The crystal structures of warwickite, ludwigite and pinakiolite

Crystal structure refinement, low- and high-temperature X-ray diffraction and Mössbauer spectroscopy study of the oxoborate ludwigite from the Iten'yurginskoe deposit

This paper reports an investigation of the chemistry, crystal structure refinement and thermal behavior (80-1650 K) of ludwigite from the Iten'yurginskoe deposit (Eastern Chukotka, Russia). Its chemical composition was determined by electron microprobe analysis, giving an empirical formula (Mg1.70Fe2+0.29Mn0.01)Σ2.00(Fe3+0.90Al0.08Mg0.02)Σ1.00O2(BO3). A refinement of the crystal structure from single-crystal X-ray diffraction data (SCXRD) was provided for the first time for ludwigite from this deposit (R = 0.047). The structure can be described as a framework composed of [MO6]n- octahedra and isolated [BO3]3- triangles located in triangular interstices of the framework. Based on a comprehensive analysis of SCXRD and Mössbauer spectroscopy data, the M1 site is occupied by Mg, M2 and M3 by Mg and Fe2+, M4 by Fe3+, Mg and Al. There are also oxo-centered [O4M4]n+ and [O2M5]n+ polyhedra building up a framework with the [BO3]3- triangles located in its hexagonal interstices. No indications of magnetic ordering are found in the temperature range investigated. The Fe2+ → Fe3+ oxidation occurs above 600 K, and is accompanied by a decrease of the unit-cell parameters and subsequent incomplete solid-phase decomposition with the formation of hematite, warwickite and magnetite. The mineral melts at temperatures above 1582 K. The thermal expansion of ludwigite is slightly anisotropic, which is explained by a dense packing of the [MO6]n- octahedra as well as a virtually perpendicular orientation of the oxo-centered double chains to each other. At room temperature, maximum expansion is along the c axis (αc = 9.1 × 10-6 K-1) and minimum expansion is in the ab plane (αa = 8.6 × 10-6, αb = 7.6 × 10-6 K-1), which is due to the preferred orientation of the [BO3]3- triangles. A comparison of the thermal behavior of three oxoborates of the ludwigite group, namely azoproite (Mg,Fe2+)2(Fe3+,Ti,Mg,Al)O2(BO3), vonsenite (Fe2+,Mg)2(Fe3+,Mn2+,Sn,Al)O2(BO3) and ludwigite (Mg,Fe2+,Mn)2(Fe3+,Al,Mg)O2(BO3), is provided.

Investigation of thermal behavior of mixed-valent iron borates vonsenite and hulsite containing [OM4]n+ and [OM5]n+ oxocentred polyhedra by in situ high-temperature Mössbauer spectroscopy, X-ray diffraction and thermal analysis

The investigation of elemental composition, crystal structure and thermal behavior of vonsenite and hulsite from the Titovskoe boron deposit in Russia is reported. The structures of the borates are described in terms of cation-centered and oxocentred polyhedra. There are different sequences of double chains and layers consisting of oxocentred [OM₄]ⁿ⁺ tetrahedra and [OM₅]ⁿ⁺ tetragonal pyramids forming a framework. Elemental composition was determined by energy-dispersive X-ray spectroscopy (EDX). Oxidation states and coordination sites of iron and tin in the oxoborates are determined using Mössbauer spectroscopy and compared with EDX and X-ray diffraction data (XRD). According to results obtained from high-temperature Mössbauer spectroscopy, the Fe²⁺ to Fe³⁺ oxidation in vonsenite and hulsite occurs at approximately 500 and 600 K, respectively. According to the high-temperature XRD data, this process is accompanied by an assumed deformation of crystal structures and subsequent solid-phase decomposition to hematite and warwickite. It is seen as a monotonic decrease of volume thermal expansion coefficients with an increase in temperature. A partial magnetic ordering in hulsite is observed for the first time with T_c ≃ 383 K. Near this temperature, an unusual change of thermal expansion coefficients is revealed. Vonsenite starts to melt at 1571 K and hulsite melts at 1504 K. Eigenvalues of thermal expansion tensor are calculated for the oxoborates as well as anisotropy of the expansion is described in comparison with their crystal structures.

Crystal Chemistry of High-Temperature Borates

In recent years borate-based crystals has attracted substantial interest among the research community. The overall importance of this family of materials is reflected in miscellaneous articles and several reviews that have been published over the years. Crystalline borate materials exhibit numerous interesting physical properties, which make them promising for further practical applications. Diversity of functional characteristics results from their high structural flexibility caused in the linkage of planar/non-planar BO₃ groups and BO₄ tetrahedra, which can occur as isolated or condensed structural units. This report is a brief review on crystal chemistry and structure features of anhydrous/high-temperature borates. Polymorphism of boron-oxygen radicals has been considered basing on cations' nature and synthesis conditions. Analysis of the laws governing borates structures and general principles of their systematics was discussed. As a result, an alternative classification of anhydrous compounds has been considered. It is based on four orders of their subdivision: (1) by the variety of anion formers, (2) by the cation charge, (3) by the N = N_M:N_B, i.e., ratio of metal atoms number to the ratio of boron atoms number (N-factor) value indicating the borate structural type (if it is known), (4) by the cation type and size.

Magnetic structure of Cu2MnBO5 ludwigite: thermodynamic, magnetic properties and neutron diffraction study

We report on the thermodynamic, magnetic properties and the magnetic structure of ludwigite-type Cu₂MnBO₅. The specific heat, low-field magnetization and paramagnetic susceptibility were studied on a single crystal and combined with powder neutron diffraction data. The temperature dependence of the specific heat and the neutron diffraction pattern reveal a single magnetic phase transition at T  =  92 K, which corresponds to the magnetic ordering into a ferrimagnetic phase. The cation distribution and the values and directions of magnetic moments of ions in different crystallographic sites are established. The magnetic moments of Cu²⁺ and Mn³⁺ ions occupying different magnetic sites in the ferrimagnetic phase are pairwise antiparallel and their directions do not coincide with the directions of the principal crystallographic axes. The small value of the magnetic moment of copper ions occupying site 2a is indicative of partial disordering of the magnetic moments on this site. The magnetization measurements show a strong temperature hysteresis of magnetization, which evidences for field-dependent transitions below the phase transition temperature.

Topochemical Approach for Transition-Metal Exchange Assisted by Copper Extrusion: from Cu2FeBO5 to Fe3BO5

All fields of today's technology are concerned with multifunctional materials, the subject of constantly expanding research. Among them, metal transition oxides occupy a strategic place because of the properties directly correlated with metal valence linked with oxygen stoichiometry. To enhance or induce new properties, knowledge of the relationships between the structural and physical characteristics is of prime importance, but a design at low scale also appears to be a powerful tool to increase the chemical reactivity and stabilize new compounds. Herein, an unexpected reaction is reported that associates the exchange of copper and iron in a 3D ludwigite lattice with the huge release of oxygen (14% by weight) at moderate temperature (<450 °C). Annealing of Cu₂FeBO₅ in a reducing atmosphere leads to the extrusion of copper and the formation of Fe₃BO₅, the micro/nanostructural state of which facilitates the partial Cu₂FeBO₅ recovery associated with the capture of oxygen in oxidizing conditions.

Crystal structure and magnetism of Fe2(OH)[B2O4(OH)]

The structure and magnetism of Fe2(OH)[B2O4(OH)] are reported. Powder x-ray diffraction reveals a characteristic structure containing two crystallographically independent zigzag-ladder chains of magnetic Fe(2+) ions. Magnetization measurements reveal a phase transition at 85 K, below which a weak spontaneous magnetization (≈ 0.15 μB/Fe) appears. Below 85 K, magnetization increases with decreasing temperature down to 70 K, below which it decreases and approaches a constant value at low temperature. The Mössbauer spectrum at room temperature is composed of two paramagnetic doublets corresponding to the two crystallographic Fe(2+) sites. Below 85 K, each doublet undergoes further splitting because of the magnetic hyperfine fields. The temperature dependence of the hyperfine field is qualitatively different for the two distinguishable Fe(2+) sites. This is responsible for the anomalous temperature dependence of the magnetization.

On the structural relations of malachite. II. The brochantite MDO polytypes

The structural relation between malachite and the brochantite MDO (maximum degree of order) polytypes is discussed. It is demonstrated that the same building blocks which form the basis of brochantite polytypism also occur in malachite. The different arrangements of these building blocks in the two mineral structures are rationalized as a result of the different coordination geometries required by the respective non-metal atoms acting as linkers. The compound stoichiometries are discussed in light of a common structured formula scheme, in which pairs of H atoms can play a similar role as single non-H atoms. An overview on the occurrence of malachite-like building blocks in several other crystal structures is given.

Mg5TiO4(BO3)2

Single crystals of pentamagnesium titanium(IV) tetraoxide bis(borate), Mg(5)TiO(4)(BO(3))(2), were prepared by slow cooling of the melt from 1623 K in air. The crystal is isostructural with the mineral ludwigite (Mg(2)FeO(2)BO(3)). The Mg and Ti atoms are coordinated by six O atoms and the B atom is coordinated by three O atoms. There are three Mg sites and one mixed site statistically occupied by Mg and Ti atoms. Atoms are at the following special positions: 2a (0, 0, 0) and 2d (0, 1/2, 1/2) for two Mg atoms, 4g (x, y, 0) for the mixed Ti/Mg site and the BO(3) group, and 4h (x, y, 1/2) for a third Mg and two oxide O atoms. MgO(6) and (Ti/Mg)O(6) octahedra are connected by sharing of edges to form zigzag folding layers along the c axis. Triangular prismatic tunnels are formed between the folding layers by sharing apical O atoms of the MgO(6) and (Ti/Mg)O(6) octahedra.

On the systematics and description of polyions of linked polyhedra

Systematics and description of polyions of both linked cation- and anion-centered polyhedra with face-, edge-, and corner-sharing are discussed. Some additions to the system introduced by Liebau (1985) for cation-centered polyions are proposed which take into account the property of linking. The concept of linkedness of the polyion,ML, and the scheme of its determination are offered. A new rule of selection of fundamental chains for layers and frameworks is proposed which also considers the property of linking. For the description of topological properties of polyhedra in the polyion a diagram of connectedness (on the base of Schlegel diagrams) and a formula of connectedness are used. For the distinction of polyhedra in the polyion the concept of topological equivalence by Wells (1970) is used and the concept of configurational equivalence is proposed. Some examples of polyions composed of anion-centered tetrahedra and polyions composed of cation-centered tetrahedra are given.