Kinetics of gallium films confined at grain boundaries

We report on a combined study, by in situ x-ray diffraction at temperatures between 293 and 80 K, differential scanning calorimetry, and coherent quasielastic neutron scattering (QENS), of thin Ga films confined at grain boundaries in Al₂O₃-Ga nanocomposites that are produced by high-energy ball milling. We find that a molar fraction of Ga of up to 0.18 does not undergo phase change in the temperature interval investigated, which contains the bulk melting temperature TMᵇᵘˡᵏ, and conclude that this Ga is located in confined layers at grain boundaries, with an estimated thickness of six atomic monolayers. In samples with a higher overall molar fraction, the Ga in excess of molar fraction 0.18 melts and crystallizes reversibly when cycled between 100 and 320 K. For such samples, QENS at 320 K, that is, 17 K above TMᵇᵘˡᵏ, indicates three distinct diffusion coefficients D for Ga: one below the resolution limit, D₁<1×10^-6cm²/s, one agreeing with the bulk liquid value, D₂=3.4×10^-5cm²/s, and a faster coefficient D₃=2.4×10^-4cm²/s. We argue that the smaller and medium diffusion coefficients, D₁ and D₂, represent, respectively, solidlike layers near the Al₂O₃-Ga interface and liquid Ga in the core of interfacial layers and in liquid droplets. The fast diffusion coefficient (D₃) may be due to diffusion on free surfaces.