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Refinement of the crystal structure of Fe6Ge6Ho
Journal of Alloys and Compounds 337 (2002) 186–188
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Refinement of the crystal structure of Fe 6 Ge 6 Ho a, b c d Lingmin Zeng *, Hugo-Frits Franzen , Jianlie Liang , Yiyou Nie a
Institute of Materials Science, Guangxi University, Nanning, Guangxi 530004, People’ s Republic of China b Ames Laboratory-DOE, Iowa State University, Ames, IA 50011, USA c Department of Physics, Guangxi Institute for Nationalities, Nanning, Guangxi, 530006, People’ s Republic of China d Department of Physics, Jiangxi Normal University, Nanchang, Jiangxi 330027, People’ s Republic of China Received 26 September 2001; accepted 6 November 2001
Abstract The crystal structure of Fe 6 Ge 6 Ho has been determined from single-crystal X-ray data. It crystallizes in the hexagonal, space group ˚ c54.050(2) A, ˚ z50.5, v591.29(3) A ˚ 3 . 2002 Elsevier Science P6 /mmm (no.191) with the Cu 7 Tb structure type and a55.102(2) A, B.V. All rights reserved. Keywords: Rare earth compounds; Transition metal compounds; Crystal structure; X-ray diffraction
1. Introduction RFe 6 Ge 6 phases (R5rare earth elements) are reported to have two structure types [1–6]. The structure of Fe 6 Ge 6 Ho has only been determined by powder diffraction methods. ˚ c54.046 A, ˚ One crystallizes in the hexagonal, a55.110 A, space group P6 /mmm with the Cu 7 Tb structure type [7]. ˚ b517.66 The other belongs to the orthogonal, a58.111 A, ˚ c55.116 A, ˚ space group Cmcm with the Fe 6 Sn 6 Tb A, ˚ b517.71070 A, ˚ structure type [8]. Results (a58.1141 A, ˚ obtained by neutron diffraction also give c55.11264 A) almost the same results [9]. In this paper, we report the refinement of the crystal structure of Fe 6 Ge 6 Ho using single crystal X-ray data.
2. Experimental details
2.1. Syntheses A hexagonal needle-shaped crystal of Fe 6 Ge 6 Ho was obtained from the ternary alloy Fe 6 Ge 6 Ho which was prepared by arc melting and annealing at 973 K for 30
*Corresponding author. E-mail address: [email protected] (L. Zeng).
days. The starting materials were iron (99.5%), germanium (99.99%) and holmium (99.7%).
2.2. Structure determinations The single crystal was glued onto a glass fiber for the structure determination. Data collection was carried out on a Rigaku AFC6R with graphite monochromatic MoK a radiation and a 12-kW rotating anode. The initial cell parameters of Fe 6 Ge 6 Ho were obtained from 25 carefully centered reflections in the range 108<2u <258. The compound belongs to the hexagonal system. Three standard reflections monitored every 150 reflections showed no significant variation in intensity throughout the data collection. Further details of the data collection are given in Table 1. The data collected were corrected for Lorentz and polarization effects as well as for absorption effects. Laue class 6 /mmm and reflection conditions (no conditions) pointed to the possible space groups of P622, P6 mm, ¯ m and P6 /mmm. The structure of Fe 6 Ge 6 Ho P6¯m2, P62 was solved using the TEXSAN [10] program package. Because the Laue point group and cell parameters are similar to those in Ref. [7], the space group P6 /mmm was chosen, and the initial atomic positions obtained by direct methods confirm that the compounds Fe 6 Ge 6 Ho and Cu 7 Tb are isostructural. The final cell parameters for Fe 6 Ge 6 Ho were obtained from the least squares analysis of 82 reflections.
0925-8388 / 02 / $ – see front matter 2002 Elsevier Science B.V. All rights reserved. PII: S0925-8388( 01 )01947-8
L. Zeng et al. / Journal of Alloys and Compounds 337 (2002) 186 – 188 Table 1 Crystal data and structure refinement for Fe 6 Ge 6 Ho Formula Formula mass Space group ˚ a (A) ˚ b (A) ˚ c (A) ˚ 3) v (A Z T (K) Dx (g cm 23 ) Diffractometer Crystal dimensions (mm) Radiation (monochromated in incident beam) Data collected No. of reflections No. of refined parameters R Rw S
Fe 6 Ge 6 Ho 935.55 P6 /mmm 5.102(2) 5.102(2) 4.050(2) 91.29(3) 0.5 293(2) 8.507 Rigaku AFC6R 0.1030.0930.26 ˚ MoK a (0.71069 A) 0
3. Results and discussion The final results for the atomic coordinates, occupancy and equivalent isotropic displacement parameters of the Fe 6 Ge 6 Ho compound are given in Table 2. These results are in good agreement with those reported in Ref. [7]. The calculated patterns using this crystal structure data are also in concord with those reported in Ref. [11]. The interatomic distances (d ), D values [D 5 (d 2 Sr ) /Sr , Dr is the sum of the respective atomic radii] for the Fe 6 Ge 6 Ho are listed in Table 3. It can be seen from Table 3 that the distances between Ho and Ge2 and the distances between ˚ respectiveGe2 and Ge2 are only 1.229(7) and 1.59(1) A, ly. The reason is that the occupancy rates of the Ho atom position (1a) and the Ge2 atom positions (2e) are only 0.528 and 0.48, respectively. Fig. 1 shows the crystal structure of Fe 6 Ge 6 Ho. The RFe 6 Ge 6 phase is known to form various superstructures of the average YCo 6 Ge 6 -structure type [1]. In Ref. [9], it was indicated that in all cases the superstructure formation originated from the ordering of the R–Ge atoms within the hexagonal channels of the Fe–Ge framework, which in turn depends on the thermal treatment. In the samples annealed at 800 and 650 8C, the R–Ge atoms are
187
Table 3 The interatomic distances (d ), D values [D 5 (d 2 Sr ) /Sr , Sr is the sum of the respective atomic radii] for Fe 6 Ge 6 Ho Atoms
˚ d (A)
D
Ho
–6Ge1 –2Ge2 –2Ge2
2.9456(9) 2.823(7) 1.227(3)
20.065 20.104
Ge1
–3Ho –3Ge1 –6Fe
2.9456(9) 2.8456(9) 2.5040(9)
20.065 0.060 20.059
Ge2
–1Ho –1Ho –1Ge2 –1Ge2 –6Fe
1.227(7) 2.823(7) 2.45(1) 1.60(1) 2.673(2)
–4Ge1 –4Ge2 –4Fe
2.5040(9) 2.673(2) 2.5510(8)
Fe
20.104 20.119 0.005 20.059 0.005 0.004
partly disordered, while samples annealed at 900 8C reveal a perfect order. In Ref. [9], it was also reported that the HoFe 6 Ge 6 sample annealed at 800 8C contained a small amount of an impurity phase (Ho 0.67 Fe 6 Ge 6 ). The results we obtained from the single crystal sample of Fe 6 Ge 6 Ho, which was annealed at 700 8C, show that Fe 6 Ge 6 Ho exists as two kinds of structure type.
Acknowledgements This research was jointly supported by the Office of the Basic Energy Sciences, Materials Sciences Division, US Department of Energy and Education Commission of Guangxi, P.R. China.
Table 2 Atomic coordinates and equivalent isotropic displacement parameters ˚ 2 ) for Fe 6 Ge 6 Ho (A Atom
Wyckoff position
x
y
z
Occ.
Beq
Ho Ge1 Ge2 Fe
1a 2c 2e 3g
0.0000 1/3 0.0000 1/2
0.0000 2/3 0.0000 0.0000
0.0000 0.0000 0.303(2) 1/2
0.528 1.0 0.48 1.0
0.70(9) 0.5(1) 0.6(2) 0.5(2)
Fig. 1. Crystal structure of Fe 6 Ge 6 Ho (large filled circles, Ho; medium open circles, Ge; small filled circles, Fe).
188
L. Zeng et al. / Journal of Alloys and Compounds 337 (2002) 186 – 188
References [1] G. Venturini, R. Welter, B. Malaman, J. Alloys Comp. 185 (1992) 99. [2] O. Oleksyn, P. Schobinger-Papamantellos, J. Rodriguez-Carvajal, E. ¨ Bruck, K.H.J. Buschow, J. Alloys Comp. 257 (1997) 36. [3] P. Schobinger-Papamantellos, O. Oleksyn, J. Rodriguez-Carvajal, E. ¨ Bruck, K.H.J. Buschow, Part 1, J. Mag. Mag. Mater. 182 (1998) 96. [4] O. Zaharko, P. Schobinger-Papamantellos, C. Ritter, J. RodriguezCarvajal, K.H.J. Buschow, Part 11, J. Mag. Mag. Mater. 1 (1998) 187. [5] O. Oleksyn, S.T. Haibach, W. Kek, in: Proceedings of Aperiodic ’97, Alpe d’Huez, France, 27–31 August, 1997, p. 359. [6] O. Oleksyn, H.-U. Nissen, R. Wessicken, Phil. Mag. Lett. 77 (1998) 275.
[7] O.Y. Mruz, P.K. Starodub, O.I. Bodak, Dopovidi Akademii Nauk Ukrains’koi Rsr, Seriya B: Geologichni, Khimichni Ta Biologichni Nauki 12 (1984) 45. [8] G. Venturini, R. Welter, B. Malaman, J. Alloys Comp. 185 (1992) 99. [9] O. Zaharko, P. Schobinger-Papamantellos, J. Rodriguez-Carvajal, K.H.J. Buschow, J. Alloys Comp. 288 (1999) 50. [10] TEXSAN Single Crystal Structure Analysis Software, Molecular Structure Corporation, 3200 Research Forest Drive, The Woodlands, TX 77381, USA, 1989. [11] L. Zeng, Powder Diffraction File 50-793, International Centre for Diffraction Data, 2000.
L
www.elsevier.com / locate / jallcom
Refinement of the crystal structure of Fe 6 Ge 6 Ho a, b c d Lingmin Zeng *, Hugo-Frits Franzen , Jianlie Liang , Yiyou Nie a
Institute of Materials Science, Guangxi University, Nanning, Guangxi 530004, People’ s Republic of China b Ames Laboratory-DOE, Iowa State University, Ames, IA 50011, USA c Department of Physics, Guangxi Institute for Nationalities, Nanning, Guangxi, 530006, People’ s Republic of China d Department of Physics, Jiangxi Normal University, Nanchang, Jiangxi 330027, People’ s Republic of China Received 26 September 2001; accepted 6 November 2001
Abstract The crystal structure of Fe 6 Ge 6 Ho has been determined from single-crystal X-ray data. It crystallizes in the hexagonal, space group ˚ c54.050(2) A, ˚ z50.5, v591.29(3) A ˚ 3 . 2002 Elsevier Science P6 /mmm (no.191) with the Cu 7 Tb structure type and a55.102(2) A, B.V. All rights reserved. Keywords: Rare earth compounds; Transition metal compounds; Crystal structure; X-ray diffraction
1. Introduction RFe 6 Ge 6 phases (R5rare earth elements) are reported to have two structure types [1–6]. The structure of Fe 6 Ge 6 Ho has only been determined by powder diffraction methods. ˚ c54.046 A, ˚ One crystallizes in the hexagonal, a55.110 A, space group P6 /mmm with the Cu 7 Tb structure type [7]. ˚ b517.66 The other belongs to the orthogonal, a58.111 A, ˚ c55.116 A, ˚ space group Cmcm with the Fe 6 Sn 6 Tb A, ˚ b517.71070 A, ˚ structure type [8]. Results (a58.1141 A, ˚ obtained by neutron diffraction also give c55.11264 A) almost the same results [9]. In this paper, we report the refinement of the crystal structure of Fe 6 Ge 6 Ho using single crystal X-ray data.
2. Experimental details
2.1. Syntheses A hexagonal needle-shaped crystal of Fe 6 Ge 6 Ho was obtained from the ternary alloy Fe 6 Ge 6 Ho which was prepared by arc melting and annealing at 973 K for 30
*Corresponding author. E-mail address: [email protected] (L. Zeng).
days. The starting materials were iron (99.5%), germanium (99.99%) and holmium (99.7%).
2.2. Structure determinations The single crystal was glued onto a glass fiber for the structure determination. Data collection was carried out on a Rigaku AFC6R with graphite monochromatic MoK a radiation and a 12-kW rotating anode. The initial cell parameters of Fe 6 Ge 6 Ho were obtained from 25 carefully centered reflections in the range 108<2u <258. The compound belongs to the hexagonal system. Three standard reflections monitored every 150 reflections showed no significant variation in intensity throughout the data collection. Further details of the data collection are given in Table 1. The data collected were corrected for Lorentz and polarization effects as well as for absorption effects. Laue class 6 /mmm and reflection conditions (no conditions) pointed to the possible space groups of P622, P6 mm, ¯ m and P6 /mmm. The structure of Fe 6 Ge 6 Ho P6¯m2, P62 was solved using the TEXSAN [10] program package. Because the Laue point group and cell parameters are similar to those in Ref. [7], the space group P6 /mmm was chosen, and the initial atomic positions obtained by direct methods confirm that the compounds Fe 6 Ge 6 Ho and Cu 7 Tb are isostructural. The final cell parameters for Fe 6 Ge 6 Ho were obtained from the least squares analysis of 82 reflections.
0925-8388 / 02 / $ – see front matter 2002 Elsevier Science B.V. All rights reserved. PII: S0925-8388( 01 )01947-8
L. Zeng et al. / Journal of Alloys and Compounds 337 (2002) 186 – 188 Table 1 Crystal data and structure refinement for Fe 6 Ge 6 Ho Formula Formula mass Space group ˚ a (A) ˚ b (A) ˚ c (A) ˚ 3) v (A Z T (K) Dx (g cm 23 ) Diffractometer Crystal dimensions (mm) Radiation (monochromated in incident beam) Data collected No. of reflections No. of refined parameters R Rw S
Fe 6 Ge 6 Ho 935.55 P6 /mmm 5.102(2) 5.102(2) 4.050(2) 91.29(3) 0.5 293(2) 8.507 Rigaku AFC6R 0.1030.0930.26 ˚ MoK a (0.71069 A) 0
3. Results and discussion The final results for the atomic coordinates, occupancy and equivalent isotropic displacement parameters of the Fe 6 Ge 6 Ho compound are given in Table 2. These results are in good agreement with those reported in Ref. [7]. The calculated patterns using this crystal structure data are also in concord with those reported in Ref. [11]. The interatomic distances (d ), D values [D 5 (d 2 Sr ) /Sr , Dr is the sum of the respective atomic radii] for the Fe 6 Ge 6 Ho are listed in Table 3. It can be seen from Table 3 that the distances between Ho and Ge2 and the distances between ˚ respectiveGe2 and Ge2 are only 1.229(7) and 1.59(1) A, ly. The reason is that the occupancy rates of the Ho atom position (1a) and the Ge2 atom positions (2e) are only 0.528 and 0.48, respectively. Fig. 1 shows the crystal structure of Fe 6 Ge 6 Ho. The RFe 6 Ge 6 phase is known to form various superstructures of the average YCo 6 Ge 6 -structure type [1]. In Ref. [9], it was indicated that in all cases the superstructure formation originated from the ordering of the R–Ge atoms within the hexagonal channels of the Fe–Ge framework, which in turn depends on the thermal treatment. In the samples annealed at 800 and 650 8C, the R–Ge atoms are
187
Table 3 The interatomic distances (d ), D values [D 5 (d 2 Sr ) /Sr , Sr is the sum of the respective atomic radii] for Fe 6 Ge 6 Ho Atoms
˚ d (A)
D
Ho
–6Ge1 –2Ge2 –2Ge2
2.9456(9) 2.823(7) 1.227(3)
20.065 20.104
Ge1
–3Ho –3Ge1 –6Fe
2.9456(9) 2.8456(9) 2.5040(9)
20.065 0.060 20.059
Ge2
–1Ho –1Ho –1Ge2 –1Ge2 –6Fe
1.227(7) 2.823(7) 2.45(1) 1.60(1) 2.673(2)
–4Ge1 –4Ge2 –4Fe
2.5040(9) 2.673(2) 2.5510(8)
Fe
20.104 20.119 0.005 20.059 0.005 0.004
partly disordered, while samples annealed at 900 8C reveal a perfect order. In Ref. [9], it was also reported that the HoFe 6 Ge 6 sample annealed at 800 8C contained a small amount of an impurity phase (Ho 0.67 Fe 6 Ge 6 ). The results we obtained from the single crystal sample of Fe 6 Ge 6 Ho, which was annealed at 700 8C, show that Fe 6 Ge 6 Ho exists as two kinds of structure type.
Acknowledgements This research was jointly supported by the Office of the Basic Energy Sciences, Materials Sciences Division, US Department of Energy and Education Commission of Guangxi, P.R. China.
Table 2 Atomic coordinates and equivalent isotropic displacement parameters ˚ 2 ) for Fe 6 Ge 6 Ho (A Atom
Wyckoff position
x
y
z
Occ.
Beq
Ho Ge1 Ge2 Fe
1a 2c 2e 3g
0.0000 1/3 0.0000 1/2
0.0000 2/3 0.0000 0.0000
0.0000 0.0000 0.303(2) 1/2
0.528 1.0 0.48 1.0
0.70(9) 0.5(1) 0.6(2) 0.5(2)
Fig. 1. Crystal structure of Fe 6 Ge 6 Ho (large filled circles, Ho; medium open circles, Ge; small filled circles, Fe).
188
L. Zeng et al. / Journal of Alloys and Compounds 337 (2002) 186 – 188
References [1] G. Venturini, R. Welter, B. Malaman, J. Alloys Comp. 185 (1992) 99. [2] O. Oleksyn, P. Schobinger-Papamantellos, J. Rodriguez-Carvajal, E. ¨ Bruck, K.H.J. Buschow, J. Alloys Comp. 257 (1997) 36. [3] P. Schobinger-Papamantellos, O. Oleksyn, J. Rodriguez-Carvajal, E. ¨ Bruck, K.H.J. Buschow, Part 1, J. Mag. Mag. Mater. 182 (1998) 96. [4] O. Zaharko, P. Schobinger-Papamantellos, C. Ritter, J. RodriguezCarvajal, K.H.J. Buschow, Part 11, J. Mag. Mag. Mater. 1 (1998) 187. [5] O. Oleksyn, S.T. Haibach, W. Kek, in: Proceedings of Aperiodic ’97, Alpe d’Huez, France, 27–31 August, 1997, p. 359. [6] O. Oleksyn, H.-U. Nissen, R. Wessicken, Phil. Mag. Lett. 77 (1998) 275.
[7] O.Y. Mruz, P.K. Starodub, O.I. Bodak, Dopovidi Akademii Nauk Ukrains’koi Rsr, Seriya B: Geologichni, Khimichni Ta Biologichni Nauki 12 (1984) 45. [8] G. Venturini, R. Welter, B. Malaman, J. Alloys Comp. 185 (1992) 99. [9] O. Zaharko, P. Schobinger-Papamantellos, J. Rodriguez-Carvajal, K.H.J. Buschow, J. Alloys Comp. 288 (1999) 50. [10] TEXSAN Single Crystal Structure Analysis Software, Molecular Structure Corporation, 3200 Research Forest Drive, The Woodlands, TX 77381, USA, 1989. [11] L. Zeng, Powder Diffraction File 50-793, International Centre for Diffraction Data, 2000.