTe型)への転移が起きる。ZnClとAlClの液体状態の構造研究は 両者の4面体ネットワークの中距離相関が1.8GPa(ZnCl)及び2.3GPa(AlCl)までの圧力増加で急激に減少することを示している。さらなる圧縮によって、両者の融体で短距離秩序の構造の変化と配位数の増加を伴った転移が起きる。AlClの転移は4GPaで急激に起きるのに対し、ZnClの転移は2-4GPaの圧力範囲で3GPaを中心に次第に起きる。よってAlClとZnCl化合物もまた、2つの現象、すなわち中距離秩序の緩慢な消失とより鋭い液体-液体配位数転移の両者が起きる例となることが示された。

The results of investigating the phase diagrams of ZnCl and AlCl halides, as well as the structure of the short range order of the corresponding melts under pressures up to 6.5 GPa, by the method of energy-dispersive X-ray diffraction are reported. When a ZnCl crystal is compressed, a phase transition occurs from the phase (HgI structure type) to the phase (distorted CdI structure, WTe type). The structural studies of the liquid state of ZnCl and AlCl indicate that the intermediate-range order decreases rapidly in the tetrahedral network of both melts as the pressure increases to 1.8 and 2.3 GPa for ZnCl and AlCl, respectively. With further compression, the transitions in both melts occur with a change in the structure of the short-range order and with an increase in the coordination number. In this case, the transition in AlCl occurs at about 4 GPa and is a sharp first order transition, whereas the transition in ZnCl occurs more smoothly in a pressure range of 2-4 GPa with a maximum intensity near 3 GPa. Thus the AlCl and ZnCl compounds exemplify the existence of two phenomena, gradual decay of intermidiate-range structural correlations and a sharper liquid-liquid coordination transition.