Bd. 73 (1982) H.
3 Thermodynamic Properties of Pd-Te Alloys 151
Thermodynamic Properties of Palladium - Tellurium Alloys
Herbert Ipser
(Institute of Inorganic Chemistry, University of Vienna, A-1090 Wien, Währingerstr. 42, Austria)
Tellurium vapor pressures of liquid and solid palladium-tellurium alloys were determined by an isopiestic method be-
tween 875 and 1350 K and between 35 and 75 at.% Te. Activities and partial molar enthalpies of tellurium were derived
from them. Integral thermodynamic properties for liquid alloys were obtained by Gibbs-Duhem integration. Allthermo-
dynamic functions show characteristic behavior around 40 at.% Te which might indicate formation of associates in the
liquid. A statistical model was successfully applied to interprete the activity data in the hexagonal PdTe-phase with NiAs-
structure. Assuming random distribution of palladium vacancies over all transition metal layers, excellent agreement
with the experimental activities was found. The energy of interaction between these vacancies was calculated to be
6500 J g-atom-"'.
Thermodynamische Eigenschaften von Palladium-Tellur-Legierungen
Die Tellurdampfdrücke von flüssigen und festen Palladium-Tellur-Legierungen wurden mit Hilfe einer isopiestischen
Methode zwischen 875 und 1350 K und zwischen 35 und 75 At.-% Te bestimmt. Daraus wurden die Aktivitäten und die
partiellen molaren Enthalpien von Tellur abgeleitet. Integrale thermodynamische Eigenschaften flüssiger Legierungen
wurden durch Gibbs-Duhem-Integration erhalten. Sämtliche thermodynamische Funktionen zeigen in der Gegend um
40 At.-% Te charakteristisches Verhalten, welches ein Hinweis auf die Ausbildung von Assoziaten in der Flüssigkeit sein
könnte. Ein theoretisches Modell wurde erfolgreich angewendet, um die Aktivitätswerte in derhexagonalen PdTe-Phase
mit NiAs-Struktur zu interpretieren. Unter der Annahme statistischer Verteilung von Palladium-Leerstellen über sämt-
liche Übergangsmetall-Schichten wurde eine ausgezeichnete Übereinstimmung mit den experimentell bestimmten
Aktivitäten festgestellt. Die Wechselwirkungsenergie zwischen diesen Leerstellen wurde zu 6500 J g-Atom-"'! ermittelt.
The palladium-tellurium phase diagram, as it appears in Finally, twashoped to gain some additional information on
literature!) to3), shows two very interesting features. Onthe the phase relationships, especially between 50 and
one hand, between 910 and 960 Ka continuous solid solu- 66.7 at.% Te, supplementing a separate investigation ofthe
tion is shown between the limiting stoichiometric composi- palladium-tellurium phase diagram!'®).
tions PdTe and PdTe, resembling the well known non-stoi-
chiometric phase in the nickel-tellurium system*)5). With Experimental Method
decreasing temperature, however, amiscibility gap opens,
separating the phases PdTe and PdTe, with rather limited The experimental details of the isopiestic method
homogeneity ranges, which exhibit the NiAs (B8,) type employed have been described previousiy by Komarek
and the Cd(OH), (C6) type structure, respectively. On the and co-workers5)!0)11)13), Since Raub and Falkenburg'”)
other hand, a deep eutectic is reported at 763K and about had demonstrated that palladium does not react with car-
36 at.% Te indicating a relatively high thermodynamic sta- bon below -1775K, graphite crucibles were preferred over
bility of the liquid phase compared to solid alloys around those made of quartz to contain the individual samples.
this composition. Only few thermodynamic measure- Starting materials for all experiments were palladium foil
ments have been reported in literature. Gronvold et al.®) (0.025 mm) with a purity of 99.95 % (Engelhardt Industries,
and Westrum et al.) measured heat capacities of PdTe Inc., New York, U.S.A.) and tellurium Iumps with a purity of
and PdTe,, respectively, between 5 and 350 K; Subbotina 99.999 % (Alfa-Ventron, Massachusetts, U.S.A.). Palladium
et al.) determined the dissociation pressure of PdTe, be- was degreased with acetone, dried, and baked out in
tween 960 and 1065 K; finally, Predel et al.°) reported ther- vacuum for 2 h at 1200 K and 10-' Pa. The graphite cru-
modynamic activities of liquid palladium-tellurium alloys. cibles, machined from high purity graphite rods (density
1.832 q cm, UT-9, Ultra Carbon Corp., Michigan, U.S.A.),
Continuing the investigation of the thermodynamic pro- were cleaned with aqua regia, whereas all quartz parts
perties of transition metal-chalcogen systems with special were treated with a mixed acid (5 % HF, 30 % HNO;,, rest
attention to the phases with NiAs- or related structures®)'0) dist. H,O); they were rinsed with distilled water and dried.
to 15), jt was the objective ofthe present study to determine The assembled isopiestic apparatus, including the graph-
the partial pressure of tellurium over the corresponding ite crucibles, was degassed by heating at 1200 K and 10!
phases in the palladium-tellurium system by an isopiestic Pa for several hours.
method and to derive therefrom the partial molar proper-
ties of tellurium. In addition, it proved to be possible to Between 50 and 100 mg palladium were weighed into each
extend the investigation to liquid alloys, where aminimum crucible on asemi-microbalance within £0.05 mg, and nor-
is reported in the integral excess Gibbs energy around the mally 20 to 25 of these crucibles were used in each run.
composition Pd,Te,°) in reasonable agreement with the The temperature of the tellurium reservoir and of each
concentration of the deep eutectic mentioned above. sample was determined by raising a calibrated Pt/Pt-10 %
