Structural and magnetic study of RFe0.5V0.5O3 (R=Y, Eu, Nd, La) perovskite compounds

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Abstract

B-site disordered RFe0.5V0.5O3 compounds, with R=La, Nd, Eu and Y, have been prepared by solid-state reaction technique and their structures and magnetic properties have been investigated through X-ray powder diffraction, time-of-flight neutron powder diffraction and magnetization measurements at temperatures ranging from 5 to 700 K. The four compounds can be described as distorted perovskites with space group symmetry Pbnm and a+bb tilt system. The studied compounds also show antiferromagnetic ordering with Neel temperatures of 299, 304, 304, and 335 K respectively. The magnetic structures of R=La, Nd and Y compounds were determined from the neutron powder diffraction as Gz with observed magnetic moments of 2.55, 2.54 and 2.69μB at 30, 40 and 40 K, respectively.

Graphical abstract

Gz type antiferromagnetic ordering of Fe and V moments in B-site disordered RFe0.5V0.5O3 perovskites (R=La, Nd, Y) with a+bb octahedral tilt and orthorhombic (Pbnm space group) structure determined by neutron powder diffraction.

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Highlights

RFe0.5V0.5O3 show a simple perovskite structure with a+bb tilting of octahedra in the Pbnm space group as RFeO3 and RVO3. ► Antiferromagnetic ordering of spins of the Gz type are observed with TN around 300 K. ► Partial but not conclusive Fe/V cation ordering is suggested by NPD data for R=La.

Introduction

Materials from the families RFeO3 and RVO3, where R is a rare earth element or Y, have been extensively studied in the past due to their interesting magnetic and structural properties. These materials crystallize with orthorhombic perovskite structures (space group Pbnm, No. 62) with lattice constants abc/√2, where the R-atoms occupy the twelve-coordinated A-sites of the perovskite structure and Fe/V atoms are located in the B-sites (centers of BO6 octahedra). The octahedra formed by the oxygen atoms are slightly rotated (tilted) around the axes of the perovskite structure, which causes the orthorhombic distortion of the unit cell. At high temperatures the magnetic properties of RFeO3 materials are determined by the indirect exchange Fe–O–Fe interactions, which lead to antiferromagnetic ordering with the Néel temperature TN in the range between 620 K for YFeO3 and 750 K for LaFeO3 [1], [2]. The orientation of the magnetic moments is of the Gx-type (magnetic moments parallel to the x-axis) and is not completely antiferromagnetic because the moments are slightly canted leading to the formation of a small net ferromagnetic component [1], [3]. For LaFeO3, it was found that the magnetic moments are directed along x with small components along y and z as well [4]. As the temperature is lowered to about 100–200 K, the competition of the Fe–Fe and R–Fe interactions leads to the so called spin–reorientation transition of the magnetic moments where they change direction from parallel to x to almost parallel to z (Gz-type antiferromagnetic). This effect has been observed for R=Nd, but not for R=Y, Eu, La. Some of the rare-earth orthoferrites (including NdFeO3) have shown a long-range C-type antiferromagnetic ordering of the magnetic moments of R-atoms below temperatures of the order of 2–6 K [1], [3]. The compounds of the RVO3 family, which are typical Mott insulators, also show intricate magnetic and structural characteristics involving correlations between octahedral tilting, lattice distortions and orbital and spin ordering. For example, YVO3 undergoes three phase transitions: a structural phase transition from orthorhombic (Pbnm) to monoclinic phase (P21/a) occurring concomitantly with G-type orbital ordering at 200 K, a magnetic transition to form a C-type antiferromagnetic arrangement of spins at 115 K and a transition to G-type spin and C-type orbital ordered state at 71 K [5], [6], [7]. On the other hand, LaVO3 undergoes a transition to a G-type spin-ordered phase at around 140 K and a structural transition only a few degrees below this temperature [5]. In earlier studies it was reported that the two transitions occur simultaneously [8]. Given this variety of effects, it could be expected that including both Fe and V atoms in the same material would give rise to new and interesting properties such as half-metallicity and magnetoresistance especially if ordered arrangement of the cations is formed. The only compounds where this idea was explored were SrLaFeVO6 and CaLaFeVO6 [9]. From powder X-ray diffraction measurements it was established that they have cubic perovskite structures with low degrees of cation ordering: 65% for SrLaFeVO6 and 69% for CaLaFeVO6 (50% corresponds to a completely disordered material and 100% to a completely ordered). In the same work by means of redox titration it was found that the oxidation state of V is 4. Both materials were found to be semiconducting and ferrimagnetically ordered with TC ∼350 K for the Sr compound and 200 K for the Ca-containing one.

In the present work we report the synthesis of the new RFe0.5V0.5O3 (R=Y, Eu, Nd, La) perovskite compounds and their characterization by means of X-ray and neutron powder diffraction and magnetic susceptibility measurements in a wide temperature range.

Section snippets

Synthesis

The samples of RFe0.5V0.5O3 (R=Y, Eu, Nd, La) were prepared by solid-state reaction of pre-fired Y2O3, Eu2O3, Nd2O3, La2O3, Fe2O3 and V2O3. The starting stoichiometric mixture of thoroughly ground oxides was pressed in a 13 mm pellet under 5 ton and first fired in Ar at 1173 K for 12 h. The materials were then re-ground, pelletized and fired in pure Ar (>99.999) at 1623 K for several 12 h periods with intermediate re-grindings. X-ray diffraction (XRD) data were collected after each firing using a

Results and discussion

From the collected X-ray data it was established that, at room temperature, all of the studied materials have the orthorhombic perovskite structure typical of GdFeO3. This structure has symmetry described by the space group Pbnm and presents octahedral tilting of the type a+bb [14], which involves in-phase rotations of the octahedra from two adjacent layers around one of the axes of the parent cubic perovskite structure and out-of-phase rotations with equal magnitudes around the other two

Conclusions

Compounds in the RFe0.5V0.5O3 system (R=La, Nd, Eu and Y) show orthorhombic Pbnm perovskite structure with a disordered arrangement of Fe3+ and V3+ cations in a tilted octahedra network. Unit cell parameters and tolerance factors evolve with rare-earth ionic radii towards a pseudo-cubic structure similar to that observed in other La perovskites. Although there seems to be no driving force for Fe/V ordering, evidence of it is observed by high-temperature neutron powder diffraction indicating a

Acknowledgments

Work at NIU was supported by the NSF-DMR-0706610 (B.D., S.K., J.M.). Argonne National Laboratory's work was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under contract DE-AC02-06CH11357 (B.D.). L.S. is indebted to PEDECIBA, CSIC and ANII (Uruguayan organizations).

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  • Cited by (2)

    • Synthesis, structural characterization and Mössbauer study of LnV <inf>0.5</inf>Fe <inf>0.5</inf>O <inf>3</inf> perovskites (Ln = Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho and Er)

      2012, Materials Research Bulletin
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      Some difficulties are on account of the question of the simultaneous stabilization of a reduced V3+ cation with the oxidized Fe3+ cation. In fact, Gateshki et al. [17] reported that several heat-treatments at 1400 °C and regrindings were necessary to obtain RFe0.5V0.5O3 (R = Y, Eu, Nd, La), which always contain small amounts of RVO4. Moreover, motivated by the new physical properties that such compounds could potentially present, we started an extensive study on this perovskite family, with most of the lanthanides and Y that we baptized as orthoferrivanadates.

    1

    Present address: PANalytical B.V., P.O. Box 13, 7600 AA Almelo, The Netherlands.

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