Structure and physical properties of nickel manganite NiMn₂O₄ obtained from nickel permanganate precursor

Abstract

In this paper we present the structural, magnetic and dielectric properties of ceramic nickel manganite NiMn₂O_4+δ produced by using nickel permanganate Ni(MnO₄)₂xH₂O as a precursor. We have characterized the NiMn₂O_4+δ stoichiometry using quantitative energy-dispersive analysis of X-rays and thermal gravimetry under reducing conditions. Increased oxygen and Mn⁴⁺ contents were detected. X-ray diffraction and Rietveld refinement of X-ray data were carried out. Temperature dependent magnetization measurements were performed and the ferri-magnetic transition was identified at ≈100 K. The ferri-magnetic moment was found to be ≈1μ_B and hysteretic magnetization vs applied field curves were obtained. Dielectric properties were measured using impedance spectroscopy. Two dielectric relaxation processes were detected, which were associated with grain boundary and bulk contributions. The Arrhenius plots of resistivity and the temperature dependent dielectric permittivity were obtained for the two relaxations by means of an equivalent circuit model based on a series of two parallel RC elements.

Introduction

Complex manganese oxides have recently evoked strong interest in various structures with different Mn valence states and Mn coordinations for example in perovskites, spinels, or pyrochlores. The manganites display a vast range of fascinating electrical and magnetic properties (colossal magnetoresistance, ferromagnetism, charge ordering and many more), which often come about due to the mixed valence states of manganese. Nickel manganite NiMn₂O₄ exhibits a partially inverse cubic spinel structure, which is well known since many years.1, 2, 3 NiMn₂O₄ is widely used in industry as the basis for the production of ceramic temperature sensors due to its electrical properties characterized by a negative temperature coefficient (NTC) of the semi-conducting electrical resistance.4, 5, 6, 7, 8, 9, 10 Several dopants can be included to improve the sensor performance.11, 12, 13 Despite the apparent spinel structure and simple chemical formula this material is surprisingly complex and keeps nowadays being revisited and prepared by different routes and in different forms: powder, thin and thick films, and single crystals.14, 15, 16, 17, 18, 19, 20, 21 The complexity of this compound is partially owned to the variability of the Ni and Mn lattice positions. Ni and Mn cations can both occupy tetrahedral and octahedral crystal sites, which are both interstitial sites within the cubic closed packed oxygen sub-lattice of the spinel structure.

The fraction of Ni occupancy on the octahedral sites corresponds to the inversion parameter v of the cubic spinel structure, which has a strong effect on the Mn valence states: The Ni fraction moving to octahedral sites is compensated by Mn going to tetrahedral sites. Such tetrahedral Mn has valence state 2+, because Mn³⁺ is unfavourable in tetrahedral four-fold coordination. The formation of tetrahedral Mn²⁺ is compensated by an equal amount of Mn⁴⁺ on the octahedral sites in order to retain charge balance, thus leading to an internal disproportionation. The driving forces for this disproportionation process are (1) the preferred octahedral coordination of Ni²⁺, and (2) the preferential 4+ valence state of Mn $(t_{2g}^3)$ in an octahedral coordination: an increasing amount of octahedral Mn³⁺ $(t_{2g}^3{e}_g^1)$ would lead to energetically unfavourable Jahn–Teller lattice distortions and ultimately to tetragonal symmetry of the spinel.

It has been found previously that v is in fact dependent on the synthesis or sintering temperature. Both, v and the Mn valence therefore depend on the preparation route and thermal history, and the physical properties vary accordingly. Thus, a common objective in many previous publications was to correlate details of the synthesis route, structure and microstructure with the observed charge transport and magnetic properties.5, 6, 22, 23, 24, 25, 26, 27 The use of Ni(MnO₄)₂xH₂O as a precursor is interesting due the high Mn⁷⁺ oxidation state. In conventional ceramic processing routes, precursor oxides such as Mn₂O₃ are used where the initial Mn valence is 3+, which corresponds to the expected average Mn valence in NiMn₂O₄. We show in this work that the permanganate precursor allows fabricating NiMn₂O₄ materials with typical physical properties.

In a previous paper we have shown that principally nickel permanganate can be used as a precursor for the synthesis of NiMn₂O₄.²⁸ Ni(MnO₄)₂xH₂O was shown to be thermally unstable, which is typical for permanganates. The compound is particularly suitable for use as a precursor for NiMn₂O₄ production, because it provides the fixed 1:2 cationic Ni:Mn ratio required. The permanganates are well known to be highly oxidizing: for Ni(MnO₄)₂xH₂O it was found that E° Mn (VII)/Mn (IV) = 1.69 V.

As concerns a possible non-stoichiometry of NiMn₂O_4+δ, the literature is abundant. A stoichiometric compound has often been reported (δ = 0), as well as non-stoichiometric variations: mostly cationic vacancies (Ni_xMn_3−x□_3δ/4 O_4+δ)29, 30, 31, 32, 33 have been mentioned, where δ depends on x and the synthesis conditions. Reports on oxygen vacancies also exist.31, 34 Therefore, we have made considerable effort in this study to correctly represent the stoichiometry and crystal structure of NiMn₂O_4+δ produced via the permanganate route. Furthermore, we have comprehensively determined the physical properties by means of temperature dependent magnetization and dielectric property measurements.