The thermodynamic properties of the selenium - tellurium system are analysed in terms of a novel regular solution model with multiple connectivity. The model is an extension of the regular solution model that discriminates between two forms of tellurium atom according to their connectivity (number of first neighbours). In the resulting ternary system, the chemical equilibrium between these two forms of tellurium is taken into account. The model can be solved in either the Bragg - Williams or the quasi-chemical approximation. A very good representation of the thermodynamic properties is obtained. Moreover, the model predicts a temperature and concentration evolution of the number of first neighbours in the liquid in close agreement with independent neutron scattering measurements. As a result, this model provides a simple insight into the mechanisms that are responsible for the complex thermodynamic and structural behaviour of the Se - Te system.

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The enthalpy of formation, hf, of the Pt-Pb alloys was measured in the 923-1316 K temperature range with respect to composition by direct reaction calorimetry (drop method) with the help of high temperature Calvet-type calorimeters. Furthermore we determined also some phase boundaries from the breaks of the h f ( x ) curves. -The thermodynamic behaviour of the melts is characterized by moderate negative deviations to ideality suggesting some chemical short-range order (h$i, = -15.88 kJ . mol-l at 923 K for xpt = 0.53). The associates are very sensible to temperature since the excess heat capacity is positive with a maximum of about 10.2 J . K-' . mol-I near xpt = 0.71. Such a behaviour differs strongly from that of the Pd-Pb system which shows a strongly negative enthalpy of mixing (&in = -38.4 kJ . mol-I) without any temperature dependence.

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Calorimetric measurements are reported which allow the enthalpic behaviour of Bi -Te melts to be established. Further work is required, however, to supplement results obtained for In-Te melts.The partial enthalpies of bismuth and tellurium in the Bi -Te melts at 755 K and those of indium and tellurium in the In-Te melts at 1010 and 987 K were measured at high dilution by direct reaction calorimetry (drop method) with the help of a Tian-Calvet hightemperature calorimeter. The limiting partial enthalpies of the components were deduced by extrapolation at infinite dilution: Ahf*." si(755 K)/kJ. mol-' = -34.0 and AhL." Te(755 K)/kJ. mol-' = -24.1 in the Bi-Te melts Ahf." i,(lO1OK)/kJ.mol-i = -75.9 and AhL" T,(lO1OK)/kJ~mol-' = -47.8 in the In-Te melts. Ahf3." ,,(987K)/kJ 'mo1-l = -75.2 and AhL" Te(987 K)/kJ.mol-' = -48.0 in the In-Te meltsAll above values are with reference to liquid metals