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Microstructure and magnetic properties of R2BaNiO5 (R = Sm, Gd, YbandLu)
233
Materials Chemistry and Physics, 34 (1993) 233-237
Microstructure Yb and Lu)
and magnetic properties
R. S&ez-Puche*, J. M. Coronado,
of R,BaNiO,
(R = Sm, Gd,
C. L. Otero-Diaz
Departamento de Quimica Znorga’nica, Facultad de Ciencias Quimicas, Universidad Complutense de Madrid, Madrid E-28040 (Spain)
and J. M. Martin-Llorente Depariamento de Q&mica Znorga’nica, Facultat de Ciencias Q&micas, Universidad de Salamanca, Salamanca (Spain) (Received
July 17, 1992; accepted
October
14, 1992)
Abstract The magnetic behavior found for R,BaNiO, (R=Sm, Gd, Yb and Lu) compounds has been explained by taking into account the two structural types in which they crystallize. For both Sm and Gd isostructural oxides above room temperature, the nickel ions are lD-antiferromagnetically ordered along the chains of octahedra characteristic of the Nd,BaNiOS structure type. Below 30 K both rare earth and nickel sublattices are simultaneously 3Dordered. In the case of Yb and Lu oxides, belonging to the SmzBaCuOS structure type, the existence of isolated distorted square pyramids (NiO,) justifies the assumption that antiferromagnetic interactions in the nickel sublattice are not operative until the temperature is as low as 10 K. Moreover, no evidence of order in the ytterbium compound has been found down to a temperature of 4.2 K. On the other hand, high-resolution electron microscopy reveals that both types of material are well ordered, without any local structural defects.
Introduction
Recently, R,BaNiO, oxides (R = rare earth element) have been the subject of numerous investigations because of their unusual and interesting structural and magnetic properties [l-3]. The Nd,BaNiO, structure type was first reported by Schifller and Mtiller-Buschbaum [4], and subsequent studies have revealed that this structure is also shown by different R,BaNiO, oxides where R =Y, Nd, Gd, Dy, Ho, Er and Tm [5, 61. This structure shows orthorhombic symmetry, space group Zrnmm, showing as the main structural feature the existence of isolated chains of flattened octahedra (NiO,) sharing corners along the u-axis. The barium and lanthanide cations form rows of (BaO,,) and (RO,) polyhedra running parallel to the u-axis. On the other hand, Yb,BaNiO, and Lu,BaNiO, show orthorhombic symmetry, space group Prima, and belong to the Sm,BaCuO, structure type [7]. The nickel atoms in this structure are situated in an isolated distorted square pyramidal environment (NiO,). Moreover, very recently we reported [l, 81 that the Tm,BaNiO, oxide, earlier described as space group Immm, is really a dimorphic oxide. A new low-temperature phase, LT-Tm,BaNiO,, has been isolated, and its structural characterization *Author
to whom correspondence
0254-0584/931$6.00
should be addressed.
by means of X-ray and neutron diffraction techniques indicates that it has orthorhombic symmetry, space group Pnma, being isostructural with Sm,BaCuO, [7]. In this paper the study of the microstructure and magnetic properties of Sm,BaNiO,, Gd,BaNiO,, Yb,BaNiO, and Lu,BaNiO, is undertaken. The very different magnetic properties of these oxides will be explained by taking into account their different structures.
Experimental
R,BaNiO, oxides were prepared as powdered samples by solid state reaction from stoichiometric amounts of high-purity R,O, oxides (99.99%), NiO (99.99%) and BaCO, (a.r.). The samples were ground after each thermal treatment in air at 1000 and 1200 “C for 48 h. The oxides were characterized by X-ray diffraction data obtained using a Siemens K-810 powder diffractometer using Cu Ka radiation and a Siemens D-500 goniometer equipped with a secondary graphite monochromator. For the electron microscopy/diffraction studies the samples were ground under ethanol, dispersed on Cu grids coated with holey carbon support films, and ex-
0 1993 - Elsevier
Sequoia.
All rights reserved
234
amined in a JEOL 200 FX instrument fitted with a double-tilt ( f 45”) goniometer stage, in order to explore the reciprocal space, and in a JEOL 4000 EX instrument for high-resolution imaging. Magnetic susceptibility measurements were made in the 4.2-300 K temperature range using a DSM-8 magneto-susceptometer. The apparatus, based on the Faraday method, was calibrated using Hg[Co(SCN),] and Gd,(SO,), .8H,O as standards. The magnetic field strength was 5570 G, with H(cWldz) = 17 kG2 cm-‘. The susceptibilities were corrected for ionic diamagnetism using the values (in lop6 e.m.u. mol-I) of -32 for Ba2+, -18 for R3+ and -12 for both 02- and Ni2+ [9].
Results and discussion X-ray powder diffraction data for the Sm,BaNiO, and Gd,BaNiO, oxides were indexed on the basis of an orthorhombic unit cell, space group Immm, whereas Yb,BaNiO, and Lu,BaNiO, oxides show orthorhombic symmetry, space group Prima. The lattice parameters obtained agree fully with those reported earlier for these oxides, which confirms the previous structural characterization [4-61. Two of these oxides, Gd,BaNiO, (space group Immm) and Yb,BaNiO, (Prima),, were examined by electron diffraction and microscopy. In both types of compounds, we observed electron micrographs showing well-defined and regular contrast, and the selected area diffraction patterns (SADP’s) show sharp diffraction spots. These structural facts are characteristic of well-ordered materials, without any local structural defects. Figure l(a) shows a typical [loo] zone axis SADP of a crystal flake of the Gd,BaNiO, sample; Fig. l(b) shows the corresponding lattice image micrograph. The traces of the atomic planes (002) and (Oil) have been indicated on the micrograph, with d-spacing values of 5.7 and 5.2 A, respectively. Figure 2(a) shows a highresolution image of a crystal of Yb,BaNiO, sample recorded with the electron beam parallel to [loll. The corresponding SADP is shown in the inset, Fig. 2(b). Note that reflections such as OM),with k = 2n + 1, appear by double diffraction e.g., g(il1) +g(lOi) =g(OlO). The image contrast is typical of a crystal wedge shape, thicker toward the right, showing a rather uniform structure without any local structural extended defects. Directions corresponding to planes such as (101) and (OlO), with d values of 6.0 and 5.6 A, respectively, have been marked on the micrograph. Similar results from crystals showing well-ordered microstructures, have been observed in the case of isostructural compounds with Ni [lo], Cu [ll, 121 and Ni doped with Zn compounds [lo].
Fig. 1. (a) SADP of a crystal of Gd*BaNiOs sample. The zone axis has been indicated on the bottom right-hand part. (b) Highresolution electron micrograph of the same crystal. Note the absence of any local structural defects.
During our observations we noticed that SADP’s from both types of material were sensitive to electron beam irradiation damage, but no streaks or superlattice reflections were observed in the diffraction patterns. The temperature dependence of the molar magnetic susceptibility of Sm,BaNiO, is shown in Fig. 3. It can be seen that the susceptibility remains almost constant between 320 and 140 K, which is characteristic of the magnetic behavior of only the contribution of Sm3+, for which the spacing of the multiplet levels is not too large compared with kT. The absence of a nickel contribution with S = 1 agrees with the one-dimensional antiferromagnetic behavior in the nickel sublattice that we reported earlier for temperatures above room temperature in the case of the isostructural compound Y2BaNiOS [lo]. This 1D behavior has also been found for the isostructural Er,BaNiO,, although it is masked by the strong paramagnetic signal of EI3’ [2]. Moreover, for this oxide it has been reported, based on neutron diffraction and heat-capacity measurements, that Ni2’
235
Fig. 2. (a) High-resolution electron micrograph of a crystal of Yb,BaNiOs sample taken with the incident electron beam parallel to [lOl] (see the inset). (b) SADP of the same crystal. Several plane directions have been marked.
and Er3+ are simultaneously ordered at ‘33 K [13, 141. The maximum in the magnetic susceptibili~, observed at 26 K for Sm,BaNiO, (Fig. 3), is indicative that this compound behaves like the isostructural Er,BaNiO, and that below 26 K both Ni2+ and Sm3+ will be antiferromagnetically ordered. Heat-capacity measurements are now in progress to conf~rrn this assumption. As shown in Fig. 4, the variation of the magnetic susceptibility with temperature for Gd,BaNiO, oxide shows marked differences compared with that of the other isostructural R,BaNiO, oxides. The maximum in the susceptibility found at low temperatures for all the isostructural R,BaNiO, compounds [2,15] is not present for Gd,BaNiO,. However, when the x--’ versus T plot is examined in detail, it is possible to visualize upward deviations of the Curie-Weiss law at about 20 K. The obtained Weiss constant has an unusually high value (- 14.7 K), which can be explained only by assuming the existence of antiferromagnetic interactions in both Gd3+ and Ni2+ sublattices. It is worth noting that in the absence of cooperative interactions the susceptibility of Gd3’ follows a quasi-Curie law, since the ground state of Gd3+ is ‘S,,, and the possible crystal field effects will be neglected [16]. The calculated magnetic moment is 7.61 BM, which is in fairly good agreement with that expected for Gd3+-only contributions (see Table 1). This result confirms, as expected, the presence of one-dimensional antiferromagnetic interactions between the Ni2+ ions along the chains of (NiO,) octahedra of this structure. Moreover, when x. Tversus Tis plotted (Fig. 4 inset), the decrease in the value of x- T at lower temperatures strongly supports the assumption of the existence of these interactions in both gadolinium and /-
3.7
;~L_------__~
200
300
of the molar magnetic
___~___._
1
T(K)
T(K) Pig. 3. Temperature dependence tibility for SmZBaNi05.
--
suscep-
Fig. 4. Variation of the reciprocat molar magnetic susceptibility with the temperature for Gd2BaNiOS. The solid line represents the Curie-Weiss law plot. Inset: the X-T vs. T plot.
236 TABLE 1. Theoretical (k) and experimental (pn) magnetic moment, Weiss constant 0 and temperature of the maximum susceptibility (T,,) for the various RzBaNi05 oxide Compound
SmzBaNiOs GdtBaNiOJ Yb,BaNi05 Lu2BaNi05 “Calculated
1.60 7.94 4.54 2.83
2.16” 7.61 4.84 2.90
-14.7 -18.3 - 18.0
24.7 10.1
at room temperature.
I
OO
100
Fig. 6. Temperature dependence susceptibility for Yb,BaNi05. Curie-Weiss law plot.
2O
I
I
100
200
--&
T(K) Fig. 5. Temperature dependence of the molar magnetic tibility for Lu,BaNi05.
suscep-
nickel sublattices. As has been pointed out [2, 151, the presence of the susceptibility maximum at low temperature for these R,BaNiO, compounds is related to the existence of strong R3’-R3+ interactions, while it has been demonstrated from neutron diffraction scattering studies at slightly higher temperatures that the R3+_Ni2’ interactions are predominant [13]. In the case of Gd,BaNiO, these former interactions will probably be operative at temperatures lower than 4.2 K. In the case of the isostructural compounds Yb,BaNiO, and Lu,BaNiO,, because they are different from the gadolinium and samarium oxides discussed above, their magnetic behaviors are quite different. The magnetic susceptibility in the case of Lu,BaNiO, (Fig. 5) obeys the Curie-Weiss law between 300 and 100 K, and the magnetic moment obtained (2.90 BM) is in good agreement with the value expected for Ni2’ with S= 1. The maximum, centered at 10.1 K, indicates the existence of antiferromagnetic interactions in the nickel sublattice. Similar results have been obtained in the case of the isostructural ‘green phases’, Y,BaCuO, and Lu,BaCuO,,
I
200
I
3c
T(K) of the reciprocal molar magnetic The solid line represents the
for which the same kinds of interactions have been reported for the same temperature range [ll, 171. Figure 6 shows the variation of the reciprocal magnetic susceptibility with temperature for Yb,BaNiO,. It shows Curie-Weiss law behavior in the temperature range 300-60 K, and the obtained value of the magnetic moment indicates that both nickel and ytterbium contribute to the susceptibility, as can be observed in Table 1. The downward deviations from linearity below 60 K can be due to the splitting of the isolated ground term ‘F, under the influence of the crystal field. Similar results have been found for different ytterbium compounds, and these deviations have been simulated from the optical spectra [18, 191. It is worth noting that preliminary results from neutron diffraction data do not reveal the existence of any magnetic interactions in either the ytterbium or nickel sublattices down to a temperature of 1.5 K [20].
Acknowledgements We thank the Centro de Microscopia Electronica (U.C.M.) for use of their facilities. This research was supported by the CYCIT project MAT 89-0768.
References 1 A. Salinas-Sanchez, R. Saez-Puche, J. Rodriguez-Carvajal and J. L. Martinez, Solid Brute Commun., 78 (1991) 481. 2 J. Amador, E. Gutierrez-Puebla, M. A. Monge, I. Rasines, C. Ruiz-Valero, F. Fernandez, R. Sbez-Puche and J. A. Campa, Phys. Rev.B, 42 (1990) 7918.
237 3 J. K. Burdett and J. F. Mitchell, J. Am. Chem. Sot., 112 (1990) 6571. 4 S. Schiffler and H. K. Miiller-Buschbaum, Z. Anorg. A&. Chem., 532 (1986) 10. 5 J. Amador, E. Gutierrez-Puebla, M. A. Monge, I. Rasines, J. A. Campa, J. M. Gomez-Salazar and C. Ruiz-Valero, Solid State Zonks, 32133 (1989) 123. 6 H. K. Miiller-Buschbaum and C. H. Lang, J. Less-Common Met., LIILZ (1988) 142; H. Mevs and H. K. Mtiller-Buschbaum, Z. Anorg. A&. Chem., 573 (1989) 128. 7 C. Michel and B. Raveau, J. Solid State Chem., 43 (1982) 73. 8 A. Salinas-Sanchez, R. Saez-Puche, F. Femfmdez, A. AndrCs, A. E. Lavat and E. J. Baran, J. Solid State Chem., 99 (1992) 63. 9 L. N. Mulay and E. A. Boudreaux (eds.), Z’heov of Molecular Paramagnetbm, Wiley, New York, 1976, p. 494. 10 R. Saez-Puche, J. M. Coronodo, C. L. Otero-Diaz and J. M. Martin-Llorente, J. Solid State Chem., 93 (1991) 461.
11 A. Salinas-Sanchez, R. Saez-Puche and M. A. Alario-France, J. Solid State Chem., 93 (1990) 461. 12 R. Norrestam, M. Hjorth and J. 0. Bovin, Z. KkistuZZogr., 183 (1988) 243. 13 J. A. Alonso, J. Amador, J. L. Martinez, I. Rasines, J. Rodriguez-Carvajal and R. Sbez-Puche, Solid State Commun., 76 (1990) 467. 14 M. Castro, R. Burriel, A. Salinas-Sanchez and R. Saez-Puche, J. Mqn. Magn. Mater., 104-107 (1992) 619. 15 R. Saez-Puche, J. M. Coronado, J. Martin-Llorente and I. Rasines, Mater. Chem. Phys., 31 (1992) 151. 16 I. Bueno, C. Parada, R. Sbez-Puche, I. L. Botto and E. J. Baran, J. Phys. Chem. Solids, 51 (1990) 1117. 17 T. Chattopadway, P. J. Brown, W. Kobler and M. Wilhelm, Europhys. Lett., 8 (1989) 685. 18 M. A. Gwo, A. T. Aldred and S. K. Chan, J. Phys. Chem. Solids, 48 (1987) 229. 19 Y. Laureiro, M. L. Veiga, F. Femandez, R. Saez-Puche, A. Jerez and C. Pica, J. Less-Common Met., 167 (1991) 387. 20 R. Saez-Puche, J. L. Martinez and J. Hernandez, unpublished work.
Materials Chemistry and Physics, 34 (1993) 233-237
Microstructure Yb and Lu)
and magnetic properties
R. S&ez-Puche*, J. M. Coronado,
of R,BaNiO,
(R = Sm, Gd,
C. L. Otero-Diaz
Departamento de Quimica Znorga’nica, Facultad de Ciencias Quimicas, Universidad Complutense de Madrid, Madrid E-28040 (Spain)
and J. M. Martin-Llorente Depariamento de Q&mica Znorga’nica, Facultat de Ciencias Q&micas, Universidad de Salamanca, Salamanca (Spain) (Received
July 17, 1992; accepted
October
14, 1992)
Abstract The magnetic behavior found for R,BaNiO, (R=Sm, Gd, Yb and Lu) compounds has been explained by taking into account the two structural types in which they crystallize. For both Sm and Gd isostructural oxides above room temperature, the nickel ions are lD-antiferromagnetically ordered along the chains of octahedra characteristic of the Nd,BaNiOS structure type. Below 30 K both rare earth and nickel sublattices are simultaneously 3Dordered. In the case of Yb and Lu oxides, belonging to the SmzBaCuOS structure type, the existence of isolated distorted square pyramids (NiO,) justifies the assumption that antiferromagnetic interactions in the nickel sublattice are not operative until the temperature is as low as 10 K. Moreover, no evidence of order in the ytterbium compound has been found down to a temperature of 4.2 K. On the other hand, high-resolution electron microscopy reveals that both types of material are well ordered, without any local structural defects.
Introduction
Recently, R,BaNiO, oxides (R = rare earth element) have been the subject of numerous investigations because of their unusual and interesting structural and magnetic properties [l-3]. The Nd,BaNiO, structure type was first reported by Schifller and Mtiller-Buschbaum [4], and subsequent studies have revealed that this structure is also shown by different R,BaNiO, oxides where R =Y, Nd, Gd, Dy, Ho, Er and Tm [5, 61. This structure shows orthorhombic symmetry, space group Zrnmm, showing as the main structural feature the existence of isolated chains of flattened octahedra (NiO,) sharing corners along the u-axis. The barium and lanthanide cations form rows of (BaO,,) and (RO,) polyhedra running parallel to the u-axis. On the other hand, Yb,BaNiO, and Lu,BaNiO, show orthorhombic symmetry, space group Prima, and belong to the Sm,BaCuO, structure type [7]. The nickel atoms in this structure are situated in an isolated distorted square pyramidal environment (NiO,). Moreover, very recently we reported [l, 81 that the Tm,BaNiO, oxide, earlier described as space group Immm, is really a dimorphic oxide. A new low-temperature phase, LT-Tm,BaNiO,, has been isolated, and its structural characterization *Author
to whom correspondence
0254-0584/931$6.00
should be addressed.
by means of X-ray and neutron diffraction techniques indicates that it has orthorhombic symmetry, space group Pnma, being isostructural with Sm,BaCuO, [7]. In this paper the study of the microstructure and magnetic properties of Sm,BaNiO,, Gd,BaNiO,, Yb,BaNiO, and Lu,BaNiO, is undertaken. The very different magnetic properties of these oxides will be explained by taking into account their different structures.
Experimental
R,BaNiO, oxides were prepared as powdered samples by solid state reaction from stoichiometric amounts of high-purity R,O, oxides (99.99%), NiO (99.99%) and BaCO, (a.r.). The samples were ground after each thermal treatment in air at 1000 and 1200 “C for 48 h. The oxides were characterized by X-ray diffraction data obtained using a Siemens K-810 powder diffractometer using Cu Ka radiation and a Siemens D-500 goniometer equipped with a secondary graphite monochromator. For the electron microscopy/diffraction studies the samples were ground under ethanol, dispersed on Cu grids coated with holey carbon support films, and ex-
0 1993 - Elsevier
Sequoia.
All rights reserved
234
amined in a JEOL 200 FX instrument fitted with a double-tilt ( f 45”) goniometer stage, in order to explore the reciprocal space, and in a JEOL 4000 EX instrument for high-resolution imaging. Magnetic susceptibility measurements were made in the 4.2-300 K temperature range using a DSM-8 magneto-susceptometer. The apparatus, based on the Faraday method, was calibrated using Hg[Co(SCN),] and Gd,(SO,), .8H,O as standards. The magnetic field strength was 5570 G, with H(cWldz) = 17 kG2 cm-‘. The susceptibilities were corrected for ionic diamagnetism using the values (in lop6 e.m.u. mol-I) of -32 for Ba2+, -18 for R3+ and -12 for both 02- and Ni2+ [9].
Results and discussion X-ray powder diffraction data for the Sm,BaNiO, and Gd,BaNiO, oxides were indexed on the basis of an orthorhombic unit cell, space group Immm, whereas Yb,BaNiO, and Lu,BaNiO, oxides show orthorhombic symmetry, space group Prima. The lattice parameters obtained agree fully with those reported earlier for these oxides, which confirms the previous structural characterization [4-61. Two of these oxides, Gd,BaNiO, (space group Immm) and Yb,BaNiO, (Prima),, were examined by electron diffraction and microscopy. In both types of compounds, we observed electron micrographs showing well-defined and regular contrast, and the selected area diffraction patterns (SADP’s) show sharp diffraction spots. These structural facts are characteristic of well-ordered materials, without any local structural defects. Figure l(a) shows a typical [loo] zone axis SADP of a crystal flake of the Gd,BaNiO, sample; Fig. l(b) shows the corresponding lattice image micrograph. The traces of the atomic planes (002) and (Oil) have been indicated on the micrograph, with d-spacing values of 5.7 and 5.2 A, respectively. Figure 2(a) shows a highresolution image of a crystal of Yb,BaNiO, sample recorded with the electron beam parallel to [loll. The corresponding SADP is shown in the inset, Fig. 2(b). Note that reflections such as OM),with k = 2n + 1, appear by double diffraction e.g., g(il1) +g(lOi) =g(OlO). The image contrast is typical of a crystal wedge shape, thicker toward the right, showing a rather uniform structure without any local structural extended defects. Directions corresponding to planes such as (101) and (OlO), with d values of 6.0 and 5.6 A, respectively, have been marked on the micrograph. Similar results from crystals showing well-ordered microstructures, have been observed in the case of isostructural compounds with Ni [lo], Cu [ll, 121 and Ni doped with Zn compounds [lo].
Fig. 1. (a) SADP of a crystal of Gd*BaNiOs sample. The zone axis has been indicated on the bottom right-hand part. (b) Highresolution electron micrograph of the same crystal. Note the absence of any local structural defects.
During our observations we noticed that SADP’s from both types of material were sensitive to electron beam irradiation damage, but no streaks or superlattice reflections were observed in the diffraction patterns. The temperature dependence of the molar magnetic susceptibility of Sm,BaNiO, is shown in Fig. 3. It can be seen that the susceptibility remains almost constant between 320 and 140 K, which is characteristic of the magnetic behavior of only the contribution of Sm3+, for which the spacing of the multiplet levels is not too large compared with kT. The absence of a nickel contribution with S = 1 agrees with the one-dimensional antiferromagnetic behavior in the nickel sublattice that we reported earlier for temperatures above room temperature in the case of the isostructural compound Y2BaNiOS [lo]. This 1D behavior has also been found for the isostructural Er,BaNiO,, although it is masked by the strong paramagnetic signal of EI3’ [2]. Moreover, for this oxide it has been reported, based on neutron diffraction and heat-capacity measurements, that Ni2’
235
Fig. 2. (a) High-resolution electron micrograph of a crystal of Yb,BaNiOs sample taken with the incident electron beam parallel to [lOl] (see the inset). (b) SADP of the same crystal. Several plane directions have been marked.
and Er3+ are simultaneously ordered at ‘33 K [13, 141. The maximum in the magnetic susceptibili~, observed at 26 K for Sm,BaNiO, (Fig. 3), is indicative that this compound behaves like the isostructural Er,BaNiO, and that below 26 K both Ni2+ and Sm3+ will be antiferromagnetically ordered. Heat-capacity measurements are now in progress to conf~rrn this assumption. As shown in Fig. 4, the variation of the magnetic susceptibility with temperature for Gd,BaNiO, oxide shows marked differences compared with that of the other isostructural R,BaNiO, oxides. The maximum in the susceptibility found at low temperatures for all the isostructural R,BaNiO, compounds [2,15] is not present for Gd,BaNiO,. However, when the x--’ versus T plot is examined in detail, it is possible to visualize upward deviations of the Curie-Weiss law at about 20 K. The obtained Weiss constant has an unusually high value (- 14.7 K), which can be explained only by assuming the existence of antiferromagnetic interactions in both Gd3+ and Ni2+ sublattices. It is worth noting that in the absence of cooperative interactions the susceptibility of Gd3’ follows a quasi-Curie law, since the ground state of Gd3+ is ‘S,,, and the possible crystal field effects will be neglected [16]. The calculated magnetic moment is 7.61 BM, which is in fairly good agreement with that expected for Gd3+-only contributions (see Table 1). This result confirms, as expected, the presence of one-dimensional antiferromagnetic interactions between the Ni2+ ions along the chains of (NiO,) octahedra of this structure. Moreover, when x. Tversus Tis plotted (Fig. 4 inset), the decrease in the value of x- T at lower temperatures strongly supports the assumption of the existence of these interactions in both gadolinium and /-
3.7
;~L_------__~
200
300
of the molar magnetic
___~___._
1
T(K)
T(K) Pig. 3. Temperature dependence tibility for SmZBaNi05.
--
suscep-
Fig. 4. Variation of the reciprocat molar magnetic susceptibility with the temperature for Gd2BaNiOS. The solid line represents the Curie-Weiss law plot. Inset: the X-T vs. T plot.
236 TABLE 1. Theoretical (k) and experimental (pn) magnetic moment, Weiss constant 0 and temperature of the maximum susceptibility (T,,) for the various RzBaNi05 oxide Compound
SmzBaNiOs GdtBaNiOJ Yb,BaNi05 Lu2BaNi05 “Calculated
1.60 7.94 4.54 2.83
2.16” 7.61 4.84 2.90
-14.7 -18.3 - 18.0
24.7 10.1
at room temperature.
I
OO
100
Fig. 6. Temperature dependence susceptibility for Yb,BaNi05. Curie-Weiss law plot.
2O
I
I
100
200
--&
T(K) Fig. 5. Temperature dependence of the molar magnetic tibility for Lu,BaNi05.
suscep-
nickel sublattices. As has been pointed out [2, 151, the presence of the susceptibility maximum at low temperature for these R,BaNiO, compounds is related to the existence of strong R3’-R3+ interactions, while it has been demonstrated from neutron diffraction scattering studies at slightly higher temperatures that the R3+_Ni2’ interactions are predominant [13]. In the case of Gd,BaNiO, these former interactions will probably be operative at temperatures lower than 4.2 K. In the case of the isostructural compounds Yb,BaNiO, and Lu,BaNiO,, because they are different from the gadolinium and samarium oxides discussed above, their magnetic behaviors are quite different. The magnetic susceptibility in the case of Lu,BaNiO, (Fig. 5) obeys the Curie-Weiss law between 300 and 100 K, and the magnetic moment obtained (2.90 BM) is in good agreement with the value expected for Ni2’ with S= 1. The maximum, centered at 10.1 K, indicates the existence of antiferromagnetic interactions in the nickel sublattice. Similar results have been obtained in the case of the isostructural ‘green phases’, Y,BaCuO, and Lu,BaCuO,,
I
200
I
3c
T(K) of the reciprocal molar magnetic The solid line represents the
for which the same kinds of interactions have been reported for the same temperature range [ll, 171. Figure 6 shows the variation of the reciprocal magnetic susceptibility with temperature for Yb,BaNiO,. It shows Curie-Weiss law behavior in the temperature range 300-60 K, and the obtained value of the magnetic moment indicates that both nickel and ytterbium contribute to the susceptibility, as can be observed in Table 1. The downward deviations from linearity below 60 K can be due to the splitting of the isolated ground term ‘F, under the influence of the crystal field. Similar results have been found for different ytterbium compounds, and these deviations have been simulated from the optical spectra [18, 191. It is worth noting that preliminary results from neutron diffraction data do not reveal the existence of any magnetic interactions in either the ytterbium or nickel sublattices down to a temperature of 1.5 K [20].
Acknowledgements We thank the Centro de Microscopia Electronica (U.C.M.) for use of their facilities. This research was supported by the CYCIT project MAT 89-0768.
References 1 A. Salinas-Sanchez, R. Saez-Puche, J. Rodriguez-Carvajal and J. L. Martinez, Solid Brute Commun., 78 (1991) 481. 2 J. Amador, E. Gutierrez-Puebla, M. A. Monge, I. Rasines, C. Ruiz-Valero, F. Fernandez, R. Sbez-Puche and J. A. Campa, Phys. Rev.B, 42 (1990) 7918.
237 3 J. K. Burdett and J. F. Mitchell, J. Am. Chem. Sot., 112 (1990) 6571. 4 S. Schiffler and H. K. Miiller-Buschbaum, Z. Anorg. A&. Chem., 532 (1986) 10. 5 J. Amador, E. Gutierrez-Puebla, M. A. Monge, I. Rasines, J. A. Campa, J. M. Gomez-Salazar and C. Ruiz-Valero, Solid State Zonks, 32133 (1989) 123. 6 H. K. Miiller-Buschbaum and C. H. Lang, J. Less-Common Met., LIILZ (1988) 142; H. Mevs and H. K. Mtiller-Buschbaum, Z. Anorg. A&. Chem., 573 (1989) 128. 7 C. Michel and B. Raveau, J. Solid State Chem., 43 (1982) 73. 8 A. Salinas-Sanchez, R. Saez-Puche, F. Femfmdez, A. AndrCs, A. E. Lavat and E. J. Baran, J. Solid State Chem., 99 (1992) 63. 9 L. N. Mulay and E. A. Boudreaux (eds.), Z’heov of Molecular Paramagnetbm, Wiley, New York, 1976, p. 494. 10 R. Saez-Puche, J. M. Coronodo, C. L. Otero-Diaz and J. M. Martin-Llorente, J. Solid State Chem., 93 (1991) 461.
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