Investigating Mechanical Properties Through Defect Chemistry in Hard Binary Phosphide Material Ta3P

We investigated the mechanical properties of Ta-rich regions of the Ta–P binary phase diagram, consisting of Ta and Ta3P (Ti3P-type) two-component samples, as well with inclusions of Fe to determine the effects on Vickers microhardness and on strength under compression loading. Different Ta concentrations in Ta3P produced diverse microstructural motifs, ranging from dispersed Ta dots (e.g., Ta81.3P18.7) and coagulated-like features (e.g., Ta84.4P15.6) to grouped lines and circular massed dot-line patterns (e.g., Ta85.9P14.1, Ta86.2P13.8) correlating with reduced load-bearing capacity and greater Vickers hardness. With increasing Ta concentration, microstructures evolved into contiguous leaf-like Ta phases (e.g., Ta90.4P9.6) and a homogenous intermediate without a pattern (e.g., Ta94.3P5.7), the latter exhibiting the highest compressive strength (712 lbs) and lowest Vickers hardness (5.76 GPa at 0.5 kgf) of Ta–P compositions. Adjusting metal concentration (Ta alone or in tandem with Fe) produced similar quantitative correlations between hardness and compression strength. Electronic structure analysis using crystal orbital Hamiltonian population (COHP) revealed that Fe interstitial defects induce distinct modifications in p orbitals of phosphorus. The proposed chemical mechanism for strengthening, involving Fe–P orbital hybridization, namely between Fe d and P p states, was observed from -2 to -4 eV in density of states (DOS) calculation, corresponding to Fe–P bonding. This localized orbital overlap between Fe d and P p states produces peak splitting features resulting in a sharp peak at -4.5 eV. A secondary shoulder near -5.25 eV originates from downshifted P pz states. Orbital-resolved DOS analysis confirmed anisotropic participation of the p orbitals from P, highlighting defects driving localized bonding rearrangements.