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Determination of deuterium site occupation in Zr4Pd2OD4.5
Journal of Alloys and Compounds 361 (2003) 108–112
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Determination of deuterium site occupation in Zr 4 Pd 2 OD 4.5 a, a a b c H.W. Brinks *, A.J. Maeland , B.C. Hauback , R.C. Bowman Jr. , J.S. Cantrell b
a Department of Physics, Institute for Energy Technology, PO Box 40, Kjeller N-2027, Norway Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109 -8099, USA c Chemistry Department, Miami University, Oxford, OH 45056, USA
Received 8 April 2003; accepted 14 April 2003
Abstract Powder neutron diffraction analysis has been used to determine hydrogen site occupation at room temperature in the lattice of the oxygen-stabilized Zr 4 Pd 2 O phase. For the composition Zr 4 Pd 2 OD 4.5 it was established that only two types of interstices, designated as D7 and D2, were occupied. Hydrogen in the D7 site is octahedrally surrounded by 6Zr. This site is nearly filled. In the D2 site hydrogen is tetrahedrally co-ordinated to 3Zr and Pd and is 1 / 3 filled. The distribution of hydrogen on the D2 sites is random. 2003 Elsevier B.V. All rights reserved. Keywords: Metal hydrides; Crystal structure; Neutron diffraction; Synchrotron radiation
1. Introduction A large number of ternary oxide alloys with the h carbide structure, i.e. cubic E9 3 Ti 2 Ni-type, has been reported [1,2]. The corresponding binary alloys (Ti 2 M or Zr 2 M where M5Co, Cu, Fe, Ni, or Pd) frequently react with hydrogen to form metastable hydrides, but in some cases disproportionate to form the more stable hydrides, TiH 2 and ZrH 2 . Many of the ternary oxides with E9 3 structure form hydride phases that are stabilized with respect to disproportionation and are therefore of interest for certain storage applications. It is also of interest to note that while the alloy Ti 2 Fe does not exist, the oxide Ti 2 FeO is known, i.e. oxygen and other non-metallic elements such as N, C or Si stabilize the h phase. While there have been a number of studies of the hydrogen absorption / desorption process in oxygen stabilized h phases, only a few have addressed the question of hydrogen location in the structure. Westlake [3,4] has applied a geometrical model, based on the generalizations that hydrogen only occupy inter˚ stices which can accommodate a sphere of at least 0.40 A and hydrogen atoms cannot at the same time occupy sites ˚ to the problem. He that are separated by less than 2.10 A, furthermore stipulates that the interstices having the largest diameter are preferentially occupied by hydrogen, and that *Corresponding author. Tel.: 147-63-806-499; fax: 147-63-810-920. E-mail address: [email protected] (H.W. Brinks). 0925-8388 / 03 / $ – see front matter 2003 Elsevier B.V. All rights reserved. doi:10.1016 / S0925-8388(03)00446-8
the lattice expansion during hydrogen site filling expands ˚ 3 per added hydrogen atom. the metal lattice by 2–3 A Stioui et al. [5] determined the deuterium sites in Ti 4 Fe 2 OD 2.25 using neutron diffraction, Rotella et al. [6] similarly determined deuterium site occupancies in Zr 3 V3 OD x (x51.86, 2.85 and 4.93), Zavaliy et al. [7] reported on deuterium site occupation in Zr 3 V3 O 0.6 D 9.6 and Cantrell et al. [8] in Zr 4 Pd 2 OD 3.5 . We report here on the hydrogen positions in Zr 4 Pd 2 OD 4.5 .
2. Experimental Zr 4 Pd 2 O was prepared by arc melting appropriate stoichiometric amounts of high purity Hf-free Zr (99.9 at.%) and Pd (99.95 at.%) with optical grade ZrO 2 . The resulting, very brittle, pellet was ground to a powder in an argon-filled glove box and returned to the arc melter where it was remelted. The pellet was turned over and remelted several times. Despite the repeated remeltings, powder X-ray diffraction (PXD) analysis indicated unreacted ZrO 2 and formation of ZrPd 2 in addition to the desired Zr 4 Pd 2 O product. Our sample was made from a previously prepared deuteride, Zr 4 Pd 2 OD 3.72 . However, since deuterium loss could have occurred in storage, desorption and reabsorption of deuterium was done to obtain an accurate composition. Desorption from this sample was carried out in a Sieverts apparatus in dynamic vacuum to 550 8C, followed
H.W. Brinks et al. / Journal of Alloys and Compounds 361 (2003) 108–112
by exposure to deuterium at room temperature at an initial pressure of 1440 Torr. The reaction occurred immediately and was evidenced by a rapid rise in temperature to 150 8C. The nominal deuterium content was determined to be 4.2 D/ f.u. at 940 Torr. High-resolution PXD and PND examinations of the deuteride were done at room temperature. Following the completion of the PND data collection, deuterium was removed from the sample under dynamic vacuum at 550 8C, and both PXD and PND patterns were obtained on deuterium free Zr 4 Pd 2 O. PND data were collected with the PUS instrument [9] at the JEEP II reactor at Kjeller, Norway. Monochromatized ˚ were obtained from a Ge(511) neutrons with l51.5549 A focusing monochromator. The detector unit consists of two banks of seven position-sensitive 3 He detectors, each covering 208 in 2u (binned in steps of 0.058). The Zr 4 Pd 2 OD 4.2 sample was transferred in the argon glove box into a 6 mm diameter silica-glass tube which was connected to a pressure chamber with 760 Torr D 2 pressure to compensate for any deuterium loss which might have occurred in the sample during manipulations in the glove box; it is our experience that hydrogen (deuterium) loss from these samples can be a problem. Immediately after the PND measurement, the sample was transferred in the glove box to a 5 mm diameter vanadium sample holder, which was then closed and returned to repeat the PND data collecting. Data obtained this way give less background. PND data on Zr 4 Pd 2 O were also collected with the sample in the rotating 5 mm vanadium holder. The PXD data were recorded at station BM1B at the Swiss–Norwegian beam line at the European Synchrotron Radiation Facility (ESRF) in Grenoble, France. Zr 4 Pd 2 OD 4.2 was sealed in a 0.3 mm diameter boron– silica–glass capillary straight after preparation and measured by PXD 2 months later. Both samples were measured in rotating 0.3 mm diameter capillaries. Intensities for Zr 4 Pd 2 OD 4.2 were measured between 2u 53.5 and 31.78 in steps of D(2u )50.018. For Zr 4 Pd 2 O, the 2u range was 7.0 to 30.58 and in steps of D(2u )50.0058. The ˚ was obtained from a channel-cut wavelength 0.49983 A (111) Si monochromator. Rietveld refinements were carried out with Fullprof (version 1.9c) [10]. The neutron scattering lengths and X-ray form-factor coefficients were taken from the Fullprof library. Pseudo-Voigt profile functions were used. The background was modeled both by cosine Fourier series polynomials and interpolation between manually chosen points.
3. Results and discussion Rietveld refinements of synchrotron PXD data were carried out for Zr 4 Pd 2 O and Zr 4 Pd 2 OD 4.2 . Both samples contain the impurity phases ZrO 2 , estimated to be 6.2 and
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Table 1 Structural data for Zr 4 Pd 2 O, determined from PXD and PND ˚ a (A) x Zr1 x Pd ˚ 2) BZr1 (A ˚ 2) BZr2 (A 2 ˚ BPd (A ) ˚ 2) BO (A R wp x2 Zr 2 Pd (wt%) ZrO 2 (wt%)
PXD
PND
12.45274 (10) 0.0580 (2) 0.7140 (2) 0.36 (6) 0.95 (12) 1.18 (9) 0.98 (–) 7.00 2.66 20.7 6.2
12.4552 (5) 0.0571 (2) 0.7147 (2) 1.01 (5) 1.34 (9) 1.25 (8) 0.98 (9) 6.87 1.45 17.2 6.5
¯ origin set 2 (centre at 3m). ¯ Space group is Fd3m, Zr is in the 48f site (x 1 / 8 1 / 8), Zr2 in 16d ( ]12 ]12 ]12 ), Pd in 32e (xxx) and O in 16c (000). Estimated standard deviation in parentheses.
8.9 wt% respectively, and in addition Zr 2 Pd and Zr 2 PdHx , estimated to be 20.7 and 16.2 wt.%. The structural data on Zr 4 Pd 2 O are summarized in Table 1. The fit for Zr 4 Pd 2 OD 4.5 (D54.5 is the refined value obtained from the neutron data) is presented in Fig. 1 and the structural data is presented in Table 2. PND data were, as noted, obtained by an in situ experiment and a subsequent experiment in a closed V container. The unit-cell dimensions were equal, and the data from the V container, which is of better quality, was used in the final refinements. Hence, Zr 4 Pd 2 OD 4.5 does not appear to have lost any hydrogen during the short handling in the glove box. The unit-cell dimensions, obtained on the PXD sample by Rietveld refinements and presented in Table 2, indicate, however, that a small amount of deuterium has been released during storage for 2 months. The lattice parameter decreased from 12.8290 (9) in the freshly made sample used in the PND experiment to 12.8060 (3) obtained 2 months later by PXD. By comparison, the unit-cell dimension for the deuterium free sample, Zr 4 Pd 2 O, determined from PXD and PND data is 12.4527 (1) and 12.4552 (5), respectively (Table 1). The volume expansion ratios (DV /Valloy ) are 0.0928 and 0.0876, respectively, from the PND and PXD parameters. The assumption of the linear relationship between DV /Valloy and hydrogen content, as previously described by Cantrell et al. [8], implies the hydride composition was Zr 4 Pd 2 OD 4.25 during the PXD measurement. The PND refinements of the Zr 4 Pd 2 O sample indicate sample impurity levels in good agreement with those deduced from the synchrotron radiation data. The PND data for Zr 4 Pd 2 OD 4.5 , however, indicate a lower amount of Zr 2 PdD x (8.5 wt.%) than expected from that seen in the deuterium free sample, while the PXD data (16.2 wt.%) are in line with expectations. The reason for this is not understood. There are at least seven possible positions for deuterium in the h structure [3] (Table 3); all were considered and tested. The general position D2 was clearly the best one.
H.W. Brinks et al. / Journal of Alloys and Compounds 361 (2003) 108–112
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Fig. 1. Rietveld refinements (upper line) of PXD data collected with BM1B, ESRF (circles) for Zr 4 Pd 2 OD 4.5 at 20 8C. Positions of Bragg reflections are shown with bars for the nuclear contribution of Zr 4 Pd 2 OD 4.5 , Zr 2 PdD x and ZrO 2 (from the top). The difference between observed and calculated intensity are shown with the bottom line.
Combinations of D2 with the other positions indicated that D7 was nearly completely filled. Monte Carlo search with the occupation numbers of all seven D positions were
Table 2 Structural data for Zr 4 Pd 2 OD 4.5 , determined from PXD and PND ˚ a (A) x Zr1 x Pd x D2 y D2 z D2 ˚ 2) BZr1 (A ˚ 2) BZr2 (A ˚ 2) BPd (A ˚ 2) BO (A ˚ 2) BD1 (A ˚ 2) BD2 (A ˚ 2) n D2 (A ˚ 2) n D7 (A R wp (%) x2 Zr 2 Pd (wt%) ZrO 2 (wt%)
PXD
PND
12.8060 (3) 0.0580 (2) 0.7092 (2) 0.6316 (–) 0.7367 (–) 0.8399 (–) 0.13 (6) 20.07 (11) 0.94 (9) 1.75 (–) 2.58 (–) 2.00 (–) 0.333 (–) 0.91 (–) 4.68 6.96 16.2 8.9
12.8290 (9) 0.0590 (3) 0.7063 (2) 0.6316 (4) 0.7367 (5) 0.8399 (5) 1.35 (8) 0.57 (9) 0.70 (10) 1.75 (15) 2.4 (2) 3.7 (4) 0.333 (6) 0.91 (2) 5.36 2.47 8.5 12.5
¯ origin set 2 (centre at 3m). ¯ Space group is Fd3m, Zr is in the 48f site (x 1 / 8 1 / 8), Zr2 in 16d ( ]12 ]12 ]12 ), Pd in 32e (xxx), O in 16c (000) D2 in 192i (xyz) and D7 in 8a (1 / 8 1 / 8 1 / 8). Estimated standard deviation in parentheses.
tested, with the same conclusion. The results are presented in Table 2 and the fit shown in Fig. 2. Bond lengths in the structure are summarized for comparison in Table 4. The addition of D1 improves the fit even more (from x 2 52.47 to x 2 52.01), but this results in a D1–D7 ˚ with both sites filled around 75%. distance of 1.61 A, Therefore, this solution was rejected. Cantrell et al. [8] also reported occupation of D2 and D7, but in addition partial occupation of D4 sites. However, occupation of D4 sites ˚ seems unlikely because D4–D4 distances are only 0.58 A ˚ and D4–D2 distances only 1.45A, and it is our conclusion that the D4 sites are empty. The D2 positions could not be completely filled because of short D–D distances within this sublattice. D(Zr 3 Pd) polyhedra are face-sharing via a common Zr 3 face with a Table 3 ¯ The possible D positions in Zr 4 Pd 2 OD x (from [3]) in space group Fd3m, ¯ origin set 2 (centre at 3m)
D1 D2 D3 D4 D5 D6 D7
Site
x
y
z
Zr 3 Pd (32e) Zr 3 Pd (192i) Zr 2 PdO (96g) Zr 2 Pd 2 (96g) Pd 4 (8b) ZrPd 3 (32e) Zr 6 (8a)
0.802 0.632 0.781 0.859 0.375 0.413 0.875
0.802 0.737 0.781 0.859 0.375 0.413 0.875
0.802 0.840 0.150 0.534 0.375 0.413 0.875
H.W. Brinks et al. / Journal of Alloys and Compounds 361 (2003) 108–112
111
Fig. 2. Rietveld refinements (upper line) of PND data collected with PUS (circles) for Zr 4 Pd 2 OD 4.5 at 20 8C. Positions of Bragg reflections are shown with bars for the nuclear contribution of Zr 4 Pd 2 OD 4.5 , Zr 2 PdD x and ZrO 2 (from the top). The difference between observed and calculated intensity are shown with the bottom line.
˚ face-sharing via a common D–D distance of 0.836 A, ˚ and edgeZr 2 Pd face with a D–D distance of 1.877 A sharing via a common ZrPd edge with a D–D distance of ˚ The sublattice is approximately one-third filled, 1.898 A. and there are no convincing indications about extra reflec-
Table 4 ˚ for Zr 4 Pd 2 OD 4.5 Selected bond lengths (in A) Zr 4 Pd 2 OD 4.5 D2–Zr1 D2–Zr1 D2–Zr2 D2–Pd D7–Zr1 D2–D2
D7–D7 D2–D7 Zr1–Zr1 Zr1–Zr2 Zr2–Zr2 Zr1–Pd Zr2–Pd
1.934 1.982 2.051 2.001 2.360 0.830 1.872 1.906 5.553 3.618 3.337 3.337 4.534 3.066 2.762
tions due to ordering of the D2 sublattice. There are unexplained intensity contributions at 54.5 and 55.68, but this is probably caused by an unknown impurity phase. A partial view of the structure is seen in Fig. 3, revealing the octahedral D7 positions and the very closely connected D2 positions.
4. Conclusions Zr 4 Pd 2 O – – – – – –
– – 3.223 3.250 4.403 2.849 2.739
Deuterium (hydrogen) occupies the octahedral (Zr 6 ) D7 interstices in oxygen-stabilized Zr 4 Pd 2 OD 4.5 ; the sites are nearly filled. In addition, 1 / 3 of the tetrahedral (Zr 3 Pd) D2 sites are filled. There is no ordering at room temperature of hydrogen on the D2 sites.
Acknowledgements The skilful assistance from the project team at the Swiss–Norwegian Beam Line, ESRF, is gratefully acknowledged.
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References [1] M.V. Nevitt, J.W. Downey, R.A. Morris, Trans. Metall. Soc. AIME 218 (1960) 1019. [2] M.V. Nevitt, in: J.H. Westbrook (Ed.), Intermetallic Compounds, Wiley, New York, 1967, p. 214. [3] D.G. Westlake, J. Chem. Phys. 79 (1983) 4532. [4] D.G. Westlake, J. Less-Common Met. 105 (1985) 69. [5] C. Stioui, D. Fruchart, A. Rouault, R. Fruchart, E. Roudaut, J. Rebiere, Mater. Res. Bull. 16 (1981) 869. [6] F.J. Rotella, H.E. Flotow, D.M. Gruen, J.D. Jorgensen, J. Chem. Phys. 79 (1983) 4522. [7] I.Yu. Zavaliy, W.B. Yelon, P.Y. Zavalij, I.V. Saldan, V.K. Pecharsky, J. Alloys Comp. 309 (2000) 75. [8] J.S. Cantrell, R.C. Bowman Jr., A.J. Maeland, J. Alloys Comp. 330–332 (2002) 191. ˚ O. Steinsvoll, K. Johansson, O. Buset, J. [9] B. Hauback, H. Fjellvag, Jørgensen, J. Neutron Res. 8 (2000) 215. [10] J. Rodriguez-Carvajal, Physica B 192 (1993) 55.
Fig. 3. Structure of a segment of Zr 4 Pd 2 OD 4.5 . D7 is surrounded by Zr 6 and D2 by Zr 3 Pd.
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Determination of deuterium site occupation in Zr 4 Pd 2 OD 4.5 a, a a b c H.W. Brinks *, A.J. Maeland , B.C. Hauback , R.C. Bowman Jr. , J.S. Cantrell b
a Department of Physics, Institute for Energy Technology, PO Box 40, Kjeller N-2027, Norway Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109 -8099, USA c Chemistry Department, Miami University, Oxford, OH 45056, USA
Received 8 April 2003; accepted 14 April 2003
Abstract Powder neutron diffraction analysis has been used to determine hydrogen site occupation at room temperature in the lattice of the oxygen-stabilized Zr 4 Pd 2 O phase. For the composition Zr 4 Pd 2 OD 4.5 it was established that only two types of interstices, designated as D7 and D2, were occupied. Hydrogen in the D7 site is octahedrally surrounded by 6Zr. This site is nearly filled. In the D2 site hydrogen is tetrahedrally co-ordinated to 3Zr and Pd and is 1 / 3 filled. The distribution of hydrogen on the D2 sites is random. 2003 Elsevier B.V. All rights reserved. Keywords: Metal hydrides; Crystal structure; Neutron diffraction; Synchrotron radiation
1. Introduction A large number of ternary oxide alloys with the h carbide structure, i.e. cubic E9 3 Ti 2 Ni-type, has been reported [1,2]. The corresponding binary alloys (Ti 2 M or Zr 2 M where M5Co, Cu, Fe, Ni, or Pd) frequently react with hydrogen to form metastable hydrides, but in some cases disproportionate to form the more stable hydrides, TiH 2 and ZrH 2 . Many of the ternary oxides with E9 3 structure form hydride phases that are stabilized with respect to disproportionation and are therefore of interest for certain storage applications. It is also of interest to note that while the alloy Ti 2 Fe does not exist, the oxide Ti 2 FeO is known, i.e. oxygen and other non-metallic elements such as N, C or Si stabilize the h phase. While there have been a number of studies of the hydrogen absorption / desorption process in oxygen stabilized h phases, only a few have addressed the question of hydrogen location in the structure. Westlake [3,4] has applied a geometrical model, based on the generalizations that hydrogen only occupy inter˚ stices which can accommodate a sphere of at least 0.40 A and hydrogen atoms cannot at the same time occupy sites ˚ to the problem. He that are separated by less than 2.10 A, furthermore stipulates that the interstices having the largest diameter are preferentially occupied by hydrogen, and that *Corresponding author. Tel.: 147-63-806-499; fax: 147-63-810-920. E-mail address: [email protected] (H.W. Brinks). 0925-8388 / 03 / $ – see front matter 2003 Elsevier B.V. All rights reserved. doi:10.1016 / S0925-8388(03)00446-8
the lattice expansion during hydrogen site filling expands ˚ 3 per added hydrogen atom. the metal lattice by 2–3 A Stioui et al. [5] determined the deuterium sites in Ti 4 Fe 2 OD 2.25 using neutron diffraction, Rotella et al. [6] similarly determined deuterium site occupancies in Zr 3 V3 OD x (x51.86, 2.85 and 4.93), Zavaliy et al. [7] reported on deuterium site occupation in Zr 3 V3 O 0.6 D 9.6 and Cantrell et al. [8] in Zr 4 Pd 2 OD 3.5 . We report here on the hydrogen positions in Zr 4 Pd 2 OD 4.5 .
2. Experimental Zr 4 Pd 2 O was prepared by arc melting appropriate stoichiometric amounts of high purity Hf-free Zr (99.9 at.%) and Pd (99.95 at.%) with optical grade ZrO 2 . The resulting, very brittle, pellet was ground to a powder in an argon-filled glove box and returned to the arc melter where it was remelted. The pellet was turned over and remelted several times. Despite the repeated remeltings, powder X-ray diffraction (PXD) analysis indicated unreacted ZrO 2 and formation of ZrPd 2 in addition to the desired Zr 4 Pd 2 O product. Our sample was made from a previously prepared deuteride, Zr 4 Pd 2 OD 3.72 . However, since deuterium loss could have occurred in storage, desorption and reabsorption of deuterium was done to obtain an accurate composition. Desorption from this sample was carried out in a Sieverts apparatus in dynamic vacuum to 550 8C, followed
H.W. Brinks et al. / Journal of Alloys and Compounds 361 (2003) 108–112
by exposure to deuterium at room temperature at an initial pressure of 1440 Torr. The reaction occurred immediately and was evidenced by a rapid rise in temperature to 150 8C. The nominal deuterium content was determined to be 4.2 D/ f.u. at 940 Torr. High-resolution PXD and PND examinations of the deuteride were done at room temperature. Following the completion of the PND data collection, deuterium was removed from the sample under dynamic vacuum at 550 8C, and both PXD and PND patterns were obtained on deuterium free Zr 4 Pd 2 O. PND data were collected with the PUS instrument [9] at the JEEP II reactor at Kjeller, Norway. Monochromatized ˚ were obtained from a Ge(511) neutrons with l51.5549 A focusing monochromator. The detector unit consists of two banks of seven position-sensitive 3 He detectors, each covering 208 in 2u (binned in steps of 0.058). The Zr 4 Pd 2 OD 4.2 sample was transferred in the argon glove box into a 6 mm diameter silica-glass tube which was connected to a pressure chamber with 760 Torr D 2 pressure to compensate for any deuterium loss which might have occurred in the sample during manipulations in the glove box; it is our experience that hydrogen (deuterium) loss from these samples can be a problem. Immediately after the PND measurement, the sample was transferred in the glove box to a 5 mm diameter vanadium sample holder, which was then closed and returned to repeat the PND data collecting. Data obtained this way give less background. PND data on Zr 4 Pd 2 O were also collected with the sample in the rotating 5 mm vanadium holder. The PXD data were recorded at station BM1B at the Swiss–Norwegian beam line at the European Synchrotron Radiation Facility (ESRF) in Grenoble, France. Zr 4 Pd 2 OD 4.2 was sealed in a 0.3 mm diameter boron– silica–glass capillary straight after preparation and measured by PXD 2 months later. Both samples were measured in rotating 0.3 mm diameter capillaries. Intensities for Zr 4 Pd 2 OD 4.2 were measured between 2u 53.5 and 31.78 in steps of D(2u )50.018. For Zr 4 Pd 2 O, the 2u range was 7.0 to 30.58 and in steps of D(2u )50.0058. The ˚ was obtained from a channel-cut wavelength 0.49983 A (111) Si monochromator. Rietveld refinements were carried out with Fullprof (version 1.9c) [10]. The neutron scattering lengths and X-ray form-factor coefficients were taken from the Fullprof library. Pseudo-Voigt profile functions were used. The background was modeled both by cosine Fourier series polynomials and interpolation between manually chosen points.
3. Results and discussion Rietveld refinements of synchrotron PXD data were carried out for Zr 4 Pd 2 O and Zr 4 Pd 2 OD 4.2 . Both samples contain the impurity phases ZrO 2 , estimated to be 6.2 and
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Table 1 Structural data for Zr 4 Pd 2 O, determined from PXD and PND ˚ a (A) x Zr1 x Pd ˚ 2) BZr1 (A ˚ 2) BZr2 (A 2 ˚ BPd (A ) ˚ 2) BO (A R wp x2 Zr 2 Pd (wt%) ZrO 2 (wt%)
PXD
PND
12.45274 (10) 0.0580 (2) 0.7140 (2) 0.36 (6) 0.95 (12) 1.18 (9) 0.98 (–) 7.00 2.66 20.7 6.2
12.4552 (5) 0.0571 (2) 0.7147 (2) 1.01 (5) 1.34 (9) 1.25 (8) 0.98 (9) 6.87 1.45 17.2 6.5
¯ origin set 2 (centre at 3m). ¯ Space group is Fd3m, Zr is in the 48f site (x 1 / 8 1 / 8), Zr2 in 16d ( ]12 ]12 ]12 ), Pd in 32e (xxx) and O in 16c (000). Estimated standard deviation in parentheses.
8.9 wt% respectively, and in addition Zr 2 Pd and Zr 2 PdHx , estimated to be 20.7 and 16.2 wt.%. The structural data on Zr 4 Pd 2 O are summarized in Table 1. The fit for Zr 4 Pd 2 OD 4.5 (D54.5 is the refined value obtained from the neutron data) is presented in Fig. 1 and the structural data is presented in Table 2. PND data were, as noted, obtained by an in situ experiment and a subsequent experiment in a closed V container. The unit-cell dimensions were equal, and the data from the V container, which is of better quality, was used in the final refinements. Hence, Zr 4 Pd 2 OD 4.5 does not appear to have lost any hydrogen during the short handling in the glove box. The unit-cell dimensions, obtained on the PXD sample by Rietveld refinements and presented in Table 2, indicate, however, that a small amount of deuterium has been released during storage for 2 months. The lattice parameter decreased from 12.8290 (9) in the freshly made sample used in the PND experiment to 12.8060 (3) obtained 2 months later by PXD. By comparison, the unit-cell dimension for the deuterium free sample, Zr 4 Pd 2 O, determined from PXD and PND data is 12.4527 (1) and 12.4552 (5), respectively (Table 1). The volume expansion ratios (DV /Valloy ) are 0.0928 and 0.0876, respectively, from the PND and PXD parameters. The assumption of the linear relationship between DV /Valloy and hydrogen content, as previously described by Cantrell et al. [8], implies the hydride composition was Zr 4 Pd 2 OD 4.25 during the PXD measurement. The PND refinements of the Zr 4 Pd 2 O sample indicate sample impurity levels in good agreement with those deduced from the synchrotron radiation data. The PND data for Zr 4 Pd 2 OD 4.5 , however, indicate a lower amount of Zr 2 PdD x (8.5 wt.%) than expected from that seen in the deuterium free sample, while the PXD data (16.2 wt.%) are in line with expectations. The reason for this is not understood. There are at least seven possible positions for deuterium in the h structure [3] (Table 3); all were considered and tested. The general position D2 was clearly the best one.
H.W. Brinks et al. / Journal of Alloys and Compounds 361 (2003) 108–112
110
Fig. 1. Rietveld refinements (upper line) of PXD data collected with BM1B, ESRF (circles) for Zr 4 Pd 2 OD 4.5 at 20 8C. Positions of Bragg reflections are shown with bars for the nuclear contribution of Zr 4 Pd 2 OD 4.5 , Zr 2 PdD x and ZrO 2 (from the top). The difference between observed and calculated intensity are shown with the bottom line.
Combinations of D2 with the other positions indicated that D7 was nearly completely filled. Monte Carlo search with the occupation numbers of all seven D positions were
Table 2 Structural data for Zr 4 Pd 2 OD 4.5 , determined from PXD and PND ˚ a (A) x Zr1 x Pd x D2 y D2 z D2 ˚ 2) BZr1 (A ˚ 2) BZr2 (A ˚ 2) BPd (A ˚ 2) BO (A ˚ 2) BD1 (A ˚ 2) BD2 (A ˚ 2) n D2 (A ˚ 2) n D7 (A R wp (%) x2 Zr 2 Pd (wt%) ZrO 2 (wt%)
PXD
PND
12.8060 (3) 0.0580 (2) 0.7092 (2) 0.6316 (–) 0.7367 (–) 0.8399 (–) 0.13 (6) 20.07 (11) 0.94 (9) 1.75 (–) 2.58 (–) 2.00 (–) 0.333 (–) 0.91 (–) 4.68 6.96 16.2 8.9
12.8290 (9) 0.0590 (3) 0.7063 (2) 0.6316 (4) 0.7367 (5) 0.8399 (5) 1.35 (8) 0.57 (9) 0.70 (10) 1.75 (15) 2.4 (2) 3.7 (4) 0.333 (6) 0.91 (2) 5.36 2.47 8.5 12.5
¯ origin set 2 (centre at 3m). ¯ Space group is Fd3m, Zr is in the 48f site (x 1 / 8 1 / 8), Zr2 in 16d ( ]12 ]12 ]12 ), Pd in 32e (xxx), O in 16c (000) D2 in 192i (xyz) and D7 in 8a (1 / 8 1 / 8 1 / 8). Estimated standard deviation in parentheses.
tested, with the same conclusion. The results are presented in Table 2 and the fit shown in Fig. 2. Bond lengths in the structure are summarized for comparison in Table 4. The addition of D1 improves the fit even more (from x 2 52.47 to x 2 52.01), but this results in a D1–D7 ˚ with both sites filled around 75%. distance of 1.61 A, Therefore, this solution was rejected. Cantrell et al. [8] also reported occupation of D2 and D7, but in addition partial occupation of D4 sites. However, occupation of D4 sites ˚ seems unlikely because D4–D4 distances are only 0.58 A ˚ and D4–D2 distances only 1.45A, and it is our conclusion that the D4 sites are empty. The D2 positions could not be completely filled because of short D–D distances within this sublattice. D(Zr 3 Pd) polyhedra are face-sharing via a common Zr 3 face with a Table 3 ¯ The possible D positions in Zr 4 Pd 2 OD x (from [3]) in space group Fd3m, ¯ origin set 2 (centre at 3m)
D1 D2 D3 D4 D5 D6 D7
Site
x
y
z
Zr 3 Pd (32e) Zr 3 Pd (192i) Zr 2 PdO (96g) Zr 2 Pd 2 (96g) Pd 4 (8b) ZrPd 3 (32e) Zr 6 (8a)
0.802 0.632 0.781 0.859 0.375 0.413 0.875
0.802 0.737 0.781 0.859 0.375 0.413 0.875
0.802 0.840 0.150 0.534 0.375 0.413 0.875
H.W. Brinks et al. / Journal of Alloys and Compounds 361 (2003) 108–112
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Fig. 2. Rietveld refinements (upper line) of PND data collected with PUS (circles) for Zr 4 Pd 2 OD 4.5 at 20 8C. Positions of Bragg reflections are shown with bars for the nuclear contribution of Zr 4 Pd 2 OD 4.5 , Zr 2 PdD x and ZrO 2 (from the top). The difference between observed and calculated intensity are shown with the bottom line.
˚ face-sharing via a common D–D distance of 0.836 A, ˚ and edgeZr 2 Pd face with a D–D distance of 1.877 A sharing via a common ZrPd edge with a D–D distance of ˚ The sublattice is approximately one-third filled, 1.898 A. and there are no convincing indications about extra reflec-
Table 4 ˚ for Zr 4 Pd 2 OD 4.5 Selected bond lengths (in A) Zr 4 Pd 2 OD 4.5 D2–Zr1 D2–Zr1 D2–Zr2 D2–Pd D7–Zr1 D2–D2
D7–D7 D2–D7 Zr1–Zr1 Zr1–Zr2 Zr2–Zr2 Zr1–Pd Zr2–Pd
1.934 1.982 2.051 2.001 2.360 0.830 1.872 1.906 5.553 3.618 3.337 3.337 4.534 3.066 2.762
tions due to ordering of the D2 sublattice. There are unexplained intensity contributions at 54.5 and 55.68, but this is probably caused by an unknown impurity phase. A partial view of the structure is seen in Fig. 3, revealing the octahedral D7 positions and the very closely connected D2 positions.
4. Conclusions Zr 4 Pd 2 O – – – – – –
– – 3.223 3.250 4.403 2.849 2.739
Deuterium (hydrogen) occupies the octahedral (Zr 6 ) D7 interstices in oxygen-stabilized Zr 4 Pd 2 OD 4.5 ; the sites are nearly filled. In addition, 1 / 3 of the tetrahedral (Zr 3 Pd) D2 sites are filled. There is no ordering at room temperature of hydrogen on the D2 sites.
Acknowledgements The skilful assistance from the project team at the Swiss–Norwegian Beam Line, ESRF, is gratefully acknowledged.
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References [1] M.V. Nevitt, J.W. Downey, R.A. Morris, Trans. Metall. Soc. AIME 218 (1960) 1019. [2] M.V. Nevitt, in: J.H. Westbrook (Ed.), Intermetallic Compounds, Wiley, New York, 1967, p. 214. [3] D.G. Westlake, J. Chem. Phys. 79 (1983) 4532. [4] D.G. Westlake, J. Less-Common Met. 105 (1985) 69. [5] C. Stioui, D. Fruchart, A. Rouault, R. Fruchart, E. Roudaut, J. Rebiere, Mater. Res. Bull. 16 (1981) 869. [6] F.J. Rotella, H.E. Flotow, D.M. Gruen, J.D. Jorgensen, J. Chem. Phys. 79 (1983) 4522. [7] I.Yu. Zavaliy, W.B. Yelon, P.Y. Zavalij, I.V. Saldan, V.K. Pecharsky, J. Alloys Comp. 309 (2000) 75. [8] J.S. Cantrell, R.C. Bowman Jr., A.J. Maeland, J. Alloys Comp. 330–332 (2002) 191. ˚ O. Steinsvoll, K. Johansson, O. Buset, J. [9] B. Hauback, H. Fjellvag, Jørgensen, J. Neutron Res. 8 (2000) 215. [10] J. Rodriguez-Carvajal, Physica B 192 (1993) 55.
Fig. 3. Structure of a segment of Zr 4 Pd 2 OD 4.5 . D7 is surrounded by Zr 6 and D2 by Zr 3 Pd.