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Yb2Ge, Eu2Ge and Eu2Si: new PbCl2-type compounds
Journal of Alloys and Compounds 348 (2003) 173–175
L
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Yb 2 Ge, Eu 2 Ge and Eu 2 Si: new PbCl 2 -type compounds a b a, F. Merlo , A. Palenzona , M. Pani * b
a Dipartimento di Chimica e Chimica Industriale, Via Dodecaneso 31, I-16146 Genova, Italy INFM and Dipartimento di Chimica e Chimica Industriale, Via Dodecaneso 31, I-16146 Genova, Italy
Received 29 May 2002; accepted 3 June 2002
Abstract The intermetallic compounds Yb 2 Ge, Eu 2 Ge and Eu 2 Si were synthesized from the elements by HF melting in tantalum crucibles. The three phases crystallize in the PbCl 2 structure type, as shown by Rietveld refinement of powder pattern intensity data. The structure of Yb 2 Ge was confirmed by a single crystal study. 2002 Elsevier Science B.V. All rights reserved. Keywords: Rare earth compounds; Crystal structure; X-ray diffraction
1. Introduction The binary compounds with general formula M 2 X (M5 Ca, Sr, Ba, Eu, Yb; X5Si, Ge, Sn, Pb) have been extensively studied in the literature [1–3]. Save for Yb 2 Sn, which crystallizes with the Ni 2 In structure, and Yb 2 Si, whose existence is still doubtful [4], all the other known phases belong to the PbCl 2 type, while no crystallographic data have yet been reported for Yb 2 Ge, Eu 2 Ge and Eu 2 Si. Dealing with the existence and the crystal structure of these three compounds, this work aims at completing the study on this M 2 X family. In the meantime this manuscript ¨ was in preparation, Pottgen et al. [5] published their results on the Eu 2 Si phase, which are in agreement with our data.
2. Experimental Metals used to prepare the alloys were Yb and Eu 99.9 wt.% and Si and Ge 99.999 wt.% pure. Small pieces of the elements, in stoichiometric amounts, were arc sealed under Ar in tantalum crucibles, melted in a HF furnace and annealed at 1273 K for 5 days. The samples so obtained were examined by standard metallographic techniques and by X-ray powder diffraction (diffractometer, Ni-filtered Cu Ka radiation, Si as internal standard). Powder intensity data for Rietveld analysis were collected with 0.028 2u steps and counting times 25–400. Owing to the high *Correspondence author. Fax: 139-010-362-8252. E-mail address: [email protected] (M. Pani).
oxidizability of the europium alloys, their X-ray samples were prepared by milling the powders under silicone grease. A single crystal of Yb 2 Ge was analysed using a MACH-3 diffractometer (graphite monochromated Mo Ka radiation). A total of 3013 reflections (of which 798 were the independent set) was collected in the 2.5–358 u range with the v –u scan mode. The absorptions effects were taken into account by c-scan data [6], using six reflections in the u range 7–228 ( m 565.1 mm 21 ; ratio between maximum and minimum transmission factor 3.5). The programs used were: Lazy-Pulverix (powder diffraction intensity calculations) [7], DBWS-9411 (Rietveld refinements) [8], SHELXL-97 (single crystal structure refinements) [9].
3. Results and discussion The micrographic analysis of all studied alloys revealed the occurrence of three phases: a few percent of primary crystals embedded in a more oxidizable matrix, and small quantities of an eutectic. Accordingly, the X-ray powder patterns showed in all cases the reflections of three phases: the M 5 X 3 phase (Mn 5 Si 3 -type Yb 5 Ge 3 ; Cr 5 B 3 -type Eu 5 Si 3 and Eu 5 Ge 3 ), the M 2 X phase (PbCl 2 type) and the M metal. Probably, the form of the phase diagram is similar in the three cases: at the liquidus curve primary crystals of the M 5 X 3 phase are initially formed, then the M 2 X phase forms peritectically (matrix) and finally an eutectic is formed between the M 2 X and elemental M. The occur-
0925-8388 / 02 / $ – see front matter 2002 Elsevier Science B.V. All rights reserved. PII: S0925-8388( 02 )00851-4
F. Merlo et al. / Journal of Alloys and Compounds 348 (2003) 173–175
174
Table 1 Crystallographic data for Yb 2 Ge, Eu 2 Ge and Eu 2 Si, PbCl 2 -type, Pnma, oP12 a Atom
x
z
˚ Yb 2 Ge a57.567(1) b54.822(1) c59.022(1) A Yb1 0.0196(2) Yb2 0.1553(2) Ge 0.2510(5)
0.6812(2) 0.0737(2) 0.3961(4)
˚ Eu 2 Ge a57.869(2) b55.077(1) c59.416(2) A Eu1 0.0196(12) Eu2 0.1545(10) Ge 0.247(2)
0.6863(7) 0.0791(10) 0.404(2)
˚ Eu 2 Si a57.821(3) b55.047(2) c59.385(4) A Eu1 0.0188(7) Eu2 0.1558(6) Si 0.248(3)
0.6826(5) 0.0790(6) 0.409(3)
a
All atoms are in 4c position. Atomic parameters were refined by Rietveld method. Cell parameters were obtained from diffractograms with Si as internal standard.
rence of the cited phases was confirmed, and the corresponding crystal structures refined by the Rietveld analysis. Table 1 reports the crystallographic data of the three M 2 X compounds. The structure of Yb 2 Ge was confirmed by a single crystal study. In order to favour the formation of primary 2:1 crystals, an alloy of nominal composition Yb 3 Ge was prepared and slowly cooled after melting: in this sample, crystals of Yb 2 Ge suitable for the single crystal analysis could be found, and the results of the single crystal refinement are reported in Table 2. Of the 20 possible M 2 X compounds, Yb 2 Sn crystallizes in the Ni 2 In structure type, Yb 2 Si most probably does not exist [4], the other 18 phases belong to the PbCl 2 type. The similarity among the closely related Co 2 Si, Co 2 P, PbCl 2 ´ and SbSI types was discussed by Flahaut and Thevet [10] on the basis of chemical criteria (coordination geometries and electronegativity differences). According to these authors, the M 2 X compounds of the present family show
Fig. 1. Trend of the average atomic volume as a function of the M metallic radius (M5Ca, Sr, Ba, Eu, Yb) in the 19 existing M 2 X compounds. The points of each series are connected by a solid line, as a guide for the eyes.
values of the b /a axial ratio within the range 0.55–0.70, which is taken as typical of the PbCl 2 type. This geometrical feature is maintained also in Yb 2 Ge, Eu 2 Ge and Eu 2 Si. Fig. 1 reports the average atomic volumes (namely the elementary cell volume divided by the number of atoms in the cell) for the 19 existing phases, as a function of the M metallic radius for coordination 12. A roughly regular trend is observed, and the non-linearity can be ascribed to different volume effects during the formation of these phases, as already described in a review article [11]. Actually, high values of volume contraction, calculated on the basis of the elemental volumes, occur for all phases, ranging from 14.9% of Eu 2 Sn to 21.4% of Sr 2 Si. The close similarity in the alloying behaviour among the bivalent rare earths and the alkaline earth metals is again confirmed, and the influence of the electron concentration accounts for the non-existence of this PbCl 2 -type M 2 X phase when M is a trivalent rare earth.
Table 2 Fractional atomic coordinates, anisotropic displacement parameters and interatomic distances of Yb 2 Ge Atom
x
y
z
˚ 2) U11 (A
˚ 2) U22 (A
˚ 2) U33 (A
˚ 2) U13 (A
Yb1 Yb2 Ge
0.02094(9) 0.15515(9) 0.2527(2)
1/4 1/4 1/4
0.68091(8) 0.07414(8) 0.3940(2)
0.0097(3) 0.0150(3) 0.0116(7)
0.0166(3) 0.0101(3) 0.0105(6)
0.0123(3) 0.0091(3) 0.0093(7)
0.0011(2) 0.0008(2) 20.0022(6)
3.127(2) 3.250(1) 3.528(2) 3.532(1) 3.571(1) 3.600(1) 3.691(1) 3.983(1) 4.071(1)
Yb2–
Ge 2 Ge Ge 2 Yb1 2 Yb1 Yb1 2 Yb2 Yb1
2.979(2) 2.991(1) 3.058(2) 3.532(1) 3.571(1) 3.600(1) 3.622(1) 3.691(1)
Ge–
Yb2 2 Yb2 Yb2 Yb1 2 Yb1 2 Yb1
2.979(2) 2.991(1) 3.058(2) 3.127(2) 3.250(1) 3.528(2)
U12 5U23 50 Yb1–
Ge 2 Ge 2 Ge 2 Yb2 2 Yb2 Yb2 Yb2 2 Yb1 2 Yb1
F. Merlo et al. / Journal of Alloys and Compounds 348 (2003) 173–175
Acknowledgements This work has received financial support from the Italian Ministero dell’Istruzione, Universita` e Ricerca by the Project ‘Leghe e composti intermetallici. Stabilita` ter` and from modinamica, proprieta` fisiche e reattivita’, Consiglio Nazionale delle Ricerche, under the ‘Progetto Finalizzato Materiali Speciali per Tecnologie Avanzate II’.
References [1] G. Bruzzone, E. Franceschi, J. Less-Common Met. 57 (1978) 201. [2] P. Villars, L.D. Calvert, in: Pearson’s Handbook of Crystallographic data for Intermetallic Phases, 2nd Edition, ASM Int, Materials Park, OH, 1991.
175
[3] A. Palenzona, P. Manfrinetti, M.L. Fornasini, J. Alloys Comp. 280 (1998) 211. [4] A. Palenzona, P. Manfrinetti, S. Brutti, G. Balducci, J. Alloys Comp. (in press). ¨ [5] R. Mishra, R.-D. Hoffmann, R. Pottgen, H. Trill, B.D. Mosel, Z. Anorg. Allg. Chem. 628 (2002) 741. [6] A.C.T. North, D.C. Phillips, F.S. Mathews, Acta Crystallogr. A24 (1968) 351. ´ J. Appl. Crystallogr. 10 (1977) 73. [7] K. Yvon, W. Jeitschko, E. Parthe, [8] R.A. Young, A. Sakthivel, T.S. Moss, C.O. Paiva-Santos, J. Appl. Crystallogr. 28 (1995) 366. [9] G.M. Sheldrick, SHELXL-97, Program for Refinement of Crystal ¨ Structures, University of Gottingen, 1997. ´ [10] J. Flahaut, F. Thevet, J. Solid State Chem. 32 (1980) 365. [11] F. Merlo, J. Phys. F: Met. Phys. 18 (1988) 1905.
L
www.elsevier.com / locate / jallcom
Yb 2 Ge, Eu 2 Ge and Eu 2 Si: new PbCl 2 -type compounds a b a, F. Merlo , A. Palenzona , M. Pani * b
a Dipartimento di Chimica e Chimica Industriale, Via Dodecaneso 31, I-16146 Genova, Italy INFM and Dipartimento di Chimica e Chimica Industriale, Via Dodecaneso 31, I-16146 Genova, Italy
Received 29 May 2002; accepted 3 June 2002
Abstract The intermetallic compounds Yb 2 Ge, Eu 2 Ge and Eu 2 Si were synthesized from the elements by HF melting in tantalum crucibles. The three phases crystallize in the PbCl 2 structure type, as shown by Rietveld refinement of powder pattern intensity data. The structure of Yb 2 Ge was confirmed by a single crystal study. 2002 Elsevier Science B.V. All rights reserved. Keywords: Rare earth compounds; Crystal structure; X-ray diffraction
1. Introduction The binary compounds with general formula M 2 X (M5 Ca, Sr, Ba, Eu, Yb; X5Si, Ge, Sn, Pb) have been extensively studied in the literature [1–3]. Save for Yb 2 Sn, which crystallizes with the Ni 2 In structure, and Yb 2 Si, whose existence is still doubtful [4], all the other known phases belong to the PbCl 2 type, while no crystallographic data have yet been reported for Yb 2 Ge, Eu 2 Ge and Eu 2 Si. Dealing with the existence and the crystal structure of these three compounds, this work aims at completing the study on this M 2 X family. In the meantime this manuscript ¨ was in preparation, Pottgen et al. [5] published their results on the Eu 2 Si phase, which are in agreement with our data.
2. Experimental Metals used to prepare the alloys were Yb and Eu 99.9 wt.% and Si and Ge 99.999 wt.% pure. Small pieces of the elements, in stoichiometric amounts, were arc sealed under Ar in tantalum crucibles, melted in a HF furnace and annealed at 1273 K for 5 days. The samples so obtained were examined by standard metallographic techniques and by X-ray powder diffraction (diffractometer, Ni-filtered Cu Ka radiation, Si as internal standard). Powder intensity data for Rietveld analysis were collected with 0.028 2u steps and counting times 25–400. Owing to the high *Correspondence author. Fax: 139-010-362-8252. E-mail address: [email protected] (M. Pani).
oxidizability of the europium alloys, their X-ray samples were prepared by milling the powders under silicone grease. A single crystal of Yb 2 Ge was analysed using a MACH-3 diffractometer (graphite monochromated Mo Ka radiation). A total of 3013 reflections (of which 798 were the independent set) was collected in the 2.5–358 u range with the v –u scan mode. The absorptions effects were taken into account by c-scan data [6], using six reflections in the u range 7–228 ( m 565.1 mm 21 ; ratio between maximum and minimum transmission factor 3.5). The programs used were: Lazy-Pulverix (powder diffraction intensity calculations) [7], DBWS-9411 (Rietveld refinements) [8], SHELXL-97 (single crystal structure refinements) [9].
3. Results and discussion The micrographic analysis of all studied alloys revealed the occurrence of three phases: a few percent of primary crystals embedded in a more oxidizable matrix, and small quantities of an eutectic. Accordingly, the X-ray powder patterns showed in all cases the reflections of three phases: the M 5 X 3 phase (Mn 5 Si 3 -type Yb 5 Ge 3 ; Cr 5 B 3 -type Eu 5 Si 3 and Eu 5 Ge 3 ), the M 2 X phase (PbCl 2 type) and the M metal. Probably, the form of the phase diagram is similar in the three cases: at the liquidus curve primary crystals of the M 5 X 3 phase are initially formed, then the M 2 X phase forms peritectically (matrix) and finally an eutectic is formed between the M 2 X and elemental M. The occur-
0925-8388 / 02 / $ – see front matter 2002 Elsevier Science B.V. All rights reserved. PII: S0925-8388( 02 )00851-4
F. Merlo et al. / Journal of Alloys and Compounds 348 (2003) 173–175
174
Table 1 Crystallographic data for Yb 2 Ge, Eu 2 Ge and Eu 2 Si, PbCl 2 -type, Pnma, oP12 a Atom
x
z
˚ Yb 2 Ge a57.567(1) b54.822(1) c59.022(1) A Yb1 0.0196(2) Yb2 0.1553(2) Ge 0.2510(5)
0.6812(2) 0.0737(2) 0.3961(4)
˚ Eu 2 Ge a57.869(2) b55.077(1) c59.416(2) A Eu1 0.0196(12) Eu2 0.1545(10) Ge 0.247(2)
0.6863(7) 0.0791(10) 0.404(2)
˚ Eu 2 Si a57.821(3) b55.047(2) c59.385(4) A Eu1 0.0188(7) Eu2 0.1558(6) Si 0.248(3)
0.6826(5) 0.0790(6) 0.409(3)
a
All atoms are in 4c position. Atomic parameters were refined by Rietveld method. Cell parameters were obtained from diffractograms with Si as internal standard.
rence of the cited phases was confirmed, and the corresponding crystal structures refined by the Rietveld analysis. Table 1 reports the crystallographic data of the three M 2 X compounds. The structure of Yb 2 Ge was confirmed by a single crystal study. In order to favour the formation of primary 2:1 crystals, an alloy of nominal composition Yb 3 Ge was prepared and slowly cooled after melting: in this sample, crystals of Yb 2 Ge suitable for the single crystal analysis could be found, and the results of the single crystal refinement are reported in Table 2. Of the 20 possible M 2 X compounds, Yb 2 Sn crystallizes in the Ni 2 In structure type, Yb 2 Si most probably does not exist [4], the other 18 phases belong to the PbCl 2 type. The similarity among the closely related Co 2 Si, Co 2 P, PbCl 2 ´ and SbSI types was discussed by Flahaut and Thevet [10] on the basis of chemical criteria (coordination geometries and electronegativity differences). According to these authors, the M 2 X compounds of the present family show
Fig. 1. Trend of the average atomic volume as a function of the M metallic radius (M5Ca, Sr, Ba, Eu, Yb) in the 19 existing M 2 X compounds. The points of each series are connected by a solid line, as a guide for the eyes.
values of the b /a axial ratio within the range 0.55–0.70, which is taken as typical of the PbCl 2 type. This geometrical feature is maintained also in Yb 2 Ge, Eu 2 Ge and Eu 2 Si. Fig. 1 reports the average atomic volumes (namely the elementary cell volume divided by the number of atoms in the cell) for the 19 existing phases, as a function of the M metallic radius for coordination 12. A roughly regular trend is observed, and the non-linearity can be ascribed to different volume effects during the formation of these phases, as already described in a review article [11]. Actually, high values of volume contraction, calculated on the basis of the elemental volumes, occur for all phases, ranging from 14.9% of Eu 2 Sn to 21.4% of Sr 2 Si. The close similarity in the alloying behaviour among the bivalent rare earths and the alkaline earth metals is again confirmed, and the influence of the electron concentration accounts for the non-existence of this PbCl 2 -type M 2 X phase when M is a trivalent rare earth.
Table 2 Fractional atomic coordinates, anisotropic displacement parameters and interatomic distances of Yb 2 Ge Atom
x
y
z
˚ 2) U11 (A
˚ 2) U22 (A
˚ 2) U33 (A
˚ 2) U13 (A
Yb1 Yb2 Ge
0.02094(9) 0.15515(9) 0.2527(2)
1/4 1/4 1/4
0.68091(8) 0.07414(8) 0.3940(2)
0.0097(3) 0.0150(3) 0.0116(7)
0.0166(3) 0.0101(3) 0.0105(6)
0.0123(3) 0.0091(3) 0.0093(7)
0.0011(2) 0.0008(2) 20.0022(6)
3.127(2) 3.250(1) 3.528(2) 3.532(1) 3.571(1) 3.600(1) 3.691(1) 3.983(1) 4.071(1)
Yb2–
Ge 2 Ge Ge 2 Yb1 2 Yb1 Yb1 2 Yb2 Yb1
2.979(2) 2.991(1) 3.058(2) 3.532(1) 3.571(1) 3.600(1) 3.622(1) 3.691(1)
Ge–
Yb2 2 Yb2 Yb2 Yb1 2 Yb1 2 Yb1
2.979(2) 2.991(1) 3.058(2) 3.127(2) 3.250(1) 3.528(2)
U12 5U23 50 Yb1–
Ge 2 Ge 2 Ge 2 Yb2 2 Yb2 Yb2 Yb2 2 Yb1 2 Yb1
F. Merlo et al. / Journal of Alloys and Compounds 348 (2003) 173–175
Acknowledgements This work has received financial support from the Italian Ministero dell’Istruzione, Universita` e Ricerca by the Project ‘Leghe e composti intermetallici. Stabilita` ter` and from modinamica, proprieta` fisiche e reattivita’, Consiglio Nazionale delle Ricerche, under the ‘Progetto Finalizzato Materiali Speciali per Tecnologie Avanzate II’.
References [1] G. Bruzzone, E. Franceschi, J. Less-Common Met. 57 (1978) 201. [2] P. Villars, L.D. Calvert, in: Pearson’s Handbook of Crystallographic data for Intermetallic Phases, 2nd Edition, ASM Int, Materials Park, OH, 1991.
175
[3] A. Palenzona, P. Manfrinetti, M.L. Fornasini, J. Alloys Comp. 280 (1998) 211. [4] A. Palenzona, P. Manfrinetti, S. Brutti, G. Balducci, J. Alloys Comp. (in press). ¨ [5] R. Mishra, R.-D. Hoffmann, R. Pottgen, H. Trill, B.D. Mosel, Z. Anorg. Allg. Chem. 628 (2002) 741. [6] A.C.T. North, D.C. Phillips, F.S. Mathews, Acta Crystallogr. A24 (1968) 351. ´ J. Appl. Crystallogr. 10 (1977) 73. [7] K. Yvon, W. Jeitschko, E. Parthe, [8] R.A. Young, A. Sakthivel, T.S. Moss, C.O. Paiva-Santos, J. Appl. Crystallogr. 28 (1995) 366. [9] G.M. Sheldrick, SHELXL-97, Program for Refinement of Crystal ¨ Structures, University of Gottingen, 1997. ´ [10] J. Flahaut, F. Thevet, J. Solid State Chem. 32 (1980) 365. [11] F. Merlo, J. Phys. F: Met. Phys. 18 (1988) 1905.