Crystal Chemical Analysis Research Articles

Tl2Se–TlInSe2–Tl4P2Se6 QUASITERNARY SYSTEM

Complex chalcogenide compound are widely used as working elements for semiconductor optical technology, thermal generation, solar power. Special attention is paid to compounds of the M2P2Se6 type (M – Ag, Cu) which due to its layer crystal structure possess promising ferroelectric, thermoelectric and electro-optical properties. Heterovalent substitutions of cations 2М2+ ® 4M1+ in the composition of M2P2Se6 type compounds must leads to deformation of the crystal structure, changing in the value of the dipole moment and, accordingly, to change the electro-physical properties.
 The Tl2Se–In2Se3–“P2Se4” system characterized by the formation of intermediate complex compounds which melts congruently TlInSe2 (1023 К), Tl4P2Se6 (758 К), TlInP2Se6 (875 К) and TlIn5Se8 (melts incongruently L+In2Se3«TlIn5Se8 at 1029 К), In4(P2Se6)3 (formed by syntactic reaction at 880 К). Triangulation of the Tl2Se–In2Se3–“P2Se4” system was shown that then divided on secondary quasiternary systems, one of them is Tl2Se–TlInSe2–Tl4P2Se6.
 Phase equilibria in the Tl2Se – TlInSe2 – Tl4P2Se6 quasiternary system were studied using classical methods of physicochemical analysis DTA (chromel-alumel thermocouple, with an accuracy of ±5 K), XRD (DRON-3-13 diffractometer, Cu Ka radiation, Ni filter, Guinier Huber G670 diffractometer, CuKα1 radiation), MSA (metallographic microscope Lomo Metam R1) in combination with the simplex method of mathematical modeling of phase equilibria in multi-component systems. Crystal structure calculation was carried out with program WinCSD.
 Investigation of physical-chemical interaction allowed to constructed perspective view of phase state diagram and liquidus surface projection of the Tl2Se–TlInSe2–Tl4P2Se6 ternary system. In the ternary system formed the boundary solid solution: a- on the basis of Tl2Se, b- on the basis of TlInSe2, g-, d-, e- on the basis of ltm-, mtm- and htm-Tl4P2Se6 (ltm, mtm, htm – low, middle and high temperature modification, respectively). The liquidus of the ternary system consists of primary crystallization areas: Tl2Se-е1-Е1-е2-Tl2Se (a phase), TlInSe2-е3-U2-U1-E1-TlInSe2 (b phase), m1-U1-E1-e2-m1 (g phase), m2-U2-U1-m1-m2 (d phase) and Tl4P2Se6-e5-U2-m2-Tl4P2Se6 (ε phase). The Tl2Se–TlInSe2–Tl4P2Se6 quasiternary system is characterized by the processes: monovariant eutectic L«htmTl4P2Se6+TlInSe2 (e5-U2, 776-693 К), monovariant eutectic L«Tl2Se+TlInSe2 (e1-Е1, 614-539 К), monovariant eutectic L«Tl2Sе+ltmTl4P2Se6 (e2-Е1, 610-539 К); monovariant peritectic L+mtmTl4P2Se6«ltmTl4P2Se6 (m1-U1, 640-620 К); monovariant peritectic L+htmTl4P2Se6« mtmTl4P2Se6 (m2-U2, 747-693 К); monovariant peritectic L+mtmTl4P2Se6«TlInSe2 (U2-U1, 693-620 К); monovariant peritectic L+ltmTl4P2Se6«TlInSe2 (U1-E1, 620-539 К). Lines of the monovariant equilibria are crossed in three point: U2 – invariant peritectic process L+htmTl4P2Se6«TlInSe2+mtmTl4P2Se6 (12 mol.% Tl2Se, 20 mol.% TlInSe2, 68 mol.% Tl4P2Se6, 693 К), U1 – invariant peritectic process L+mtmTl4P2Se6«TlInSe2+ltmTl4P2Se6 (38 mol.% Tl2Se, 9 mol.% TlInSe2, 53 mol.% Tl4P2Se6, 620 К), E1 – invariant eutectic process L « Tl2Se+TlInSe2+ltmTl4P2Se6 (47 mol.% Tl2Se, 7 mol.% TlInSe2, 46 mol.% Tl4P2Se6, 539 К). New complex compounds were not observed in the ternary system. Limited solid solutions on the basis of TlInSe2, Tl4P2Se6 initial compounds are not up to 5–8 mol%.
 Crystal-structure studies of Tl2Se, TlInSe2 and Tl4P2Se6 complex chalcogenides were carried out by a powder method, refinement of the structural parameters – by the Rietveld method. The lattice parameters are: Tl2Se – Р4/n, а=8,540; с=12,380 Å, TlInSe2 – I4/mcm, a=8.064, c=6.833 Å, Tl4P2Se6 – P121/c1, a=12.239, b=9.055, c=12.328 Å, b=98.83. Crystal-chemical analysis of the compounds showed that they are characterized by a mixed ion-covalent type of chemical bond. During the transition from the binary Tl2Se to TlInSe2 ternary compound the covalent component of the In–Se bond is enhanced, the opposite change is observed for Tl4P2Se6, an increase in the ion component of the Tl–Se bond. The study of the mechanisms of formation of solid solutions showed that with the reciprocal solubility of the TlInSe2, Tl4P2Se6 ternary compounds characterized by the formation of substitution structure, the dissolution of Tl2Se in ternary selenides follows the substitution and subtraction mechanism.

Sol—Gel Synthesis and Structure Formation of Manganese Zirconium (Titanium) Phosphates

Compounds Mn0.5Ti2(PO4)3 and Mn0.5Zr2(PO4)3 and Mn0.5+2xZr2 – x(PO4)3 (0 < x ≤0.35) solid solution were prepared by two variants of the sol-gel method using inorganic and organic reagents and were characterized using X-ray diffraction and IR spectroscopy. Mn0.5Ti2(PO4)3, a compound with an NaZr2(PO4)3 (NZP) structure, is formed at 600°C and is stable up to 950°C. Mn0.5Zr2(PO4)3 has dimorphism; its low-temperature phase having the Sc2(WO4)3 (SW) structure was prepared at 650°C, and the high-temperature NZP phase, at 1200°C. Mn0.5 + 2xZr2−x(PO4)3 solid solution crystallizes in an SW-type structure; it is thermally unstable at temperatures above 900°C. The thermal stability of samples decays as x rises. p ]The numbers of the stretching and bending vibrations in an $${\rm{PO}}_4^{3 - }$$ ion in the IR spectra of NZP and SW ortho-phosphates agree with factor-group analysis for space group R3 and P21/n. Structure refinement was carried out for the low-temperature Mn0.5Zr2(PO4)3 phase (space group P21/n, a = 8.861(3) A, b = 8.869(2) A, c = 12.561(3) A, β = 89.51(2)°) and for the solid solution. The basis of the structures is a framework built of corner-sharing tetrahedra PO4 and octahedra ZrO6 or (Mn,Zr)O6. The framework interstices are occupied by cations Mn2+ in tetrahedral oxygen coordination. A comparative crystal-chemical analysis of the morpho-tropic series of M0.5Zr2(PO4)3 phosphates (M stands for a metal in the oxidation state +2) elucidated a relationship between structural features.

Bond valence parameters for alkali– and alkaline-earth–oxygen pairs: derivation and application to metal−organic compounds

New bond valence R0 parameters were derived for alkali– and alkaline-earth–oxygen pairs encountered in metal−organic compounds. A total of 7693 metal−oxygen bond lengths were extracted from structures deposited in the Cambridge Structural Database (CSD) and containing at least one carboxylate ligand bound to the metal. One hundred structures were individually analyzed for each metal. R0 values were computed using two different approaches: geometric mean and graphical fitting. In both cases, the bond valence parameter b was fixed at 0.37 Å. R0 values obtained using a geometric mean approach were 1.744 (Na–O), 2.094 (K–O), 2.228 (Rb–O), 2.412 (Cs–O), 1.661 (Mg–O), 1.933 (Ca–O), 2.090 (Sr–O), and 2.265 Å (Ba–O). These parameters yielded more accurate metal bond valence sums (BVS) and lower root-mean square deviations (RMSD) than those computed using R0 values currently available in the literature,1 which had been derived from the structures of inorganic oxides. The most significant improvement was observed for sodium (this work mean BVS = 1.01 v.u., RMSD = 0.116 v.u.; literature mean BVS = 1.18 v.u., RMSD = 0.226 v.u.; v.u.: valence units) and magnesium (this work mean BVS = 2.00 v.u., RMSD = 0.038 v.u.; literature mean BVS = 2.18 v.u., RMSD = 0.100 v.u.). More accurate bond valence sums and lower variances were also observed when the new set of R0 values was applied to metal–organic compounds featuring oxygenated ligands such as ethers, ketones, aldehydes, alcohols, and esthers. These parameters should be applicable to the structural and crystal-chemical analysis of metal−organic compounds ranging from coordination complexes to organic−inorganic extended hybrids.

Crystal structure, quantum chemical and Hirshfeld surface analysis of substituted imidazo-thiadiazole-5-carbaldehyde

Abstract In the present study, we have reported crystal structure of 6-(4-methoxyphenyl)-2-(naphthalen-1-ylmethyl)imidazo[2,1-b][1,3,4]thiadiazole-5-carbaldehyde (C23H17N3O2S) via single crystal X-ray diffraction. The compound crystallizes in the Triclinic, P 1 ¯ space group. The crystal structure shows intermolecular C – H…O, N – H…N, and π…π interactions. Quantum Theory of Atoms in Molecules (QTAIM) analysis confirmed presence of these interactions with acceptable topological parameters. The PIXEL Coulomb London Pauli (CLP) module was employed to characterize energetics associated with all interactions. Hirshfeld 2D fingerprint plots helped in evaluating intermoleular interactions quantitatively. The molecular electrostatic potential (MEP) was plotted to identify the reaction sites of the molecule. Interestingly, the centrosymmetric molecule exhibits nonlinear optical (NLO) properties calculated using Density Functional Theory (DFT). The mean and hyperpolarizability values are found to be 42.6314 × 10−24 and 11.4248 × 10−30 esu respectively indicating that the compound would be a potential candidate for the NLO applications.

The Coordination Polyhedra BCn in Crystal Structures

The method of intersecting spheres and Voronoi–Dirichlet polyhedra are used for crystal chemical analysis of 3817 structures comprising 4533 crystallographically nonequivalent sorts of boron atoms contained in BCn coordination polyhedra. In these coordination polyhedra, boron has coordination numbers (CNs) from 2 to 6, the CN 4 being most typical. With increasing boron CN, the average B–C interatomic distances of the BCn polyhedra increase by 0.01–0.13 A, while the average radii of the spheres whose volumes are equal to the volumes of the Voronoi–Dirichlet polyhedra of boron atoms virtually do not vary within the determination error. The characteristic features of intermolecular contacts in the structures of two crystalline hydrates, Fe[B(CN)4]2 · 2H2O and Fe[B(CN)4]3 · 6H2O, are considered according to the method of Voronoi–Dirichlet molecular polyhedra.

Distribution of Substitutional Alloying Elements and Interstitial Impurities in In Situ Multicomponent Composites Based on the Nb–Si System

The distribution of substitution alloying elements in the structure of hexagonal silicide Nb5Si3 of the γ-modification, which serves as a reinforcing phase of the composite, and in the Nb solid solution matrix of a high-temperature natural composite based on Nb–Si is studied. Studies were carried out on samples in a cast state and after homogenizing thermal treatment. A structural model of the γ-modification of silicide Nb5Si3 is constructed. On the basis of the crystal-chemical analysis of the structure of hexagonal silicide, possible causes of phase instability of the composite after thermal treatment are revealed.

Vanadium clusters formation in geometrically frustrated spinel oxide AlV2O4.

The spinel oxide AlV2O4 is a unique material, in which the formation of clusters is accompanied by atomic, charge and orbital ordering and a rhombohedral lattice distortion. In this work a theory of the structural phase transition in AlV2O4 is proposed. This theory is based on the study of the order-parameter symmetry, thermodynamics, electron density distribution, crystal chemistry and mechanisms of formation of the atomic and orbital structures of the rhombohedral phase. It is established that the critical order parameter is transformed according to irreducible representation k9(τ4) (in Kovalev notation) of the Fd \bar{3}m space group. Knowledge of the order-parameter symmetry allows us to show that the derived AlV2O4 rhombohedral structure is a result of displacements of all atom types and the ordering of Al atoms (1:1 order type in tetrahedral spinel sites), V atoms (1:1:6 order type in octahedral sites) and O atoms (1:1:3:3 order type), and the ordering of dxy, dxz and dyz orbitals. Application of the density functional theory showed that V atoms in the Kagomé sublattice formed separate trimers. Also, no sign of metallic bonding between separate vanadium trimers in the heptamer structure was found. The density functional theory study and the crystal chemical analysis of V-O bond lengths allowed us to assume the existence of dimers and trimers as main clusters in the structure of the AlV2O4 rhombohedral modification. The trimer model of the low-symmetry AlV2O4 structure is proposed. Within the Landau theory of phase transitions, typical diagrams of possible phase states are built. It is shown that phase states can be changed as a first-order phase transition close to the second order in the vicinity of tricritical points of the phase diagrams.

Jahn–Teller Effect in the [CuEn3]CrO4 Structure

A change in the coordination of the copper atom in the crystal structure of [CuEn3]CrO4 (En is ethylenediamine) in studied in the range 150–300 K. According to the single crystal X-ray diffraction (XRD) data at 150 K, the single crystal has a complicated twining character based on the triclinic cell (a = 9.027(2) A, b = 13.335(3) A, c = 13.339(3) A, α = 71.77(3)°, β = 70.53(3)°, γ = 70.42(3)°) composed of two crystallographically independent [CuEn3]2+ complex cations. The coordination of copper atoms is a distorted square bipyramid; four short Cu–N distances lie in the range 2.049-2.082 A; two long ones are 2.415 A and 2.470 A. According to the differential scanning calorimetry (DSC) data, near 218 K there is a phase transition. The single crystal XRD experiment performed at 250 K (a = 15.6992(19) A, c = 9.7573(13) A, V = 2082.6(6) A3, space group P\(\bar 3\)c1 (No. 165), Z = 6) shows that chromate anions are disordered over three positions about the с axis, and Cu–N distances are 2.120-2.177 A. According to the DSC data, on further heating the structure undergoes yet another two alterations (260 K and 270 K) due to the disordering of oxygen atoms of chromate anions and the subsequent equalization of Cu–N distances. At 300 K in the structure (a = 9.0778(6) A, c = 9.7715(5) A, V = 697.4 A3, space group P\(\bar 3\)c (No. 163), Z = 2) all Cu–N distances are 2.155 A, and chromate anions are disordered over six positions about the с axis. A comparative crystal chemical analysis of the packing of the studied structures is carried out.

Hydrothermal Synthesis, Study, and Classification of Microporous Uranium Silicates and Germanates.

Four novel uranyl silicates and germanates with framework structures, K4Na2(UO2)3(Si2O7)2·3H2O, K4Na2(UO2)3(Ge2O7)2·3H2O, H3O(UO2)2(HGe2O7)·2H2O, and Na2(UO2)GeO4, have been synthesized by means of the hydrothermal method. The structures of the title compounds were refined by single-crystal X-ray diffraction and characterized by Raman spectroscopy. We used the method of secondary building units (SBUs) for a crystal chemical analysis of the 3D framework and their topologies. The framework of the K4Na2(UO2)3(T2O7)2·3H2O (T = Si, Ge) series exhibits large 14-membered rings and smaller 8-membered rings which are built upon [UT4] pentamers. The internal size of the largest pores is approximately 12.39 × 3.33 Å2. H3O(UO2)2(HGe2O7)·2H2O is based on 10-membered rings with intermediate sized pores. They are built upon [U2Ge2] tetramers with 7-fold-coordinated U. The internal dimension of the pores in H3O(UO2)2(HGe2O7)·2H2O is smaller compared to the K4Na2(UO2)3(T2O7)2·3H2O (T = Si, Ge) series with ∼5.91 × 5.33 Å2. Its topology is similar to several uranium germanate synthetic phases and silicate minerals, especially α- and β-uranophane which are constructed from similar building units. A novel 3D framework type of Na2(UO2)GeO4 with 8-membered rings demonstrates the smallest free volume in the family of porous uranium germanates. It crystallizes in tetragonal symmetry and is built upon corner sharing of [UGe4] pentamers. The size of the channels is ∼6.76 × 4.27 Å2. The vibrational bands in Raman spectra were associated with pyro-(Si2O7)6- and -(Ge2O7)6- groups, with the Ge-OH bond and with H3O+ cations, confirming the results of the X-ray crystallographic structural characterization. We systemized existing uranyl silicates and germanates based on their building units and chemical composition. We found a simple structural dependence between synthetic conditions and chemical composition.

Metal-loaded pollucite-like aluminophosphates: dissymmetrisation of crystal structures and physical properties

Two aluminophosphate analogues of the mineral pollucite with the general formula Cs2(M,Al)3P3O12 (where M = Cu or Mn) have been synthesized by high-temperature flux and structurally characterized using the single-crystal X-ray diffraction. Both samples crystallize in cubic I4132 space group, Z = 8, with a = 13.5911(5) and a = 13.8544(7) for Cu- and Mn-loaded phases, respectively. Their framework structures are based on the ANA-type topology and exhibit the partial ordering of the metal (M/Al) and phosphorus (P) cations over the tetrahedral sites. The regular changes in cell dimensions and volumes in the row Cs2(Cu,Al)3P3O12→Cs2(Mn,Al)3P3O12 obviously correspond to increasing radii of the transition metal. The crystal chemical analysis of both pollucite-like phases show correlations between the difference in the radii size of tetrahedral cations and the degree of distortion of flexible ANA-type framework due to decreasing of the intertetrahedral angles (T–O–T). Magnetic susceptibility measurements indicate that both compounds are paramagnets in the temperature range of 2–300 K.

Conditions for the formation of a non-autonomous phase at the structural deformation of complex vanadium oxides

A new previously unknown effect of a reversible transition from a single-phase system to a heterophase system containing a non-autonomous phase was observed during thermal and chemical deformations of the Zn2-2xCd2xV2O7 structure.The role of local symmetry in the formation of the non-autonomous phase is shown on the basis of X-ray diffraction studies in situ and a comparative crystal-chemical analysis of the structural deformations of isoform monoclinic solid solutions of zinc and copper pyrovanadates with zero volumetric thermal expansion.

A study of structural non-stoichiometry with respect to oxygen in RBaCo4O7+x single crystals

Impurity-free RBaCo4O7+x (R = Y, Lu) crystals are grown and the evolution of their structure with varying oxygen concentration in the range 0 ≤ x ≤ 1.3 is studied. The structural features of the studied samples required to develop an extended strategy of the X-ray crystallographic experiment, which made it possible to obtain more accurate data on unit cell parameters and additional information on the features of superstructure modulation and diffuse scattering, along with the phase composition of crystalline samples. A crystal chemical analysis of the known structural models of RBaCo4O7+x is performed and suggestions are made about possible structural changes occurring when the oxygen concentration increases up to the limit.

Occurrence and origin of pore-lining chlorite and its effectiveness on preserving porosity in sandstone of the middle Yanchang Formation in the southwest Ordos Basin

The occurrence and origin of pore-lining chlorites in sandstone were studied using mineralogical and crystal chemical analysis and their effectiveness on preserving porosity was discussed. During the sedimentary period of the middle Yanchang Formation, the two provenance and sedimentary systems of the southwest and northeast developed in the southwest Ordos Basin. In the Northeast of the study area, the content of biotite flakes is approximately 10vol%. Pore-lining chlorites prefer to grow on the surfaces of fine-grained detrital quartz and feldspar but not on ductile grains such as biotite. Pore-lining chlorites that developed during the early stage of eodiagenesis (<90°C) probably locally increased the compressive strength of the rock, in places with residual intergranular pores. The chlorites within different sand bodies show slight variations in their chemical composition. After initial mechanical compaction, the formation of authigenic chlorites (brunsvigite and diabantite) was probably related to the hydrolysis and alteration of biotite fragments and the migration of diagenetic fluid. In the Northeast, only 2vol% of the chlorite rims significantly inhibited quartz overgrowth, preserving the primary porosity. However, they probably blocked and delayed the dissolution of acids to feldspar. The porosity probably decreased when the authigenic chlorite content exceeded 9.38vol%. In the Southwest, the authigenic chlorite is rare and quartz-overgrown cement is widespread, causing poor reservoir quality. Authigenic chlorites increase with the content of biotite flakes in the northeast and southwestern parts of the study area, but excess ductile biotite flakes and authigenic chlorites have negative effects on reservoir quality.

NiMn2O4 spinel as a material for supercapacitors with a pseudocapacity effect

Nanopowders of the NiMn2O4 spinel of a grain size of about 30 nm are obtained by the method of coprecipitation. The specific capacity in an aqueous electrolyte (1 М LiNO3) is determined by the method of cyclic voltammetry. Using computer simulation, the peculiarities of the mechanism of intercalation of Li+ into a NiMn2O4 spinel are studied by the method of crystal chemical analysis.

Stereochemistry of thorium in oxygen-containing compounds

The Voronoi–Dirichlet polyhedra (VDP) and the method of intersecting spheres were used to perform a crystal-chemical analysis of all compounds studied to date whose structures contain rhodium atoms surrounded by oxygen atoms. All Rh atoms were found to have a coordination number (CN) equal to 6 and to form coordination polyhedra of two types, namely, the distorted octahedra RhO6 and RhO5Rh. A coordination number of 6 was found for all Rh(IV), Rh3.5+, and Rh(III) atoms, while a CN of 5+1 was found typical only of Rh2.5+ and Rh(II) atoms. The effect of the valence state of Rh on the main features of their crystal-chemical role in the structures is considered in terms of the 18-electron rule.

Cation-Dependent Structural Evolution in A2Th(TVO4)2(A = Li, Na, K, Rb, Cs; T = P and As) Series

A new alkali thorium phosphate family, A2Th(PO4)2 (A = Li, Na, K, Rb, Cs), has been synthesized and systematically investigated. The structural evolution of this new family was compared with that of the previously reported arsenic-based family A2Th(AsO4)2 (A = Li, Na, K, Rb, Cs). Similar compositions of the phosphorus- and arsenic-containing series allowed their direct comparison in order to reveal the influence of the size and nature of either monovalent or pnictogen cations on the crystal structure and related properties. Detailed crystal chemical analyses suggest that two main driving forces are in charge for the structural evolution, i.e., the size of alkali cations as well as the local environment of the thorium polyhedra and the TO4 (T = P, As) tetrahedra. Raman spectra were measured for all studied phases and bands have been assigned. The density functional theory calculations revealed that the local environment of the PO4 tetrahedra may be responsible for shifts of PO4 vibrational bands observed i...

Oxy-foitite, [] (Fe2+ Al2)Al6 (Si6 O18 )(BO3)3 (OH)3 O, a new mineral species of the tourmaline supergroup

Oxy-foitite, □(Fe 2+ Al 2 )Al 6 (Si 6 O 18 )(BO 3 ) 3 (OH) 3 O, is a new mineral of the tourmaline supergroup. It occurs in high-grade migmatitic gneisses of pelitic composition at the Cooma metamorphic Complex (New South Wales, Australia), in association with muscovite, K-feldspar and quartz. Crystals are black with a vitreous luster, sub-conchoidal fracture and gray streak. Oxy-foitite has a Mohs hardness of ∼7, and has a calculated density of 3.143 g/cm 3 . In plane-polarized light, oxy-foitite is pleochroic ( O = dark brown and E = pale brown), uniaxial negative. Oxy-foitite belongs to the trigonal crystal system, space group R 3 m , a = 15.9387(3) A, c = 7.1507(1) A and V = 1573.20(6) A 3 , Z = 3. The crystal structure of oxy-foitite was refined to R 1 = 1.48% using 3247 unique reflections from single-crystal X-ray diffraction using Mo K α radiation. Crystal-chemical analysis resulted in the empirical structural formula: X ( □ 0.53 Na 0.45 Ca 0.01 K 0.01 ) ∑ 1.00 Y ( Al 1.53 Fe 2 + 1.16 Mg 0.22 Mn 2 + 0.05 Zn 0.01 Ti 4 + 0.03 ) ∑ 3.00 Z ( Al 5.47 Fe 3 + 0.14 Mg 0.39 ) ∑ 6.00 [ ( Si 5.89 Al 0.11 ) ∑ 6.00 O 18 ] ( BO 3 ) 3 V ( OH ) 3 W [ O 0.57 F 0.04 ( OH ) 0.39 ] ∑ 1.00 . Oxy-foitite belongs to the X -site vacant group of the tourmaline-supergroup minerals, and shows chemical relationships with foitite through the substitution Y Al 3+ + W O 2− → Y Fe 2+ + W (OH) 1− .

Crystal-chemical characteristics of nontronites from bottom sediments of Pacific ocean

A crystal-chemical analysis of the nontronite samples formed in deep-water sediments of the underwater Juan-de-Fuca ridge in the Pacific ocean has been performed using powder X-ray diffraction, IR spectroscopy, and Mossbauer spectroscopy. A comparison with the previously investigated nontronites from different regions of the Sea of Okhotsk showed that the structural features of these formations are due to the difference in the physicochemical parameters of their crystallization. The values of the basal interplanar spacing d001 (within 11–13 A) in the samples analyzed are determined by the degree of hydration and cation filling of the interlayer space, while the differences in the IR spectra are due to isomorphic substitutions in the structure. The character of cation distribution and the nature and concentration of stacking faults in nontronite structures are determined. The differences in the composition, structure, and properties of nontronites of different origin are confirmed by theoretical calculations of their structural parameters.

Influence of the octahedral cationic-site occupancies on the framework vibrations of Li-free tourmalines, with implications for estimating temperature and oxygen fugacity in host rocks

Tourmalines, XY 3 Z 6 T 6 O 18 (BO 3 ) 3 V 3 W, are excellent petrogenetic indicators as they capture the signature of the host-rock bulk composition. Raman spectra of tourmalines can be used as fingerprints for species identification and crystal-chemical analysis. While Li-bearing species are directly distinguishable by the shape of the OH-stretching vibrations, the discrimination of Mg- and Fe-dominant species can be hindered by the coexistence of at least two types of octahedrally coordinated R n+ cations. Thirty Li-free tourmaline samples comprising 14 different species were studied by Raman spectroscopy and electron microprobe. All nine Fe 3+ -bearing samples were also analyzed by single-crystal X-ray diffraction and Mossbauer spectroscopy. The Raman scattering analysis shows that Mg-dominant species can be immediately distinguished from Fe-dominant species by the shape of the vibrational modes at ~200–240 cm −1 arising from the YO 6 vibrations. Trivalent Fe can be observed and quantified by shifts of the framework vibrations toward lower wavenumbers. The position of the main ZO 6 vibrational mode (275–375 cm −1 ) can be used to determine the Z Fe 3+ content, while the Y Fe 3+ content can be inferred from the position of the peak at ~315 cm −1 . Fits to the data points indicate that the homovalent substitution of Fe 3+ for Al 3+ leads to a considerably larger downward shift of the ZO 6 vibrational mode than the heterovalent substitution Mg 2+ for Al 3+ . The intensity ratio of the two major YO 6 vibrational modes (200–240 cm −1 ) of the fully characterized Fe 3+ -bearing samples reflects the amount of Y-site Mg and thus can be used to deduce the site-occupancy disorder of Mg over the Y and Z site for tourmaline species with Mg ≤2 apfu. By combining the information from framework and OH-stretching vibrations, Raman spectroscopy alone can be used as a micrometer-scale sensitive non-destructive method for the analysis of tourmaline crystal chemistry including trivalent Fe, which is the major tracer for oxygen fugacity and central for intersite geothermometry.

Crystal-chemical features of thermal polymorphism of actinides

Crystal-chemical analysis of actinide baric polymorphs was performed on the basis of Voronoi–Dirichlet (VD) tessellation. The second moment of inertia of the VD polyhedron (G3) and the displacement of the atom nucleus from the center of gravity of its VD polyhedron (DA) were suggested as descriptors acting as criteria for revealing 5f interactions between the actinide atoms. Pressure elevation is accompanied, as a rule, by an increase in G3 for Th, Pa, Am, Cm, Bk, and Cf, whereas for U and Pu it leads to a decrease in G3. Topologically equivalent flat (Th-hP) or corrugated (Pa-hP, α-U, Вk-III) 44 networks are formed in these high-pressure phases owing to the shortest (covalent) bonds between the actinide atoms, whereas a diamond-like framework is formed in the structures of An-III or An-IV (An = Am, Cm, Cf) and Cm-V, and Pu4 molecules are formed in hexagonal Pu-hP.