Interaction of tantalum, titanium and phosphorus at 1070 K: Phase diagram and structural chemistry

Abstract

The solid state phase diagram for the equilibria of the ternary Ta-Ti-P system was established by X-ray powder diffraction at T = 1070 K in the region up to 67 at.% P. Isostructural compounds Ta3P and Ti3P (both Ti3P-type) form a continuous series of substitution-type solid solutions. The majority of binary compounds form substitution-type solid solutions, in which the substitution of Ti for Ta, or Ta for Ti, takes place. The crystal structure investigation of the new ternary phosphide demonstrates that this phase is isotypical to the OsGe2 structure type and has a small homogeneity range, which can be described by the formula Та1-xТіxР2 (x = 0.07(3)−0.145(7), space group C2/m, a = 8.8556(9)−8.8446(2), b = 3.2654(4)−3.2605(1), c = 7.4846(8)−7.4741(1) Å, β = 119.307(2)−119.308(1)°). Our density functional theory (DFT) electronic structure calculations show that the stoichiometric composition TaP2 has no electronic instability, although it does not exist under the thermal conditions used in present study. The addition of a small amount of Ti leads to a ternary phase Та1-xТіxР2 (x = 0.07(3)−0.145(7)) with a higher thermal stability compared to TaP2.