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.

(Cs X)Cu5O2(PO4)2 ( X = Cl, Br, I): A Family of Cu2+ S = 1/2 Compounds with Capped-Kagomé Networks Composed of OCu4 Units

Three new salt inclusion compounds (Cs X)Cu₅O₂(PO₄)₂ ( X = Cl, Br, I), phosphate analogues of the kagomé mineral averievite, are reported. Their crystal structures are composed of trigonal networks of corner-sharing OCu₄ anion-centered tetrahedra, forming capped-kagomé planes, which can also be regarded as two-dimensional slices along the [111] direction of a pyrochlore lattice. Magnetization and heat capacity measurements reveal strong geometric frustration of this network and complex magnetic behavior. X-ray and neutron diffraction studies show that all three compounds undergo a trigonal-to-monoclinic phase transition upon cooling, with a first-order phase transition seen in CsBr and CsI analogues. Along with the previously reported (CsCl)Cu₅O₂(VO₄)₂, these three new compounds belong to a large family of OCu₄-based networks, which are a playground for studying frustrated quantum magnetism.

Antiferromagnetic spin-frustrated layers of corner-sharing Cu4 tetrahedra on the kagome lattice in volcanic minerals Cu5O2(VO4)2(CuCl), NaCu5O2(SeO3)2Cl3, and K2Cu5Cl8(OH)4·2H2O

The objective of the present work was to analyze the possibility of realization of quantum spin liquids in three volcanic minerals-averievite (Cu₅O₂(VO₄)₂(CuCl)), ilinskite (NaCu₅O₂(SeO₃)₂Cl₃), and avdononite (K₂Cu₅Cl₈(OH)₄·2H₂O)-from the crystal chemistry point of view. Based on the structural data, the sign and strength of magnetic interactions have been calculated and the geometric frustrations serving as the main reason of the existence of spin liquids have been investigated. According to our calculations, the magnetic structures of averievite and ilinskite are composed of antiferromagnetic (AFM) spin-frustrated layers of corner-sharing Cu₄ tetrahedra on the kagome lattice. However, the direction of nonshared corners of tetrahedra is different in them. The oxygen ions centering the OCu₄ tetrahedra in averievite and ilinskite provide the main contribution to the formation of AFM interactions along the tetrahedra edges. The local electric polarization in averievite and the possibility of spin configuration fluctuations due to vibrations of tetrahedra-centering oxygen ions have been discussed. The existence of structural phase transitions accompanied with magnetic transitions was assumed in ilinskite because of the effect of a lone electron pair by Se⁴⁺ ions. As was demonstrated through comparison of averievite and avdoninite, at the removal of centering oxygen ions from tetrahedra, the magnetic structure of the pyrochlore layer present in averievite transformed into an openwork curled net with large cells woven from corner-sharing open AFM spin-frustrated tetrahedra ('butterflies') in avdoninite.

Derivation of bond-valence parameters for some cation-oxygen pairs on the basis of empirical relationships between ro and b

Relationships between the ro and b bond-valence parameters are investigated for ideal and distorted coordination polyhedra. For some ranges of the b values, this relationship can be approximated by linear functions, which are used to derive bond-valence parameters for some cation-oxygen pairs (ro , b [Å]): Cu2+—O (1.679, 0.36), Pb2+—O (1.963, 0.49), Hg2+—O (1.924, 0.38), Bi3+—O (1.990, 0.48), Sb3+—O (1.885, 0.53), Y3+—O (2.028, 0.35), La3+—O (2.086, 0.45), Nd3+—O (2.021, 0.46).

Conformation of single chains of anion-centred edge-sharing tetrahedra

The conformation of single chains of edge-sharing [XA4] anion-centred tetrahedra is discussed. The conformation can be interpreted as an adaptation of tetrahedral chains to the large halide ions. This adaptation is very reminiscent of that of silicate chains to large cations (Ca, Na, REE). Large anions rotate the [X2A6] groups of two edge sharing [XA4] tetrahedra just as large cations rotate corner sharing [SiO4] tetrahedra in silicates. As a result of conformation, vierer chains of anion-centred tetrahedra are formed.

Crystal chemistry of inorganic compounds based on chains of oxocentered tetrahedra III. Crystal structure of georgbokiite, Cu5O2(SeO3)2Cl2 1

The monoclinic crystal structure of georgbokiite, a new mineral with the chemical composition Cu5O2(SeO3)2Cl2 (a = 6.030(1) Å), b = 13.744(3) Å, c = 5.562(1) Å, β = 95.75(1)°, V = 458.6(2) Å3, Z = 2, P21/c) has been refined to an R index of 0.043 [wR = 0.108). The structure is similar to that previously determined for synthetic Cu5Se2O8Cl2. It is based on the unbranched zweier chains {uB, 1χ 1} [O2Cu5] running parallel to the c axis which are composed of [OCu4] oxocentered copper tetrahedra linked through corners and edges in turn. The pyramidal (SeO3) groups are in “face-to-face” positions relative to the [OCu4] tetrahedra and together with the oxocentered chains form the more complex chains {[O2Cu5](SeO3)2} lying side by side in the (010) plane. The chloride anions are located between the chains to link them into a three-dimensional framework. The main structural features of other inorganic compounds based on single chains of oxocentered condensed copper tetrahedra are discussed in comparison.

Crystal structure of Pb10O7(OH)2F2(SO4) and crystal chemistry of lead oxysulfate minerals and inorganic compounds

The crystal structure of Pb10O7(OH)2F2(SO4) (triclinic, P1̅, a = 9.6637(16), b = 10.1922(17), c = 11.1407(18) Å, α = 110.978(3), β = 91.795(3), γ = 107.020(3)°, V = 968.2(3) Å3) has been solved by direct methods and refined to R1 = 0.050 (wR = 0.089). There are ten symmetrically unique Pb2+ cations in the structure all of which have highly asymmetric coordination polyhedra owing to presence of stereoactive 6s 2 lone pairs of electrons. The structure is based upon [O7Pb10] zigzag chains of edge-sharing OPb4 oxocentered tetrahedra. The observed [O7Pb10] chain is the most complex chain of oxocentered tetrahedra known so far and was not observed previously in structures of inorganic compounds. The chains run along [011] and their plane is parallel to (111). The chains form sheets parallel to (111). The SO4) tetrahedra, F– anions and OH groups are between the sheets providing their linkage into three-dimensional structure. A comparison of the structure with other Pb oxysulfates is given in the framework of theory of complexes of anion-centered tetrahedra.

Minerals with antiperovskite structure: a review

Antiperovskites or inverse perovskites are inorganic compounds with a perovskite structure but with cations replaced by anions and vice versa. It is shown that there exists a number of minerals whose structures can be regarded as based upon antiperovskite units composed of face-and corner-sharing octahedra. The structures of sulphohalite, galeite, schairerite, kogarkoite, nacaphite, arctite, polyphite, hatrurite and related compounds are considered. The 3C, 2H, 5H, 7H, 9Rand 15Rantiperovskite polytypes are described. In some cases, antiperovskite modules are intercalated with intermediate ions and complexes.

On the Structure of Tetralead(II) Complexes with OH Bridges

Using Hartree-Fock, B3LYP and MP2 treatments, the optimal geometries and corresponding electronic structures of tetrahedral [Pb4(μ3-OH)n]q and [Pb4O(μ3-OH)n]q-2 complex cations with total charges q = 8 - n , n = 2, 3, 4, are investigated. After OH- removal, the central oxygen atom in [Pb4O(μ3-OH)4]2+ is shifted to the apical position in [Pb4O(μ3-OH)3]3+ whereas the [Pb4(μ3-OH)2]6+ and [Pb4O(μ3-OH)2]4+ complex cations are unstable. Direct Pb-Pb and O-O interactions are weakly antibonding in all the systems under study. The clusters are held together exclusively by relatively weak Pb-O bonds. A higher stability of the complex cations with a larger number of OH- bridges may be confirmed.