Thermochemistry of the intermetallic compounds RuTi, RuZr, and RuHf
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I.
INTRODUCTION
D U R I N G the past two-three years we have developed a new method ("high temperature mixing calorimetry") for thermochemical studies of refractory materials such as borides 1'2'3 and silicides. 4'5'6 This method differs from traditional ones such as combustion, solution, and direct reaction calorimetry. It involves dropping mixtures of solids from 298 K into a high temperature calorimeter in which they generate a homogeneous liquid mixture. This multicomponent liquid mixture is formed either (a) from several solid elements or (b) from a solid mixture of the same overall composition as under (a), but which now includes the compound which is being studied. The difference between the two heat effects associated with (a) and (b) yields the standard enthalpy of formation. Due to the great scarcity of reliable thermodynamic data for intermetallic compounds, and in view of their theoretical and practical significance, we attempted to use this method also for such compounds. It was first applied to the intermetallic compound PdTi. 7 The success of this investigation proved that we had a viable experimental method; it led us to initiate a systematic study of the thermochemistry of intermetallic compounds formed between early and late transition metals. So far this research has emphasized the equiatomic compounds of Group IV metals (Ti, Zr, Hf) with metals of the Pd- and Pt-groups. In recent communications we have reported on the enthalpies of formation of PdTi, 7 of PdZr and PdHf, 8 and of RhTi, RhZr, and RhHf. 9 In the present work our study is extended to RuTi, RuZr, and RuHf. Since there are no published experimental values for the enthalpies of formation of these compounds, our new results will be compared with estimated and predicted values. In the future these investigations will be extended also to other binary compounds between early and late transition metals.
later applied to silicides, 4'5'6 and most recently also to intermetallic compounds, v'8'9 Based on our experience with the intermetallic compounds of Pd and Rh with Ti, Zr, and Hf we originally attempted to dissolve the corresponding Ru compounds in a (Cu + Ge) melt at 1400 K. Different alloy compositions were tested in the calorimeter and the solidified liquid alloys examined by SEM and energy dispersive X-ray analysis. We found, however, that the RuMe compounds did not dissolve completely in the (Cu + Ge) melts used. We then tried (Pt + Ge) and ( P d + Ge) melts of different compositions, and found that the compounds dissolved completely in these melts. Even so, to increase confidence in our results, we decided to make use of two different liquid alloys [(Pd + Ge) and (Pt + Ge)] of somewhat different composition. (For RuTi we also made use of a third liquid alloy composition by increasing the amount of RuTi in the sample. The reasons for this will be explained below.) The alloy compositions actually used will be given in the section on "Results". The standard enthalpies of formation were obtained from the heat effects associated with t
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