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Neutron diffraction study of the structure of the A15-type deuteride Ti3SbD2.6
Journal of Alloys and Compounds, 210 (1994) 27-29 JALCOM 1175
27
Neutron diffraction study of the structure of the A15-type deuteride Ti3SbD2.6 A.V. Skripov ~'*, A.A. P o d l e s n y a k b'* a n d P. F i s c h e r b *Max-Planck-Institut fiir Metallforschung, Institut fiir Physik, D-70569 Stuttgart (Germany) bLaboratorium fiir Neutronenstreuung, ETH Ziirich and PSI Filligen, CH-5232 I,Tlligen PSI (Switzerland) (Received November 2, 1993)
Abstract Neutron diffraction measurements on the A15-type deuteride Ti3SbD2.6 (space group symmetry Pm3n) at room temperature have shown that deuterium occupies only tetrahedral d sites formed by four Ti atoms. This is consistent with the predictions of the chemical affinity model. The possibility of i site occupation suggested on the basis of recent proton spin-lattice relaxation measurements in Ti3SbH~ is not supported by the present results.
1. Introduction A number of A3B intermetallic compounds with the cubic A15 structure are known to absorb considerable amounts of hydrogen [1-6]. The host lattice usually retains the A15 structure after hydrogen absorption. However, little is known about the properties of the hydrogen sublattice in these compounds. The only A15type hydride for which the sites occupied by hydrogen have been determined from neutron diffraction experiments is Nb3SnH~.o [7]. In this compound H atoms have been found to occupy the sixfold d positions of the space group Pm3n, i.e. the tetrahedral interstitial sites (formed by four Nb atoms) on the faces of the unit cell. Complete occupancy of these sites corresponds to the composition A3BH3. However, a number of the A15-type hydrides with higher H content have been prepared recently, including Ti3IrH3.g [5], Nb3AuH4.3, NbolrH4.7, NboPtHsa and Nb3OsH4.o [3]. These results indicate that other interstitial sites may be occupied by hydrogen in A15 compounds (either exclusively or in addition to d sites). Possible alternative sites are the 16-fold i sites (coordinated by one B and three A atoms) on the space diagonals of the unit cell. These sites form closely spaced pairs between each pair of B atoms in the [111] direction. Occupation of i sites is favourable for the occurrence of the localized hydrogen motion which has been recently reported for the A15-type Ti3SbHx [8]. *Permanent address: Institute of Metal Physics, Urals Branch of the Academy of Sciences, Ekaterinburg 620219, Russian Federation.
The aim of the present work is to determine the interstitial sites occupied by hydrogen in Ti3Sb from neutron diffraction measurements on the deuteride Ti3SbD2.6. To our best knowledge, this is the first neutron diffraction experiment on a deuteride of an A15-type compound. In particular, our purpose is to test the possibility of i site occupation suggested on the basis of the proton spin-lattice relaxation measurements in Ti3SbHx (0.6~x~<2.4) [8]. Previous studies of the properties of the Ti3SbHx system include also the measurements of the hydrogen solubility and lattice parameters (0~
2. Experimental details The sample of Ti3Sb was prepared by arc melting the appropriate amounts of high purity Ti and Sb under a helium pressure of 0.5 bar. According to the X-ray diffraction analysis, the sample had the cubic A15 structure with lattice parameter a = 5.216/~, the amount of extra phases being of the order of 5%. Small pieces of the TiaSb ingot were charged with D2 gas using a Sieverts-type vacuum system. The hot extraction analysis of deuterium content yielded the composition Ti3SbD2.40).
The neutron diffraction measurements were performed using the double-axis multicounter diffractometer [10] at the Saphir reactor at the Paul Scherrer Institute in Villigen (high resolution mode, A= 1.6984(1) /~, Ge(311) monochromator, vertical focusing). The powdered sample sealed under helium in a cylindrical
0925-8388/94/$07.00 © 1994 Elsevier Science S.A. All rights reserved SSDI 0925-8388(94)01175-H
A.V. Skn'pov et. al. /Neutron diffraction of A15-Ti3SbDz6
28
vanadium container of 10 mm diameter was oscillated in order to reduce possible problems related to preferred orientation. •
...':""
3. Results and discussion
The observed neutron diffraction pattern at room temperature is shown in Fig. 1. In addition to the lines belonging to the A15-type phase, a number of extra low intensity peaks have been found. These could be identified as contributions due to b.c.c, vanadium (in the container walls) and the metastable Ti3Sb phase (structural type Ni3Sn, a=4.25 /~, c=5.58 /k). The diffraction pattern for the main phase has been refined using the FULLPROF program [11] based on the neutron scattering lengths from ref. 12. The pattern has been corrected for absorption as measured by transmission (/zr= 0.178). Isotropic temperature factors have been assumed for all atoms in order to keep the number of fitting parameters at a reasonable level. As noted above, the most probable sites occupied by D atoms in the A15-type Ti3Sb are the tetrahedral d sites with [Ti4] coordination and i sites with [Ti3Sb] coordination. The unit cell of Ti3Sb with the positions of d and i sites is shown in Fig. 2. Since i sites form closely spaced pairs in the [111] direction with a separation of about 0.77/~, only one i site in a pair can be occupied at a given time owing to the "blocking" effect [13]. Deuterium atoms may also prefer to occupy 8-fold e sites (in the centres of each pair of i sites) with the triangular bipyramidal [TiaSb2] coordination. I
I
I I III
III
III
°°f 40.
0
.>, o~ C C
III
II
•
obs.
---
calc.
diff.
O. 60. A O
c-
40.
z
20.
m i
O.
m
.i r_a-~
25
I - ~.J',-L . . . . . .
50 2-Theta
J~L-,. . . .
75
,dJ... . . . . . .
100
•
sb
Ti Fig. 2. Unit cell of Ti3Sb with the positions of d and i sites. T A B L E 1. Structural parameters, isotropic Debye-Waller factors Bi~o and occupation numbers for TiaSbD2.6 at 295 K, as refined from neutron diffraction data (estimated standard deviations in parentheses) Atom
Site
x
y
z
Bi,o
Occupancy ~
Ti Sb D
6c 2a 6d
0.25 0 0.5
0 0 0
0.5 0 0.25
0.49(7) 0.29(7) 1.08(5)
1 1 0.87(5)
(A2)
Space group Pm3n (no. 223); lattice parameter a =5.3602(2)/~; agreement factors Ri = 6.68%, R ~ = 5.51%, R=~p= 2.02%. aRatio of the number of occupied sites to the total number of sites of this type.
I
20.
P
.
I.'~--
1
10. 0. -10.
125
(Degrees)
Fig. 1. Observed (top), calculated (middle) and difference (bottom) neutron diffraction patterns of Ti3SbDz6, A= 1.6984 A, T = 295 K.
The occupation of similar bipyramidal sites has been reported, for example, for ZrBe2D1.49 [14]. In our refinement we have allowed for the possibility of d, i and e site occupation. The main result of the refinement is that only d sites are found to be occupied by D atoms. Attempts to fix even a small occupancy of i or e sites yield strong disagreement between the observed and calculated line intensities. Results of the refinement are presented in Table 1; the calculated and difference diffraction patterns are included in Fig. 1. In agreement with the chemical analysis, the refined occupancy of d sites corresponds to the composition TiaSbD2.6O ). The value of the lattice parameter for our sample is in a general agreement with the results of X-ray diffraction measurements on Ti3SbHx with lower H contents (x ~ 2.4) [4, 8]. In fact, this value appears to be somewhat lower than that obtained from a linear extrapolation of the low x results [4, 8] to x = 2.6 (aext=5.38/~). However, the possibility of an isotope effect in the lattice parameter should be taken into account. The distance between
A.V. Skripov et al. /Neutron diffraction of A15-Ti3SbDz6
deuterium and the nearest-neighbour Ti atoms in is 1.895 /~, being close to the corresponding value (1.92 /k) in the binary deuteride TiD2. Such a similarity of metal-hydrogen distances in intermetallic hydrides and the corresponding elemental hydrides is typical of a large number of compounds [15, 16]. Although the volume of tetrahedra forming i sites in the A15 lattice is larger than that of d sites, deuterium atoms prefer to occupy d sites in Ti3Sb. This can be understood in terms of the chemical affinity model [17] predicting the preferential occupation of interstices surrounded by a larger number of hydride-forming elements (in our case, Ti). Similar trends are observed for many hydrides of intermetallic compounds [16]. The possibility of i site occupation in Ti3SbHx suggested on the basis of proton spin-lattice relaxation measurements [8] is not supported by our present results. This means that the geometry of hydrogen motion in Ti3Sb remains to be elucidated. It may be clarified by quasi-elastic neutron scattering experiments. Another problem to be solved is to find out what types of interstitial sites are occupied by hydrogen in A15 hydrides with x>~ 3 [3, 5]. As noted above, the complete occupancy of d sites corresponds to x = 3. We plan to perform neutron diffraction measurements on A15 deuterides with x>~ 3 in the near future. Ti3SbD2. 6
Acknowledgments We are grateful to U. Knell for deuteration of the sample and to H. Wipf for helpful discussions and encouragement. A.V.S. acknowledges financial support from the Alexander von Humboldt Foundation. A.A.P.
29
is grateful to the Swiss National Science Foundation for financial support of a post-doctoral position.
References 1 P.R. Sahm, Phys. Lett. A, 26 (1968) 459. 2 S.Z. Huang, T. Skoskiewicz, C.W. Chu and J.L. Smith, Phys. Rev. B, 22 (1980) 137. 3 V.E. Antonov, T.E. Antonova, I.T. Belash, O.V. Zharikov, A.I. Latynin, A.V. Palnichenko and V.I. Rashchupkin, Soy. Phys. Solid State, 31 (1989) 1659. 4 K.V.S. Rama Rao, M. Mrowietz and A. Weiss, Ber. Bunsenges. Phys. Chem., 86 (1982) 1135. 5 M. Schlereth and H. Wipf, J. Phys.: Condens. Matter, 2 (1990) 6929. 6 M. Baier, R. Wordel, F.E. Wagner, T.E. Antonova and V.E. Antonov, J. Less-Common Met., 172 (1991) 358. 7 L.J. Vieland, A.W. Wicklund and J.G. White, Phys. Rev. B, 11 (1975) 3311. 8 A.V. Skripov, M. Yu. Belyaev and S.A. Petrova, J. Phys.: Condens. Matter, 4 (1992) L537. 9 K.V.S. Rama Rao, H. Sturm, B. Elschner and A. Weiss, Phys. Lett. ,4, 93 (1983) 492. 10 J. Schefer, P. Fischer, H. Heer, A. Isacson, M. Koch and R. Thut, Nucl. Instrum. Methods A, 288 (1990) 477. 11 J. Rodrigues-Carvajal, Abstracts of the Satellite Meeting on Powder Diffraction, XVth Congr. of International Union of Crystallography, Toulouse, 1990, p. 127. 12 V.F. Sears, Neutron News, 3 (1992) 26. 13 A.C. Switendick, Z. Phys. Chem., N.F., 117 (1979) 89. 14 A.F. Andresen, K. Otnes and A.J. Maeland, J. Less-Common Met., 89 (1983) 201. 15 V.A. Somenkov and A.V. Irodova, J. Less-Common Met., 101 (1984) 481. 16 K. Yvon and P. Fischer, in L. Schlapbach (ed.), Hydrogen in Intermetallic Compounds I, Springer, Berlin, 1988, p. 87. 17 I. Jacob and D. Shaltiel, J. Less-Common Met., 65 (1979) 117.
27
Neutron diffraction study of the structure of the A15-type deuteride Ti3SbD2.6 A.V. Skripov ~'*, A.A. P o d l e s n y a k b'* a n d P. F i s c h e r b *Max-Planck-Institut fiir Metallforschung, Institut fiir Physik, D-70569 Stuttgart (Germany) bLaboratorium fiir Neutronenstreuung, ETH Ziirich and PSI Filligen, CH-5232 I,Tlligen PSI (Switzerland) (Received November 2, 1993)
Abstract Neutron diffraction measurements on the A15-type deuteride Ti3SbD2.6 (space group symmetry Pm3n) at room temperature have shown that deuterium occupies only tetrahedral d sites formed by four Ti atoms. This is consistent with the predictions of the chemical affinity model. The possibility of i site occupation suggested on the basis of recent proton spin-lattice relaxation measurements in Ti3SbH~ is not supported by the present results.
1. Introduction A number of A3B intermetallic compounds with the cubic A15 structure are known to absorb considerable amounts of hydrogen [1-6]. The host lattice usually retains the A15 structure after hydrogen absorption. However, little is known about the properties of the hydrogen sublattice in these compounds. The only A15type hydride for which the sites occupied by hydrogen have been determined from neutron diffraction experiments is Nb3SnH~.o [7]. In this compound H atoms have been found to occupy the sixfold d positions of the space group Pm3n, i.e. the tetrahedral interstitial sites (formed by four Nb atoms) on the faces of the unit cell. Complete occupancy of these sites corresponds to the composition A3BH3. However, a number of the A15-type hydrides with higher H content have been prepared recently, including Ti3IrH3.g [5], Nb3AuH4.3, NbolrH4.7, NboPtHsa and Nb3OsH4.o [3]. These results indicate that other interstitial sites may be occupied by hydrogen in A15 compounds (either exclusively or in addition to d sites). Possible alternative sites are the 16-fold i sites (coordinated by one B and three A atoms) on the space diagonals of the unit cell. These sites form closely spaced pairs between each pair of B atoms in the [111] direction. Occupation of i sites is favourable for the occurrence of the localized hydrogen motion which has been recently reported for the A15-type Ti3SbHx [8]. *Permanent address: Institute of Metal Physics, Urals Branch of the Academy of Sciences, Ekaterinburg 620219, Russian Federation.
The aim of the present work is to determine the interstitial sites occupied by hydrogen in Ti3Sb from neutron diffraction measurements on the deuteride Ti3SbD2.6. To our best knowledge, this is the first neutron diffraction experiment on a deuteride of an A15-type compound. In particular, our purpose is to test the possibility of i site occupation suggested on the basis of the proton spin-lattice relaxation measurements in Ti3SbHx (0.6~x~<2.4) [8]. Previous studies of the properties of the Ti3SbHx system include also the measurements of the hydrogen solubility and lattice parameters (0~
2. Experimental details The sample of Ti3Sb was prepared by arc melting the appropriate amounts of high purity Ti and Sb under a helium pressure of 0.5 bar. According to the X-ray diffraction analysis, the sample had the cubic A15 structure with lattice parameter a = 5.216/~, the amount of extra phases being of the order of 5%. Small pieces of the TiaSb ingot were charged with D2 gas using a Sieverts-type vacuum system. The hot extraction analysis of deuterium content yielded the composition Ti3SbD2.40).
The neutron diffraction measurements were performed using the double-axis multicounter diffractometer [10] at the Saphir reactor at the Paul Scherrer Institute in Villigen (high resolution mode, A= 1.6984(1) /~, Ge(311) monochromator, vertical focusing). The powdered sample sealed under helium in a cylindrical
0925-8388/94/$07.00 © 1994 Elsevier Science S.A. All rights reserved SSDI 0925-8388(94)01175-H
A.V. Skn'pov et. al. /Neutron diffraction of A15-Ti3SbDz6
28
vanadium container of 10 mm diameter was oscillated in order to reduce possible problems related to preferred orientation. •
...':""
3. Results and discussion
The observed neutron diffraction pattern at room temperature is shown in Fig. 1. In addition to the lines belonging to the A15-type phase, a number of extra low intensity peaks have been found. These could be identified as contributions due to b.c.c, vanadium (in the container walls) and the metastable Ti3Sb phase (structural type Ni3Sn, a=4.25 /~, c=5.58 /k). The diffraction pattern for the main phase has been refined using the FULLPROF program [11] based on the neutron scattering lengths from ref. 12. The pattern has been corrected for absorption as measured by transmission (/zr= 0.178). Isotropic temperature factors have been assumed for all atoms in order to keep the number of fitting parameters at a reasonable level. As noted above, the most probable sites occupied by D atoms in the A15-type Ti3Sb are the tetrahedral d sites with [Ti4] coordination and i sites with [Ti3Sb] coordination. The unit cell of Ti3Sb with the positions of d and i sites is shown in Fig. 2. Since i sites form closely spaced pairs in the [111] direction with a separation of about 0.77/~, only one i site in a pair can be occupied at a given time owing to the "blocking" effect [13]. Deuterium atoms may also prefer to occupy 8-fold e sites (in the centres of each pair of i sites) with the triangular bipyramidal [TiaSb2] coordination. I
I
I I III
III
III
°°f 40.
0
.>, o~ C C
III
II
•
obs.
---
calc.
diff.
O. 60. A O
c-
40.
z
20.
m i
O.
m
.i r_a-~
25
I - ~.J',-L . . . . . .
50 2-Theta
J~L-,. . . .
75
,dJ... . . . . . .
100
•
sb
Ti Fig. 2. Unit cell of Ti3Sb with the positions of d and i sites. T A B L E 1. Structural parameters, isotropic Debye-Waller factors Bi~o and occupation numbers for TiaSbD2.6 at 295 K, as refined from neutron diffraction data (estimated standard deviations in parentheses) Atom
Site
x
y
z
Bi,o
Occupancy ~
Ti Sb D
6c 2a 6d
0.25 0 0.5
0 0 0
0.5 0 0.25
0.49(7) 0.29(7) 1.08(5)
1 1 0.87(5)
(A2)
Space group Pm3n (no. 223); lattice parameter a =5.3602(2)/~; agreement factors Ri = 6.68%, R ~ = 5.51%, R=~p= 2.02%. aRatio of the number of occupied sites to the total number of sites of this type.
I
20.
P
.
I.'~--
1
10. 0. -10.
125
(Degrees)
Fig. 1. Observed (top), calculated (middle) and difference (bottom) neutron diffraction patterns of Ti3SbDz6, A= 1.6984 A, T = 295 K.
The occupation of similar bipyramidal sites has been reported, for example, for ZrBe2D1.49 [14]. In our refinement we have allowed for the possibility of d, i and e site occupation. The main result of the refinement is that only d sites are found to be occupied by D atoms. Attempts to fix even a small occupancy of i or e sites yield strong disagreement between the observed and calculated line intensities. Results of the refinement are presented in Table 1; the calculated and difference diffraction patterns are included in Fig. 1. In agreement with the chemical analysis, the refined occupancy of d sites corresponds to the composition TiaSbD2.6O ). The value of the lattice parameter for our sample is in a general agreement with the results of X-ray diffraction measurements on Ti3SbHx with lower H contents (x ~ 2.4) [4, 8]. In fact, this value appears to be somewhat lower than that obtained from a linear extrapolation of the low x results [4, 8] to x = 2.6 (aext=5.38/~). However, the possibility of an isotope effect in the lattice parameter should be taken into account. The distance between
A.V. Skripov et al. /Neutron diffraction of A15-Ti3SbDz6
deuterium and the nearest-neighbour Ti atoms in is 1.895 /~, being close to the corresponding value (1.92 /k) in the binary deuteride TiD2. Such a similarity of metal-hydrogen distances in intermetallic hydrides and the corresponding elemental hydrides is typical of a large number of compounds [15, 16]. Although the volume of tetrahedra forming i sites in the A15 lattice is larger than that of d sites, deuterium atoms prefer to occupy d sites in Ti3Sb. This can be understood in terms of the chemical affinity model [17] predicting the preferential occupation of interstices surrounded by a larger number of hydride-forming elements (in our case, Ti). Similar trends are observed for many hydrides of intermetallic compounds [16]. The possibility of i site occupation in Ti3SbHx suggested on the basis of proton spin-lattice relaxation measurements [8] is not supported by our present results. This means that the geometry of hydrogen motion in Ti3Sb remains to be elucidated. It may be clarified by quasi-elastic neutron scattering experiments. Another problem to be solved is to find out what types of interstitial sites are occupied by hydrogen in A15 hydrides with x>~ 3 [3, 5]. As noted above, the complete occupancy of d sites corresponds to x = 3. We plan to perform neutron diffraction measurements on A15 deuterides with x>~ 3 in the near future. Ti3SbD2. 6
Acknowledgments We are grateful to U. Knell for deuteration of the sample and to H. Wipf for helpful discussions and encouragement. A.V.S. acknowledges financial support from the Alexander von Humboldt Foundation. A.A.P.
29
is grateful to the Swiss National Science Foundation for financial support of a post-doctoral position.
References 1 P.R. Sahm, Phys. Lett. A, 26 (1968) 459. 2 S.Z. Huang, T. Skoskiewicz, C.W. Chu and J.L. Smith, Phys. Rev. B, 22 (1980) 137. 3 V.E. Antonov, T.E. Antonova, I.T. Belash, O.V. Zharikov, A.I. Latynin, A.V. Palnichenko and V.I. Rashchupkin, Soy. Phys. Solid State, 31 (1989) 1659. 4 K.V.S. Rama Rao, M. Mrowietz and A. Weiss, Ber. Bunsenges. Phys. Chem., 86 (1982) 1135. 5 M. Schlereth and H. Wipf, J. Phys.: Condens. Matter, 2 (1990) 6929. 6 M. Baier, R. Wordel, F.E. Wagner, T.E. Antonova and V.E. Antonov, J. Less-Common Met., 172 (1991) 358. 7 L.J. Vieland, A.W. Wicklund and J.G. White, Phys. Rev. B, 11 (1975) 3311. 8 A.V. Skripov, M. Yu. Belyaev and S.A. Petrova, J. Phys.: Condens. Matter, 4 (1992) L537. 9 K.V.S. Rama Rao, H. Sturm, B. Elschner and A. Weiss, Phys. Lett. ,4, 93 (1983) 492. 10 J. Schefer, P. Fischer, H. Heer, A. Isacson, M. Koch and R. Thut, Nucl. Instrum. Methods A, 288 (1990) 477. 11 J. Rodrigues-Carvajal, Abstracts of the Satellite Meeting on Powder Diffraction, XVth Congr. of International Union of Crystallography, Toulouse, 1990, p. 127. 12 V.F. Sears, Neutron News, 3 (1992) 26. 13 A.C. Switendick, Z. Phys. Chem., N.F., 117 (1979) 89. 14 A.F. Andresen, K. Otnes and A.J. Maeland, J. Less-Common Met., 89 (1983) 201. 15 V.A. Somenkov and A.V. Irodova, J. Less-Common Met., 101 (1984) 481. 16 K. Yvon and P. Fischer, in L. Schlapbach (ed.), Hydrogen in Intermetallic Compounds I, Springer, Berlin, 1988, p. 87. 17 I. Jacob and D. Shaltiel, J. Less-Common Met., 65 (1979) 117.