Usuki Takeshi, Bokova Maria, Kassem Mohammad, Ohara Koji, Hannon Alex C, Bychkov Eugene
Faculty of Science , Yamagata University , Yamagata 990-8560 , Japan.
LPCA , Université du Littoral Côte d'Opale , Dunkerque 59140 , France.
J Phys Chem B. 2019 Dec 5;123(48):10260-10266. doi: 10.1021/acs.jpcb.9b08307. Epub 2019 Nov 25.
Group 13 trihalides MY (M = Al, Ga, and In; Y = Cl, Br, and I) mostly having a dimeric MY molecular structure in the solid state and a mixture of MY dimers and MY monomers in the vapor phase are potential candidates for entropy-driven liquid-liquid transition MY ⇄ 2MY at elevated temperatures. Using pulsed neutron diffraction and high-energy X-ray scattering supported by structural modeling, we show a dimer molecular structure of liquid GaCl above the melting point at 351 K and midway between the boiling point (474 K) and the critical temperature (694 K) with almost hidden characteristic evolution toward a possible liquid-liquid transition. In contrast to edge-sharing (ES) dimers in solid and vapor of symmetry, the ES GaCl molecules in the melt have a puckered structure of the central four-membered ring with shorter Cl-Cl (2.90-3.09 Å) and longer Ga-Ga (3.20-3.26 Å) second-neighbor correlations. The elongation of Ga-Ga intramolecular distances with increasing temperature simultaneously with diminished Cl-Cl nearest neighbor contacts destabilizes the ES dimers, indicating the first step toward dimer dissociation.
第13族三卤化物MY（M = Al、Ga和In；Y = Cl、Br和I）在固态时大多具有二聚体MY分子结构，在气相中是MY二聚体和MY单体的混合物，是高温下熵驱动液 - 液转变MY ⇄ 2MY的潜在候选物。利用脉冲中子衍射和结构建模支持的高能X射线散射，我们展示了在351 K熔点以上、沸点（474 K）和临界温度（694 K）中间温度下液态GaCl的二聚体分子结构，其朝着可能的液 - 液转变几乎有隐藏的特征演化。与具有对称结构的固态和气相中的边共享（ES）二聚体不同，熔体中的ES GaCl分子具有中心四元环的褶皱结构，其第二近邻Cl-Cl（2.90 - 3.09 Å）相关性较短，Ga-Ga（3.20 - 3.26 Å）相关性较长。随着温度升高，Ga-Ga分子内距离的伸长同时伴随着Cl-Cl最近邻接触的减少，使ES二聚体不稳定，这表明了二聚体解离的第一步。
