Journal of Alloys and Compounds 313 (2000) L10–L14

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Letter

Neodymium trihydride, NdH 3 , with tysonite type structure a

b

G. Renaudin , P. Fischer , K. Yvon

a,c ,

*

a

b

` , CH-1211 Geneva, Switzerland Laboratoire de Cristallographie, Universite´ de Geneve ¨ Neutronenstreuung, Paul Scherrer Institute and ETHZ, CH-5303 Villigen, Switzerland Laboratorium f ur c School of Science, Griffith University, 4111 Qld., Australia Received 15 August 2000; accepted 26 August 2000

Abstract The crystal structure of trigonal neodymium trihydride has been refined by neutron powder diffraction on a two-phase deuteride sample ˚ c56.8778(2) A, ˚ T5293 K, LaF 3 (tysonite) type structure). The hcp metal atom host structure (NdD 3 , space group P3¯c1, a56.6581(2) A, ˚ and contains three sorts of deuterium atoms, one occupying tetrahedral interstices with bond distances in the range Nd–D52.36–2.58 A, ˚ One of the triangular sites is ordered two having nearly triangular metal coordinations with bond distances of Nd–D52.24 and 2.26 A. and the other disordered, in contrast to previous work on rare-earth analogues that suggested both sites to be disordered. NdD 3 coexists with a cubic phase of composition NdD 2.61 in which deuterium atoms occupy tetrahedral and octahedral interstices of a ccp metal atom ˚ host structure (space group Fm3¯m, a55.4324(2) A).  2000 Elsevier Science B.V. All rights reserved. Keywords: Neodymium hydrides; Neutron diffraction; Crystal structure determination

1. Introduction Neodymium is the lightest lanthanide in the series of trivalent rare-earth elements (R) that forms hydrides having both cubic close packed (ccp) and hexagonal close packed (hcp) metal atom arrangements [1]. Lighter lanthanides (R5La, Ce, Pr) form hydrides having ccp metal arrangements only, at least under ordinary conditions. Complete structure data for RH 3 hydrides having hcp metal arrangements have so far only been reported for R5Ho [2], Y [3] and Dy [4]. The results suggest a trigonal LaF 3 (tysonite) type structure in which the anion sublattice is partially disordered. In DyD 3 [4], for example, the deuterium atoms occupy up to five crystallographically different sites of which one is ordered and four are disordered. The present paper reports structure data on the trigonal neodymium analogue NdD 3 . It will be shown that the deuterium distribution in this phase can be described by a model having only three atomic sites of which two are ordered and one is disordered. Data on the coexisting phase NdD 21x having a ccp metal arrangement are also presented. At a deuterium content of x50.61 its structure

*Corresponding author. E-mail address: [email protected] (K. Yvon).

has cubic symmetry, in contrast to the tetragonal symmetry reported for the concentration range 0.30,x,0.38 [5]. The data on both phases are of interest for theoretical calculations and a better understanding of the metal–hydrogen interactions in R–H systems.

2. Experimental

2.1. Sample preparation Samples were prepared by hydrogenation (deuteration) of neodymium metal (pieces, 99.99% purity) in an autoclave. The temperature was increased to 823 K at a hydrogen (deuterium) gas pressure of about 90 bar for 1 day. The autoclave was air-quenched and opened in an argon-filled glove box. The polycrystalline products had a dark brown colour and were sensitive to air and moisture. They consisted of two hydride (deuteride) phases, one having trigonal symmetry and the other cubic symmetry (see 2.2). Attempts to obtain single phase products by changing the synthesis conditions failed. Decreasing the temperature and cooling speed, for example, did not allow to destabilise the cubic phase, as was suggested during a previous study [6] that described the latter as a metastable high-temperature phase.

0925-8388 / 00 / $ – see front matter  2000 Elsevier Science B.V. All rights reserved. PII: S0925-8388( 00 )01200-7

G. Renaudin et al. / Journal of Alloys and Compounds 313 (2000) L10 –L14

2.2. X-ray diffraction The samples were characterised by X-ray powder diffraction at room temperature by using a Guinier camera with sealed Lindeman capillaries (CuKa radiation) and a Bruker D8 diffractometer (Bragg–Brentano geometry, CuKa 1 radiation) equipped by an air tight sample holder. Corundum (a-Al 2 O 3 ) powder was used as an internal ˚ c512.9894 A). ˚ The diffraction standard (a54.7583 A, patterns showed mixtures of trigonal and cubic hydride (deuteride) phases. For the trigonal phases the patterns were consistent with a hcp metal atom substructure. However, a structure refinement by the Rietveld method showed that the Nd atoms were shifted towards positions of lower symmetry parallel to the basal plane, thus leading to the trigonal superstructure of tysonite (space group P3¯c1, NdD 3 : Nd on 6 f x,0,1 / 4; etc, with x Nd 50.3252(9)). The cell parameters (Table 1) were consistent with those ˚ [6]) and the reported for NdD 3 (a53.84, c56.80 A ˚ [7]), fluoride analogue NdF 3 (a57.030(2), c57.200(2) A except that the former values refer to a substructure. For the cubic phases the patterns were consistent with a ccp metal atom substructure. No superstructure peaks and no broadening or splitting of the diffraction peaks were detected, in contrast to previous work on NdH 21x [5] for which a tetragonal distortion was reported for hydrogen contents in the range 0.30,x,0.38. Cell parameters are given in Table 1. The cell volume of the cubic phase ˚ 3 ) is bigger than that of the substructure of (V5160.3 A ˚ 3 ) [5], but smaller than tetragonal NdD 2.36 (V/ 25159.2 A that of the metastable cubic high-temperature phase (a5 ˚ V5163.1 A ˚ 3 ) reported in [6]. 5.464 A,

2.3. Neutron diffraction The hydrogen distributions were investigated by neutron diffraction on the deuteride. A sample of | 4 g mass was enclosed in a cylindrical vanadium container of 8-mm I.D. and placed on the HRPT [8] powder diffractometer at PSI, ˚ 2u range 5–1598; step size 2u 5 Villigen ( l51.494 A; 0.058; T5293 K). The transmission factor was calculated (mR50.38) and the data corrected accordingly. The structure refinement by FULLPROF [9] included the binary phases NdD 3 and NdD 21x . No impurity phase was detected, and vanadium contributed only marginally to the diffraction

Table 1 Cell parameters of neodymium hydrides and deuterides

Cubic NdX 2.61 Trigonal NdX 3

X5H a

X5D

˚ a55.4486(9) A ˚ a56.6629(3) A ˚ c56.8853(4) A

˚ a55.4324(2) A ˚ a56.6581(2) A ˚ c56.8778(2) A

a Hydrogen content of hydride assumed to be identical to deuterium content of deuteride.

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¯ pattern. For trigonal NdD 3 (space group P3c1) deuterium was put on the anion positions of the tysonite type structure. One site (D1, site 12 g, site symmetry 1) is near the centre of the tetrahedral interstices, and two sites are off-centre octahedral interstices having respectively triangular (D3, site 2 a, site symmetry 32), or nearly triangular metal co-ordinations (D2, site 4 d, site symmetry 3). Preliminary refinements showed that D2 was ordered, whereas D3 was displaced from the (ordered) in-plane position 2 a (0,0,1 / 4 etc) to the (disordered) off-plane (split-atom) position 4(c) (0,0,z etc, z|0.18, symmetry 3). In order to find a reliable value for z the isotropic atomic displacement amplitudes of D3 were constrained to those of D2 which has a similar (but not identical) atom environment as D3. The resulting model differs from those reported for tysonite type deuterides such as YD 3 [3] and DyD 3 [4] by the following details. The positional coordinates z of D2 and D3 are not constrained, D2 is not a split-atom position, and the deuterium distribution close to D3 does not result from a superposition of an in-plane position 2 a and an off-plane position 4 c. For cubic NdD 21x (space group Fm3¯m) deuterium was put into tetrahedral interstices (D1) with full occupancy and into octahedral interstices (D2) with partial occupancy. In a first step D2 was put at the centre of the metal octahedra (site 4 b, 1 / 2,1 / 2,1 / 2, etc. symmetry m3¯m), and its occupancy was refined while constraining its displacement amplitudes. The value found (occ.50.610(5)) indicated the overall composition of this phase to be NdD 2.61 . In a second step D2 was allowed to move along k111l to a site of lower symmetry (site 32 f, x,x,x, etc, symmetry 3 m), while its occupancy was fixed to the value found previously (0.61 / 8). During the last refinement cycles the following 28 parameters were allowed to vary: zero position (one), scale factors (two), profile (eight), cell (three), atom positions (seven), preferred orientation (two) and isotropic atomic displacement (five). The molar ratio between the trigonal and cubic phase was found to be 0.90. The diffraction patterns are shown in Fig. 1 and refinement results are listed in Table 2. A list of selected interatomic distances is given in Table 3, and the deuterium atom environments in the trigonal phase are illustrated in Fig. 2.

3. Results and discussion The overall structure of NdD 3 is isotypic to tysonite and the rare-earth analogues RD 3 (R5Ho [2], Y [3], Dy [4]). However, its detailed deuterium distribution, in particular that in the vicinity of the sites having triangular (or nearly triangular) metal co-ordinations (D2 and D3), differs from the anion distributions in tysonite and the above deuterides. In tysonite and HoD 3 [2] the anion sites at the metal triangles are ordered, one being situated above the plane of a triangle (symmetry 3) and the other in the plane of another triangle (symmetry 32). In NdD 3 one site (D2)

G. Renaudin et al. / Journal of Alloys and Compounds 313 (2000) L10 –L14

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Fig. 1. Observed (a), difference (b) and calculated (c1: NdD 2.61 , c2: NdD 3 ) neutron powder diffraction patterns for a Nd–D two-phase sample ( l 51.494 ˚ A).

˚ above the plane of a is ordered and situated at |0.45 A triangle, whereas the other (D3) is a disordered ‘split˚ above and below the atom’ position situated at |0.17 A plane of a triangle (Fig. 2). In YD 3 and DyD 3 the deuterium distributions at both triangles appear to be disordered. In DyD 3 they were described [4] as a superposition of two split-atom positions and a position at the centre of the metal triangle, thus leading to a total of five crystallographic different, partially occupied deuterium

sites. Moreover, the co-ordinates and occupancies of these sites were constrained such as to render the deuterium atom distributions around both metal triangles chemically equivalent. According to the present work these distributions are chemically non-equivalent inasmuch as both metal triangles differ with respect to dimensions and atomic environments. In NdD 3 the metal triangles are ˚ than smaller around (ordered) D2 (Nd–Nd53.83 A) ˚ around (disordered) D3 (Nd–Nd53.86 A), and the

Table 2 Refinement results a on neutron powder diffraction data (T5295 K, estimated standard deviations in parentheses) Phase

Atom

Site

x /a

y /b

z /c

˚ 2) Biso (A

Occupancy

NdD 2.61 (Fm3¯ m, Z54) Dx 56.19 g / cm 3

Nd D1 D2

4a 8c 32 f

0 1/4 0.469(2)

0 1/4 0.469(2)

0 1/4 0.469(2)

0.67(5) 1.85(8)b 1.85(8)b

1(2) 1(2) 0.610(2) / 8 c

NdD 3 (P3¯ c1, Z56) Dx 5 5.67 g / cm 3

Nd D1 D2 D3

6f 12 g 4d 4c

0.335(1) 0.3456(8) 1/3 0

0 0.3218(8) 2/3 0

1/4 0.0954(3) 0.1850(7) 0.225(1)

0.33(3) 1.84(4) 1.57(5)b 1.57(5)b

1(2) 1(2) 1(2) 0.5(2)

a

R p 59.259%, R wp 57.56%, x 2 54.42; R bragg 5 2.88% (NdD 2.61 ), 5.85% (NdD 3 ). Constrained. c Previously refined to 0.610(5) by assuming overall isotropic displacement amplitudes. b

G. Renaudin et al. / Journal of Alloys and Compounds 313 (2000) L10 –L14 Table 3 ˚ (T5295 K, estimated standard deviation in Interatomic distances (A) parentheses) NdD 2.61

NdD 3

a

Nd–8D1 –D2 –D2 –12Nd

2.3524(1) 2.56(1) 2.89(1) 3.8413(1)

D2–D1 –3D1 –3D1 –D1 –3Nd –3Nd Nd–D3 –2D2 –2D1 –2D1 –2D1 –2D1 –4Nd –2Nd –2Nd –4Nd

2.06(1) 2.27(1) 2.46(1) 2.64(1) 2.56(1) 2.89(1) 2.237(7) 2.259(3) 2.361(6) 2.380(2) 2.453(8) 2.576(8) 3.834(8) 3.863(8) 4.081(5) 4.099(3)

D2 a –3Nd –3D1 –3D1 –3D1

2.259(3) 2.418(6) 2.587(6) 2.851(6)

D1–4Nd –D2 –D2 –D2 –D2 –6D1

2.3524(1) 2.06(1) 2.27(1) 2.46(1) 2.64(1) 2.7163(1)

D1 a –D1 –Nd –Nd –D3 –D2 –Nd –D3 –Nd –2D1 –D2 –D1

2.144(3) 2.361(6) 2.380(2) 2.398(6) 2.418(6) 2.453(8) 2.546(6) 2.576(8) 2.584(7) 2.587(6) 2.589(5)

D3 a –3Nd –3D1 –3D1

2.237(7) 2.398(6) 2.546(6)

˚ are listed. Only D–D distances less than 3.1 A

deuterium environment around the former triangle is less symmetric than around the latter (Fig. 2). As expected for low co-ordination environments the Nd–D bond distances within the metal triangles are rather short, i.e. Nd–D25 ˚ for the ordered deuterium site (smaller metal 2.26 A ˚ for the disordered deuterium triangle) and Nd–D352.24 A site (bigger metal triangle). The Nd–D bond distances within the metal tetrahedra are much longer, i.e. Nd–D15 ˚ and compare well with those in other Nd 2.36–2.58 A containing solid state metal deuterides containing 4-coordinate deuterium such as NdD 2.36 [5], Nd 2 Fe 17 D 4.8 [10] ˚ 2.33 A, ˚ and 2.37A, ˚ and NdCo 5 D 2.8 [11] (Nd–D52.32 A, respectively). A structural aspect which has not yet been discussed in detail with tysonite type metal deuterides are the displacements of the deuterium atoms away from the centre of the tetrahedral and octahedral metal interstices. Those in the tetrahedral interstices appear to be governed mainly by repulsive interactions between the deuterium anions. In fact, the close proximity of these interstices in hcp metal arrangements (they share faces, as opposed to edges in fcc arrangements) leads to rather close deuterium–deuterium contacts. In NdD 3 those along the c direction (see Fig. 2) ˚ This is are the closest in the structure (D1–D152.14 A). ˚ [2]), YD 3 (2.14 A ˚ [3]), and also true for HoD 3 (2.04 A ˚ [4]). In the absence of any displacements DyD 3 (2.14 A (i.e. if the deuterium ions were to occupy the centres of regular metal tetrahedra) these distances would be even

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˚ in NdD 3 ) and thus energetically unshorter (|1.56 A favourable. Clearly, a more favourable configuration (i.e. an increase of D–D contact distances and a decrease of D–D repulsions) is achieved by a rearrangement of the tetrahedra such that they become elongated along c (see Nd–Nd, Nd–D and D–D distances in Fig. 2). The displacements in the octahedral interstices appear to be governed mainly by the propensity of deuterium to maximize its attractive interactions with the metal. The observed shifts of D2 and D3 away from the centre positions having six relatively long (i.e. weak) D–Nd bonds allows these atoms to acquire an energetically more favourable metal configuration having three short (i.e. strong) D–Nd bonds (Fig. 2). Repulsive interactions between deuterium anions involving these sites appear to play a minor role because of the relatively long distances involved (D2– ˚ D3–D152.40 A). ˚ As a result of the interplay D152.42 A, between the repulsive D–D and attractive Nd–D interactions the hcp metal substructure of NdD 3 acquires trigonal symmetry, its cell parameter a is enlarged by a factor of Œ]3 and its axial ratio cŒ]3 /a 5 1.79 increases beyond that of an ideal hcp metal arrangement (c /a51.63). The overall structure of cubic NdD 2.61 is similar to that of tetragonal NdD 2.36 [5] except that the deuterium content of the former is higher and its deuterium distribution less ordered. The same situation also occurs with cerium deuteride for which deuterium rich cubic CeD 2.51 is less ordered than deuterium poor tetragonal CeD 2.29 [12]. In all these compounds deuterium occupies tetrahedral interstices with full occupancy and octahedral interstices with partial occupancies. In cubic NdD 2.61 the metal–deuterium bond ˚ for tetrahedral interstices, Nd– distances (Nd–D152.35 A ˚ D252.56 and 2.89 A for octahedral interstices) compare well with those in tetragonal NdD 2.36 (Nd–D52.32–2.40 ˚ for tetrahedral interstices and Nd–D52.42–2.88 A ˚ for A octahedral interstices). As in cubic LaD 3 [13,14] and CeD 3 [15] those in the octahedral interstices depend critically on the deuterium displacements towards the metal triangles (i.e. along k111l). These displacements have presumably the same origin as those in the trigonal deuteride structure. However, compared to the latter they are more strongly limited by repulsive interactions between deuterium anions in octahedral and tetrahedral interstices. In fact, the contact distance between both anion sites in cubic NdD 2.61 (D1– ˚ is shorter than in trigonal NdD 3 (D1–D2. D252.06 A) ˚ 2.40 A). Interestingly, the corresponding contact distances ˚ which in tetragonal NdD 2.36 are longer (D–D52.26 A) suggests that repulsive D–D interactions in this more ordered modification play a less important role.

Acknowledgements This work was supported by the Swiss National Science Foundation and the Swiss Federal Office of Energy.

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G. Renaudin et al. / Journal of Alloys and Compounds 313 (2000) L10 –L14

Fig. 2. Deuterium atom environments in trigonal NdD 3 , viewed approximately perpendicular to the trigonal axis. Ordered deuterium sites D1 (top) and D2 (middle), disordered split-atom site D3 (bottom). Metal–deuterium and metal–metal bond distances (left), and deuterium–deuterium contact distances ˚ Large spheres Nd, medium spheres D1 (light) and D2 (dark), small spheres D3 (split atom site). Point group symmetries 1 (D1) and 3 (D2, (right) in A. D3).

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