On the constitution and thermodynamic modelling of the system Zr-Ni-Sn

Abstract

In continuation of our optimization of {Ti,Zr}NiSn-based thermoelectrics, we herein determined the constitution of the system Zr-Ni-Sn (liquidus surface and isothermal section at 950 degrees C) employing X-ray powder diffraction (XPD) and electron probe micro analysis (EPMA) of about 80 ternary alloys in as cast and annealed state. The system is characterized by the existence of four ternary compounds labelled tau(1) to tau(4). At 950 degrees C no significant homogeneity regions are found for the compounds tau(1)-ZrNiSn (Half-Heusler phase, MgAgAs-type), tau(2)-ZrNi2Sn (Heusler phase, MnCu2Al-type) and tau(4)-Zr6NiSn2 (K2UF6-type, ordered Fe2P-type), but tau(3)-Zr2+xNi2+ySn1-x-y exhibits a significant solution range for 0.0 <= x <= 0.25 and 0.0 <= y <= 0.06. Extended solid solutions starting from binary phases at 950 degrees C have been evaluated for Zr-5(Ni-x square Sn-y(1-x-y))(4) (filled Mn5Si3 - Ti5Ga4-type; 0 <= x <= 0.216, 0.002 <= y <= 1) and Zr1-x(Ni1-ySny)(5+x) (AuBe5-type) reaching a maximum solubility at x = 0.022, y = 0.146 (the symbol square denotes a vacancy). From differential thermal analysis (DTA) measurements a complete liquidus surface has been elucidated revealing congruent melting for tau(1)-ZrNiSn (at 1465 +/- 10 degrees C) and tau(2)-ZrNi2Sn at 1469 +/- 10 degrees C, but incongruent melting for tau(3)-Zr2Ni2Sn (pseudobinary peritectic formation: l + Zr-5(Ni-x square Sn-y(1-x-y))(4) <-> tau(3) at 1406 +/- 10 degrees C), and for tau(4)-Zr6NiSn2 (ternary P-type reaction: L + Zr-5(Ni-x square Sn-y(1-x-y))(4) + (Zr) <-> tau(4)-Zr6NiSn2 at 1124 +/- 8 degrees C). A Schultz-Scheil diagram for the solidification behavior was constructed for the entire diagram. X-ray single crystal data have defined precise atom site occupancies in tau(4)-Zr6NiSn2 (R-F2 = 0.0113) as well as close to the end point of the solid solution Zr5Ni1-xSn3 (stuffed Mn5Si3-type; x = 0.21, R-F2 = 0.0238) and isostructural Hf5Ni1-xSn3 (x = 0.26, R-F2 = 0.0242). As thermodynamic data in the ternary system were only available in the literature for ZrNiSn, heat of formation data were supplied by our DFT calculations for ZrNi2Sn, Zr2Ni2Sn and Zr6NiSn2 as well as for the solid solutions Zr-5(Ni-x square ySn(1-x-y))(4) and Zr1-x(Ni1-ySny)(5+x) for y = 0 and x = 0.25 or y = 0.20. For these cases DFT calculations also provide details on electronic properties and bonding. Thermodynamic CALPHAD calculation was performed with the Pandat software and resulted in reasonably good agreement for all the 29 invariant reaction isotherms involving the liquid.