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: V₁^P (P deficient in N₃P₂) and V₂^P (P deficient in N₃P₁). V₁^P was a slightly more stable point defect than V₂^P by 0.20 eV. The P diffusions to vacancies (V₁^P and V₂^P) had large diffusion barriers of more than 1 eV, except the P diffusion path along the [0001] direction through an interstitial site in Ni₃P₁ (I_1→2^P) and then to V₁^P, which showed the lowest energy barrier of about 0.18 eV. The DFT calculations suggested that the two adjacent vacancies (both V₁^P) 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.