Self-diffusion coefficients of H2O molecules in Na-rich hectorite gel were measured by 'H nuclear magnetic resonance (NMR), Spin-echo pulse sequences with magnetic field gradient pulses for the translational diffusion measurement were applied to the hectorite gel at the Larmor frequency of 20 MHz. Effects of clay fraction (0-51.2 wt. %) and temperature (20.0-60.3°C) were studied. The results show: (1) Phenomenologically, the self-diffusion coefficient, D, of 1H2O in the clay gel is expressed by the normalized diffusivity, D/D0 = exp(−0.0257w), where D0 is the water self-diffusivity in bulk water at temperature and w is the weight fraction of the hectorite (wt. %). (2) The activation energy of H2O diffusivity in the hectorite gel is nearly equal to that in bulk water. Hence, the normalized diffusivity, D/D0, obeys a temperature-independent curve. (3) The exponential dependence of D/D0 on w for w <30 wt. % is explained by a random-walk model, in which free or unbound H2O molecules migrate in the geometrically complex and tortuous pore structure of randomly scattered clay-mineral grains.

Computer simulation techniques have been used to investigate defect formation and the diffusion of Ca and Mg in diopside. It was found that isolated, non-interacting CaO and MgO Schottky defects had the lowest formation energies (3.66 and 3.97 eV respectively); oxygen Frenkel defects are the most favourable oxygen defects (formation energies 3.93 eV). Magnesium and calcium self-diffusion in the c-direction of diopside is easiest by a vacancy mechanism involving either direct jumps along the c-direction, or double jumps in the b-c plane. In the extrinsic regime, diffusion activation energies for Mg are predicted to be 9.82 eV (direct route) and 1.97 eV (double jump route); for Ca diffusion, activation energies are predicted to be 6.62 eV (direct route) and 5.63 eV (double jump route). If additional vacancies (oxygen or magnesium) are present in the vicinity of the diffusion path, Ca migration energies fall to 1.97–2.59 eV. At elevated temperatures in the intrinsic regime, diffusion activation energies of ⩾ 5.95 eV are predicted for Mg self-diffusion and 9.29–10.28 eV for Ca self-diffusion. The values for Ca diffusion are comparable with published experimental data. It is inferred that a divacancy mechanism may operate in diopside crystals.

High entropy alloys are multicomponent alloys, which consist of five or more elements in equiatomic or nearly equiatomic concentrations. These materials are hypothesized to show significantly decreased self-diffusivities. For the first time, diffusion of all constituent elements in equiatomic CoCrFeNi and CoCrFeMnNi single crystals and additionally solute diffusion of Mn in the quaternary alloy is investigated using the radiotracer technique, thereby the tracer diffusion coefficients of 57Co, 51Cr, 59Fe, 54Mn, and 63Ni are determined at a temperature of 1373 K. The components are characterized by significantly different diffusion rates, with Mn being the fastest element and Ni and Co being the slowest ones. Furthermore, solute diffusion of Cu in the CoCrFeNi single crystal is investigated in the temperature range of 973–1173 K using the 64Cu isotope. In the quaternary alloy, Cu is found to be a fast diffuser at the moderate temperatures below 1273 K and its diffusion rate follows the Arrhenius law with an activation enthalpy of about 149 kJ/mol.