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Mixed valency of europium in magnetically ordered EuFe4Al8
Solid State Communications,Vol. 28, pp.67—70. ©Pergamon Press Ltd. 1978. Printed in Great Britain.
0038—1098/78/1001—0067 $ 02.00/0
MIXED VALENCY OF EUROPIUM IN MAGNETICALLY ORDERED EuFe4AI8* I. Felner and I. Nowik Racah Institute of Physics, The Hebrew University, Jerusalem, Israel (Received 18 June 1978 by W. Low) X-ray, susceptibility and Mässbauer effect (ME) studies of EuFe4AI8 and EuFe3.5Cu0.5M8 were performed. The susceptibility and the “Fe ME studies indicate an antiferromagnetic 151EuME studies phase show atransition temperature of the dependent iron sublattic atshift isomer 140 typical K. The of a mixed valent state with an interconflguration excitation energy (ICE) of 1100K and width400K. At temperatures below 140 K also the Eu ions become magnetically ordered. In EuFe 3~5Cu0~5A18 the europium ICE is strongly dependent on local environment. WE REPORT HERE an extensive study of a new mixed valency [1] system, EuFe4A18, in which both Eu and Fe are magnetically ordered at temperatures below 140K. The system was investigated by X-ray to determine its exact crystal structure. Its magnetic susceptibility was measured to determine its magnetic structure 51Eu Mössbauer studies were performed and “Fe and ‘ to determine magnetic properties, hyperfine interactions
PAR vibrating sample magnetometer, model 155. Figure 1 shows the susceptibility curve, the magnetic phase transition at 140 K is evident. We have also measured the susceptibility curves of other RFe4A18 systems [2]. The systems R = La, Ce, Lu, Y show a single magnetictophase transitionordering at aboutof120—130K corresponding the magnetic the iron sublattice. In the RFe 4A18 systems [2] (R = Pr, Nd, Sm, Gd, and the temperature dependence of the Eu mixed valent Th, Dy, Ho, Er, Tm, Yb) a second antiferromagnetic state. A study of EuFe3.5Cu0~5Al8has determined the transition was observed at about 20K which is intersensitivity of the Eu mixed valent state to local environ- preted as the magnetic ordering of the independent rare ment. earth sublattice. In EuFe4A18 the Eu sublattice does not The compound EuFe4A18 and all other RFe4AI8 have an independent magnetic transition temperature. (R = rare earth) [2] were preparedby melting stiochioThe Eu ion is in a mixed valent state, evidenced from metric amounts of the elements in an argon atmosphere the unit cell size relative to the other rare earths (Fig. 2) using an induction furnace. Powder samples were exam- and from the ~ Mossbauer studies. The rise in suscepined by X-ray diffraction using a Phillips diffractometer tibility observed at 15 K (Fig. 1) is probably due to a With CUKa radiation with a Ni filter and graphite mono- spin reorientation of the iron sublattice, a similar rise is chromator. The X-ray spectrum (peak positions and observed in LaFe4AI8 and CeFe4AL8 [2]. intensities) was analyzed by an iterative least-squares Mossbauer studies of EuFe4A18 were performed fitting procedure to yield the lattice parameters and using the 14.4 keY transition in “Fe (using a source of exact positions of the atoms in the unit cell. It turns “Co in Cu) at 4.1—300K, and using the 21.6 keV tran51Eu (using a ‘51SmF out that EuFe4A18 crystallizes in the tetragonal body sition in ‘ 3 source) at 4.1—500K. centered structure, space group (14/mmm). Each unit The “Fe Mossbauer spectra are shown in Fig. 3. cell [c = 5.051(3), a = 8.784(4)] contains two formula Though all iron crystallographic sites are equivalent, the units. The Eu ions occupy the 2(a) sites (position (000)), spectra are composed of at least three subspectra, correthe Fe occupy the 8(f) sites (position (1/4, 1/4, 1/4)) sponding to inequivalent Fe magnetic sites. One may and the Al occupy the 8(i) (0.343(5), 0, 0) and 8(j) imagine that the antiferromagnetism of the Fe sublattice (0.282(4), 1/2, 0) sites. More details of the full analysis is of oscillatory nature, with many magnetic iron of the X-ray spectrum of EuFe4Al8 and the other inequivalent sites. Another possible interpretation of the RFe4A18 compounds is given elsewhere [21. observed spectra is that the inequivalent sites are due to The magnetic susceptibility of EuFe4A18 (4.1— a partial (~ 20%) Fe occupation of the Al sites. The 300 K in fields up to 17 kOe) has been measured by a temperature dependence of the hyperfmne field of the dominant iron site (>80%) is shown in Fig. 1. * This research was supported in part by the Israel ‘~EuMossbauerspectra (Fig. 4) below the magAcademy of Science and Humanities. netic ordering temperature of the iron sublattice show 67
68
MIXED VALENCY OF EUROPIUM IN MAGNETICALLY ORDERED EuFe4Al8
0.75
Vol.28, No. 1
-150w
Neff( Eu ~uFe~
-
10o~ U. ‘a U-
0.25
(Fe)
—
-
0.0c
50~ 0. 0
0
50
100 TEPI~ERATURE (K)
150
Fig. 1. The magnetic susceptibility of EuFe4AI8 (a), the hyperfine field acting on the Fe nuclei (b) and the hyperfme field acting on the Eu nuclei (c). 57Fe in Eu~~ 8
1.00
3)
~
-
0.99-
V(A
EE
1.00
~
I I II La Ce Pr Nd Sm
273K
I I I Eu Gd Tb
I I I Dy Ho Er Tm
~\%\y~13oK Yb Lu
W ,—
u.9~
~ 1.00 Fig. 2. Unit cell volume ofRFe 4AL8 (R = rare earth). 15tEu that an Observing induced magnetic field is acting on the of the Eu nuclei. the temperature dependence hyperfine field (Fig. 1) one can derive the exchange field acting onthe Eu ions. Assuming that the Eu ions are predominantly trivalent (0
mental zero exchange spectrum field proves aI eq~~~QI that single the absorption iron sublattice width has canted ism). the 140K, Mössbauer The whereas antiferromagnetic effective spectra atassuming temperatures quadrupole was structure above (weak <30 140K Mc ferromagnetderived sec’ itline was below froma 170(10) Mc sec1. Above Kinteraction the quadrupole interaction was obtained bycubic a 140 least to be that observed in Cssquare fit to the experi1). 2NaEuC16 (2.1field mmsec It is concluded that the magnetic hyperfme and thus also the exchange field, point at an angle 55 ±100 to the fourfold symmetry c-axis. This assumes that the electric field gradient is mainly composed of the lattice and Stark contributions [3] and thus is along the c axis. This is justified since here the exchange induced electric field gradient [3] is very small. 151Eu isomer shift pay ouraattention to the (Fig.We 5).now We observe temperature dependent isomer shift, typical to Eu in the mixed valent state. An analysis of the curve shown in Fig. S along the lines given in the letter discussing the EuRh 2 system [4], yields the
o.gg ~ 1.00 ~
,
.~
-J
0.96 1.00 0.99 0.98
--
I
I
-2
-1 VELOCITY
0
1 (mrn/sec)
2
Fig. 3. Mossbauer spectra of “Fe in EuFe 4AI8. solid curves are theoretical spectra composed ofThe three subspectra (intensities 0.82,0.11,0.07, isomer shift 0.16(3) mm sec1. At 4.1 K the quadrupole interactions eqQ are 3.1(2), 1.2(2), 0.5(2) Mc sec’ and the hyperfine fields are 116(2), 68(2) and 24(3) kOe for the three —
sites, respectively).
Vol. 28, No. 1
MIXED VALENCY OF EUROPIUM IN MAGNETICALLY ORDERED EuFe4Al8
1.00
513 K 0.99
1.00 1.00
69
5¼~in EuFe
~V~W
4A18
‘
273 K
~
~
0.98
-
273 K
1.00
0.99
150K
CO
~
0.99
‘3
0.98
I
I
0.99 ‘3 ‘a
‘a
I ‘a -I
~
120K
~
0.97
0.98
T~~OK
1.00,
0.99
1.00
0.99-
I
-20
—10
10
0
VELOCITY
20
—10
(mm/sec)
0 VELOCITY
10
20
(mm/sec)
151Eu in EuFe Fig. 4. Mössbauer spectra of 4A18 and EuFe3.5Cu0.5A18. The solid curves are theoretical spectra. The EuFe3.5Cu0.5A18 spectra are composed of 4 lines with intensities corresponding to Eu ions with 0, 1, 2 and 3 Cu first nearest neighbors. In the analysis of the spectra below 140K the magnetic hyperfine interactions were taken into account. 1
~u In EuFe4A18
interconfiguration excitation energy (1100 ±lOOK) and its width (400 ±100 K). This means that at room ternperature the Eu ions has 19% divalent nature and at
~
0 —1
~
~
.~.—.
‘
--. U,
1-
500K 30% divalent nature. This result is reasonably consistent with the unit cell size (Fig. 2). Since in EuCu4A18 we found that the Eu ion is divalent we studied several EuFe4.~Cu~Al8 systems. The Mossbauer spectra of EuFe3.5Cu0•5Si2 (Fig. 4) can be analyzed in terms of four sub-spectra corresponding to Eu-ions with different local environments. Assuming random probability of Cu—Fe exchange, the relative number of Eu ions with 0, 1,2, 3 (out of eight) Cu first nearest neighbors is 0.343, 0.393, 0.196, 0.056, respectively. The isomer shift of the various Mässbauer sub-spectra (Fig. 5)
_~.
-2
-7
~aQCu 8 EuFe3sCuo5M4~1Cu
neighbor to Eu
1v2 Cu
-9
~O3Cu -10
-11
~
241n çuFe4Ala
-12 ,Eu 0
100
200
~ 300
400
500
T~nperature (K) Fig.151Eu 5. The temperature dependence of the isomer shift in EuFe of 4A18 and in EuFe3.5Cu0.5A18.
show themodel sensitivity of thethat Eu ion The analysis valency of [4], the shows isomer shift ICE in to terms oflocal Euof inenvironment. the mixed EuFe 3.5Cu0.5A18 is 1200K when the Eu has no Cu nearest neighbor. It becomes 900 K for Eu ions with one Cu neighbor 400K (slightly temperature dependent) for Eu ions with 2 Cu neighbors and below 400K —
70
MIXED VALENCY OF EUROPIUM IN MAGNETICALLY ORDERED EuFe4AI8
(ahnost stable 2+) for Eu ions with 3 or more Cu neighbors. The analysis assumed the same width 400 K for all excitation energies, S3 = 0.6 mm sec’ and S2 S3 = 13mm sec’ (see [4]). It is concluded that in EuFe4A18 the Eu ion is in a mixed valent state, nevertheless, at low temperatures it responds to the exchange field produced by the neigh. boring magnetically ordered iron ions and a magnetic 3~cases [3]. The hyperfme field is induced, as in pure Eu —
—
Vol. 28, No.1
magnetic susceptibility at temperatures above TN is also consistent with an effective moment of the Eu ion in a mixed valent state (the effective moment of iron is assumed to be ~ ji~ as in all other RFe4AI8 systems [2]). From the EuFe3.5Cu0.5Al8 study we conclude that the Eu valency is extremely sensitive to local environment, when three out of eight first nearest Fe neighbors are replaced by Cu the Eu ion becomes divalent.
REFERENCES 1.
Valence Instabilities and Narrow Band Phenomena (Edited by PARKS R.D.). Plenum Press, New York (1976).
2.
FELNER I. & NOWIK I., J. Phys. Chem. Solids (in press).
3.
GILAT G. & NOWIK I., Phys. Rev. 130, 1361 (1963); BAUMINTER E.R., NOWIK I. & OFER S., Phys. Lett. 29A, 199 (1969).
4.
NOWIK I., CAMPAGNA M. & WERTHEIM G.K.,Phys. Rev. Lett. 38,43 (1977).
0038—1098/78/1001—0067 $ 02.00/0
MIXED VALENCY OF EUROPIUM IN MAGNETICALLY ORDERED EuFe4AI8* I. Felner and I. Nowik Racah Institute of Physics, The Hebrew University, Jerusalem, Israel (Received 18 June 1978 by W. Low) X-ray, susceptibility and Mässbauer effect (ME) studies of EuFe4AI8 and EuFe3.5Cu0.5M8 were performed. The susceptibility and the “Fe ME studies indicate an antiferromagnetic 151EuME studies phase show atransition temperature of the dependent iron sublattic atshift isomer 140 typical K. The of a mixed valent state with an interconflguration excitation energy (ICE) of 1100K and width400K. At temperatures below 140 K also the Eu ions become magnetically ordered. In EuFe 3~5Cu0~5A18 the europium ICE is strongly dependent on local environment. WE REPORT HERE an extensive study of a new mixed valency [1] system, EuFe4A18, in which both Eu and Fe are magnetically ordered at temperatures below 140K. The system was investigated by X-ray to determine its exact crystal structure. Its magnetic susceptibility was measured to determine its magnetic structure 51Eu Mössbauer studies were performed and “Fe and ‘ to determine magnetic properties, hyperfine interactions
PAR vibrating sample magnetometer, model 155. Figure 1 shows the susceptibility curve, the magnetic phase transition at 140 K is evident. We have also measured the susceptibility curves of other RFe4A18 systems [2]. The systems R = La, Ce, Lu, Y show a single magnetictophase transitionordering at aboutof120—130K corresponding the magnetic the iron sublattice. In the RFe 4A18 systems [2] (R = Pr, Nd, Sm, Gd, and the temperature dependence of the Eu mixed valent Th, Dy, Ho, Er, Tm, Yb) a second antiferromagnetic state. A study of EuFe3.5Cu0~5Al8has determined the transition was observed at about 20K which is intersensitivity of the Eu mixed valent state to local environ- preted as the magnetic ordering of the independent rare ment. earth sublattice. In EuFe4A18 the Eu sublattice does not The compound EuFe4A18 and all other RFe4AI8 have an independent magnetic transition temperature. (R = rare earth) [2] were preparedby melting stiochioThe Eu ion is in a mixed valent state, evidenced from metric amounts of the elements in an argon atmosphere the unit cell size relative to the other rare earths (Fig. 2) using an induction furnace. Powder samples were exam- and from the ~ Mossbauer studies. The rise in suscepined by X-ray diffraction using a Phillips diffractometer tibility observed at 15 K (Fig. 1) is probably due to a With CUKa radiation with a Ni filter and graphite mono- spin reorientation of the iron sublattice, a similar rise is chromator. The X-ray spectrum (peak positions and observed in LaFe4AI8 and CeFe4AL8 [2]. intensities) was analyzed by an iterative least-squares Mossbauer studies of EuFe4A18 were performed fitting procedure to yield the lattice parameters and using the 14.4 keY transition in “Fe (using a source of exact positions of the atoms in the unit cell. It turns “Co in Cu) at 4.1—300K, and using the 21.6 keV tran51Eu (using a ‘51SmF out that EuFe4A18 crystallizes in the tetragonal body sition in ‘ 3 source) at 4.1—500K. centered structure, space group (14/mmm). Each unit The “Fe Mossbauer spectra are shown in Fig. 3. cell [c = 5.051(3), a = 8.784(4)] contains two formula Though all iron crystallographic sites are equivalent, the units. The Eu ions occupy the 2(a) sites (position (000)), spectra are composed of at least three subspectra, correthe Fe occupy the 8(f) sites (position (1/4, 1/4, 1/4)) sponding to inequivalent Fe magnetic sites. One may and the Al occupy the 8(i) (0.343(5), 0, 0) and 8(j) imagine that the antiferromagnetism of the Fe sublattice (0.282(4), 1/2, 0) sites. More details of the full analysis is of oscillatory nature, with many magnetic iron of the X-ray spectrum of EuFe4Al8 and the other inequivalent sites. Another possible interpretation of the RFe4A18 compounds is given elsewhere [21. observed spectra is that the inequivalent sites are due to The magnetic susceptibility of EuFe4A18 (4.1— a partial (~ 20%) Fe occupation of the Al sites. The 300 K in fields up to 17 kOe) has been measured by a temperature dependence of the hyperfmne field of the dominant iron site (>80%) is shown in Fig. 1. * This research was supported in part by the Israel ‘~EuMossbauerspectra (Fig. 4) below the magAcademy of Science and Humanities. netic ordering temperature of the iron sublattice show 67
68
MIXED VALENCY OF EUROPIUM IN MAGNETICALLY ORDERED EuFe4Al8
0.75
Vol.28, No. 1
-150w
Neff( Eu ~uFe~
-
10o~ U. ‘a U-
0.25
(Fe)
—
-
0.0c
50~ 0. 0
0
50
100 TEPI~ERATURE (K)
150
Fig. 1. The magnetic susceptibility of EuFe4AI8 (a), the hyperfine field acting on the Fe nuclei (b) and the hyperfme field acting on the Eu nuclei (c). 57Fe in Eu~~ 8
1.00
3)
~
-
0.99-
V(A
EE
1.00
~
I I II La Ce Pr Nd Sm
273K
I I I Eu Gd Tb
I I I Dy Ho Er Tm
~\%\y~13oK Yb Lu
W ,—
u.9~
~ 1.00 Fig. 2. Unit cell volume ofRFe 4AL8 (R = rare earth). 15tEu that an Observing induced magnetic field is acting on the of the Eu nuclei. the temperature dependence hyperfine field (Fig. 1) one can derive the exchange field acting onthe Eu ions. Assuming that the Eu ions are predominantly trivalent (0
mental zero exchange spectrum field proves aI eq~~~QI that single the absorption iron sublattice width has canted ism). the 140K, Mössbauer The whereas antiferromagnetic effective spectra atassuming temperatures quadrupole was structure above (weak <30 140K Mc ferromagnetderived sec’ itline was below froma 170(10) Mc sec1. Above Kinteraction the quadrupole interaction was obtained bycubic a 140 least to be that observed in Cssquare fit to the experi1). 2NaEuC16 (2.1field mmsec It is concluded that the magnetic hyperfme and thus also the exchange field, point at an angle 55 ±100 to the fourfold symmetry c-axis. This assumes that the electric field gradient is mainly composed of the lattice and Stark contributions [3] and thus is along the c axis. This is justified since here the exchange induced electric field gradient [3] is very small. 151Eu isomer shift pay ouraattention to the (Fig.We 5).now We observe temperature dependent isomer shift, typical to Eu in the mixed valent state. An analysis of the curve shown in Fig. S along the lines given in the letter discussing the EuRh 2 system [4], yields the
o.gg ~ 1.00 ~
,
.~
-J
0.96 1.00 0.99 0.98
--
I
I
-2
-1 VELOCITY
0
1 (mrn/sec)
2
Fig. 3. Mossbauer spectra of “Fe in EuFe 4AI8. solid curves are theoretical spectra composed ofThe three subspectra (intensities 0.82,0.11,0.07, isomer shift 0.16(3) mm sec1. At 4.1 K the quadrupole interactions eqQ are 3.1(2), 1.2(2), 0.5(2) Mc sec’ and the hyperfine fields are 116(2), 68(2) and 24(3) kOe for the three —
sites, respectively).
Vol. 28, No. 1
MIXED VALENCY OF EUROPIUM IN MAGNETICALLY ORDERED EuFe4Al8
1.00
513 K 0.99
1.00 1.00
69
5¼~in EuFe
~V~W
4A18
‘
273 K
~
~
0.98
-
273 K
1.00
0.99
150K
CO
~
0.99
‘3
0.98
I
I
0.99 ‘3 ‘a
‘a
I ‘a -I
~
120K
~
0.97
0.98
T~~OK
1.00,
0.99
1.00
0.99-
I
-20
—10
10
0
VELOCITY
20
—10
(mm/sec)
0 VELOCITY
10
20
(mm/sec)
151Eu in EuFe Fig. 4. Mössbauer spectra of 4A18 and EuFe3.5Cu0.5A18. The solid curves are theoretical spectra. The EuFe3.5Cu0.5A18 spectra are composed of 4 lines with intensities corresponding to Eu ions with 0, 1, 2 and 3 Cu first nearest neighbors. In the analysis of the spectra below 140K the magnetic hyperfine interactions were taken into account. 1
~u In EuFe4A18
interconfiguration excitation energy (1100 ±lOOK) and its width (400 ±100 K). This means that at room ternperature the Eu ions has 19% divalent nature and at
~
0 —1
~
~
.~.—.
‘
--. U,
1-
500K 30% divalent nature. This result is reasonably consistent with the unit cell size (Fig. 2). Since in EuCu4A18 we found that the Eu ion is divalent we studied several EuFe4.~Cu~Al8 systems. The Mossbauer spectra of EuFe3.5Cu0•5Si2 (Fig. 4) can be analyzed in terms of four sub-spectra corresponding to Eu-ions with different local environments. Assuming random probability of Cu—Fe exchange, the relative number of Eu ions with 0, 1,2, 3 (out of eight) Cu first nearest neighbors is 0.343, 0.393, 0.196, 0.056, respectively. The isomer shift of the various Mässbauer sub-spectra (Fig. 5)
_~.
-2
-7
~aQCu 8 EuFe3sCuo5M4~1Cu
neighbor to Eu
1v2 Cu
-9
~O3Cu -10
-11
~
241n çuFe4Ala
-12 ,Eu 0
100
200
~ 300
400
500
T~nperature (K) Fig.151Eu 5. The temperature dependence of the isomer shift in EuFe of 4A18 and in EuFe3.5Cu0.5A18.
show themodel sensitivity of thethat Eu ion The analysis valency of [4], the shows isomer shift ICE in to terms oflocal Euof inenvironment. the mixed EuFe 3.5Cu0.5A18 is 1200K when the Eu has no Cu nearest neighbor. It becomes 900 K for Eu ions with one Cu neighbor 400K (slightly temperature dependent) for Eu ions with 2 Cu neighbors and below 400K —
70
MIXED VALENCY OF EUROPIUM IN MAGNETICALLY ORDERED EuFe4AI8
(ahnost stable 2+) for Eu ions with 3 or more Cu neighbors. The analysis assumed the same width 400 K for all excitation energies, S3 = 0.6 mm sec’ and S2 S3 = 13mm sec’ (see [4]). It is concluded that in EuFe4A18 the Eu ion is in a mixed valent state, nevertheless, at low temperatures it responds to the exchange field produced by the neigh. boring magnetically ordered iron ions and a magnetic 3~cases [3]. The hyperfme field is induced, as in pure Eu —
—
Vol. 28, No.1
magnetic susceptibility at temperatures above TN is also consistent with an effective moment of the Eu ion in a mixed valent state (the effective moment of iron is assumed to be ~ ji~ as in all other RFe4AI8 systems [2]). From the EuFe3.5Cu0.5Al8 study we conclude that the Eu valency is extremely sensitive to local environment, when three out of eight first nearest Fe neighbors are replaced by Cu the Eu ion becomes divalent.
REFERENCES 1.
Valence Instabilities and Narrow Band Phenomena (Edited by PARKS R.D.). Plenum Press, New York (1976).
2.
FELNER I. & NOWIK I., J. Phys. Chem. Solids (in press).
3.
GILAT G. & NOWIK I., Phys. Rev. 130, 1361 (1963); BAUMINTER E.R., NOWIK I. & OFER S., Phys. Lett. 29A, 199 (1969).
4.
NOWIK I., CAMPAGNA M. & WERTHEIM G.K.,Phys. Rev. Lett. 38,43 (1977).