- Email: [email protected]
Hydrogen in the A15 compound V3Ga: 51V and 1H nuclear magnetic resonance study
Journal of Alloys and Compounds, 209 (1994) 111-116 JALCOM 1022
111
Hydrogen in the A15 compound V3Ga: 51V and 1H nuclear magnetic resonance study A . V . Skripov* Max-Planck-Institut fiir Metallforschung, Institut fiir Physik, W-70569 Stuttgart (Germany)
Y u . G. C h e r e p a n o v Institute of Metal Physics, Urals Branch of the Academy of Sciences, Ekatetinburg 620219 (Russian Federation)
H. Wipf Institut fi~r FestkOrperphysik, Technische Hochschule Darmstadt, W-6100 Darmstadt (Germany) (Received July 7, 1993; in final form October 20, 1993)
Abstract Nuclear magnetic resonance (NMR) measurements of the 51V and IH spin-lattice relaxation times and the 5~V Knight shifts in the A15-type compounds V3GaHx (x=0, 0.5 and 1.67) have been performed over the temperature range 11-450 K. The 5~V N M R data show that the density of electron states at the Fermi level decreases strongly with increasing x. For both samples with x > 0 the proton free-induction decay has two well-resolved components in the range 180-380 K, indicating the presence of mobile and static H atoms on the N M R frequency scale.
I. Introduction
The intermetallic compound V3Ga belongs to the family of A15-type superconductors showing a number of unusual physical properties [1]. It is the only vanadium-based A15 compound which is known to absorb considerable amounts of hydrogen [2]. The host lattice retains the A15 structure after hydrogen absorption, so that a continuous series of solid solutions V3GaHx (x ~<2) are formed [3, 4]. Previous studies of the properties of the V 3 G a - H system include measurements of hydrogen solubility [4], lattice parameters [2-5], superconducting transition temperature Tc [2, 5], specific heat [5] and hydrogen vibration energies [3]. The value of Tc is found to decrease strongly with increasing hydrogen content [2, 5]. Little is known about the properties of the hydrogen sublattice in A15 compounds, including the positions and mobility of H atoms. The only A15-type hydride for which the sites occupied by hydrogen have been determined from neutron diffraction measurements is NbaSnHLo [6]. In this case H atoms have been found to occupy the six-fold 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. The results *Permanent address: Institute of Metal Physics, Ekaterinburg 620219, Russian Federation.
of inelastic neutron-scattering experiments on V3GaHx [3] are also consistent with d site occupancy. It should be noted that the complete occupancy of these sites corresponds to the maximum hydrogen content x = 3. However, a number of A15-type hydrides with x > 3 have been prepared recently, including Ti3IrH3.8 [4], NbaAuH4.3, Nb3IrH4.7, NbaPtH5a a n d Nb3OsH4.0 [7].
These results indicate that other interstitial sites may be occupied by H in A15 compounds. Possible alternative sites are the 16-fold i sites forming closely spaced pairs on the space diagonals of the unit cell [8]. Occupation of/sites is favourable for the occurrence of low frequency localized hydrogen motion which has been found for Ti3SbHx [8]. Nuclear magnetic resonance (NMR) measurements in metal-hydrogen systems can give microscopic information on hydrogen mobility and hydrogen-induced changes in the electronic structure [9]. In the present work we report the results of 51V and 1H NMR studies of the electronic properties and hydrogen motion in the V3Ga-H system. Our results are consistent with the existence of two types of hydrogen motion with different frequency scales. 2. Experimental details
The details of the preparation of V3GaHx have been described in refs. 3 and 4. Measurements were made
0925-8388/94/$07.00 © 1994 Elsevier Science S.A. All rights reserved SSD! 0925-8388(93)01022-V
A.V. Skripov et aL / NMR study of hydrogen in V3Ga
112
on samples with x---0, 0.5 and 1.67. X-ray diffraction studies at room temperature have shown that all the samples are single-phase compounds with the cubic A15-type host metal structure. The lattice parameters ao are 4.827, 4.864 and 4.960/~ for x = 0, 0.5 and 1.67 respectively. The structural data are consistent with the results of previous studies [2-5]. NMR measurements on powder samples were performed using a Bruker SXP pulse spectrometer at the frequencies to/2~-= 20.5 MHz (s~V), 35, 51 and 90 MHz ( 1 H ) . 51V NMR spectra were recorded by integrating the echo signal and sweeping the magnetic field. Spin-lattice relaxation times T1 were determined from the recovery of the free-induction decay (FID) signal after the saturation pulse sequence (saV) or after a single inverting r.f. pulse (all).
3. Results and discussion
3.1. ~2V Knight shifts and spin-lattice relaxation rates The s~V NMR spectrum of V3Ga at room temperature is a typical powder pattern of a nuclear spin I = 7 with axially symmetric electric quadrupole interaction. For V3GaHo.5 and V3GaH1.67 the quadrupole satellite lines are not resolved because of the broad distribution of electric field gradients at V sites. The 5xV Knight shift values Kv have been determined from the positions of the central line maxima with respect to the 5~V resonance line in an aqueous solution of LiVO3. The Kv values measured at room temperature are presented in Table 1. It should be noted that for V3Ga the 51V Knight shift decreases with decreasing temperature. This temperature dependence is consistent with previous observations [10, 11]; it is believed to result from the narrow peak in the density of electron states, N(E), near the Fermi level. For VaGaHo.5 and V3GaH1.67 the measured Kv values are nearly temperature independent. As can be seen from Table 1, the 5~V Knight shift strongly increases with increasing hydrogen content. For all the studied samples the 5~V spin-lattice relaxation can be fitted by an exponential function. The temperature dependences of the 5W spin-lattice relaxation rates T~- ~ in V3GaHx measured in the range 130-440 K are shown in Fig. 1. Typical statistical errors T A B L E 1.51V Knight shifts and electronic contributions to proton spin-lattice relaxation rates in V3GaHx Sample
K v (295 K) (%)
(TIcT) -1 (s -I K - t )
V3Ga VaGaH0.s VaGaHl.67
0.573 + 0.006 0.606 + 0.007 0.732 + 0.010
(2.10 + 0.09) x 10 -2 (0.50 + 0.04) × 10 -2
1000 [ 800 ~| _
zxV~Ga •
•
V~GaHo.5 Gall 1.67 3
•
V
A
A• &
A _
•
.
0
100
200
T (K)
300
4-00
500
Fig. 1. Temperature dependence of stV spin-lattice relaxation rate in VaGa, V3GaHo.5 and V3GaH1.67 measured at 20.5 MHz. Solid lines show linear fits to data at T < 4 0 0 K (V3GaHo.5) and T < 3 2 0 K (VaGaH1.67).
in T1-1 are of the order of 5%. In normal metals T1-1 is usually proportional to temperature. For VaGa the measured TI-I(T) shows strong deviations from such a behaviour, the value of (T1T) -~ increasing with decreasing temperature. This is consistent with previous measurements [12, 13]. As in the case of Kv(T), the temperature dependence of (/'1T)- ~is believed to result from the N(E) peak near the Fermi level. For hydrogencontaining samples VaGaHx the relaxation rate is nearly proportional to temperature at T<400 K (x=0.5) and at T<340 K (x= 1.67). The low temperature values of /'1 -~ decrease with increasing x. For both V3GaHo.5 and V3GaH1.67 the 5~V relaxation rate starts to increase rapidly at T> 400 K. This feature can be attributed to the excitation of hydrogen motion on the NMR frequency scale. The additional contribution to the 5~V relaxation rate results from the interaction between the nuclear quadrupole moments and the fluctuating electric field gradients at V sites due to hopping of H atoms. This mechanism is consistent with the proton relaxation data to be discussed below. In transition metal compounds the Knight shift is usually determined by the sum of three main contributions, 1
tZBNA
(Ho&+ Ho~bXorb+ HdXd)
(1) where Ks is the contact contribution due to s-electrons, Korb is the orbital contribution of d-electrons and Kd is the core polarization spin contribution of d-electrons. Each of these contributions to the Knight shift is proportional to the corresponding contribution to the magnetic susceptibility X; X, and Xd are the spin susceptibilities of s- and d-electrons respectively and Xorb
A.V. Skripov et al. / NMR study of hydrogen in V3Ga
is the orbital susceptibility of d-electrons. Hs, Horb and Hd are the appropriate hyperfine fields at nuclear sites, /Xn is the Bohr magneton and NA is the Avogadro number. In a first approximation the spin susceptibilities gs and A'dare proportional to the corresponding densities of electron states at the Fermi level, Ns(Ev) and N,~(Ev). The value of Nd(EF) is usually much higher than that of Ns(EF), i.e. Xd >> Xs- Since the core polarization hyperfine field is negative [14], the observed increase in Kv with increasing x suggests a decrease in N~(Ev). This is also supported by measurements of the 51V spin-lattice relaxation rate. In the low temperature region where the motional contribution to the 51V relaxation rate is negligible, T1-1 can be written as [14,
15] T, -1 = 2h,yZk, T[H 2N 2(EF) + (pHSrb .q_qHd2)Ud2(T) ]
(2) where T is the nuclear gyromagnetic ratio for 51V and the dimensionless factors p and q (not exceeding unity) are determined by admixture coefficients of different d-orbital states at the Fermi level. The effective (temperature-dependent) square of the density of d-electron states at the Fermi level, Nd2(T), is given by
N 2(73 = - f
Nd2(E)dE
(3)
where f is the Fermi-Dirac function; as T ~ 0 , NdZ(T) =Nje(Ev). The experimental data on T1-1 show that the temperature dependence of N~2(T) is important only for V3Ga. For V3GaHo.s and V3GaHL67 N,Z(T) is nearly constant below 300 K. This means that the density of d-electron states in these compounds does not show strong changes over an energy interval of about kBT" near Ev. Hydrogen absorption is expected to result in a shift of the Fermi level away from the No(E) peak. Assuming that the hyperfine fields at vanadium sites are nearly constant in the studied range of x, we can estimate the relative changes in N(EF) for V3GaHx using the low temperature T~-~ data. The upper limits of N(Ev) values for V3GaHo.5 and V3GaH~.67 are found to be equal to 71% and 36% of the N(Ev) value for V3Ga respectively. These results are consistent with estimates of N(Ev) based on specific heat data for V3GaH~ [5].
corresponds to static protons; however, it becomes long above 390 K, so that a separation of the two components is no longer possible at T>390 K. The spin-lattice relaxation times for the two components have been found to differ strongly. Selective T 1 measurements were performed using variable data acquisition delays. In order to measure T1 of the longer FID component, we started data acquisition 70/zs after the second (reading) r.f. pulse, i.e. when the shorter FID component was expected to vanish. For the shorter FID component we started the 4 /zs period of data acquisition a few microseconds after the reading pulse. First we consider the behaviour of the spin-lattice relaxation rate Tls -1 for the shorter FID component giving the main contribution to the signal amplitude. The temperature dependences of Tls -~ in V3GaHo.5 and V3GaHL67 measured at three frequencies are shown in Figs. 2 and 3 respectively. Typical statistical errors in T~s-I are of the order of 4%. Figure 4 shows the low temperature part of the data at 90 MHz for both samples. It can be seen that at T< 150 K the relaxation rate is nearly proportional to temperature, as expected for the electronic (Korringa) contribution Tlc-I=CT. The values of C=(TleT) -~ are presented in Table 1. Since in transition metal-hydrogen systems C is approximately proportional to No2(EF) [9], our proton relaxation results suggest that N,,(EF) decreases with increasing x, in agreement
.50
O
150
100
L 2O
T•
For both samples with x > 0 the proton free-induction decay consists of two well-resolved components in the range 180-380K. The intensity of the longer FID component is small (about 8% of the total FID intensity at 300 K), decreasing with decreasing temperature. This longer component corresponds to protons mobile on the NMR frequency scale. The shorter FID component
O
5O
I c~
O
°~,o• T~
0
F-
160
240
320 T (K)
10 [ [~e
0
400
alii~i
© zx •
•
3.2. 1H spin-lattice relaxation rates
113
0
o~
100
200 500 T (K)
.A. ~ •
A 35 MHz o 51 MHz • 90 MHz
400
500
Fig. 2. T e m p e r a t u r e d e p e n d e n c e o f p r o t o n s p i n - l a t t i c e relaxation rate for s h o r t e r F I D c o m p o n e n t in V3GaHo. 5 m e a s u r e d at 35, 51 a n d 90 M H z . Inset: t e m p e r a t u r e d e p e n d e n c e of p r o t o n s p i n - l a t t i c e relaxation rate for longer F I D c o m p o n e n t in V3GaHo. 5 at s a m e f r e q u e n c i e s .
A.I4. Skripov et aL / NMR study of hydrogen in I/'3Ga
114
15
100 [
7.
10
o
5O ZX
lloe
I
A
o3
• 010
0
I
240
160
32O
T (K)
zx
400 zx z~
%0
5
o
A
•
•
A ,55 MHz 0 51 MHz • 90 MHz
~0
0
d,o o~o 0
100
61 I
I
,
I
200 300 T (K)
i
r
400
I
500
Fig. 3. Temperature dependence of proton spin-lattice relaxation rate for shorter FID component in V3GaHI.67 measured at 35, 51 and 90 MHz. Inset: temperature dependence of proton spin-lattice relaxation rate for longer FID component in V3GaHL67 at same frequencies.
• V3GaHo. 5
5 I
~-~2 S~
1
0
4O
8O T (K)
120
160
Fig. 4. Low temperature part of temperature dependence of proton spin-lattice relaxation rate in VaGaH0.5 and V3GaH1.67 measured at 90 MHz. Solid lines show linear fits to data.
with the 51V data (Section 3.1). Above 150 K the proton relaxation rate shows deviations from the linear temperature dependence; these are especially pronounced for V3GaH1.67 (Fig. 3). The deviations are also accompanied by the appearance of a certain frequency dependence of Tls-1. Most probably such a behaviour is
due to the presence of mobile protons giving rise to the longer FID component. As noted above, at T> 390 K the shorter FID component becomes long. This means that protons contributing to this component become mobile on the NMR frequency scale. In this region the measured proton relaxation rate increases rapidly and shows a strong frequency dependence. (Some of the data points are not shown in Figs. 2 and 3, being out of the scale of the plots.) We may conclude that this temperature region corresponds to the excitation of the long-range diffusion for the main fraction of H atoms. We now consider the behaviour of the spin-lattice relaxation rate for the long FID component, T1L -1. Selective measurements of T1L-1 were only possible in the range 180-380 K, the low temperature limit being determined by the small amplitude of the longer FID component. The recovery of the FID amplitude measured with the 70 ~s data acquisition delay can be well described by a sum of two exponential functions with strongly differing time constants. The slower part of the recovery is found to have a time constant nearly equal to Tls; it is expected to result from the tail of the shorter FID component. The faster part of the recovery is expected to represent the intrinsic relaxation time for the longer FID component, T1L. The temperature dependences of T~L-1 at three frequencies are shown in the insets of Figs. 2 and 3. The statistical errors in T1L-~ shown in these plots are higher than for Tls-1 because of the small amplitude of the longer FID component. It can be seen that for both samples T1L-J(T) exhibits a peak, the values of T~L-] being nearly frequency independent on the high temperature slope of the peak and strongly frequency dependent on the low temperature slope of the peak. Such a behaviour is typical for relaxation due to atomic motion. The relaxation rate maximum is known to occur when the atomic hopping rate is approximately equal to the resonance frequency to. In order to elucidate the origin of the two FID components, we consider several possibilities. In many cases such a structure of FID results from coexisting phases with different hydrogen mobilities. However, our X-ray diffraction measurements have not revealed the presence of a second phase. Another argument against phase coexistence in our system comes from the fact that the observed amplitude of the longer FID component shows considerable changes with temperature. It should be noted, however, that the amount of hydrogen in the second phase may be lower than the relative amplitude of the longer FID component, since the apparent amplitude of the shorter component is expected to be reduced because of the finite "dead time" of a receiver. On the basis of available experimental data we cannot exclude the existence of a
A.V. Skripov et al. / NMR study of hydrogen in V3Ga
second phase (e.g. some kind of finely dispersed or amorphous phase), although this possibility can hardly explain all the results. Another possibility is that the longer FID component originates from a fraction of H atoms participating in a fast localized motion. The relevant example is the localized motion of H atoms trapped by lattice defects or impurities [16]. However, in this case the number of such mobile atoms is expected to increase with decreasing temperature, in contrast with the observed decrease in the amplitude of the longer FID component upon cooling. The alternative model of localized motion implies a partial occupation of i sites by H atoms. As noted above, i sites form closely spaced pairs on the space diagonals of the unit cell. In this case the longer FID component originates from a small fraction of H atoms hopping between i sites forming a pair. The shorter FID component corresponds to the main fraction of H atoms occupying d sites and remaining static on the NMR frequency scale below 390 K. Since this model implies that the site energies of hydrogen at i sites are higher than those at d sites, the amplitude of the longer FID component is expected to decrease with decreasing temperature. For such a model the effects of spin diffusion should be taken into account. In our case the effectiveness of spin diffusion is expected to be reduced owing to the following factors. (1) Because of site blocking, only the next-nearest-neighbour d sites (at a distance of about 3 /~ from the centre of a pair of i sites) can be occupied. (2) The dominant contribution to the proton linewidth in V 3 G a H . arises from H - V dipolar interactions. In this case the spin diffusion coefficient is known to decrease approximately in the ratio of the proton linewidth arising from H - H interactions to that arising from all nuclear interactions [17, 18]. A further reduction of the spin diffusion may be related to the fact that the two proton spin systems have strongly differing linewidths. Under these conditions the bottle-neck is the transfer of spin polarization between the two spin systems. The estimated spin diffusion coefficient for this transfer is about 10-14 cm 2 s-1, i.e. the rate of spin flips is comparable with the direct relaxation rate due to dipole-dipole interaction between the two spin systems. As noted above, the mobile proton system is likely to contribute to the measured Tls- 1 values, resulting in their increase above 150 K. This model is also consistent with the observed maximum values of TIL -1. Taking into account V - H and Ga-H dipole-dipole interactions and using eqn. (5) of ref. 19, we can estimate (Tin- 1)m~xfor a hydrogen atom hopping between the closely spaced i sites in V3GaH1.67 as (Tm-a)max-37 s -1 at 51 MHz. This value is of the same order of magnitude as the corresponding exper-
115
imental one (Fig. 3); the difference may be ascribed to the contribution of H - H dipolar interactions. It should be noted, however, that there is no direct evidence for the occupation of i sites in V 3 G a H .. Thus the model of localized motion between i sites should be considered only as one of the possible explanations of the data.
4. Conclusions The main results of our 51V and iH NMR measurements in the A15-type V3Ga-H system may be summarized as follows. The density of electron states at the Fermi level decreases strongly with increasing hydrogen content. Proton NMR measurements have revealed the presence of both static and mobile H atoms on the NMR frequency scale in the temperature range 180-380 K. The main fraction of H atoms remains static (hopping rates lower than 5 × 10 4 S-1) below 380 K and participates in the long-range diffusion with hopping rates higher than 5 x 104 s- 1 above 390 K. A small fraction of H atoms (about 8% at 300 K) shows a fast motion with characteristic frequencies of the order of 5X 108 s -1 at room temperature. This mobile component may originate from H diffusion in a second phase (which for some reason cannot be detected by X-ray diffraction) or from localized motion of a minor fraction of H atoms.
Acknowledgments The financial support from the Alexander von Humboldt Foundation (A.V.S.) and from the Russian Foundation for Fundamental Research (Yu.G.C.) is gratefully acknowledged.
References 1 S.V. Vonsovsky, Yu.A. Izyumov and E.Z. Kurmaev, Superconductivity of Transition Metals, Their Alloys and Compounds, Springer, Berlin, 1982. 2 S.Z. Huang, T. Skoskiewicz, C.W. Chu and J.L. Smith, Phys. Rev. B, 22 (1980) 137. 3 M. Schlereth, D. Steinbinder, H. Wipf, H.J. Lauter and A. Magerl, Z. Phys. Chem. N.F., 164 (1989) 1065. 4 M. Schlereth and H. Wipf, J. Phys.: Condens. Matter, 2 (1990) 6929. 5 B. Cort, G.R. Stewart, S.Z. Huang, R.L. Meng and C.W. Chu, Phys. Rev. B, 24 (1981) 5058. 6 L.J. Vieland, A.W. Wicklund and J.G. White, Phys. Rev. B, 11 (1975) 3311. 7 V.E. Antonov, T.E. Antonova, I.T. Belash, O.V. Zharikov, A.I. Latynin, A.V. Palnichenko and V.I. Rashchupkin, F/z. Tverd. Tela, 31 (1989) 12 (Engl. Transl.: Soy. Phys. - Solid State, 31 (1989) 1659).
116
A.V. Skripov et al. / N M R study of hydrogen in V3Ga
8 A.V. Skripov, M.Yu. Belyaev and S.A. Petrova, J. Phys.: Condens. Matter, 4 (1992) L537. 9 R.M. Cotts, in G. Alefeld and J. V61kl (eds.), Hydrogen in Metals I, Springer, Berlin, 1978, p. 227. 10 W.E. Blumberg, J. Eisinger, V. Jaccarino and B.T. Matthias, Phys. Rev. Lett., 5 (1960) 149. 11 A.V. Skripov and A.P. Stepanov, Fiz. Met. Metull., 55 (1983) 90 (Engl. transl.: Phys. Met. Metall., 55 (1) (1983) 77). 12 B.G. Silbernagel, M. Weger, W.G. Clark and J.H. Wernick, Phys. Rev., 153 (1967) 535. 13 A.V. Skripov and A.P. Stepanov, Fiz. Tverd. Tela, 23 (1981) 966 (Engl. transl.: Soy. Phys. - Solid State, 23 (1981) 560).
14 A. Narath, in A.J. Freeman and R.B. Frankel (eds.), Hyperfine Interactions, Academic, New York, 1967, p. 287. 15 A.V. Skripov and A.P. Stepanov, Phys. Status Solidi B, 126 (1984) 557. 16 H. Wipf, D. Steinbinder, K. Neumaier, Gutsmiedl, A. Magerl and A.J. Dianoux, Europhys. Lett., 4 (1987) 1379. 17 A. Abragam, The Principles of Nuclear Magnetism, Clarendon, Oxford, 1961, p. 328. 18 T.T. Phua, B.J. Beaudry, D.T. Peterson, D.R. Torgeson, R.G. Barnes, M. Belhoul, G.A. Styles and E.F.W. Seymour, Phys. Rev. B, 28 (1983) 6227. 19 A.V. Skripov, S.V. Rychkova, M.Yu. Belyaev and A.P. Stepanov, Z Phys.: Condens. Matter, 2 (1990) 7195.
111
Hydrogen in the A15 compound V3Ga: 51V and 1H nuclear magnetic resonance study A . V . Skripov* Max-Planck-Institut fiir Metallforschung, Institut fiir Physik, W-70569 Stuttgart (Germany)
Y u . G. C h e r e p a n o v Institute of Metal Physics, Urals Branch of the Academy of Sciences, Ekatetinburg 620219 (Russian Federation)
H. Wipf Institut fi~r FestkOrperphysik, Technische Hochschule Darmstadt, W-6100 Darmstadt (Germany) (Received July 7, 1993; in final form October 20, 1993)
Abstract Nuclear magnetic resonance (NMR) measurements of the 51V and IH spin-lattice relaxation times and the 5~V Knight shifts in the A15-type compounds V3GaHx (x=0, 0.5 and 1.67) have been performed over the temperature range 11-450 K. The 5~V N M R data show that the density of electron states at the Fermi level decreases strongly with increasing x. For both samples with x > 0 the proton free-induction decay has two well-resolved components in the range 180-380 K, indicating the presence of mobile and static H atoms on the N M R frequency scale.
I. Introduction
The intermetallic compound V3Ga belongs to the family of A15-type superconductors showing a number of unusual physical properties [1]. It is the only vanadium-based A15 compound which is known to absorb considerable amounts of hydrogen [2]. The host lattice retains the A15 structure after hydrogen absorption, so that a continuous series of solid solutions V3GaHx (x ~<2) are formed [3, 4]. Previous studies of the properties of the V 3 G a - H system include measurements of hydrogen solubility [4], lattice parameters [2-5], superconducting transition temperature Tc [2, 5], specific heat [5] and hydrogen vibration energies [3]. The value of Tc is found to decrease strongly with increasing hydrogen content [2, 5]. Little is known about the properties of the hydrogen sublattice in A15 compounds, including the positions and mobility of H atoms. The only A15-type hydride for which the sites occupied by hydrogen have been determined from neutron diffraction measurements is NbaSnHLo [6]. In this case H atoms have been found to occupy the six-fold 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. The results *Permanent address: Institute of Metal Physics, Ekaterinburg 620219, Russian Federation.
of inelastic neutron-scattering experiments on V3GaHx [3] are also consistent with d site occupancy. It should be noted that the complete occupancy of these sites corresponds to the maximum hydrogen content x = 3. However, a number of A15-type hydrides with x > 3 have been prepared recently, including Ti3IrH3.8 [4], NbaAuH4.3, Nb3IrH4.7, NbaPtH5a a n d Nb3OsH4.0 [7].
These results indicate that other interstitial sites may be occupied by H in A15 compounds. Possible alternative sites are the 16-fold i sites forming closely spaced pairs on the space diagonals of the unit cell [8]. Occupation of/sites is favourable for the occurrence of low frequency localized hydrogen motion which has been found for Ti3SbHx [8]. Nuclear magnetic resonance (NMR) measurements in metal-hydrogen systems can give microscopic information on hydrogen mobility and hydrogen-induced changes in the electronic structure [9]. In the present work we report the results of 51V and 1H NMR studies of the electronic properties and hydrogen motion in the V3Ga-H system. Our results are consistent with the existence of two types of hydrogen motion with different frequency scales. 2. Experimental details
The details of the preparation of V3GaHx have been described in refs. 3 and 4. Measurements were made
0925-8388/94/$07.00 © 1994 Elsevier Science S.A. All rights reserved SSD! 0925-8388(93)01022-V
A.V. Skripov et aL / NMR study of hydrogen in V3Ga
112
on samples with x---0, 0.5 and 1.67. X-ray diffraction studies at room temperature have shown that all the samples are single-phase compounds with the cubic A15-type host metal structure. The lattice parameters ao are 4.827, 4.864 and 4.960/~ for x = 0, 0.5 and 1.67 respectively. The structural data are consistent with the results of previous studies [2-5]. NMR measurements on powder samples were performed using a Bruker SXP pulse spectrometer at the frequencies to/2~-= 20.5 MHz (s~V), 35, 51 and 90 MHz ( 1 H ) . 51V NMR spectra were recorded by integrating the echo signal and sweeping the magnetic field. Spin-lattice relaxation times T1 were determined from the recovery of the free-induction decay (FID) signal after the saturation pulse sequence (saV) or after a single inverting r.f. pulse (all).
3. Results and discussion
3.1. ~2V Knight shifts and spin-lattice relaxation rates The s~V NMR spectrum of V3Ga at room temperature is a typical powder pattern of a nuclear spin I = 7 with axially symmetric electric quadrupole interaction. For V3GaHo.5 and V3GaH1.67 the quadrupole satellite lines are not resolved because of the broad distribution of electric field gradients at V sites. The 5xV Knight shift values Kv have been determined from the positions of the central line maxima with respect to the 5~V resonance line in an aqueous solution of LiVO3. The Kv values measured at room temperature are presented in Table 1. It should be noted that for V3Ga the 51V Knight shift decreases with decreasing temperature. This temperature dependence is consistent with previous observations [10, 11]; it is believed to result from the narrow peak in the density of electron states, N(E), near the Fermi level. For VaGaHo.5 and V3GaH1.67 the measured Kv values are nearly temperature independent. As can be seen from Table 1, the 5~V Knight shift strongly increases with increasing hydrogen content. For all the studied samples the 5~V spin-lattice relaxation can be fitted by an exponential function. The temperature dependences of the 5W spin-lattice relaxation rates T~- ~ in V3GaHx measured in the range 130-440 K are shown in Fig. 1. Typical statistical errors T A B L E 1.51V Knight shifts and electronic contributions to proton spin-lattice relaxation rates in V3GaHx Sample
K v (295 K) (%)
(TIcT) -1 (s -I K - t )
V3Ga VaGaH0.s VaGaHl.67
0.573 + 0.006 0.606 + 0.007 0.732 + 0.010
(2.10 + 0.09) x 10 -2 (0.50 + 0.04) × 10 -2
1000 [ 800 ~| _
zxV~Ga •
•
V~GaHo.5 Gall 1.67 3
•
V
A
A• &
A _
•
.
0
100
200
T (K)
300
4-00
500
Fig. 1. Temperature dependence of stV spin-lattice relaxation rate in VaGa, V3GaHo.5 and V3GaH1.67 measured at 20.5 MHz. Solid lines show linear fits to data at T < 4 0 0 K (V3GaHo.5) and T < 3 2 0 K (VaGaH1.67).
in T1-1 are of the order of 5%. In normal metals T1-1 is usually proportional to temperature. For VaGa the measured TI-I(T) shows strong deviations from such a behaviour, the value of (T1T) -~ increasing with decreasing temperature. This is consistent with previous measurements [12, 13]. As in the case of Kv(T), the temperature dependence of (/'1T)- ~is believed to result from the N(E) peak near the Fermi level. For hydrogencontaining samples VaGaHx the relaxation rate is nearly proportional to temperature at T<400 K (x=0.5) and at T<340 K (x= 1.67). The low temperature values of /'1 -~ decrease with increasing x. For both V3GaHo.5 and V3GaH1.67 the 5~V relaxation rate starts to increase rapidly at T> 400 K. This feature can be attributed to the excitation of hydrogen motion on the NMR frequency scale. The additional contribution to the 5~V relaxation rate results from the interaction between the nuclear quadrupole moments and the fluctuating electric field gradients at V sites due to hopping of H atoms. This mechanism is consistent with the proton relaxation data to be discussed below. In transition metal compounds the Knight shift is usually determined by the sum of three main contributions, 1
tZBNA
(Ho&+ Ho~bXorb+ HdXd)
(1) where Ks is the contact contribution due to s-electrons, Korb is the orbital contribution of d-electrons and Kd is the core polarization spin contribution of d-electrons. Each of these contributions to the Knight shift is proportional to the corresponding contribution to the magnetic susceptibility X; X, and Xd are the spin susceptibilities of s- and d-electrons respectively and Xorb
A.V. Skripov et al. / NMR study of hydrogen in V3Ga
is the orbital susceptibility of d-electrons. Hs, Horb and Hd are the appropriate hyperfine fields at nuclear sites, /Xn is the Bohr magneton and NA is the Avogadro number. In a first approximation the spin susceptibilities gs and A'dare proportional to the corresponding densities of electron states at the Fermi level, Ns(Ev) and N,~(Ev). The value of Nd(EF) is usually much higher than that of Ns(EF), i.e. Xd >> Xs- Since the core polarization hyperfine field is negative [14], the observed increase in Kv with increasing x suggests a decrease in N~(Ev). This is also supported by measurements of the 51V spin-lattice relaxation rate. In the low temperature region where the motional contribution to the 51V relaxation rate is negligible, T1-1 can be written as [14,
15] T, -1 = 2h,yZk, T[H 2N 2(EF) + (pHSrb .q_qHd2)Ud2(T) ]
(2) where T is the nuclear gyromagnetic ratio for 51V and the dimensionless factors p and q (not exceeding unity) are determined by admixture coefficients of different d-orbital states at the Fermi level. The effective (temperature-dependent) square of the density of d-electron states at the Fermi level, Nd2(T), is given by
N 2(73 = - f
Nd2(E)dE
(3)
where f is the Fermi-Dirac function; as T ~ 0 , NdZ(T) =Nje(Ev). The experimental data on T1-1 show that the temperature dependence of N~2(T) is important only for V3Ga. For V3GaHo.s and V3GaHL67 N,Z(T) is nearly constant below 300 K. This means that the density of d-electron states in these compounds does not show strong changes over an energy interval of about kBT" near Ev. Hydrogen absorption is expected to result in a shift of the Fermi level away from the No(E) peak. Assuming that the hyperfine fields at vanadium sites are nearly constant in the studied range of x, we can estimate the relative changes in N(EF) for V3GaHx using the low temperature T~-~ data. The upper limits of N(Ev) values for V3GaHo.5 and V3GaH~.67 are found to be equal to 71% and 36% of the N(Ev) value for V3Ga respectively. These results are consistent with estimates of N(Ev) based on specific heat data for V3GaH~ [5].
corresponds to static protons; however, it becomes long above 390 K, so that a separation of the two components is no longer possible at T>390 K. The spin-lattice relaxation times for the two components have been found to differ strongly. Selective T 1 measurements were performed using variable data acquisition delays. In order to measure T1 of the longer FID component, we started data acquisition 70/zs after the second (reading) r.f. pulse, i.e. when the shorter FID component was expected to vanish. For the shorter FID component we started the 4 /zs period of data acquisition a few microseconds after the reading pulse. First we consider the behaviour of the spin-lattice relaxation rate Tls -1 for the shorter FID component giving the main contribution to the signal amplitude. The temperature dependences of Tls -~ in V3GaHo.5 and V3GaHL67 measured at three frequencies are shown in Figs. 2 and 3 respectively. Typical statistical errors in T~s-I are of the order of 4%. Figure 4 shows the low temperature part of the data at 90 MHz for both samples. It can be seen that at T< 150 K the relaxation rate is nearly proportional to temperature, as expected for the electronic (Korringa) contribution Tlc-I=CT. The values of C=(TleT) -~ are presented in Table 1. Since in transition metal-hydrogen systems C is approximately proportional to No2(EF) [9], our proton relaxation results suggest that N,,(EF) decreases with increasing x, in agreement
.50
O
150
100
L 2O
T•
For both samples with x > 0 the proton free-induction decay consists of two well-resolved components in the range 180-380K. The intensity of the longer FID component is small (about 8% of the total FID intensity at 300 K), decreasing with decreasing temperature. This longer component corresponds to protons mobile on the NMR frequency scale. The shorter FID component
O
5O
I c~
O
°~,o• T~
0
F-
160
240
320 T (K)
10 [ [~e
0
400
alii~i
© zx •
•
3.2. 1H spin-lattice relaxation rates
113
0
o~
100
200 500 T (K)
.A. ~ •
A 35 MHz o 51 MHz • 90 MHz
400
500
Fig. 2. T e m p e r a t u r e d e p e n d e n c e o f p r o t o n s p i n - l a t t i c e relaxation rate for s h o r t e r F I D c o m p o n e n t in V3GaHo. 5 m e a s u r e d at 35, 51 a n d 90 M H z . Inset: t e m p e r a t u r e d e p e n d e n c e of p r o t o n s p i n - l a t t i c e relaxation rate for longer F I D c o m p o n e n t in V3GaHo. 5 at s a m e f r e q u e n c i e s .
A.I4. Skripov et aL / NMR study of hydrogen in I/'3Ga
114
15
100 [
7.
10
o
5O ZX
lloe
I
A
o3
• 010
0
I
240
160
32O
T (K)
zx
400 zx z~
%0
5
o
A
•
•
A ,55 MHz 0 51 MHz • 90 MHz
~0
0
d,o o~o 0
100
61 I
I
,
I
200 300 T (K)
i
r
400
I
500
Fig. 3. Temperature dependence of proton spin-lattice relaxation rate for shorter FID component in V3GaHI.67 measured at 35, 51 and 90 MHz. Inset: temperature dependence of proton spin-lattice relaxation rate for longer FID component in V3GaHL67 at same frequencies.
• V3GaHo. 5
5 I
~-~2 S~
1
0
4O
8O T (K)
120
160
Fig. 4. Low temperature part of temperature dependence of proton spin-lattice relaxation rate in VaGaH0.5 and V3GaH1.67 measured at 90 MHz. Solid lines show linear fits to data.
with the 51V data (Section 3.1). Above 150 K the proton relaxation rate shows deviations from the linear temperature dependence; these are especially pronounced for V3GaH1.67 (Fig. 3). The deviations are also accompanied by the appearance of a certain frequency dependence of Tls-1. Most probably such a behaviour is
due to the presence of mobile protons giving rise to the longer FID component. As noted above, at T> 390 K the shorter FID component becomes long. This means that protons contributing to this component become mobile on the NMR frequency scale. In this region the measured proton relaxation rate increases rapidly and shows a strong frequency dependence. (Some of the data points are not shown in Figs. 2 and 3, being out of the scale of the plots.) We may conclude that this temperature region corresponds to the excitation of the long-range diffusion for the main fraction of H atoms. We now consider the behaviour of the spin-lattice relaxation rate for the long FID component, T1L -1. Selective measurements of T1L-1 were only possible in the range 180-380 K, the low temperature limit being determined by the small amplitude of the longer FID component. The recovery of the FID amplitude measured with the 70 ~s data acquisition delay can be well described by a sum of two exponential functions with strongly differing time constants. The slower part of the recovery is found to have a time constant nearly equal to Tls; it is expected to result from the tail of the shorter FID component. The faster part of the recovery is expected to represent the intrinsic relaxation time for the longer FID component, T1L. The temperature dependences of T~L-1 at three frequencies are shown in the insets of Figs. 2 and 3. The statistical errors in T1L-~ shown in these plots are higher than for Tls-1 because of the small amplitude of the longer FID component. It can be seen that for both samples T1L-J(T) exhibits a peak, the values of T~L-] being nearly frequency independent on the high temperature slope of the peak and strongly frequency dependent on the low temperature slope of the peak. Such a behaviour is typical for relaxation due to atomic motion. The relaxation rate maximum is known to occur when the atomic hopping rate is approximately equal to the resonance frequency to. In order to elucidate the origin of the two FID components, we consider several possibilities. In many cases such a structure of FID results from coexisting phases with different hydrogen mobilities. However, our X-ray diffraction measurements have not revealed the presence of a second phase. Another argument against phase coexistence in our system comes from the fact that the observed amplitude of the longer FID component shows considerable changes with temperature. It should be noted, however, that the amount of hydrogen in the second phase may be lower than the relative amplitude of the longer FID component, since the apparent amplitude of the shorter component is expected to be reduced because of the finite "dead time" of a receiver. On the basis of available experimental data we cannot exclude the existence of a
A.V. Skripov et al. / NMR study of hydrogen in V3Ga
second phase (e.g. some kind of finely dispersed or amorphous phase), although this possibility can hardly explain all the results. Another possibility is that the longer FID component originates from a fraction of H atoms participating in a fast localized motion. The relevant example is the localized motion of H atoms trapped by lattice defects or impurities [16]. However, in this case the number of such mobile atoms is expected to increase with decreasing temperature, in contrast with the observed decrease in the amplitude of the longer FID component upon cooling. The alternative model of localized motion implies a partial occupation of i sites by H atoms. As noted above, i sites form closely spaced pairs on the space diagonals of the unit cell. In this case the longer FID component originates from a small fraction of H atoms hopping between i sites forming a pair. The shorter FID component corresponds to the main fraction of H atoms occupying d sites and remaining static on the NMR frequency scale below 390 K. Since this model implies that the site energies of hydrogen at i sites are higher than those at d sites, the amplitude of the longer FID component is expected to decrease with decreasing temperature. For such a model the effects of spin diffusion should be taken into account. In our case the effectiveness of spin diffusion is expected to be reduced owing to the following factors. (1) Because of site blocking, only the next-nearest-neighbour d sites (at a distance of about 3 /~ from the centre of a pair of i sites) can be occupied. (2) The dominant contribution to the proton linewidth in V 3 G a H . arises from H - V dipolar interactions. In this case the spin diffusion coefficient is known to decrease approximately in the ratio of the proton linewidth arising from H - H interactions to that arising from all nuclear interactions [17, 18]. A further reduction of the spin diffusion may be related to the fact that the two proton spin systems have strongly differing linewidths. Under these conditions the bottle-neck is the transfer of spin polarization between the two spin systems. The estimated spin diffusion coefficient for this transfer is about 10-14 cm 2 s-1, i.e. the rate of spin flips is comparable with the direct relaxation rate due to dipole-dipole interaction between the two spin systems. As noted above, the mobile proton system is likely to contribute to the measured Tls- 1 values, resulting in their increase above 150 K. This model is also consistent with the observed maximum values of TIL -1. Taking into account V - H and Ga-H dipole-dipole interactions and using eqn. (5) of ref. 19, we can estimate (Tin- 1)m~xfor a hydrogen atom hopping between the closely spaced i sites in V3GaH1.67 as (Tm-a)max-37 s -1 at 51 MHz. This value is of the same order of magnitude as the corresponding exper-
115
imental one (Fig. 3); the difference may be ascribed to the contribution of H - H dipolar interactions. It should be noted, however, that there is no direct evidence for the occupation of i sites in V 3 G a H .. Thus the model of localized motion between i sites should be considered only as one of the possible explanations of the data.
4. Conclusions The main results of our 51V and iH NMR measurements in the A15-type V3Ga-H system may be summarized as follows. The density of electron states at the Fermi level decreases strongly with increasing hydrogen content. Proton NMR measurements have revealed the presence of both static and mobile H atoms on the NMR frequency scale in the temperature range 180-380 K. The main fraction of H atoms remains static (hopping rates lower than 5 × 10 4 S-1) below 380 K and participates in the long-range diffusion with hopping rates higher than 5 x 104 s- 1 above 390 K. A small fraction of H atoms (about 8% at 300 K) shows a fast motion with characteristic frequencies of the order of 5X 108 s -1 at room temperature. This mobile component may originate from H diffusion in a second phase (which for some reason cannot be detected by X-ray diffraction) or from localized motion of a minor fraction of H atoms.
Acknowledgments The financial support from the Alexander von Humboldt Foundation (A.V.S.) and from the Russian Foundation for Fundamental Research (Yu.G.C.) is gratefully acknowledged.
References 1 S.V. Vonsovsky, Yu.A. Izyumov and E.Z. Kurmaev, Superconductivity of Transition Metals, Their Alloys and Compounds, Springer, Berlin, 1982. 2 S.Z. Huang, T. Skoskiewicz, C.W. Chu and J.L. Smith, Phys. Rev. B, 22 (1980) 137. 3 M. Schlereth, D. Steinbinder, H. Wipf, H.J. Lauter and A. Magerl, Z. Phys. Chem. N.F., 164 (1989) 1065. 4 M. Schlereth and H. Wipf, J. Phys.: Condens. Matter, 2 (1990) 6929. 5 B. Cort, G.R. Stewart, S.Z. Huang, R.L. Meng and C.W. Chu, Phys. Rev. B, 24 (1981) 5058. 6 L.J. Vieland, A.W. Wicklund and J.G. White, Phys. Rev. B, 11 (1975) 3311. 7 V.E. Antonov, T.E. Antonova, I.T. Belash, O.V. Zharikov, A.I. Latynin, A.V. Palnichenko and V.I. Rashchupkin, F/z. Tverd. Tela, 31 (1989) 12 (Engl. Transl.: Soy. Phys. - Solid State, 31 (1989) 1659).
116
A.V. Skripov et al. / N M R study of hydrogen in V3Ga
8 A.V. Skripov, M.Yu. Belyaev and S.A. Petrova, J. Phys.: Condens. Matter, 4 (1992) L537. 9 R.M. Cotts, in G. Alefeld and J. V61kl (eds.), Hydrogen in Metals I, Springer, Berlin, 1978, p. 227. 10 W.E. Blumberg, J. Eisinger, V. Jaccarino and B.T. Matthias, Phys. Rev. Lett., 5 (1960) 149. 11 A.V. Skripov and A.P. Stepanov, Fiz. Met. Metull., 55 (1983) 90 (Engl. transl.: Phys. Met. Metall., 55 (1) (1983) 77). 12 B.G. Silbernagel, M. Weger, W.G. Clark and J.H. Wernick, Phys. Rev., 153 (1967) 535. 13 A.V. Skripov and A.P. Stepanov, Fiz. Tverd. Tela, 23 (1981) 966 (Engl. transl.: Soy. Phys. - Solid State, 23 (1981) 560).
14 A. Narath, in A.J. Freeman and R.B. Frankel (eds.), Hyperfine Interactions, Academic, New York, 1967, p. 287. 15 A.V. Skripov and A.P. Stepanov, Phys. Status Solidi B, 126 (1984) 557. 16 H. Wipf, D. Steinbinder, K. Neumaier, Gutsmiedl, A. Magerl and A.J. Dianoux, Europhys. Lett., 4 (1987) 1379. 17 A. Abragam, The Principles of Nuclear Magnetism, Clarendon, Oxford, 1961, p. 328. 18 T.T. Phua, B.J. Beaudry, D.T. Peterson, D.R. Torgeson, R.G. Barnes, M. Belhoul, G.A. Styles and E.F.W. Seymour, Phys. Rev. B, 28 (1983) 6227. 19 A.V. Skripov, S.V. Rychkova, M.Yu. Belyaev and A.P. Stepanov, Z Phys.: Condens. Matter, 2 (1990) 7195.