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Journal of Research of the National Bureau of Standards. Section A, Physics and Chemistry logoLink to Journal of Research of the National Bureau of Standards. Section A, Physics and Chemistry
. 1968 Jan-Feb;72A(1):27–37. doi: 10.6028/jres.072A.004

Phase Relations Between Palladium Oxide and the Rare Earth Sesquioxides in Air

C L McDaniel 1, S J Schneider 1
PMCID: PMC6640586  PMID: 31824074

Abstract

The equilibrium phase relations were determined in an air environment between PdO and each of the following: La2O3, NH2O3, Sm2O3, Eu2O3, Gd2O3, Dy2O3, Ho2O3, Y2O3, Er2O3, Tm2O3, Yb2O3, and Lu2O3. In air PdO dissociates to Pd metal at 800 °C. The dissociation of PdO is apparently a reversible process. The Nd2O3-PdO and Sm2O3-PdO systems were studied in detail inasmuch as they typified several of the Ln2O3-PdO systems. Three compounds, 2Nd2O3 · PdO, metastable Nd2O3 · PdO, Nd2O3 2PdO occur in the Nd2O3-PdO system. The 2:1, 1:1, and 1:2 compounds, of unknown symmetry, dissociate or decompose at 1135, 860, and 1085 °C, respectively. The 2:1 compound dissociates to the solid phases, Nd2O3 and Pd. No further reactions occur between Nd2O3 and Pd up to 1300 °C Three compounds, 2:1, metastable 1:1, and 1:2 occur in the Sm2O3-PdO and Eu2O3-PdO systems. Two compounds, 2:1 and 1:2 occur in the La2O3-PdO system. Other compounds detected were the 1:1 and 1:2 in the Gd2O3-PdO system and the metastable 1:1 in the Dy2O3-PdO system. Each of these compounds subsequently dissociated upon heating. No apparent reaction occurred between PdO and either Ho2O3, Y2O3, Er2O3, Tm2O3, Yb2O3, or Lu2O3.

Keywords: Dissociation, equilibrium, Ln2O3:PdO compounds, Ln2O3-PdO systems, phase relations

1. Introduction

This study is part of a program to determine what effect, if any, various Pt-group metals have upon the metal oxides when heated together in an oxidizing environment. Several of the Pt-group metals have a strong tendency to oxidize when heated in air at moderate temperatures. At higher temperatures in air, these oxides volatilize and dissociate to one solid phase, the metal. Previous work [1, 2]1 has shown that iridium dioxide reacts with other refractory oxides at moderate temperatures. Considering the fact that several of the Pt-group metals are used as secondary standards on the International Practical Temperature Scale (IPTS)2 [3] as well as container materials, it is important to better understand the behavior of these metals in an air environment. This work presents the results of an investigation of the phase relations between palladium oxide (PdO) and the rare earth sesquioxides (Ln2O3) in air.

The Nd2O3-PdO and Sm2O3-PdO systems were studied in detail and were found to be quite similar in many respects. The present study was broadened somewhat to include PdO in combination with each of the following sesquioxides: La2O3, Eu2O3, Gd2O3, Dy2O3, Ho2O3, Y2O3, Er2O3, Tm2O3, Yb2O3, and Lu2O3. A limited study seemed adequate for these ten systems due to their similarity with the Nd2O3-PdO system or to their apparent lack of reaction.

Palladium (Pd) oxidizes to PdO when heated in air at moderate temperatures. Palladium oxide rather than Pd metal was selected as one end member of the system, because the latter oxidizes too slowly in air. By utilizing PdO, an approach to equilibrium could be achieved more readily. The study would still reflect, however, the behavior in air of Pd metal in combination with other oxides.

Palladium has the structure of face-centered cubic copper with a = 3.8898 Å [4]. The freezing point of Pd is 1552 °C, a value which is given as a secondary reference point on the International Practical Temperature Scale of 1948. The structure of PdO has been described on the basis of a tetragonal unit cell with a = 3.0434 Å and c = 5.337 Å [5]. Upon heating, PdO has been reported to dissociate to Pd and a vapor phase at 870 °C in 1 atm oxygen [6].

The stable modification of neodymium sesquioxide (Nd2O3) has the hexagonal A type (a = 3.831 Å, c = 5.999 Å) [7] rare earth oxide structure at the temperatures investigated in this study. Samarium sesquioxide (Sm2O3) has been reported to crystallize in the C form at low temperatures and to invert directly and irreversibly in air to the B type monoclinic structure at about 950 °C [8]. The unit cell dimensions of B type Sm2O3 were reported by Roth and Schneider [8] as a = 14.16 Å, b = 3.621 Å, c = 8.84 Å, and β = 100.05°. The melting points of Nd2O3 and Sm2O3 have been reported to be over 2000 °C [9].

2. Materials

All starting materials employed in this study had a purity of 99.7 percent or greater. With the exception of PdO and Y2O3, the oxides were used in other investigations and their spectrochemical analyses were reported previously [10, 11]. The PdO and Y2O3 samples were found by general qualitative spectrochemical analysis3 to have the following impurities:

PdO: 0.01–0.1%, Fe and Si;

  • 0.001–0.01% each Al, Ba, Ca, Cu, Mg, Pt, and Sr;

  • < 0.001% each Ag, Mn, and Pb

Y2O3: 0.01–0.1%, Ca;

  • 0.001–0.01% each Al, Ho, Tm, and Yb;

  • 0.0001–0.001% each Cu, Fe, Lu, and Mg.

3. Experimental Procedure

Specimens were prepared from 0.4 g batches of various combinations of PdO and the rare earth oxides. Calculated amounts of each oxide, corrected for ignition loss, were weighed to the nearest milligram. Each batch was thoroughly hand mixed, placed in fused silica tubes (sealed at one end) and fired in a muffle furnace for a minimum of 18 hr at 770 °C and at 780 °C. Succeeding each heat treatment, the materials were thoroughly hand mixed and examined by x-ray diffraction techniques.

Following the preliminary heat treatments, portions of each batch were placed in the open silica tubes and fired in a platinum alloy wire-wound quench furnace at various temperatures for different periods of time. The specimen was air quenched by quickly pulling the tube from the furnace. Equilibrium was assumed to have been achieved when the x-ray pattern showed no change after successive heat treatments or when the data were consistent with the results from a previous set of experiments.

Sealed platinum tubes were employed as specimen containers for the experiments having prolonged heat treatments below the dissociation temperatures. Sealed tubes were utilized in an attempt to maintain composition and to obtain maximum reaction. The use of fused silica tubes instead of platinum was necessary because Pd, frequently found as a decomposition product, readily reacts with platinum. On the other hand, the silica tube did not appear to influence or react with the various oxide samples.

Temperatures in the quench furnace were measured with a 100 percent Pt versus 90 percent Pt-10 percent Rh thermocouple. All reported temperatures pertaining to quench furnace data are considered accurate to within ±5 °C. The precision of the measurements was ±2 °C.

All specimens were examined by x-ray diffraction at room temperature using a high angle recording Geiger counter diffractometer and Ni-filtered Cu radiation.

4. Results and Discussion

4.1. Nd2O3-PdO and Sm2O3-PdO Systems in Air

The equilibrium phase diagram for the Nd2O3-PdO system in air is given in figure 1. The diagram was constructed from the data listed in table 1. The solid circles indicate the compositions and temperatures of the experiments conducted. It should be emphasized that figure 1 represents a composite of the Nd2O3-Pd and Nd2O3-PdO systems in the Nd-Pd-Oxygen ternary. At the lower temperatures, the oxygen content of the specimens closely conforms to the compositions represented by the pseudobinary system, Nd2O3-PdO. At the higher temperatures, the compositions of the solid phases change by an apparent oxygen loss to those indicated by the Nd2O3-Pd join. Figure 1 is a pseudobinary representation of a portion of the ternary system. This method of illustration has been employed by a number of investigators [1, 12].

Figure 1. Phase equilibrium diagram for the Nd2O3-PdO system in air.

Figure 1.

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Dotted lines indicate metastahie 1:1 compound and decomposition temperature. Italic lettering indicates metastable phase assemblages. ● – compositions and temperatures of experiments conducted.

Table 1. Experimental data for compositions in the Ln2O3-PdO systems.

System Composition Heat treatmenta X-ray diffraction analysesb Remarks
Temp. Time
mole % °C hr
Nd2O3-PdO 75:25 780 18 2:1 + Nd2O3 + 1:1 Nonequilibrium.
c 1000 18 2:1 + Nd2O3
c 1063 3 2:1 + Nd2O3
c 1103 2 2:1 + Nd2O3 Quenched in ice water.
d 1125 2 2:1 + Nd2O3
c 1130 62 2:1 + Nd2O3 Quenched in ice water.
d 1130 1.5 2:1 + Nd2O3
d 1135 1.5 2:1 + Nd2O3 + Pd Nonequilibrium.
d 1140 2 2:1 + Nd2O3 + Pd Nonequilibrium.
71:29 780 22 2:1 + Nd2O3 + 1:1 Nonequilibrium.
c 1000 78 2:1 + Nd2O3
66.6:33.3 780 18 2:1 + Nd2O3 + 1:1 Nonequilibrium.
790 18 2:1 + Nd2O3 + 1:1 Nonequilibrium.
800 20 2:1 + Nd2O3 + 1:1 Nonequilibrium.
900 71 2:1 + Nd2O3 Nonequilibrium. e
c 1000 18 2:1
c 1050 504 2:1
c 1070 2 2:1 Quenched in ice water.
c 1085 2 2:1
1115 20 2:1 + Nd2O3 + Pd Nonequilibrium; reheat of 1205 °C specimen.
d 1115 2 2:1
d 1125 2 2:1
d 1130 2 2:1
d 1135 2 2:1 + Pd + Nd2O3 Nonequilibrium.
1205 1 Nd2O3 + Pd
1300 1 Nd2O3 + Pd
62:38 780 18 2:1 + 1:1 + Nd2O3 Nonequilibrium.
845 20 2:1 + 1:1
c 1000 78 2:1 + 1:2
50:50 780 18 1:1 + Nd2O3 + PdO Nonequilibrium.
c 830 20 1:1 Quenched in ice water.
842 15 1:1 + Nd2O3 Nonequilibrium.e
845 144 2:1 + 1:2 Reheat of 902 °C specimen.
855 3 1:1
860 2 1:1 + 1:2 Nonequilibrium.
865 2 1:1 + 1:2 + 2:1 Nonequilibrium.
902 18 2:1 + 1:2
c 1000 18 2:1 + 1:2
c 1070 78 2:1 + 1:2 Quenched in ice water.
1080 2 2:1 + 1:2
1085 2 2:1 + 1:2 + Pd Nonequilibrium.
1090 2 2:1 + 1:2 + Pd Nonequilibrium.
1100 4 2:1 + Pd + 1:2 Nonequilibrium.
1200 18 Nd2O3 + Pd
1250 2 Nd2O3 + Pd
38:62 780 21 1:2 + 1:1 + PdO Nonequilibrium.
c 845 20 1:2+ 1:1 Quenched in ice water.
c 1000 78 1:2 + 2:1
33.3:66.6 780 18 1:2+ 1:1 + PdO Nonequilibrium.
c800 120 1:2
900 2 1:2
925 2 1:2
970 120 1:2 + Nd2O3 + Pd Nonequilibrium; reheat 1150 °C specimen.
975 2 1:2
c 1000 18 1:2 Quenched in ice water.
c 1000 48 1:2
1025 2 1:2
1040 2 1:2
c 1050 110 1:2
d 1060 2 1:2
d 1070 3 1:2
d 1080 2 1:2
d 1085 2 1:2 + Pd + 2:1 Nonequilibrium.
d 1090 1.5 1:2 + Pd + 2:1 Nonequilibrium.
1110 18 2:1 + Pd
1130 2 2:1 + Pd
1150 1 Nd2O3 + Pd
29:71 780 21 1:2 + 1:1 + PdO Nonequilibrium.
1000 3 1:2 + Pd
1020 1.5 1:2 + Pd
25:75 780 18 1:2 + 1:1 + PdO Nonequilibrium.
1000 3 1:2 + Pd
1050 1.5 1:2 + Pd
1075 2 1:2 + Pd
1085 3 Pd + 2:1 + 1:2 Nonequilibrium.
1110 2 Pd + 2:1
1150 2 Pd + Nd2O3
1300 1.5 Pd + Nd2O3
0:100 780 18 PdO
795 2 PdO
800 2 PdO + Pd Nonequilibrium.
810 4 Pd
825 1.5 Pd
850 3 Pd
900 3 Pd
1200 2 Pd
Sm2O3-PdO 75:25 780 18 2:1 + Sm2O3 + 1:1 Nonequilibrium.
c 1103 3 2:1 + Sm2O3 Quenched in ice water.
1140 1 2:1 + Sm2O3 + Pd Nonequilibrium.
66.6:33.3 780 19 2:1 + Sm2O3 + 1:1 Nonequilibrium.
790 120 2:1+ Sm2O3 + PdO Nonequilibrium; reheat 1200 °C specimen.
c 1000 20 2:1 + Sm2O3 Nonequilibrium.e
c 1000 24 2:1
d 1100 2.5 2:1
d 1110 2 2:1
d 1115 2 2:1 + Pd + Sm2O3 Nonequilibrium.
d 1125 1.5 2:1 + Pd + Sm2O3 Nonequilibrium.
1200 2 Sm2O3 + Pd
50:50 780 21 1:1 + 1:2+ Sm2O3 Nonequilibrium.
800 144 2:1 + 1:2 Reheat of 1000 °C specimen.
c 830 20 1:1 Quenched in ice water.
830 5 1:1
850 2 1:1 Reheat of 830 °C 20 hr specimen.
880 2.5 1:1 Reheat of 830 °C 20 hr specimen.
c 902 18 1:1 Quenched in ice water.
915 1.5 1:1 Reheat of 830 °C 20 hr specimen.
935 2 1:1 Reheat of 830 °C 20 hr specimen.
940 2 1:1 + 1:2 + 2:1 Nonequilibrium; reheat of 830 °C 20 hr specimen.
1000 20 2:1 + 1:2
1100 3 2:1 + Pd + 1:2 Nonequilibrium.
1200 2.5 Sm2O3 + Pd
33.3:66.6 780 21 1:2 + 1:1 + Sm2O3 Nonequilibrium.
790 120 1:2 + PdO + Sm2O3 Nonequilibrium; reheat of 1150 °C specimen.
c 800 54 1:2 Quenched in ice water.
c 1000 20 1:2
d 1055 2.5 1:2
d 1060 2 1:2 + Pd Nonequilibrium.
d 1075 2 1:2 + Pd + Sm2O3 Nonequilibrium.
1150 1 Sm2O3 + Pd
25:75 780 21 1:2 + PdO + Sm2O3 Nonequilibrium.
1050 1.5 1:2 + Pd Quenched in ice water.
La2O3-PdO 66.6:33.3 780 20 La2O3 + PdO + 1:2 Nonequilibrium.
c 1000 18 2:1 + La2O3 Nonequilibrium.e
c 1000 72 2:1 + La2O3 Nonequilibrium.e
c 1070 66 2:1 Ouenched in ice water
1150 2 2:1 Reheat of 1070 °C specimen.
1160 3 2:1 Reheat of 1070 °C specimen.
1185 2 2:1 Reheat of 1070 °C specimen.
1190 2 2:1 + La2O3 + Pd Nonequilibrium; reheat of 1070 °C specimen.
50:50 780 20 La2O3 + PdO + 1:2 Nonequilibrium.
850 144 2:1 + 1:2
c 1000 18 2:1 + 1:2
1000 144 2:1 + 1:2 + Pd Nonequilibrium; reheat of 1310 °C specimen.
1310 2 La2O3 + Pd + La(OH)3 La2O3 commonly hydrates at room temperature.
33.3:66.6 780 20 La2O3 + PdO + 1:2 Nonequilibrium.
c 1000 18 1:2 Quenched in ice water.
c 1070 66 1:2
d 1085 2 1:2
d 1090 2 1:2
d 1100 2.5 1:2
d 1105 2 1:2 + Pd Nonequilibrium.
d 1110 2 1:2 + Pd + 2:1 Nonequilibrium.
d 1140 2 2:1 + Pd Nonequilibrium.
Eu2O3-PdO 66.6:33.3 780 18 Eu2O3 + PdO + 1:1 + 2:1 Nonequilibrium.
c 900 10 2:1 + Eu2O3 Nonequilibrium.e
c 950 48 2:1 + Eu2O3 Nonequlibriume; quenched in ice water.
c 1050 36 2:1 + Eu2O3 Nonequilibrium.e
1085 2 2:1 + Eu2O3 + Pd Nonequilibriume; reheat of 1050 °C specimen.
1090 2 2:1 + Eu2O3 + Pd Nonequilibriume; reheat of 1050 °C specimen.
1095 2 2:1 + Eu2O3 + Pd Nonequilibriume; reheat of 1050 °C specimen.
50:50 780 18 Eu2O3 + 1:1 + PdO Nonequilibrium.
800 144 1:2 + 2:1 + Eu2O3 Nonequilibrium; reheat of 1000 °C 20 hr specimen.
c 850 67 1:1 + Eu2O3 Nonequilibrium.e
c 900 168 1:1 + Eu2O3 Nonequilibrium.e
960 2 1:1 + Eu2O3 Nonequilibriume; reheat of 900 °C specimen.
980 2 1:1 + Eu2O3 Nonequilibrium.e
985 2 1:1 + Eu2O3 + 1:2 Nonequilibrium.
990 2 1:1 + Eu2O3 + 1:2 Nonequilibrium.
1000 20 1:2 + 2:1 + Eu2O3 Nonequilibrium.
1000 144 1:2 + 2:1 + Eu2O3 Nonequilibrium; reheat of 1250 °C specimen.
1250 2 Eu2O3 + Pd
33.3:66.6 780 18 1:2 + PdO + Eu2O3 + 1:1 Nonequilibrium.
c 900 10 1:2 + Eu2O3 Nonequilibriume; quenched in ice water.
c 950 96 1:2
1040 2 1:2 Reheat of 950 °C specimen.
1045 2 1:2 + Pd + 2:1 Nonequilibrium; reheat of 950 °C specimen.
1050 2 1:2 + Pd + 2:1 Nonequilibrium; reheat of 950 °C specimen.
1055 2 1:2 + Pd + 2:1 Nonequilibrium; reheat of 950 °C specimen.
Gd2O3-PdO 66.6:33.3 780 18 Gd2O3 + 1:1 + PdO Nonequilibrium.
c 900 144 1:1 + Gd2O3
50:50 780 18 Gd2O3 + 1:1 + PdO Nonequilibrium.
c 900 20 1:1 + Gd2O3 Nonequilibrium.e
900 72 1:2 + Gd2O3 + Pd Nonequilibrium; reheat of 1100 °C specimen.
1000 2.5 1:1 + Gd2O3 Nonequilibriume; reheat of 900 °C 20 hr specimen.
1005 2 1:1 + Gd2O3 + 1:2 Nonequilibrium; reheat of 900 °C 20 hr specimen.
1010 3 1:1 + Gd2O3 + 1:2 Nonequilibrium; reheat of 900 °C 20 hr specimen.
1020 2 1:1 + Gd2O3 + 1:2 Nonequilibrium; reheat of 900 °C 20 hr specimen.
1040 2 Gd2O3 + Pd + 1:2 Nonequilibrium; reheat of 900 °C 20 hr specimen.
1100 3 Gd2O3 + Pd
33.3:66.6 780 18 1:1 + 1:2 + Gd2O3 Nonequilibrium.
c 900 144 1:2 + Gd2O3 Nonequilibrium.e
c 950 28 1:2 + 1:1 Nonequilibriume; quenched in ice water.
1015 2 1:2 + Gd2O3 Nonequilibriume; reheat of 950 °C specimen.
1020 2 1:2 + Gd2O3 Nonequilibriume; reheat of 950 °C specimen.
1025 2 1:2 + Gd2O3 + Pd Nonequilibrium; reheat of 950 °C specimen.
1030 2 1:2 + Gd2O3 + Pd Nonequilibrium; reheat of 950 °C specimen.
1035 2 1:2 + Gd2O3 + Pd Nonequilibrium; reheat of 950 °C specimen.
1040 2 1:2 + Pd + Gd2O3 Nonequilibrium; reheat of 950 °C specimen.
Dy2O3-pdO 66.6:33.3 780 18 1:1 + PdO + Dy2O3 Nonequilibrium.
850 67 1:1 + Dy2O3
c 900 18 1:1 + Dy2O3 Quenched in ice water.
c 1000 144 1:1 + Dy2O3
50:50 780 18 1:1 + Dy2O3 + PdO Nonequilibrium.
c 900 68 1:1 + Dy2O3 Nonequilibrium.e
790 120 Dy2O3 + PdO + Pd Nonequilibrium; reheat of 1250 °C specimen.
1000 144 Dy2O3 + Pd Reheat of 1250 °C specimen.
1010 2 1:1 + Dy2O3 Reheat of 900 °C specimen.
1025 2 1:1 + Dy2O3 Reheat of 900 °C specimen.
1030 2 1:1 + Dy2O3 + Pd Nonequilibrium; reheat of 900 °C specimen.
1250 2 Dy2O3 + Pd
33.3:66.6 780 18 1:1 + PdO + Dy2O3 Nonequilibrium.
c 900 20 1:1 + Pd
1050 120 Pd + Dy2O3
Ho2O3-pdO 50:50 780 18 Ho2O3 + PdO
c 790 20 Ho2O3 + PdO
800 2 Ho2O3 + PdO + Pd Nonequilibrium.
900 120 Ho2O3 + Pd
Y2O3-pdO 50:50 780 18 Y2O3 + PdO
c 790 20 Y2O3 + PdO
900 120 Y2O3 + Pd
1000 144 Y2O3 + Pd
Er2O3-PdO 50:50 780 18 Er2O3 + PdO
c 790 20 Er2O3 + PdO
900 120 Er2O3 + Pd
Tm2O3-PdO 50:50 780 18 Tm2O3 + PdO
c 790 18 Tm2O3 + PdO
900 120 Tm2O3 + Pd
1000 23 Tm2O3 + Pd
Yb2O3-PdO 50:50 780 18 Yb2O3 + PdO
c 790 18 Yb2O3 + PdO
900 120 Yb2O3 + Pd
Lu2O3-PdO 50:50 780 21 Lu2O3 + PdO
c 790 18 Lu2O3 + PdO
900 68 Lu2O3 + Pd
1000 25 Lu2O3 + Pd
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a

All specimens were heat treated at 770 °C, a minimum of 18 hr. Unless otherwise indicated, fused silica tubes (sealed at one end) were used for specimen containers and were air quenched.

b

The phases identified are given in order of the relative amount present at room temperature.

c

Sealed platinum tubes were used for specimen containers.

d

Reheat of 1000 °C specimen.

e

PdO probably lost by volatilization.

Palladium oxide was found to dissociate to Pd metal and presumably oxygen at 800 ±5 °C in air at atmospheric pressure. This value compares favorably with the data (870 °C in one atmosphere oxygen) given by Bell et al., in their study of the Pd-oxygen system [6]. The dissociation of PdO is a reversible process. Palladium oxide was first heated above 800 °C, until only Pd was present. The same material was then reheated at 790 °C and the x-ray data indicated only PdO.

Raub [13] reports that palladium takes up appreciable quantities of oxygen into solid solution when the metal is heated in oxygen at 1200 °C. Raub’s concluions were based on weight gain data with no x-ray results given. Chaston [14] concluded the weight gain of Pd observed by Raub was due to the oxidation of base metal impurities. The x-ray diffraction data obtained in the present study show no indication of solid solution of oxygen in Pd, when heated in an air environment.

Three intermediate compounds, 2:1, 1:1, and 1:2 occur in the Nd2O3-PdO system. The stable 2:1 and 1:2 compounds dissociate to two solid phases and a vapor phase, presumably oxygen at 1135 and 1085 °C, respectively. The 1:1 phase was found to decompose at about 860 °C to the 2:1 and 1:2 compounds. In order to establish the stability of the three compounds, appropriate mixtures were heated first above and then below their respective decomposition or dissociation temperatures. Only the 2:1 and 1:2 compounds reformed from their decomposition products. Prolonged heat treatments (up to 6 days) failed to reform the 1:1 compound, indicating perhaps it is metastable phase that forms only on heating.

A literature search did not reveal any compounds structurally similar to the Nd2O3-PdO phases. The x-ray powder patterns of the compounds were not similar to those reported by Barry and Roy [15] for the 2:1, 1:1, and 1:2 rare earth oxide-calcium oxide compounds. The unindexed x-ray diffraction powder patterns for the three compounds found in the present study are given in table 2.

Table 2. X-ray diffraction powder data for Nd2O3-PdO compounds.

(CuKα radiation)a

2Nd2O3 · PdOb Nd2O3 · Pdc Nd2O3 · 2PdOb
d I/I0 d I/I0 d I/I0
3.2840 15 3.1895 17 4.2387 13
3.0064 100 2.8768 100 3.4568 12
2.8723 45 2.8280 60 3.2733 20
2.8643 81 2.0732 12 2.9303 28
2.8038 45 1.9980 33 2.8228 76
2.1401 21 1.9556 8 2.7792 41
2.0827 20 1.6715 18 2.5924 100
2.0170 8 1.6423 27 2.3178 71
1.9746 19 1.4393 7 2.1372 12
1.9313 12 1.4120 9 2.1185 10
1.7640 8 1.2641 5 2.0732 6
1.7199 9 2.0347 9
1.6888 12 2.0179 54
1.6800 22 1.8192 20
1.6628 4 1.7804 8
1.6137 11 1.7287 11
1.6019 11 1.6634 7
1.5784 10 1.6148 18
1.4342 6 1.5898 11
1.5732 25
1.5674 33
1.5230 21
1.4669 15
1.4181 7
1.3913 11
1.3807 3
1.2971 11
1.2753 8
1.2281 4
1.1870 7
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a

d – interplanar spacing, I/I0 – relative intensity.

b

X-ray pattern obtained from specimen heat treated at 1000 °C for 18 hr.

c

X-ray pattern obtained from specimen heat treated at 830 °C for 20 hr.

At temperatures above 800 °C the system no longer can be represented by the Nd2O3-PdO join. The system changes through dissociation and at 1135 °C becomes the true binary Nd2O3-Pd. Up to 1300 °C, Nd2O3 and Pd do not react in the solid state.

The equilibrium phase diagram for the Sm2O3-PdO system in air is given in figure 2. The pertinent data are listed in table 1. The diagram is similar to the Nd2O3-PdO system in many respects in that three intermediate compounds 2:1, metastable 1:1, and 1:2 also occur. These phases dissociate or decompose at 1115, 940, and 1060 °C, respectively.

Figure 2. Phase equilibrium diagram for the Sm2O3-PdO system in air.

Figure 2.

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Dotted lines indicate metastable 1:1 compound and decomposition temperature. Italic lettering indicates metastable phase assemblages. ● – compositions and temperatures of experiments conducted.

4.2. Other Ln2O3-PdO Systems in Air

Mixtures were prepared from PdO and each of the following sesquioxides: La2O3, Eu2O3, Gd2O3, Dy2O3, Ho2O3, Y2O3, Er2O3, Tm2O3, Yb2O3, and Lu2O3. The phase equilibrium diagrams for the various Ln2O3-PdO systems in air are given in figure 3. The experimental data are tabulated in table 1. It is evident that the Nd2O3-PdO system is representative in a general way of the other Ln2O3-PdO systems. The La2O3-PdO diagram indicates the occurrence of the 2:1 and the 1:2 compounds. The 1:1 compound was not detected. Three compounds, 2:1, metastable 1:1, and 1:2 occur in the Eu2O3-PdO system. The 1:2 and metastable 1:1 compounds occur in the Gd2O3-PdO system. The 2Gd2O3 · PdO compound was not detected. In the Dy2O3-PdO system, only the metastable 1:1 compound was detected.

Figure 3. Proposed phase equilibrium diagrams for various Ln2O3-PdO systems in air.

Figure 3.

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For clarity experimental points are not included, see table 1 for exact compositions and temperatures studied. Dotted lines indicate metastable 1:1 compound and decomposition temperature. Italic lettering indicates metastable phase assemblages.

The remaining systems of either Ho2O3, Y2O3, Er2O3, Tm2O3, Yb2O3, or Lu2O3 with PdO are rather simple and straightforward inasmuch as there was no detectable reaction between end members in the solid state. The diagrams indicate only the dissociation of PdO, the point at which the system reverts to the true Ln2O3-Pd system.

Table 3 summarizes the results obtained in this study. Listed are the systems investigated, size of the rare earth cation, and the dissociation temperatures of the Ln2O3-PdO compounds. The dissociation temperatures of the 2:1 compounds decrease as the size of the rare earth cation decreases. The 1:2 compounds were found to dissociate in a similar manner. However, the decomposition temperatures of the metastable 1:1 compounds increase as the size of the rare earth cation decreases. As expected, the Ln2O3-PdO compounds with the same molecular ratio appear to have similar x-ray patterns. The patterns indicate appropriate shift in d-spacings to account for difference in ionic size of the rare earth cations.

Table 3. Dissociation temperatures of Ln2O3 · PdO compounds.

All radii of the rare earth cations taken from Arhens [16] with the exception of Y+3 which was taken from Roth and Schneider [8]. Parenthesis indicate decomposition temperatures of metastable 1:1 compounds.

SYSTEM RADIUS OF Ln+3 Å
 DISSOCIATION TEMP. °C
2 Ln2O3 · PdO Ln2O3 · PdO Ln2O3 · 2 PdO
La2O3 - PdO 1.14 1190 — 1105
Nd2O3 - PdO 1.04 1135 (860) 1085
Sm2O3 - PdO 1.00 1115 (940) 1060
Eu2O3 - PdO 0.98 1085 (985) 1045
Gd2O3 - PdO 0.97 — (1005) 1025
Dy2O3 - PdO 0.92 — (1030) —
Ho2O3 - PdO 0.91 — — —
Y2O3 - PdO 0.91 — — —
Er2 O3 - PdO 0.89 — — —
Tm2O3 - PdO 0.87 — — —
Yb2O3 - PdO 0.86 — — —
Lu2O3 - PdO 0.85 — — —
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It should be noted that the proposed diagrams pertain only to the phase relations of the systems in an air environment at atmospheric pressure. Any change in oxygen pressure would change the equilibrium diagram. In an air environment, precaution should be taken when utilizing Pd metal in combination with some of the rare earth oxides, since the data indicate the tendency to form new phases.

4.3. Summary

Equilibrium phase diagrams for systems involving PdO and various rare earth oxides were determined in air. Selected mixtures in the systems were studied by x-ray diffraction techniques after various heat treatments. Palladium, in air, oxidizes to PdO at moderate temperatures. The dissociation temperature of PdO in air at atmospheric pressure was established at 800 ±5 °C. This dissociation was found to be a reversible process. Palladium oxide reacts with a number of oxides to form binary compounds. The pseudobinary system Nd2O3-PdO exemplified the typical type of reaction and was studied in detail. Three compounds, 2Nd2O3·PdO, Nd2O3·PdO (metastable), and Nd2O3·2PdO occur in the system. The 2:1, 1:1, and 1:2 compounds, of unknown symmetry dissociate or decompose at 1135, 860, and 1085 °C, respectively. Above 1135 °C the system corresponds to the Nd2O3-Pd binary system. No further reaction appears to take place between Nd2O3 and Pd up to 1300 °C. Similar type compounds were found to exist in other Ln2O3-PdO systems.

Three compounds, 2:1, 1:1, and 1:2, occur in the Sm2O3-PdO and Eu2O3-PdO systems. Two compounds, 2:1 and 1:2, occur in the La2O3-PdO system. Other compounds detected were the 1:1 and 1:2 in the Gd2O3-PdO system and the 1:1 in the Dy2O3-PdO system. Each of these compounds also dissociated upon heating at temperatures above the dissociation temperature of PdO. Mixtures of either Ho2O3, Y2O3, Er,O3, Tm2O3, Yb2O3, or Lu2O3 with PdO did not react in the solid state.

Footnotes

1

Figures in brackets indicate the literature references at the end of this paper.

2

This scale (IPTS) applies to all temperatures listed in this paper.

3

The spectrochemical analyses were performed by the Spectrochemical Analysis Section of the National Bureau of Standards.

5. References

Articles from Journal of Research of the National Bureau of Standards. Section A, Physics and Chemistry are provided here courtesy of National Institute of Standards and Technology

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