Standard molar enthalpies of formation of MeAl (Me = Ru, Rh, Os, Ir)
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INTRODUCTION
D U R I N G recent years, there has been a greatly increased interest in the phase stability and chemical bonding of intermetallic compounds. Evidence for this interest can be found in a number of theoretical and semiempirical models t~-sj which have been developed in order to predict the enthalpy of formation of binary alloy phases formed by transition metals. However, due to the scarcity of experimental information, it has been very difficult to test many of these predictions. We have for a long time pursued a research program which attempts to provide reliable experimental information on the thermochemistry of binary alloy systems. In the course of these investigations, we have determined the enthalpies of formation for borides, silicides, germanides, and intermetallic compounds of transition metals. Very recently, we have extended this study to transition metal aluminides and have reported on the standard enthalpies of formation of PdA1, PtA1, ScAI~ 78, YA12, and LaA12.Eg] In the present work, we have investigated a series of additional MeAl compounds (Me = Ru, Rh, Os, Ir) by calorimetric measurements. We will compare our results with available values for other Group VIII aluminides and with predicted values. II.
EXPERIMENTAL
The experiments were carried out at (1473 _ 2) K in a modified Setaram-type high-temperature calorimeter. Details of this apparatus and of the calorimeter liner assembly have already been given in our earlier reports.t~~ Table I reports on the metallic purity and describes the materials used in this work. The rhodium powder was obtained from Engelhard. Ruthenium, osmium, iridium, and aluminum powders were purchased from JohnsonMatthey AESAR group. The lots were A10A03, J24A05, 127A01, and 18918, respectively. Beryllium oxide (BeO) crucibles were used rather than boron nitride (BN). The reacted products were examined by scanning electron microscopy (SEM) and by energy-
dispersive X-ray (EDX) analysis. Powder X-ray diffraction also was carried out on each compound. Calibration was achieved by dropping small pieces of 2-mm-diameter high-purity copper wire from room temperature into the calorimeter at 1473 K. The enthalpy of pure copper was taken from Hultgren e t a l . : t121 46,465 J/mole at 1473 K. Within a single series of measurements, the calibrations were reproducible to -+ 1.5 pet. III.
RESULTS A N D DISCUSSION
A survey of the phase diagrams E~3'~4jshows that all of the studied MeAl (Me = Ru, Rh, Os, It) compounds have melting temperatures above 2273 K. Although the direct combination method may not give good results for very refractory compounds, we adopted this method, since one of the components (aluminum) is liquid above 933 K. Also, very strong interactions were expected in these systems. The transition metal and aluminum powders were weighed carefully in 1 : 1 ratio and mixed thoroughly. This mixture was compressed into 4-mm-diameter pellets which were dropped from room temperature into the calorimeter at (1473 -+ 2) K. After the measurements, the alloy product
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