Elsevier

Journal of Solid State Chemistry

Volume 205, September 2013, Pages 57-63
Journal of Solid State Chemistry

Magnetocrystalline interactions and oxidation state determination of Mn(2−x)V(1+x)O4 (x=0, 1/3 and 1) magnetorresistive spinel family

https://doi.org/10.1016/j.jssc.2013.06.025Get rights and content

Highlights

  • Determination of oxidation state of the metallic ions in Mn(2−x)V(1+x)O4 (x=0,1/3,1) by XAS and XES techniques.

  • The ionic models found are Mn2+2V4+O4, Mn2+5/3V3.5+4/3O4 and Mn2+V3+O4.

  • EPR spectra correspond almost exclusively to a resonance of Mn2+.

Abstract

Oxidation states of transition metal cations in spinels-type oxides are sometimes extremely difficult to determine by conventional spectroscopic methods. One of the most complex cases occurs when there are different cations, each one with several possible oxidation states, as in the case of the magnetoresistant Mn(2−x)V(1+x)O4 (x=0, 1/3 and 1) spinel-type family. In this contribution we describe the determination of the oxidation state of manganese and vanadium in Mn(2−x)V(1+x)O4 (x=0, 1/3,1) spinel-type compounds by analyzing XANES and high-resolution X-ray fluorescence spectra. The ionic models found are Mn2+2V4+O4, Mn2+5/3V3.5+4/3O4 and Mn2+V3+2O4. Combination of the present results with previous data provided a reliable cation distribution model. For these spinels, single magnetic electron paramagnetic resonance (EPR) lines are observed at 480 K showing the interaction among the different magnetic ions. The analysis of the EPR parameters show that g-values and relative intensities are highly influenced by the concentration and the high-spin state of Mn2+. EPR broadening linewidth is explained in terms of the bottleneck effect, which is due to the presence of the fast relaxing V3+ ion instead of the weak Mn2+ (S state) coupled to the lattice. The EPR results, at high temperature, are well explained assuming the oxidation states of the magnetic ions obtained by the other spectroscopic techniques.

Graphical abstract

View of the crystallographic structure of a spinel. It shows as an example one of the models of ion distribution determined for the spinels Mn(2−x)V(1+x)O4 (x=0, 1/3,1).

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Introduction

In the design of oxides with potential technological applications, a detailed knowledge of the electronic configurations of the constituent elements plays an essential role. From the simple presence of magnetic correlations, to the more exotic phenomena related to them, all are controlled by the electronic structure of the material. The spinel case is very interesting in this regard and has been widely studied, as it shows a variety of effects due to different electronic correlations related to chemical composition (number of electrons) and crystal structure (where those electrons are located, and how they can interact with each other).

Materials belonging to the spinel family have an AB2X4 stoichiometry, where A and B are cations of either 2+ and 3+ charge or of 4+ and 2+ charge, and X represents O2− or chalcogenides (like S2− and Te2−). The structure is based on the cubic close-packing of anions with the cations filling 1/8 of the tetrahedral (Td) and 1/2 of the octahedral (Oh) sites. The general formula for the cationic distribution is (A1−γBγ)Τd [B2−γ Aγ]Oh O2−4, were γ is the degree of inversion (0≤γ≤1) and Td and Oh indicate tetrahedral and octahedral sites, respectively [1]. Besides, the octahedral sites are interconnected in a geometrically frustrated pyrochlore-type network.

Cation distribution between Oh and Td sites is crucial, since it determines the magnetic and electrical properties of the spinels; thus, the knowledge of this distribution, as well as the oxidation state, is important to explain, control and predict the physical properties of this family of compounds [2], [3].

Since the magnetoresistance effect (MR) was discovered in spinel-type oxides [4], [5], these materials have been the subject of renewed studies in the field of material sciences. A few years ago we succeeded in synthesizing a family of spinel-type oxides, Mn(2−x)V(1+x)O4 (x=0, 1/3 and 1) [6], [7]. The composition with x=1, which can be easily obtained, has been thoroughly studied [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20]. Its properties can be satisfactorily explained assuming that Mn is divalent and V is trivalent, based on the most stable oxidation states for these metals and the coincidence with experimental evidence reported in [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19]. Our objective was to substitute V3+ for V4+, while keeping Mn as Mn2+. This would allow us to study the effect of two types of cations, one with electrons located only in t2g orbitals (V3+ and V4+), and another one with electrons in eg and t2g orbitals (Mn2+). However, the presence of V4+ was only inferred, and had not been confirmed up to now.

A MR effect was reported in a study of structural, magnetic and electrical transport properties of Mn(2−x)V(1+x)O4 (x=0, 1/3 and 1), which belong to the spinel-type family [6], [7]. Electrical conduction in these samples occurs through small polarons [7]. This, coupled with the magnetostriction studied for the end-member of the series with x=1 [16], suggests that magnetoresistance could be the result of the interaction of these two effects. To elucidate this important question it is necessary to determine the oxidation states and distribution between Td and Oh sites of Mn and V.

Albeit neutron diffraction techniques are probably the most widely used method to determine cation distribution between Oh and Td sites, the oxidation state determination is not trivial. Even after considering some other indirect evidences and the redox potentials of V and Mn, the results did not allow an unambiguous assignment of site occupancies and oxidation states [6], [7].

In general, electron paramagnetic resonance (EPR) is a good technique for determining the oxidation state of diluted magnetic ions. On the contrary, in the spinel system described in this contribution, the interpretation of the paramagnetic resonance is not trivial because the magnetic ions (vanadium and manganese) are concentrated and their orbitals are overlapped, which improve the exchange interaction and it dominates over the Zeeman interaction. However, a careful analysis of the experimental parameters obtained from the resonance lines, as the magnetic field resonance, linewidth and EPR intensity, can give information to infer the oxidation state of the magnetic ions, the magnetic interactions involved in the system and the relative importance to determine the magnetic order observed.

In this contribution we present a detailed study of electron paramagnetic resonance spectra complemented with high-resolution X-ray emission spectroscopy (XES) and X-ray absorption spectroscopy (XAS), measured in members of the spinel family Mn(2−x)V(1+x)O4 (x=0, 1/3 and 1), using synthetic standards with known oxidation states of Mn and V for comparison. Our objective is to determine oxidation states and discuss their implications on electrical and magnetic properties.

Section snippets

Mn(2−x)V(1+x)O4 (x=0, 1/3 and 1) spinels

These samples have been synthesized by solid state reaction in sealed evacuated quartz ampoules [6]. Symmetry and unit cell dimensions have been determined using high-resolution synchrotron X-ray powder diffraction (HRS-XRPD) patterns measured at room temperature (RT). Crystal structure, cation site occupancies and magnetic structure have been refined using neutron powder diffraction (NPD) patterns obtained at RT. The details of these studies are discussed in Ref. [6]. The electrical transport

Oxidation states of V and Mn determined using XES

Several spectral changes are related to the chemical environment surrounding the atoms of interest (Mn or V) [30], [31]. In order to quantify the oxidation states, it is necessary to select those parameters that vary linearly, as evidenced from data collected on standards.

In the case of Mn compounds we used the integral of the absolute values of the difference spectra (hereafter IAD) value as reference parameters. The IAD [36] value for a spectrum with respect to a reference spectra Iref(E) is

Conclusions

The model regarding the oxidation states of V and Mn and the site occupancies in Mn(2−x)V(1+x)O4 spinels proposed by Pannunzio Miner et al. [6], [7] was refined using a combination of data taken from XAS, XES and EPR measurements. The combination of these techniques with powder neutron diffraction data is a powerful tool to determine univocally oxidation states in mixed oxides with variable oxidation states transition metal cations and thus, explain or predict magnetic and transport properties

Acknowledgments

The research was supported by LNLS-Brazilian Synchrotron Light Laboratory (projects D12A XRD1-9843, and D04B-XAFS1-10693). Financial supports from the Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET), Agencia Nacional de promoción científica y tecnológica (ANPCyT) and from the Secretaría de Ciencia y Tecnología de la Universidad Nacional de Córdoba (UNC) are gratefully acknowledged. The authors thank R. E. Carbonio and F. Colombo and anonymous reviewers for helpful comments.

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