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 (Formula presented) 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 (Formula presented) 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 (Formula presented) indicates three distinct diffusion coefficients (Formula presented) for Ga: one below the resolution limit, (Formula presented) one agreeing with the bulk liquid value, (Formula presented) and a faster coefficient (Formula presented) We argue that the smaller and medium diffusion coefficients, (Formula presented) and (Formula presented) represent, respectively, solidlike layers near the (Formula presented) interface and liquid Ga in the core of interfacial layers and in liquid droplets. The fast diffusion coefficient (Formula presented) may be due to diffusion on free surfaces.