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Local environment effects on the IV → 3+ transition in CePd3Bx
Journal
108
LOCAL ENVIRONMENT E. BEAUREPAIRE, L.M.S.E.S. France
(Laboratoire
EFFECTS
P. PANISSOD Associt! au CNRS
of Magnetism
and Magnetic
Materials 47&48 (1985) 108-110 North-Holland, Amsterdam
ON THE IV + 3+ TRANSITION
IN CePd,B,
and J.P. KAPPLER no. 306), Universit.6 Louis Pasteur,
4, rue Blaise Pascal, 67070 Strasbourg
Cedex.
A detailed analysis of the bulk susceptibility and “B NMR data in CePd,B, shows that Ce atoms become trivalent as soon as they are surrounded by at least 3 boron atoms, the others remaining in the IV state. Electronic relaxation times are also deduced which increase by two orders of magnitude with increasing x from 0.04 to 0.55.
The effect of boron addition in the interstitial sites of the IV CePd, compound has been reported first by Dhar et al. [l]: the lattice anomaly of CePd, among the REPd, compounds is suppressed in CePd,B, where Ce is in the 3’ ionic state. Our NMR investigation of the CePd,B, system was performed in order to study at a local scale the effect of boron on the valence state of Ce. Bulk susceptibilities have also been measured, which agree with the previous results, so that average magnetic properties can be compared with the local ones as measured by “B NMR. Samples for x = 0.04, 0.065 and 0.09 were studied by NMR together with CePd,B,,,, and YPd 3B,,,, as reference samples (3+ and non-magnetic, respectively). Although the existence of an ordered CePd, (AuCu,-like) structure is not well established the NMR spectra do not indicate the presence of any electric field gradient on B sites which implies that, at least around boron in the low B content compounds the structure is ordered, the B atoms being surrounded by a Pd octahedron and a Ce cube. This is well understood in terms of available volume for the B atoms. For high B concentration the broadening of the NMR line due to the demagnetizing field associated with the high bulk susceptibility does not allow to draw any conclusion about the chemical order in the compounds. Two striking observations have been made on the “B spectra in the low B content range and at low temperature: i) the spectra are asymmetric (fig. 1) which implies a distribution of Knight shifts; ii) relaxation times T, are also distributed along the resonance line, the nuclei with a small Knight shift (NM) having a relaxation time up to two orders of magnitude longer than those with a strong Knight shift 0304-8853/85/$03.30 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V.
(M) (fig. 2). Both observations are consistent since the Knight shift K and the relaxation rate T,-’ measure the local susceptibilities (static for K and dynamic, Q, for T; ’ ); they imply that “ magnetic” and “ non-magnetic” Ce atoms coexist in the samples at low temperature: NM borons are surrounded by non-magnetic Ce atoms (in the IV state) while M borons have magnetic Ce neighbours (in the 3+ state). At high temperature the two populations cannot be distinguished: the spectra are symmetrical and an unique short r, is observed (as in CePd,B,,,,). This is consistent with the Ce atoms being all “magnetic” (T is higher than any “spin fluctuation temperature” T,,). Owing to their very different T1 the two kinds of B atoms can be easily separated in the spectra and counted (fig. 1). The bulk susceptibility as well can be separated easily in a magnetic component (following a Curie Law or a CePd,B behaviour) and a non-magnetic one (temperature independent at low T’s) [2]. To go further in the quantitative analysis the relative populations of 3+ (magnetic) Ce and IV (non-magnetic Ce) at 4.2 K and those of NM and M boron are compared to probabilities computed in a simple environment model assuming
30G
H
Fig. 1. “B spectrum in CePd,B,,, NM and M boron contributions
(4.2 K) and its separation (see text). (w = 14.5 MHz).
in
109
E. Beourepaire et al. / IV + 3 + transition in CePd,B,
% z 0
E 0, 0 .? t
l/1
20-
A
2
3
lo-
0
_FO
,
L!z I
Fig. 2. Temperature dependence of the “B relaxation time in CePd,B, ent parts and 0.09 tively)is
compounds. For x = 0.06 the extreme T,‘s on differof the spectra (NM and M) are shown. For x = 0.04 only the majority contribution (NM and M, respecshown.
that the Ce valence state depends only on the number of boron atoms among the 8 nearest interstitial sites: Ce atoms would be in the trivalent state when they have at least n boron neighbours. Obviously n is larger than 1 otherwise all B atoms would be surrounded by Ce3+. On fig. 3 it can be seen that the proportion of Ce3+ deduced from susceptibility data and that of NM boron deduced from NMR data are both consistent with Ce atoms being in the trivalent state for n = 3. However, in fig. 3b, an overestimation of NM boron by our model is not surprising because of the influence of the second neighbour shell on the magnetic environment of boron. Thus the transition from the IV state in CePd, to the trivalent state in CePd,B occurs locully as soon as a Ce atom has three boron atoms in the nearest interstitial sites. Ce atoms that have less than 3 boron neighbours are still non-magnetic at low temperature. However, this transition is obviously not purely local and collective effects also take place as indicated for example by the rapid increase of the lattice parameter at low B content; it can also be seen from the thermal dependence of the suceptibility and from Ti measurements that TSF for Ce atoms that are still intermediate valent is considerably reduced (about 80 and 20 K for x = 0.04 and 0.09, respectively) with respect to that in CePd, (= 150 K). Correspondingly the addition of boron rapidly lengthens the electronic relaxation time re (fig. 4) that can be deduced from TI measurements following standard relaxation theory. According to Knight shift measurements to be presented elsewhere, relaxation occurs
0
5
10
15
x%
Fig. 3. Solid lines: proportion of trivalent Ce and B atoms with no Ce3+ neighbour (NM) computed in a local environment model for n = 2 or 3 (see text). 0: proportion of trivalent Ce deduced from bulk susceptibility; n : proportion of NM boron deduced from NMR.
mostly through the direct dipolar interaction between Ce moments and B nuclei, hence T’,S reported here concern only those Ce atoms that are rather close to B atoms (the totality for x = 0.55, about 30% for x = 0.04). As shown on fig. 4 the transition from the IV regime to the trivalent state is characterized by strong changes of
fi/T(meV)
10K
10
T(K)
100
300
Fig. 4. Thermal and concentration dependence of the Ce electronic relaxation rate (in units of energy h/r,). Solid lines: neutron quasielastic line width (CePd,: Holland-Moritz et al. [3]; Ce$: Horn et al. [4]).
110
E. Beaurepaire et al. / IV
the electronic relaxation time behaviour. In the trivalent state (x = 0.55) the thermal variation of re (proportional to T-“2) is qualitatively similar to that observed in “Kondo lattice” systems (Ce$, CeAl,, ). This behaviour is also roughly observed for x = 0.09 which for most of the B atoms are close to trivalent Ce atoms. Oppositely for x = 0.04 re is temperature independent above 100 K as in CePd, (but 10 times longer) and other IV systems; however a strong decrease of TV is observed at low temperature down to values comparable to that in CePd,: this could be correlated to an increase of the Ce valence (as often observed in mixed valent systems) (51. For x = 0.065 the two behaviours are mixed at low temperature as one probes through B NMR both IV and trivalent Ce atoms with comparable probabilities. These measurements have thus emphasized the role of local environment effects in the valence transition of cerium and as a consequence the coexistence of cerium
3 + transitton in CePd,B,
atoms in various valence states in CePd,B, samples. Preliminary measurements in CePd,Be, compounds also show similar effects which are certainly quite general as far as valence transitions are obtained by alloying.
References VI SK. Dhar, SK. Malik and R. Vijayaraghavan, Phya. Rev. 824 (1981) 6182. PI J.P. Kappler, M.J. Besnus. E. Beaurepaire, A. Meyer. J. Sereni and G. Nieva, J. Magn. Magn. Mat. 47&48 (1985)
111. [31 E. Holland-Moritz, D. Wohlleben and M. Loewenhaupt, Phys. Rev. B25 (1982) 7482. I41S. Horn, F. Steglich, M. Loewenhaupt and E. HollandMoritz, Physica 107B (1981) 103. J. Magn. Magn. Mat. 47&48 151 See e.g. E. Holland-Moritz, (1985) 127.
108
LOCAL ENVIRONMENT E. BEAUREPAIRE, L.M.S.E.S. France
(Laboratoire
EFFECTS
P. PANISSOD Associt! au CNRS
of Magnetism
and Magnetic
Materials 47&48 (1985) 108-110 North-Holland, Amsterdam
ON THE IV + 3+ TRANSITION
IN CePd,B,
and J.P. KAPPLER no. 306), Universit.6 Louis Pasteur,
4, rue Blaise Pascal, 67070 Strasbourg
Cedex.
A detailed analysis of the bulk susceptibility and “B NMR data in CePd,B, shows that Ce atoms become trivalent as soon as they are surrounded by at least 3 boron atoms, the others remaining in the IV state. Electronic relaxation times are also deduced which increase by two orders of magnitude with increasing x from 0.04 to 0.55.
The effect of boron addition in the interstitial sites of the IV CePd, compound has been reported first by Dhar et al. [l]: the lattice anomaly of CePd, among the REPd, compounds is suppressed in CePd,B, where Ce is in the 3’ ionic state. Our NMR investigation of the CePd,B, system was performed in order to study at a local scale the effect of boron on the valence state of Ce. Bulk susceptibilities have also been measured, which agree with the previous results, so that average magnetic properties can be compared with the local ones as measured by “B NMR. Samples for x = 0.04, 0.065 and 0.09 were studied by NMR together with CePd,B,,,, and YPd 3B,,,, as reference samples (3+ and non-magnetic, respectively). Although the existence of an ordered CePd, (AuCu,-like) structure is not well established the NMR spectra do not indicate the presence of any electric field gradient on B sites which implies that, at least around boron in the low B content compounds the structure is ordered, the B atoms being surrounded by a Pd octahedron and a Ce cube. This is well understood in terms of available volume for the B atoms. For high B concentration the broadening of the NMR line due to the demagnetizing field associated with the high bulk susceptibility does not allow to draw any conclusion about the chemical order in the compounds. Two striking observations have been made on the “B spectra in the low B content range and at low temperature: i) the spectra are asymmetric (fig. 1) which implies a distribution of Knight shifts; ii) relaxation times T, are also distributed along the resonance line, the nuclei with a small Knight shift (NM) having a relaxation time up to two orders of magnitude longer than those with a strong Knight shift 0304-8853/85/$03.30 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V.
(M) (fig. 2). Both observations are consistent since the Knight shift K and the relaxation rate T,-’ measure the local susceptibilities (static for K and dynamic, Q, for T; ’ ); they imply that “ magnetic” and “ non-magnetic” Ce atoms coexist in the samples at low temperature: NM borons are surrounded by non-magnetic Ce atoms (in the IV state) while M borons have magnetic Ce neighbours (in the 3+ state). At high temperature the two populations cannot be distinguished: the spectra are symmetrical and an unique short r, is observed (as in CePd,B,,,,). This is consistent with the Ce atoms being all “magnetic” (T is higher than any “spin fluctuation temperature” T,,). Owing to their very different T1 the two kinds of B atoms can be easily separated in the spectra and counted (fig. 1). The bulk susceptibility as well can be separated easily in a magnetic component (following a Curie Law or a CePd,B behaviour) and a non-magnetic one (temperature independent at low T’s) [2]. To go further in the quantitative analysis the relative populations of 3+ (magnetic) Ce and IV (non-magnetic Ce) at 4.2 K and those of NM and M boron are compared to probabilities computed in a simple environment model assuming
30G
H
Fig. 1. “B spectrum in CePd,B,,, NM and M boron contributions
(4.2 K) and its separation (see text). (w = 14.5 MHz).
in
109
E. Beourepaire et al. / IV + 3 + transition in CePd,B,
% z 0
E 0, 0 .? t
l/1
20-
A
2
3
lo-
0
_FO
,
L!z I
Fig. 2. Temperature dependence of the “B relaxation time in CePd,B, ent parts and 0.09 tively)is
compounds. For x = 0.06 the extreme T,‘s on differof the spectra (NM and M) are shown. For x = 0.04 only the majority contribution (NM and M, respecshown.
that the Ce valence state depends only on the number of boron atoms among the 8 nearest interstitial sites: Ce atoms would be in the trivalent state when they have at least n boron neighbours. Obviously n is larger than 1 otherwise all B atoms would be surrounded by Ce3+. On fig. 3 it can be seen that the proportion of Ce3+ deduced from susceptibility data and that of NM boron deduced from NMR data are both consistent with Ce atoms being in the trivalent state for n = 3. However, in fig. 3b, an overestimation of NM boron by our model is not surprising because of the influence of the second neighbour shell on the magnetic environment of boron. Thus the transition from the IV state in CePd, to the trivalent state in CePd,B occurs locully as soon as a Ce atom has three boron atoms in the nearest interstitial sites. Ce atoms that have less than 3 boron neighbours are still non-magnetic at low temperature. However, this transition is obviously not purely local and collective effects also take place as indicated for example by the rapid increase of the lattice parameter at low B content; it can also be seen from the thermal dependence of the suceptibility and from Ti measurements that TSF for Ce atoms that are still intermediate valent is considerably reduced (about 80 and 20 K for x = 0.04 and 0.09, respectively) with respect to that in CePd, (= 150 K). Correspondingly the addition of boron rapidly lengthens the electronic relaxation time re (fig. 4) that can be deduced from TI measurements following standard relaxation theory. According to Knight shift measurements to be presented elsewhere, relaxation occurs
0
5
10
15
x%
Fig. 3. Solid lines: proportion of trivalent Ce and B atoms with no Ce3+ neighbour (NM) computed in a local environment model for n = 2 or 3 (see text). 0: proportion of trivalent Ce deduced from bulk susceptibility; n : proportion of NM boron deduced from NMR.
mostly through the direct dipolar interaction between Ce moments and B nuclei, hence T’,S reported here concern only those Ce atoms that are rather close to B atoms (the totality for x = 0.55, about 30% for x = 0.04). As shown on fig. 4 the transition from the IV regime to the trivalent state is characterized by strong changes of
fi/T(meV)
10K
10
T(K)
100
300
Fig. 4. Thermal and concentration dependence of the Ce electronic relaxation rate (in units of energy h/r,). Solid lines: neutron quasielastic line width (CePd,: Holland-Moritz et al. [3]; Ce$: Horn et al. [4]).
110
E. Beaurepaire et al. / IV
the electronic relaxation time behaviour. In the trivalent state (x = 0.55) the thermal variation of re (proportional to T-“2) is qualitatively similar to that observed in “Kondo lattice” systems (Ce$, CeAl,, ). This behaviour is also roughly observed for x = 0.09 which for most of the B atoms are close to trivalent Ce atoms. Oppositely for x = 0.04 re is temperature independent above 100 K as in CePd, (but 10 times longer) and other IV systems; however a strong decrease of TV is observed at low temperature down to values comparable to that in CePd,: this could be correlated to an increase of the Ce valence (as often observed in mixed valent systems) (51. For x = 0.065 the two behaviours are mixed at low temperature as one probes through B NMR both IV and trivalent Ce atoms with comparable probabilities. These measurements have thus emphasized the role of local environment effects in the valence transition of cerium and as a consequence the coexistence of cerium
3 + transitton in CePd,B,
atoms in various valence states in CePd,B, samples. Preliminary measurements in CePd,Be, compounds also show similar effects which are certainly quite general as far as valence transitions are obtained by alloying.
References VI SK. Dhar, SK. Malik and R. Vijayaraghavan, Phya. Rev. 824 (1981) 6182. PI J.P. Kappler, M.J. Besnus. E. Beaurepaire, A. Meyer. J. Sereni and G. Nieva, J. Magn. Magn. Mat. 47&48 (1985)
111. [31 E. Holland-Moritz, D. Wohlleben and M. Loewenhaupt, Phys. Rev. B25 (1982) 7482. I41S. Horn, F. Steglich, M. Loewenhaupt and E. HollandMoritz, Physica 107B (1981) 103. J. Magn. Magn. Mat. 47&48 151 See e.g. E. Holland-Moritz, (1985) 127.