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The La–Y–Ag system: isothermal section at 700°C and hardness of the intermetallic phases
Journal of Alloys and Compounds 313 (2000) L19–L22
L
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Letter
The La–Y–Ag system: isothermal section at 7008C and hardness of the intermetallic phases b ¨ O.O. Shcherban a , N.Z. Vityk a , R.E. Gladyshevskii a , *, R. Flukiger b
a Department of Inorganic Chemistry, Ivan Franko L’ viv National University, 6, vul. Kyryla i Mefodiya, UA-79005 L’ viv, Ukraine ´ ` Condensee ´ , Universite´ de Geneve ` , 24, quai Ernest Ansermet, CH-1211 Geneve ` , Switzerland Departement de Physique de la Matiere
Received 31 August 2000; accepted 2 September 2000
Abstract The isothermal section of the phase diagram of the La–Y–Ag ternary system at 7008C was studied. The existence of two continuous solid solutions, (La xY 12x ) 14 Ag 51 and (La xY 12x )Ag, was established. The solubility of La in YAg 2 does not exceed 2.5 at.%. The intermetallic phases show higher microhardness than pure Ag. The samples could be easily deformed by hot rolling. 2000 Elsevier Science B.V. All rights reserved. Keywords: Rare earth alloys; X-ray diffraction; Crystal structure; Phase diagram
1. Introduction Pure silver and silver-based solid solutions are commonly used as sheath materials for high-T c superconducting (HTSC) tapes. Intermetallic substrates with an atom arrangement more similar to that of the HTSC phases may improve the grain contacts in the superconducting ceramic. The crystal structures of all superconducting cuprates are of layered character, the metal atoms forming a square ˚ mesh within the layers with a translation period of |3.8 A. The ideal structures of the HTSC are tetragonal or orthorhombic [1]. Among the different structures of intermetallic compounds, the atoms form square mesh in the f.c.c. Cu type (space group Fm-3 m), b.c.c. W type (Im-3 m) and related structure types [2]. Pure silver crystallizes with a Cu-type ˚ Except for Pd and Au, which form structure, a54.08 A. continuous solid solutions with Ag, the solubility of other metals in Ag is limited; the lattice parameter remains approximately the same within all solid solutions. The intermetallic compounds LaAg, YAg and YAg 2 crystallize with ordered structures that are substitution variants of the W type. The former two are representatives of the cubic CsCl type (Pm-3 m), the latter of the tetragonal MoSi 2 *Corresponding author. Tel.: 141-22-702-6078; fax: 141-22-7026869.
(I4 /mmm) type. Their lattice parameters are close to the short parameters of the HTSC, the tetragonal structure having c three times larger than a (Fig. 1).
2. Experiment Alloys of the La–Y–Ag ternary system, each with a total weight of 1 g, were prepared from pure elements (La: 99.84%, Y: 99.99%, Ag: 99.98%) by arc melting. Samples wrapped in Mo foil were annealed at 7008C for 3 months in an evacuated silica tube. X-ray phase analyses were carried out using the powder diffractometers DRON-4.0 and Philips PW1820 (Cu Ka radiation). Diffraction data were collected from stress-relieved powders (annealed at 3008C for 1 h) and from flat polished surfaces of alloys. Structural refinements were performed by the Rietveld method using the DBWS-9807 program [3]. The domain size was evaluated considering a pseudo-Voight profile function [4]. Microstructural analyses were performed using a NEOPHOT-30 microscope. The hardness was measured by the Vickers method with a PMT-3 microhardness meter. The density was determined by the weighing method. Two samples, of nominal composition La 11 Y 22 Ag 67 and La 25 Y 25 Ag 50 , in form of 1.5 mm-thick plates, were rolled at 350–4008C at a rate of 50–100
0925-8388 / 00 / $ – see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S0925-8388( 00 )01219-6
O.O. Shcherban et al. / Journal of Alloys and Compounds 313 (2000) L19 –L22
L20
Fig. 1. Atom arrangement in the W type (a), YAg (CsCl structure type) (b), YAg 2 (MoSi 2 structure type) (c) and YBa 2 Cu 3 O 7 (d).
mm / min down to a thickness of |0.8 mm. These samples were also characterized by X-ray diffraction.
3. Results and discussion An isothermal section of the phase diagram of the La–Y–Ag ternary system was constructed based on X-ray phase analysis of 29 samples. The existence of the binary compounds La 14 Ag 51 , Y 14 Ag 51 (Gd 14 Ag 51 structure type), YAg 2 (MoSi 2 ), LaAg 2 (KHg 2 ), LaAg and YAg (CsCl) [5–10] was confirmed. LaAg 5 (possibly CaCu 5 structure
Fig. 2. Isothermal section at 7008C of the phase diagram of the La–Y–Ag system (,40 at.% Ag: two-phase region (La xY 12x )Ag1(La xY 12x )).
type), which forms through a peritectic reaction, was not observed under the experimental conditions used here. The existence of two continuous solid solutions, (La xY 12x ) 14 Ag 51 and (La xY 12x )Ag, extending between the corresponding isostructural binary compounds, was established (Fig. 2). Microstructural studies and the change of the lattice parameters show that the solubility of La in YAg 2 does not exceed 2.5 at.% (Table 1). An extended two-phase region was observed between (La xY 12x )Ag and (La xY 12x ). As expected, the lattice parameter of the (La xY 12x )Ag solid solution increases nearly linearly with increasing La content (Fig. 3). The density determined by the weighing method is smaller than the X-ray density (Table 2) and the volume fraction of the voids increases slightly with increasing La content, reaching 1% for LaAg, in good agreement with the microstructural observations. The microhardness of the ternary samples is larger than that of the binary alloys. For the (La xY 12x )Ag solid solution the microhardness increases linearly from each binary composition, a maximum being observed at the composition (La 0.5 Y 0.5 )Ag (see Fig. 3). This can be explained by a possible short-range ordering of the Y and La atoms in the structure. The behavior of the microhardness is similar to the change of the lattice parameter and, consequently, the solid solution can be divided into two regions characterized by linear functions with different slopes. Along the line (La xY 12x )Ag 2 the microhardness increases up to 5 at.% La and then remains constant (see Table 1). For ternary samples with high Ag content (Fig. 4) the microhardness is higher than for pure Ag (68.1 kg / mm 2 ) and is proportional to the density. Domain size studies, based on X-ray diffraction profile ˚ For analysis, showed an average domain size of |200 A. cubic (La xY 12x )Ag with high Y content the domain size is larger in the planes h110j than in h100j, however, with
O.O. Shcherban et al. / Journal of Alloys and Compounds 313 (2000) L19 –L22
L21
Table 1 Crystallographic and microstructural parameters of the (La xY 12x )Ag 2 solid solution (MoSi 2 structure type) Composition
Y 33.3 Ag 66.7 La 2.5 Y 30.8 Ag 66.7 La 5.0 Y 28.3 Ag 66.7 La 8.3 Y 25.0 Ag 66.7 La 10.0 Y 23.3 Ag 66.7
˚ Cell parameters, A
H, kg / mm
a
c
3.712(2) 3.721(2) 3.728(3)
9.263(8) 9.296(6) 9.291(9)
186(3) 210(5) 233(7) 233(8) 159(4)
increasing La content the domains become larger in the planes h100j. For the (La xY 12x )Ag 2 solid solution no relation between the domain size in different crystallographic directions and composition could be detected.
2
Domain size D
( hkl)
˚ ,A
(110)
(103)
(200)
(213)
232 290 80
159 144 154
137 70 175
419 232 176
The samples could be easily hot-rolled down to 50% of their initial thickness. As expected, samples with lower Ag content require smaller reduction steps and lower temperatures (due to the high reactivity of rare-earth metals in air). X-ray diffraction revealed a change in the composition after hot rolling, a temperature of 350–4008C being sufficient to oxidize yttrium to Y 2 O 3 . The observed and calculated X-ray diffraction patterns for the sample La 11 Y 22 Ag 67 after hot rolling are presented in Fig. 5. It appears that lanthanum was not oxidized, but formed with silver the intermetallic compound La 14 Ag 51 , which was stable under the conditions applied here. The remaining silver was found in the oxide AgO, which was included in the refinement considering a NaCl-type structure.
4. Conclusion The compounds LaAg, YAg and YAg 2 and their solid solutions are characterized by crystallographic parameters similar to those of HTSC, their mechanical properties represent a good compromise between ductility and stiffness, however, the compounds are not stable in contact with oxygen. Appropriate substitution may reduce or avoid
Fig. 3. Microhardness, lattice parameter, density and domain size vs. La content in the (La xY 12x )Ag solid solution (CsCl structure type).
Fig. 4. Microhardness vs. La content in the (La xY 12x ) 14 Ag 51 solid solution (Gd 14 Ag 51 structure type).
O.O. Shcherban et al. / Journal of Alloys and Compounds 313 (2000) L19 –L22
L22
Table 2 Crystallographic and microstructural parameters of the (La xY 12x )Ag solid solution (CsCl structure type) Composition
Y 50 Ag 50 La 12.5 Y 37.5 Ag 50 La 25.0 Y 25.0 Ag 50 La 37.5 Y 12.5 Ag 50 La 50 Ag 50
˚ a, A
3.623(1) 3.655(2) 3.706(2) 3.725(2) 3.737(2)
Dm , g / cm
3
6.74(2) 6.74(2) 6.78(1) 6.99(1) 7.24(5)
H, kg / mm
159(5) 176(5) 199(5) 171(5) 151(5)
2
Domain size D
( hkl)
˚ ,A
(110)
(200)
(211)
219 226 128 117 143
145 100 90 318 637
134 105 81 123 219
Fig. 5. Experimental, calculated and difference patterns (R p 50.082) for a hot-rolled sample of nominal composition La 11 Y 22 Ag 67 ; (1) Al (sample holder); ˚ (3) Y 2 O 3 (29 wt.%): a-Mn 2 O 3 structure type, space group Ia-3, (2) AgO (17 wt.%): NaCl structure type, space group Fm-3 m, a54.9519(5) A; ˚ (4) La 14 Ag 51 (54 wt.%): Gd 14 Ag 51 structure type, space group P6 /m, a512.925(2), c59.498(2) A. ˚ a510.618(1) A;
oxidation, making it possible to produce a substrate material with favorable atom arrangement for, for instance, YBa 2 Cu 3 O 7 growth.
References [1] R.E. Gladyshevskii, Ph. Galez, in: P. PooleJr. (Ed.), Handbook of Superconductivity, Academic Press, San Diego, 2000, pp. 267–431, Chapter 8. ´ L. Gelato, B. Chabot, M. Penzo, K. Cenzual, R. [2] E. Parthe, Gladyshevskii, TYPIX. Standardized Data and Crystal Chemical Characterization of Inorganic Structure Types, Vol. 1–4, SpringerVerlag, Berlin, 1993 / 1994.
[3] R.A. Young, A. Sakthivel, T.S. Moss, C.O. Paiva-Santos, J. Appl. Cryst. 28 (1995) 366. [4] Th.H. de Keijser, J.I. Langford, E.J. Mittemeijer, A.B.P. Vogels, J. Appl. Cryst. 15 (1982) 308. [5] K.A. Gschneidner Jr., F.W. Calderwood, Bull. Alloy Phase Diagrams 4 (1983). [6] E.I. Gladyshevskii, O.I. Bodak, Crystal Chemistry of Intermetallic Compounds of Rare-Earth Metals, Vyshcha Shkola, L’vov, 1982, In Russian. [7] S. Delfino, R. Ferro, R. Capelli, A. Borsese, Z. Metallkd. 65 (1974) 781. [8] O.D. McMasters, K.A. Gshneidner, R.F. Venteicher, Acta Crystallogr. B26 (1970) 1224. [9] A. Iandelli, A. Pallezona, J. Less-Common Met. 15 (1968) 273. [10] J.L. Moriarty, J.E. Humphreys, R.O. Gordon, N.C. Baenziger, Acta Crystallogr. 21 (1966) 840.
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www.elsevier.com / locate / jallcom
Letter
The La–Y–Ag system: isothermal section at 7008C and hardness of the intermetallic phases b ¨ O.O. Shcherban a , N.Z. Vityk a , R.E. Gladyshevskii a , *, R. Flukiger b
a Department of Inorganic Chemistry, Ivan Franko L’ viv National University, 6, vul. Kyryla i Mefodiya, UA-79005 L’ viv, Ukraine ´ ` Condensee ´ , Universite´ de Geneve ` , 24, quai Ernest Ansermet, CH-1211 Geneve ` , Switzerland Departement de Physique de la Matiere
Received 31 August 2000; accepted 2 September 2000
Abstract The isothermal section of the phase diagram of the La–Y–Ag ternary system at 7008C was studied. The existence of two continuous solid solutions, (La xY 12x ) 14 Ag 51 and (La xY 12x )Ag, was established. The solubility of La in YAg 2 does not exceed 2.5 at.%. The intermetallic phases show higher microhardness than pure Ag. The samples could be easily deformed by hot rolling. 2000 Elsevier Science B.V. All rights reserved. Keywords: Rare earth alloys; X-ray diffraction; Crystal structure; Phase diagram
1. Introduction Pure silver and silver-based solid solutions are commonly used as sheath materials for high-T c superconducting (HTSC) tapes. Intermetallic substrates with an atom arrangement more similar to that of the HTSC phases may improve the grain contacts in the superconducting ceramic. The crystal structures of all superconducting cuprates are of layered character, the metal atoms forming a square ˚ mesh within the layers with a translation period of |3.8 A. The ideal structures of the HTSC are tetragonal or orthorhombic [1]. Among the different structures of intermetallic compounds, the atoms form square mesh in the f.c.c. Cu type (space group Fm-3 m), b.c.c. W type (Im-3 m) and related structure types [2]. Pure silver crystallizes with a Cu-type ˚ Except for Pd and Au, which form structure, a54.08 A. continuous solid solutions with Ag, the solubility of other metals in Ag is limited; the lattice parameter remains approximately the same within all solid solutions. The intermetallic compounds LaAg, YAg and YAg 2 crystallize with ordered structures that are substitution variants of the W type. The former two are representatives of the cubic CsCl type (Pm-3 m), the latter of the tetragonal MoSi 2 *Corresponding author. Tel.: 141-22-702-6078; fax: 141-22-7026869.
(I4 /mmm) type. Their lattice parameters are close to the short parameters of the HTSC, the tetragonal structure having c three times larger than a (Fig. 1).
2. Experiment Alloys of the La–Y–Ag ternary system, each with a total weight of 1 g, were prepared from pure elements (La: 99.84%, Y: 99.99%, Ag: 99.98%) by arc melting. Samples wrapped in Mo foil were annealed at 7008C for 3 months in an evacuated silica tube. X-ray phase analyses were carried out using the powder diffractometers DRON-4.0 and Philips PW1820 (Cu Ka radiation). Diffraction data were collected from stress-relieved powders (annealed at 3008C for 1 h) and from flat polished surfaces of alloys. Structural refinements were performed by the Rietveld method using the DBWS-9807 program [3]. The domain size was evaluated considering a pseudo-Voight profile function [4]. Microstructural analyses were performed using a NEOPHOT-30 microscope. The hardness was measured by the Vickers method with a PMT-3 microhardness meter. The density was determined by the weighing method. Two samples, of nominal composition La 11 Y 22 Ag 67 and La 25 Y 25 Ag 50 , in form of 1.5 mm-thick plates, were rolled at 350–4008C at a rate of 50–100
0925-8388 / 00 / $ – see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S0925-8388( 00 )01219-6
O.O. Shcherban et al. / Journal of Alloys and Compounds 313 (2000) L19 –L22
L20
Fig. 1. Atom arrangement in the W type (a), YAg (CsCl structure type) (b), YAg 2 (MoSi 2 structure type) (c) and YBa 2 Cu 3 O 7 (d).
mm / min down to a thickness of |0.8 mm. These samples were also characterized by X-ray diffraction.
3. Results and discussion An isothermal section of the phase diagram of the La–Y–Ag ternary system was constructed based on X-ray phase analysis of 29 samples. The existence of the binary compounds La 14 Ag 51 , Y 14 Ag 51 (Gd 14 Ag 51 structure type), YAg 2 (MoSi 2 ), LaAg 2 (KHg 2 ), LaAg and YAg (CsCl) [5–10] was confirmed. LaAg 5 (possibly CaCu 5 structure
Fig. 2. Isothermal section at 7008C of the phase diagram of the La–Y–Ag system (,40 at.% Ag: two-phase region (La xY 12x )Ag1(La xY 12x )).
type), which forms through a peritectic reaction, was not observed under the experimental conditions used here. The existence of two continuous solid solutions, (La xY 12x ) 14 Ag 51 and (La xY 12x )Ag, extending between the corresponding isostructural binary compounds, was established (Fig. 2). Microstructural studies and the change of the lattice parameters show that the solubility of La in YAg 2 does not exceed 2.5 at.% (Table 1). An extended two-phase region was observed between (La xY 12x )Ag and (La xY 12x ). As expected, the lattice parameter of the (La xY 12x )Ag solid solution increases nearly linearly with increasing La content (Fig. 3). The density determined by the weighing method is smaller than the X-ray density (Table 2) and the volume fraction of the voids increases slightly with increasing La content, reaching 1% for LaAg, in good agreement with the microstructural observations. The microhardness of the ternary samples is larger than that of the binary alloys. For the (La xY 12x )Ag solid solution the microhardness increases linearly from each binary composition, a maximum being observed at the composition (La 0.5 Y 0.5 )Ag (see Fig. 3). This can be explained by a possible short-range ordering of the Y and La atoms in the structure. The behavior of the microhardness is similar to the change of the lattice parameter and, consequently, the solid solution can be divided into two regions characterized by linear functions with different slopes. Along the line (La xY 12x )Ag 2 the microhardness increases up to 5 at.% La and then remains constant (see Table 1). For ternary samples with high Ag content (Fig. 4) the microhardness is higher than for pure Ag (68.1 kg / mm 2 ) and is proportional to the density. Domain size studies, based on X-ray diffraction profile ˚ For analysis, showed an average domain size of |200 A. cubic (La xY 12x )Ag with high Y content the domain size is larger in the planes h110j than in h100j, however, with
O.O. Shcherban et al. / Journal of Alloys and Compounds 313 (2000) L19 –L22
L21
Table 1 Crystallographic and microstructural parameters of the (La xY 12x )Ag 2 solid solution (MoSi 2 structure type) Composition
Y 33.3 Ag 66.7 La 2.5 Y 30.8 Ag 66.7 La 5.0 Y 28.3 Ag 66.7 La 8.3 Y 25.0 Ag 66.7 La 10.0 Y 23.3 Ag 66.7
˚ Cell parameters, A
H, kg / mm
a
c
3.712(2) 3.721(2) 3.728(3)
9.263(8) 9.296(6) 9.291(9)
186(3) 210(5) 233(7) 233(8) 159(4)
increasing La content the domains become larger in the planes h100j. For the (La xY 12x )Ag 2 solid solution no relation between the domain size in different crystallographic directions and composition could be detected.
2
Domain size D
( hkl)
˚ ,A
(110)
(103)
(200)
(213)
232 290 80
159 144 154
137 70 175
419 232 176
The samples could be easily hot-rolled down to 50% of their initial thickness. As expected, samples with lower Ag content require smaller reduction steps and lower temperatures (due to the high reactivity of rare-earth metals in air). X-ray diffraction revealed a change in the composition after hot rolling, a temperature of 350–4008C being sufficient to oxidize yttrium to Y 2 O 3 . The observed and calculated X-ray diffraction patterns for the sample La 11 Y 22 Ag 67 after hot rolling are presented in Fig. 5. It appears that lanthanum was not oxidized, but formed with silver the intermetallic compound La 14 Ag 51 , which was stable under the conditions applied here. The remaining silver was found in the oxide AgO, which was included in the refinement considering a NaCl-type structure.
4. Conclusion The compounds LaAg, YAg and YAg 2 and their solid solutions are characterized by crystallographic parameters similar to those of HTSC, their mechanical properties represent a good compromise between ductility and stiffness, however, the compounds are not stable in contact with oxygen. Appropriate substitution may reduce or avoid
Fig. 3. Microhardness, lattice parameter, density and domain size vs. La content in the (La xY 12x )Ag solid solution (CsCl structure type).
Fig. 4. Microhardness vs. La content in the (La xY 12x ) 14 Ag 51 solid solution (Gd 14 Ag 51 structure type).
O.O. Shcherban et al. / Journal of Alloys and Compounds 313 (2000) L19 –L22
L22
Table 2 Crystallographic and microstructural parameters of the (La xY 12x )Ag solid solution (CsCl structure type) Composition
Y 50 Ag 50 La 12.5 Y 37.5 Ag 50 La 25.0 Y 25.0 Ag 50 La 37.5 Y 12.5 Ag 50 La 50 Ag 50
˚ a, A
3.623(1) 3.655(2) 3.706(2) 3.725(2) 3.737(2)
Dm , g / cm
3
6.74(2) 6.74(2) 6.78(1) 6.99(1) 7.24(5)
H, kg / mm
159(5) 176(5) 199(5) 171(5) 151(5)
2
Domain size D
( hkl)
˚ ,A
(110)
(200)
(211)
219 226 128 117 143
145 100 90 318 637
134 105 81 123 219
Fig. 5. Experimental, calculated and difference patterns (R p 50.082) for a hot-rolled sample of nominal composition La 11 Y 22 Ag 67 ; (1) Al (sample holder); ˚ (3) Y 2 O 3 (29 wt.%): a-Mn 2 O 3 structure type, space group Ia-3, (2) AgO (17 wt.%): NaCl structure type, space group Fm-3 m, a54.9519(5) A; ˚ (4) La 14 Ag 51 (54 wt.%): Gd 14 Ag 51 structure type, space group P6 /m, a512.925(2), c59.498(2) A. ˚ a510.618(1) A;
oxidation, making it possible to produce a substrate material with favorable atom arrangement for, for instance, YBa 2 Cu 3 O 7 growth.
References [1] R.E. Gladyshevskii, Ph. Galez, in: P. PooleJr. (Ed.), Handbook of Superconductivity, Academic Press, San Diego, 2000, pp. 267–431, Chapter 8. ´ L. Gelato, B. Chabot, M. Penzo, K. Cenzual, R. [2] E. Parthe, Gladyshevskii, TYPIX. Standardized Data and Crystal Chemical Characterization of Inorganic Structure Types, Vol. 1–4, SpringerVerlag, Berlin, 1993 / 1994.
[3] R.A. Young, A. Sakthivel, T.S. Moss, C.O. Paiva-Santos, J. Appl. Cryst. 28 (1995) 366. [4] Th.H. de Keijser, J.I. Langford, E.J. Mittemeijer, A.B.P. Vogels, J. Appl. Cryst. 15 (1982) 308. [5] K.A. Gschneidner Jr., F.W. Calderwood, Bull. Alloy Phase Diagrams 4 (1983). [6] E.I. Gladyshevskii, O.I. Bodak, Crystal Chemistry of Intermetallic Compounds of Rare-Earth Metals, Vyshcha Shkola, L’vov, 1982, In Russian. [7] S. Delfino, R. Ferro, R. Capelli, A. Borsese, Z. Metallkd. 65 (1974) 781. [8] O.D. McMasters, K.A. Gshneidner, R.F. Venteicher, Acta Crystallogr. B26 (1970) 1224. [9] A. Iandelli, A. Pallezona, J. Less-Common Met. 15 (1968) 273. [10] J.L. Moriarty, J.E. Humphreys, R.O. Gordon, N.C. Baenziger, Acta Crystallogr. 21 (1966) 840.