Phosphorous Diffusion Through Ni2P—Low Energy Diffusion Path and Its Unique Local Structure

Phosphorous (P) diffusion in bulk Ni2P was investigated by the density functional theory (DFT) to find the origin of the low-temperature P diffusion into the surface. The Ni2P bulk structure consists of two types of layers, Ni3P2 and Ni3P1, stacked along the [0001] direction. Two types of P vacancies in Ni2P were studied: V1P (P deficient in N3P2) and V2P (P deficient in N3P1). V1P was a slightly more stable point defect than V2P by 0.20 eV. The P diffusions to vacancies (V1P and V2P) had large diffusion barriers of more than 1 eV, except the P diffusion path along the [0001] direction through an interstitial site in Ni3P1 (I1→2P) and then to V1P, which showed the lowest energy barrier of about 0.18 eV. The DFT calculations suggested that the two adjacent vacancies (both V1P) allow the local rearrangement of the structure to form a tetrahedral structure at the intermediate state. We have proposed a new diffusion mechanism in the intermetallic compound named the interstitial–vacancy diffusion mechanism.