Thermodynamics of the (Pt+Ga) system

The integral enthalpy of mixing (Delta mixH(m)(o) = f(x(Pt))) of the (Pt + Ga) liquid system was measured using a very high temperature calorimeter in the temperature and molar fraction ranges 1071 < T/K < 1465 and 0 < x(Pt) < 0.61 respectively. It can be described by the following equation: Delta(mix)H(m)(o) x(Pt)(1-x(Pt)). xi(x(Pt)) kJ mol(-1), with xi(x(Pt)) = -150.19-308.28x(Pt) +126.28r(Pt2) +131.88x(Pt3)- 14.31x(PT4). This function is negative with a minimum Delta mixH(m)(o) = -64.5 +/- 2 kJ mol(-1) at x(Pt) = 0.52 +/- 0.01, and is independent of temperature within experimental error. The limiting partial molar enthalpy of supercooled liquid platinum in liquid gallium, deduced from experiments performed between 1169 K and 1180 K, is Delta(mix)h(m)(o) (Pt supercooled liq in infinity liq Ga) = -151 +/- 5 kJ mol(-1). However, by extrapolation of the xi-function to x(Pt) = 1, the limiting enthalpy of Ga in supercooled liquid Pt was obtained: Delta(mix)h(m)(o) (Ga liq in infinity supercooled liq Pt) = -215 +/- 30 kJ mol(-1). Moreover, from calorimetric measurements performed at 1350 K, the enthalpies of formation and of fusion of the PtGa definite compound were determined: Delta(for)H(m)(o)(PtGa) = 69.5 kJ mol(-1), Delta(fus)H(m)(o)(PtGa) = +5.1 kJ mol(-1). In addition, some points of the liquidus were deduced: x(Pt) = 0.15, with T = 1071 K; x(Pt) = 0.21, with T = 1125 K; x(Pt) = 0.54(5), with T = 1175 K; x(Pt) = 0.54, with T = 1325 K; x(Pt) = 0.42(S) and x(Pt) = 0.53, with T = 1350 K then x(Pt) = 0.47, and x(Pt) = 0.52, with T = 1366 K. For two compositions (x(Pt) = 0.5 and x(Pt) = 0.62(5)), the molar heat capacities were measured between 423 and 763 K: The enthalpies of mixing were compared with the data (i) previously obtained with solid alloys, (ii) predicted by Miedema's empirical model, (iii) measured for the homologous systems (Pd + Ga), (Pd + ln) and (Pt + ln). The energetic effect is due to a transfer from Ga to Pt of 2.5 electrons at most in the highest Pt-band.