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The BaAg system
Journal of the Less-Common
Metals, 128 (1987)
259 - 264
259
THE Ba-Ag SYSTEM G. BRUZZONE,
M. FERRETTI
and F. MERLO
Istituto di Chimica Fisica, Universitci di Genova, Corso Europa, 16132 Genova (Italy) (Received
Palazzo delle Scienze,
July 31,1986)
Summary The Ba-Ag system was studied by thermal analysis, metallography and X-ray diffraction. Four intermediate phases were identified: BaAg, and BaAg, which melt congruently at 715 “C and 770 “C respectively and BaAg and BasAg, which form peritectically at 560 “C and 360 “C respectively. Three eutectics were formed: between silver and BaAg, at 700 “C and 86.0 at.% Ag, between BaAg, and BaAg, at 655 “C and 76.5 at.% Ag and between BasAg, and barium at 340 “C and 22.0 at.% Ag. The crystal structures of BaAg, (CaCus-type), BaAg, (CeCu,-type) and BaAg (FeB-type) were confirmed. The phase BasAg, was found to be isotypic with ErsNiz. The BaAg, phase showed a homogeneity range, the maximum width extending from BaAg,, to BaAg, in the high-temperature region. The mechanism of formation for the silver-rich phases of the solid solution was studied by diffractometric investigation of a single crystal of the composition BaAg,,,, .
1. Introduction The phase diagrams of the binary systems formed by calcium, strontium and barium with copper, silver and gold have been extensively studied, and only for the Ba-Au system is no thermal information available. Nevertheless, the constitution of the Ba-Ag system, determined by Weibke [ 11, is open to discussion. The reported intermediate phases (BaAg,, Bas Ag, and Ba? Ag,) are structurally unknown, whilst in the literature three different compounds are reported: BaAg, (CaCus type) [2], BaAg, (CeCu* type) [ 31 and BaAg (FeB type) [ 41. The present work reports the thermal results of the redetermination of the Ba-Ag system. 2. Experimental details The metals used were barium (purity 99.5%, Fluka, Switzerland) and silver (purity 99.999%, Koch-Light, U.K.). The samples were prepared in 0022-5088/87/$3.50
@ Elsevier Sequoia/Printed
in The Netherlands
260
molybdenum containers, arc welded under an argon atmosphere. Differential thermal analysis was carried out using a Pt-PtlO%Rh thermocouple, with an accuracy of k5 “C. Metallographic examination was made in air only for the silver-rich samples. For barium contents higher than 40 at.%, the alloys were polished under dried paraffin oil, owing to their increasing reactivity. All the samples were examined by X-ray powder methods. In some cases single-crystal data were also obtained; the intensity collection was made using an Enraf Nonius CAD-4 four-circle diffractometer. Both powders and crystals were sealed under vacuum in glass capillaries. The values of the cell constants were obtained from powder photographs or from Guinier patterns (using KC1 as an internal standard). In the case of single crystals, the angular values of 25 diffractometer-measured reflections were used. The powder pattern intensities were calculated by means of LAZY PULVERIX [ 51. 3. Results The equilibrium phase diagram of the Ba-Ag system is shown in Fig. 1. The occurrence’of four intermediate phases is recognized: BaAg, and BaAg, which melt congruently at 715 “C and 770 “C respectively, and BaAg and BasAg, which form peritectically at 560 “C and 360 “C respectively. Three eutectics are formed: between silver metal and BaAg, at 700 “C and 86.0 at.% Ag, between BaAg, and BaAg, at 655 “C and 76.5 at.% Ag and between BasAg, and barium metal at 340 “C and 22.0 at.% Ag. weight
Percent
sdver
600
Fig. 1. The Ba-Ag phase diagram.
261
TABLE 1 Structural data for the Ba-Ag intermetallic compounds Compound
Structure type
Space group and Pearson’s symbol
Lattice parameters a b
(A) c
Reference and X-ray methoda
BwQz
Er3Ni2
R;, hR45
10.385(5)
19.332(5)
t.w.; D
BaAg
FeB
Prima, oP8
8.657(3)
4.982(3)
6.651(3)
[4]; C
BaAg2
CeCu2
Imma, 0112
4.958 4.952(l)
8.063 8.058(3)
8.453 &X447(2)
t31; G t.w.; P
BaAgs
CaCu,
PG/mmm, hP6
5.720 5.803(l)
4.645 4.612(l)
t.w.; G
%.w. = this work; D = single crystal diffractometer nier method; C = single crystal photographic data.
[2l;P
values; P = powder method; G = Gui-
The crystallographic data obtained for the four compounds are reported in Table 1, together with the literature data. The crystal structure of BaAg, (CeCuz type) and BaAg (FeB type) were confirmed. The hitherto unknown Ba3Ag, phase was assumed to be isotypic with ErsNiZ [6] on the basis of geometrical single-crystal data (unit cell parameters and systematic extinctions); the powder intensity calculation confirmed the proposed structure. As several CaCu,-type phases are known to form extended solid solubility ranges, a lot of effort was dedicated to the part of the diagram around the BaAg, composition. A series of samples with nominal composition ranging from BaAg,, to BaAg, were prepared in iron crucibles, melted and quenched in water. After micrographic and X-ray examinations, the samples were annealed at 500 “C for two months and then quenched. The values of the cell constants, axial ratios and volumes per unit cell for quenched and annealed samples are graphically reported as a function of the composition in Fig. 2. The maximum extent of the homogeneity range occurs in the high-temperature region, and includes BaAg,, to BaAg,. At 500 “C the homogeneity field is restricted to the range from BaAg,., to BaAg,. The crystallographic parameters show a large range of values when going from the barium-richest sample to the silver-richest sample: 5.826 f a < 5.681 A; 4.598 < c < 4.698 A; 0.789 < c/a < 0.827; 135.2 > V> 131.3 A3. As found in other systems, the CaCus-type structure also allows strong stoichiometric deviations from the ideal 1:5 composition. In order to define the crystallographic mechanism of formation of the solid solution, a singlecrystal study was carried out on a sample of composition BaisAgs,. Intensities were collected using graphite-monochromated MO Ko radiation. The data were corrected for Lorentz and polarization factors; the absorption effects were accounted for by the semiempirical correction based on the azimuthal scan data of four top reflections [7]. Full-matrix least-squares refinement based on the F were made with SHELX-76 [8], taking atomic
262
a
582__
__136'
--4.66
4.66
V
5.78__
__134
--462
(164__130
--136
V
__I34
5.74__
0.84--
=/, --cl8
OK?--
570__
%
4.58
a
wO__
01)0--
__132
V
-4.62
V c
c
5.78__
__132
5.74__
a
582__
130
0.82-0.80--
% * 5.66 _
I
I
4
3
I 5
I 6
0.78--
0.?8--
I 7
566
I 4
3
n mlLbA&))
(a)
I
I
I
I
5
6
7
n hnacq,)
tbf
Fig. 2. The variation of lattice parameters, cell volume and axial ratio values with composition in the homogeneity region around BaAgs: (a) quenched samples; (b) annealed samples (500 “C). The full symbols refer to single-crystal data of BaAg,,.
dispersion corrections from International data collection and reduction, the final positional and thermal parameters and the agreement index are given in Table 2. The anisotropic refinement was limited to the barium atoms and to the silver position with a full occupancy factor, The atomic content per unit to the formula BaAg6.,4(4j. cell resulted in Ba0.s0(1jAg5.40(3), corresponding The measured density (8.74 Mg me3) agrees well with the calculated value (8.75 Mg mP3), These single-crystal results appear closely similar to those of YbCu,., , obtained using powder intensity data [lo]. scattering
factors
and anomalous
Tables [9 1. Details on the intensity
TABLE 2 Positional and thermal parameters of BaAg6.74 Atom
Position
Occupancy
-Q(l) A#) Ag(3) Ag(4) Ba
61 3g 2e 2~ la
17.6(3) 100 19.9(4) 47.2(8) 80.1(5)
(%)
x
Y
z
u (A21
0.2905(6) 112 0 l/3 0
2x 0 0 213 0
0 112 0.2921(12) 0 0
0.0121(6) 0.0153(3)8 0.0181(10) 0.0127(5) 0.0204(4)a
a = 5.685(l) a, c = 4.695(l) A, space group PGlmmm, dcale = 8.75 Mg rnB3, dabs = 8.74 Mg mW3.Crystal size is 0.01 mm x 0.09 mm X 0.10 mm, /.I(MoK~) = 25.0 mm-‘, scan mode w - 8, 0 range 2 < 6 Q 35”, 1255 total reflections, 146 independent reflections, 143 reflectionswith F, > 20(F,), weights w = l/{02(F0) + O.O0025F,*}, Ranis = 0.038. The thermal parameters are defined by T = expi-8n2U(sin 8/k)*}. avalues correspond to equivalent U’s derived from the anisotropic parameters by the The estimated standard deviations in parentheses expression U,, = 1/3CiCjUi*CJi* Uj*-UiUj. refer to the last significant figure.
263
Concerning the barium-rich side of the homogeneity range, the preliminary single-crystal photographic patterns appear of poor quality, with diffuse spots; this can be ascribed to the occurrence of disorder. This prevented a complete structural analysis.
4. Discussion The comparison between the thermal results presented in this paper and the previous data, reported in Hansen’s compilation [ll], shows some general agreement on the form of the liquidus curve and the positions of the eutectics. However, to improve the agreement the two phases “Ba*Ag,” and “BaJAgS” would have to be replaced by BaAg,, and the “BaAg,” phase would have to be replaced by a homogeneity range around BaAg,. The intermediate concentration region in the previous diagram is considerably modified, and interpreted according to the occurrence of BaAg and Ba3Ag,. shows that the silver-rich phases The structural analysis of BaAg,,, with the CaCus type cell, form as already observed for YbCub.s [lo]. The stoichiometric change corresponds to the occupation of barium vacancies by pairs of silver atoms, and it is accompanied by a relaxation of the silver atoms at z = 0 towards the region of the substitution. Arrhenius et al. [12] also found a wide homogeneity range for the BaAuS compound, with crystallographic parameters (cell constants, volume and axial ratio values) changing with composition in the same manner as shown in Fig. 2. This suggests the same atomic substitution and relaxation mechanism for both gold and silver 1: 5 phases. The Ba-Ag and Ba-Au systems show a nearly complete crystallographic similarity. Four of the hitherto known Ba-Au phases, BaAu, [2], BaAu [ 131 and BasAu* [14] are isotypic with the corresponding silver compounds, and BaAuz [15] crystallizes in the AIBz-type structure, which is closely related to the CeCuz type. The comparison between the experimental unit cell volumes and the stoichiometric sums of the elemental atomic volumes shows that phase formation is accompanied by large volume contractions. The values of AV/V are 10.6 (Ba3Agz), 10.9 (BaAg), 13.6 (BaAg,), 9.5 (BaAg,); 17.8 (BasAu*), 18.4 (BaAu), 15.4 (BaAu*), 13.9 (BaAu,). The higher values found for the gold phases are clearly related to the higher electronegativity difference. Moreover, in the Ba-Ag system the greatest volume contraction occurs for BaAg, while in the Ba-Au phases AV/V is largest for BaAu and Ba3Au,. This means that a relatively higher thermal stability of the barium-rich compounds may be expected in the Ba-Au system. This would confirm an empirical rule, already observed [16]: the phase with the highest melting point shows an increasing barium content if the position of the partner element changes from the top to the bottom in the same group of the periodic table.
264
References 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
F. Weibke, 2. anorg. allg. Chem., 193 (1930) 297. T. Heumann, Nachr. Akad. Wiss. GBttingen, II, A (1950) 1. W. Kiister and J. Meixner, 2. Metallkd., 56 (1965) 695. F. Merlo and M. L. Fornasini, Acta Crystallogr. Sect. B, 37 (1981) 500. K. Yvon, W. Jeitschko and E. Parthe, J. Appl. Crystallogr., 10 (1977) 73. J. M. Moreau, D. Paccard and D. Gignoux, Acta Crystallogr. Sect. B, 30 (1974) 2122. A. C. T. North, D. C. Phillips and F. S. Mathews, Acta Crystallogr. A, 24 (1968) 351. G. M. Sheldrick, SHELX 76: A program for crystal structure determination, University of Cambridge, 1976. J. A. Ibers and W. C. Hamilton (eds.), International Tables for X-ray Crystallography, Vol. 4, Kynoch, Birmingham, 1974, p. 71. J. Hornstra and K. H. J. Buschow, J. Less-Common Met., 27 (1972) 123. M. Hansen and K. Anderko, Constitution of binary alloys, McGraw-Hill, New York, 1965. G. Arrhenius, Ch. J. Raub, D. C. Hamilton and B. T. Matthias, Phys. Rev. Lett., I I (1963) 313. M. L. Fornasini, J. Solid State Chem., 59 (1985) 60. M. L. Fornasini, F. Merlo and M. Pani, Rev. Chim. Miner., 22 (1985) 791. G. Bruzzone, Rend. Accad. Nae. Lincei, 48 (1970) 235. G. Bruzzone and F. Merlo, J. Less-Common Met., 85 (1982) 285.
Metals, 128 (1987)
259 - 264
259
THE Ba-Ag SYSTEM G. BRUZZONE,
M. FERRETTI
and F. MERLO
Istituto di Chimica Fisica, Universitci di Genova, Corso Europa, 16132 Genova (Italy) (Received
Palazzo delle Scienze,
July 31,1986)
Summary The Ba-Ag system was studied by thermal analysis, metallography and X-ray diffraction. Four intermediate phases were identified: BaAg, and BaAg, which melt congruently at 715 “C and 770 “C respectively and BaAg and BasAg, which form peritectically at 560 “C and 360 “C respectively. Three eutectics were formed: between silver and BaAg, at 700 “C and 86.0 at.% Ag, between BaAg, and BaAg, at 655 “C and 76.5 at.% Ag and between BasAg, and barium at 340 “C and 22.0 at.% Ag. The crystal structures of BaAg, (CaCus-type), BaAg, (CeCu,-type) and BaAg (FeB-type) were confirmed. The phase BasAg, was found to be isotypic with ErsNiz. The BaAg, phase showed a homogeneity range, the maximum width extending from BaAg,, to BaAg, in the high-temperature region. The mechanism of formation for the silver-rich phases of the solid solution was studied by diffractometric investigation of a single crystal of the composition BaAg,,,, .
1. Introduction The phase diagrams of the binary systems formed by calcium, strontium and barium with copper, silver and gold have been extensively studied, and only for the Ba-Au system is no thermal information available. Nevertheless, the constitution of the Ba-Ag system, determined by Weibke [ 11, is open to discussion. The reported intermediate phases (BaAg,, Bas Ag, and Ba? Ag,) are structurally unknown, whilst in the literature three different compounds are reported: BaAg, (CaCus type) [2], BaAg, (CeCu* type) [ 31 and BaAg (FeB type) [ 41. The present work reports the thermal results of the redetermination of the Ba-Ag system. 2. Experimental details The metals used were barium (purity 99.5%, Fluka, Switzerland) and silver (purity 99.999%, Koch-Light, U.K.). The samples were prepared in 0022-5088/87/$3.50
@ Elsevier Sequoia/Printed
in The Netherlands
260
molybdenum containers, arc welded under an argon atmosphere. Differential thermal analysis was carried out using a Pt-PtlO%Rh thermocouple, with an accuracy of k5 “C. Metallographic examination was made in air only for the silver-rich samples. For barium contents higher than 40 at.%, the alloys were polished under dried paraffin oil, owing to their increasing reactivity. All the samples were examined by X-ray powder methods. In some cases single-crystal data were also obtained; the intensity collection was made using an Enraf Nonius CAD-4 four-circle diffractometer. Both powders and crystals were sealed under vacuum in glass capillaries. The values of the cell constants were obtained from powder photographs or from Guinier patterns (using KC1 as an internal standard). In the case of single crystals, the angular values of 25 diffractometer-measured reflections were used. The powder pattern intensities were calculated by means of LAZY PULVERIX [ 51. 3. Results The equilibrium phase diagram of the Ba-Ag system is shown in Fig. 1. The occurrence’of four intermediate phases is recognized: BaAg, and BaAg, which melt congruently at 715 “C and 770 “C respectively, and BaAg and BasAg, which form peritectically at 560 “C and 360 “C respectively. Three eutectics are formed: between silver metal and BaAg, at 700 “C and 86.0 at.% Ag, between BaAg, and BaAg, at 655 “C and 76.5 at.% Ag and between BasAg, and barium metal at 340 “C and 22.0 at.% Ag. weight
Percent
sdver
600
Fig. 1. The Ba-Ag phase diagram.
261
TABLE 1 Structural data for the Ba-Ag intermetallic compounds Compound
Structure type
Space group and Pearson’s symbol
Lattice parameters a b
(A) c
Reference and X-ray methoda
BwQz
Er3Ni2
R;, hR45
10.385(5)
19.332(5)
t.w.; D
BaAg
FeB
Prima, oP8
8.657(3)
4.982(3)
6.651(3)
[4]; C
BaAg2
CeCu2
Imma, 0112
4.958 4.952(l)
8.063 8.058(3)
8.453 &X447(2)
t31; G t.w.; P
BaAgs
CaCu,
PG/mmm, hP6
5.720 5.803(l)
4.645 4.612(l)
t.w.; G
%.w. = this work; D = single crystal diffractometer nier method; C = single crystal photographic data.
[2l;P
values; P = powder method; G = Gui-
The crystallographic data obtained for the four compounds are reported in Table 1, together with the literature data. The crystal structure of BaAg, (CeCuz type) and BaAg (FeB type) were confirmed. The hitherto unknown Ba3Ag, phase was assumed to be isotypic with ErsNiZ [6] on the basis of geometrical single-crystal data (unit cell parameters and systematic extinctions); the powder intensity calculation confirmed the proposed structure. As several CaCu,-type phases are known to form extended solid solubility ranges, a lot of effort was dedicated to the part of the diagram around the BaAg, composition. A series of samples with nominal composition ranging from BaAg,, to BaAg, were prepared in iron crucibles, melted and quenched in water. After micrographic and X-ray examinations, the samples were annealed at 500 “C for two months and then quenched. The values of the cell constants, axial ratios and volumes per unit cell for quenched and annealed samples are graphically reported as a function of the composition in Fig. 2. The maximum extent of the homogeneity range occurs in the high-temperature region, and includes BaAg,, to BaAg,. At 500 “C the homogeneity field is restricted to the range from BaAg,., to BaAg,. The crystallographic parameters show a large range of values when going from the barium-richest sample to the silver-richest sample: 5.826 f a < 5.681 A; 4.598 < c < 4.698 A; 0.789 < c/a < 0.827; 135.2 > V> 131.3 A3. As found in other systems, the CaCus-type structure also allows strong stoichiometric deviations from the ideal 1:5 composition. In order to define the crystallographic mechanism of formation of the solid solution, a singlecrystal study was carried out on a sample of composition BaisAgs,. Intensities were collected using graphite-monochromated MO Ko radiation. The data were corrected for Lorentz and polarization factors; the absorption effects were accounted for by the semiempirical correction based on the azimuthal scan data of four top reflections [7]. Full-matrix least-squares refinement based on the F were made with SHELX-76 [8], taking atomic
262
a
582__
__136'
--4.66
4.66
V
5.78__
__134
--462
(164__130
--136
V
__I34
5.74__
0.84--
=/, --cl8
OK?--
570__
%
4.58
a
wO__
01)0--
__132
V
-4.62
V c
c
5.78__
__132
5.74__
a
582__
130
0.82-0.80--
% * 5.66 _
I
I
4
3
I 5
I 6
0.78--
0.?8--
I 7
566
I 4
3
n mlLbA&))
(a)
I
I
I
I
5
6
7
n hnacq,)
tbf
Fig. 2. The variation of lattice parameters, cell volume and axial ratio values with composition in the homogeneity region around BaAgs: (a) quenched samples; (b) annealed samples (500 “C). The full symbols refer to single-crystal data of BaAg,,.
dispersion corrections from International data collection and reduction, the final positional and thermal parameters and the agreement index are given in Table 2. The anisotropic refinement was limited to the barium atoms and to the silver position with a full occupancy factor, The atomic content per unit to the formula BaAg6.,4(4j. cell resulted in Ba0.s0(1jAg5.40(3), corresponding The measured density (8.74 Mg me3) agrees well with the calculated value (8.75 Mg mP3), These single-crystal results appear closely similar to those of YbCu,., , obtained using powder intensity data [lo]. scattering
factors
and anomalous
Tables [9 1. Details on the intensity
TABLE 2 Positional and thermal parameters of BaAg6.74 Atom
Position
Occupancy
-Q(l) A#) Ag(3) Ag(4) Ba
61 3g 2e 2~ la
17.6(3) 100 19.9(4) 47.2(8) 80.1(5)
(%)
x
Y
z
u (A21
0.2905(6) 112 0 l/3 0
2x 0 0 213 0
0 112 0.2921(12) 0 0
0.0121(6) 0.0153(3)8 0.0181(10) 0.0127(5) 0.0204(4)a
a = 5.685(l) a, c = 4.695(l) A, space group PGlmmm, dcale = 8.75 Mg rnB3, dabs = 8.74 Mg mW3.Crystal size is 0.01 mm x 0.09 mm X 0.10 mm, /.I(MoK~) = 25.0 mm-‘, scan mode w - 8, 0 range 2 < 6 Q 35”, 1255 total reflections, 146 independent reflections, 143 reflectionswith F, > 20(F,), weights w = l/{02(F0) + O.O0025F,*}, Ranis = 0.038. The thermal parameters are defined by T = expi-8n2U(sin 8/k)*}. avalues correspond to equivalent U’s derived from the anisotropic parameters by the The estimated standard deviations in parentheses expression U,, = 1/3CiCjUi*CJi* Uj*-UiUj. refer to the last significant figure.
263
Concerning the barium-rich side of the homogeneity range, the preliminary single-crystal photographic patterns appear of poor quality, with diffuse spots; this can be ascribed to the occurrence of disorder. This prevented a complete structural analysis.
4. Discussion The comparison between the thermal results presented in this paper and the previous data, reported in Hansen’s compilation [ll], shows some general agreement on the form of the liquidus curve and the positions of the eutectics. However, to improve the agreement the two phases “Ba*Ag,” and “BaJAgS” would have to be replaced by BaAg,, and the “BaAg,” phase would have to be replaced by a homogeneity range around BaAg,. The intermediate concentration region in the previous diagram is considerably modified, and interpreted according to the occurrence of BaAg and Ba3Ag,. shows that the silver-rich phases The structural analysis of BaAg,,, with the CaCus type cell, form as already observed for YbCub.s [lo]. The stoichiometric change corresponds to the occupation of barium vacancies by pairs of silver atoms, and it is accompanied by a relaxation of the silver atoms at z = 0 towards the region of the substitution. Arrhenius et al. [12] also found a wide homogeneity range for the BaAuS compound, with crystallographic parameters (cell constants, volume and axial ratio values) changing with composition in the same manner as shown in Fig. 2. This suggests the same atomic substitution and relaxation mechanism for both gold and silver 1: 5 phases. The Ba-Ag and Ba-Au systems show a nearly complete crystallographic similarity. Four of the hitherto known Ba-Au phases, BaAu, [2], BaAu [ 131 and BasAu* [14] are isotypic with the corresponding silver compounds, and BaAuz [15] crystallizes in the AIBz-type structure, which is closely related to the CeCuz type. The comparison between the experimental unit cell volumes and the stoichiometric sums of the elemental atomic volumes shows that phase formation is accompanied by large volume contractions. The values of AV/V are 10.6 (Ba3Agz), 10.9 (BaAg), 13.6 (BaAg,), 9.5 (BaAg,); 17.8 (BasAu*), 18.4 (BaAu), 15.4 (BaAu*), 13.9 (BaAu,). The higher values found for the gold phases are clearly related to the higher electronegativity difference. Moreover, in the Ba-Ag system the greatest volume contraction occurs for BaAg, while in the Ba-Au phases AV/V is largest for BaAu and Ba3Au,. This means that a relatively higher thermal stability of the barium-rich compounds may be expected in the Ba-Au system. This would confirm an empirical rule, already observed [16]: the phase with the highest melting point shows an increasing barium content if the position of the partner element changes from the top to the bottom in the same group of the periodic table.
264
References 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
F. Weibke, 2. anorg. allg. Chem., 193 (1930) 297. T. Heumann, Nachr. Akad. Wiss. GBttingen, II, A (1950) 1. W. Kiister and J. Meixner, 2. Metallkd., 56 (1965) 695. F. Merlo and M. L. Fornasini, Acta Crystallogr. Sect. B, 37 (1981) 500. K. Yvon, W. Jeitschko and E. Parthe, J. Appl. Crystallogr., 10 (1977) 73. J. M. Moreau, D. Paccard and D. Gignoux, Acta Crystallogr. Sect. B, 30 (1974) 2122. A. C. T. North, D. C. Phillips and F. S. Mathews, Acta Crystallogr. A, 24 (1968) 351. G. M. Sheldrick, SHELX 76: A program for crystal structure determination, University of Cambridge, 1976. J. A. Ibers and W. C. Hamilton (eds.), International Tables for X-ray Crystallography, Vol. 4, Kynoch, Birmingham, 1974, p. 71. J. Hornstra and K. H. J. Buschow, J. Less-Common Met., 27 (1972) 123. M. Hansen and K. Anderko, Constitution of binary alloys, McGraw-Hill, New York, 1965. G. Arrhenius, Ch. J. Raub, D. C. Hamilton and B. T. Matthias, Phys. Rev. Lett., I I (1963) 313. M. L. Fornasini, J. Solid State Chem., 59 (1985) 60. M. L. Fornasini, F. Merlo and M. Pani, Rev. Chim. Miner., 22 (1985) 791. G. Bruzzone, Rend. Accad. Nae. Lincei, 48 (1970) 235. G. Bruzzone and F. Merlo, J. Less-Common Met., 85 (1982) 285.