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Crystal structure of a quaternary Nd–Ni–Si–B compound-refinement of synchrotron radiation data
L
Journal of Alloys and Compounds 264 (1998) 232–235
Crystal structure of a quaternary Nd–Ni–Si–B compound-refinement of synchrotron radiation data E. Wu*, H. Zhang, S.J. Campbell, S.R. Bulcock
1
School of Physics, University College, The University of New South Wales, Australian Defence Force Academy, Canberra, ACT 2600, Australia Received 16 May 1997
Abstract High-resolution synchrotron-radiation powder data have been obtained for a new quaternary Nd–Ni–B–Si compound with the nominal composition of NdNi 8 SiB 3 . The data were collected on a (573 mm radius) cylindrical cassette diffractometer using imaging plates. The crystal structure of the new compound has been determined and refined by Rietveld analysis. The initial structural model and space group ¨ for the refinement is based on insight derived from earlier Mossbauer and TEM analyses. Refinement of the powder data leads to a ˚ and c57.8731(2) A ˚ in the space group P4 / nmm. The refined compound is tetragonal unit-cell with lattice parameters a511.2260(2) A found to have a composition of NdNi 9.7 Si 1.3 B |4 . The atomic coordinates for the Nd, Ni and Si atoms have been determined. Small amounts of the impurity phases, NdNi 5 and Nd 3 Ni 13 B 2 , have also been determined and refined in the analysis. 1998 Elsevier Science S.A. Keywords: Synchrotron diffraction; Rietveld refinement; Structure determination
1. Introduction
2. Experimental
A quaternary phase of the R–T–Si–B (R: rare-earth, T: transition-metal) system which fills a gap between the important ternary rare-earth transition-metal systems R–T– B and R–T–Si, has recently been reported [1,2]. The overall structural and magnetic properties of two of these compounds with overall compositions, NdCo 8 SiB 3 and NdNi 8 SiB 3 have been investigated by X-ray diffraction ¨ and Mossbauer spectroscopy [1–3], with the symmetry of the compounds having been determined from an electron diffraction analysis [4]. These studies have provided important information for the study of the crystal structure of the new phase. High-resolution synchrotron-radiation powder data has recently been collected for the NdNi 8 SiB 3 sample, and, building on from these earlier studies [1–4], this paper reports the determination of the crystal structure of this phase from a Rietveld profile refinement of the synchrotron data.
A sample of nominal composition of NdNi 8 SiB 3 was synthesised by arc melting high purity elements of Nd (99.9 wt.%), Ni (99.9 wt.%), Si (99.9 wt.%) and B (99.999 wt.%). The sample was then wrapped in a protective Ta foil and annealed in sealed evacuated quartz tubes at 900 8C for two weeks. The synchrotron data was collected on a multipurpose diffractometer at the Australian National Beamline Facility (ANBF) at the Photon Factory of the National Laboratory for High Energy Physics (KEK) in Tsukuba, Japan. The ANBF beamline monochromator is a Si(111) channel cut crystal located about 11 m from the radiation source. The diffractometer is a cylindrical cassette camera mounted 2 with 4003200 mm imaging plate (IP) storage phosphor detectors. The ground powder of the specimen was inserted in a 0.3 mm glass capillary and mounted in a sample spinner goniometer in the camera cassette, at a distance of 14 m from the source of the synchrotron radiation. A powder pattern was collected at room temperature (2361 8C) from a set of two IPs clamped on each side of the cassette covering the 2u range 65 to 640 8. The IPs were exposed for 10 min in the cassette which was evacuated to a
*Corresponding author. 1 Present address: Electron Microscope Unit (F09), University of Sydney, NSW 2006, Australia. 0925-8388 / 98 / $19.00 1998 Elsevier Science S.A. All rights reserved. PII S0925-8388( 97 )00274-0
E. Wu et al. / Journal of Alloys and Compounds 264 (1998) 232 – 235
pressure of about 0.1 Torr. A high-resolution data set of intensity versus scattering angle 2u, with a step size of 0.01 8, was extracted from the IP images. The synchrotron ˚ from a wavelength was calibrated to be 0.62649 A measurement of a standard Si powder sample.
3. Structural refinements and results The structure of the compound was analysed using a Rietveld refinement program RIET2 [5]. Peaks were modelled using the pseudo-Voigt profile function and corrected for asymmetry in the form of a sum of five peaks method [6]. The background was modelled using a refinable fourth-order polynomial. The intensity of the Bragg peaks was modified for a minor preferred orientation on [110] using the March–Dollase function, and corrected for the effects of sample absorption by applying the transmission factor assuming a real sample cylindrical radius of 0.1 mm [5]. Determination of the atomic coordinates in the unit cell for an unknown structure can rarely be carried out based on powder diffraction data only. Most of the intensity modelling programs were written to calculate the intensities from a proposed structural arrangement to test the model [7]. In the case of this quaternary Nd–Ni–Si–B compound, the space group of the structure has been determined on the basis of electron diffraction studies [4], which considered two space groups P4 / n and P4 / nmm, and identified P4 / nmm as the most likely space group for the phase. The initial structural model was established from in¨ formation derived from Mossbauer effect spectroscopy and TEM data [2–4]. The main feature of the room tempera¨ ture Mossbauer spectrum [2,3] is a singlet, indicating a highly symmetric crystal field environment around the 57 Fe atoms introduced as a dilute dopant in the compound. A simple explanation for this is that the transition metal sublattice is isostructurally related to a close packed fcc lattice, and a structural model for the transition metal atoms was developed along these lines [3], with the Nd atoms assumed to be in a few positions of high symmetry. The atomic arrangement in a unit-cell of this initial model can only be described by the lower symmetry space group P4 / n which displays two strong reflection peaks, consistent with those observed experimentally. The refinement appears to converge after a few cycles of refinements on the Ni positions in this model, with some of the atomic positions for the Ni atoms having been found to change significantly. These changes allow constraints to be imposed on the Ni atoms in accordance with the atomic coordinate positions 16k, 8i and 8j of the higher symmetry space group of P4 / nmm proposed from the TEM analysis [4]. However, an unacceptably high temperature factor was derived for a set of 8i positions, and the Ni atoms in this set were accordingly replaced by the lighter Si atoms.
233
Table 1 The atomic coordinates, thermal and population parameters for NdNi 9.7 Si 1.3 B |4 (cf. Fig. 1) Refinement of Space group P4 / nmm (No. 129) origin choice 2 ˚ c57.8731(2) A, ˚ Z56, Dcal 58.0 g cm 23 , Dmeas 57.8 g a511.2260(2) A, 23 cm R p 55.45%, R wp 57.35%, R exp 52.92%, R B 53.37% for 226 reflections in NdNi 9.7 Si 1.3 B |4 ; R B 52.98% for 25 reflections of NdNi 5 ; R B 53.52% for 60 reflections of Nd 3 Ni 13 B 2 . Site
x
y
z
˚ 2) Biso (A
Nd(1) 2b Nd(2) 4d Ni(1) 2c Ni(2) 8i 1 Ni(3) 8i 2 Ni(4) 8i 3 Ni(5) 8j 1 Ni(6) 8j 2 Ni(7) 16k Si 8i
3/4 0 1/4 1/4 1/4 1/4 0.3659(4) 0.3608(4) 0.4108(4) 1/4
1/4 0 1/4 0.0203(4) 0.6405(4) 0.0839(5) x x 0.5759(4) 0.929(1)
1/2 0 0.467(1) 0.474(1) 0.889(1) 0.973(1) 0.249(1) 0.701(1) 0.6671(4) 0.176(2)
0.1(1) 0.2(1) 0.1(2) 20.2(1) 0.2(1) 20.2(1) 0.1(1) 0.2(1) 20.2(1) 0.4(3)
Some additional peaks which could not be indexed in the refinements were, however, determined through further analysis to be due to the impurity phases, NdNi 5 and Nd 3 Ni 13 B 2 . The structural information for these two compounds were inserted into the Rietveld program and refinement of the data for these three phases — the compound of interest and the two impurities — carried out simultaneously. The quantitative analysis of the refinement of the whole pattern led to impurity contents of 10 wt.% NdNi 5 and 7 wt.% Nd 3 Ni 13 B 2 , thus corresponding to a main phase composition of |NdNi 9.2 Si 1.4 B 4 in the material of overall starting composition NdNi 8 SiB 3 . The refined structural parameters and agreement values are reported in Table 1. Plots of the observed, calculated and difference profiles for the Rietveld refinement are shown in Fig. 1, with the unit cell determined for NdNi 9.7 Si 1.3 B |4 , (as viewed along the [001] direction) shown in Fig. 2.
4. Discussion On the basis of the refined structure, the elements Nd, Ni and Si have the composition NdNi 9.7 Si 1.3 , in which the calculated composition of Ni is slightly higher than the composition determined from the refinement. Nonetheless, the overall composition is reasonably close to the composition of NdNi 9.2 Si 1.4 B 4 derived from the quantitative analysis. This deviation of |0.5% in the Ni concentration as based on the quantitative analysis is acceptable. In this refined formula, the compositions for the Si and Ni are fixed according to their distinct crystallographic sets, whereas in our preparations of this and related compounds
234
E. Wu et al. / Journal of Alloys and Compounds 264 (1998) 232 – 235
Fig. 1. Rietveld refinement of the synchrotron diffraction pattern obtained for the quaternary Nd–Ni–Si–B compound. The fit also includes refinement of NdNi 5 and Nd 3 Ni 13 B 2 impurity phases. The difference profile for the refinement is also shown along with the reflection markers for the P4 / nmm structure of the new phase (top set of markers); NdNi 5 (middle markers) and Nd 3 Ni 13 B 2 (bottom markers).
Fig. 2. The refined unit cell of NdNi 9.7 Si 1.3 B |4 as viewed along [001]. The boundary indicates the a axis; the large open circles are Nd atoms; the small circles are Ni atoms and the small open circles are the Si sites. The atomic positions (in %) along the c axis are labelled for each atom.
E. Wu et al. / Journal of Alloys and Compounds 264 (1998) 232 – 235
[8], it has been noticed that the Si and Ni components can be varied over a certain range of the composition. We have therefore assumed that Si atoms can be mixed with Ni atoms in some of the Ni sites in the unit cell, similar to the approach taken in other structural studies of Nd–Ni–Si compounds [9,10]. However, the refined occupancies for the Si atoms on any of the Ni sites were too small to provide a considerable composition variation for the present NdNi 9.7 Si 1.3 B |4 compound. Further investigation is required to clarify the difference between the experimental behaviour and the apparent Si, Ni mixing based on the refinement of the present compound. The refinement (Table 1) reveals that very small and even negative temperature factors have been derived for the Ni atoms; this could be due to the influence of the B atoms which are unaccounted for at this stage. A current neutron diffraction investigation of this compound will clarify this point. An interesting feature of this structure (Fig. 2) is that the coordinates of the Ni atoms in the 2c, 8i 1 , 8j 1 and 8j 2 sites are distorted only slightly from their positions as proposed initially for an ideal close packed fcc sublattice model [3]. The 2c sites remain in a highly symmetrical environments with each of the Ni atoms surrounded by 12 Ni atoms from half of the 8i 1 , 8j 1 and 8j 2 sites which have similar ˚ In the unit-cell as a interatomic distances (2.52–2.57 A). whole, the calculated interatomic distances are in the ˚ for the following ranges: for the Ni–Ni atoms 2.1–2.6 A; ˚ for the Ni–Nd atoms 2.9–3.3 A; ˚ Ni–Si atoms 2.3–2.7 A; ˚ These values correand for the Si–Nd atoms 3.2–3.3 A. spond well with those obtained for Nd–Ni–Si compounds [9–11]. On the basis of our original structural model [3], and the calculated interstitial spaces, there could be another set of 2c sites for Ni or Si atoms created a distance of about c / 2 from the current 2c sites. The refinement agreement values were found to improve on introduction of atoms at these sites, but only allowed very low fractional occupancies for Ni or Si atoms in these sites. This behaviour could occur as a result of the unallocated B atoms occurring in these sites. However, because of the relatively low scattering factor for B, at this stage no attempt has been made to locate the B atoms on refinement of the synchrotron data. As mentioned
235
above, a neutron diffraction study of the corresponding Nd–Ni–Si– 11 B compound of this phase is underway. The relatively large neutron scattering length and low neutron absorption coefficient for 11 B will assist in enabling us to determine the positions of the B atoms in the unit cell.
Acknowledgements The authors are grateful to Australian National Beamline Facility (ANBF) for support and access to the multipurpose synchrotron diffractometer. The authors thank Dr. D.J. Cookson of Australian Nuclear Science and Technology Organisation and Dr. E.M.A. Gray of Griffith University for their assistance in data collection and Dr. H.S. Li of the University of New South Wales and Dr. B.A. Hunter of Australian Nuclear Science and Technology Organisation for helpful discussions on aspects of this work. The work was supported by ARC grants, leading to the award of a Postgraduate Research Scholarship (HZ) and a Research Associateship. HZ also acknowledges the award of an Overseas Postgraduate Research Scholarship.
References [1] E. Wu, G.H.J. Wantenaar, S.J. Campbell, H.S. Li, J. Phys. Cond. Matt. 5 (1993) L457. [2] H. Zhang, S.J. Campbell, S.R. Bulcock, A.V.J. Edge, E. Wu, Physica B 229 (1997) 333. [3] E. Wu, H. Zhang, S.J. Campbell, Solid State Communications, (submitted, 1997). [4] S.R. Bulcock, H. Zhang, E. Wu, S.J. Campbell, to be published. [5] R.J. Hill, C.J. Howard, B.A. Hunter, Rep. AAEC / M112, 1995, Australian Atomic Energy Commission, Lucas Heights. [6] C.J. Howard, J. Appl. Cryst. 15 (1982) 615. [7] D.K. Smith, eds. D.L. Bish, J.E. Post, Rev. Mineral. 20 (1991) p. 183 [8] H. Zhang, S.J. Campbell, H.S. Li, E. Wu, in preparation. [9] I. Mayer, M. Tassa, J. Less-Common Metals 19 (1969) 173. [10] I. Mayer, I. Felner, J. Solid State Chem. 7 (1973) 292. [11] J.M. Barandiaran, D. Gignoux, D. Schmitt, J.C. Gomez Sal, J. Rodriguez Fernandez, J. Magn. Magn. Mater. 69 (1987) 61.
Journal of Alloys and Compounds 264 (1998) 232–235
Crystal structure of a quaternary Nd–Ni–Si–B compound-refinement of synchrotron radiation data E. Wu*, H. Zhang, S.J. Campbell, S.R. Bulcock
1
School of Physics, University College, The University of New South Wales, Australian Defence Force Academy, Canberra, ACT 2600, Australia Received 16 May 1997
Abstract High-resolution synchrotron-radiation powder data have been obtained for a new quaternary Nd–Ni–B–Si compound with the nominal composition of NdNi 8 SiB 3 . The data were collected on a (573 mm radius) cylindrical cassette diffractometer using imaging plates. The crystal structure of the new compound has been determined and refined by Rietveld analysis. The initial structural model and space group ¨ for the refinement is based on insight derived from earlier Mossbauer and TEM analyses. Refinement of the powder data leads to a ˚ and c57.8731(2) A ˚ in the space group P4 / nmm. The refined compound is tetragonal unit-cell with lattice parameters a511.2260(2) A found to have a composition of NdNi 9.7 Si 1.3 B |4 . The atomic coordinates for the Nd, Ni and Si atoms have been determined. Small amounts of the impurity phases, NdNi 5 and Nd 3 Ni 13 B 2 , have also been determined and refined in the analysis. 1998 Elsevier Science S.A. Keywords: Synchrotron diffraction; Rietveld refinement; Structure determination
1. Introduction
2. Experimental
A quaternary phase of the R–T–Si–B (R: rare-earth, T: transition-metal) system which fills a gap between the important ternary rare-earth transition-metal systems R–T– B and R–T–Si, has recently been reported [1,2]. The overall structural and magnetic properties of two of these compounds with overall compositions, NdCo 8 SiB 3 and NdNi 8 SiB 3 have been investigated by X-ray diffraction ¨ and Mossbauer spectroscopy [1–3], with the symmetry of the compounds having been determined from an electron diffraction analysis [4]. These studies have provided important information for the study of the crystal structure of the new phase. High-resolution synchrotron-radiation powder data has recently been collected for the NdNi 8 SiB 3 sample, and, building on from these earlier studies [1–4], this paper reports the determination of the crystal structure of this phase from a Rietveld profile refinement of the synchrotron data.
A sample of nominal composition of NdNi 8 SiB 3 was synthesised by arc melting high purity elements of Nd (99.9 wt.%), Ni (99.9 wt.%), Si (99.9 wt.%) and B (99.999 wt.%). The sample was then wrapped in a protective Ta foil and annealed in sealed evacuated quartz tubes at 900 8C for two weeks. The synchrotron data was collected on a multipurpose diffractometer at the Australian National Beamline Facility (ANBF) at the Photon Factory of the National Laboratory for High Energy Physics (KEK) in Tsukuba, Japan. The ANBF beamline monochromator is a Si(111) channel cut crystal located about 11 m from the radiation source. The diffractometer is a cylindrical cassette camera mounted 2 with 4003200 mm imaging plate (IP) storage phosphor detectors. The ground powder of the specimen was inserted in a 0.3 mm glass capillary and mounted in a sample spinner goniometer in the camera cassette, at a distance of 14 m from the source of the synchrotron radiation. A powder pattern was collected at room temperature (2361 8C) from a set of two IPs clamped on each side of the cassette covering the 2u range 65 to 640 8. The IPs were exposed for 10 min in the cassette which was evacuated to a
*Corresponding author. 1 Present address: Electron Microscope Unit (F09), University of Sydney, NSW 2006, Australia. 0925-8388 / 98 / $19.00 1998 Elsevier Science S.A. All rights reserved. PII S0925-8388( 97 )00274-0
E. Wu et al. / Journal of Alloys and Compounds 264 (1998) 232 – 235
pressure of about 0.1 Torr. A high-resolution data set of intensity versus scattering angle 2u, with a step size of 0.01 8, was extracted from the IP images. The synchrotron ˚ from a wavelength was calibrated to be 0.62649 A measurement of a standard Si powder sample.
3. Structural refinements and results The structure of the compound was analysed using a Rietveld refinement program RIET2 [5]. Peaks were modelled using the pseudo-Voigt profile function and corrected for asymmetry in the form of a sum of five peaks method [6]. The background was modelled using a refinable fourth-order polynomial. The intensity of the Bragg peaks was modified for a minor preferred orientation on [110] using the March–Dollase function, and corrected for the effects of sample absorption by applying the transmission factor assuming a real sample cylindrical radius of 0.1 mm [5]. Determination of the atomic coordinates in the unit cell for an unknown structure can rarely be carried out based on powder diffraction data only. Most of the intensity modelling programs were written to calculate the intensities from a proposed structural arrangement to test the model [7]. In the case of this quaternary Nd–Ni–Si–B compound, the space group of the structure has been determined on the basis of electron diffraction studies [4], which considered two space groups P4 / n and P4 / nmm, and identified P4 / nmm as the most likely space group for the phase. The initial structural model was established from in¨ formation derived from Mossbauer effect spectroscopy and TEM data [2–4]. The main feature of the room tempera¨ ture Mossbauer spectrum [2,3] is a singlet, indicating a highly symmetric crystal field environment around the 57 Fe atoms introduced as a dilute dopant in the compound. A simple explanation for this is that the transition metal sublattice is isostructurally related to a close packed fcc lattice, and a structural model for the transition metal atoms was developed along these lines [3], with the Nd atoms assumed to be in a few positions of high symmetry. The atomic arrangement in a unit-cell of this initial model can only be described by the lower symmetry space group P4 / n which displays two strong reflection peaks, consistent with those observed experimentally. The refinement appears to converge after a few cycles of refinements on the Ni positions in this model, with some of the atomic positions for the Ni atoms having been found to change significantly. These changes allow constraints to be imposed on the Ni atoms in accordance with the atomic coordinate positions 16k, 8i and 8j of the higher symmetry space group of P4 / nmm proposed from the TEM analysis [4]. However, an unacceptably high temperature factor was derived for a set of 8i positions, and the Ni atoms in this set were accordingly replaced by the lighter Si atoms.
233
Table 1 The atomic coordinates, thermal and population parameters for NdNi 9.7 Si 1.3 B |4 (cf. Fig. 1) Refinement of Space group P4 / nmm (No. 129) origin choice 2 ˚ c57.8731(2) A, ˚ Z56, Dcal 58.0 g cm 23 , Dmeas 57.8 g a511.2260(2) A, 23 cm R p 55.45%, R wp 57.35%, R exp 52.92%, R B 53.37% for 226 reflections in NdNi 9.7 Si 1.3 B |4 ; R B 52.98% for 25 reflections of NdNi 5 ; R B 53.52% for 60 reflections of Nd 3 Ni 13 B 2 . Site
x
y
z
˚ 2) Biso (A
Nd(1) 2b Nd(2) 4d Ni(1) 2c Ni(2) 8i 1 Ni(3) 8i 2 Ni(4) 8i 3 Ni(5) 8j 1 Ni(6) 8j 2 Ni(7) 16k Si 8i
3/4 0 1/4 1/4 1/4 1/4 0.3659(4) 0.3608(4) 0.4108(4) 1/4
1/4 0 1/4 0.0203(4) 0.6405(4) 0.0839(5) x x 0.5759(4) 0.929(1)
1/2 0 0.467(1) 0.474(1) 0.889(1) 0.973(1) 0.249(1) 0.701(1) 0.6671(4) 0.176(2)
0.1(1) 0.2(1) 0.1(2) 20.2(1) 0.2(1) 20.2(1) 0.1(1) 0.2(1) 20.2(1) 0.4(3)
Some additional peaks which could not be indexed in the refinements were, however, determined through further analysis to be due to the impurity phases, NdNi 5 and Nd 3 Ni 13 B 2 . The structural information for these two compounds were inserted into the Rietveld program and refinement of the data for these three phases — the compound of interest and the two impurities — carried out simultaneously. The quantitative analysis of the refinement of the whole pattern led to impurity contents of 10 wt.% NdNi 5 and 7 wt.% Nd 3 Ni 13 B 2 , thus corresponding to a main phase composition of |NdNi 9.2 Si 1.4 B 4 in the material of overall starting composition NdNi 8 SiB 3 . The refined structural parameters and agreement values are reported in Table 1. Plots of the observed, calculated and difference profiles for the Rietveld refinement are shown in Fig. 1, with the unit cell determined for NdNi 9.7 Si 1.3 B |4 , (as viewed along the [001] direction) shown in Fig. 2.
4. Discussion On the basis of the refined structure, the elements Nd, Ni and Si have the composition NdNi 9.7 Si 1.3 , in which the calculated composition of Ni is slightly higher than the composition determined from the refinement. Nonetheless, the overall composition is reasonably close to the composition of NdNi 9.2 Si 1.4 B 4 derived from the quantitative analysis. This deviation of |0.5% in the Ni concentration as based on the quantitative analysis is acceptable. In this refined formula, the compositions for the Si and Ni are fixed according to their distinct crystallographic sets, whereas in our preparations of this and related compounds
234
E. Wu et al. / Journal of Alloys and Compounds 264 (1998) 232 – 235
Fig. 1. Rietveld refinement of the synchrotron diffraction pattern obtained for the quaternary Nd–Ni–Si–B compound. The fit also includes refinement of NdNi 5 and Nd 3 Ni 13 B 2 impurity phases. The difference profile for the refinement is also shown along with the reflection markers for the P4 / nmm structure of the new phase (top set of markers); NdNi 5 (middle markers) and Nd 3 Ni 13 B 2 (bottom markers).
Fig. 2. The refined unit cell of NdNi 9.7 Si 1.3 B |4 as viewed along [001]. The boundary indicates the a axis; the large open circles are Nd atoms; the small circles are Ni atoms and the small open circles are the Si sites. The atomic positions (in %) along the c axis are labelled for each atom.
E. Wu et al. / Journal of Alloys and Compounds 264 (1998) 232 – 235
[8], it has been noticed that the Si and Ni components can be varied over a certain range of the composition. We have therefore assumed that Si atoms can be mixed with Ni atoms in some of the Ni sites in the unit cell, similar to the approach taken in other structural studies of Nd–Ni–Si compounds [9,10]. However, the refined occupancies for the Si atoms on any of the Ni sites were too small to provide a considerable composition variation for the present NdNi 9.7 Si 1.3 B |4 compound. Further investigation is required to clarify the difference between the experimental behaviour and the apparent Si, Ni mixing based on the refinement of the present compound. The refinement (Table 1) reveals that very small and even negative temperature factors have been derived for the Ni atoms; this could be due to the influence of the B atoms which are unaccounted for at this stage. A current neutron diffraction investigation of this compound will clarify this point. An interesting feature of this structure (Fig. 2) is that the coordinates of the Ni atoms in the 2c, 8i 1 , 8j 1 and 8j 2 sites are distorted only slightly from their positions as proposed initially for an ideal close packed fcc sublattice model [3]. The 2c sites remain in a highly symmetrical environments with each of the Ni atoms surrounded by 12 Ni atoms from half of the 8i 1 , 8j 1 and 8j 2 sites which have similar ˚ In the unit-cell as a interatomic distances (2.52–2.57 A). whole, the calculated interatomic distances are in the ˚ for the following ranges: for the Ni–Ni atoms 2.1–2.6 A; ˚ for the Ni–Nd atoms 2.9–3.3 A; ˚ Ni–Si atoms 2.3–2.7 A; ˚ These values correand for the Si–Nd atoms 3.2–3.3 A. spond well with those obtained for Nd–Ni–Si compounds [9–11]. On the basis of our original structural model [3], and the calculated interstitial spaces, there could be another set of 2c sites for Ni or Si atoms created a distance of about c / 2 from the current 2c sites. The refinement agreement values were found to improve on introduction of atoms at these sites, but only allowed very low fractional occupancies for Ni or Si atoms in these sites. This behaviour could occur as a result of the unallocated B atoms occurring in these sites. However, because of the relatively low scattering factor for B, at this stage no attempt has been made to locate the B atoms on refinement of the synchrotron data. As mentioned
235
above, a neutron diffraction study of the corresponding Nd–Ni–Si– 11 B compound of this phase is underway. The relatively large neutron scattering length and low neutron absorption coefficient for 11 B will assist in enabling us to determine the positions of the B atoms in the unit cell.
Acknowledgements The authors are grateful to Australian National Beamline Facility (ANBF) for support and access to the multipurpose synchrotron diffractometer. The authors thank Dr. D.J. Cookson of Australian Nuclear Science and Technology Organisation and Dr. E.M.A. Gray of Griffith University for their assistance in data collection and Dr. H.S. Li of the University of New South Wales and Dr. B.A. Hunter of Australian Nuclear Science and Technology Organisation for helpful discussions on aspects of this work. The work was supported by ARC grants, leading to the award of a Postgraduate Research Scholarship (HZ) and a Research Associateship. HZ also acknowledges the award of an Overseas Postgraduate Research Scholarship.
References [1] E. Wu, G.H.J. Wantenaar, S.J. Campbell, H.S. Li, J. Phys. Cond. Matt. 5 (1993) L457. [2] H. Zhang, S.J. Campbell, S.R. Bulcock, A.V.J. Edge, E. Wu, Physica B 229 (1997) 333. [3] E. Wu, H. Zhang, S.J. Campbell, Solid State Communications, (submitted, 1997). [4] S.R. Bulcock, H. Zhang, E. Wu, S.J. Campbell, to be published. [5] R.J. Hill, C.J. Howard, B.A. Hunter, Rep. AAEC / M112, 1995, Australian Atomic Energy Commission, Lucas Heights. [6] C.J. Howard, J. Appl. Cryst. 15 (1982) 615. [7] D.K. Smith, eds. D.L. Bish, J.E. Post, Rev. Mineral. 20 (1991) p. 183 [8] H. Zhang, S.J. Campbell, H.S. Li, E. Wu, in preparation. [9] I. Mayer, M. Tassa, J. Less-Common Metals 19 (1969) 173. [10] I. Mayer, I. Felner, J. Solid State Chem. 7 (1973) 292. [11] J.M. Barandiaran, D. Gignoux, D. Schmitt, J.C. Gomez Sal, J. Rodriguez Fernandez, J. Magn. Magn. Mater. 69 (1987) 61.