Crystal Chemical Reasons Research Articles

Syntheses and Characterization of the Mixed-Valent Manganese(II/III) Fluorides Mn2F5 and Mn3F8

We obtained single crystals of the binary mixed-valent fluorides Mn2F5 and Mn3F8 using a high-pressure/high-temperature approach. Mn2F5 crystallizes isotypic to CaCrF5 in the monoclinic space group C2/c (No. 15), with a = 8.7078(8) Å, b = 6.1473(6) Å, c = 7.7817(7) Å, β = 117.41(1)°, V = 369.80(6) Å3, Z = 4, and mC28 at T = 173 K. Mn3F8 crystallizes in the monoclinic space group P21 (No. 4) with a = 5.5253(2) Å, b = 4.8786(2) Å, c = 9.9124(4) Å, β = 92.608(2)°, V = 266.92(2) Å3, Z = 2, and mP22 at T = 183 K and presents a new structure type. Crystal-chemical reasoning, CHARDI calculations, and quantum-chemical calculations allowed for the assignment of the oxidation states of the Mn atoms. In both bulk compounds, MnF2 was present as an impurity, as evidenced by powder X-ray diffraction and IR and Raman spectroscopy.

Investigation about iron(III) incorporation into layered double hydroxides: Compositional and structural properties of Mg2FeyAl(1−y)(OH)6-Cl and Zn2FeyAl(1−y)(OH)6-Cl

Layered Double Hydroxides (LDH) and related nanocomposites have attracted much attention for biomedical applications and the development of LDH drug carriers composed by endogenous metals such as iron is of obvious interest. However, most of the studies reported so far on iron-containing LDH, mainly focusing on the applications, suffer from insufficient data about the synthesis and the characterization of these materials. In this study, it is addressed compositional and structural properties of two series of LDH materials, Mg2FeyAl(1−y)-Cl and Zn2FeyAl(1−y)-Cl with a M2+/M3+ molar ratio (R) equal to 2 and 0 ≤ y ≤ 1. By combining crystal-chemical reasoning, Rietveld refinements and pair distribution function analysis (PDF), it was possible to differentiate between contributions from crystalline and amorphous components. Concerning Mg-series, for y > 0.5, the compositions were found to slightly deviate from those expected with an increase in the value of R tending to 3. For Zn-series, more heterogeneous samples were obtained with the presence of amorphous 2-line ferrihydrite clearly demonstrated by PDF analysis. As well as providing a reliable approach to the characterization of Fe-LDH, this study gives useful elements for better understanding and interpreting the results reported in the literature regarding these phases.

Targeting complex plutonium oxides by combining crystal chemical reasoning with density-functional theory calculations: the quaternary plutonium oxide Cs2PuSi6O15.

The stability of the novel Pu(iv) silicate, Cs2PuSi6O15, was predicted from a combination of crystal chemical reasoning and DFT calculations and confirmed by its synthesis via flux crystal growth. Formation enthalpies of the A2MSi6O15 (A = Na-Cs; M = Ce, Th, U-Pu) compositional family were calculated and indicated the Cs-containing phases should preferentially form in the Cmc21 structure type, consistent with previous experimental findings and the novel phases produced in this work, Cs2PuSi6O15 and Cs2CeSi6O15. The formation enthalpies of a second set of compositions, A2MSi3O9, were also calculated and a comparison between the two compositional families correctly predicted A2MSi6O15 to be on average more stable than A2MSi3O9.

Special Spatial Symmetry of Mineral Structures

Formation of mineral structures is afected both by energetic and crystal chemical barriers: limitations of possible choice of positions of structural units in a crystal lattice caused by features of its Fedorov group. The correspondence of structures of thermodynamically instable mineral phases to priority (most likely) crystal chemical types provides for their kinetic stability. At the same time, crystallization of rare structure is possible only upon conditions of its energetic stability, because the rarity of the Fedorov group indicates its low «crystal chemical feasibility» of the corresponding structure. The phenomenon of rare Fedorov groups is stable, since it is caused by fundamental crystal chemical reasons. It is most striking in a framework structure of a crystal lattice, because the mineral phases can contain 24 various types of 3D-periodical framework structures. The minerals with framework structure can exhibit the efect of supersymmetry, i.e., stability of the structure with chemically identical but symmetrically independent structural units. This efect may be typical of framework structures corresponding to Fedorov groups P42m, P4m2, I42m, I4m2, P222, C222, F222, I222, P23, F23, I23, P432, P4232, F432, P422, I422, P4222, P6222 and P6422. In particular, this structural efect is typical of cubic modifcation of lazurite with Fedorov group P23.

Phase Diagrams of Solid Solutions of Relaxor Ferroelectrics from the Dielectric Spectroscopy Data

The dielectric spectra of Pb(1–z)Ba z (Mg1/3Nb2/3) m (Zn1/3Nb2/3) y (Ni1/3Nb2/3) n TixO3 (x = 0.25–0.4, y = 0.1130–0.0842, m = 0.4844–0.1298, n = 0.1266–0.4726, z = 0–0.15) ceramics with substitution in both A and B crystallographic positions of the perovskite structure are studied. The system demonstrates a transition from the relaxor state to the normal ferroelectric state in both cases: when the concentration of lead titanate grows and the concentration of barium is reduced. On the basis of experimental results, the x–T and z–T phase diagrams are plotted. Despite different crystal chemical reasons of the relaxor state emergence in the investigated solid solutions, their diagrams demonstrate an evident similarity. We have revealed the disappearance of the temperature hysteresis at the transition to the relaxor state in both cases, which has allowed us to make an assumption of the existence of tricritical points on the corresponding diagrams.

Фазовые диаграммы твердых растворов сегнетоэлектриков-релаксоров по данным диэлектрической спектроскопии

AbstractThe dielectric spectra of Pb(_1– z )Ba_ z (Mg_1/3Nb_2/3)_ m (Zn_1/3Nb_2/3)_ y (Ni_1/3Nb_2/3)_ n TixO_3 ( x = 0.25–0.4, y = 0.1130–0.0842, m = 0.4844–0.1298, n = 0.1266–0.4726, z = 0–0.15) ceramics with substitution in both A and B crystallographic positions of the perovskite structure are studied. The system demonstrates a transition from the relaxor state to the normal ferroelectric state in both cases: when the concentration of lead titanate grows and the concentration of barium is reduced. On the basis of experimental results, the x–T and z–T phase diagrams are plotted. Despite different crystal chemical reasons of the relaxor state emergence in the investigated solid solutions, their diagrams demonstrate an evident similarity. We have revealed the disappearance of the temperature hysteresis at the transition to the relaxor state in both cases, which has allowed us to make an assumption of the existence of tricritical points on the corresponding diagrams.

Foitit-magneziofoitit a rózsa-hegyi ércesedésből, avagy a nagybörzsönyi turmalin újravizsgálata

Turmalin jelenléte a nagybörzsönyi ércesedésben már 60 éve ismert. A Rózsa-hegy É-i felén, az ún. rózsabányai ércesedés területén erősen kovásodott kőzet üregeiben, illetve később képződött ásványok (kvarc, kalcit, sziderit, pirrhotin, szfalerit) zárványaiként általában 1 mm-nél rövidebb, színtelen, tűs kristályokat, tűs-sugaras, vagy nemezszerű halmazokat alkot. A korábban készült nedveskémiai elemzés alapján a szakirodalomba drávitként bekerült turmalin az újabb elektron-mikroszondás vizsgálatok alapján zónás, és a foitit-magneziofoitit ásványfajokat képviseli. A vasban és magnéziumban legdúsabb elemzési pontokon mért összetétel (31 anionra számolva): ([]0,63Na0,29Ca0,08)Σ=1,00 (Fe2+1,26Mg0,89Al0,79)Σ=2,94 Al6,00 (Si5,88Al0,12)Σ=6,00 O18 (BO3)3 (OH)4,00, illetve ([]0,45Na0,37Ca0,18)Σ=1,00 (Mg1,55Fe2+0,48Al0,83Ti0,01)Σ=2,87 Al6,00 (Si5,67Al0,33)Σ=6,00 O18 (BO3)3 (OH)4,00. A röntgen-pordiffrakciós felvételből számított rácsállandók: a = 15,901(6) Å; c = 7,190(4) Å, V = 1574(1) Å3. A turmalin hidrotermás oldatból vált ki, melynek hőmérséklete 260°C fölött lehetett. A képződő turmalin kémiáját elsősorban a hidrotermás oldat összetétele határozta meg, ebben a tekintetben a mellékkőzetek összetétele, illetve az uralkodó hőmérséklet- és nyomásviszonyok csak másodrangúak voltak. A kristályokon megfigyelhető kémiai zónásság elsősorban a Fe ↔ Mg helyettesítésből ered. A zónásság oka az lehet, hogy a Fe-gazdag hidrotermás oldatok összetételét a Mg-ban és Ca-ban gazdagabb leszivárgó vizek megváltoztatták. A turmalin tűs habitusa kristálykémiai okokra vezethető vissza, ugyanis a magas Al-tartalom és a viszonylag alacsony Fe+Mg-tartalom a c-tengely irányában való gyors növekedését segítette elő.

Correlation between the d-value and the M2 +:M3 + cation ratio in Mg–Al–CO3 layered double hydroxides

Twenty-six samples of natural layered double hydroxides (LDHs: hydrotalcite, pyroaurite and quintinite) from various localities worldwide have been studied by means of single-crystal, powder X-ray diffraction and electron microprobe analysis. High crystallinity of samples allowed for the good accuracy of the measured d-values and reliable values of the M2+:M3+ ratios. Based on the obtained data, it has been found out that d-value for LDHs depends upon the M2+:M3+ ratio that can be explained by purely crystal chemical reasons. The ideal d-values for M2+:M3+=2:1 (quintinite group) and 3:1 (hydrotalcite group) are 7.56 and 7.80Å, respectively, whereas intermediate cation ratios correspond to the intermediate d-values. The observed quantitative correlation between d-value and M2+:M3+ cation ratio for carbonate LDHs can be described by the linear function d=0.2385×R+7.0852 (R=M2+:M3+), which allows for the estimation of M2+:M3+ on the basis of d-value and vice versa.

Layered precursors for new zeolitic materials: Synthesis and characterization of B-RUB-39 and its condensation product B-RUB-41

Condensation of layered silicate precursors leads to new, all silica zeolite frameworks. In order to introduce catalytic functionality, boron has been substituted into the silicate layer of RUB-39 in a single step synthesis process. Condensation of the silicate layer to the zeolite framework of RUB-41, RRO framework structure type, preserved B as constituent of the material. Analysis of structural details obtained from Rietveld analysis of powder diffraction data, 11B and 29Si NMR experiments of the as synthesized precursor as well as of the zeolite condensation product, and crystal chemical reasoning indicates segregation of B on one specific T-site. This T-site is buried in the silicate anionic layer of the precursor shielding the additional negative charge introduced by the trivalent T-atom.

The P21/m- C2/m phase transition in synthetic amphiboles in the system Li2ONa2OMgOSiO2H2O: a high-T FTIR study

The P 2 1 / m ↔ C 2/ m phase-transition has been studied by high- T FTIR analysis on a series of synthetic amphiboles in the Li 2 O–Na 2 O–MgO–SiO 2 –H 2 O (LNMSH) system. Spectra were collected in the T range 25–450 °C on KBr disks. All examined amphiboles have P 2 1 / m symmetry at room T . Their OH-stretching FTIR spectrum consists of two main bands at ~3740 and 3715 cm −1 . At the transition temperature ( T c ), these bands merge into one single absorption centred at ~3720 cm −1 , and no further change is observed beyond this T . Significant modifications consisting in peak shifting and band broadening are also observed in the MIR (medium infrared) 1300–640 cm −1 region. T c values for the different compositions were estimated based on various methods; the most reliable procedure is considered to be the fit of Landau 2-4-6 potentials using band shifts observed in the MIR region. The T c values obtained for all samples are consistent with previous results obtained on two members of the series examined here by single-crystal or synchrotron powder H T -XRD (high- T X-ray diffraction). They correlate linearly with the aggregate cation radius at M (4) [ T c (°C) = 803–533 〈 r M (4) 〉; R 2 = 0.97]. This work thus provides a measure of the role played by the size of the M (4) polyhedron in determining the T c in simple chemical systems where the B-site occupancy (and geometry) is the only variable. The slope of the equation is far less steep in the LNMSH system than in cummingtonite; crystal-chemical reasons for this behaviour are discussed, and the local order between A and monovalent B cations is suggested to be the major constraint. In more complex systems, inspection of the available data shows that other factors such as the aggregate size of the strip of octahedra must be taken into account.

Structural and electrical properties of the 2Bi 2O 3·3ZrO 2 system

Powder mixtures of α-Bi 2O 3 (bismite) and monoclinic m-ZrO 2 (baddeleyite) in the molar ratio 2:3 were mechanochemically and thermally treated with the goal to examine the phases, which may appear during such procedures. The prepared samples were characterized by X-ray powder diffraction, differential scanning calorimetry (DSC), electrical measurements, as well as scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The mechanochemical reaction leads to the gradual formation of a nanocrystalline phase, which resembles δ-Bi 2O 3, a high-temperature Bi 2O 3 polymorph. Isothermal sintering in air at a temperature of 820 °C for 24 h followed by quenching to room temperature yielded a mixture of ZrO 2-stabilized β-Bi 2O 3 and m-ZrO 2 phases, whereas in slowly cooled products, the complete separation of the initial α-Bi 2O 3 and m-ZrO 2 constituents was observed. The dielectric permittivity of the sintered samples significantly depended on the temperature. The sintered and quenched samples exhibited a hysteresis dependence of the dielectric shift, showing that the ZrO 2-doped β-Bi 2O 3 phase possess ferroelectric properties, which were detected for the first time. This fact, together with Rietveld refinement of the β-Bi 2O 3/m-ZrO 2 mixture based on neutron powder diffraction data showed that ZrO 2-doped β-Bi 2O 3 has a non-centrosymmetric structure with P 4 ¯ 2 1 c as the true space group. The ZrO 2 content in the doped β-Bi 2O 3 and the crystal chemical reasons for the stabilization of the β-Bi 2O 3 phase by the addition of m-ZrO 2 are discussed.

Structure redetermination and high pressure behaviour of AgCuO 2

AgCuO 2 has been obtained via a low temperature route. The structure has been redetermined with Rietveld refinements using neutron powder diffraction data [ C2/m, a=6.0756(3), b=2.8088(1), c=5.8728(3) Å, β=107.987(4)°, Z=2, R p=6.36%, R wp=7.22%]. The crystal structure consists of chains of ladder-like CuO 4-rectangles running along [010], interconnected by AgO 2-dumbbells. For physical and crystal chemical reasons the oxidation states +I for silver, and +III for copper, respectively, have been assigned. High-pressure X-ray powder diffraction experiments have been made. AgCuO 2 is stable with respect to decomposition and structural distortion up to 36.3 GPa. The zero pressure bulk modulus of AgCuO 2 is estimated as 118.2 GPa.

AgCuO2—Synthesis, Crystal Structure, and Structural Relationships with CuO and AgIAgIIIO2

The new silver cuprate AgCuO2 was synthesized by applying a low-temperature route. AgCuO2 precipitates as a microcrystalline black and thermally labile powder from concentrated aqueous solutions of copper and silver nitrate under alkaline and oxidizing conditions. Using the positional parameters of AgAgO2 as a starting set, Rietveld refinements have been performed using X-ray and neutron powder data (P21/c, a=5.8657(3), b=2.8062(2), and c=6.0770(3) Å, β=108.106(4)°, Z=2, Rp=6.61%, Rwp=5.00% for joint refinement). The crystal structure consists of chains of ladder-like CuO4 rectangles running along [010], interconnected by AgO2 dumbbells. For physical and crystal chemical reasons the oxidation states +I for silver and +III for copper, respectively, have been assigned. Similar to AgAgO2 and CuO the metal cations form a fcc structure with oxygen at positions slightly displaced from the centers of the octahedral vacancies. The small differences from the AgAgO2 structure can be traced back to a different off-center position in the octahedral void.

Crystal-chemical reasons for the immiscibility of periclase and wustite under lithospheric P,T conditions

Crystal-chemical reasons for the immiscibility of periclase and wustite under lithospheric P,T conditions

RUB-23: a new microporous lithosilicate containing spiro-5 building units

A new microporous lithosilicate, Cs 10(Li,H) 14[Li 8Si 40O 96]·12H 2O, was synthesised under hydrothermal conditions. The crystal structure has been solved and refined from single-crystal data in the space group P2 1 to R=14%. The three-dimensional (3D) silicate framework contains lithium as cations on T sites in the centre of spiro-5 units that are charge-balanced by non-framework caesium, lithium and hydrogen cations. The hydrate water in the pores can be removed, and the structure is stable up to ∼300°C. The experimental density of the as-synthesised crystal is 2.62 g cm −3; the framework density is calculated to be 17.7 T/1000 Å. The channel system is one-dimensional for 10-membered ring (10-MR) channels which are connected to 8-MRs running parallel to the 10-MRs. The connection is established by 8-MR pore openings. In principle, this allows the 3D diffusion of (at least) small molecules. Chemical analysis, 29Si, 7Li and 133Cs solid-state nuclear magnetic resonance (NMR) spectroscopy, thermal analysis, infrared (IR) spectroscopy and X-ray structure analysis are all consistent and agree with crystal–chemical reasoning based on the flexibility of [TO 4] units in silicates which are required to build 3-MRs as observed in spiro-5 units.

Optimization of site occupancies in minerals using quadratic programming

Atomic sites with multiple substituents are common in minerals, and correct site assignment of substituents in structure refinement is of fundamental importance. Substituents must be assigned to particular sites to fit the observed site scattering and chemical analysis, but the assignments are rarely made with mathematical rigor. We propose a quadratic programming approach to calculating optimal site assignments, thereby providing crystallographers with a mathematically robust starting point for the determination of site occupancies. Our program, O CCQP, implements this approach within the widely used MATLAB programming environment. User-defined weights may be assigned to the structural formula, site scattering, and bond-valence sums. The program is useful for evaluation of site occupancies in newly refined structures and re-evaluation of previously published structures with ad hoc site assignments. For larger problems, mineralogists incorporate additional information (e.g., spectroscopic analyses) and adjust site occupancies in an attempt to mimic observed bond lengths, sitescattering values, and quantitative chemical data. Rarely, however, are these adjustments made with mathematical rigor. They invariably incorporate simplifying assumptions based on crystal-chemical reasoning. Examples of such simplifications include substituting only species of like-valence at a site, assuming certain species cannot even be considered at a site, assuming only two species may occupy a site, and assuming certain sites are fully occupied. We present a model for optimizing the occupancies of multiply occupied sites of crystals. The approach is based on crystal-structure data and chemical analyses of the compound, without making prior assumptions. Furthermore, this method provides a flexible means of evaluating the trade-off inherent in assigning occupancies on the basis of a single criterion, such as satisfying the observed structural formula exactly. Multiple criteria may be used and their relative importance adjusted interactively. The optimized occupancies obtained by this method provide a rigorous starting point for evaluation by crystallographers; the optimization yields excellent results from minimal assumptions.

X-ray powder diffraction and NMR-spectroscopic investigations on a porous zincosilicate related to the zeolite VPI-7 (VSV)

Structural investigations on a new porous zincosilicate phase, VPI-7#, which is related to VPI-7 were carried out using high resolution X-ray powder diffraction and multinuclear NMR experiments ( 29Si CP MAS and 23Na DOR). The combination of these complementary techniques allowed for the identification of VPI-7# and an understanding of the structural differences between VPI-7# and VPI-7. VPI-7# crystallizes in the triclinic space group P1 with cell parameters a = 40.12(2) A ̊ , b = 10.296(5) A ̊ , c = 10.246(5) A ̊ , α = 90.74(2) °, β = 91.66(2) ° and γ = 87.89(2) °. Taking into account the closely related unit cell parameters and the similarity of the chemical shifts of the 29Si NMR signals in the spectra of VPI-7# and VPI-7 the framework topology of the two zincosilicates is the same. Based on a preliminary Rietveld refinement the Zn-ordering in the zincosilicate framework of VPI-7# is also the same as in VPI-7. 23Na DOR experiments of VPI-7 and VPI-7# show an increase in the number of symmetrically inequivalent, however, well ordered Na-sites. This leads to the conclusion, that the ordering scheme of the nonframework constituents Na and H 2O is different. The conclusion is based on experimental facts as well as crystal chemical reasoning since the number of atoms in the asymmetric unit (216 framework atoms in general sites) by far exceeds the potential of a Rietveld analysis.

The incommensurately modulated(1 − x)Ta 2O 5· xWO 3, 0 ≤ x ≤ 0.267 solid solution

The phase(1 − x)Ta 2O 5 · WO 3, 0 ≤ x ≤ 0.267 has been studied by X-ray powder diffraction and transmission electron microscopy. It was previously described as an infinite series of anion-deficient, α-UO 3-type “line phases,” with compositions resulting from intergrowths of different blocks made up by small numbers of α-UO 3-type cells. More correctly(1 − x)Ta 2O 5· xWO 3, 0 ≤ x ≤ 0.267 is described as an incommensurately modulated structure with a linearly composition-dependent primary modulation wave-vector q prim. = q b*. The underlying orthorhombically distorted α-UO 3-type parent structure has space group symmetry Cmmm ( a ≈ 6.20−6.14, b ≈ 3.66, c ≈ 3.89−3.85Å). Characteristic extinction conditions imply a superspace group symmetry of P : Cmmmm : s, −1,1. The four previously reported crystal structures in the solid solution field are examined by means of apparent valence calculations. Crystal chemical reasons are proposed for the width of the composition range,0 ≤ x ≤ 0.267, observed for the title phase.

Incommensurately modulated ZrO 2− xF 2 x: 0.698 ≤ x ≤ 0.714

The compound ZrO 2− x F 2 x has been studied by X-ray powder diffraction and transmission electron microscopy. It was previously described as a series of anion-deficient, α-UO 3-type “line phases,” with compositions in between resulting from unit cell scale intergrowths of the line phases. More correctly ZrO 2− x F 2 x is described as an incommensurately modulated structure with composition-dependent primary modulation wave-vector, q = x b∗. The underlying orthorhombically distorted α-UO 3-type parent structure has space group symmetry Cmmm ( a ≈ 6.44, b ≈ 3.83, c ≈ 4.07 Å). Characteristic extinction conditions imply a superspace group symmetry of P: Cmmm: s, −1,1. The previously reported crystal structure of Zr 7O 9F 10 is Fourier decomposed in these terms. Crystal chemical reasons are proposed for the very limited composition range, 0.698 ≤ x ≤ 0.714, observed for the title compound.