I.V. Koval'chuck, R. Cerny, R.V. Denys - Crystal structure of K-Hf9Mo4SiD168 deuteride - страница 1

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Chemistry of Metals and Alloys

Chem. Met. Alloys 1 (2008) 180-184 Ivan Franko National University of Lviv www. chemetal-j ournal. org

Crystal structure of K-Hf9Mo4SiD168 deuteride

I.V. KOVAL'CHUCK1, R. CERNY2 R.V. DENYS1, I.Yu. ZAVALIY1*

Physico-Mechanical Institute of NAS of Ukraine, 5 Naukova Str., 79601 Lviv, Ukraine 2 Laboratory of Crystallography, University of Geneva, 24 quai Ernest-Ansermet, CH-1211 Geneva 4, Switzerland * Corresponding author. Tel.: +380-32-2296833; fax: +380-32-2649427; e-mail: zavaliy@ipm.lviv.ua

Received November 22, 2007; accepted May 20, 2008; available on-line September 10, 2008

The K-Hf9Mo4SiD168 deuteride was prepared by deuteration of the intermetallic alloy at room temperature and 0.1 MPa D2 pressure. Synchrotron X-ray and neutron powder diffraction studies revealed that the initial crystal structure is preserved upon deuterium absorption (sp.gr. P63/mmc, a = 8.9553(1), c = 9.0947(2) A, Da/a = 4.0%; Dc/c = 5.5%; DV/V = 14.1%; DV/at.D = 2.33 A3). The deuterium atoms occupy four interstitial positions: two tetrahedral Hf2Mo2 and Hf3Si sites and two triangular Hf3 sites.

Hafnium alloys / Hydrides / Crystal structure / X-ray and neutron diffraction analysis

Introduction

Intermetallic K-phases with Hf9Mo4B structure type [1] were found in a number of ternary Hf-Mo-X systems (X = Si, P, S, Ge, As, Se) [2]. Phase analytical and crystallographic results indicate that Hf®Mo substitution in the 6h and 2a sites is the cause of variations in the composition and lattice parameters [3,4]. In our previous work we have synthesised the Zr9V4SH~23 hydride and analysed the possible structure of its hydrogen sublattice. A neutron powder diffraction study of the Zr9V4SD~23 deuteride was carried out to localize the deuterium atoms [5]. A number of K-phases, Zr9Mo4NiOx (x = 0^3) and Hf9Mo4Ge, as well as their hydrides, have been synthesized and characterised [6]. A comparison of the hydrogenation capacities of Zr9Mo4NiOx compounds with the Zr9V4SD~23 structural data allowed us to suggest that the presence of oxygen atoms in octahedral interstices blocks filling of Zr3 triangular faces by hydrogen atoms, thus decreasing the maximum hydrogen storage capacity by 25%. In this work we present results on the preparation and crystal structure of the deuteride of the K-Hf9Mo4Si phase.

Experimental details

A Hf9Mo4Si alloy was prepared by arc melting of the constituent elements followed by high temperature annealing in an evacuated quartz ampoule (1170°C, 5 h). The deuteride of this alloy was synthesised by gas-solid reaction at room temperature and 0.1 MPa D2 pressure. The deuterium content was measured by a standard volumetric technique. The parent and deuterated alloys were examined by X-ray powder diffraction (DRON-3.0, Cu-Ka radiation and Bruker D8, Cu-Ka1 radiation). The crystal structure of the deuteride was determined by a joint Rietveld refinement of synchrotron X-ray (SNBL, ESRF, France, l = 0.3748 A) and neutron powder diffraction data (Paul Scherrer Institute, Switzerland, HRPT instrument, l = 1.494 A) using GSAS software [7].

Results and discussion

The K-Hf9Mo4Si phase with Hf9Mo4B structure type was found as the main constituent phase in the parent alloy. The presence of Hf2Si (sp.gr. I4/mcm, a = 6.5268(8), c = 5.212(1) A), HfMo2 (sp.gr. Fd-3m, a = 7.5696(6) A), and p-Hf07Mo03 (sp.gr. Im-3m, a = 3.4167(3) A) as additional phases is in agreement with the phase diagram of the Hf-Mo-Si system. The content of the K-Hf9Mo4Si phase in the parent alloy was ~ 60 wt.% according to the Rietveld refinement of

the powder X-ray diffraction (XRD) data. XRD profiles of the parent alloy are presented in Fig. 1 and the corresponding crystal structure data for the main K-Hf9Mo4Si phase are described in Table 1.

The hydrogen (deuterium) absorption capacity of the alloy obtained by volumetric measurements was 1.12 D/M (M=Hf, Mo and Si). The analysis of the powder diffraction data showed that all the constituent phases formed corresponding deuterides. A multiphase Rietveld refinement on synchrotron and neutron diffraction data (Fig. 2) showed the following phase composition of the deuterated alloy: HfjMo4SiD16.8 (57.1(1) wt.%);

Table 1 Crystallographic parameters of K-Hf9Mo4Si.

Sp.gr. P63/mmc, a = 8.6116(6), c = 8.6188(8) A, V = 553.53(8) A3.

Atom

Site

x/a

y/b

z/c

Hf1

6h

0.5386(6)

0.0772(12)

1/4

Hf2

12k

0.1994(4)

0.3988(8)

0.0522(6)

Mo1

2a

0

0

0

Mo2

6h

0.8914(8)

0.7828(16)

1/4

Si

2c

1/3

2/3

1/4

Note: atomic displacement parameters Uiso for all atoms were set to 0.005 A2.

Table 2 Crystallographic parameters of the Hf9Mo4SiD168 deuteride (filled Hf9Mo4B type). Sp.gr. P63/mmc, a = 8.9553(1), c = 9.0947(2) A, V = 631.66(4) A3.

Atom

Site

Surrounding

x/a

y/b

z/c

UKoX100 (A2)

Occupation

Hf1

6h

-

0.5401(1)

0.0802(2)

1/4

0.73(2)

1.0(-)

Hf2

12k

-

0.20185(8)

0.4037(2)

0.0526(1)

= Uso(Hf1)

1.0(-)

Mo1

2a

-

0

0

0

1.10(6)

1.0(-)

Mo2

6h

-

0.8958(2)

0.7916(3)

1/4

= UKo(Mo1)

1.0(-)

Si

2c

-

1/3

2/3

1/4

1.7(3)

1.0(-)

D1

24l1

T: Hf1Hf22Mo2

0.3383(4)

0.0419(3)

0.1251(3)

1.61(5)

0.816(6)

D3

2d

A: Hf13

1/3

2/3

3/4

= Uso(D1)

0.999(17)

D4

12k4

T: Hf1Hf22Si

0.428(1)

0.572(1)

0.148(2)

= Uso(D1)

0.219(5)

D5

12k5

A: Hf1Hf22

0.5862(2)

0.1724(5)

0.0470(5)

= Uso(D1)

0.781(5)

Note: T: tetrahedral, A: triangular interstices.

Table 3 Crystallographic parameters of Hf2SiD (filled CuAl2 type). Sp.gr. I4/mcm, a = 6.5126(2), c = 5.4124(3) A, V = 229.56(2) A3.

Atom

Site

Surrounding

x/a

y/b

z/c

Uso><100 (A2)

Occupation

Hf

Si D

8h 4a 4b

T: Hf4

0.1649(2)

0

0

0.6649(2) 0

1/2

0

1/4 1/4

0.20(3)

0.7(2)

2.8(2)

1.0(-)

1.0(-)

1.02(2)

Table 4 Crystallographic parameters of HfMo2D13 (cubic Laves phase deuteride). Sp.gr. Fd-3m [setting 2], a = 7.6799(2) A, V = 452.97(2) A3.

Atom

Site

Surrounding

x/a

y/b

z/c

Uiso>100 (A2)

Occupation

Hf

Mo

D

8b

16c

96g

T: Hf2Mo2

3/8

0

0.203(2)

3/8 0

0.547(2)

3/8 0

0.370(2)

1.52(6) 1.24(6) 2.5(5)

1.0(-) 1.0(-) 0.109(6)

Table 5 Crystallographic parameters of Hf07Mo03D17 (e-ZrH2 type). Sp.gr. I4/mmm, a = 3.3046(4), c = 4.4940(9) A, V = 49.07(1) A3.

Atom

Site

Surrounding

x/a

y/b

z/c

Uso>100 (A2)

Occupation

M

2a

-

0

0

0

1.06(7)

0.73 Hf + 0.27 Mo

D

4d

T: M4

0

1/2

1/4

2.0(-)

0.86(2)

Hf2SiD (13.0(1) wt.%), HfMo2D13 (14.7(1) wt.%) and Hf07Mo03D17 (15.2(1) wt.%). The total hydrogenation capacity, calculated from these data, is 1.02 D/M, which is close to the result of the volumetric measurements. The obtained crystal structure data are summarized in Tables 2-5. The crystal structure of the K-Hf9Mo4SiD168 deuteride is shown in Fig. 3.

Similarly to other studied K-phase hydrides [5,6], Hf9Mo4SiD16.8 preserves the initial symmetry of the metal matrix. The relative changes in the unit cell parameters during its formation (Da/a = 4.0%; Dc/c = 5.5%; DV/V = 14.1%; AV/at.D = 2.33 A3) are comparable to those of Zr9Mo4NiOxHy and Hf9Mo4GeH16.0 [6], which may indicate similarities of the hydrogen sublattice of these hydrides. An analysis of the types of interstitial site available for hydrogen insertion and the possible models for the hydrogen sublattice of K-phase hydrides was performed in [5]. The refinement on neutron diffraction data allowed us to locate four crystallographic sites occupied by D

I.V. Koval'chuck et al., Crystal structure of K-Hf9Mo4SiD16.8 deuteride

Fig. 1 Observed (+), calculated (line) and difference (bottom line) X-ray powder diffraction patterns of the parent alloy. Vertical bars indicate Bragg positions of the constituent phases (from top to bottom): Hf9Mo4Si, Hf2Si, Hf0.7Mo0.3 and HfMo2. R-factors of the refinement: Rwp=4.73%, Rp=3.74%, /=1.35.

(a)

(b)

Fig. 2 Observed (+), calculated (line) and difference (bottom line) SRXRD (a) and PND (b) patterns of the deuterated alloy. Vertical bars indicate Bragg positions of the constituent phases (from bottom to top): Hf9Mo4SiD168; Hf2SiD; HfMo2D15 and Hf07Mo03D17. R-factors: PND: Rwp=3.53%, Rp=2.72%; SXRD: Rwp=7.37%, Rp=5.90%; combined: Rwp=5.94%, Rp=3.99%; C=2.95.

Fig. 3 Crystal structure of K-Hf9Mo4SiD168.

Zr1

D4...Si = 1.74 A

D4...S = 2.93 A

S atom shifts from initial 2c site to 6h site (occupied by 1/3)

Fig 4 Comparison of structure fragments of Hf9Mo4SiD16.8 (left) and Zr9V4SD~23 (right).

atoms (see Table 2, the D sites are labelled according to [5]). Contrary to the Zr9V4SD~23 structure, the D2 site (HfMo3 interstice, which corresponds to ZrV3 in Zr9V4SD~23) is vacant, which can be explained by the smaller hydrogen affinity of Mo compared to V. Another difference between the structures of Hf9Mo4SiD16.8 and Zr9V4SD~23 [5] is that the atoms of the p-element (Si) in Hf9Mo4SiD16.8 remain in the initial position, whereas the sulphur atoms are shifted from the 3-fold axis (2c position) by 1.1 A into a 6h position, filled by 1/3. (see Fig. 4). Such a displacement is attributed to a repulsive deuterium-sulphur interaction (all observed deuterium-sulphur distances are longer than 2.9 A), and allows the filling of Hf3 faces (D4) by deuterium. The D4 atom in the structure of Hf9Mo4SiD16.8 is shifted from the triangular Hf3 face towards a Si atom and occupies the centre of a Hf3Si tetrahedron. Thus, we observe direct contact  between  the  p-element  and deuterium

(dD4-D5 = 1.74(2) A).

The D4 site is occupied by 22 % only due to self-blocking  (dD4-D4 = 1.85(3) A)  and filling  of the neighbouring D5 position (dD4-D5 = 1.79(2) A). The sum of the occupancy factors of the D4 and D5 sites cannot exceed unity. The D1 and D3 sites do not have any D sites in their nearest surrounding and can be filled up completely. However, a full occupancy of only the D3 site is observed, giving a refined deuterium content of 16.8(1) D/f.u. The maximum possible capacity of the deuteride, taking into account blocking between the D4 and D5 sites, is 19 D/f.u. The distances between D atoms and the metal/silicon atoms in their surrounding are presented in Table 6. They are in agreement with the sum of the radii of these atoms. The separation between the nearest deuterium atoms in the Hf9Mo4SiD16.8 structure exceeds 2 A.

Conclusions

The Hf9Mo4SiD16.8 deuteride has been synthesized. The crystal structure of this deuteride has been studied by synchrotron X-ray and neutron powder diffraction.

Table 6 Selected interatomic distances in the structure of Hf9Mo4SiD168.

Atoms

d, A

Atoms

d, A

Hf1...4 D1

2.013(3)

Hf2...2 D1

2.042(3)

Hf1...D3

1.963(2)

Hf2...2 D5

1.998(2)

Hf1...2 D5

1.980(4)

Hf2...2 D4

2.019(8)

Hf1.2 D4

1.97(2)

Mo2...4 D1

1.898(3)

Hf2...2 D1

2.007(3)

Si...6 D4

1.74(2)

The Hf2SiD, HfMo2D1.3 and Hf0.7Mo0.3D1.7 deuterides

were found as additional phases. It has been shown that K-Hf9Mo4SiD16.8 belongs to the filled Hf9Mo4B-type structure (sp.gr. P63/mmc, a = 8.9553(1) A, c = 9.0947(2)     A,     Da/a = 4.0%;     Dc/c = 5.5%;

DV/V = 14.1%; DV/at.D = 2.33 A3). The deuterium

atoms occupy four crystallographic positions: two types of tetrahedral interstice, Hf1Hf22Mo2 and Hf1Hf22Si; and two types of triangular face, Hf13 and

Hf1Hf22.

Acknowledgements

This work was supported by YSF INTAS grant

Ref.No:06-1000019-6490.  The  help  of Denys

Sheptyakov (Paul Scherrer Institute, Switzerland) with the neutron powder diffraction experiment is highly appreciated.

References

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P. Rogl, H. Nowotny, F. Benesowsky, Monatsh.

Chem. 104 (1973) 182.

A. Harsta, J. Solid State Chem. 57 (1985) 362. R.  Mackay, H.F.  Franzen, New Zirconium Kappa Phases, Z. Anorg. Allg. Chem. 616 (1992)

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I.V. Koval'chuck, R. Cerny, R.V. Denys - Crystal structure of K-Hf9Mo4SiD168 deuteride