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Heats of formation of dysprosium-bismuth alloys
115
Journal of the Less-Common Metals, 55 (1977) 115 - 120 0 Elsevier Sequoia S.A., Lausanne -Printed in the Netherlands
HEATS
OF FORMATION
A. BORSESE, G. BORZONE, Istituto
OF DYSPROSIUM-BISMUTH
ALLOYS
R. FERRO and S. DELFINO
di Chimica Generale ed lnorganica
dell’Uniuersitd
di Genova,
Genoa (Italy)
(Received February 2,1977)
Summary
Heats of formation in the Dy-Bi system have been measured using a small furnace isoperibol calorimeter. The composition and states of the alloys were checked by chemical, metallographic and X-ray analyses. For the Dy-Bi compounds the following values were obtained for reaction in the solid state at 300 K : Dy, +%Bis, AH = -19.5 + 0.5 kcal (g atom)-1 (3~= 0); DyBi, AH = -23.0 + 0.5 kcal (g atom))‘. The results obtained are compared with those of other rare earth bismuthides and are discussed briefly.
Introduction
In the course of our systematic measurement of the heats of formation of binary alloys of the rare earths, the examination was recently performed of the compounds of the Group V elements. The following systems have been already examined: Y-Bi [l] , Bi-Nd [2], Bi-La [3], Bi-Pr [4], Sb-Nd [ 51 and Sb-Y [13] ; the heats of formation of Dy-Bi alloys have now been measured. The Dy-Bi system The complete phase diagram has not yet been determined. The sketch shown in Fig. 1 is based on suggestions by Gambino [6]. As in almost all the R-Bi systems, the maximum of the melting temperatures corresponds to the 1:l compound (m.p. about 2000 “C). According to what is known from X-ray diffraction data the existing phases are Dy,+,Bis (orthorhombic, Y,Bis type; a = 8.160s, b = 9.421,, c = 11.9341) and DyBi (NaCl type; a = 6.24g1 (Dy rich); a = 6.250s (Bi rich); alloys annealed at 700 “C) [12]. Experimental
Preparation of the alloys and calorimetric measurements The nominal purities of the metals used were Dy 99.9% and Bi 99.999%.
116
Dy 10
20 30
at %
Bi
70
80
90
Bi
Fig. 1. Heats of formation and phase diagram for Dy-Bi alloys (sketched according to Gambino [6], taking into account the results of the Al3 measurements and the crystal data of Yoshihara et al. [ 12 ] ).
Reference samples were prepared by melting mixtures of the two metals, enclosed under argon in Ta crucibles, in an induction furnace. These samples were then annealed. The calorimetric measurements were carried out using isoperibol small furnace calorimeters [ 7,8] inside which fine mixtures of the two elements were heated until the reaction commenced (the reaction temperature was about 600 “C, as for Y-Bi, in contrast to the tempe~ture of 400 “C needed for the bismuthides of the light rare earths). The heat evolved in the reaction was evaluated by following the temperature of the external surface of the calorimeter by means of a multijunction thermopile. The electrical energy used (both in the reaction and in subsequent calibration runs) was measured by using a standard watt-hour meter. The alloys prepared in the calorimeter were then subjected to the examinations described below, in order to check that they had reached equilibrium. The results were compared with those obtained for the reference samples.
117
Chemical
analysis
After dissolution in aqua regia the separation of the two elements was effected by precipitation of Bi as sulphide; Bi was then determined with 3hydroxyquinoline and Dy with oxalic acid. Metallographic
examination
Metallography was performed on the largest section of the specimen using standard techniques. Typical photomicrographs of some of the alloys prepared in the calorimeter are shown in Figs. 2 and 3.
Fig. 2. Alloy 1, 20 at.% Bi (265X).
X-ray examination
The X-ray examination was carried out on the various samples using the Debye method (powders prepared under argon) with Cu K, radiation.
Results The experimental data are reported in Table 1 and summarized in Fig. 1 All the values can be considered as measured at 300 K, because the sample inside the calorimeter is surrounded by a thermostat at 27.0 f 0.01 “C and during the measurements it cools down to this temperature. The error of f 0.5 kcal (g atom))1 ascribed to all measurements is considered to include instrumental effects and uncertainties caused by small variations in composition or by the eventual quenching of disorder etc. from a certain temperature higher than 300 K. Owing either to the large grain size of the powders or to inadequate initial heating, not all the samples prepared reacted completely inside the calorimeter. In Table 1 are reported the alloys for which the reaction reached equilibrium; four alloys are also reported for which only a partial reaction of the mixture was observed. These data may be useful in defining limiting values of AH. The behaviour of alloys 2 and 4, which gave X-ray diffraction
118 TABLE 1 Heats of formation of solid dysprosium-bismuth Dyl -$i, Alloy number
Analytical (or nominal) composition xBi
Gl il $1 61 9 10 11 12 13 14
alloys at 300 K: (1 - x)Dy + xBi =
(0.20) (0.30) (0.30) 0.316 0.338 0.375 (0.40) 0.423 (0.43) (0.47) (0.52) 0.580 (0.65) (0.75)
~fcnm
(kcal (g atom)-1
+ 0.5)
- 9.45 i-131
-15.0, i-13.7 I -18.15 [-11.41 -l9.82 [-18.41 -20.93 -22.59 -21.6 -18.0 -15.49 -11.33
For the alloys 2,4,6 and 8 the reaction in the calorimeter was not complete. However, the data obtained may be useful as limiting values.
patterns corresponding to the DyBi compound, may be useful in evaluating the extension of the peritectic line. By extrapolation from the reported data we obtain the following most probable values of AH,,,, for the solid state reaction at 300 K : Dy5+%Big, AH = -19.5 * 0.5 kcal (g atom))’ (LX= 0); DyBi, AH = -23.0 k 0.5 kcal (g atom))l. These values should be correct even if other intermediate phases exist. During this investigation, no indications of the existence of other compounds were observed. We must, however, remember that the R-Bi systems are generally characterized by the presence of a number of incongruently melting phases which are often formed with extreme difficulty and sluggishness (the values of AH,,,, are generally very close to those of the proper mixtures of the more easily formed compounds).
Discussion First of all it may be useful to make a comparison of the experimental data with those calculated from the model suggested by Miedema [9] . In Fig. 4 are reported the experimental values of the heat of formation of the Y-Bi and Dy-Bi alloys (the similarity shown by the alloys of Dy and Y is well known).
119
In the same figure is reported the curve of AHform uersus composition, calculated by using the relationship suggested by Miedema [Q] as a first approximation : AH = ~(c){-~e(A~*)2
+ @(a~)~
-
R)23
The following values of the constants originally indicated have been used : P = 0.85 V-l, Q/P = 0.175 (eV)2 (density units)2, R/P = 1.36 (eV)2; 23 is, of course, the conversion factor between eV atom-’ and kcal (g atom))‘. For Bi we have assumed the elementary constants @ai = 4.3, nsi = 1.2 (by analogy with results obtained for other bismuthides) and for the rare earths nny = ny = 1.4 and Qtny= rgy = 3 (Miedema has indicated Gy = 2.95; the accepted expe~men~l values of the work function [lo] are 3.1 and 3.09 for Y and Dy , respectively). We note in Fig. 4 the good agreement between the experimental and calculated values,
Fig. 4. Comparison between observed and calculated (according to Miedema [S]) heats of formation for Y-Bi and Dy-Bi alloys.
As the values of the heats of formation of some other bismuthides of the rare earths are already known, it may be useful to summarise the data now available. This is done in Fig. 5, where for the compounds RBi and R4Bi3 the trend of AH,,,,, in agreement with that suggested by Gschneidner [ II] , is related to the trend of the volume ratio relative to La (V, BI,/VR)/ (VI,, Bi fV,,). We note that the volume ratio increases slightly with atomic numft;er?The data so far known for A&,,, seem to lie on a straight line of similar slope.
Fig. 5. Comparison between the heats of formation and the volume ratios (relative to La, computed according to Gschneidner [ 111) for rare earth bismuthides.
References 1 2 3 4 6 6 7 8 9 10 11 12 13
R. Ferro, A. Borsese, R. Capelli and S. Delfino, Thermochim. Acta, 8 (1974) 387. A. Borsese, R. Capelli, S. Delfino and R. Ferro, Thermochim. Acta, 8 (1974) 393. A. Borsese, R. Capelli, 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. Borsese, R. Ferro, R, Capelli and S. Delfino, J. Less-Common Met., 55 (1977) 77. R. J. Gambino, J. Less-Common Met., 12 (1967) 344. R. Ferro, R. Capelli and A. Borsese, submitted for publication. R. Capelli, R. Ferro and A. Borsese, ~ermochim. Acta, 10 (1974) 13. A. R. Miedema, J, Less-Common Met., 32 (1973) 117. R. Miedema, R. Room and F. R. de Boer, J. Less-Common Met., 41(1975) 283. V. S. Fomenko, in G. V. Samsonov (ed.), Handbook of Thermoionic Properties, Plenum Press, New York, 1966. K. A. Gschneidner, Jr., J. Less-Common Met., 17 (1969) 1. K. Yoshihara, J. B. Taylor, L. D. Calvert and J. G. Despault, J. Less-Common Met., 41(1975) 329. A. Borsese, G. Borzone, A. Saccone and R. Ferro, J. Less-Common Met., 52 (1976) 123.
Journal of the Less-Common Metals, 55 (1977) 115 - 120 0 Elsevier Sequoia S.A., Lausanne -Printed in the Netherlands
HEATS
OF FORMATION
A. BORSESE, G. BORZONE, Istituto
OF DYSPROSIUM-BISMUTH
ALLOYS
R. FERRO and S. DELFINO
di Chimica Generale ed lnorganica
dell’Uniuersitd
di Genova,
Genoa (Italy)
(Received February 2,1977)
Summary
Heats of formation in the Dy-Bi system have been measured using a small furnace isoperibol calorimeter. The composition and states of the alloys were checked by chemical, metallographic and X-ray analyses. For the Dy-Bi compounds the following values were obtained for reaction in the solid state at 300 K : Dy, +%Bis, AH = -19.5 + 0.5 kcal (g atom)-1 (3~= 0); DyBi, AH = -23.0 + 0.5 kcal (g atom))‘. The results obtained are compared with those of other rare earth bismuthides and are discussed briefly.
Introduction
In the course of our systematic measurement of the heats of formation of binary alloys of the rare earths, the examination was recently performed of the compounds of the Group V elements. The following systems have been already examined: Y-Bi [l] , Bi-Nd [2], Bi-La [3], Bi-Pr [4], Sb-Nd [ 51 and Sb-Y [13] ; the heats of formation of Dy-Bi alloys have now been measured. The Dy-Bi system The complete phase diagram has not yet been determined. The sketch shown in Fig. 1 is based on suggestions by Gambino [6]. As in almost all the R-Bi systems, the maximum of the melting temperatures corresponds to the 1:l compound (m.p. about 2000 “C). According to what is known from X-ray diffraction data the existing phases are Dy,+,Bis (orthorhombic, Y,Bis type; a = 8.160s, b = 9.421,, c = 11.9341) and DyBi (NaCl type; a = 6.24g1 (Dy rich); a = 6.250s (Bi rich); alloys annealed at 700 “C) [12]. Experimental
Preparation of the alloys and calorimetric measurements The nominal purities of the metals used were Dy 99.9% and Bi 99.999%.
116
Dy 10
20 30
at %
Bi
70
80
90
Bi
Fig. 1. Heats of formation and phase diagram for Dy-Bi alloys (sketched according to Gambino [6], taking into account the results of the Al3 measurements and the crystal data of Yoshihara et al. [ 12 ] ).
Reference samples were prepared by melting mixtures of the two metals, enclosed under argon in Ta crucibles, in an induction furnace. These samples were then annealed. The calorimetric measurements were carried out using isoperibol small furnace calorimeters [ 7,8] inside which fine mixtures of the two elements were heated until the reaction commenced (the reaction temperature was about 600 “C, as for Y-Bi, in contrast to the tempe~ture of 400 “C needed for the bismuthides of the light rare earths). The heat evolved in the reaction was evaluated by following the temperature of the external surface of the calorimeter by means of a multijunction thermopile. The electrical energy used (both in the reaction and in subsequent calibration runs) was measured by using a standard watt-hour meter. The alloys prepared in the calorimeter were then subjected to the examinations described below, in order to check that they had reached equilibrium. The results were compared with those obtained for the reference samples.
117
Chemical
analysis
After dissolution in aqua regia the separation of the two elements was effected by precipitation of Bi as sulphide; Bi was then determined with 3hydroxyquinoline and Dy with oxalic acid. Metallographic
examination
Metallography was performed on the largest section of the specimen using standard techniques. Typical photomicrographs of some of the alloys prepared in the calorimeter are shown in Figs. 2 and 3.
Fig. 2. Alloy 1, 20 at.% Bi (265X).
X-ray examination
The X-ray examination was carried out on the various samples using the Debye method (powders prepared under argon) with Cu K, radiation.
Results The experimental data are reported in Table 1 and summarized in Fig. 1 All the values can be considered as measured at 300 K, because the sample inside the calorimeter is surrounded by a thermostat at 27.0 f 0.01 “C and during the measurements it cools down to this temperature. The error of f 0.5 kcal (g atom))1 ascribed to all measurements is considered to include instrumental effects and uncertainties caused by small variations in composition or by the eventual quenching of disorder etc. from a certain temperature higher than 300 K. Owing either to the large grain size of the powders or to inadequate initial heating, not all the samples prepared reacted completely inside the calorimeter. In Table 1 are reported the alloys for which the reaction reached equilibrium; four alloys are also reported for which only a partial reaction of the mixture was observed. These data may be useful in defining limiting values of AH. The behaviour of alloys 2 and 4, which gave X-ray diffraction
118 TABLE 1 Heats of formation of solid dysprosium-bismuth Dyl -$i, Alloy number
Analytical (or nominal) composition xBi
Gl il $1 61 9 10 11 12 13 14
alloys at 300 K: (1 - x)Dy + xBi =
(0.20) (0.30) (0.30) 0.316 0.338 0.375 (0.40) 0.423 (0.43) (0.47) (0.52) 0.580 (0.65) (0.75)
~fcnm
(kcal (g atom)-1
+ 0.5)
- 9.45 i-131
-15.0, i-13.7 I -18.15 [-11.41 -l9.82 [-18.41 -20.93 -22.59 -21.6 -18.0 -15.49 -11.33
For the alloys 2,4,6 and 8 the reaction in the calorimeter was not complete. However, the data obtained may be useful as limiting values.
patterns corresponding to the DyBi compound, may be useful in evaluating the extension of the peritectic line. By extrapolation from the reported data we obtain the following most probable values of AH,,,, for the solid state reaction at 300 K : Dy5+%Big, AH = -19.5 * 0.5 kcal (g atom))’ (LX= 0); DyBi, AH = -23.0 k 0.5 kcal (g atom))l. These values should be correct even if other intermediate phases exist. During this investigation, no indications of the existence of other compounds were observed. We must, however, remember that the R-Bi systems are generally characterized by the presence of a number of incongruently melting phases which are often formed with extreme difficulty and sluggishness (the values of AH,,,, are generally very close to those of the proper mixtures of the more easily formed compounds).
Discussion First of all it may be useful to make a comparison of the experimental data with those calculated from the model suggested by Miedema [9] . In Fig. 4 are reported the experimental values of the heat of formation of the Y-Bi and Dy-Bi alloys (the similarity shown by the alloys of Dy and Y is well known).
119
In the same figure is reported the curve of AHform uersus composition, calculated by using the relationship suggested by Miedema [Q] as a first approximation : AH = ~(c){-~e(A~*)2
+ @(a~)~
-
R)23
The following values of the constants originally indicated have been used : P = 0.85 V-l, Q/P = 0.175 (eV)2 (density units)2, R/P = 1.36 (eV)2; 23 is, of course, the conversion factor between eV atom-’ and kcal (g atom))‘. For Bi we have assumed the elementary constants @ai = 4.3, nsi = 1.2 (by analogy with results obtained for other bismuthides) and for the rare earths nny = ny = 1.4 and Qtny= rgy = 3 (Miedema has indicated Gy = 2.95; the accepted expe~men~l values of the work function [lo] are 3.1 and 3.09 for Y and Dy , respectively). We note in Fig. 4 the good agreement between the experimental and calculated values,
Fig. 4. Comparison between observed and calculated (according to Miedema [S]) heats of formation for Y-Bi and Dy-Bi alloys.
As the values of the heats of formation of some other bismuthides of the rare earths are already known, it may be useful to summarise the data now available. This is done in Fig. 5, where for the compounds RBi and R4Bi3 the trend of AH,,,,, in agreement with that suggested by Gschneidner [ II] , is related to the trend of the volume ratio relative to La (V, BI,/VR)/ (VI,, Bi fV,,). We note that the volume ratio increases slightly with atomic numft;er?The data so far known for A&,,, seem to lie on a straight line of similar slope.
Fig. 5. Comparison between the heats of formation and the volume ratios (relative to La, computed according to Gschneidner [ 111) for rare earth bismuthides.
References 1 2 3 4 6 6 7 8 9 10 11 12 13
R. Ferro, A. Borsese, R. Capelli and S. Delfino, Thermochim. Acta, 8 (1974) 387. A. Borsese, R. Capelli, S. Delfino and R. Ferro, Thermochim. Acta, 8 (1974) 393. A. Borsese, R. Capelli, 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. Borsese, R. Ferro, R, Capelli and S. Delfino, J. Less-Common Met., 55 (1977) 77. R. J. Gambino, J. Less-Common Met., 12 (1967) 344. R. Ferro, R. Capelli and A. Borsese, submitted for publication. R. Capelli, R. Ferro and A. Borsese, ~ermochim. Acta, 10 (1974) 13. A. R. Miedema, J, Less-Common Met., 32 (1973) 117. R. Miedema, R. Room and F. R. de Boer, J. Less-Common Met., 41(1975) 283. V. S. Fomenko, in G. V. Samsonov (ed.), Handbook of Thermoionic Properties, Plenum Press, New York, 1966. K. A. Gschneidner, Jr., J. Less-Common Met., 17 (1969) 1. K. Yoshihara, J. B. Taylor, L. D. Calvert and J. G. Despault, J. Less-Common Met., 41(1975) 329. A. Borsese, G. Borzone, A. Saccone and R. Ferro, J. Less-Common Met., 52 (1976) 123.