Experimental measurements of the correlation factor for solute diffusion

取代原子通过跃迁到邻近的空晶格位而扩散。但是，刚跃起的原子可能会向后跃进空位，因此，与假设的随机游走过程相比，原子的净“向前运动”被所谓的相关因子所减少。对于宿主原子，相关因子只是一个反映扩散机制和扩散子晶格几何形状的数值常数。另一方面，溶质原子与空位相互作用，改变空位的局部浓度和微观跳跃速率。这种溶质-空位相互作用使得溶质扩散的相关因子对原子相互作用和温度的细节高度敏感。它的测量应该有助于确定扩散机制，解释相互作用，并通过原子模拟促进扩散速率的可靠预测。到目前为止，还没有直接确定溶质扩散的相关因素。目前的建议旨在通过结合在同一矩阵中溶质的扩散系数和跳速的互补测量结果来首次做到这一点。这些将分别使用放射性示踪技术和摄动角相关光谱进行测量。在一个常数范围内，相关系数就是扩散系数和跳速的比值。选择具有常见L12结构的A3B相作为基准体系进行研究。这些有序相的扩散引起了极大的兴趣，因为存在许多可能的扩散机制，并且可能发生不同活化能的溶质的亚晶格间和亚晶格内跳跃。确定组分和温度对相关因子的影响。具体来说，镉溶质扩散的相关因素将在不同温度下测量In3Gd相的富in和贫in成分。不同的扩散机制可以在两种成分上运行，这将通过使用具有相反相边界成分的样品来获得。nster大学（放射性示踪剂实验室）和美国华盛顿州立大学（摄动角相关实验室）最先进的设备将使测量具有所需的精度。在没有任何先前测量的情况下，结果可能会令人惊讶。

Substitutional atoms diffuse by jumping into neighboring, vacant lattice sites. But an atom that has just jumped may jump backwards into the vacancy so that, compared with a hypothetical random walk process, the net 'forward movement' of the atom is decreased by a so-called correlation factor. For host atoms, the correlation factor is simply a numerical constant reflecting the diffusion mechanism and geometry of the diffusion sublattice. Solute atoms, on the other hand, interact with vacancies and modify the local concentration of vacancies and microscopic jump rates. Such solute-vacancy interactions make the correlation factor for solute diffusion highly sensitive to details of the atomic interactions and temperature. Its measurement should help identify diffusion mechanisms, interpret the interactions, and facilitate reliable predictions of diffusion rates via atomistic simulations. As yet there has been no direct determination of a correlation factor for solute diffusion. The present proposal aims to do that for the first time by combining results of complementary measurements of the diffusion coefficient and jump-rate of a solute in the same matrix. These will be measured using the radiotracer technique and perturbed angular correlation spectroscopy, respectively. To within a constant, the correlation factor is simply the ratio of the diffusion coefficient and the jump-rate. A3B phases having the common L12 structure have been chosen for study as a benchmark system. Diffusion in these ordered phases has attracted enormous interest because there exists a number of possible diffusion mechanisms and likely occurrence of inter- and intra-sublattice jumps for solutes with different activation energies. The influence of composition and temperature on the correlation factor will be determined. Specifically, correlation factors for diffusion of cadmium solutes will be measured for In-rich and In-poor compositions of the In3Gd phase at a variety of temperatures. Different diffusion mechanisms may operate at the two compositions, which will be obtained using samples having the opposing phase boundary compositions. State-of-the-art facilities at Münster University (radiotracer laboratory) and at Washington State University, USA (perturbed angular correlation laboratory) will enable measurements to be made having the required accuracy. In the absence of any previous measurements, results are likely to surprise.