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Structure and magnetic ordering of the “free-electron” rare-earth halides RE2X5 (RE-Ce, Pr; X=Br, I)
Journal of Magnetism and Magnetic Materials 104-107 (1992) 1201-1203 North-Holland
Structure and magnetic ordering of the "free-electron" rare-earth halides RE2X 5 (RE-= Ce, Pr; X = Br, I) L. Keller ~', P. Fischer a, A. Furrer ~, K. Kr~imer and A.W. Hewat d
b
G. Meyer b, H.U. Giidel c
" Laboratory for Neutron Scattering, ETH Ziirich, CH-5232 Villigen PSI, Switzerland Institut fiir Anorganische Chemie, Unic'ersitiit Hannot~er, W-3000 Hannot,er, Germany c lnstitut fiir Anorganische Chemie, Unicersitiit Bern, CH-3000 Bern, Switzerland d lnstitut Laue-Langez,in, F-38042 Grenoble Cedex, France
Magnetic and neutron diffraction investigations performed on the rare-earth halides RE2X5=(RE3+)2(X )5(e ) (RE = Pr, Ce; X = Br, I) showed that these compounds possess remarkably high magnetic ordering temperatures. Pr2Br 5 orders antiferromagnetically at T N = 50(1) K, corresponding to the Shubnikov space group P21/m', i.e. k = 0. Below T = 20 K there is a second magnetic phase transition causing an abrupt increase of the magnetic moments. At T N = 40(2) K, CezBr 5 shows a similar antiferromagnetic ordering corresponding to k = 0. There is no evidence for a second phase transition at lower temperatures. Below TN = 37(1) K, Pr2I 5 orders antiferromagnetically corresponding to k = [½, 0,0] and the Shubnikov space group Pd2~. Structural investigations and X-ray absorption near-edge spectra showed that the compounds contain rare-earth ions RE 3+ as the "free-electron" formula (RE 3+ )2(X-)5(e-) suggests, although measurements of the average electrical conductivities of polycrystalline Pr2X s (X = Br, I) and Ce2Br 5 indicate semiconducting behaviour in the "powder average". Presumably the delocalized electron causes via RKKY interactions the high ordering temperatures. For some of the diiodides of the lanthanides R E I 2 (RE = La, Ce, Pr, Nd, Gd), metallic behaviour had been suggested [1,2], which was later on established [3] so that these halides should be formulated as, for example, ( L a 3 + ) ( I ) 2 ( e ), attesting to their "free-electron" behaviour. A similar formulation should be possible for R E 2 X 5 ( R E = L a , Ce, Pr; X = B r , I), i.e. ( R E 3 + ) 2 ( X - ) 5 ( e ) . They are isostructural and crystallize according to the space group P 2 J m [4], Preliminary bulk magnetic measurements on Pr2X 5 (X = Br, I) and Ce2Br ~ had revealed very interesting magnetic properties with remarkably high ordering temperatures at 50, 37 and 40 K respectively. Further magnetic phase transitions at T = 20 (Pr2Br 5) and 9 K (Pr2l 5) are indicated by second peaks in the magnetic susceptibility. The paramagnetic Curie temperatures Op --~ 40 K for both Pr2Br 5 and Pr2I 5 indicate substantial ferromagnetic interactions [5]. Ce2Br 5 shows an antiferromagnetic phase transition at about 40 K and a negative Curie temperature Op ~ - 3 5 K. In order to establish the long-range magnetic ordering, neutron scattering experiments were performed at the reactor Saphir (PSI, Wiirenlingen) and at ILL. R E z X 5 (RE = Pr, Ce; X = Br, I) crystallize according to the space group P 2 J m , Z = 2. Seven- ( R E D and eight-coordinate (RE2) rare-earth ions are connected to double chains along [010]. The double chains form layers approximately parallel to (101), which are connected to a three-dimensional structure over rather long bromide contacts. The room temperature distances P r - B r are in good agreement with p r 3 + - B r
distances in K2PrBrs, attesting to a localized valence state of Pr 3+ of 4f 2 with one delocalized electron per formula unit. This was confirmed by X-ray near-edge spectra. At the Pr L-edge only one absorption edge was found which by comparison with PrCl 3 and PrI 3 doubtlessly could be attributed to a Pr 4f 2 state. At lower temperatures, additional Bragg peaks resulting from long-range magnetic ordering occur in the neutron scattering patterns of all compounds R E 2 X 5 ( R E = Pr, Ce; X = Br, I). Pr 2Br 5 shows two magnetic ordering p h e n o m e n a at 50 and 20 K, respectively, corresponding to k = 0 and the Shubnikov space group P 2 j m ' . Below T = 20 K the ordered magnetic moments of the two inequivalent Pr 3+ ions increase abruptly from /d,pr I = 1.2/x B and ~Pr2 = 2.2/xB at 30 K to the saturation values /XprI = 1.9p~ B and /Zpr2 = 2.6p~B. The scattering angles and intensities of the magnetic Bragg peaks of CexBr 5 indicate similar magnetic ordering as in PrzBr 5. The magnetic moments are (as expected for Ce) smaller than in the Pr compound: J'~Cel ~ 1.1/xB and /XCe2--~ 1.4p~ B. There is no evidence for a second phase transition at lower temperatures. Pr2I 5 undergoes a magnetic phase transition at T N = 37(1) K. The magnetic peaks may be indexed by doubling of the a-axis, i.e. k = [½, 0, 0]. Then all magnetic peaks have odd h indices, implying antiferromagnetic coupling along [100]. Therefore, the magnetic unit cell results from the chemical cell by an antitranslation in the [100] direction. The magnetic structure corresponds to the Shubnikov space group Pa21 with
0312-8853/92/$05.00 © 1992 - Elsevier Science Publishers B.V. All rights reserved
1202
L. Keller et al. / Structure and magnetic ordering of Re2X 5
ordered magnetic moments at saturation: ~U,pr 1 = 2.1/x B and ]d~Pr2= I ' 8 / J ' B " The temperature dependence of the magnetic Bragg peak (100) does not indicate the second magnetic phase transition around 9 K which was observed in bulk magnetic investigations within the error limits. Similar neutron scattering experiments on Ce2I s and inelastic neutron measurements to search for crystal fields on R E 2 X 5 ( R E = Pr, Ce; X = Br, I) are in progress. The results show clearly that in the rare-earth halides R E 2 X 5 ( R E = Pr, Ce; X = Br, I) the rare-earth ions are trivalent with one " f r e e " electron per formula unit. According to conductivity measurements this " f r e e " electron is not delocalized in the sense of a metallic conductor. The compounds are semi-conductors in the " p o w d e r average"• An explanation for this behaviour was found through extended-Hiickel calculations which showed that the " f r e e " electrons are localized in weak R E - R E bonds. At low temperatures the compounds are insulators and the R E - R E bonds provide good superexchange pathways causing high magnetic ordering temperatures. The band structure shows two very narrow bands for the " f r e e " electrons being half filled in the sense of a Mott insulator [6]. A striking result of the neutron scattering experiments is the fact that the isostructural bromides and
1,2
1,0
-
OO
0,8-
° 0,6-
0,4
.
0,2 . 0,0
.
|
10
•
i
20
•
i
•
30 Temperature
i
40
.
l
50
•
60
[K]
Fig. 1. Temperature dependence of reduced magnetization for CezBr 5.
iodides do not have the same magnetic structure. They correspond to k = 0 and k = [½, 0, 0], respectively. In both cases the resulting magnetic structure consists of p r e d o m i n a n t l y ferromagnetically c o u p l e d double chains, which order antiferromagnetically at low temperatures. Presumably due to crystal field effects the
Fig. 2. Magnetic structure of PrzBr 5.
L. Keller et al. / Structure and magnetic ordering of Re2Xs
1203
i
I I !
?
a
•
Prl
oPr2 Fig. 3, Magnetic structure of Pr2I 5.
Table 1 Magnetic moments [/ZB] or Pr2X 5 (X = Br, I) and Ce2Br 5 corresponding to the two inequivalent rare-earth sites Pr 2 Br s 30 K ]'£RE1 P~x /.~y /z z
1.2 (2) 1.1 (2) 0 0.5 (1)
]£RE2 ~ /Zy /xz
2.2 (2) -- 1.6 ( 2 ) 0 - 1.5 (1)
5K 1.9 (3) 1.6 (4) 0 0.9 (2) 2.6 (2)
Pr 215 5K 2.14 (9) 1.54 (8) 1.47 (6) 0.2 (1) 1.8 (1)
-- 2.0 ( 4 )
-- 1 . 0 4 ( 7 )
0 - 1.6 (2)
1.18 (9) - 0 . 8 8 (8)
Ce 2Brs 1.5 K
o r d e r e d m a g n e t i c m o m e n t s o f R E 1 a n d R E 2 in R E 2 X 5 at s a t u r a t i o n a p p e a r to b e significantly r e d u c e d b e l o w t h e free ion value g ~ B ~ l ) a n d t u r n out to b e very d i f f e r e n t f r o m e a c h o t h e r .
~ 1.1
References
(in progress)
[1] [2] [3] [4] [5] [6]
~ 1.4 (in progress)
J.D. Corbett et al., Adv. Chem. 71 (1967) 56. J.D. Corbett, Inorg. Chem. 22 (1983) 2669. J.H. Burrow et al., J. Phys. C 20 (1987) 4115. Th. Schleid et al., Z. Anorg. Allg. Chem. 97 (1987) 552. K. Kr~imer et al., J. Solid State Chem., to be published. H.-J. Meyer and R. Hoffmann, J. Solid State Chem., in press.
Structure and magnetic ordering of the "free-electron" rare-earth halides RE2X 5 (RE-= Ce, Pr; X = Br, I) L. Keller ~', P. Fischer a, A. Furrer ~, K. Kr~imer and A.W. Hewat d
b
G. Meyer b, H.U. Giidel c
" Laboratory for Neutron Scattering, ETH Ziirich, CH-5232 Villigen PSI, Switzerland Institut fiir Anorganische Chemie, Unic'ersitiit Hannot~er, W-3000 Hannot,er, Germany c lnstitut fiir Anorganische Chemie, Unicersitiit Bern, CH-3000 Bern, Switzerland d lnstitut Laue-Langez,in, F-38042 Grenoble Cedex, France
Magnetic and neutron diffraction investigations performed on the rare-earth halides RE2X5=(RE3+)2(X )5(e ) (RE = Pr, Ce; X = Br, I) showed that these compounds possess remarkably high magnetic ordering temperatures. Pr2Br 5 orders antiferromagnetically at T N = 50(1) K, corresponding to the Shubnikov space group P21/m', i.e. k = 0. Below T = 20 K there is a second magnetic phase transition causing an abrupt increase of the magnetic moments. At T N = 40(2) K, CezBr 5 shows a similar antiferromagnetic ordering corresponding to k = 0. There is no evidence for a second phase transition at lower temperatures. Below TN = 37(1) K, Pr2I 5 orders antiferromagnetically corresponding to k = [½, 0,0] and the Shubnikov space group Pd2~. Structural investigations and X-ray absorption near-edge spectra showed that the compounds contain rare-earth ions RE 3+ as the "free-electron" formula (RE 3+ )2(X-)5(e-) suggests, although measurements of the average electrical conductivities of polycrystalline Pr2X s (X = Br, I) and Ce2Br 5 indicate semiconducting behaviour in the "powder average". Presumably the delocalized electron causes via RKKY interactions the high ordering temperatures. For some of the diiodides of the lanthanides R E I 2 (RE = La, Ce, Pr, Nd, Gd), metallic behaviour had been suggested [1,2], which was later on established [3] so that these halides should be formulated as, for example, ( L a 3 + ) ( I ) 2 ( e ), attesting to their "free-electron" behaviour. A similar formulation should be possible for R E 2 X 5 ( R E = L a , Ce, Pr; X = B r , I), i.e. ( R E 3 + ) 2 ( X - ) 5 ( e ) . They are isostructural and crystallize according to the space group P 2 J m [4], Preliminary bulk magnetic measurements on Pr2X 5 (X = Br, I) and Ce2Br ~ had revealed very interesting magnetic properties with remarkably high ordering temperatures at 50, 37 and 40 K respectively. Further magnetic phase transitions at T = 20 (Pr2Br 5) and 9 K (Pr2l 5) are indicated by second peaks in the magnetic susceptibility. The paramagnetic Curie temperatures Op --~ 40 K for both Pr2Br 5 and Pr2I 5 indicate substantial ferromagnetic interactions [5]. Ce2Br 5 shows an antiferromagnetic phase transition at about 40 K and a negative Curie temperature Op ~ - 3 5 K. In order to establish the long-range magnetic ordering, neutron scattering experiments were performed at the reactor Saphir (PSI, Wiirenlingen) and at ILL. R E z X 5 (RE = Pr, Ce; X = Br, I) crystallize according to the space group P 2 J m , Z = 2. Seven- ( R E D and eight-coordinate (RE2) rare-earth ions are connected to double chains along [010]. The double chains form layers approximately parallel to (101), which are connected to a three-dimensional structure over rather long bromide contacts. The room temperature distances P r - B r are in good agreement with p r 3 + - B r
distances in K2PrBrs, attesting to a localized valence state of Pr 3+ of 4f 2 with one delocalized electron per formula unit. This was confirmed by X-ray near-edge spectra. At the Pr L-edge only one absorption edge was found which by comparison with PrCl 3 and PrI 3 doubtlessly could be attributed to a Pr 4f 2 state. At lower temperatures, additional Bragg peaks resulting from long-range magnetic ordering occur in the neutron scattering patterns of all compounds R E 2 X 5 ( R E = Pr, Ce; X = Br, I). Pr 2Br 5 shows two magnetic ordering p h e n o m e n a at 50 and 20 K, respectively, corresponding to k = 0 and the Shubnikov space group P 2 j m ' . Below T = 20 K the ordered magnetic moments of the two inequivalent Pr 3+ ions increase abruptly from /d,pr I = 1.2/x B and ~Pr2 = 2.2/xB at 30 K to the saturation values /XprI = 1.9p~ B and /Zpr2 = 2.6p~B. The scattering angles and intensities of the magnetic Bragg peaks of CexBr 5 indicate similar magnetic ordering as in PrzBr 5. The magnetic moments are (as expected for Ce) smaller than in the Pr compound: J'~Cel ~ 1.1/xB and /XCe2--~ 1.4p~ B. There is no evidence for a second phase transition at lower temperatures. Pr2I 5 undergoes a magnetic phase transition at T N = 37(1) K. The magnetic peaks may be indexed by doubling of the a-axis, i.e. k = [½, 0, 0]. Then all magnetic peaks have odd h indices, implying antiferromagnetic coupling along [100]. Therefore, the magnetic unit cell results from the chemical cell by an antitranslation in the [100] direction. The magnetic structure corresponds to the Shubnikov space group Pa21 with
0312-8853/92/$05.00 © 1992 - Elsevier Science Publishers B.V. All rights reserved
1202
L. Keller et al. / Structure and magnetic ordering of Re2X 5
ordered magnetic moments at saturation: ~U,pr 1 = 2.1/x B and ]d~Pr2= I ' 8 / J ' B " The temperature dependence of the magnetic Bragg peak (100) does not indicate the second magnetic phase transition around 9 K which was observed in bulk magnetic investigations within the error limits. Similar neutron scattering experiments on Ce2I s and inelastic neutron measurements to search for crystal fields on R E 2 X 5 ( R E = Pr, Ce; X = Br, I) are in progress. The results show clearly that in the rare-earth halides R E 2 X 5 ( R E = Pr, Ce; X = Br, I) the rare-earth ions are trivalent with one " f r e e " electron per formula unit. According to conductivity measurements this " f r e e " electron is not delocalized in the sense of a metallic conductor. The compounds are semi-conductors in the " p o w d e r average"• An explanation for this behaviour was found through extended-Hiickel calculations which showed that the " f r e e " electrons are localized in weak R E - R E bonds. At low temperatures the compounds are insulators and the R E - R E bonds provide good superexchange pathways causing high magnetic ordering temperatures. The band structure shows two very narrow bands for the " f r e e " electrons being half filled in the sense of a Mott insulator [6]. A striking result of the neutron scattering experiments is the fact that the isostructural bromides and
1,2
1,0
-
OO
0,8-
° 0,6-
0,4
.
0,2 . 0,0
.
|
10
•
i
20
•
i
•
30 Temperature
i
40
.
l
50
•
60
[K]
Fig. 1. Temperature dependence of reduced magnetization for CezBr 5.
iodides do not have the same magnetic structure. They correspond to k = 0 and k = [½, 0, 0], respectively. In both cases the resulting magnetic structure consists of p r e d o m i n a n t l y ferromagnetically c o u p l e d double chains, which order antiferromagnetically at low temperatures. Presumably due to crystal field effects the
Fig. 2. Magnetic structure of PrzBr 5.
L. Keller et al. / Structure and magnetic ordering of Re2Xs
1203
i
I I !
?
a
•
Prl
oPr2 Fig. 3, Magnetic structure of Pr2I 5.
Table 1 Magnetic moments [/ZB] or Pr2X 5 (X = Br, I) and Ce2Br 5 corresponding to the two inequivalent rare-earth sites Pr 2 Br s 30 K ]'£RE1 P~x /.~y /z z
1.2 (2) 1.1 (2) 0 0.5 (1)
]£RE2 ~ /Zy /xz
2.2 (2) -- 1.6 ( 2 ) 0 - 1.5 (1)
5K 1.9 (3) 1.6 (4) 0 0.9 (2) 2.6 (2)
Pr 215 5K 2.14 (9) 1.54 (8) 1.47 (6) 0.2 (1) 1.8 (1)
-- 2.0 ( 4 )
-- 1 . 0 4 ( 7 )
0 - 1.6 (2)
1.18 (9) - 0 . 8 8 (8)
Ce 2Brs 1.5 K
o r d e r e d m a g n e t i c m o m e n t s o f R E 1 a n d R E 2 in R E 2 X 5 at s a t u r a t i o n a p p e a r to b e significantly r e d u c e d b e l o w t h e free ion value g ~ B ~ l ) a n d t u r n out to b e very d i f f e r e n t f r o m e a c h o t h e r .
~ 1.1
References
(in progress)
[1] [2] [3] [4] [5] [6]
~ 1.4 (in progress)
J.D. Corbett et al., Adv. Chem. 71 (1967) 56. J.D. Corbett, Inorg. Chem. 22 (1983) 2669. J.H. Burrow et al., J. Phys. C 20 (1987) 4115. Th. Schleid et al., Z. Anorg. Allg. Chem. 97 (1987) 552. K. Kr~imer et al., J. Solid State Chem., to be published. H.-J. Meyer and R. Hoffmann, J. Solid State Chem., in press.