Crystallographic investigation of the Co–B–O system

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.

Expanding transition metal borate chemistry to include main group elements: high-pressure synthesis and structural relation of β-MgB4O7

The high-pressure/high-temperature synthesis of β-MgB4O7 at 9 GPa and 1200 °C is presented. The determination of its crystal structure by single-crystal X-ray diffraction has shown that the compound is isotypic to the transition metal borates with the structure type of β-MB4O7 (M = Mn–Zn). β-MgB4O7 crystallizes in the orthorhombic space group Cmcm with the lattice parameters a = 10.848(2), b = 6.513(2), and c = 5.144(1) Å. The structural relation between the normal-pressure phase α-MgB4O7 to the here presented high-pressure phase β-MgB4O7 and to other reference compounds is discussed.

Temperature- and Field-Induced Transformation of the Magnetic State in Co2.5Ge0.5BO5

A tetravalent-substituted cobalt ludwigite Co_2.5Ge_0.5BO₅ has been synthesized using the flux method. The compound undergoes two magnetic transitions: a long-range antiferromagnetic transition at T_N1 = 84 K and a metamagnetic one at T_N2 = 36 K. The sample-oriented magnetization measurements revealed a fully compensated magnetic moment along the a- and c-axes and an uncompensated one along the b-axis leading to high uniaxial anisotropy. A field-induced enhancement of the ferromagnetic correlations at T_N2 is observed in specific heat measurements. The DFT+GGA calculation predicts the spin configuration of (↑↓↓↑) as a ground state with a magnetic moment of 1.37 μ_B/f.u. The strong hybridization of Ge(4s, 4p) with O (2p) orbitals resulting from the high electronegativity of Ge⁴⁺ is assumed to cause an increase in the interlayer interaction, contributing to the long-range magnetic order. The effect of two super-superexchange pathways Co²⁺-O-B-O-Co²⁺ and Co²⁺-O-M4-O-Co²⁺ on the magnetic state is discussed.

Efficient metal borate catalysts for oxidative dehydrogenation of propane

Boron-based catalysts have attracted attention due to their high selectivity to light olefins in the catalytic oxidative dehydrogenation of propane (ODHP) in the recent years, however, which still faced the...

Crystal structure of BaMnB2O5 containing structurally isolated manganese oxide sheets

In an attempt to search for mixed alkaline-earth and transition metal pyroborates, the title compound, barium manganese(II) pyroborate, has been synthesized by employing a flux method. The structure of BaMnB2O5 is composed of MnO5 square pyramids that form Mn2O8 dimers by edge-sharing and of pyroborate units ([B2O5]4-) that are composed of two corner-sharing trigonal-planar BO3 units. These building blocks share corners to form ∞2[MnB2O5]2- layers extending parallel to (100). The Ba2+ cations reside in the gap between two manganese pyroborate slabs with a coordination number of nine. The title compound forms an inter-esting spiral framework propagating along the 21 screw axis. The structure is characterized by two alternating layers, which is relatively rare among known transition-metal-based pyroborate compounds.