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Orthorhombic structure of γ-TiFeD≈2
39
QRTHORHOMBIC STRUCTURE OF y-TiFeD,,
*
P. FISCHER*, J. SCHEFER’T, K. YVONb, L. SCHLAPBACHC and T. RIESTERERC aLabor fiir Neutronenstreuung, Wiirenlingen (Switzerland) bLuboratoire
de Cristaflographie,
Eidgendissische
Z’echnische Hochschule
Wniuersit~, CH-1211
cLaboratorium f& ~e~tk~F~er~hysik, Ziirich ~Switze~~a~d~
Eidge~~~~~he
Ziirich,
CH-5303
Gendue {Switzerland) ~e~h~~che
Hochachufe,
CH-8093
(Received May 7,1936)
summary The crystal structure of the hydrogen storage system y-TiFeD1.wcs, was reinvestigated by neutron powder diffraction profile analysis at room temperature and 113 f 17 bar D2 pressure. In contrast with previous work which assumed a structure of monoclinic symmetry a structure of orthorhombic symmetry was assumed: space group Ctnmm; lattice parameters a = T-029(4) a, b = 6.233(2) A, c = 2.835(l) 8; titanium in equipoint 4h (x, 0, L), x = 0.223(Z); iron in 4i (0, y, 0), y = 0.2887(7); Dl in 4e (4, 1, 0); D$ in 2c ($-, 0, f ); D3 in 2a (0, 0,O); agreement values after profile refinement R,, = 0.160, R,= 0.085, RF = 0.060; (sin e/x),,,,, = 0.59 A-l; 85 inequivalent contributing reflections hkE; 10 refined structural parameters. Refinements based on the less symmetric monoclinic model yield virtually the same fit and structure but require four more structural parameters. The structure is characterized by three types of octahedral metal atom interstices; two of type Ti,Fe, occupied by deuterium atoms Dl and D3 and one of type T&Fe, occupied by deuterium atom D2. The former are 100% occupied and the latter are 91% occupied. The shortest interatomic distances are Fe-D1 = 1.77 A, Ti-D2 = 1.95 a and Dl-D3 = 2.35 a.
1. Introduction As it is a hydrogen storage system, the intermetallic compound TiFe is one of the low-temperature prototype materials in automotive applications *Paper presented at the International Symposium on the Properties and Applications of Metal Hydrides V, Mau~u~~on, France, May 25 - 30,1986. pfresent address: Brookhaven National Laboratory, Upton, NY 11973 (U.S.A.) 0022~50%%/%7/$3.50
0 Eisevier Sequoia/Printed in The Netherlands
40
(see for example ref. 1). It is a Pauli paramagnet with a CsCl-type structure and forms a cubic a-phase TiFeH,s_r [Z, 31, an orthorhombic P-phase TiFeH, _iV4[4 - 7 J and a y-phase TiFeH m2with hydrogen. For the latter compound a monoclinic structure model was first published by Thompson et al. [8] and confirmed by other groups [5,7,9]. It is based on the space group A, b, = 2.8301(5) A and P2/m and lattice constants a, = c, = 4.704(l) fl= 96.97( 5)” for the deuteride [ 81. The titanium atoms occupy equipoints 2n (LX,i, z), x = 0.29(2), z = 0.23(2) and the iron atoms occupy positions 2m (x, 0, z), x = 0.203(3), z = 0.721(3). The deuterium atoms are distributed over four octahedral sites, Dl(la: 0, 0, 0), D2(ld: L, 0, 0), D3(lg: 1 0 L ) and D4(lf: 0 L,L ). The metal interstices Dl - D8 correspond to occupied by deuterium atoms. ~~or~~ation T&Fe 2, which2 are completely The less probable sites D4 of type TizFe4 are 85% filled by deuterium atoms in y-TiFeD 1.9p The lattice metric parameters a, = e, and atomic coordinates of the monoclinic model have led one of the present authors to the assumpwith space tion that the true symmetry of y-TiFeD m;2could be orthorhombic group Cmmm [lo]. Although Thompson et al. judged refinement in this space group unsatisfactory [8] it is the purpose of the present neutron diffraction study to verify this more symmetric structure model.
2. Expe~ent~
details
A pure stoichiometric powder sample of TiFe was prepared and activated as described in ref. 11. The neutron diffraction measurements were performed at room temperature in situ with Dz pressures up to 130 bar on the multidetector powder spectrometer DMC, situated at reactor Saphir, Wilrenlingen. With Ge(311) as the monochromator the neutron wavelength h = 1.189 A was chosen. The sample was contained in a special vanadium cylinder of 8 mm inner diameter, 1.5 mm wall thickness and approximately 5 cm height. To avoid hydrogen corrosion problems an inside gold coating of 0.01 mm thickness was used. By means of the m~tide~c~r covering a scattering angle range of 79.8” with an angular step of 0.2” the effects of deuterium absorption and desorption were quickly recognized. Extensive neutron diffraction measurements using two detector positions, i.e. resulting in an angular step of A28 = O.l”, were made at zero deuterium pressure (vacuum), 37 t 3 bar, 100 rt 5 bar and 113 f 17 bar for the scattering angle range 10 < 28 g 89.9”. The observed intensities were corrected for absorption and incoherent scattering according to the measured transmission (~.lr= 0.19). The results for the y phase are shown in Figs. 1 -3. The TiFe diagram corresponds to a pure single-phase sample. The patterns for 100 and 113 bar Dz pressure are rather similar in agreement with the absorption isotherm published in ref. 5. Presumably owing to slight sample inhomogeneities such as varying grain size, a pure ‘y-phase state could not be achieved even at the highest D, pressures. Thus an estimated con~ibution of the order
41
of 18 vol.% arising from the parameters [ 51 in a two-phase surement. The presence of the third diffraction peaks (see published in ref. 13 were used.
pz phase was included with fixed structure profile refinement [ 121 of the 113 bar meapz phase is particularly visible in the first and Figs. 1 - 3). The neutron scattering lengths
a !
I!II
I II
/I
II
Y
‘f-TiFeDl_gq + kTiFeDl.4 113 bar D2
yobs-Ycalc
n,
Fig. 1. Observed (points, A28 = O.l”, h = 1.189 8) and calculated (line, space group Cmmm) neutron diffraction patterns for y-TiFeD wz at 113 f 17 bar Dz pressure and room temperature. The vertical bars indicate 20 positions of hkl reflections (upper __ values for the &-TiFeDt.4 impurity phase).
10-
u-TiFeDj*gq Cmmm
i
m: 5-
29 Fig. 2. Calculated neutron diffraction
(dw)
pattern of single-phase orthorhombic
r-TiFeDrSw
42
iv-TiFeDl.94
IO
P2/m
ii d "0 5 2"
A
(deg) Fig. 3. Calculated
neutron
diffraction
pattern
”
of pure y-TiFeD1,H
for space group P2lm.
3. Data analysis, results and discussion
Starting from a distorted CsCl-type structure with a unit cell which is doubled along two cubic axes compared with the arphase, the following o~horhombic structure model of r-TiFeD,? with space group Cmmm can be derived (Fig. 4). The titanium atoms are centered on equipoints 4h (x, 0, $), x = + and the iron atoms occupy positions 4i (0, y, 0), y = f . The deuterium atoms fully occupy two octahedral sites of the type TieFez, Dl (4e: $, il 0) and D3 (2a: 0, 0,O) and partly occupy (91%) one site of the type T12Fe4, D2 (2~: 4 , 0,; ). Except for a change in origin the monoclinic unit cell is related to the orthorhombic cell by a, = -a/2 + b/2, b, = c and c, = a/2 + b/2, which results in a, = c, = (a2 + b2)1’2/2, tg(@B) = a/b, x, = -x + Y, Ym=Zt %l = x + Y. For the sake of comparison both structure models were refined. The resulting structuraf parameters are summarized in Table 1. The equivalence of the atomic sites between the orthorhombic and monoclinic structures can be seen from the last column in Table 1. For the orthorhombic structure with 10 structural parameters derived from 85 inequivalent hkl values, (sin 0/A),, = 0.59 A-l, the agreement values [12] R,, = 16.0%, R, = 8.5% and RF = 6.0% were obtained for weighted profile, integrated intensities and structure factors respectively. Observed and calculated profile intensities are shown in Fig. 1. Virtually the same fit (see Table 1) despite four more structural parameters (14) was obtained for the monoclinic P2/m model. Thus within the present experimental resolution Ad/d Z 6 X low3 (d = lattice spacing) the validity of the higher, i.e. orthorhombic symmetry can be concluded for y-TiFeD+ The refined composition TiFeD 1.94f 0.,,3 corresponds closely to TiFeD2. Within experimental errors the octahedral metal interstices Dl and D3 with a maximum number of hydride forming ti~ium atom neighbours (coordination Ti,Fe,) are
43 Y-TiFeD2
bm Fig. 4. Orthorhombic structure of y-TiFeD Is% (unit cell shown by thick lines), compared with the previous monoclinic cell (with axes a,, b,, c,), which is illustrated by broken lines. The octahedral coordinations (TiJ?ea) of deuterium atoms Dl and D3 and (TizFe4) of D2 are indicated by bonds. TABLE 1 Structural parameters of r-TiFeDlaw for space groups Cmmm and P2/m determined by neutron diffraction (the transformed coordinates from the refinement are listed for comparison with those of the monoclinic cell)
= @P bb
Cmmm
P21m
~~~forrned
7.029(4a) 6.233(2e) 2.835(la)
4.711(68) 2.84(1=) 4.684( 6’) 96.88( 2)
4.697( 2) 2835(l) 4.697(2) 96.87(4)
0.223(2)
0.28(l) 0.22(l) 0.214(7)
0.277(2) 0.223(2) 0.2113(7)
0.712(7)
0.7113(7)
1 .OOd l.OOd 1 .OOd 0.87(2)
1.00(2) 0.99(2) 1.00(2) 0.91(Z)
0.22(6) 1.4(l)
0.24(5) 1.4(l)
0.22(6) 1.4(l)
0.160 0.085 0.060
0.161 0.086 0.061
0.160 0.085 0.060
&)b xTi
s!ri XFe
0.2887(7)
YFe =Fe
1.00(2) 0.91(2) 0.99(2)
PI)lC PDZ’ pD3c
PDIC pD2’ PD3’ PD4C
;%@ R WPf RIf RF~
coordinafes
(A2)
aEstimated standard deviations include wavelength uncertainty Ax. Estimated standard deviations are given within parentheses and relate to the last digit. ba, b, c, 0 are lattice constants. cP = site occupancy. d Constrained value. eB = temperature factor. The same thermal parameters were used for the impurity phase fi2-TiFeDr -4. fR wp, RI and RF are agreement vafues [ 121.
44 TABLE 2 Shortest interatomic distances in yTiFeDIsw Atoms
interatomic
Ti-D2 l%D3 Ti-Dl Fe-D1 Fe-D3 Fe-D2 Dl-Fe Dl-Ti D2-Fe D2-Ti D3-Fe D3-Ti Dl-D3
1.96(l) 2.11(l) 2.115(l) 1.774(l) 1.800(4) 1.935(3) 1.774(l) 2.115(l) 1.935(3) 1.95(l) l&00(4) 2.11(l) 2.349(l)
(2x) (4x) (4~) (2x) (2x) (4x)
distance”
for space group Cmmm (A)
aEstimated standard deviations are in parentheses.
completely occupied by deuterium atoms, whereas the occupation number of the less attractive sites (D2) with coordination Ti,Fe4 amounts to (91 + 2)s. In the TiFeD, system the latter interstices become populated only at large deuterium concentrations. The shortest interatomic distances in the structure (see Table 2) are those involving the iron atoms: Fe-D1 = 1.77 & similar to those in o-TiFeD0_057 (Fe-D = 1.49 A) [3] and Mg,FeD6 (Fe-D = 1.56 A) [14]. The Ti-D2 distance of 1.95 A is close to that in TiD, (Ti-D = 1.92 A) 1153. Th e minimum deuterium separation Dl-D3 = 2.35 A is consistent with the “2.1 A rule” characteristic for ordered metal hydride structures. Compared with cubic TiFe with lattice constant a = 2.975 A [ll] the metal lattice in y-TiFeD, is expanded by 18 vol.% and is considerably distorted. The density of hydrogen (deu~~um) amounts to 6.4 X 1O22 crnw3 in y-TiFeH,, i.e. it exceeds that of liquid hydrogen by approximately 50%. Acknowledgments We thank the Labor fur Neutronenstreuung, ETH Ziirich, for support of the present investigation and in particular M. Koch for manufacturing the special vanadium container for the pressure experiments. Support by the Swiss National Energy Research Foundation (NEFF) and Science Foundation (SNF) is gratefully acknowledged. References 1 H. Buchner, p. 153.
Energiespeicherung
in Metallhydriden,
Springer-Verlag,
Wien, 1982,
45 2 J. J. Reilly and R. H. Wiswall Jr., J. Inorg. Chem., 13 (1974) 218. 8 P. Thompson, F. Feidinger, J. J. Reilly, L. M. Corliss and J. M. Hastings, J. Phys. F, 10 (1980) L57. 4 P. Thompson, M. A. Pick, F. Reidinger, L. M. Corliss, J. M. Hastings and J. J. Reilly, J. Phys. F, 8 (1978) L75. 5 J. Schefer, P. Fischer, W. Halg, F. Stucki, L. Schlapbach and A. F. Andresen, Mater. Res. Bull., 14 (1979) 1281. 6 W. Schafer, E. Lebsanft and A. Bliisius, 2. Phys. Chem., N.F., 115 (1979) 201. 7 D. Fruchart, M. Commandre, D. Sauvage, A. Rouault and R. Tellgren, J. LessCommon Met., 74 (1980) 55. 8 P. Thompson, J. J. Reilly, F. Reidinger, J. M. Hastings and L. M. Corliss, J. Phys. F, 9 (1979) L61. 9 W. Schafer, G. Will and T. Schober, Mater. Res. Bull., 15 (1980) 627. 10 K. Yvon, J. Less-Common Met., 103 (1984) 53. 11 P. Fischer, W. Kiilg, L. Schlapbach, F. Stucki and A. F. Andresen, Mater. Res. Bull., 13 (1978) 931. 12 P. E. Werner, S. Salome, G. Malmros and J. 0. Thomas, J. Appl. Crystallogr., 12 (1979) 107. 13 V. F. Sears, Rep. AECL-8490, Chalk River, Canada, 1984. 14 J.-J. Didisheim, P. Zolliker, K. Yvon, P. Fischer, J. Schefer, M. Gubelmann and A. F. Williams, Znorg. Chem., 23 (1984) 1953. 15 S. S. Sidhu, L. Heaton and D. D. Zauberis, Acta Crystallogr., 9 (1956) 612.
QRTHORHOMBIC STRUCTURE OF y-TiFeD,,
*
P. FISCHER*, J. SCHEFER’T, K. YVONb, L. SCHLAPBACHC and T. RIESTERERC aLabor fiir Neutronenstreuung, Wiirenlingen (Switzerland) bLuboratoire
de Cristaflographie,
Eidgendissische
Z’echnische Hochschule
Wniuersit~, CH-1211
cLaboratorium f& ~e~tk~F~er~hysik, Ziirich ~Switze~~a~d~
Eidge~~~~~he
Ziirich,
CH-5303
Gendue {Switzerland) ~e~h~~che
Hochachufe,
CH-8093
(Received May 7,1936)
summary The crystal structure of the hydrogen storage system y-TiFeD1.wcs, was reinvestigated by neutron powder diffraction profile analysis at room temperature and 113 f 17 bar D2 pressure. In contrast with previous work which assumed a structure of monoclinic symmetry a structure of orthorhombic symmetry was assumed: space group Ctnmm; lattice parameters a = T-029(4) a, b = 6.233(2) A, c = 2.835(l) 8; titanium in equipoint 4h (x, 0, L), x = 0.223(Z); iron in 4i (0, y, 0), y = 0.2887(7); Dl in 4e (4, 1, 0); D$ in 2c ($-, 0, f ); D3 in 2a (0, 0,O); agreement values after profile refinement R,, = 0.160, R,= 0.085, RF = 0.060; (sin e/x),,,,, = 0.59 A-l; 85 inequivalent contributing reflections hkE; 10 refined structural parameters. Refinements based on the less symmetric monoclinic model yield virtually the same fit and structure but require four more structural parameters. The structure is characterized by three types of octahedral metal atom interstices; two of type Ti,Fe, occupied by deuterium atoms Dl and D3 and one of type T&Fe, occupied by deuterium atom D2. The former are 100% occupied and the latter are 91% occupied. The shortest interatomic distances are Fe-D1 = 1.77 A, Ti-D2 = 1.95 a and Dl-D3 = 2.35 a.
1. Introduction As it is a hydrogen storage system, the intermetallic compound TiFe is one of the low-temperature prototype materials in automotive applications *Paper presented at the International Symposium on the Properties and Applications of Metal Hydrides V, Mau~u~~on, France, May 25 - 30,1986. pfresent address: Brookhaven National Laboratory, Upton, NY 11973 (U.S.A.) 0022~50%%/%7/$3.50
0 Eisevier Sequoia/Printed in The Netherlands
40
(see for example ref. 1). It is a Pauli paramagnet with a CsCl-type structure and forms a cubic a-phase TiFeH,s_r [Z, 31, an orthorhombic P-phase TiFeH, _iV4[4 - 7 J and a y-phase TiFeH m2with hydrogen. For the latter compound a monoclinic structure model was first published by Thompson et al. [8] and confirmed by other groups [5,7,9]. It is based on the space group A, b, = 2.8301(5) A and P2/m and lattice constants a, = c, = 4.704(l) fl= 96.97( 5)” for the deuteride [ 81. The titanium atoms occupy equipoints 2n (LX,i, z), x = 0.29(2), z = 0.23(2) and the iron atoms occupy positions 2m (x, 0, z), x = 0.203(3), z = 0.721(3). The deuterium atoms are distributed over four octahedral sites, Dl(la: 0, 0, 0), D2(ld: L, 0, 0), D3(lg: 1 0 L ) and D4(lf: 0 L,L ). The metal interstices Dl - D8 correspond to occupied by deuterium atoms. ~~or~~ation T&Fe 2, which2 are completely The less probable sites D4 of type TizFe4 are 85% filled by deuterium atoms in y-TiFeD 1.9p The lattice metric parameters a, = e, and atomic coordinates of the monoclinic model have led one of the present authors to the assumpwith space tion that the true symmetry of y-TiFeD m;2could be orthorhombic group Cmmm [lo]. Although Thompson et al. judged refinement in this space group unsatisfactory [8] it is the purpose of the present neutron diffraction study to verify this more symmetric structure model.
2. Expe~ent~
details
A pure stoichiometric powder sample of TiFe was prepared and activated as described in ref. 11. The neutron diffraction measurements were performed at room temperature in situ with Dz pressures up to 130 bar on the multidetector powder spectrometer DMC, situated at reactor Saphir, Wilrenlingen. With Ge(311) as the monochromator the neutron wavelength h = 1.189 A was chosen. The sample was contained in a special vanadium cylinder of 8 mm inner diameter, 1.5 mm wall thickness and approximately 5 cm height. To avoid hydrogen corrosion problems an inside gold coating of 0.01 mm thickness was used. By means of the m~tide~c~r covering a scattering angle range of 79.8” with an angular step of 0.2” the effects of deuterium absorption and desorption were quickly recognized. Extensive neutron diffraction measurements using two detector positions, i.e. resulting in an angular step of A28 = O.l”, were made at zero deuterium pressure (vacuum), 37 t 3 bar, 100 rt 5 bar and 113 f 17 bar for the scattering angle range 10 < 28 g 89.9”. The observed intensities were corrected for absorption and incoherent scattering according to the measured transmission (~.lr= 0.19). The results for the y phase are shown in Figs. 1 -3. The TiFe diagram corresponds to a pure single-phase sample. The patterns for 100 and 113 bar Dz pressure are rather similar in agreement with the absorption isotherm published in ref. 5. Presumably owing to slight sample inhomogeneities such as varying grain size, a pure ‘y-phase state could not be achieved even at the highest D, pressures. Thus an estimated con~ibution of the order
41
of 18 vol.% arising from the parameters [ 51 in a two-phase surement. The presence of the third diffraction peaks (see published in ref. 13 were used.
pz phase was included with fixed structure profile refinement [ 121 of the 113 bar meapz phase is particularly visible in the first and Figs. 1 - 3). The neutron scattering lengths
a !
I!II
I II
/I
II
Y
‘f-TiFeDl_gq + kTiFeDl.4 113 bar D2
yobs-Ycalc
n,
Fig. 1. Observed (points, A28 = O.l”, h = 1.189 8) and calculated (line, space group Cmmm) neutron diffraction patterns for y-TiFeD wz at 113 f 17 bar Dz pressure and room temperature. The vertical bars indicate 20 positions of hkl reflections (upper __ values for the &-TiFeDt.4 impurity phase).
10-
u-TiFeDj*gq Cmmm
i
m: 5-
29 Fig. 2. Calculated neutron diffraction
(dw)
pattern of single-phase orthorhombic
r-TiFeDrSw
42
iv-TiFeDl.94
IO
P2/m
ii d "0 5 2"
A
(deg) Fig. 3. Calculated
neutron
diffraction
pattern
”
of pure y-TiFeD1,H
for space group P2lm.
3. Data analysis, results and discussion
Starting from a distorted CsCl-type structure with a unit cell which is doubled along two cubic axes compared with the arphase, the following o~horhombic structure model of r-TiFeD,? with space group Cmmm can be derived (Fig. 4). The titanium atoms are centered on equipoints 4h (x, 0, $), x = + and the iron atoms occupy positions 4i (0, y, 0), y = f . The deuterium atoms fully occupy two octahedral sites of the type TieFez, Dl (4e: $, il 0) and D3 (2a: 0, 0,O) and partly occupy (91%) one site of the type T12Fe4, D2 (2~: 4 , 0,; ). Except for a change in origin the monoclinic unit cell is related to the orthorhombic cell by a, = -a/2 + b/2, b, = c and c, = a/2 + b/2, which results in a, = c, = (a2 + b2)1’2/2, tg(@B) = a/b, x, = -x + Y, Ym=Zt %l = x + Y. For the sake of comparison both structure models were refined. The resulting structuraf parameters are summarized in Table 1. The equivalence of the atomic sites between the orthorhombic and monoclinic structures can be seen from the last column in Table 1. For the orthorhombic structure with 10 structural parameters derived from 85 inequivalent hkl values, (sin 0/A),, = 0.59 A-l, the agreement values [12] R,, = 16.0%, R, = 8.5% and RF = 6.0% were obtained for weighted profile, integrated intensities and structure factors respectively. Observed and calculated profile intensities are shown in Fig. 1. Virtually the same fit (see Table 1) despite four more structural parameters (14) was obtained for the monoclinic P2/m model. Thus within the present experimental resolution Ad/d Z 6 X low3 (d = lattice spacing) the validity of the higher, i.e. orthorhombic symmetry can be concluded for y-TiFeD+ The refined composition TiFeD 1.94f 0.,,3 corresponds closely to TiFeD2. Within experimental errors the octahedral metal interstices Dl and D3 with a maximum number of hydride forming ti~ium atom neighbours (coordination Ti,Fe,) are
43 Y-TiFeD2
bm Fig. 4. Orthorhombic structure of y-TiFeD Is% (unit cell shown by thick lines), compared with the previous monoclinic cell (with axes a,, b,, c,), which is illustrated by broken lines. The octahedral coordinations (TiJ?ea) of deuterium atoms Dl and D3 and (TizFe4) of D2 are indicated by bonds. TABLE 1 Structural parameters of r-TiFeDlaw for space groups Cmmm and P2/m determined by neutron diffraction (the transformed coordinates from the refinement are listed for comparison with those of the monoclinic cell)
= @P bb
Cmmm
P21m
~~~forrned
7.029(4a) 6.233(2e) 2.835(la)
4.711(68) 2.84(1=) 4.684( 6’) 96.88( 2)
4.697( 2) 2835(l) 4.697(2) 96.87(4)
0.223(2)
0.28(l) 0.22(l) 0.214(7)
0.277(2) 0.223(2) 0.2113(7)
0.712(7)
0.7113(7)
1 .OOd l.OOd 1 .OOd 0.87(2)
1.00(2) 0.99(2) 1.00(2) 0.91(Z)
0.22(6) 1.4(l)
0.24(5) 1.4(l)
0.22(6) 1.4(l)
0.160 0.085 0.060
0.161 0.086 0.061
0.160 0.085 0.060
&)b xTi
s!ri XFe
0.2887(7)
YFe =Fe
1.00(2) 0.91(2) 0.99(2)
PI)lC PDZ’ pD3c
PDIC pD2’ PD3’ PD4C
;%@ R WPf RIf RF~
coordinafes
(A2)
aEstimated standard deviations include wavelength uncertainty Ax. Estimated standard deviations are given within parentheses and relate to the last digit. ba, b, c, 0 are lattice constants. cP = site occupancy. d Constrained value. eB = temperature factor. The same thermal parameters were used for the impurity phase fi2-TiFeDr -4. fR wp, RI and RF are agreement vafues [ 121.
44 TABLE 2 Shortest interatomic distances in yTiFeDIsw Atoms
interatomic
Ti-D2 l%D3 Ti-Dl Fe-D1 Fe-D3 Fe-D2 Dl-Fe Dl-Ti D2-Fe D2-Ti D3-Fe D3-Ti Dl-D3
1.96(l) 2.11(l) 2.115(l) 1.774(l) 1.800(4) 1.935(3) 1.774(l) 2.115(l) 1.935(3) 1.95(l) l&00(4) 2.11(l) 2.349(l)
(2x) (4x) (4~) (2x) (2x) (4x)
distance”
for space group Cmmm (A)
aEstimated standard deviations are in parentheses.
completely occupied by deuterium atoms, whereas the occupation number of the less attractive sites (D2) with coordination Ti,Fe4 amounts to (91 + 2)s. In the TiFeD, system the latter interstices become populated only at large deuterium concentrations. The shortest interatomic distances in the structure (see Table 2) are those involving the iron atoms: Fe-D1 = 1.77 & similar to those in o-TiFeD0_057 (Fe-D = 1.49 A) [3] and Mg,FeD6 (Fe-D = 1.56 A) [14]. The Ti-D2 distance of 1.95 A is close to that in TiD, (Ti-D = 1.92 A) 1153. Th e minimum deuterium separation Dl-D3 = 2.35 A is consistent with the “2.1 A rule” characteristic for ordered metal hydride structures. Compared with cubic TiFe with lattice constant a = 2.975 A [ll] the metal lattice in y-TiFeD, is expanded by 18 vol.% and is considerably distorted. The density of hydrogen (deu~~um) amounts to 6.4 X 1O22 crnw3 in y-TiFeH,, i.e. it exceeds that of liquid hydrogen by approximately 50%. Acknowledgments We thank the Labor fur Neutronenstreuung, ETH Ziirich, for support of the present investigation and in particular M. Koch for manufacturing the special vanadium container for the pressure experiments. Support by the Swiss National Energy Research Foundation (NEFF) and Science Foundation (SNF) is gratefully acknowledged. References 1 H. Buchner, p. 153.
Energiespeicherung
in Metallhydriden,
Springer-Verlag,
Wien, 1982,
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