Journal of Alloys and Compounds 351 (2003) 151–157
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Structural studies of deuterides of yttrium carbide J.P. Maehlen, V.A. Yartys*, B.C. Hauback Institute for Energy Technology, P.O. Box 40, Kjeller N-2027, Norway Received 16 August 2002; received in revised form 30 August 2002; accepted 30 August 2002
Abstract The yttrium carbide Y 2 C and two related deuterides Y 2 CD 2.55 and Y 2 CD 2.0 , were studied by powder neutron and synchrotron X-ray diffraction. During deuteration the Y sublattice undergoes a transformation from cubic (ABCA . . . ) to the hexagonal (ABA . . . ) close packing of the Y layers. Carbon atoms orderly occupy each second of the available Y 6 octahedra, both in the initial carbide and in the deuterides. The Y–Y distances in the C-occupied octahedra are considerably shorter than in the empty ones. In the saturated deuteride, Y 2 CD 2.55 , D atoms occupy three different sites, including Y 4 tetrahedra and two types of Y 6 octahedra. On a transition to the lower deuteride Y 2 CD 2.0 , deuterium is completely removed from both types of octahedra leaving completely occupied Y 4 sites. The latter transformation is accompanied by lattice expansion. 2002 Elsevier Science B.V. All rights reserved. Keywords: Rare earth compounds; Gas–solid reaction; Crystal structure; Neutron diffraction
1. Introduction In recent years, carbon materials have attracted considerable interest as possible hydrogen storage materials with high weight efficiency. However, by now the nature of chemical bonding between hydrogen and carbon in those materials is not completely understood. On the other hand, metal hydrides have been extensively studied and the general principles of these hydrides are well established. In a paper by Lobier [1] the structures of two hydride phases of Y 2 C (Y 2 CH 2.7 and Y 2 CH 2 ) were presented. For the lower hydride Y 2 CH 2 it was reported that hydrogen mainly occupied Y 4 tetragonal sites. Before complete occupation of Y 4 tetrahedra, hydrogen started occupying adjacent Y 6 octahedral sites in Y 2 CH 2 . The fractional hydrogen occupation numbers reported was 0.89 and 0.22 for the tetrahedral and octahedral sites, respectively. For the hydride Y 2 CH 2.7 almost complete H occupation of the Y 4 tetrahedra was reported (fractional occupation of 0.98). In addition, two different Y 6 octahedral positions were partially occupied, both with equal fractional occupation numbers of 0.74. A volume contraction on a transition from the lower deuteride to Y 2 CH 2.7 was observed. In both *Corresponding author. Fax: 147-63-812-905. E-mail address:
[email protected] (V.A. Yartys).
structures, the hydrogen atoms were reported to interact most prominently with the Y atoms and not with C atoms. This work was aimed on studies of the system Y 2 C–D, where an interaction between C-sublattice and deuterium absorption–desorption properties can be revealed.
2. Experimental details Yttrium (Goodfellow) with a purity of 99.9% and graphite (Goodfellow) with spectral purity (99.9999%) were used for the preparation of the alloy. The alloy Y 2 C was made by arc melting in argon atmosphere. It was vacuum annealed at 1075 K for 3 weeks and waterquenched afterwards. To reduce incoherent scattering contribution during the powder neutron diffraction (PND) experiments, deuterium (purity 99.8%) loaded samples were used instead of hydrogen. The deuteration was performed by application of the direct gas–solid state reaction. After activation at 625 K in dynamic secondary vacuum, Y 2 C readily absorbed deuterium. A final pressure of 3.7 bar was used for saturation, where volumetric calculations suggested the composition Y 2 CD 2.460.1 . A lower deuteride Y 2 CD 2.0 and the intermetallic carbide Y 2 C were prepared from the
0925-8388 / 02 / $ – see front matter 2002 Elsevier Science B.V. All rights reserved. PII: S0925-8388( 02 )01028-9
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Table 1 Summary of the diffraction data (298 K)
Data collection Wavelength ˚ ( l (A)) Angle range (2u ) Step lengths (D2u ) Refinement (software) Number of data points Number of background parameters used Number of profile parameters used Total number of refined parameters
Y2C
Y 2 CD 2.0
SRPXD
PND
SRPXD
PND
0.50056
1.5554
0.50095
1.5554
3.5–34.1
10–130
4.0–26.3
10–130
0.006
0.05
0.005
0.05
Rietveld (GSAS) 5099
Combined Rietveld (GSAS) 2400 4460
Rietveld (GSAS) 2400
12
10
6
12
10
8
6
10
25
Y 2 CD 2.55
36
35
atoms occupy in an ordered way each second layer of the available Y 6 octahedra. These layers are stacked along [0 0 1] in the sequence filled–empty–filled–empty, etc. For convenience of presentation we will name the layers made by carbon-containing Y 6 octahedra as ‘the carbon layers’. The crystal structure of Y 2 C is shown in Fig. 1, Table 2 includes the unit cell data, and the crystal structure data are presented in Table 3.
3.2. Saturated deuteride Y2 CD2.55 The crystal structure of the saturated deuteride Y 2 CD 2.55 was refined on the basis of the PND data. Deuteration of Y 2 C is accompanied by changes of the space group ] ] symmetry (R3 m→P31 m) and unit cell parameters (a deut | ] Œ3 a Y C , c deut | 1 / 3 c Y C ) with close to zero volume 2 2 expansion. The Y sublattice in Y 2 CD 2.55 can be represented as a two-layer hexagonal close packing of Y layers (ABA . . . ). ] A description of Y 2 CD 2.55 in the space group P31 m involves four octahedral sites (1a, 1b, 2c and 2d) and two
saturated deuteride under dynamic vacuum at selected temperatures. Synchrotron powder X-ray diffraction (SRPXD) data were collected at the Swiss–Norwegian Beam Line at ˚ for the lower deuteride, ESRF, Grenoble ( l50.50095 A ˚ l50.50056 A for the carbide, scintillation detectors). PND data were collected with the PUS high-resolution two-axis diffractometer [2] at the JEEP II reactor, Kjeller, ˚ focusing Ge(511) monochromator, Norway ( l51.5554 A; position sensitive detectors). The diffraction data for the deuterides and the carbide were analysed according to the Rietveld-type method [3] using the program GSAS [4]. The backgrounds were modelled with cosine Fourier series polynomials and the peak shapes were described by a multi-term Simpson’s rule integration of the pseudo-Voigt function [5,6]. For the lower deuteride a combined Rietveld analysis was performed for the PND and SRPXD data. Details concerning the collected diffraction data are given in Table 1.
3. Results and discussion
3.1. Y2 C carbide The crystal structure data for the carbide were derived from SRPXD data using a sample of Y 2 C obtained after completing one deuterium absorption–desorption cycle. In agreement with Atoji [7] refinement showed the formation ˚ of a trigonal structure (space group R3¯m; a53.625 A, ˚ c517.964 A) that can be represented as a cubic ABCA . . . stacking of the closed packed Y nets. Carbon
Fig. 1. The crystal structure of Y 2 C. Two types of layers, ‘carbon layers’ and the ‘non-carbon layers’ are shown.
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Table 2 Synthesis conditions, crystallographic data and reliability factors for the PND/ SRPXD refinements. Calculated standard deviations in parentheses Y 2 CD 2.55( 3 )
Y 2 CD 2.00( 6 )
Y2C
Synthesis
5 bar D 2 at 298 K
D 2 desorption a from Y 2 CD 2.55 at 775 K
Complete D 2 desorption a from Y 2 CD 2.55 at 1075 K
Space group: ˚ a (A) ˚ c (A) ˚ 3) V (A b Z (DV /Vcarbide )?100 c Specific volume d R p (%) Rw p (%) x2
¯ m (No. 162) P31 6.3124(3) 5.9320(4) 204.70(2) 3 0.14 34.12 5.1 6.5 1.7
P3¯ m1 (No. 164) 3.6556(1) 5.9973(2) 69.406(4) 1 1.86 34.70 12.6 e 17.6 e 5.5 e
R3¯ m (No. 166) 3.62483(8) 17.9640(5) 204.410(9) 3 – 34.06 4.5 6.1 3.4
a
Desorption of D 2 was performed under secondary vacuum. Number of formula units in the cell. c DV defined as Vdeuteride 2Vcarbide . d Specific volume defined as Vcell /N, where N5number of Y atoms in the cell. e Reliability factor from combined Rietveld refinement (SRPXD and PND). b
tetrahedral sites (6k 1 and 6k 2 ). The carbon atoms occupy every second Y 6 octahedron in an ordered way, forming layers perpendicular to [001]. The structure consists of two non-equivalent layers; the carbon layers with carbon atoms in the Y 6 octahedra and with all the Y 4 tetrahedra empty, and the non-carbon layers (or deuterium layers) where all Table 3 Crystal structure data (atomic coordinates a , occupation (n) and isotropic ˚ 2 )) derived from Rietveld refinement of temperature factors (Uiso 3100 A SRPXD data for Y 2 C, PND and SRPXD data for Y 2 CD 2.0 and PND data for Y 2 C 2.55 Y2C Y in 6c Y in 2d Y in 6k
C C C D
in in in in
3a 1a 2c 2d
D1 in 1b D2 in 2d D3 in 6k
z Uiso z Uiso x z Uiso Uiso Uiso Uiso z Uiso n Uiso n Uiso n x z Uiso n
Y 2 CD 2.0
Y 2 CD 2.55
0.25793(5) 0.34 0.2265(3) 0.15 0.3511(4) 0.2431(4) 0.47 0.44 0.23
0.32 0.32
0.607(1) 3.26 1.00(3) 1.88 0.19(3) 1.88 0.79(1) 0.3077(4) 0.6254(4) 1.88 0.977(8)
Calculated standard deviations in parentheses. a Occupied positions are for Y 2 C (space group R3¯ m): 3a (0,0,0), 6c (0,0,z); for Y 2 CD 2.0 (space group P3¯ m1 ): 1a (0,0,0), 2d (1 / 3,2 / 3,z); for ¯ m): 1a (0,0,0), 1b (0,0,1 / 2), 2c (1 / 3,2 / 3,0), Y 2 CD 2.55 (space group P31 2d (1 / 3,2 / 3,1 / 2) and 6k (x,0,z).
interstitials are occupied by deuterium atoms. Compared to the initial carbide the carbon layers are shifted in the a,b-direction upon deuteration (see Fig. 2), giving a transformation from a ccp yttrium sublattice to a hcp yttrium sublattice. There are three different D-occupied sites, labelled D1, D2 and D3 (see Fig. 3). The D1 is located inside an ˚ The ‘expanded’ Y 6 octahedron (cavity of radius 0.97 A). D2 is located in the irregular ‘shrinked’ Y 6 -octahedron ˚ Finally, the D3 is coordinated by (cavity of radius 0.83 A). ˚ The a deformed Y 4 tetrahedron (cavity of radius 0.54 A). observed, calculated and difference plots of the Rietveldtype refinement are shown in Fig. 4. The significantly different occupation numbers of the octahedral sites (n D1 /n D2 50.19 / 0.79) can be attributed to the differences in size of the octahedra. The reduced occupancy is characteristic for a too large site. Similar regularities have been observed earlier for deuterides of Tb 3 Ni 6 Al 2 and Zr 3 Fe [8–11]. Relevant interatomic distances for Y 2 CD 2.55 are given in Table 4. The shortest interatomic Y–Y and D–D distances, 3.443(5) and ˚ respectively, are slightly smaller in Y 2 CD 2.55 2.080(5) A, than in the structure of the binary deuteride YD 3 [12] ˚ D–D52.14 A). ˚ (Y–Y53.64 A;
3.3. Lower deuteride Y2 CD2.0 Thermal desorption spectroscopy study of Y 2 CD 2.55 showed that desorption starts at 425 K and is completed at 1075 K. Three desorption peaks, at 555, 785 (the largest one) and 1060 K are observed. The two latter peaks are rather wide and partially overlapping. A constant desorption temperature of 775 K in vacuum was chosen for the synthesis of a lower deuteride. From combined Rietveld-type refinement of the PND
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Fig. 2. Comparison of the crystal structures of Y 2 C, Y 2 CD 2.55 and Y 2 CD 2.0 . The neighbouring layers of C-filled Y 6 (carbon layers) in Y 2 C are shifted by (1 / 3, 2 / 3) while in Y 2 CD 2.5 and Y 2 CD 2.0 they lay above each other along the c-axis. The indexes a 1 /b 1 /c 1 , a 2 /b 2 /c 2 and a 3 /b 3 /c 3 correspond to the unit cell axis of Y 2 CD 2.55 , Y 2 CD 2.0 and Y 2 C, respectively.
and SRPXD data it became evident that the crystal structure of the lower deuteride is described by the space ] group P3 m1 and by a unit cell with three times smaller ] volume compared to Y 2 CD 2.55 (a Y 2 CD 2.0 | 1 /Œ3 a Y 2 CD 2.55 , c Y 2 CD 2.0 | c Y 2 CD 2.55 ). The composition is Y 2 CD 2.0 . Only one site, the Y 4 tetrahedron, remains occupied by deuterium, corresponding to the D3 site in the saturated deuteride. In contrast to earlier work [1], no evidence of D occupation of octahedral sites was found. The transformation Y 2 CD 2.55 →Y 2 CD 2.0 is accompanied by a volume expansion, leading to an increase in size of the Y 4 ˚ The most significant tetrahedra from r50.54 to r50.60 A.
difference for the interatomic distances is an increase of the separation between the carbon and the deuterium atoms ˚ in the fully saturated deuteride (for the rising from 2.95 A ˚ in the lower deuteride (see Table 4). D3 site) to 3.16 A
4. Discussion When the carbide is subjected to deuterium absorption the carbon containing Y 6 octahedral layers shift in the a,b-direction (see Fig. 2), transforming the packing from
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Fig. 3. Presentation of the crystal structures of Y 2 CD 2.0 – 2.55 as layers. (a) D1 filled Y 6 octahedra and D2 filled Y 6 octahedra in the structure of Y 2 CD 2.55 ; (b) D3 (Y 2 CD 2.55 ) and D (Y 2 CD 2.0 ) filled Y 4 tetrahedra and (c) stacking of C-centred Y 6 octahedra and D filled Y 4 tetrahedra in the structures of Y 2 CD 2.55 and Y 2 CD 2.0 . Only the non-carbon layers are shown in (a) and (b).
the edge sharing Y 4 tetrahedra in the carbide into the face sharing Y 4 tetrahedra in the deuterides. There are two types of nonequivalent Y 4 -tetrahedra in the Y 2 C structure; one inside the carbon layer (coordinates ˚ with carbon (1 / 3, 2 / 3, 0.86); cavity of radius 0.46 A) atoms bounded to three of its triangular faces, and one inside the non-carbon layer (coordinates (1 / 3, 2 / 3, 0.61); ˚ with a carbon atom connected to cavity of radius 0.60 A) only one of the triangular faces. Upon deuteration, deuterium first enters the latter site. This induces a transformation in the metal sublattice, shifting the C-filled Y 6 octahedra away from the D-occupied tetrahedra. The number of nonequivalent tetrahedra in the deuteride is not changed. One tetrahedron has carbon atoms connected to three of its triangular faces (with interstice to carbon ˚ respectively). The distances of 2.1, 2.3 and 2.3 A,
deuterium atoms do not occupy these tetrahedra. The second type of tetrahedron does not have any carbon atoms connected to its triangular faces. These tetrahedra are nearly completely filled by D. This indicates strong C– H(D) repulsive interactions. Furthermore, a complete deuterium occupation of both tetrahedral sites gives a very ˚ and is short D–D interatomic distance (less than 1.5 A), ˚ not acceptable according to the ‘2 A’ rule [13,14]. Upon increased loading of deuterium, two other crystallographically nonequivalent Y 6 octahedra are occupied; the smaller one appeared to be the preferred site. The maximum deuterium content achievable in the model corresponds to the stoichiometry Y 2 CD 3 . Assuming that atoms are spherical, the ideal positions for the interstitial deuterium atoms (i.e., the positions allowing for the largest spherical interstitial atoms) have been found to
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Fig. 4. Powder neutron diffraction pattern for Y 2 CD 2.55 showing observed (circles), calculated (upper line) and difference (bottom line) plot. The positions of the Bragg peaks are shown as bars.
be very close to the observed ones (the largest displace˚ ment 0.04 A), indicating the absence of strong H–H repulsion effects and significant deformations of the metal sublattice. A common feature for both the carbide and its corresponding deuterides is that the Y–Y distances in the Coccupied octahedra are considerably shorter than in the empty (carbide) or deuterium occupied ones. In Y 2 C, the shortest interatomic distance between yttrium atoms sur˚ while the corresponding rounding carbon atoms is 3.423 A, shortest Y–Y distances in the vacant Y 6 octahedra is 3.890 ˚ The corresponding distances for Y 2 CD 2.0 are 3.441 and A. ˚ and for Y 2 CD 2.55 3.442 and 3.839 A, ˚ respective3.901 A, ly. This means a shorter distance between the Y nets in the carbon layers compared to the non-carbon layers. Defining the metal net distance ratio (R net ) as the distance between neighbouring Y nets in the non-carbon layers divided by
the distance between neighbouring Y nets in the carbon layers, the following values are obtained for Y 2 C, Y 2 CD 2.0 and Y 2 CD 2.55 , respectively: R net (Y 2 C)51.21, R net (Y 2 CD 2.0 )51.21 and R net (Y 2 CD 2.55 )51.06. In the saturated deuteride, the occupation of D in Y 6 octahedra reduces the differences in the Y–Y distances in the carbon layers and the non-carbon layers due to Y–H attractive interaction. With deuterium in the tetrahedral sites only (Y 2 CD 2.0 ), the stacking distances between the yttrium nets are almost unchanged compared to the initial alloy. The main increase in the volume on deuteration is attributed to the enlargements in the basal plane. Upon the transition from Y 2 CD 2.0 to Y 2 CD 2.55 , the deuterium atoms start to occupy the octahedra, and the distances between the yttrium nets in the carbon layer increase, while the distances between the yttrium nets in the non-carbon layer decrease.
Table 4 ˚ in Y 2 CD 2.55 , Y 2 CD 2.0 , Y 2 C and YD 3 Selected interatomic distances (A) Atoms
Y 2 CD 2.55
Y 2 CD 2.0
Y2C
YD 3 a
Y–Y Y–C Y–D C–D D–D C–C
3.442(5)–3.839(5) 2.507(2)–2.644(3) 2.234(3)–2.290(5) 2.951(5) 2.080(5)–2.848(5) 3.644(3)
3.441(3)–3.901(3) 2.510(1) 2.283(6)–2.334(3) 3.162(1) 2.471(5) 3.6556(1)
3.423(1)–3.890(1) 2.4929(5) – – – 3.6248(1)
3.637(2)–3.935(2) – 2.105(2)–2.518(3) – 2.136(3)–2.794(3) –
Calculated standard deviations in parentheses. a Data from Ref. [12], T5295 K.
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The increase of the unit cell volume during the hydrogenation of hydride-forming alloys is mainly due to the expansion of tetrahedra accommodating hydrogen atoms. The octahedral interstitials are generally larger and do not need to expand significantly to host hydrogen atoms. On transformation from Y 2 CD 2.0 to Y 2 CD 2.55 the extra deuterium atoms are absorbed in rather big octahedral sites ˚ The Y–H (with an initial cavity radius of about 0.9 A). bonding contracts these octahedra in the same way as Y–C bonds contract the C containing Y 6 octahedra compared to the empty octahedra in Y 2 C and Y 2 CD 2.0 . This is the main reason for the observed volume contraction of the saturated deuteride compared to the lower one. The contraction going from the lower to the saturated deuteride is well known for the hydrides formed by rare earth metals (e.g., the La–D system, Ce–D system) [15– 18]. Since the deuterides of the yttrium carbide can be viewed as layered compounds, where one layer contains yttrium–deuterium–yttrium and the other layer contains yttrium–carbon–yttrium slabs, locally, around the deuterium atoms, the compound resembles a rare earth deuteride. The present study shows strong differences between two Y 6 sites and proposes an explanation of these differences, in contrast with the earlier work [1].
5. Conclusions Studies of the crystal structures of Y 2 C-based deuterides show that their structural chemistry is governed by the interplay of different atomic interactions, Y–Y, Y–C, C– H(D) and H–H (D–D). They are dominated by strong repulsion between C and H(D) manifested by rebuilding of the Y sublattice in order to avoid simultaneous occupation of the neighbouring CY 6 octahedra and DY 4 tetrahedra. The difference in occupancy of the available tetrahedral Y 4 and octahedral Y 6 sites by H(D) was considered and explained on the basis of their size and surroundings.
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Acknowledgements We are grateful to the Swiss Norwegian Beam Line at ESRF for the possibility to collect high quality diffraction data and to Dr. D. Fruchart, Laboratoire de Cristallographie at CNRS, Grenoble for the opportunity to use their arc melting facility. This work received a support from Norsk Hydro ASA and Norwegian Research Council.
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