Structural and magnetic characterization of barbosalite Fe3(PO4)2(OH)2

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Abstract

Single crystals and polycrystalline samples of barbosalite Fe3(PO4)2(OH)2 iron phosphate were successfully synthesized by hydrothermal method at low pressure and temperature. Careful investigations of the crystal structure by high resolution X-ray diffraction and transmission electron microscopy evidence a complex barbosalite framework where trimers of face sharing FeO6 octahedra along the <110> direction are connected by an additional Fe site, partly occupied. Despite this structural disorder, the spins order in a rather simple magnetic structure below 160 ​K. The corresponding magnetic point group of P21 makes the barbosalite a promising candidate for the design of new multiferroics.

Graphical abstract

Double barbosalite structure in the P21 space group: PO4 tetrahedrons (yellow), FeO6 octahedrons (red and orange) and where the blue octahedrons correspond to the additional Fe7 site.

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Introduction

Natural iron hydroxyl phosphates are minerals with a very rich crystal chemistry in connection with complex structures that are able to adapt several contents of Fe, OH or H2O. The study of such materials, to determine their compositions and to establish relationships between structures, microstructures and functionalities, is often motivated by their applications related to energy and catalysis. For instance, Li3Fe2(PO4)3 or LiFePO4 used as cathode materials in batteries have attracted much attention, according to the Li insertion made possible by the Fe3+/Fe2+ redox couple [[1], [2], [3], [4]]. In that respect, the Fe3(PO4)2(OH)2 barbosalite, belonging to the P21/c MgAl2(PO4)2(OH)2 lazurite group, has just been recently shown to be less performing than LiFePO4 with the possibility to exchange only 0.7 Li at 2.6 ​V [5]. However, barbosalite exhibits the best catalytic properties of the Fe–P–OH–H system [6,7].

The presence of high spin Fe3+/Fe2+ cations in these compounds make them interesting candidates for magnetoelectric properties. LiFePO4, a member of the LiMPO4 olivines, is an antiferromagnet with TN ​≈ ​50 ​K [8,9] that crystallizes in the same structure as the magnetoelectric members LiNiPO4 and LiCoPO4 [10,11]. In that context, it becomes worthy to study the magnetic structure of the Fe3(PO4)2(OH)2 barbosalite. This is also motivated by the presence in this structure of Fe trimers formed by face-shared FeO6 octahedra [12,13]. Indeed, the presence of such trimers, in which a ferrous cation is surrounded by two ferric cations, could be involved in the multiferroic behaviour of the Fe3BO5 ludwigite [14].

G.J. Redhammer et al. [15] have reinvestigated the barbosalite and characterized its magnetic properties, revealing a magnetic transition at ​≅ ​160 ​K, but the magnetic structure and its temperature dependence are still unknown. In this context, we have undertaken a detailed study of the Fe3(PO4)2(OH)2 barbosalite for its structural and magnetic properties, by using X-ray, neutron and electron diffraction and magnetometry, on both single crystals and polycrystalline samples.

Section snippets

Experimental section

Two types of hydrothermal synthesis were used, leading to barbosalite crystals with different sizes but with similar structural and physical properties.

  • (i)

    The reactants FeCl2radical dot6H2O [1.2096 g], FeCl3 [0.9869 ​g] and (NH4)2HPO4 [0.8034 ​g] were dissolved independently in distilled water by magnetic stirring during 30 ​min approximatively. The mixtures were transferred to a 63 ​mL Teflon-lined stainless steel autoclave (80% degree of filling) and magnetically stirred at room temperature leading to the

Structural characterizations

The RT SXRPD pattern of the polycrystalline sample can be indexed using the P21/n space group and lattice parameters: a ​= ​7.3231 ​Å, b ​= ​7.4687 ​Å, c ​= ​7.4073 ​Å and β ​= ​118.565°, in agreement with the previous reports [5,15]. The barbosalite framework is described as consisting of Fe3+-Fe2+-Fe3+ face sharing FeO6 octahedra trimers, oriented along the <110> direction, and connected together by phosphate tetrahedra [15] (Fig. 1a and b). In this model, two iron sites are necessary to

Discussion and conclusion

The monoclinic structure proposed initially for the barbosalite Fe3(PO4)2(OH)2 [15] is characterised by the existence of Fe3+-Fe2+-Fe3+ “trimers”, also-called h-clusters, specific to iron phosphates materials [19], with short Fe–Fe equal distances within the trimers; those trimers are themselves connected by PO4 tetrahedra, leading to a dense structural arrangement. In this structural framework, disorder is observed by TEM, with the existence and coexistence of several types of super structures

CRediT authorship contribution statement

M. Poienar: Conceptualization, Funding acquisition, Project administration, Writing - original draft, Validation, Formal analysis, Investigation. F. Damay: Writing - review & editing, Validation, Formal analysis, Investigation. J. Rouquette: Writing - review & editing, Validation, Formal analysis, Investigation. V. Ranieri: Validation, Formal analysis, Investigation. S. Malo: Validation, Formal analysis, Investigation. A. Maignan: Supervision, Writing - review & editing, Validation, Formal

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgement

Financial support for this work was provided by the joint French-Romanian project ANR-UEFISCDI, contracts no. 8 RO-FR/01.01.2013, code PN-II-ID-JRP-2011-2-0056/ ANR-12-IS08-0003, COFeIn. This work is dedicated to our colleague Dr R. Baies†, from Timişoara, who was involved in the early stage of this work.

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