Structures and magnetic properties of rare earth rhenium oxides Ln3ReO7 (Ln = Gd, Tb, and Dy)

Journal of Alloys and Compounds 374 (2004) 79–83

Structures and magnetic properties of rare earth rhenium oxides Ln3ReO7 (Ln = Gd, Tb, and Dy) Yukio Hinatsu∗ , Makoto Wakeshima, Noriyoshi Kawabuchi, Nobuyuki Taira Division of Chemistry, Graduate School of Science, Hokkaido University, Sapporo 060-0810, Japan

Abstract Polycrystalline samples of Ln3 ReO7 (Ln = Gd, Tb, and Dy) have been prepared. The structures of these compounds were determined by X-ray diffraction measurements. They crystallize in a superstructure of cubic fluorite (space group, Cmcm for Ln = Gd and Tb; C2221 for Ln = Dy). The Re ion is octahedrally coordinated by six oxygens and the distorted octahedra share corners forming a zig-zag chain parallel to the c-axis. The samples have been characterized by magnetic susceptibility and specific heat measurements. The compounds show complex magnetic behavior at low temperatures. The Gd, Tb, and Dy compounds show magnetic transitions at 7.0, 14.0, and 2.8 K, respectively. The magnetic behavior below the transition temperatures is complicated, and there is a large difference in the temperature dependence of the magnetic susceptibility measured under zero-field-cooled (ZFC) condition and under field-cooled (FC) condition. © 2003 Elsevier B.V. All rights reserved. Keywords: Rare earths; Rhenium; Oxides; One-dimensional structures; Magnetic properties

1. Introduction Compounds of the general formula Ln3 MO7 have attracted great interests, because of their one-dimensional structural features and possible related magnetic properties. The parent structure of this family of compounds, La3 NbO7 , was first determined by Rossell [1]. The structure is an orthorhombic superstructure of the cubic fluorite-type (lattice parameter ac ) with space group Cmcm √ and unit-cell parameters aorth ≈ 2ac , borth ≈ corth ≈ 2ac . The M5+ cation is octahedrally coordinated by six oxygen ions and the octahedra share corners forming a zig-zag chain parallel to the c-axis. In this structure, slabs are formed in the bc-plane, in which one-dimensional MO6 chain runs parallel to the c-axis alternating with rows of edge-shared LnO8 pseudo cubes consisting of one-third of Ln ions. These slabs are separated by the remaining two-thirds of Ln ions which is seven-coordinated by oxygen ions [1]. The interchain M–M distance is about 6.6 Å compared with the corresponding intrachain distance of 3.7 Å, which suggests that these compounds may exhibit one-dimensional electronic behavior. Most of the rare earth metals, however, have a nonzero spin, which could lead to long-range order at some finite temperature due to Ln–M coupling. ∗

Corresponding author. E-mail address: [email protected] (Y. Hinatsu).

0925-8388/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2003.11.067

Recently, several papers have been published dealing with the magnetic properties of compounds containing Ru at the M-site [2–10]. As for Ln3 ReO7 compounds, Wltschek et al. [11] first analyzed the crystal structure of Sm3 ReO7 by measuring the X-ray diffraction with a single crystal, and the crystal structure is found to be orthorhombic with space group Cmcm. Lam et al. [12] also refined the structures of Pr3 ReO7 and Nd3 ReO7 with the same space group Cmcm. Magnetic susceptibility measurements showed that Sm3 ReO7 is paramagnetic down to 4 K [11]. For Pr3 ReO7 and Nd3 ReO7 , the Re5+ ions display a local magnetic moment consistent with S = 1 and both show evidence for cooperative magnetic effects at low temperatures [12]. In this study, we have prepared Ln3 ReO7 for heavier rare earths Ln = Gd, Tb, and Dy. Through X-ray diffraction measurements, their crystal structures have been determined. The magnetic susceptibility and specific heat measurements have been performed from 1.8 K to room temperature.

2. Experimental 2.1. Sample preparation As starting materials, Ln2 O3 , ReO2 , and ReO3 were used. For preparation of Tb3 ReO7 , Tb4 O7 , Re, and ReO2 were used. They were weighed in an appropriate metal ratio and

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the mixtures were ground in an agate mortar. Then the mixtures were sealed in an evacuated platinum tube, and heated at 1300–1400 ◦ C for 6–12 h with several intermediate grindings. The structures were refined with the Rietveld method. 2.2. X-ray diffraction analysis Powder X-ray diffraction measurements were carried out in the region of 10◦ ≤ 2θ ≤ 120◦ using Cu K␣ radiation on a Rigaku MultiFlex diffractometer equipped with a curved graphite monochromator. The structures were refined with the Rietveld method. 2.3. Magnetic susceptibility measurements The temperature-dependence of the magnetic susceptibility was measured in an applied field of 0.1 T over the temperature range of 1.8 K ≤ T ≤ 300 K, using a SQUID magnetometer (Quantum Design, MPMS5S). The susceptibility measurements were performed either using zero-field-cooling (ZFC) and field-cooling (FC) conditions. The former was measured upon heating the sample to 300 K under the applied magnetic field of 0.1 T after zero-field-cooling to 1.8 K. The latter was measured upon cooling the sample from 300 to 1.8 K at 0.1 T.

Table 1 Atomic positional parameters and lattice parameters for Ln3 ReO7 (Ln = Gd, Tb, and Dy) Atom

Site

x

y

z

B/Å2

Gd3 ReO7 a Gd(1) Gd(2) Re O(1) O(2) O(3)

4b 8g 4a 4c 16h 8g

0 0.2740(1) 0 1/2 0.1324(9) 0.370(1)

1/2 0.2959(2) 0 0.424(2) 0.193(1) 0.030(2)

0 1/4 0 1/4 0.039(1) 1/4

0.68(7) 0.25(7) 0.10(5) 1.2(2) 1.2(2) 1.2(2)

Tb3 ReO7 b Tb(1) Tb(2) Re O(1) O(2) O(3)

4b 8g 4a 4c 16h 8g

0 0.2713(2) 0 1/2 0.142(1) 0.379(2)

1/2 0.2913(2) 0 0.436(3) 0.196(1) 0.019(2)

0 1/4 0 1/4 0.042(1) 1/4

0.69(8) 0.30(7) 0.18(5) 1.2(2) 1.2(2) 1.2(2)

Dy3 ReO7 c Dy(1) Dy(2) Re O(1) O(2) O(3) O(4) O(5)

4b 8c 4b 8c 8c 4a 4a 4a

0 0.2431(2) 0 0.174(1) 0.137(1) 0.084(1) 0.105(1) 0.090(1)

0.4956(2) 0.2519(2) 0 0.200(1) 0.743(1) 1/2 1/2 0

1/4 0 1/4 0.224(1) 0.303(1) 0 1/2 0

0.97(7) 0.80(6) 0.40(6) 0.73(10) 0.73(10) 0.73(10) 0.73(10) 0.73(10)

Note. Definition of reliability factors Rwp and RI are given as follows:


2.4. Specific heat measurements Rwp =

Specific heat measurements were performed using a relaxation technique by a commercial heat capacity measuring system (Quantum Design, PPMS) in the temperature range of 1.8–300 K. The sintered sample in the form of a pellet was mounted on a thin alumina plate with Apiezon for better thermal contact.

3. Results and discussion 3.1. Preparation and crystal structure X-ray diffraction measurements show that the desired compounds Ln3 ReO7 (Ln = Gd, Tb, and Dy) could be prepared and that very small amounts of impurities remained in the desired compounds; they were unreacted starting materials Ln2 O3 . This is presumably a consequence of loss of the volatile rhenium oxides. In order to remove these impurities, the samples were washed with diluted hydrochloric acid. After this treatment, the single-phase compounds could be obtained. Following the crystal structure analysis for Sm3 ReO7 by Wltschek et al. [11], we also tried to analyze the crystal structures for Ln3 ReO7 with Ln = Gd and Tb, and they have the orthorhombic symmetry with space group Cmcm. Table 1 lists the lattice parameters and atomic positional parameters for Gd3 ReO7 and Tb3 ReO7 . For the X-ray diffraction profile of Dy3 ReO7 , there exist many very weak diffraction peaks

w(|F(o)| − |F(c)|)2  w|F(o)|2



1/2 and

RI =

|Ik (o) − Ik (c)|  . Ik (o)

a Space group: Cmcm, a = 10.593(1) Å, b = 7.3955(3) Å, c = 7.4534(4) Å, Rwp = 10.71%, RI = 3.35%. b Space group: Cmcm, a = 10.510(2) Å, b = 7.3640(1) Å, c = 7.4140(1) Å, Rwp = 12.74%, RI = 3.81%. c Space group: C222 , a = 10.561(1) Å, b = 7.4613(6) Å, c = 1 7.4567(7) Å, Rwp = 14.08%, RI = 4.68%.

which could not be indexed based on the Cmcm and other space groups such as Pnma (for Ln3 NbO7 ) [13] and P21 21 21 (for Ln3 MoO7 ) [14]. Allpress and Rossell [15] reported that the crystal structures of Ln3 MO7 (M = Ta and Sb) for smaller Ln cations were well described with space group C2221 , and Rossell [1] analyzed the detailed crystal structure of Y3 TaO7 . We tried to analyze the X-ray diffraction profile for Dy3 ReO7 with various space groups. All the reflections could be successfully indexed with the space group C2221 . The refinement of one model in C2221 converged rapidly, producing a low value for the residual and acceptably low values for the calculated standard deviations of refined parameters (Rwp = 14.08%, RI = 4.68%). Table 1 also lists the lattice parameters and atomic coordinates for Dy3 ReO7 . Fig. 1 shows the crystal structure of Dy3 ReO7 . The orthorhombic structures have features in common for Ln3 ReO7 (Ln = Gd and Tb), and Dy3 ReO7 . Thus, slabs may be recognized, parallel to (1 0 0), in which rows of MO6 , lie in the direction (0 0 1), alternating with parallel rows of Ln(1) cations in eight-fold coordination. Slabs

Y. Hinatsu et al. / Journal of Alloys and Compounds 374 (2004) 79–83

Fig. 1. The crystal structure of Dy3 ReO7 .

are stacked with all rows parallel, and between the slabs are Ln(2) cations in seven-fold coordination. In Dy3 ReO7 , the ReO6 octahedra and LnO8 cubes are more distorted than in the other Ln3 ReO7 (Ln = Pr, Nd, Sm, Gd, and Tb) structures [11,12]. 3.2. Magnetic properties Any of Gd3 ReO7 , Tb3 ReO7 , and Dy3 ReO7 shows an antiferromagnetic transition at low temperatures. Fig. 2a shows the temperature dependence of the magnetic susceptibility

(a)

(b) Fig. 2. (a) Temperature dependence of magnetic susceptibility for Gd3 ReO7 . The inset shows its detailed temperature dependence below 20 K. (b) Temperature dependence of specific heat for Gd3 ReO7 . The inset shows its detailed temperature dependence below 20 K.

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for Gd3 ReO7 in the temperature range of 1.8–300 K, indicating the antiferromagnetic ordering at 7.0 K. Below this temperature, the divergence between the ZFC and FC magnetic susceptibility has been observed, i.e., the ZFC susceptibility decreases with decreasing temperature, while the FC susceptibility increases with decreasing temperature. The corresponding anomaly has been observed in the temperature dependence of the specific heat, as shown in Fig. 2b. The Curie–Weiss relationship has been found for the susceptibilities above 30 K. The derived fitting constants are C = 22.0 emu K/mol and θ = −10.3 K. This Curie constant gives the effective magnetic moment of Gd3 ReO7 to be 13.26 µB . Three Gd3+ ions and one Re5+ ion contribute to the paramagnetism of this compound. Since the theoretical effective magnetic moments of Gd3+ and Re5+ ions are 7.94 and 2.83 µB , respectively, the expected effective magnetic moment (∼14.04  µB ) for Gd3 ReO7 is estimated from the relation µeff = µ2Re5+ + 3µ2 3+ . The value obtained Gd from the experiment is lower than the calculated value. This result may suggest that the magnetic ions in this compound are affected by the crystal field to some extent. The negative Weiss constant indicates that the predominant magnetic interaction in Gd3 ReO7 is antiferromagnetic. We consider that the divergence between the ZFC and FC susceptibilities for Gd3 ReO7 is due to the hysteresis behavior of the ferromagnetic component of the magnetic moment. Measurements of the variation of magnetization as a function of magnetic field for Gd3 ReO7 at various temperatures shows that small magnetic hysteresis has been found below the magnetic transition temperature (at 1.8 and 5.0 K), while no hysteresis has been observed at 10 and 15 K. These results indicate that there exists a small ferromagnetic component in the magnetic moments which is produced by not a completely antiparallel alignment of the magnetic moments, i.e., the Gd3 ReO7 is not an ideal antiferromagnet. We consider that this is due to the results of a low crystal symmetry of these compounds. That is, the Dzyaloshinsky–Moriya (D–M) interaction can exist between the magnetically ordered elements, which results in the existence of a weak ferromagnetic component in their susceptibilities. Gd3 TaO7 which is isomorphous with Gd3 ReO7 , shows no magnetic anomaly down to 1.8 K [16]. In the Gd3 TaO7 , only the Gd3+ ions are paramagnetic. Compared with the magnetic properties on Gd3 TaO7 , the Re5+ –Gd3+ magnetic coupling should enhance the Gd3+ –Gd3+ magnetic coupling, which causes the increase of the magnetic transition temperature and yields the weak ferromagnetic properties observed in the Gd3 ReO7 below 7 K. Fig. 3a shows the temperature dependence of the magnetic susceptibility for Tb3 ReO7 . Below 20 K, the divergence between the ZFC and FC susceptibilities has been observed, and at 14 K, the characteristic temperature dependence of susceptibilities for the antiferromagnet has been observed. The corresponding specific heat anomaly has been observed at 14 K, as shown in Fig. 3b. In addition, another specific

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Y. Hinatsu et al. / Journal of Alloys and Compounds 374 (2004) 79–83

(a)

(a)

(b)

(b)

Fig. 3. (a) Temperature dependence of magnetic susceptibility for Tb3 ReO7 . The inset shows its detailed temperature dependence below 25 K. (b) Temperature dependence of specific heat for Tb3 ReO7 . The inset shows its detailed temperature dependence below 25 K.

Fig. 4. (a) Temperature dependence of magnetic susceptibility for Dy3 ReO7 . The inset shows its detailed temperature dependence below 6 K. (b) Temperature dependence of specific heat for Dy3 ReO7 . The inset shows its detailed temperature dependence below 6 K.

heat anomaly is observed at ca. 5 K. The magnetic susceptibility measurements on Tb3 TaO7 show the antiferromagnetic transition at ca. 5 K due to the magnetic interaction between terbium ions [16]. Therefore, we consider that the interaction at 5 K is due to the one between terbium ions, and that the magnetic interaction at 14 K is due to the antiferromagnetic interaction between Re5+ ions. Of course, both the magnetic moments of Re5+ and Tb3+ ions mutually interact, and therefore the corresponding specific heat peaks have become very broad. In the temperature range between 100 and 300 K, the Curie–Weiss law is found, which gives the Curie constant C = 34.5 emu K/mol and Weiss constant θ = −14.8 K. Since the calculated Curie constant is 36.4 emu/mol, it can be said that the oxidation state of terbium ion is +3. Fig. 4a and b shows the temperature dependences of the magnetic susceptibility and specific heat for Dy3 ReO7 . The antiferromagnetic behavior has been observed at 2.8 K, and the ZFC and FC magnetic susceptibilities diverse below this temperature. The results of the magnetic susceptibility measurements on Dy3 TaO7 (which is isomorphous with Dy3 ReO7 ) show the existence of antiferromagnetic transition at ca. 3 K [16]. Compared with the magnetic properties on Dy3 TaO7 , it is discussed that the antiferromagnetic be-

havior observed at 2.8 K for Dy3 ReO7 is due to the magnetic interaction between Dy3+ ions. Long-range magnetic ordering ascribable to the interaction between Re5+ ions was not observed, which may be due to the fact that the magnetic moment of Re5+ ion is much smaller than that of Dy3+ ion. In the temperature range where the Curie–Weiss relationship holds, the Curie and Weiss constants are determined to be C = 42.7 emu K/mol and θ = −5.4 K. This Curie constant gives the effective magnetic moment of Dy3 ReO7 to be 18.49 µB . In the same way as the case for Gd3 ReO7 , the effective magnetic moment expected for Dy3 ReO7 is calculated to be 18.66 µB . The experimental magnetic moment is close to this calculated moment for Dy3 ReO7 .

4. Summary Polycrystalline samples of Ln3 ReO7 (Ln = Gd, Tb, and Dy) have been prepared for the first time. They crystallize in a superstructure of cubic fluorite (space group Cmcm for Ln = Gd and Tb; C2221 for Ln = Dy). Any of these compounds (Ln = Gd, Tb, and Dy) shows magnetic transitions at 7.0, 14.0, and 2.8 K, respectively, and their ZFC and FC

Y. Hinatsu et al. / Journal of Alloys and Compounds 374 (2004) 79–83

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