Chemical pressure effects on structural, dielectric and magnetic properties of solid solutions Mn3−xCoxTeO6
Graphical abstract
Introduction
Recent intense research on multiferroic materials has led to the discovery of new types of compounds with spin and dipole ordering and to a better understanding of the fundamental physical processes and interactions behind their complex physical behavior [1], [2], [3], [4]. Many of these single-phase materials possess long-wavelength geometrically frustrated spin networks. In this class of antiferromagnetic (AFM) materials, incommensurate (ICM) magnetic structures with spiral-spin order occur in which any inversion-symmetry breaking event can induce a spontaneous electric polarization via the spin–orbit coupling. However, the origin of the magnetoelectric coupling in these systems is complicated and essential aspects of their properties still are not fully understood. Also, only few single-phase multiferroics have so far been discovered [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17]. Therefore, it is of great interest to find new single-phase materials that show multiferroic properties. Up to date the search for new multiferroic materials has mainly been focused on perovskite-based materials due to their compositional flexibility and because the mechanisms that govern the properties of this structure type are well understood [15], [16], [17].
However, in recent years also transition metal orthotellurates, A3TeO6, with corundum-related structures, where A = Mn, Co, Ni, Cu, have been extensively studied as possible multiferroics [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30], [31], [32]. This family of materials is known to exhibit various properties as a result of compositional flexibility. The structure of many A3TeO6 (A2ATeO6) phases is related to the double perovskites A2BB′O6 [33]; however, the presence of additional crystallographically non-equivalent sites for the magnetic cations provides extra degrees of freedom for manipulation with the structure. Among these potential spin-spiral multiferroics, Mn3TeO6 (MTO), in which non-Jahn-Teller Mn2+ (3d5) cations carry S = 5/2 spins, is a promising example. MTO was first described and prepared in powder form by Bayer [34] who found the compound to be isotypic to the corundum-related Mg3TeO6 family. The interest in A3TeO6 phases also increased after Kosse et al. [35], [36] had identified them as a new class of ferroelectrics.
It was recently reported [27] that MTO enters a complex long-range magnetically ordered state below 23 K. In this structure, the incommensurate magnetic propagation vector k = [0, 0, 0.4302(1)] splits the unique Mn site into two magnetically different orbits. One orbit forms a perfect helix with the spiral axis along the c-axis while the other orbit has a sine wave character along the c-axis. The loss of inversion symmetry due to the helical spin ordering may introduce ferroelectric polarization in MTO.
Co3TeO6 (CTO) belongs to the family of transition-metal orthotellurates. It crystallizes in the monoclinic space group C2/c [21], [28]. The Co2+ ions are located in five crystallographically distinct sites, four of which are octahedrally coordinated and one tetrahedrally coordinated; the corresponding CoO6 and CoO4 polyhedra are connected by corner-, edge-, and face-sharing. through CoO bonds. The exchange interactions between the Co2+ ions in Co3TeO6 are sufficiently strong to result in long-range magnetic ordering. This compound exhibits magnetic field driven electric polarization at low temperatures, indicating strong coupling between magnetic and electric dipoles [37].
In this study A-site substitution of Mn by Co in the solid solution series Mn3−xCoxTeO6 (0 ≤ x ≤ 3) (MCTO) is investigated with regard to structural, dielectric and magnetic properties. We surprisingly find that all samples up to x = 2.4 adopt the trigonal corundum-related structure of pure Mn3TeO6 (space group ), i.e. not even substitution of 80% of the Mn2+ ions with Co2+ transfers MCTO to the monoclinic structure of pure Co3TeO6. The lattice parameters of MCTO decrease linearly, the magnetic ordering temperature increases and the level of magnetic frustration decreases with increasing Co2+ content.
Section snippets
Single crystals
Single crystals of MTO were grown by chemical transport reactions [23]. A mixture of MnO and TeO3 in the stoichiometric ratio 3:1 was thoroughly ground, pressed into a pellet and placed in a silica ampoule which was evacuated, sealed, and heated within 3 h to 1103 K and kept at this temperature for 3 days. This material was mixed with PtCl2 and loaded in an evacuated and sealed silica ampoule which was heated in a temperature gradient 1103 → 1023 K. At this temperature, PtCl2 decomposes with release
Structural characterization
From thermal analyses it was found that formation of MCTO phases starts in the temperature range of 900–920 K, and single-phase samples are obtained after annealing at 1000 K. According to the elemental analyses performed on 20 different crystallites, the metal composition of the MCTO samples is quite close to the expected ratios and permits to conclude that the sample stoichiometry is the nominal one. The oxygen content, as determined by thermogravimetric analysis, is also in agreement with the
Discussion
Viewing the MCTO system as substituting Co for Mn in Mn3TeO6, it retains the trigonal structure of the parent compound up to more than 80% of Co substitution (possibly 90%). Using the opposing view of MCTO, i.e. substituting Mn for Co in Co3TeO6, transforms the monoclinic structure of the parent compound to the trigonal MTO structure already at less than 20% Mn substitution. This game of words highlights a key finding of this study of MCTO – the stability of the higher symmetrical structure of
Concluding remarks
Mn3−xCoxTeO6 solid solutions preserves the trigonal corundum-related structure of MTO up to x = 2.4 at ambient and low temperature conditions. The lattice parameters a and c decrease linearly with the increase in x, whereas the c/a ratio increases continuously with x.
A key feature of the structure is a close packing of strongly distorted hexagonal oxygen layers parallel to (0 0 1), with (Mn/Co) and two distinct Te atoms in the octahedral interstices. The two TeO6 octahedra are fairly regular
Acknowledgements
Financial support of this research from the Swedish Research Council (VR), the Göran Gustafsson Foundation and the Russian Foundation for Basic Research is gratefully acknowledged. We thank S. Yu. Stefanovich for his technical assistance with the SHG experiments. The authors are also grateful to M.I. Aroyo for stimulating discussions and assistance related to the tools hosted in the Bilbao Crystallographic Server.
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