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Temperature induced valence transition in (YbxIn1-x)Cu2 (x = 0.35, 0.50, 0.55)
Journal of Magnetism and Magnetic Materials 76 & 77 (1988) 159-160 North-Holland, Amsterdam
159
TEMPERATURE I N D U C E D VALENCE T R A N S I T I O N IN (Ybx In i_ x)Cu z (x = 0.35, 0.50, 0.55) H. MOLLER, E. BAUER, E. G R A T Z Institute o/Experimental Physics TU- Vienna, A -1040 Wien, Austria
K. Y O S H I M U R A , T. N I T T A and M. M E K A T A Dept. of Applied Physics Fukuji Unit'. Bunl~vo Fukuji 910, Japan The temperature induced valence transition of Yb in the pseudobinary (Yb, ln 1 ~)Cu 2 series is studied by means of dc susceptibility, electrical resistivity, thermal expansion and low temperature X-ray diffraction. All these experiments exhibit that this transition from the Yb 3+ state to an intermediate valence state takes place around 40 K within a few degrees.
1. Introduction Recently a temperature induced valence transition of the Yb-ion in the pseudobinary compound (Yb0.4In0.6)Cu2 has been reported [1]. Later on it could be shown that this transition appears in a rather wide concentration range in this cubic Laves phase system [2]. In the present paper we studied the nature of the transition for different concentrations (x = 0.35, 0.5, 0.55) performing dc susceptibility, electrical resistivity, thermal expansion and low temperature X-ray diffraction measurements. The samples were prepared in an argon-furnace followed by a heat treatment at 8 0 0 ° C over one week. The cubic Laves phase structure was found for all these concentrations.
2. Discussion Fig. 1 shows the inverse magnetic susceptibility versus temperature. The discontinuities in 1/X visible near 40 K indicate the valence transition temperature Tv. Above Tv all samples exhibit a Curie-Weiss behaviour. The following values for the paramagnetic Curie temperature and the effective magnetic moment have been found: x = 0.35, 0=-17 K; x = 0 . 5 , 0 = - 1 5 K; x = 0 . 5 5 , 0 = - 3 6 K. The observed effective moments are very close to the value for the free Yb-ion (/~eff = 4.54 B), indicating the 3 + state of the Yb-ion above T,, in these compounds. Magnetization measurements performed in pulsed fields up to about 380 kOe reveal that metamagnetic transitions occur in fields of about H v = 300 kOe at 4.2 K, irrespective of the Yb
concentration. The inset in fig. 1 shows the M vs. H curves for (Yb0.sIn05)Cu 2 at T = 4.2 K and T = 77 K (below and above Tv). As can be seen from this inset, no metamagnetic transition is observable at 77 K ( T > Tv). It can be assumed that both transitions, caused by temperature and field, are of the same nature, since the thermal energy (kBTv) and the field energy (ktBju0H,.) are of the same order of magnitude. Fig. 2 shows the thermal expansion Al/l o in the temperature range from 10 to 300 K. The inset gives the temperature dependence of the absolute values of the lattice constant and the volume for (Yb0. sIn0.s)Cu 2- The temperature dependence of the unit cell parameter (a), was obtained by averaging the position of the [422], [333] and [440] X-ray diffraction line at a certain temperature. As can be seen from fig. 2 the lattice parameters decrease in a common way from 300 K down to about 50 K. For x = 0.35 and 0.5 a very sharp enlargement of the lattice parameters sets in near
~ 77K
2°I
'5° t /'-~
,o~
/ / ~:';
0
,"/
~-o 55
:
x-O.50
7[K]
50
100
t50
200
250
300
Fig. 1. Inverse magnetic suszeptibility X 1 versus temperature o f ( Y b , In 1 • ) C u > Inset: Magnetisation versus field for x = 0.5 at T = 4.2 and 77 K.
0304-8853/88/$03.50 © Elsevier Science Publishers B.V.
160
H. Mfiller et al. / Valence transition m ( Yh, In i
T o' -10~ I 7 ~,,5 2"0l" , ,zo ,
I
x=035 x:055 x:050
120 ÷
s°I-
O[ z~2s
2
O'~
'°t
0
20 (tUosinos)Cu2 0
50
T[H]
lO0
150
20O
250
versus temperature of Fig. 2. Thermal expansion A I / I . (Yb, lnl )Cu ~ and (Lu 0.5 In 0.5)Cu 2- Inset: Lattice parameter ( a ) and volume (V) of the cubic Laves phase (Yb0. 5In 0 O C u versus temperature.
42 K, while a less sharp change was found for x = 0.55. This increase of the cell dimension is consistent with the expected valence change from Yb 3+ towards the larger Yb 2+ ion. Below T,. no splitting or at least a broadening of the X-ray line profile could be detected within the experimental error for any concentration. As in our previous paper [3] we have estimated the magnitude of the valence change from the change in the unit cell dimension. Details of these procedure are described there. The magnitude of the increase of the cell dimension for different concentrations was obtained by comparing A l / l o of the different Yb concentrations to the corresponding isostructural (Lu~.Inl_~)Cu 2 compounds (in fig. 2 AI/I o for (Lu0.5In0.5)Cu2 is included for comparison). This estimation of the valence change as a function of x gives a maximum valence reduction of about 0.1 in the vicinity of x = 0.45. Fig. 3 shows the temperature dependence of the electrical resistivity O for the different concentrations. For comparison we have included the p vs. T curve of the isostructural nonmagnetic compound (Lu0.5In0.5)Cu 2. While the Lu compound
~) ( ' u ,
Fig.
3.
~
/
-7/
_---
~
.,=035
~
,., Tfnj
50
100
z50
200
250
300
Electrical resistivity # versus temperature (Yb, In~ , )Cu 2 and L u . . y l n . s )Cu,.
of
shows a smooth decrease of P with decreasing temperature tending to a rather high residual resistivity P0, the o ( T ) curve of the Yb compounds drops (by a factor of 10 for x = 0.5 and x = 0.55 and by a factor of 2 for x = 0.35) toward low P, values. It can be expected that the discontinuity in p vs. T around T, is connected with the disapearance of the spin disorder scattering (Om~g) below T~, but due to the small de Gennes factor of Yb (note 0m,g is proportional to the de Gennes factor [4]) this cannot be the only reason. The fairly low resistivity below T,., where the Yb is of intermediate valence, is certainly also associated with a drastic change in the density of states at 7",. References [1] I. Felner and I. Novik, Phys. Rev. B33 (1986) 617. [2] T. Shimizu, K. Yoshimura, T. Nina, T. Sakakibara, T. Goto and M. Mekata, J. Phys. Soc. Japan (1988) 57. [3] 1. Felner, I. Novik, D. Vaknin, U. Potzel, J. Moser, G.M. Kalvius, G. W o r t m a n m G. Schmiester, G. Hilscher, E. Gratz, Ch. Schmitzer, N. Pillmayr, K.G. Prasad, H. de Waard and H. Pinto, Phys. Rev. B35 (1987) 6956. [4] E. Gratz and M.J. Zuckermann, in: Handbook on the Physics and Chemistry of Rare Earth, eds. K.A. Gschneidner Jr. and L. Eyring (North-Holland. Amsterdam, 1982) p. 117.
159
TEMPERATURE I N D U C E D VALENCE T R A N S I T I O N IN (Ybx In i_ x)Cu z (x = 0.35, 0.50, 0.55) H. MOLLER, E. BAUER, E. G R A T Z Institute o/Experimental Physics TU- Vienna, A -1040 Wien, Austria
K. Y O S H I M U R A , T. N I T T A and M. M E K A T A Dept. of Applied Physics Fukuji Unit'. Bunl~vo Fukuji 910, Japan The temperature induced valence transition of Yb in the pseudobinary (Yb, ln 1 ~)Cu 2 series is studied by means of dc susceptibility, electrical resistivity, thermal expansion and low temperature X-ray diffraction. All these experiments exhibit that this transition from the Yb 3+ state to an intermediate valence state takes place around 40 K within a few degrees.
1. Introduction Recently a temperature induced valence transition of the Yb-ion in the pseudobinary compound (Yb0.4In0.6)Cu2 has been reported [1]. Later on it could be shown that this transition appears in a rather wide concentration range in this cubic Laves phase system [2]. In the present paper we studied the nature of the transition for different concentrations (x = 0.35, 0.5, 0.55) performing dc susceptibility, electrical resistivity, thermal expansion and low temperature X-ray diffraction measurements. The samples were prepared in an argon-furnace followed by a heat treatment at 8 0 0 ° C over one week. The cubic Laves phase structure was found for all these concentrations.
2. Discussion Fig. 1 shows the inverse magnetic susceptibility versus temperature. The discontinuities in 1/X visible near 40 K indicate the valence transition temperature Tv. Above Tv all samples exhibit a Curie-Weiss behaviour. The following values for the paramagnetic Curie temperature and the effective magnetic moment have been found: x = 0.35, 0=-17 K; x = 0 . 5 , 0 = - 1 5 K; x = 0 . 5 5 , 0 = - 3 6 K. The observed effective moments are very close to the value for the free Yb-ion (/~eff = 4.54 B), indicating the 3 + state of the Yb-ion above T,, in these compounds. Magnetization measurements performed in pulsed fields up to about 380 kOe reveal that metamagnetic transitions occur in fields of about H v = 300 kOe at 4.2 K, irrespective of the Yb
concentration. The inset in fig. 1 shows the M vs. H curves for (Yb0.sIn05)Cu 2 at T = 4.2 K and T = 77 K (below and above Tv). As can be seen from this inset, no metamagnetic transition is observable at 77 K ( T > Tv). It can be assumed that both transitions, caused by temperature and field, are of the same nature, since the thermal energy (kBTv) and the field energy (ktBju0H,.) are of the same order of magnitude. Fig. 2 shows the thermal expansion Al/l o in the temperature range from 10 to 300 K. The inset gives the temperature dependence of the absolute values of the lattice constant and the volume for (Yb0. sIn0.s)Cu 2- The temperature dependence of the unit cell parameter (a), was obtained by averaging the position of the [422], [333] and [440] X-ray diffraction line at a certain temperature. As can be seen from fig. 2 the lattice parameters decrease in a common way from 300 K down to about 50 K. For x = 0.35 and 0.5 a very sharp enlargement of the lattice parameters sets in near
~ 77K
2°I
'5° t /'-~
,o~
/ / ~:';
0
,"/
~-o 55
:
x-O.50
7[K]
50
100
t50
200
250
300
Fig. 1. Inverse magnetic suszeptibility X 1 versus temperature o f ( Y b , In 1 • ) C u > Inset: Magnetisation versus field for x = 0.5 at T = 4.2 and 77 K.
0304-8853/88/$03.50 © Elsevier Science Publishers B.V.
160
H. Mfiller et al. / Valence transition m ( Yh, In i
T o' -10~ I 7 ~,,5 2"0l" , ,zo ,
I
x=035 x:055 x:050
120 ÷
s°I-
O[ z~2s
2
O'~
'°t
0
20 (tUosinos)Cu2 0
50
T[H]
lO0
150
20O
250
versus temperature of Fig. 2. Thermal expansion A I / I . (Yb, lnl )Cu ~ and (Lu 0.5 In 0.5)Cu 2- Inset: Lattice parameter ( a ) and volume (V) of the cubic Laves phase (Yb0. 5In 0 O C u versus temperature.
42 K, while a less sharp change was found for x = 0.55. This increase of the cell dimension is consistent with the expected valence change from Yb 3+ towards the larger Yb 2+ ion. Below T,. no splitting or at least a broadening of the X-ray line profile could be detected within the experimental error for any concentration. As in our previous paper [3] we have estimated the magnitude of the valence change from the change in the unit cell dimension. Details of these procedure are described there. The magnitude of the increase of the cell dimension for different concentrations was obtained by comparing A l / l o of the different Yb concentrations to the corresponding isostructural (Lu~.Inl_~)Cu 2 compounds (in fig. 2 AI/I o for (Lu0.5In0.5)Cu2 is included for comparison). This estimation of the valence change as a function of x gives a maximum valence reduction of about 0.1 in the vicinity of x = 0.45. Fig. 3 shows the temperature dependence of the electrical resistivity O for the different concentrations. For comparison we have included the p vs. T curve of the isostructural nonmagnetic compound (Lu0.5In0.5)Cu 2. While the Lu compound
~) ( ' u ,
Fig.
3.
~
/
-7/
_---
~
.,=035
~
,., Tfnj
50
100
z50
200
250
300
Electrical resistivity # versus temperature (Yb, In~ , )Cu 2 and L u . . y l n . s )Cu,.
of
shows a smooth decrease of P with decreasing temperature tending to a rather high residual resistivity P0, the o ( T ) curve of the Yb compounds drops (by a factor of 10 for x = 0.5 and x = 0.55 and by a factor of 2 for x = 0.35) toward low P, values. It can be expected that the discontinuity in p vs. T around T, is connected with the disapearance of the spin disorder scattering (Om~g) below T~, but due to the small de Gennes factor of Yb (note 0m,g is proportional to the de Gennes factor [4]) this cannot be the only reason. The fairly low resistivity below T,., where the Yb is of intermediate valence, is certainly also associated with a drastic change in the density of states at 7",. References [1] I. Felner and I. Novik, Phys. Rev. B33 (1986) 617. [2] T. Shimizu, K. Yoshimura, T. Nina, T. Sakakibara, T. Goto and M. Mekata, J. Phys. Soc. Japan (1988) 57. [3] 1. Felner, I. Novik, D. Vaknin, U. Potzel, J. Moser, G.M. Kalvius, G. W o r t m a n m G. Schmiester, G. Hilscher, E. Gratz, Ch. Schmitzer, N. Pillmayr, K.G. Prasad, H. de Waard and H. Pinto, Phys. Rev. B35 (1987) 6956. [4] E. Gratz and M.J. Zuckermann, in: Handbook on the Physics and Chemistry of Rare Earth, eds. K.A. Gschneidner Jr. and L. Eyring (North-Holland. Amsterdam, 1982) p. 117.