Polyoxometalate clusters in minerals: review and complexity analysis

Recent advances in atomic cluster synthesis: a perspective from chemical elements

Despite its potential significance, "cluster chemistry" remains a somewhat marginalized topic within the chemistry field. However, atomic clusters with their unusual and unique structures and properties represent a novel material group situated between molecules and nanoparticles or solid matter, judging from both scientific standpoints and historical backgrounds. Surveying an entire material group, including all substances that can be regarded as a cluster, is essential for establishing cluster chemistry as a more prominent chemistry field. This review aims to provide a comprehensive understanding by categorizing, summarizing, and reviewing clusters, focusing on their constituent elements in the periodic table. However, because numerous disparate synthetic processes have been individually developed to date, their straightforward and uniform classification is a challenging task. As such, comprehensively reviewing this field from a chemical composition viewpoint presents significant obstacles. It should be therefore noted that despite adopting a synthetic method-based classification in this review, the discussions presented herein could entail inaccuracies. Nevertheless, this unorthodox viewpoint unfolds a new scientific perspective which accentuates the common ground between different development processes by emphasizing the lack of a definitive border between their synthetic methods and material groups, thus opening new avenues for cementing cluster chemistry as an attractive chemistry field.

Electrochemistry of Uranyl Peroxide Solutions during Electrospray Ionization

The ionization of uranyl triperoxide monomer, [(UO₂)(O₂)₃]^4- (UT), and uranyl peroxide cage cluster, [(UO₂)₂₈(O₂)_42 - x(OH)_2x]^28- (U₂₈), was studied with electrospray ionization mass spectrometry (ESI-MS). Experiments including tandem mass spectrometry with collision-induced dissociation (MS/CID/MS), use of natural water and D₂O as solvent, and use of N₂ and SF₆ as nebulizer gases, provide insight into the mechanisms of ionization. The U₂₈ nanocluster under MS/CID/MS with collision energies ranging from 0 to 25 eV produced the monomeric units UO_x^- (x = 3-8) and UO_xH_y^- (x = 4-8, y = 1, 2). UT under ESI conditions yielded the gas-phase ions UO_x^- (x = 4-6) and UO_xH_y^- (x = 4-8, y = 1-3). Mechanisms that produce the observed anions in the UT and U₂₈ systems are: (a) gas-phase combinations of uranyl monomers in the collision cell upon fragmentation of U₂₈, (b) reduction-oxidation resulting from the electrospray process, and (c) ionization of surrounding analytes, creating reactive oxygen species that then coordinate to uranyl ions. The electronic structures of anions UO_x^- (x = 6-8) were investigated using density functional theory (DFT).

(Na,Li)3(Cl,OH)[Cu3OAl(PO4)3]: a first salt-inclusion aluminophosphate oxocuprate with a new type of crystal structure

The synthesis and characterization of a first salt-inclusion aluminophosphate oxocuprate, (Na,Li)₃(Cl,OH)[Cu₃OAl(PO₄)₃], obtained as single crystals, is reported. A novel phase, with a strongly pseudo-orthorhombic structure, is described as a monoclinic crystal structure established by the study of a pseudomerohedric microtwin. It was investigated using scanning electron microscopy, microprobe analysis and low-temperature X-ray diffraction. The composite crystal structure represents an original framework assembled from Cu-centered polyhedra, AlO₆ octahedra and PO₄ tetrahedra with channels, which incorporate the Na/Li salt component [(Na,Li)₃(Cl,OH)]²⁺ that ensures electroneutrality of the compound. Layers of strongly corrugated chains of Cu-centered octahedra with shared edges and linked by PO₄ tetrahedra are shown to be topologically identical with the layers also built from Cu-centered polyhedra and AsO₄/VO₄ tetrahedra forming the crystal structure of a fumarolic mineral aleutite, (M_0.5Cl)[Cu₅O₂(AsO₄)(VO₄)] [Siidra et al. (2019). MinMag, 83, 847-853]. `Sawtooth chains' and pairs of Cu-centered octahedra inherent in the title structure may be of interest in solid-state physics, engaging studies in the field of low-dimensional and frustrated magnetism.

Pascoite Minerals and Potential Application of NMR Spectroscopy

The 20 minerals encompassing the pascoite family of decavanadate isopolyanion-containing [V10O28]6− minerals include a few minerals, such as rakovanite, that have been described as containing a protonated decavanadate anion. Rakovanite was originally assigned the formula Na3[H3V10O28]•15H2O and now is redefined with an ideal formula (NH4)3Na3[V10O28]•12H2O. Nuclear magnetic resonance (NMR) and particularly 51V NMR spectroscopy is an informative method used to describe the protonation state and speciation in both solid and solution states of materials in the chemical and life sciences. However, 51V NMR spectroscopy has not yet been used experimentally to distinguish the protonation state of the decavanadate ion of leaching solutions and thus contributing to the discussion regarding the controversial protonation states of decavanadate ions in gunterite, rakovanite, and nashite. In contrast, the morphology and crystal structure for apatites, vanadinite, pyromorphite, and mimetite was related to 207Pb NMR chemical shifts, assisting in describing the local environments of these minerals. NMR spectroscopy could be a useful method if used in the future for decavanadate-containing minerals. Currently, partial reduction of two Pascoite minerals (caseyite and nashite) is proposed and accordingly could now effectively be investigated using a different magnetic resonance technique, EPR spectroscopy.

A Novel Mineral-like Copper Phosphate Chloride with a Disordered Guest Structure: Crystal Chemistry and Magnetic Properties

Novel copper phosphate chloride has been obtained under middle-temperature hydrothermal conditions. Its crystal structure was established based on the low-temperature X-ray diffraction data: Na₂Li_0.75(Cs,K)_0.5[Cu₅(PO₄)₄Cl]·3.5(H₂O,OH), sp. gr. C2/m, a = 19.3951(8) Å, b = 9.7627(3) Å, c = 9.7383(4) Å, β = 99.329(4)°, T = 150 K, MoK_α (λ = 0.71073 Å), R = 0.049. The crystal structure includes tetrameric copper clusters as the main building blocks, which are built of four CuO₄Cl pyramids sharing apical Cl vertices. The clusters are combined through phosphate groups and additional copper-centered polyhedra to form two mostly ordered periodic layers. Between the layers and inside the framework channels, alkali ions, H₂O molecules, or OH groups are statistically distributed. Na₂Li_0.75(Cs,K)_0.5[Cu₅(PO₄)₄Cl]·3.5(H₂O,OH) is a synthetic modification of a sampleite-polymorph of the lavendulan mineral group and represents a new member in a mero-plesiotype series of copper phosphates and arsenates, for which the crystal structures contain two-periodic [Cu₄X(TO₄)₄]_∞ modules (T = As, P; X = Cl, O). Magnetically, this phase exhibits the phase transition at T_C = 6.5 K, below which it possesses a weak ferromagnetic moment.

Crystallographic complexity partition analysis

We present an illustrative analysis of the complexity of a crystal structure based on the application of Shannon’s entropy formula in the form of Krivovichev’s complexity measures and extended according to the contributions of distinct discrete probability distributions derived from the atomic numbers and the Wyckoff multiplicities and arities of the atoms and sites constituting the crystal structure, respectively. The results of a full crystallographic complexity partition analysis for the intermetallic phase Mo3Al2C, a compound of intermediate structural complexity, are presented, with all calculations performed in detail. In addition, a partial analysis is discussed for the crystal structures of α- and β-quartz.

Information Entropy in Chemistry: An Overview

Basic applications of the information entropy concept to chemical objects are reviewed. These applications deal with quantifying chemical and electronic structures of molecules, signal processing, structural studies on crystals, and molecular ensembles. Recent advances in the mentioned areas make information entropy a central concept in interdisciplinary studies on digitalizing chemical reactions, chemico-information synthesis, crystal engineering, as well as digitally rethinking basic notions of structural chemistry in terms of informatics.

Hydrothermal Synthesis and a Composite Crystal Structure of Na6Cu7BiO4(PO4)4[Cl,(OH)]3 as a Candidate for Quantum Spin Liquid

A novel sodium bismuth oxo-cuprate phosphate chloride, Na₆Cu₇BiO₄(PO₄)₄[Cl_2.23(OH)_0.77], containing square-kagomé layers of Cu²⁺ has been synthesized by hydrothermal techniques. The compound crystallizes in the tetragonal space group P4/nmm, a = 10.0176(4), c = 10.8545(6), Z = 2, V = 1089.3(1) ų, R₁ = 0.021, wR = 0.053, S = 1.32. Its composite crystal structure includes [O₄Cu₆Bi]⁷⁺ layers, which are formed by the clusters of oxygen-centered tetrahedra [OCu₃Bi]. These positively charged two periodic fragments are intercalated in a negatively charged [CuNa₆Cl₃(PO₄)₄]^7- matrix built by Na-centered polyhedra, PO₄ tetrahedra, and CuO₄Cl pyramids. The composite character of the crystal structure of Na₆Cu₇BiO₄(PO₄)₄[Cl_2.23(OH)_0.77], as well as the way of its self-assembly, are discussed in close connection with the sulfohalite Na₆ClF(SO₄)₂ salt. It is shown that the "host-guest" model of the formation of the tetragonal Na₆Cu₇BiO₄(PO₄)₄[Cl_2.23(OH)_0.77] phase is due to the group-subgroup symmetry relation with the cubic crystal structure of mineral sulfohalite and is also supported by the crystallization condition in excess sodium chloride. The magnetic subsystem of Na₆Cu₇BiO₄(PO₄)₄[Cl_2.23(OH)_0.77] is represented by a dense square-kagomé network of 2Cu1 and 4Cu2 ions, decorated with weakly bonded Cu3 ions. Measurements of magnetization and heat capacity indicate the absence of long-range order up to 2 K, which makes this compound a candidate for a highly demanded spin liquid.

Complexity Parameters for Molecular Solids

Structural complexity measures based on Shannon information entropy are widely used for inorganic crystal structures. However, the application of these parameters for molecular crystals requires essential modification since atoms in inorganic compounds usually possess more degrees of freedom. In this work, a novel scheme for the calculation of complexity parameters (HmolNet, HmolNet,tot) for molecular crystals is proposed as a sum of the complexity of each molecule, the complexity of intermolecular contacts, and the combined complexity of both. This scheme is tested for several molecular crystal structures.

Natrophosphate, Arctic Mineral and Nuclear Waste Phase: Structure Refinements and Chemical Variability

The crystal structures of natural (Mt. Koashva, Khibiny alkaline massif, Kola Peninsula, Russian Arctic) and synthetic (obtained from an aqueous solution of sodium phosphate and sodium fluoride (1:1) by evaporation at room temperature (RT)) natrophosphate, Na7(PO4)2F·19H2O, have been investigated using single-crystal X-ray diffraction analysis. Natrophosphate and its synthetic analogue are cubic, Fd-3c, a = 27.6942(3) Å (natrophosphate at RT), a = 27.6241(4) Å (natrophosphate at 100 K), a = 28.1150(12) Å (synthetic analogue at RT), a = 27.9777(7) Å (synthetic analogue at 100 K). The crystal structure is based upon the super-octahedral [Na6(H2O)18F]5+ polycationic complexes consisting of six edge-linked Na6(OH2)5F octahedra sharing one common fluorine vertex. The A site is statistically occupied by Na and H2O with the prevalence of H2O with the refined occupancy factors O:Na equal to 0.53:0.47 for natrophosphate and 0.75:0.25 for its synthetic analogue. The coordination of the A site in synthetic natrophosphate is enlarged compared to the natural sample, which agrees well with its higher occupancy by H2O molecules. The general formula of natrophosphates can be written as Na6+xHxF(PO4)2·(19 + x)H2O, where x = 0–1. The chemical variability of natrophosphate allows to explain the discrepancies in its solubility reported by different authors. The information-based parameters of structural complexity are equal to 3.713 bit/atom and 2109.177 bit/cell that allows to classify natrophosphate as a structurally very complex mineral.

Reactivity, Formation, and Solubility of Polyoxometalates Probed by Calorimetry

Room temperature calorimetry methods were developed to describe the energy landscapes of six polyoxometalates (POMs), Li-U24, Li-U28, K-U28, Li/K-U60, Mo132, and Mo154, in terms of three components: enthalpy of dissolution (ΔHdiss), enthalpy of formation of aqueous POMs (ΔHf,(aq)), and enthalpy of formation of POM crystals (ΔHf,(c)). ΔHdiss is controlled by a combination of cation solvation enthalpy and the favorability of cation interactions with binding sites on the POM. In the case of the four uranyl peroxide POMs studied, clusters with hydroxide bridges have lower ΔHf,(aq) and are more stable than those containing only peroxide bridges. In general for POMs, the combination of calorimetric results and synthetic observations suggest that spherical topologies may be more stable than wheel-like clusters, and ΔHf,(aq) can be accurately estimated using only ΔHf,(c) values owing to the dominance of the clusters in determining the energetics of POM crystals.

Producing highly complicated materials. Nature does it better

Through the years, mineralogical studies have produced a tremendous amount of data on the atomic arrangement and mineral properties. Quite often, structural analysis has led to elucidate the role played by minor components, giving interesting insights into the physico-chemical conditions of mineral crystallization and allowing the description of unpredictable structures that represented a body of knowledge critical for assessing their technological potentialities. Using such a rich database, containing many basic acquisitions, further steps became appropriate and possible, into the directions of more advanced knowledge frontiers. Some of these frontiers assume the name of modularity, complexity, aperiodicity, and matter organization at not conventional levels, and will be discussed in this review.