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Heats of formation of cerium-bismuth alloys
31
Journal of the Less-Common Metals, 58 (1978) 31 - 36 0 Elsevier Sequoia S.A., Lausanne - Printed in the Netherlands
HEATS
OF FORMATION
G. BORZONE,
A. BORSESE,
Istituto di Chimica (Received
Generale
OF CERIUM-BISMUTH
A. CALABRETTA dell’Uniuersitci
ALLOYS
and R. FERRO
di Genoua,
Genoa
(Italy)
June 30, 1977)
Summary In the cerium-bismuth system the heats of formation have been measured by using an isoperibol direct calorimeter. The composition and equilibrium state of the samples have been checked by chemical, metallographic and X-ray analyses. The following values have been obtained for AH,,, (reaction in the solid state at 300 K) of the five Ce-Bi compounds (kcal (g atom)-l f 0.5): CezBi, -19.5; CeSBis, -21.9; CedBia, -24.2; CeBi, -27.4; CeBip, ? -18.5. The results obtained are discussed briefly and compared with those of other similar systems.
Introduction In the course of a systematic investigation of the alloy thermochemistry of the rare earths, we now report data concerning the Ce-Bi system. The thermodynamics of the Ce-Bi liquid solutions have been previously reported in the literature. Extremely large negative deviations from Raoult’s law have been observed and evidence of marked interaction between the alloy components has been obtained [ 131 . The heats of formation of sclid alloys have now been measured.
Experimental Calorimetric
measurements
The choice of a particular calorimetric technique is strictly conditioned by the general characteristics of the materials under consideration. The measurement of the heat of formation AH,,,, was carried out using an isoperibol direct calorimeter [l] already satisfactorily employed for a number of similar substances. The alloys were prepared directly in the calorimeter; a fine mixture of the powders of the two elements was heated inside the
32
calorimeter until the reaction started. The total heat evolved was measured by using a multijunction thermopile. Of course, owing to transformations such as the peritectic reactions, it is sometimes possible to have only a partial reaction and not to reach the equilibrium state. For this reason calorimetric measurements have been carried out on the complete range of compositions, in order to discover the trend of AH,,,; the state of the various samples has been controlled by using the preparative and analytical techniques described below. Moreover, in order to improve the reliability of the data we plan to carry c.rt independent measurements using different calorimetric methods. Preparation
of the alloys
The metals employed 99.999%, respectively. Reference
were Ce and Bi, of nominal purities 99.9% and
samples
A number of alloys have been prepared as reference samples. These have been obtained by melting the metals in MO crucibles closed by welding under argon. These samples were then annealed, on the basis of the indication given by the thermal data for the analogous Nd-Bi alloys. The phase diagram of Ce-Bi alloys has been studied [2] in the past, but more recent investigations [ 3 - 51 have shown the necessity for considerable revision. Calorimetric
samples
In order to have a priming temperature as low as possible, the two metals were first reduced to fine powders (this operation was carried out under argon for Ce) and then carefully mixed and compacted inside an iron crucible, which was subsequently closed by plasma welding under argon. The following examinations were carried out on all the samples. Chemical
analysis
After dissolution in aqua regia the separation of the two elements was effected by precipitation of Bi as sulphide. Bi, after redissolution, was then determined with %hydroxyquinoline and Ce with oxalic acid. Metallographic
examinations
All the calorimetric samples were examined metallographically. In order to evaluate the thermochemical data, samples which did not appear to be in the equilibrium state (three-phase samples, partial peritectic transformations, etc.) were rejected. X-ray examinations
These were carried out using the Debye method with Cu K, radiation, and by identifying the known crystal structures of the various phases which are listed in Table 1. Generally, for calorimetric alloys with compositions between Ce and CeBi, the powder photographs confirmed the existence of the equilibrium phases. For the Bi-rich alloys, however, the diffraction lines of the CeBiz phase were not observed.
33 TABLE
1
Crystal
data of Ce-Bi
compounds type
Unit cell dimensions
Refs.
(A)
Compound
Structure
CezBi
tetragonal tll2 LazSb
a = 4.5911,
c = 18.1539
3
CegBi3
hexagonal hP16 MngSis
a = 9.5313,
c = 6.5871
3
CeqBia
cubic cl28 anti-ThaPd
a = 9.6736
CeBi
cubic cF8 NaCl
a = 6.5055, a = 6.5066,
Ce-rich, 700 “C Bi-rich, 700 “C
CeBiz
triclinic aP? LaBi
a = 6.5280, (Y = 91.515”
b = 13.0564, 0 = 103.02”,
3, 5, 6 3,738 c = 11.8516 y = 92.20”
3
Results The data obtained are given in Table 2 and in the lower part of Fig. 1. We note two discontinuities in the plot of AH,,,,, at the compositions 37.5 TABLE
2
Heats of formation xBi = Cel_,Bi,
of cerium-bismuth
alloys
in the solid state at 300 K; (1 -x)Ce
Analytical composition (xni + 0.003)
AH measured (kcal (g atom))l
1 2a 3 4 5 6a
0.22 0.30 0.35 0.375 0.40 0.42
7 8 9a 10a 11 12 13 14 15a 16
0.425 0.45 0.475 0.48 0.50 0.52 0.55 0.60 0.65 0.70
-12.9 (-19.4) -20.7 -21.9 -23.0 (-28.3) - 23.8 -25.2 (-24.1) (-25.3) -28.0 -25.7r, -25.1 -22.6 (-23.7) -16.0
Alloy
number
aFor these alloys, either the reaction have been observed.
in the calorimeter
was not complete
+
50.5) -
or side reactions
34
Fig. 1. Ce-Bi alloys. Heat of formation of the alloys in the solid state and sketch of the possible form of the phase diagram. 0, experimental points; (O), for these alloys the reaction in the calorimeter was not complete; [O] , in the course of the calorimetric synthesis of these alloys, side reactions were observed.
and 50 at.% Bi. In the upper part of the figure a conjectural picture of the shape of the phase diagram is shown. This has been sketched on the basis of suggestions by Gschneidner [4] and by analogy with the Nd-Bi [9] system. From the reported data the following values for the heat of formation are obtained (kcal (g atom)-‘, reaction in the solid state at 300 K): CesBi, -19.5 + 0.5; CesBis, -21.9 f 0.5; Ce,Bia, -24.2 + 0.5; CeBi, -27.4 f 0.5; CeBis, ‘? -18.5. Only a limiting value has been given for CeBip because of the difficulty of obtaining samples at equilibrium in this composition range.
35
Conclusions The data obtained are compared in Fig. 2 with those for the other light rare earth bismuthides [ 10 - 121. A comparison of the trend of AH,,, with the phase diagram is possible only for the Nd-Bi system. The other RBi phase diagrams so far determined are not completely reliable, but the other systems are expected to be similar, as has been suggested in Fig. 1 for the Ce-Bi diagram. As shown in Fig. 2, the curves of AH,,, versus composition have a similar shape for the various R-Bi systems, although the Pr-Bi alloys seem to be significantly less exothermic than those of the other light rare earths. R
2
‘0
n
20__3 ia
...&..
Nd -c--
Fig. 2. Summary of the trends of AH form for the light rare earth bismuthides. The points represent the values of AH form for the compounds so far examined, the existence of which has been independently confirmed by X-ray diffraction measurements.
As far as the Ce-Bi alloys are concerned, we observe that the value of previously reported for CeBiz must be considered a limiting value. This appears to be in agreement with the results of Bayanov [ 131. In his paper, thermochemical data have been given for the most B&rich compounds in the La-Bi and Ce-Bi systems. The reported heats of formation of the RBiX (RBia) compounds are around 66 kcal mol-’ for the reaction at 900 K, or, considering only the correction for the heat of fusion of Bi, around 60 - 61 keal mol-’ (about 20 kcal (g atom)-1 ) at room temperature. This value is in good agreement with the value of AHform previously measured for NdBia (-21 of:0.5 kcal (g atom)-l).
AH,,,
36
Acknowledgments The financial help obtained from the Italian Consiglio Nazionale delle Ricerche is acknowledged with thanks. The authors also wish to thank Mr. B. Brusasco, Mr. M. Cappai and Mr. A. Cavanna for their assistance.
References
10 11 12 13
R. Capelli, R. Ferro and A. Borsese, Thermochim. Acta, 10 (1974) 13. R. Vogel, Z. Anorg. Chem., 84 (1914) 327. K. Yoshihara, J. B. Taylor, L. D. Calvert and J. G. Despault, J. Less-Common Met., 41 (1975) 329. K. A. Gschneidner, Jr. and M. E. Verkade, Publ. No. 15RIC-7, Iowa State University, 1974. R. J. Gambino, J. Less-Common Met., 12 (1967) 344. D. Hohnke and E. Parthe, Acta Crystallogr., 21 (1966) 435. A. Iandelli, Atti Accad. Naz. Lincei Cl. Sci. Fis. Mat. Nat. Rend., 37 (1964) 160. G. L. Glcese, Atti Accad. Naz. Lincei Cl. Sci. Fis. Mat. Nat. Rend., 40 (1966) 629. G. F. Kobzenko and V. B. Chernogorenko, Doki. Akad. Nauk. Ukr. SSR, Ser. A, 32 (1970) 945. A. Borsese, R. Capelli, S. Delfino and R. Ferro, Thermochim. Acta, 8 (1974) 393. A. Borsese, R. Capelfi, S. Delfino and R. Ferro, Thermochim. Acta, 9 (1974) 313. A. Borsese, R. Ferro, R. Capelli and S. Delfino, Thermochim. Acta, 11 (1975) 205. A. P. Bayanov, Russ. Chem. Rev., 44 (1975) 122.
Journal of the Less-Common Metals, 58 (1978) 31 - 36 0 Elsevier Sequoia S.A., Lausanne - Printed in the Netherlands
HEATS
OF FORMATION
G. BORZONE,
A. BORSESE,
Istituto di Chimica (Received
Generale
OF CERIUM-BISMUTH
A. CALABRETTA dell’Uniuersitci
ALLOYS
and R. FERRO
di Genoua,
Genoa
(Italy)
June 30, 1977)
Summary In the cerium-bismuth system the heats of formation have been measured by using an isoperibol direct calorimeter. The composition and equilibrium state of the samples have been checked by chemical, metallographic and X-ray analyses. The following values have been obtained for AH,,, (reaction in the solid state at 300 K) of the five Ce-Bi compounds (kcal (g atom)-l f 0.5): CezBi, -19.5; CeSBis, -21.9; CedBia, -24.2; CeBi, -27.4; CeBip, ? -18.5. The results obtained are discussed briefly and compared with those of other similar systems.
Introduction In the course of a systematic investigation of the alloy thermochemistry of the rare earths, we now report data concerning the Ce-Bi system. The thermodynamics of the Ce-Bi liquid solutions have been previously reported in the literature. Extremely large negative deviations from Raoult’s law have been observed and evidence of marked interaction between the alloy components has been obtained [ 131 . The heats of formation of sclid alloys have now been measured.
Experimental Calorimetric
measurements
The choice of a particular calorimetric technique is strictly conditioned by the general characteristics of the materials under consideration. The measurement of the heat of formation AH,,,, was carried out using an isoperibol direct calorimeter [l] already satisfactorily employed for a number of similar substances. The alloys were prepared directly in the calorimeter; a fine mixture of the powders of the two elements was heated inside the
32
calorimeter until the reaction started. The total heat evolved was measured by using a multijunction thermopile. Of course, owing to transformations such as the peritectic reactions, it is sometimes possible to have only a partial reaction and not to reach the equilibrium state. For this reason calorimetric measurements have been carried out on the complete range of compositions, in order to discover the trend of AH,,,; the state of the various samples has been controlled by using the preparative and analytical techniques described below. Moreover, in order to improve the reliability of the data we plan to carry c.rt independent measurements using different calorimetric methods. Preparation
of the alloys
The metals employed 99.999%, respectively. Reference
were Ce and Bi, of nominal purities 99.9% and
samples
A number of alloys have been prepared as reference samples. These have been obtained by melting the metals in MO crucibles closed by welding under argon. These samples were then annealed, on the basis of the indication given by the thermal data for the analogous Nd-Bi alloys. The phase diagram of Ce-Bi alloys has been studied [2] in the past, but more recent investigations [ 3 - 51 have shown the necessity for considerable revision. Calorimetric
samples
In order to have a priming temperature as low as possible, the two metals were first reduced to fine powders (this operation was carried out under argon for Ce) and then carefully mixed and compacted inside an iron crucible, which was subsequently closed by plasma welding under argon. The following examinations were carried out on all the samples. Chemical
analysis
After dissolution in aqua regia the separation of the two elements was effected by precipitation of Bi as sulphide. Bi, after redissolution, was then determined with %hydroxyquinoline and Ce with oxalic acid. Metallographic
examinations
All the calorimetric samples were examined metallographically. In order to evaluate the thermochemical data, samples which did not appear to be in the equilibrium state (three-phase samples, partial peritectic transformations, etc.) were rejected. X-ray examinations
These were carried out using the Debye method with Cu K, radiation, and by identifying the known crystal structures of the various phases which are listed in Table 1. Generally, for calorimetric alloys with compositions between Ce and CeBi, the powder photographs confirmed the existence of the equilibrium phases. For the Bi-rich alloys, however, the diffraction lines of the CeBiz phase were not observed.
33 TABLE
1
Crystal
data of Ce-Bi
compounds type
Unit cell dimensions
Refs.
(A)
Compound
Structure
CezBi
tetragonal tll2 LazSb
a = 4.5911,
c = 18.1539
3
CegBi3
hexagonal hP16 MngSis
a = 9.5313,
c = 6.5871
3
CeqBia
cubic cl28 anti-ThaPd
a = 9.6736
CeBi
cubic cF8 NaCl
a = 6.5055, a = 6.5066,
Ce-rich, 700 “C Bi-rich, 700 “C
CeBiz
triclinic aP? LaBi
a = 6.5280, (Y = 91.515”
b = 13.0564, 0 = 103.02”,
3, 5, 6 3,738 c = 11.8516 y = 92.20”
3
Results The data obtained are given in Table 2 and in the lower part of Fig. 1. We note two discontinuities in the plot of AH,,,,, at the compositions 37.5 TABLE
2
Heats of formation xBi = Cel_,Bi,
of cerium-bismuth
alloys
in the solid state at 300 K; (1 -x)Ce
Analytical composition (xni + 0.003)
AH measured (kcal (g atom))l
1 2a 3 4 5 6a
0.22 0.30 0.35 0.375 0.40 0.42
7 8 9a 10a 11 12 13 14 15a 16
0.425 0.45 0.475 0.48 0.50 0.52 0.55 0.60 0.65 0.70
-12.9 (-19.4) -20.7 -21.9 -23.0 (-28.3) - 23.8 -25.2 (-24.1) (-25.3) -28.0 -25.7r, -25.1 -22.6 (-23.7) -16.0
Alloy
number
aFor these alloys, either the reaction have been observed.
in the calorimeter
was not complete
+
50.5) -
or side reactions
34
Fig. 1. Ce-Bi alloys. Heat of formation of the alloys in the solid state and sketch of the possible form of the phase diagram. 0, experimental points; (O), for these alloys the reaction in the calorimeter was not complete; [O] , in the course of the calorimetric synthesis of these alloys, side reactions were observed.
and 50 at.% Bi. In the upper part of the figure a conjectural picture of the shape of the phase diagram is shown. This has been sketched on the basis of suggestions by Gschneidner [4] and by analogy with the Nd-Bi [9] system. From the reported data the following values for the heat of formation are obtained (kcal (g atom)-‘, reaction in the solid state at 300 K): CesBi, -19.5 + 0.5; CesBis, -21.9 f 0.5; Ce,Bia, -24.2 + 0.5; CeBi, -27.4 f 0.5; CeBis, ‘? -18.5. Only a limiting value has been given for CeBip because of the difficulty of obtaining samples at equilibrium in this composition range.
35
Conclusions The data obtained are compared in Fig. 2 with those for the other light rare earth bismuthides [ 10 - 121. A comparison of the trend of AH,,, with the phase diagram is possible only for the Nd-Bi system. The other RBi phase diagrams so far determined are not completely reliable, but the other systems are expected to be similar, as has been suggested in Fig. 1 for the Ce-Bi diagram. As shown in Fig. 2, the curves of AH,,, versus composition have a similar shape for the various R-Bi systems, although the Pr-Bi alloys seem to be significantly less exothermic than those of the other light rare earths. R
2
‘0
n
20__3 ia
...&..
Nd -c--
Fig. 2. Summary of the trends of AH form for the light rare earth bismuthides. The points represent the values of AH form for the compounds so far examined, the existence of which has been independently confirmed by X-ray diffraction measurements.
As far as the Ce-Bi alloys are concerned, we observe that the value of previously reported for CeBiz must be considered a limiting value. This appears to be in agreement with the results of Bayanov [ 131. In his paper, thermochemical data have been given for the most B&rich compounds in the La-Bi and Ce-Bi systems. The reported heats of formation of the RBiX (RBia) compounds are around 66 kcal mol-’ for the reaction at 900 K, or, considering only the correction for the heat of fusion of Bi, around 60 - 61 keal mol-’ (about 20 kcal (g atom)-1 ) at room temperature. This value is in good agreement with the value of AHform previously measured for NdBia (-21 of:0.5 kcal (g atom)-l).
AH,,,
36
Acknowledgments The financial help obtained from the Italian Consiglio Nazionale delle Ricerche is acknowledged with thanks. The authors also wish to thank Mr. B. Brusasco, Mr. M. Cappai and Mr. A. Cavanna for their assistance.
References
10 11 12 13
R. Capelli, R. Ferro and A. Borsese, Thermochim. Acta, 10 (1974) 13. R. Vogel, Z. Anorg. Chem., 84 (1914) 327. K. Yoshihara, J. B. Taylor, L. D. Calvert and J. G. Despault, J. Less-Common Met., 41 (1975) 329. K. A. Gschneidner, Jr. and M. E. Verkade, Publ. No. 15RIC-7, Iowa State University, 1974. R. J. Gambino, J. Less-Common Met., 12 (1967) 344. D. Hohnke and E. Parthe, Acta Crystallogr., 21 (1966) 435. A. Iandelli, Atti Accad. Naz. Lincei Cl. Sci. Fis. Mat. Nat. Rend., 37 (1964) 160. G. L. Glcese, Atti Accad. Naz. Lincei Cl. Sci. Fis. Mat. Nat. Rend., 40 (1966) 629. G. F. Kobzenko and V. B. Chernogorenko, Doki. Akad. Nauk. Ukr. SSR, Ser. A, 32 (1970) 945. A. Borsese, R. Capelli, S. Delfino and R. Ferro, Thermochim. Acta, 8 (1974) 393. A. Borsese, R. Capelfi, S. Delfino and R. Ferro, Thermochim. Acta, 9 (1974) 313. A. Borsese, R. Ferro, R. Capelli and S. Delfino, Thermochim. Acta, 11 (1975) 205. A. P. Bayanov, Russ. Chem. Rev., 44 (1975) 122.