Investigation in the ternary Ta-Ni-P system: Solid state phase equilibria at T=1070 K, crystal and electronic structures of new ternary phosphides

Abstract

The solid state phase diagram for the ternary Ta-Ni-P system was established at T=1070 K in the region of up to 67 at% of P by means of X-ray powder diffraction methods (PXRD). Six ternary compounds, namely Ta4NiP (Nb4CoSi-type), Ta1.10–0.82Ni0.90–1.18P (TiNiSi-type), TaNiP2 (NbNiP2-type), Ta5.00–4.81Ni4.00–4.19P4 (Nb5Cu4Si4-type), TaNi2P (own structure type) and Ta1–0.08(1)Ni0.08(1)P2 (OsGe2-type) have been confirmed to exist. Rather minor Ta/Ni homogeneity ranges have been found for α-Ta3-xNixP (x=0.2) (Ті3Р-type), Ta1-xNixP (x=0.18) (NbAs-type), Ni3-xTaxP (x=0.2) (Ni3P-type) and Ni2-xTaxP (x=0.25) (Fe2P-type). The crystal structure of Ta4.811(9)Ni4.189(9)P4 has been refined from single crystal X-ray diffraction data (Nb5Cu4Si4-type, space group I4/m,a =9.8474(17), c=3.5182(7) Å, R1=0.0283, wR2=0.0470), while that of the new TaNi2P compound was determined by means of PXRD. This phosphide crystalizes in its own structure type (space group Pnma, a=8.3588(3), b=3.5208(1), c=6.7051(3) Å, RІ=0.044, RР=0.161). A new isostructural Fe-compound, TaFe2P (a=8.358(2), b=3.5194(7), c=6.703(1) Å), was also synthetized. The electronic structures of Ta5Ni4P4 and TaNi2P were analyzed using the tight-binding linear muffin-tin orbital (TB-LMTO) and extended Hückel methods.