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Gd–Sb system: standard enthalpies of formation of solid alloys and crystal structure of Gd16Sb39
Intermetallics 8 (2000) 189±194
Gd±Sb system: standard enthalpies of formation of solid alloys and crystal structure of Gd16Sb39 G. Borzonea, M.L. Fornasinib, N. Parodia, R. Ferroa,* a
Dipartimento di Chimica e Chimica Industriale, Sezione di Chimica Inorganica e Metallurgia, via Dodecaneso, 31, 16146 Genova, Italy b Dipartimento di Chimica e Chimica Industriale, Sezione di Chimica Fisica, via Dodecaneso, 31, 16146 Genova, Italy Received 27 July 1999; accepted 17 August 1999
Abstract The data obtained in the investigation of Gd±Sb alloys are reported. This investigation was performed using metallographic and Xray diraction analyses and calorimetric measurements of the enthalpies of formation at 300 K. The values of f H (kJ/mol of atoms 2) for the following compounds were obtained for the reaction in the solid state at 300 K: Gd5Sb3: ÿ110; Gd4Sb3: ÿ116; GdSb: ÿ120; Gd16Sb39: ÿ78. The crystal structures of Gd5Sb3 (hP16-Mn5Si3 type), Gd4Sb3 (cI28-anti-Th3P4 type) and GdSb (cF8-NaCl type) have been con®rmed. The crystal structure of the compound previously called ``GdSb2'' was determined by single-crystal methods and the composition Gd16Sb39 was found. # 2000 Elsevier Science Ltd. All rights reserved. Keywords: A. Rare earth intermetallics; B. Thermodynamic and thermochemical properties; B. Crystal chemistry of intermetallics; F. Calorimetry
1. Introduction In the framework of our research on the binary compounds of the rare earth metals with the elements of the 15th group, the results obtained in the investigation of the Gd±Sb system are here reported. Systematic thermochemical investigations of the rare earth (R=Y, La, Ce, Pr, Nd, Sm and Dy) antimonides have previously been carried out by our group [1]. An optimization of the thermodynamic data has also been performed for selected systems [2]. 2. Literature data on the Gd±Sb system 2.1. Crystal structures and phase diagram Reliable phase diagram data are available for only a few R±Sb systems. The experimental determination of these data indeed, is very dicult owing to the very high melting temperatures of the compounds, the high reactivity of the samples and the high volatility of antimony. As for the Gd±Sb system, the following compounds have been described: Gd5Sb3 (hP16-Mn5Si3 type), Gd4Sb3 * Corresponding author. Tel.:+39-010-353-6113; fax: +39-010-362-5051. E-mail address: [email protected] (R. Ferro).
(cI28-anti-Th3P4 type) and GdSb (cF8-NaCl type) [4±6]. For GdSb a solid state transformation at 1840 C was proposed. The existence of the GdSb2 phase of unknown structure was also suggested [6] (a GdSb2 oC24-LaSb2 type high pressure phase [7] was also reported). Subsequently, for the antimony-richest compound, in a preliminary communication, Altmeyer and Jeitschko [8,9] suggested a stoichiometry R2Sb5 (71.42 at% Sb); Gd2Sb5 was described as monoclinic, Dy2Sb5 type [9]. In this work, on the basis of a complete crystal structure determination relevant to the Gd±Sb phases, a slightly dierent stoichiometry has been attributed to this phase, that is, Gd16Sb39 (70.91 at% Sb). The crystal-structural data available in the literature are summarized in Table 1, together with the results obtained in this work. For the Gd±Sb system an experimental determination of the phase diagram has been reported by Abdusalyamova et al. [6]. This diagram, slightly modi®ed, however, following the results obtained during the present investigation, is shown in Fig. 1. 2.2. Gd±Sb thermodynamics Thermodynamic data for the formation functions of Gd±Sb alloys have previously been obtained by direct calorimetry [13,14], by electromotive force measurements
0966-9795/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved. PII: S0966-9795(99)00097-7
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G. Borzone et al. / Intermetallics 8 (2000) 189±194
Table 1 Crystal structure data of the Gd±Sb system Phase
Structural type
Prototype
Lattice parameters (pm and ) a
b
Ref c
a Gd b Gd Gd5Sb3
hP2 cI2 hP16
Mg W Mn5Si3
363.36 406 897.5(4) 897.0 897.4 899.0± 901.5
578.10
[3] [3] [4] [5] [6] This work
Gd4Sb3
cI28
anti-Th3P4
922.4(5) 922.4 922.8±923.2
[10] [11] This work
aGdSb
cF8
NaCl
621.7 621.8 621.0 621.3±622.0
[12] [12] [6] This work
bGdSb
±
±
Gd16Sb39
mS110
Gd16Sb39
634.3(3) 633.6 634.2 633±638
[6] 5739.5(9) 5740(2)
carried out in the 380±560 C temperature range [15,16], in the 365±630 C temperature range [17] and by vapour pressure measurements [18] within the temperature range 1850±1935 C. The results obtained in these investigations for the enthalpies of formation may be summarized by the following values of the f H (kJ/mol of atoms): Gd5Sb3: ÿ141.92.5 [15] at T=440 C; Gd4Sb3: ÿ145.13.0 [15] at T=440 C; Gd0.52Sb0.48: ÿ137.13.0 [16] at T480 C; GdSb: ÿ131.0 [18] at T=25 C; GdSb: ÿ102.56.0 [13] at T= 420 C; GdSb2: ÿ96.01.0 [17] at T=530 C. On the basis of the unlikely trend of these data vs composition and by comparison with the f H data relevant to the other rare earth antimonides, a revision seemed to be useful and worthwhile. In this paper the data obtained on the entire range of composition by using a direct calorimetry method are reported. 3. Experimental details Gadolinium (99.9 mass%) and antimony (99.99 mass% nominal purities) were used as starting materials. A number of alloys of selected compositions were prepared by melting the metals in Ta crucibles (which were sealed by welding under argon). These alloys were prepared for use as ``reference alloys'' against those prepared in the calorimeter. For the antimony-richest alloys, owing to the high volatility of Sb and its reactivity, the
415.1(1) 415.3(2)
1320.9(2) 1320.9(3)
99.21(1) 99.21(1)
This work single crystal This work powder diract.
preparation was made in Al2O3 crucibles placed inside Mo crucibles sealed under argon. The calorimetric measurements were performed using a direct isoperibolic aneroid calorimeter [19,20]. Its construction was based on the principles of an instrument described by Kubaschewski and Dench [21]. The calorimeter consists of a thick aluminium cylinder containing two small electric furnaces, which are used for starting the reaction in the sample and for electric calibration, respectively. The sample (about 10±15 g) is a mixture of ®ne metal powders enclosed in a gas-tight stainless steel crucible sealed by electric welding which is then inserted in the calorimeter and, after thermal equilibration, is heated until the reaction starts. The temperature of the calorimeter was followed by a multiple-junction thermopile (dierentially connected to the similar thermopile of another calorimeter, identical to the ®rst, used as a reference). The two calorimeters, symmetrically inserted in an aluminium block were surrounded by a water ultrathermostat at 270.01 C and the formation heats were considered measured at 300 K because the sample in the calorimeter cooled down to this temperature during the measurement. The error of 2 kJ/mol of atoms, at, generally ascribed to all measurements, is considered to include both the instrumental errors and any uncertainties due to small compositional variations or to possible quenching of disorder (or metastable situation) from a certain temperature higher than 300 K. The complete range of composition was investigated and the dierent
G. Borzone et al. / Intermetallics 8 (2000) 189±194
samples prepared in the calorimeter (also those used as ``reference alloys'') were studied by X-ray powder diffraction, optical and electron microscopy and electron probe microanalysis, in order to check whether the equilibrium state had been reached and also to check for side reactions. The X-ray analysis of the various alloys was carried out by the Debye method and by powder diractometry using Cu and Fe K radiations. Powder photographs were used both for phase analysis and identi®cation and for lattice parameter measurements. These were re®ned by a least-squares ®t using the Nelson±Riley function. The observed diraction intensities were compared with the calculated values obtained by a computer program (Lazy Pulverix [22]). In this work, moreover, single crystals of the phase Gd16Sb39 were searched for in a sample of composition 80 at% Sb. After several trials, a needle-shaped single crystal was found and intensities were collected by an
191
Enraf-Nonius CAD-4 diractometer with graphitemonochromated Mo K radiation. Lattice constants were obtained from 25 diractometer measured re¯ections in the range =20±22 . Metallographic examination was carried out after SiC paper, diamond paste polishing and after etching in air or in dilute alcoholic HNO3 solution. An example of the metallographic appearance of a 32.5 at%Sb calorimetric specimen alloy is shown in Fig. 2. Notice that this direct synthesis of the alloys carried out in the calorimeter by the same technique often referred to as `combustion synthesis', as well as being useful for the enthalpy data often succeeds in producing well crystallized and uniform samples. 4. Results 4.1. Thermochemical data The results obtained in the measurements of the heats of formation are listed in Table 2. These data were used to trace the trend of the f H vs composition (see Fig. 1) and to evaluate the most probable values of the f H for the dierent compounds. These values are listed at the bottom of Table 2. We may notice the high exothermicity of these alloys and the correspondence of the f H trend with the shape of the phase diagram. In particular the trend is characterized by a minimum at the atomic composition corresponding to the maximum melting point and by slight discontinuities for the other intermediate compounds. Similar trends in f H have been observed for other rareearth antimonide alloys. These values, moreover, ®t very well within those obtained for the isostructural rareearth compounds formed by the other rare-earth elements.
Fig. 1. The Gd±Sb system. The phase diagram [after [6] with some modi®cations (this work)] is shown together with the trend of the standard formation enthalpy of the solid alloys at room temperature (this work).
Fig. 2. Scanning electron micrograph of a Gd±Sb alloy at 32.5 at% Sb synthesized in the calorimeter during the measurement of the formation enthalpy. Etching in air. Polygonal crystals of Gd5Sb3 and Gd at the grain boundaries.
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G. Borzone et al. / Intermetallics 8 (2000) 189±194
Table 2 Heats of formation of solid Gd±Sb alloys at 300 Ka
Table 3 Crystal data of Gd16Sb39
Nominal composition at% Sb
fH (KJ/mol of atoms)
Nominal composition at% Sb
fH (KJ/mol of atoms)
28.0 32.5 36.0 38.0 40.0 44.0 49.0 51.0 52.0
ÿ82.5 ÿ94.2 ÿ106.6 ÿ106.8 ÿ111.5 ÿ114.0 ÿ117.7 ÿ112.8 ÿ112.7
52.7 54.0 55.0 57.0 62.0 65.0 69.0 76.0
ÿ116.7 ÿ115.8 ÿ118.8 ÿ108.1 ÿ98.3 ÿ89.3 ÿ72.3 ÿ58.1
a Calorimetric data for the reaction: (1ÿx) aGd(solid)+xSb(solid)= Gd(1ÿx)Sbx(solid). Interpolated values of the fH (KJ/mol of atoms 2) for the various compounds: Gd5Sb3 ÿ110; Gd4Sb3 ÿ116; GdSb: ÿ120; Gd16Sb39 ÿ78.
Considering, for instance, the equiatomic RSb compounds, we may notice small dierences between the values relevant to a R metal and its neighbouring ones within an overall trend indicating, along the lanthanide series, a progressive decrease of the exothermic heat of formation. On the basis of the data obtained by calorimetric methods, the following values of the standard enthalpies of formation may be indeed considered (kJ/mol of atoms, room temperature): LaSb: ÿ1312 [23]; CeSb: ÿ126.42 [24]; PrSb: ÿ1262 [25]; NdSb: ÿ1232 [26]; SmSb: ÿ1222 [27]; GdSb: ÿ1202 (this work); DySb: ÿ1142 [1]; ErSb: ÿ1114 [2]. As a concluding remark on the thermochemistry of these alloys, a comparison with the literature data may be noteworhty. We may underline the much more negative values reported in literature for the Gd-rich region of the system. On the basis of the comments previously reported, we believe that these values are not reliable. We may notice, however, that similar discrepancies have been observed for the R-rich regions of other R±X systems (with X=p-block element such as Al, Si, Ge) especially when enthalpy data obtained by indirect method (by emf or vapour pressure) were considered. This point has been previously discussed [28]. 4.2. Crystal-structure data The lattice parameters of the dierent phases are listed in Table 1. As for Gd16Sb39, its crystal structure has been determined in this work. The relevant data are reported in Table 3. The programs used were SIR92 [29] for the structure solution and SHELXL-97 [30] for the structure re®nement, with anisotropic displacement parameters and weights w 1= 2 F 20 0:0824P2 , where P F20 2Fc2 =3. Table 4 lists atomic coordinates, standardized by Structure Tidy [31] and equivalent isotropic displacement parameters. The
Pearson code Space group Formula units Crystal size (mm3) Scan mode Range in h,k,l Total number of re¯ections Absorption correction Highest/lowest transmission Independent re¯ections Re¯ections with F0 > 4 F0 Number of parameters wR F 2 , all data RF0 > 4 F0 Goodness of ®t, S min ; max e=A3
mS110 C2/m (No. 12) Z=2 0.0150.0200.140 o 74; 0+5; 0+17 4219 psi-scan and spherical corrections 1.77 4047 2072 173 0.183 0.076 0.942 ÿ5.8, 8.0
Table 4 Atomic coordinates and equivalent isotropic displacement parameters of Gd16Sb39 Atom Position x
y
z
Ueq (pm2)a Occ.
Gd1 Gd2 Gd3 Gd4 Gd5 Gd6 Gd7 Gd8 Sb1b Sb2 Sb3b Sb4 Sb5 Sb6 Sb7 Sb8 Sb9 Sb10 Sb11 Sb12 Sb13 Sb14 Sb15 Sb16 Sb17b Sb18 Sb19 Sb20 Sb21 Sb22 Sb23b
0 0 0 0 0 0 0 0 0.288(3) 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1/2 0
0.0826(2) 0.3565(2) 0.6216(2) 0.3473(2) 0.0261(2) 0.3039(2) 0.2792(2) 0.4352(2) 0.1285(8) 0.3404(3) 0.102(1) 0.9169(3) 0.6160(3) 0.3169(3) 0.8786(3) 0.5962(2) 0.1589(2) 0.5826(2) 0.2751(3) 0.0240(3) 0.5601(2) 0.2661(2) 0.5439(2) 0.1027(3) 0.115(1) 0.1763(3) 0.1724(3) 0.1615(2) 0.1286(2) 1/2 0
105(5) 82(5) 85(5) 90(5) 107(5) 81(5) 92(5) 98(5) 140(30) 117(7) 170(40) 139(8) 88(7) 117(7) 105(7) 82(7) 99(7) 83(7) 112(7) 132(7) 95(7) 85(7) 96(7) 139(8) 150(40) 145(8) 98(7) 102(7) 116(7) 180(10) 160(80)
a b
4i 4i 4i 4i 4i 4i 4i 4i 8j 4i 4i 4i 4i 4i 4i 4i 4i 4i 4i 4i 4i 4i 4i 4i 4i 4i 4i 4i 4i 2d 2a
0.09076(4) 0.14227(4) 0.19150(4) 0.21329(4) 0.34118(4) 0.39716(4) 0.46671(4) 0.55200(4) 0.0067(2) 0.01055(5) 0.0130(2) 0.04298(5) 0.06273(5) 0.08217(5) 0.11904(5) 0.13563(5) 0.16908(5) 0.24356(5) 0.26548(6) 0.27452(6) 0.31771(5) 0.33886(5) 0.39106(5) 0.42031(6) 0.5046(2) 0.55662(6) 0.62960(5) 0.70695(5) 0.80595(6) 0 0
0.222(9) 0.236(9)
0.26(1)
0.124(5)
Ueq is de®ned as one third of the trace of the orthogonalized Uij tensor. Isotropically re®ned.
powder diractogram of this phase was indexed by means of Lazy Pulverix [22], and lattice parameters resulted in very good agreement with those derived from a single crystal. As shown in Fig. 3 the structure of Gd16Sb39 is
G. Borzone et al. / Intermetallics 8 (2000) 189±194
193
the antimony atoms show a large variety in their environment. The most symmetrical Sb22 is surrounded by a parallelepiped of 4 Gd+4 Sb. The same kind of atoms, but in a dierent way, surround Sb6, Sb8, Sb9, Sb10, Sb14, Sb15 and Sb16, all showing the same coordination polyhedron, while for Sb21 this polyhedron is defective, lacking a Gd atom. Seven atoms (2 Sb +5 Gd) are also found around Sb7 and Sb19. With coordination number six we have Sb11 and Sb20 (3 Sb +3 Gd), Sb12 (6 Sb, in the form of a distorted octahedron), Sb5 and Sb13 (6 Gd, in the form of a trigonal prism). The Sb atoms with partial occupancy complete the coordination of four other atoms: Sb2, Sb4, Sb18 and Gd7. While Sb1, Sb3 and Sb17 enter into the environment of more atoms, Sb23, showing the smallest occupation factor, contributes to the coordination of Sb4 only. The shortest distances are Sb12±Sb20 of 282.4(4) pm and Gd4±Sb13 of 310.9(3) pm. As a ®nal comment we may remark that the structure of Gd16Sb39 shows features similar to those reported for the orthorhombic (U1/2Ho1/2)3Sb7 compound [32].
Acknowledgements
Fig. 3. Projection of the Gd16Sb39 structure along the b axis. Open and stippled circles represent atoms at y=0 and y=1/2, respectively. Large circles: Gd; small circles: Sb. The trigonal prisms forming eightand four-membered columnar units are outlined.
characterized by columns of Sb trigonal prisms running parallel to the b axis. All prisms are centered by Gd atoms and form eight- and four-membered patterns, by sharing lateral edges. Two large channels parallel to the b axis occur, where the columnar units of prisms leave room for other atoms. One of these, centered on the position x=1/4, z=0, is ®lled by Sb12, which does not participate in forming trigonal prisms and is coordinated only with Sb atoms. The other channel, centered on the position x=0, z=0 and ®lled by four atoms with statistical occupation, represents the disorder point of this structure. A third smaller channel at the centre of the eight-membered pattern is occupied by Sb22. The coordination polyhedron of the gadolinium atoms is always the same, an Sb trigonal prism tricapped on the side faces by three other Sb atoms, while
Planning and development of the studies here presented form a part of an Italian National Research Project entitled ``Leghe e composti intermetallici. StabilitaÁ termodinamica, proprietaÁ ®siche e reattivitaÁ''. The authors would like to thank the Italian Ministero dell' UniversitaÁ e della Ricerca Scienti®ca e Tecnologica (Programmi di Ricerca Scienti®ca di Rilevante Interesse Nazionale) for ®nancial support. Partial ®nancial support has also been obtained from the Italian Consiglio Nazionale delle Ricerche under the ``Progetto Finalizzato Materiali Speciali per Tecnologie Avanzate II''.
References [1] Ferro R, Borzone G, Cacciamani G. Thermochim Acta 1988;129:99±113. [2] Cacciamani G, Borzone G, Parodi N, Ferro R. Z Metallkd 1996;87:562±7. [3] Gschneidner Jr. KA, Calderwood FW. In: Gschneidner Jr. KA, Eyring L, editors. Handbook on the physics and chemistry of rare earths, vol 8. Amsterdam, The Netherlands: Elsevier, 1986. p. 1. [4] Rieger W, Parthe E. Acta Crystallogr B 1968;24B:456±8. [5] Abdusalyamova MN, Abulkhaev VD, Chechernikov VI. Inorg Mater [Trans Neorg Mater] 1981;17(12):1696±8. [6] Abdusalyamova MN, Burnashev OR, Mironov KY. J LessCommon Met 1986;125:1±6. [7] Eatough NL, Hall HT. Inorg Chem 1969;8:1439±45. [8] Altmeyer RO, Jeitschko W. Z Kristallogr 1989;186:5±7. [9] Altmeyer RO, Jeitschko W. Z Kristallogr 1988;182:3±5. [10] Holtzberg F, McGuire TR, Methfessel S, Suits JC. J Appl Phys 1964;35:1033±8.
194
G. Borzone et al. / Intermetallics 8 (2000) 189±194
[11] Gambino RJ. J Less-Common Metals 1967;12:344±52. [12] Villars P, Calvert LD. In: Pearson's handbook of crystallographic data for intermetallic phases. Materials Park OH, USA: ASM, 1991. [13] Pratt, JN, Chua KS. Thermodynamic studies of some rare-earth alloys, ®nal report min tech agreement no. 2027/048/RL. University of Birmingham, 1970. [14] Chua KS, Pratt JN. Thermochim Acta 1974;8:409±21. [15] Goryacheva VI, Messauden N, Nikol'skaya AV, Gerasimova YaI. Vestn Mosk Univ, Ser 2, Khim 1977;18:749. [16] Goryacheva VI, Nikol'skaya AV, Gerasimov YaI. Dokl Akad Nauk SSSR 1971;197:389±92. [17] Kober VI, Nichkov IF, Raspopin SP, Kuzminnykh VM. Splavy Redk Met Osobymi Fiz Svoistvami: Redkozem Blagorodn Met. 1983; 132±5 [18] Viksman GSh, Gordienko SP. Izv Akad Nauk SSSR, Neorg Mater 1984;20:1441±4. [19] Capelli R, Ferro R, Borsese A. Thermochim Acta 1974;10:13±21. [20] Ferro R, Borzone G, Parodi N, Cacciamani G. J Phase Equil 1994;15:317±29.
[21] Kubaschewski O, Dench WA. Acta Metall 1955;3:339. [22] Yvon K, Jeitschko W, Parthe E. J Appl Crystallogr 1977;10:73. [23] Borzone G, Borsese A, Saccone A, Ferro R. J Less-Common Met 1979;65:253±62. [24] Borsese A, Borzone G, Mazzone D, Ferro R. J Less-Common Met 1981;79:57±63. [25] Borzone G, Borsese A, Zanicchi G, Ferro R. J Therm Anal 1982;25:433±40. [26] Borsese A, Ferro R, Capelli R, Del®no S. J Less-Common Met 1977;55:77±83. [27] Borzone G, Borsese A, Del®no S, Ferro R. Z Metallkd 1985;76:208±13. [28] Borzone G, Raggio R, Ferro R. Phys Chem Chem Phys 1999;1:1487±500. [29] Altomare A, Cascarano G, Giacovazzo C, Guagliardi A, Burla MC, Polidori G, Camalli M. J Appl Crystallogr 1994;27:435. [30] Sheldrick GM, SHELXL-97, Program for re®nement of crystal structures. University of GoÈttingen, Germany, 1997. [31] Gelato LM, Parthe E. J Appl Crystallogr 1987;20:139. [32] Schmidt T, Jeitschko W. Z Kristallogr 1998;15:58.
Gd±Sb system: standard enthalpies of formation of solid alloys and crystal structure of Gd16Sb39 G. Borzonea, M.L. Fornasinib, N. Parodia, R. Ferroa,* a
Dipartimento di Chimica e Chimica Industriale, Sezione di Chimica Inorganica e Metallurgia, via Dodecaneso, 31, 16146 Genova, Italy b Dipartimento di Chimica e Chimica Industriale, Sezione di Chimica Fisica, via Dodecaneso, 31, 16146 Genova, Italy Received 27 July 1999; accepted 17 August 1999
Abstract The data obtained in the investigation of Gd±Sb alloys are reported. This investigation was performed using metallographic and Xray diraction analyses and calorimetric measurements of the enthalpies of formation at 300 K. The values of f H (kJ/mol of atoms 2) for the following compounds were obtained for the reaction in the solid state at 300 K: Gd5Sb3: ÿ110; Gd4Sb3: ÿ116; GdSb: ÿ120; Gd16Sb39: ÿ78. The crystal structures of Gd5Sb3 (hP16-Mn5Si3 type), Gd4Sb3 (cI28-anti-Th3P4 type) and GdSb (cF8-NaCl type) have been con®rmed. The crystal structure of the compound previously called ``GdSb2'' was determined by single-crystal methods and the composition Gd16Sb39 was found. # 2000 Elsevier Science Ltd. All rights reserved. Keywords: A. Rare earth intermetallics; B. Thermodynamic and thermochemical properties; B. Crystal chemistry of intermetallics; F. Calorimetry
1. Introduction In the framework of our research on the binary compounds of the rare earth metals with the elements of the 15th group, the results obtained in the investigation of the Gd±Sb system are here reported. Systematic thermochemical investigations of the rare earth (R=Y, La, Ce, Pr, Nd, Sm and Dy) antimonides have previously been carried out by our group [1]. An optimization of the thermodynamic data has also been performed for selected systems [2]. 2. Literature data on the Gd±Sb system 2.1. Crystal structures and phase diagram Reliable phase diagram data are available for only a few R±Sb systems. The experimental determination of these data indeed, is very dicult owing to the very high melting temperatures of the compounds, the high reactivity of the samples and the high volatility of antimony. As for the Gd±Sb system, the following compounds have been described: Gd5Sb3 (hP16-Mn5Si3 type), Gd4Sb3 * Corresponding author. Tel.:+39-010-353-6113; fax: +39-010-362-5051. E-mail address: [email protected] (R. Ferro).
(cI28-anti-Th3P4 type) and GdSb (cF8-NaCl type) [4±6]. For GdSb a solid state transformation at 1840 C was proposed. The existence of the GdSb2 phase of unknown structure was also suggested [6] (a GdSb2 oC24-LaSb2 type high pressure phase [7] was also reported). Subsequently, for the antimony-richest compound, in a preliminary communication, Altmeyer and Jeitschko [8,9] suggested a stoichiometry R2Sb5 (71.42 at% Sb); Gd2Sb5 was described as monoclinic, Dy2Sb5 type [9]. In this work, on the basis of a complete crystal structure determination relevant to the Gd±Sb phases, a slightly dierent stoichiometry has been attributed to this phase, that is, Gd16Sb39 (70.91 at% Sb). The crystal-structural data available in the literature are summarized in Table 1, together with the results obtained in this work. For the Gd±Sb system an experimental determination of the phase diagram has been reported by Abdusalyamova et al. [6]. This diagram, slightly modi®ed, however, following the results obtained during the present investigation, is shown in Fig. 1. 2.2. Gd±Sb thermodynamics Thermodynamic data for the formation functions of Gd±Sb alloys have previously been obtained by direct calorimetry [13,14], by electromotive force measurements
0966-9795/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved. PII: S0966-9795(99)00097-7
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G. Borzone et al. / Intermetallics 8 (2000) 189±194
Table 1 Crystal structure data of the Gd±Sb system Phase
Structural type
Prototype
Lattice parameters (pm and ) a
b
Ref c
a Gd b Gd Gd5Sb3
hP2 cI2 hP16
Mg W Mn5Si3
363.36 406 897.5(4) 897.0 897.4 899.0± 901.5
578.10
[3] [3] [4] [5] [6] This work
Gd4Sb3
cI28
anti-Th3P4
922.4(5) 922.4 922.8±923.2
[10] [11] This work
aGdSb
cF8
NaCl
621.7 621.8 621.0 621.3±622.0
[12] [12] [6] This work
bGdSb
±
±
Gd16Sb39
mS110
Gd16Sb39
634.3(3) 633.6 634.2 633±638
[6] 5739.5(9) 5740(2)
carried out in the 380±560 C temperature range [15,16], in the 365±630 C temperature range [17] and by vapour pressure measurements [18] within the temperature range 1850±1935 C. The results obtained in these investigations for the enthalpies of formation may be summarized by the following values of the f H (kJ/mol of atoms): Gd5Sb3: ÿ141.92.5 [15] at T=440 C; Gd4Sb3: ÿ145.13.0 [15] at T=440 C; Gd0.52Sb0.48: ÿ137.13.0 [16] at T480 C; GdSb: ÿ131.0 [18] at T=25 C; GdSb: ÿ102.56.0 [13] at T= 420 C; GdSb2: ÿ96.01.0 [17] at T=530 C. On the basis of the unlikely trend of these data vs composition and by comparison with the f H data relevant to the other rare earth antimonides, a revision seemed to be useful and worthwhile. In this paper the data obtained on the entire range of composition by using a direct calorimetry method are reported. 3. Experimental details Gadolinium (99.9 mass%) and antimony (99.99 mass% nominal purities) were used as starting materials. A number of alloys of selected compositions were prepared by melting the metals in Ta crucibles (which were sealed by welding under argon). These alloys were prepared for use as ``reference alloys'' against those prepared in the calorimeter. For the antimony-richest alloys, owing to the high volatility of Sb and its reactivity, the
415.1(1) 415.3(2)
1320.9(2) 1320.9(3)
99.21(1) 99.21(1)
This work single crystal This work powder diract.
preparation was made in Al2O3 crucibles placed inside Mo crucibles sealed under argon. The calorimetric measurements were performed using a direct isoperibolic aneroid calorimeter [19,20]. Its construction was based on the principles of an instrument described by Kubaschewski and Dench [21]. The calorimeter consists of a thick aluminium cylinder containing two small electric furnaces, which are used for starting the reaction in the sample and for electric calibration, respectively. The sample (about 10±15 g) is a mixture of ®ne metal powders enclosed in a gas-tight stainless steel crucible sealed by electric welding which is then inserted in the calorimeter and, after thermal equilibration, is heated until the reaction starts. The temperature of the calorimeter was followed by a multiple-junction thermopile (dierentially connected to the similar thermopile of another calorimeter, identical to the ®rst, used as a reference). The two calorimeters, symmetrically inserted in an aluminium block were surrounded by a water ultrathermostat at 270.01 C and the formation heats were considered measured at 300 K because the sample in the calorimeter cooled down to this temperature during the measurement. The error of 2 kJ/mol of atoms, at, generally ascribed to all measurements, is considered to include both the instrumental errors and any uncertainties due to small compositional variations or to possible quenching of disorder (or metastable situation) from a certain temperature higher than 300 K. The complete range of composition was investigated and the dierent
G. Borzone et al. / Intermetallics 8 (2000) 189±194
samples prepared in the calorimeter (also those used as ``reference alloys'') were studied by X-ray powder diffraction, optical and electron microscopy and electron probe microanalysis, in order to check whether the equilibrium state had been reached and also to check for side reactions. The X-ray analysis of the various alloys was carried out by the Debye method and by powder diractometry using Cu and Fe K radiations. Powder photographs were used both for phase analysis and identi®cation and for lattice parameter measurements. These were re®ned by a least-squares ®t using the Nelson±Riley function. The observed diraction intensities were compared with the calculated values obtained by a computer program (Lazy Pulverix [22]). In this work, moreover, single crystals of the phase Gd16Sb39 were searched for in a sample of composition 80 at% Sb. After several trials, a needle-shaped single crystal was found and intensities were collected by an
191
Enraf-Nonius CAD-4 diractometer with graphitemonochromated Mo K radiation. Lattice constants were obtained from 25 diractometer measured re¯ections in the range =20±22 . Metallographic examination was carried out after SiC paper, diamond paste polishing and after etching in air or in dilute alcoholic HNO3 solution. An example of the metallographic appearance of a 32.5 at%Sb calorimetric specimen alloy is shown in Fig. 2. Notice that this direct synthesis of the alloys carried out in the calorimeter by the same technique often referred to as `combustion synthesis', as well as being useful for the enthalpy data often succeeds in producing well crystallized and uniform samples. 4. Results 4.1. Thermochemical data The results obtained in the measurements of the heats of formation are listed in Table 2. These data were used to trace the trend of the f H vs composition (see Fig. 1) and to evaluate the most probable values of the f H for the dierent compounds. These values are listed at the bottom of Table 2. We may notice the high exothermicity of these alloys and the correspondence of the f H trend with the shape of the phase diagram. In particular the trend is characterized by a minimum at the atomic composition corresponding to the maximum melting point and by slight discontinuities for the other intermediate compounds. Similar trends in f H have been observed for other rareearth antimonide alloys. These values, moreover, ®t very well within those obtained for the isostructural rareearth compounds formed by the other rare-earth elements.
Fig. 1. The Gd±Sb system. The phase diagram [after [6] with some modi®cations (this work)] is shown together with the trend of the standard formation enthalpy of the solid alloys at room temperature (this work).
Fig. 2. Scanning electron micrograph of a Gd±Sb alloy at 32.5 at% Sb synthesized in the calorimeter during the measurement of the formation enthalpy. Etching in air. Polygonal crystals of Gd5Sb3 and Gd at the grain boundaries.
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G. Borzone et al. / Intermetallics 8 (2000) 189±194
Table 2 Heats of formation of solid Gd±Sb alloys at 300 Ka
Table 3 Crystal data of Gd16Sb39
Nominal composition at% Sb
fH (KJ/mol of atoms)
Nominal composition at% Sb
fH (KJ/mol of atoms)
28.0 32.5 36.0 38.0 40.0 44.0 49.0 51.0 52.0
ÿ82.5 ÿ94.2 ÿ106.6 ÿ106.8 ÿ111.5 ÿ114.0 ÿ117.7 ÿ112.8 ÿ112.7
52.7 54.0 55.0 57.0 62.0 65.0 69.0 76.0
ÿ116.7 ÿ115.8 ÿ118.8 ÿ108.1 ÿ98.3 ÿ89.3 ÿ72.3 ÿ58.1
a Calorimetric data for the reaction: (1ÿx) aGd(solid)+xSb(solid)= Gd(1ÿx)Sbx(solid). Interpolated values of the fH (KJ/mol of atoms 2) for the various compounds: Gd5Sb3 ÿ110; Gd4Sb3 ÿ116; GdSb: ÿ120; Gd16Sb39 ÿ78.
Considering, for instance, the equiatomic RSb compounds, we may notice small dierences between the values relevant to a R metal and its neighbouring ones within an overall trend indicating, along the lanthanide series, a progressive decrease of the exothermic heat of formation. On the basis of the data obtained by calorimetric methods, the following values of the standard enthalpies of formation may be indeed considered (kJ/mol of atoms, room temperature): LaSb: ÿ1312 [23]; CeSb: ÿ126.42 [24]; PrSb: ÿ1262 [25]; NdSb: ÿ1232 [26]; SmSb: ÿ1222 [27]; GdSb: ÿ1202 (this work); DySb: ÿ1142 [1]; ErSb: ÿ1114 [2]. As a concluding remark on the thermochemistry of these alloys, a comparison with the literature data may be noteworhty. We may underline the much more negative values reported in literature for the Gd-rich region of the system. On the basis of the comments previously reported, we believe that these values are not reliable. We may notice, however, that similar discrepancies have been observed for the R-rich regions of other R±X systems (with X=p-block element such as Al, Si, Ge) especially when enthalpy data obtained by indirect method (by emf or vapour pressure) were considered. This point has been previously discussed [28]. 4.2. Crystal-structure data The lattice parameters of the dierent phases are listed in Table 1. As for Gd16Sb39, its crystal structure has been determined in this work. The relevant data are reported in Table 3. The programs used were SIR92 [29] for the structure solution and SHELXL-97 [30] for the structure re®nement, with anisotropic displacement parameters and weights w 1= 2 F 20 0:0824P2 , where P F20 2Fc2 =3. Table 4 lists atomic coordinates, standardized by Structure Tidy [31] and equivalent isotropic displacement parameters. The
Pearson code Space group Formula units Crystal size (mm3) Scan mode Range in h,k,l Total number of re¯ections Absorption correction Highest/lowest transmission Independent re¯ections Re¯ections with F0 > 4 F0 Number of parameters wR F 2 , all data RF0 > 4 F0 Goodness of ®t, S min ; max e=A3
mS110 C2/m (No. 12) Z=2 0.0150.0200.140 o 74; 0+5; 0+17 4219 psi-scan and spherical corrections 1.77 4047 2072 173 0.183 0.076 0.942 ÿ5.8, 8.0
Table 4 Atomic coordinates and equivalent isotropic displacement parameters of Gd16Sb39 Atom Position x
y
z
Ueq (pm2)a Occ.
Gd1 Gd2 Gd3 Gd4 Gd5 Gd6 Gd7 Gd8 Sb1b Sb2 Sb3b Sb4 Sb5 Sb6 Sb7 Sb8 Sb9 Sb10 Sb11 Sb12 Sb13 Sb14 Sb15 Sb16 Sb17b Sb18 Sb19 Sb20 Sb21 Sb22 Sb23b
0 0 0 0 0 0 0 0 0.288(3) 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1/2 0
0.0826(2) 0.3565(2) 0.6216(2) 0.3473(2) 0.0261(2) 0.3039(2) 0.2792(2) 0.4352(2) 0.1285(8) 0.3404(3) 0.102(1) 0.9169(3) 0.6160(3) 0.3169(3) 0.8786(3) 0.5962(2) 0.1589(2) 0.5826(2) 0.2751(3) 0.0240(3) 0.5601(2) 0.2661(2) 0.5439(2) 0.1027(3) 0.115(1) 0.1763(3) 0.1724(3) 0.1615(2) 0.1286(2) 1/2 0
105(5) 82(5) 85(5) 90(5) 107(5) 81(5) 92(5) 98(5) 140(30) 117(7) 170(40) 139(8) 88(7) 117(7) 105(7) 82(7) 99(7) 83(7) 112(7) 132(7) 95(7) 85(7) 96(7) 139(8) 150(40) 145(8) 98(7) 102(7) 116(7) 180(10) 160(80)
a b
4i 4i 4i 4i 4i 4i 4i 4i 8j 4i 4i 4i 4i 4i 4i 4i 4i 4i 4i 4i 4i 4i 4i 4i 4i 4i 4i 4i 4i 2d 2a
0.09076(4) 0.14227(4) 0.19150(4) 0.21329(4) 0.34118(4) 0.39716(4) 0.46671(4) 0.55200(4) 0.0067(2) 0.01055(5) 0.0130(2) 0.04298(5) 0.06273(5) 0.08217(5) 0.11904(5) 0.13563(5) 0.16908(5) 0.24356(5) 0.26548(6) 0.27452(6) 0.31771(5) 0.33886(5) 0.39106(5) 0.42031(6) 0.5046(2) 0.55662(6) 0.62960(5) 0.70695(5) 0.80595(6) 0 0
0.222(9) 0.236(9)
0.26(1)
0.124(5)
Ueq is de®ned as one third of the trace of the orthogonalized Uij tensor. Isotropically re®ned.
powder diractogram of this phase was indexed by means of Lazy Pulverix [22], and lattice parameters resulted in very good agreement with those derived from a single crystal. As shown in Fig. 3 the structure of Gd16Sb39 is
G. Borzone et al. / Intermetallics 8 (2000) 189±194
193
the antimony atoms show a large variety in their environment. The most symmetrical Sb22 is surrounded by a parallelepiped of 4 Gd+4 Sb. The same kind of atoms, but in a dierent way, surround Sb6, Sb8, Sb9, Sb10, Sb14, Sb15 and Sb16, all showing the same coordination polyhedron, while for Sb21 this polyhedron is defective, lacking a Gd atom. Seven atoms (2 Sb +5 Gd) are also found around Sb7 and Sb19. With coordination number six we have Sb11 and Sb20 (3 Sb +3 Gd), Sb12 (6 Sb, in the form of a distorted octahedron), Sb5 and Sb13 (6 Gd, in the form of a trigonal prism). The Sb atoms with partial occupancy complete the coordination of four other atoms: Sb2, Sb4, Sb18 and Gd7. While Sb1, Sb3 and Sb17 enter into the environment of more atoms, Sb23, showing the smallest occupation factor, contributes to the coordination of Sb4 only. The shortest distances are Sb12±Sb20 of 282.4(4) pm and Gd4±Sb13 of 310.9(3) pm. As a ®nal comment we may remark that the structure of Gd16Sb39 shows features similar to those reported for the orthorhombic (U1/2Ho1/2)3Sb7 compound [32].
Acknowledgements
Fig. 3. Projection of the Gd16Sb39 structure along the b axis. Open and stippled circles represent atoms at y=0 and y=1/2, respectively. Large circles: Gd; small circles: Sb. The trigonal prisms forming eightand four-membered columnar units are outlined.
characterized by columns of Sb trigonal prisms running parallel to the b axis. All prisms are centered by Gd atoms and form eight- and four-membered patterns, by sharing lateral edges. Two large channels parallel to the b axis occur, where the columnar units of prisms leave room for other atoms. One of these, centered on the position x=1/4, z=0, is ®lled by Sb12, which does not participate in forming trigonal prisms and is coordinated only with Sb atoms. The other channel, centered on the position x=0, z=0 and ®lled by four atoms with statistical occupation, represents the disorder point of this structure. A third smaller channel at the centre of the eight-membered pattern is occupied by Sb22. The coordination polyhedron of the gadolinium atoms is always the same, an Sb trigonal prism tricapped on the side faces by three other Sb atoms, while
Planning and development of the studies here presented form a part of an Italian National Research Project entitled ``Leghe e composti intermetallici. StabilitaÁ termodinamica, proprietaÁ ®siche e reattivitaÁ''. The authors would like to thank the Italian Ministero dell' UniversitaÁ e della Ricerca Scienti®ca e Tecnologica (Programmi di Ricerca Scienti®ca di Rilevante Interesse Nazionale) for ®nancial support. Partial ®nancial support has also been obtained from the Italian Consiglio Nazionale delle Ricerche under the ``Progetto Finalizzato Materiali Speciali per Tecnologie Avanzate II''.
References [1] Ferro R, Borzone G, Cacciamani G. Thermochim Acta 1988;129:99±113. [2] Cacciamani G, Borzone G, Parodi N, Ferro R. Z Metallkd 1996;87:562±7. [3] Gschneidner Jr. KA, Calderwood FW. In: Gschneidner Jr. KA, Eyring L, editors. Handbook on the physics and chemistry of rare earths, vol 8. Amsterdam, The Netherlands: Elsevier, 1986. p. 1. [4] Rieger W, Parthe E. Acta Crystallogr B 1968;24B:456±8. [5] Abdusalyamova MN, Abulkhaev VD, Chechernikov VI. Inorg Mater [Trans Neorg Mater] 1981;17(12):1696±8. [6] Abdusalyamova MN, Burnashev OR, Mironov KY. J LessCommon Met 1986;125:1±6. [7] Eatough NL, Hall HT. Inorg Chem 1969;8:1439±45. [8] Altmeyer RO, Jeitschko W. Z Kristallogr 1989;186:5±7. [9] Altmeyer RO, Jeitschko W. Z Kristallogr 1988;182:3±5. [10] Holtzberg F, McGuire TR, Methfessel S, Suits JC. J Appl Phys 1964;35:1033±8.
194
G. Borzone et al. / Intermetallics 8 (2000) 189±194
[11] Gambino RJ. J Less-Common Metals 1967;12:344±52. [12] Villars P, Calvert LD. In: Pearson's handbook of crystallographic data for intermetallic phases. Materials Park OH, USA: ASM, 1991. [13] Pratt, JN, Chua KS. Thermodynamic studies of some rare-earth alloys, ®nal report min tech agreement no. 2027/048/RL. University of Birmingham, 1970. [14] Chua KS, Pratt JN. Thermochim Acta 1974;8:409±21. [15] Goryacheva VI, Messauden N, Nikol'skaya AV, Gerasimova YaI. Vestn Mosk Univ, Ser 2, Khim 1977;18:749. [16] Goryacheva VI, Nikol'skaya AV, Gerasimov YaI. Dokl Akad Nauk SSSR 1971;197:389±92. [17] Kober VI, Nichkov IF, Raspopin SP, Kuzminnykh VM. Splavy Redk Met Osobymi Fiz Svoistvami: Redkozem Blagorodn Met. 1983; 132±5 [18] Viksman GSh, Gordienko SP. Izv Akad Nauk SSSR, Neorg Mater 1984;20:1441±4. [19] Capelli R, Ferro R, Borsese A. Thermochim Acta 1974;10:13±21. [20] Ferro R, Borzone G, Parodi N, Cacciamani G. J Phase Equil 1994;15:317±29.
[21] Kubaschewski O, Dench WA. Acta Metall 1955;3:339. [22] Yvon K, Jeitschko W, Parthe E. J Appl Crystallogr 1977;10:73. [23] Borzone G, Borsese A, Saccone A, Ferro R. J Less-Common Met 1979;65:253±62. [24] Borsese A, Borzone G, Mazzone D, Ferro R. J Less-Common Met 1981;79:57±63. [25] Borzone G, Borsese A, Zanicchi G, Ferro R. J Therm Anal 1982;25:433±40. [26] Borsese A, Ferro R, Capelli R, Del®no S. J Less-Common Met 1977;55:77±83. [27] Borzone G, Borsese A, Del®no S, Ferro R. Z Metallkd 1985;76:208±13. [28] Borzone G, Raggio R, Ferro R. Phys Chem Chem Phys 1999;1:1487±500. [29] Altomare A, Cascarano G, Giacovazzo C, Guagliardi A, Burla MC, Polidori G, Camalli M. J Appl Crystallogr 1994;27:435. [30] Sheldrick GM, SHELXL-97, Program for re®nement of crystal structures. University of GoÈttingen, Germany, 1997. [31] Gelato LM, Parthe E. J Appl Crystallogr 1987;20:139. [32] Schmidt T, Jeitschko W. Z Kristallogr 1998;15:58.