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Neutron scattering study of the dynamics of hydrogen and deuterium solved in crystalline Pd9Si2
Physica B 241 243 (1998) 332 334
ELSEVIER
Neutron scattering study of the dynamics of hydrogen and deuterium solved in crystalline Pd9Si 2 C. K a r m o n i k a'*, T.J. U d o v i c a, Q . H u a n g a, J.J. R u s h a, Y. A n d e r s s o n b, T . B . F l a n a g a n c aNIST Center for Neutron Research, National Institute of Standards" and Technology, Gaithersburg MD 20899, USA b Institute of Chemistry, Department ~?['lnorganic Chemistry, Universi~ q[" Uppsala, S-751 21 Uppsala, Sweden ~Department of Chemistry, Universi O, of Vermont, Burlington VT 05405, USA
Abstract Using neutron powder diffraction and neutron vibrational spectroscopy, we were able to determine the crystallographic sites for hydrogen/deuterium uptake in crystalline PdgSi 2. By means of quasielastic neutron scattering, a localized motion was revealed between two of these sites, which is consistent with the temperature-dependent occupation visible in powder diffraction refinements. ,~ 1998 Elsevier Science B.V. All rights reserved. Kevwords: Palladium-silicon compounds; Hydrogen uptake; Quasielastic neutron scattering; Localized motion
1. Introduction
2. Experimental details
The investigation of the structure [1] and hydrogen-absorbing abilities [2] of palladium-based alloys, e.g. palladium-phosphorus and palladiumsilicon compounds, has been the subject of intensive studies in recent years. Here we present a short summary of a comprehensive neutron scattering s t u d y - carried out on crystalline PdgSi2 as a model system - with the aim of determining the H / D sites (by means of neutron powder diffraction (NPD)) and elucidating the H and D dynamics (using neutron vibrational spectroscopy (NVS) and quasielastic neutron scattering (QENS)).
The experiments were carried out at the Neutron Beam Split-Core Reactor (NBSR) at the National Institute of Standards and Technology (NIST). Using the FCS spectrometer with a wavelength of 4.8A. for the QENS measurements, an elastic energy resolution of 73~teV half width at half maximum (HWHM) is achieved. The NVS experiments were performed using the BT4 spectrometer with the Cu(220) monochromator and the low-resolution Be filter analyzer (final energy 3 meV). The diffraction patterns were collected using the BT1 high-resolution powder diffractometer. Details of the sample preparation and the experimental setup are given in Ref. [3].
* Corresponding author. Fax: 1-(301)921-9847;e-mail: christof(~rrdjazz.nist.gov.
0921-4526/98/$19.00 (" 1998 ElsevierScienceB.V. All rights reserved Pll S092 1 -4526(97)00577-2
333
C. Karmonik et al. / Physica B 24l 243 (1998) 332 334 3. Results
3.1.
out to be difficult and suggests a low amount for all temperatures.
Vibrational spectroscopy
Fig. 1 compares the spectra for the deuterated and the hydrogenated samples at 18 K. A preliminary data analysis indicates the existence of two different sites for deuterium further related to as site (1) and site (2). To interpret the spectra of the hydrogenated sample, a contribution of a third site (site (3)) had to be taken into consideration [3] containing approx. 30% of the absorbed hydrogen. The existence of this third site could be confirmed by studying the sample with different hydrogen contents. The hydrogen in this site reveals a different temperature behaviour. While the modes of the deuterium and hydrogen attributed to site (1) and (2) soften in the temperature range 18-300 K, there is no such effect visible for the modes of site (3).
3.3. Quasielastic scattering
The results of the NVS and powder diffraction experiments suggest the existence of a localized motion between sites (1) and (2). In the hydrogenated sample however, there also seems to exist some in the investigated temperature range immobile amount of hydrogen in site (3) which appears as an elastic part in the quasielastic spectra. As it is not possible to separate this elastic part from the EISF of the localized motion, we performed measurements also on the deuterated system despite the considerably lower incoherent scattering cross section of that isotope to be able to compare the parameters of the motion in that simpler system (only two occupied sites) with the results of the hydrogenated sample.
3.2. P o w d e r diffraction
In PdgSi2D:,, two crystallographically different sites are found for deuterium, in agreement with the NVS results. Both sites are neighbouring pyramidal interstices formed by five Pd atoms [4]. The Fourier difference map indicates different amounts of deuterium in these sites. Moreover, refinements carried out at different temperatures indicate a change of the occupation ratio. At 16 K only one site seems to be occupied, whereas at room temperature, around one-third of the D occupies the second interstice. Due to the lower coherent scattering cross section of hydrogen, the data analysis of the PdgSi2H~ diffraction pattern is more difficult; however there is evidence for the occupation of an additional third site. As for deuterium, refinements suggest that hydrogen mainly occupies site (1), the amount of hydrogen in that position changing from 0.36 _+ 0.02 at 18 K to 0.24 _+ 0.02 at room temperature. For the third site, tentatively assigned to be of octahedral nature, no mentionable change of the amount of hydrogen could be found within the error limits in that temperature range (0.07 _+ 0.01 at 18 K and 0.10 _+ 0.01 at room temperature}. The determination of the occupation of site (2) turned
"" .' © ¢-
o,'%'
¢; ~,
~
¢~, .J
'. ~.
hydrogenated i
.l...a
~,~ ~ 30 '
deuterated ~5
40 ' 5; 6'0 70 ' Energy Transfer [meV]
80 '
Fig. 1. NVS spectra of the PdSiDo.z and the PdSiHo.~s at 18K.
Fig. 2. Schematic view of the proton motion between the two pyramidal sites.
334
C. Karmonik et al. / Physica B 241 243 (1998) 332-334
For each temperature, the data for both isotopes are in excellent agreement with the motion of a particle in an asymmetric double well potential [5]. (A more detailed description of the data analysis will be given in Ref. [3].) In that analysis, 30% of the incorporated hydrogen was expected to be immobile and located at site (3). Fig. 2 schematically summarizes the hydrogen motion between site (1) and site (2).
amount or are empty. This might be explained by the different distances of these interstices to the silicon atoms: site (1) is 3.17~, and site (2) 2.88~, apart from the nearest Si-atom. The temperaturedependent occupation of site (1) and site (2) found in powder diffraction refinements could be explained by a localized motion between these sites; the mean jump distance of 1.74 + 0.03 ~, is in good agreement with the crystallographic distance of 1.59 + 0.08 ,A.
4. Discussion and conclusion Using different neutron scattering techniques, three different sites for the H / D uptake in crystalline Pd9Si2 could be identified: two are of pyramidal nature, i.e. surrounded by five palladium atoms, the third site might be attributed to an octahedral environment. The occupation of these sites differs for each isotope: One pyramidal site contains most or all of the hydrogen/deuterium while the other sites only incorporate a small
References [1] Y. Andersson, Chemica Scripta 26 A (1986)99. [2] T.B. Flanagan, H. Noh, A. Craft, Y. Andersson, J. Solid State Chem. 120 (1995)90. [3] C. Karmonik,T.J. Udovic,Q. Huang, J.J. Rush, Y. Andersson, T.B. Flanagan, to be published. [4] T.J. Udovic,J.J Rush, T.B. Flanagan, H. Nob, Y. Andersson, J. AlloysCompounds 253 & 254 (1997)255. [5] Bee,Quasielastic Neutron Scattering,Adam Hilger,Bristol, 1988.
ELSEVIER
Neutron scattering study of the dynamics of hydrogen and deuterium solved in crystalline Pd9Si 2 C. K a r m o n i k a'*, T.J. U d o v i c a, Q . H u a n g a, J.J. R u s h a, Y. A n d e r s s o n b, T . B . F l a n a g a n c aNIST Center for Neutron Research, National Institute of Standards" and Technology, Gaithersburg MD 20899, USA b Institute of Chemistry, Department ~?['lnorganic Chemistry, Universi~ q[" Uppsala, S-751 21 Uppsala, Sweden ~Department of Chemistry, Universi O, of Vermont, Burlington VT 05405, USA
Abstract Using neutron powder diffraction and neutron vibrational spectroscopy, we were able to determine the crystallographic sites for hydrogen/deuterium uptake in crystalline PdgSi 2. By means of quasielastic neutron scattering, a localized motion was revealed between two of these sites, which is consistent with the temperature-dependent occupation visible in powder diffraction refinements. ,~ 1998 Elsevier Science B.V. All rights reserved. Kevwords: Palladium-silicon compounds; Hydrogen uptake; Quasielastic neutron scattering; Localized motion
1. Introduction
2. Experimental details
The investigation of the structure [1] and hydrogen-absorbing abilities [2] of palladium-based alloys, e.g. palladium-phosphorus and palladiumsilicon compounds, has been the subject of intensive studies in recent years. Here we present a short summary of a comprehensive neutron scattering s t u d y - carried out on crystalline PdgSi2 as a model system - with the aim of determining the H / D sites (by means of neutron powder diffraction (NPD)) and elucidating the H and D dynamics (using neutron vibrational spectroscopy (NVS) and quasielastic neutron scattering (QENS)).
The experiments were carried out at the Neutron Beam Split-Core Reactor (NBSR) at the National Institute of Standards and Technology (NIST). Using the FCS spectrometer with a wavelength of 4.8A. for the QENS measurements, an elastic energy resolution of 73~teV half width at half maximum (HWHM) is achieved. The NVS experiments were performed using the BT4 spectrometer with the Cu(220) monochromator and the low-resolution Be filter analyzer (final energy 3 meV). The diffraction patterns were collected using the BT1 high-resolution powder diffractometer. Details of the sample preparation and the experimental setup are given in Ref. [3].
* Corresponding author. Fax: 1-(301)921-9847;e-mail: christof(~rrdjazz.nist.gov.
0921-4526/98/$19.00 (" 1998 ElsevierScienceB.V. All rights reserved Pll S092 1 -4526(97)00577-2
333
C. Karmonik et al. / Physica B 24l 243 (1998) 332 334 3. Results
3.1.
out to be difficult and suggests a low amount for all temperatures.
Vibrational spectroscopy
Fig. 1 compares the spectra for the deuterated and the hydrogenated samples at 18 K. A preliminary data analysis indicates the existence of two different sites for deuterium further related to as site (1) and site (2). To interpret the spectra of the hydrogenated sample, a contribution of a third site (site (3)) had to be taken into consideration [3] containing approx. 30% of the absorbed hydrogen. The existence of this third site could be confirmed by studying the sample with different hydrogen contents. The hydrogen in this site reveals a different temperature behaviour. While the modes of the deuterium and hydrogen attributed to site (1) and (2) soften in the temperature range 18-300 K, there is no such effect visible for the modes of site (3).
3.3. Quasielastic scattering
The results of the NVS and powder diffraction experiments suggest the existence of a localized motion between sites (1) and (2). In the hydrogenated sample however, there also seems to exist some in the investigated temperature range immobile amount of hydrogen in site (3) which appears as an elastic part in the quasielastic spectra. As it is not possible to separate this elastic part from the EISF of the localized motion, we performed measurements also on the deuterated system despite the considerably lower incoherent scattering cross section of that isotope to be able to compare the parameters of the motion in that simpler system (only two occupied sites) with the results of the hydrogenated sample.
3.2. P o w d e r diffraction
In PdgSi2D:,, two crystallographically different sites are found for deuterium, in agreement with the NVS results. Both sites are neighbouring pyramidal interstices formed by five Pd atoms [4]. The Fourier difference map indicates different amounts of deuterium in these sites. Moreover, refinements carried out at different temperatures indicate a change of the occupation ratio. At 16 K only one site seems to be occupied, whereas at room temperature, around one-third of the D occupies the second interstice. Due to the lower coherent scattering cross section of hydrogen, the data analysis of the PdgSi2H~ diffraction pattern is more difficult; however there is evidence for the occupation of an additional third site. As for deuterium, refinements suggest that hydrogen mainly occupies site (1), the amount of hydrogen in that position changing from 0.36 _+ 0.02 at 18 K to 0.24 _+ 0.02 at room temperature. For the third site, tentatively assigned to be of octahedral nature, no mentionable change of the amount of hydrogen could be found within the error limits in that temperature range (0.07 _+ 0.01 at 18 K and 0.10 _+ 0.01 at room temperature}. The determination of the occupation of site (2) turned
"" .' © ¢-
o,'%'
¢; ~,
~
¢~, .J
'. ~.
hydrogenated i
.l...a
~,~ ~ 30 '
deuterated ~5
40 ' 5; 6'0 70 ' Energy Transfer [meV]
80 '
Fig. 1. NVS spectra of the PdSiDo.z and the PdSiHo.~s at 18K.
Fig. 2. Schematic view of the proton motion between the two pyramidal sites.
334
C. Karmonik et al. / Physica B 241 243 (1998) 332-334
For each temperature, the data for both isotopes are in excellent agreement with the motion of a particle in an asymmetric double well potential [5]. (A more detailed description of the data analysis will be given in Ref. [3].) In that analysis, 30% of the incorporated hydrogen was expected to be immobile and located at site (3). Fig. 2 schematically summarizes the hydrogen motion between site (1) and site (2).
amount or are empty. This might be explained by the different distances of these interstices to the silicon atoms: site (1) is 3.17~, and site (2) 2.88~, apart from the nearest Si-atom. The temperaturedependent occupation of site (1) and site (2) found in powder diffraction refinements could be explained by a localized motion between these sites; the mean jump distance of 1.74 + 0.03 ~, is in good agreement with the crystallographic distance of 1.59 + 0.08 ,A.
4. Discussion and conclusion Using different neutron scattering techniques, three different sites for the H / D uptake in crystalline Pd9Si2 could be identified: two are of pyramidal nature, i.e. surrounded by five palladium atoms, the third site might be attributed to an octahedral environment. The occupation of these sites differs for each isotope: One pyramidal site contains most or all of the hydrogen/deuterium while the other sites only incorporate a small
References [1] Y. Andersson, Chemica Scripta 26 A (1986)99. [2] T.B. Flanagan, H. Noh, A. Craft, Y. Andersson, J. Solid State Chem. 120 (1995)90. [3] C. Karmonik,T.J. Udovic,Q. Huang, J.J. Rush, Y. Andersson, T.B. Flanagan, to be published. [4] T.J. Udovic,J.J Rush, T.B. Flanagan, H. Nob, Y. Andersson, J. AlloysCompounds 253 & 254 (1997)255. [5] Bee,Quasielastic Neutron Scattering,Adam Hilger,Bristol, 1988.