- Email: [email protected]
Magnetic properties of RRhO3 (R=rare earth)
Journal of Alloys and Compounds 350 (2003) 24–29
L
www.elsevier.com / locate / jallcom
Magnetic properties of RRhO 3 (R5rare earth) a a a, b c T. Taniguchi , W. Iizuka , Y. Nagata *, T. Uchida , H. Samata a
College of Science and Engineering, Aoyama Gakuin University, 6 -16 -1 Chitosedai, Setagaya, Tokyo 157 -8572, Japan b Tokyo Institute of Polytechnics, Iiyama, Atsugi, Kanagawa 243 -0297, Japan c Faculty of Mercantile Marine Science, Kobe University of Mercantile Marine, Fukaeminami, Higashinada, Kobe 658 -0022, Japan Received 3 July 2002; received in revised form 5 August 2002; accepted 5 August 2002
Abstract RRhO 3 (R5rare earth except Ce and Pm) was prepared by a solid-state reaction, and its crystallographic, magnetic, and electric properties were investigated. RRhO 3 has an orthorhombic perovskite-type structure of the space group Pbnm. RRhO 3 shows Curie–Weiss paramagnetism above 5 K. On the other hand, EuRhO 3 shows antiferromagnetic behavior. The Rh 31 ion, which seems to be in the low-spin state, has a very small effective magnetic moment (Peff 50.295 mB / ion). The Peff value of the RRhO 3 compounds shows the same dependence on the number of 4f electrons as the gJ value of the rare-earth ions. The rare-earth ions make a major contribution to the magnetic moment of RRhO 3 . The resistivity of all RRhO 3 shows an activation-type (or semiconductor-like) temperature dependence. 2002 Elsevier Science B.V. All rights reserved. Keywords: Rare earth compounds; Transition metal compounds; Solid state reaction; Electrical transport; Magnetic measurements
1. Introduction As a result of the discovery of superconductivity in Sr 2 RuO 4 [1], numerous studies have been conducted on ruthenium oxides such as MRuO 3 and M 2 RuO 4 . However, very few studies have been performed on other 4d transition metal oxides. This seems to be due to the peculiar magnetic properties of the 4d transition metal ions. In general, since 4d electrons have a larger spatial extent, very strong spin-orbit coupling and a large ligand-field effect are expected [2]. When the energy splitting caused by the spin-orbit coupling is equivalent to that of the multiplets, a higher-energy term often mixes into the lowest-lying term. For this reason, 4d transition metal ions often have extremely small magnetic susceptibilities that cannot be explained by Hund’s rule, which explains the magnetism of iron-group ions. This seems to make the study on oxides with 4d transition metal ions difficult. Among 4d transition metal oxides, perovskite-type Rh oxides (or orthorhodites) seem interesting; however, there are few systematic studies on these oxides [3–7]. The trivalent rhodium ion with six 4d electrons is expected to have no appreciable magnetic moment in the low-spin state. This seems to be the reason for the lack of systematic *Corresponding author. E-mail address: [email protected] (Y. Nagata).
studies on orthorhodites. However, since 4d transition metal oxides are very interesting candidates for use as metallic conductors, catalysts, anode material of photoelectrolytic cells, and superconductors, it seems important to investigate their magnetic and electric properties in detail. In this study, in order to understand the behavior of the Rh ion as well as the rare-earth ions in orthorhodites, RRhO 3 compounds (R5rare earth except Ce and Pm) were synthesized, and their crystallographic, magnetic, and electric properties were studied.
2. Experimental Polycrystalline specimens of RRhO 3 (R5rare earth except Ce and Pm) were prepared by the usual solid-state reaction method. High-purity reagents of Rh 2 O 3 (99.9%) and R 2 O 3 (or Tb 4 O 7 ) (99.9%) were mixed by an agate motor so as to compose stoichiometric RRhO 3 , and the mixture was calcined at 950 8C for 10 h in an oxygen atmosphere (1 atm) after being pelletized under 100 kg / cm 2 pressure. The mixing, pelletization, and calcination were repeated three times. After being reground and pressed into a pellet, the mixture was sintered at 1100– 1300 8C for 12 h in an oxygen atmosphere (1 atm). The chemical composition and homogeneity of the specimens were characterized by electron-probe micro-
0925-8388 / 02 / $ – see front matter 2002 Elsevier Science B.V. All rights reserved. PII: S0925-8388( 02 )00969-6
T. Taniguchi et al. / Journal of Alloys and Compounds 350 (2003) 24–29
analysis (EPMA) using a wavelength-dispersive spectrometer. The crystal structure of the specimens was characterized by X-ray powder diffraction using Cu Ka radiation and subsequent refinement of the diffraction data using the Rietveld method. The magnetic properties were characterized at temperatures from 5 to 300 K using a SQUID magnetometer under applied magnetic fields of 5 Oe to 10 kOe. The electrical resistivity was measured by a DC four-probe method. Electrical contacts were established by using gold wires (50 mm) and gold paste.
3. Results and discussion
3.1. Crystallographic properties X-ray powder diffraction profiles for specimens of RRhO 3 (R5rare earth except Ce and Pm) are shown in Fig. 1. All the diffraction data were refined well assuming
Fig. 1. X-ray powder diffraction profiles for specimens of RRhO 3 (R5 rare earth except Ce and Pm).
25
an orthorhombic perovskite-type structure of the space group Pbnm. Refined orthorhombic lattice constants are shown in Fig. 2(a) as a function of the number n of 4f ] electrons of rare-earth ions. Constants a and c /Œ2 show a monotonic decrease as the number n of 4f electrons increases, while constant b increases for less than half of the 4f electrons (n#7) and tends to decrease after reaching a maximum at n57, which corresponds to the Gd 31 ion. The behavior of the lattice constants is consistent with that reported by Shannon and Shaplygin [5,7]. Fig. 2(b) shows the n dependence of the orthorhombicity, which is given by (b2a) /(b1a). The orthorhombicity increases monotonically as the n increases. When the Goldschmidt’s ionic radii of La 31 50.122 nm, Lu 31 50.099 nm, Rh 31 50.069 nm, and O 22 50.132 nm are used, the tolerance factor t is in the range of 0.894 (LaRhO 3 ).t.0.813 (LuRhO 3 ). The tolerance factor tends to decrease when the number n of the 4f electrons increases. This, together with the behavior of orthorhombicity, means that the crystal lattice of RRhO 3 is distorted as the number of 4f electrons increases. It is well known that the BO 6 octahedron in the perovskite-type ABO 3 tilts when t,0.9 and the perovskite-type crystal lattice transforms from a cubic (or rhombohedral) to an
Fig. 2. (a) Refined orthorhombic lattice constants of RRhO 3 (R5rare earth except Ce and Pm) and (b) orthorhombicity (b2a) /(b1a) as a function of the number n of 4f electrons of rare-earth ions.
26
T. Taniguchi et al. / Journal of Alloys and Compounds 350 (2003) 24–29
orthorhombic structure. Presumably, the tilt of the RhO 6 octahedron increases when the ionic radius of the rareearth ion decreases with the lanthanoid contraction, and the orthorhombicity then increases.
3.2. Magnetic properties Fig. 3 shows the magnetic-field dependence of magnetization measured for specimens of RRhO 3 (R5rare earth except Ce and Pm) at 5 K. The magnetization curves of all the specimens show linear and reversible field dependence, suggesting paramagnetism at 5 K. Fig. 4 shows the temperature dependence of molar magnetic susceptibility x measured for a specimen of RRhO 3 under an applied magnetic field of 1 kOe. The x (T ) shows a characteristic of Curie paramagnetism, and the magnitude of the susceptibility depends strongly on the type of rare-earth ion. Fig. 5 shows the temperature dependence of the reciprocal susceptibility x 21 that is deduced from the
Fig. 4. Temperature dependence of molar magnetic susceptibility x measured for specimens of RRhO 3 (R5rare earth except Ce and Pm) in an applied magnetic field of 1 kOe.
Fig. 3. Magnetic-field dependence of magnetization measured for specimens of RRhO 3 (R5rare earth except Ce and Pm) at 5 K.
data in Fig. 4. x 21 (T ) with linear temperature dependence was fitted by the Curie–Weiss law. The Curie constant C, the asymptotic Curie temperature Q, and parameter x0 , which were used for the fitting, are listed in Table 1. The effective magnetic moment Peff , which was deduced from the Curie constant, is also listed in Table 1 and shown along with the theoretical moment gJ in Fig. 6(a) as a function of the number n of 4f electrons. The Peff of 0.295 mB was obtained for LaRhO 3 (n50). Since La 31 is a non-magnetic ion, the moment is attributed to the Rh 31 ion. However, since the Rh 31 ion with six 4d electrons is in the low-spin state, no appreciable magnetic moment can be expected. Therefore, the moment Peff 50.295 mB seems to be induced by the peculiarity of 4d electrons. It is well known that 4d electrons have a larger spatial extent than iron-group ions; then, a very strong spin-orbit coupling, a small coulomb interaction between d electrons, and a slightly large ligand-field effect are expected [2]. When the
T. Taniguchi et al. / Journal of Alloys and Compounds 350 (2003) 24–29
27
Fig. 6. (a) The effective magnetic moment Peff and the theoretical moment gJ as a function of the number n of 4f electrons. (b) Asymptotic Curie temperature Q of RRhO 3 as a function of n.
Fig. 5. Temperature dependence of the reciprocal susceptibility x is deduced from the data in Fig. 4.
21
that
Table 1 Curie constant C, asymptotic Curie temperature Q, parameter x0 , and effective magnetic moment Peff for specimens of RRhO 3 Rare earth
x0 (10 23 emu / mol)
C (emu / mol)
Q (K)
Peff ( mB )
La Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu
0.046 0.091 1.022 0.932 0.000 0.047 5.014 5.197 15.18 9.207 0.000 2.597 0.002
0.011 1.252 1.102 0.040 2.779 7.579 9.824 11.90 11.72 9.970 7.144 0.955 0.007
22.74 213.9 216.4 22.64 2297 25.23 23.74 22.82 22.20 24.19 213.9 20.78 20.87
0.295 3.167 2.971 0.569 4.717 7.791 8.870 9.764 9.687 8.936 7.564 2.766 0.242
energy splitting caused by the spin-orbit coupling is equal to the energy separation of the multiplets, it is likely that the higher-energy term mixes into the lowest-lying term. In this case, an extremely small effective moment, which cannot be explained by Hund’s rule, will appear. The moment 0.295 mB of the Rh 31 ion is consistent with the reported values [2]. Peff (n) shows almost the same n dependence as that of the gJ of rare-earth ions, suggesting that the magnetic moment of RRhO 3 is attributed to the moment of rare-earth ions. The asymptotic Curie temperature Q of RRhO 3 is shown in Fig. 6(b) as a function of the number n of 4f electrons. The Q values of all specimens are negative, suggesting antiferromagnetism. However, an antiferromagnetic transition was not observed at temperatures above 5 K. Q appears to be independent of the gJ of rare-earth ions. Rare-earth ions seem to make no contribution to the formation of an antiferromagnetic order. Among RRhO 3 oxides, EuRhO 3 shows interesting magnetic behavior. Fig. 7 shows the temperature dependence of magnetic susceptibility and reciprocal susceptibility measured for a specimen of EuRhO 3 under an applied magnetic field of 1 kOe. The magnetic susceptibility x
28
T. Taniguchi et al. / Journal of Alloys and Compounds 350 (2003) 24–29
Fig. 7. Temperature dependence of magnetic susceptibility and reciprocal susceptibility measured for a specimen of EuRhO 3 in an applied magnetic field of 1 kOe.
Fig. 8. Temperature dependence of normalized electrical resistivity measured for specimens of RRhO 3 . The inset shows the activation energy Ea for specimens of RRhO 3 as a function of the number n of 4f electrons.
shows peculiar behavior, and a shoulder is observed in the x (T ) at |80 K. This resembles a characteristic of the antiferromagnetic transition. When the x 21 (T ) in Fig. 7 was fitted by the Curie–Weiss law at temperatures above 160 K, Q 5 2297 K and Peff 54.72 mB were obtained. Since the asymptotic Curie temperature Q has negative value, it is quite likely that an antiferromagnetic order exists at temperatures below 80 K. It is well known that Eu 31 has a considerably large Peff (|3.61 mB ) that is due to the mixing of a higher term and the lowest-lying term [8,9]. However, the Peff of EuRhO 3 cannot be explained by the moments of Rh and Eu 31 ions. This discrepancy seems to be due to a mixed valence of Eu ion. It is well known that Eu 21 as well as Eu 31 is stable in Eu oxides. When a small number of Eu 21 ions coexist with Eu 31 ions in EuRhO 3 , some of the Rh 31 ions will change to the Rh 41 ion to attain a charge compensation. Since Rh 41 with a 4d 5 configuration has S51 / 2 in the low-spin state, a magnetic moment of 1.73 mB is expected for Rh 41 . In this case, the magnetic interaction between Rh ions must be enhanced by the appearance of Rh 41 ions, and the antiferromagnetic transition temperature would shift to higher temperatures. Moreover, it is well known that Eu 21 with a 4f 7 configuration has a theoretical moment of 7.94 mB and an experimental moment of 8.48 mB [10]. Therefore, the effective moment Peff of EuRhO 3 must be enhanced by the appearance of Eu 21 . When this Peff is explained by roughly assuming the moments of 4.72, 0.295, 1.73, 3.61, and 8.48 mB for EuRhO 3 , Rh 31 , Rh 41 , Eu 31 , and Eu 21 , respectively, |15% of Eu ions must exist as Eu 21 ions. However, the magnetism of EuRhO 3 is still unclear. Details of the magnetism must be investigated by using single crystalline specimen.
3.3. Electric properties Fig. 8 shows the temperature dependence of normalized electrical resistivity measured for RRhO 3 . The resistivity has been normalized by a value at 280 K. The r /r280 of all specimens shows semiconductor-like behavior, and the resistivity increases as the temperature is decreased. The r (T ) at temperatures above 200 K could be fitted by the activation-type formula which is given by r (T )5 r0 exp(Ea /kT ), where r0 , Ea , and k are the constant, the activation energy, and the Boltzmann’s constant, respectively. The activation energy Ea is shown in the inset of Fig. 8 as a function of the number n of 4f electrons. However, Ea shows a tendency to rise for heavy rare-earth ions. Unlike the behavior of the effective magnetic moment Peff (n), no systematic change is observed in the Ea (n). The magnetic moment of rare-earth ions seems irrelevant to the transport properties of RRhO 3 . Rh ions are thought to make a major contribution to the semiconductor-like transport properties of RRhO 3 . Although the reason for the rise in Ea is unclear at present, the change in the lattice constants may have a relation to the behavior of Ea . As shown in Fig. 2, the lattice constants tend to decrease for heavy rare-earth ions. The change in the crystal lattice may modify the band scheme of RRhO 3 and increase Ea .
4. Conclusion RRhO 3 (R5rare earth except Ce and Pm) was prepared by the solid-state reaction, and its crystallographic, magnetic, and electric properties were investigated. RRhO 3 has an orthorhombic perovskite-type structure of the space
T. Taniguchi et al. / Journal of Alloys and Compounds 350 (2003) 24–29
group Pbnm. RRhO 3 shows Curie–Weiss paramagnetism above 5 K; on the other hand, EuRhO 3 shows antiferromagnetic behavior. The Rh 31 ion, which seems to be in the low-spin state, has a very small effective magnetic moment (Peff 50.295 mB / ion). The Peff of RRhO 3 shows the same dependence on the number of 4f electrons as that of the gJ of rare-earth ions. Rare-earth ions make a major contribution to the magnetic moment of RRhO 3 . The resistivity of all RRhO 3 shows activation-type (or semiconductor-like) temperature dependence.
Acknowledgements A part of the work done at Aoyama Gakuin University was supported by The Foundation for the Scientific Research of Material Science and The Private School High-Tech Research Center Program of the Ministry of Education, Science, Sports and Culture, Japan.
29
References [1] Y. Maeno, H. Hashimoto, K. Yoshida, S. Nishizaki, T. Fujita, J.G. Bednorz, F. Lichtenberg, Nature 372 (1994) 532–534. [2] J.H. Van Vleck, in: The Theory of Electric and Magnetic Susceptibilities, Oxford University Press, Oxford, 1965, p. 311. [3] A. Wold, B. Post, E. Banks, J. Am. Chem. Soc. 79 (1957) 6365– 6366. [4] A. Wold, R.J. Arnott, W.J. Croft, Inorg. Chem. 2 (1963) 972–974. [5] R.D. Shannon, Acta Crystallogr. B26 (1970) 447–449. [6] L.L. Kochergina, V.V. Fomichev, O.I. Kondratov, I.S. Shaplygin, K.I. Petrov, Zh. Neorg. Khim. 25 (1980) 2082–2088. [7] I.S. Shaplygin, I.I. Prosychev, V.B. Lazarev, Zh. Neorg. Khim. 31 (1986) 2870–2875. [8] J.H. Van Vleck, in: The Theory of Electric and Magnetic Susceptibilities, Oxford University Press, Oxford, 1965, p. 243. [9] B. Cabrera, Comptes Rendus 180 (1925) 668–671. [10] A.R. Mackintosh, J. de Phys. 32 (1971) Cl482–Cl487.
L
www.elsevier.com / locate / jallcom
Magnetic properties of RRhO 3 (R5rare earth) a a a, b c T. Taniguchi , W. Iizuka , Y. Nagata *, T. Uchida , H. Samata a
College of Science and Engineering, Aoyama Gakuin University, 6 -16 -1 Chitosedai, Setagaya, Tokyo 157 -8572, Japan b Tokyo Institute of Polytechnics, Iiyama, Atsugi, Kanagawa 243 -0297, Japan c Faculty of Mercantile Marine Science, Kobe University of Mercantile Marine, Fukaeminami, Higashinada, Kobe 658 -0022, Japan Received 3 July 2002; received in revised form 5 August 2002; accepted 5 August 2002
Abstract RRhO 3 (R5rare earth except Ce and Pm) was prepared by a solid-state reaction, and its crystallographic, magnetic, and electric properties were investigated. RRhO 3 has an orthorhombic perovskite-type structure of the space group Pbnm. RRhO 3 shows Curie–Weiss paramagnetism above 5 K. On the other hand, EuRhO 3 shows antiferromagnetic behavior. The Rh 31 ion, which seems to be in the low-spin state, has a very small effective magnetic moment (Peff 50.295 mB / ion). The Peff value of the RRhO 3 compounds shows the same dependence on the number of 4f electrons as the gJ value of the rare-earth ions. The rare-earth ions make a major contribution to the magnetic moment of RRhO 3 . The resistivity of all RRhO 3 shows an activation-type (or semiconductor-like) temperature dependence. 2002 Elsevier Science B.V. All rights reserved. Keywords: Rare earth compounds; Transition metal compounds; Solid state reaction; Electrical transport; Magnetic measurements
1. Introduction As a result of the discovery of superconductivity in Sr 2 RuO 4 [1], numerous studies have been conducted on ruthenium oxides such as MRuO 3 and M 2 RuO 4 . However, very few studies have been performed on other 4d transition metal oxides. This seems to be due to the peculiar magnetic properties of the 4d transition metal ions. In general, since 4d electrons have a larger spatial extent, very strong spin-orbit coupling and a large ligand-field effect are expected [2]. When the energy splitting caused by the spin-orbit coupling is equivalent to that of the multiplets, a higher-energy term often mixes into the lowest-lying term. For this reason, 4d transition metal ions often have extremely small magnetic susceptibilities that cannot be explained by Hund’s rule, which explains the magnetism of iron-group ions. This seems to make the study on oxides with 4d transition metal ions difficult. Among 4d transition metal oxides, perovskite-type Rh oxides (or orthorhodites) seem interesting; however, there are few systematic studies on these oxides [3–7]. The trivalent rhodium ion with six 4d electrons is expected to have no appreciable magnetic moment in the low-spin state. This seems to be the reason for the lack of systematic *Corresponding author. E-mail address: [email protected] (Y. Nagata).
studies on orthorhodites. However, since 4d transition metal oxides are very interesting candidates for use as metallic conductors, catalysts, anode material of photoelectrolytic cells, and superconductors, it seems important to investigate their magnetic and electric properties in detail. In this study, in order to understand the behavior of the Rh ion as well as the rare-earth ions in orthorhodites, RRhO 3 compounds (R5rare earth except Ce and Pm) were synthesized, and their crystallographic, magnetic, and electric properties were studied.
2. Experimental Polycrystalline specimens of RRhO 3 (R5rare earth except Ce and Pm) were prepared by the usual solid-state reaction method. High-purity reagents of Rh 2 O 3 (99.9%) and R 2 O 3 (or Tb 4 O 7 ) (99.9%) were mixed by an agate motor so as to compose stoichiometric RRhO 3 , and the mixture was calcined at 950 8C for 10 h in an oxygen atmosphere (1 atm) after being pelletized under 100 kg / cm 2 pressure. The mixing, pelletization, and calcination were repeated three times. After being reground and pressed into a pellet, the mixture was sintered at 1100– 1300 8C for 12 h in an oxygen atmosphere (1 atm). The chemical composition and homogeneity of the specimens were characterized by electron-probe micro-
0925-8388 / 02 / $ – see front matter 2002 Elsevier Science B.V. All rights reserved. PII: S0925-8388( 02 )00969-6
T. Taniguchi et al. / Journal of Alloys and Compounds 350 (2003) 24–29
analysis (EPMA) using a wavelength-dispersive spectrometer. The crystal structure of the specimens was characterized by X-ray powder diffraction using Cu Ka radiation and subsequent refinement of the diffraction data using the Rietveld method. The magnetic properties were characterized at temperatures from 5 to 300 K using a SQUID magnetometer under applied magnetic fields of 5 Oe to 10 kOe. The electrical resistivity was measured by a DC four-probe method. Electrical contacts were established by using gold wires (50 mm) and gold paste.
3. Results and discussion
3.1. Crystallographic properties X-ray powder diffraction profiles for specimens of RRhO 3 (R5rare earth except Ce and Pm) are shown in Fig. 1. All the diffraction data were refined well assuming
Fig. 1. X-ray powder diffraction profiles for specimens of RRhO 3 (R5 rare earth except Ce and Pm).
25
an orthorhombic perovskite-type structure of the space group Pbnm. Refined orthorhombic lattice constants are shown in Fig. 2(a) as a function of the number n of 4f ] electrons of rare-earth ions. Constants a and c /Œ2 show a monotonic decrease as the number n of 4f electrons increases, while constant b increases for less than half of the 4f electrons (n#7) and tends to decrease after reaching a maximum at n57, which corresponds to the Gd 31 ion. The behavior of the lattice constants is consistent with that reported by Shannon and Shaplygin [5,7]. Fig. 2(b) shows the n dependence of the orthorhombicity, which is given by (b2a) /(b1a). The orthorhombicity increases monotonically as the n increases. When the Goldschmidt’s ionic radii of La 31 50.122 nm, Lu 31 50.099 nm, Rh 31 50.069 nm, and O 22 50.132 nm are used, the tolerance factor t is in the range of 0.894 (LaRhO 3 ).t.0.813 (LuRhO 3 ). The tolerance factor tends to decrease when the number n of the 4f electrons increases. This, together with the behavior of orthorhombicity, means that the crystal lattice of RRhO 3 is distorted as the number of 4f electrons increases. It is well known that the BO 6 octahedron in the perovskite-type ABO 3 tilts when t,0.9 and the perovskite-type crystal lattice transforms from a cubic (or rhombohedral) to an
Fig. 2. (a) Refined orthorhombic lattice constants of RRhO 3 (R5rare earth except Ce and Pm) and (b) orthorhombicity (b2a) /(b1a) as a function of the number n of 4f electrons of rare-earth ions.
26
T. Taniguchi et al. / Journal of Alloys and Compounds 350 (2003) 24–29
orthorhombic structure. Presumably, the tilt of the RhO 6 octahedron increases when the ionic radius of the rareearth ion decreases with the lanthanoid contraction, and the orthorhombicity then increases.
3.2. Magnetic properties Fig. 3 shows the magnetic-field dependence of magnetization measured for specimens of RRhO 3 (R5rare earth except Ce and Pm) at 5 K. The magnetization curves of all the specimens show linear and reversible field dependence, suggesting paramagnetism at 5 K. Fig. 4 shows the temperature dependence of molar magnetic susceptibility x measured for a specimen of RRhO 3 under an applied magnetic field of 1 kOe. The x (T ) shows a characteristic of Curie paramagnetism, and the magnitude of the susceptibility depends strongly on the type of rare-earth ion. Fig. 5 shows the temperature dependence of the reciprocal susceptibility x 21 that is deduced from the
Fig. 4. Temperature dependence of molar magnetic susceptibility x measured for specimens of RRhO 3 (R5rare earth except Ce and Pm) in an applied magnetic field of 1 kOe.
Fig. 3. Magnetic-field dependence of magnetization measured for specimens of RRhO 3 (R5rare earth except Ce and Pm) at 5 K.
data in Fig. 4. x 21 (T ) with linear temperature dependence was fitted by the Curie–Weiss law. The Curie constant C, the asymptotic Curie temperature Q, and parameter x0 , which were used for the fitting, are listed in Table 1. The effective magnetic moment Peff , which was deduced from the Curie constant, is also listed in Table 1 and shown along with the theoretical moment gJ in Fig. 6(a) as a function of the number n of 4f electrons. The Peff of 0.295 mB was obtained for LaRhO 3 (n50). Since La 31 is a non-magnetic ion, the moment is attributed to the Rh 31 ion. However, since the Rh 31 ion with six 4d electrons is in the low-spin state, no appreciable magnetic moment can be expected. Therefore, the moment Peff 50.295 mB seems to be induced by the peculiarity of 4d electrons. It is well known that 4d electrons have a larger spatial extent than iron-group ions; then, a very strong spin-orbit coupling, a small coulomb interaction between d electrons, and a slightly large ligand-field effect are expected [2]. When the
T. Taniguchi et al. / Journal of Alloys and Compounds 350 (2003) 24–29
27
Fig. 6. (a) The effective magnetic moment Peff and the theoretical moment gJ as a function of the number n of 4f electrons. (b) Asymptotic Curie temperature Q of RRhO 3 as a function of n.
Fig. 5. Temperature dependence of the reciprocal susceptibility x is deduced from the data in Fig. 4.
21
that
Table 1 Curie constant C, asymptotic Curie temperature Q, parameter x0 , and effective magnetic moment Peff for specimens of RRhO 3 Rare earth
x0 (10 23 emu / mol)
C (emu / mol)
Q (K)
Peff ( mB )
La Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu
0.046 0.091 1.022 0.932 0.000 0.047 5.014 5.197 15.18 9.207 0.000 2.597 0.002
0.011 1.252 1.102 0.040 2.779 7.579 9.824 11.90 11.72 9.970 7.144 0.955 0.007
22.74 213.9 216.4 22.64 2297 25.23 23.74 22.82 22.20 24.19 213.9 20.78 20.87
0.295 3.167 2.971 0.569 4.717 7.791 8.870 9.764 9.687 8.936 7.564 2.766 0.242
energy splitting caused by the spin-orbit coupling is equal to the energy separation of the multiplets, it is likely that the higher-energy term mixes into the lowest-lying term. In this case, an extremely small effective moment, which cannot be explained by Hund’s rule, will appear. The moment 0.295 mB of the Rh 31 ion is consistent with the reported values [2]. Peff (n) shows almost the same n dependence as that of the gJ of rare-earth ions, suggesting that the magnetic moment of RRhO 3 is attributed to the moment of rare-earth ions. The asymptotic Curie temperature Q of RRhO 3 is shown in Fig. 6(b) as a function of the number n of 4f electrons. The Q values of all specimens are negative, suggesting antiferromagnetism. However, an antiferromagnetic transition was not observed at temperatures above 5 K. Q appears to be independent of the gJ of rare-earth ions. Rare-earth ions seem to make no contribution to the formation of an antiferromagnetic order. Among RRhO 3 oxides, EuRhO 3 shows interesting magnetic behavior. Fig. 7 shows the temperature dependence of magnetic susceptibility and reciprocal susceptibility measured for a specimen of EuRhO 3 under an applied magnetic field of 1 kOe. The magnetic susceptibility x
28
T. Taniguchi et al. / Journal of Alloys and Compounds 350 (2003) 24–29
Fig. 7. Temperature dependence of magnetic susceptibility and reciprocal susceptibility measured for a specimen of EuRhO 3 in an applied magnetic field of 1 kOe.
Fig. 8. Temperature dependence of normalized electrical resistivity measured for specimens of RRhO 3 . The inset shows the activation energy Ea for specimens of RRhO 3 as a function of the number n of 4f electrons.
shows peculiar behavior, and a shoulder is observed in the x (T ) at |80 K. This resembles a characteristic of the antiferromagnetic transition. When the x 21 (T ) in Fig. 7 was fitted by the Curie–Weiss law at temperatures above 160 K, Q 5 2297 K and Peff 54.72 mB were obtained. Since the asymptotic Curie temperature Q has negative value, it is quite likely that an antiferromagnetic order exists at temperatures below 80 K. It is well known that Eu 31 has a considerably large Peff (|3.61 mB ) that is due to the mixing of a higher term and the lowest-lying term [8,9]. However, the Peff of EuRhO 3 cannot be explained by the moments of Rh and Eu 31 ions. This discrepancy seems to be due to a mixed valence of Eu ion. It is well known that Eu 21 as well as Eu 31 is stable in Eu oxides. When a small number of Eu 21 ions coexist with Eu 31 ions in EuRhO 3 , some of the Rh 31 ions will change to the Rh 41 ion to attain a charge compensation. Since Rh 41 with a 4d 5 configuration has S51 / 2 in the low-spin state, a magnetic moment of 1.73 mB is expected for Rh 41 . In this case, the magnetic interaction between Rh ions must be enhanced by the appearance of Rh 41 ions, and the antiferromagnetic transition temperature would shift to higher temperatures. Moreover, it is well known that Eu 21 with a 4f 7 configuration has a theoretical moment of 7.94 mB and an experimental moment of 8.48 mB [10]. Therefore, the effective moment Peff of EuRhO 3 must be enhanced by the appearance of Eu 21 . When this Peff is explained by roughly assuming the moments of 4.72, 0.295, 1.73, 3.61, and 8.48 mB for EuRhO 3 , Rh 31 , Rh 41 , Eu 31 , and Eu 21 , respectively, |15% of Eu ions must exist as Eu 21 ions. However, the magnetism of EuRhO 3 is still unclear. Details of the magnetism must be investigated by using single crystalline specimen.
3.3. Electric properties Fig. 8 shows the temperature dependence of normalized electrical resistivity measured for RRhO 3 . The resistivity has been normalized by a value at 280 K. The r /r280 of all specimens shows semiconductor-like behavior, and the resistivity increases as the temperature is decreased. The r (T ) at temperatures above 200 K could be fitted by the activation-type formula which is given by r (T )5 r0 exp(Ea /kT ), where r0 , Ea , and k are the constant, the activation energy, and the Boltzmann’s constant, respectively. The activation energy Ea is shown in the inset of Fig. 8 as a function of the number n of 4f electrons. However, Ea shows a tendency to rise for heavy rare-earth ions. Unlike the behavior of the effective magnetic moment Peff (n), no systematic change is observed in the Ea (n). The magnetic moment of rare-earth ions seems irrelevant to the transport properties of RRhO 3 . Rh ions are thought to make a major contribution to the semiconductor-like transport properties of RRhO 3 . Although the reason for the rise in Ea is unclear at present, the change in the lattice constants may have a relation to the behavior of Ea . As shown in Fig. 2, the lattice constants tend to decrease for heavy rare-earth ions. The change in the crystal lattice may modify the band scheme of RRhO 3 and increase Ea .
4. Conclusion RRhO 3 (R5rare earth except Ce and Pm) was prepared by the solid-state reaction, and its crystallographic, magnetic, and electric properties were investigated. RRhO 3 has an orthorhombic perovskite-type structure of the space
T. Taniguchi et al. / Journal of Alloys and Compounds 350 (2003) 24–29
group Pbnm. RRhO 3 shows Curie–Weiss paramagnetism above 5 K; on the other hand, EuRhO 3 shows antiferromagnetic behavior. The Rh 31 ion, which seems to be in the low-spin state, has a very small effective magnetic moment (Peff 50.295 mB / ion). The Peff of RRhO 3 shows the same dependence on the number of 4f electrons as that of the gJ of rare-earth ions. Rare-earth ions make a major contribution to the magnetic moment of RRhO 3 . The resistivity of all RRhO 3 shows activation-type (or semiconductor-like) temperature dependence.
Acknowledgements A part of the work done at Aoyama Gakuin University was supported by The Foundation for the Scientific Research of Material Science and The Private School High-Tech Research Center Program of the Ministry of Education, Science, Sports and Culture, Japan.
29
References [1] Y. Maeno, H. Hashimoto, K. Yoshida, S. Nishizaki, T. Fujita, J.G. Bednorz, F. Lichtenberg, Nature 372 (1994) 532–534. [2] J.H. Van Vleck, in: The Theory of Electric and Magnetic Susceptibilities, Oxford University Press, Oxford, 1965, p. 311. [3] A. Wold, B. Post, E. Banks, J. Am. Chem. Soc. 79 (1957) 6365– 6366. [4] A. Wold, R.J. Arnott, W.J. Croft, Inorg. Chem. 2 (1963) 972–974. [5] R.D. Shannon, Acta Crystallogr. B26 (1970) 447–449. [6] L.L. Kochergina, V.V. Fomichev, O.I. Kondratov, I.S. Shaplygin, K.I. Petrov, Zh. Neorg. Khim. 25 (1980) 2082–2088. [7] I.S. Shaplygin, I.I. Prosychev, V.B. Lazarev, Zh. Neorg. Khim. 31 (1986) 2870–2875. [8] J.H. Van Vleck, in: The Theory of Electric and Magnetic Susceptibilities, Oxford University Press, Oxford, 1965, p. 243. [9] B. Cabrera, Comptes Rendus 180 (1925) 668–671. [10] A.R. Mackintosh, J. de Phys. 32 (1971) Cl482–Cl487.