Dimeric Molecular Structure of Molten Gallium Trichloride and a Hidden Evolution toward a Possible Liquid–Liquid Transition

• Published in: The journal of physical chemistry. B 2019-12, Vol.123 (48), p.10260-10266
• Main Authors: Usuki, Takeshi, Bokova, Maria, Kassem, Mohammad, Ohara, Koji, Hannon, Alex C, Bychkov, Eugene
• Format: Article
• Language: English
• Subjects: Physics
• ISSN: 1520-6106; 1520-5207
• DOI: 10.1021/acs.jpcb.9b08307

Group 13 trihalides MY3 (M = Al, Ga, and In; Y = Cl, Br, and I) mostly having a dimeric M2Y6 molecular structure in the solid state and a mixture of M2Y6 dimers and MY3 monomers in the vapor phase are potential candidates for entropy-driven liquid–liquid transition M2Y6 ⇄ 2MY3 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 GaCl3 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 D 2h symmetry, the ES Ga2Cl6 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.