Structural Study of Nano-Sized Gahnite (ZnAl2O4): From the Average to the Local Scale

Spinel gahnite (ZnAl2O4) has been obtained through a hydrothermal synthesis method with a grain size of about 2 nm. The sample was calcined for a few hours at two different temperatures (800 and 900 °C) in order to obtain larger grain sizes to be analyzed by means of powder diffraction with the Rietveld method, and by means of total scattering with the Pair Distribution Function (PDF) method. The idea is to compare the average to the local structure, as a function of increasing grain size. The total scattering data were collected at the European Synchrotron Radiation Facility (ESRF), Grenoble. The samples have been also characterised by means of high resolution Transmission Electron Microscopy (TEM), showing an increasing grain size up to about 9 nm. The average structure presented variations in the inversion degree and an increase in grain size. TEM observations demonstrated that the small crystals are well crystallised: the high resolution images neatly showed the atomic planes, even in the smallest particles. However, the average structure did not properly fit the PDF data in the local region, owing to a slightly different coordination among the octahedra. A new structural model is proposed for the local region of the PDF, that helped our understanding of the differences between a real nanostructured sample and that of a microcrystalline one. The oxygen disorder, due to the inversion grade of the spinel, is demonstrates to be at the basis of the local deviation. No signals of interstitial Zn atoms were detected

Phase stability, elastic behavior, and pressure-induced structural evolution of kalsilite: A ceramic material and high-T/high-P mineral

The phase stability, elastic behavior, and pressure-induced structural evolution of a natural metamorphic kalsilite (ideal formula KAlSiO4) from Punalur (Kerala district in southern India), with P31c symmetry and a K/Na molar ratio of ~350, has been investigated by in situ X-ray single-crystal diffraction up to ~7 GPa with a diamond-anvil cell under hydrostatic conditions. At high-pressure, a previously unreported iso-symmetric first-order phase transition occurs at ~3.5 GPa. The volume compression of the two phases is described by third-order Birch-Murnaghan equations-of-state: V0 = 201.02(1) A3, KT0 = 59.7(5) GPa, K' = 3.5(3) for the low-P polymorph, and V0 = 200.1(13) A3, KT0 = 44(8) GPa, K' = 6.4(20) for the high-P polymorph. The pressure-induced structural evolution in kalsilite up to 7 GPa appears to be completely reversible. The compression of both phases involves tetrahedral rotations around [0001], which close up the channels within the framework. In addition, compression of the low-pressure phase involves tilting of the tetrahedra. The major structural change at the phase transition is an increase in the tilting of the tetrahedra, but with a reversion of the tetrahedral rotations to the value found at ambient conditions. This behavior is in distinct contrast to that of nepheline, which has a tetrahedral framework of the same topology

Cation distribution and valence in synthetic Al-Mn-O and Fe-Mn-O spinels under varying fO2 conditions

The spinel-group minerals, found in a range of igneous rocks, are resistant toweathering and can incorporate several multivalent elements, meaning they have the potential to provide insight into the redox conditions of parental magmas. Naturally occurring spinel can contain varying quantities of Mn, an element which occurs terrestrially and extra-terrestrially as Mn2+, Mn3+, Mn4+ and Mn5+. However, a lack of information on the effects of oxygen fugacity (fO2 ) on: (1) Mn valence state and cation distribution; and (2) on spinel-melt partitioning means that the potential for a Mn-in-spinel oxy-barometer remains largely untested. Here, we use electron probe microanalysis, micro-focus X-ray Absorption Near Edge Structure (XANES) spectroscopy and single-crystal X-ray diffraction (SC-XRD) to investigate cation distribution and valence state in spinels in the Al-Mn-O and Fe-Mn-O systems synthesized at ambient pressure under varying fO2 conditions. In contrast to previous studies, we find that the spectral resolution of the Mn K-edge XANES spectra is insufficient to provide quantitative data onMn valence state and site occupancy, although it does verify that Mn is incorporated as both Mn2+ and Mn3+, distributed over tetrahedral and octahedral sites. Combination of data from XANES and SC-XRD refinements can, however, be used to model Mn, Al and Fe valence and site occupancy. It would be expected thatMn-Fe spinels have the potential to record fO2 conditions in parental melts due to changes to the octahedral site under conditions that were more reducing. However, decoupling the effects of temperature and oxygen fugacity on the TFe3+-TMn2+ exchange in the Mn-Fe spinels remains challenging. In contrast, little variation is noted in Mn-Al spinels as a function of fO2 , implying that crystal chemistry and cation site geometry may significantly influence cation distribution, and by inference, crystal-melt partitioning, in spinel-group minerals

Crystal structure determination of Ca5Nb5O17

Compounds of the homologous series AnBnO3n+2 (where A = Ca, La or SR and B = Ti or NB) with n = 4, 4.5, 5, 6, and 7 have attracted attention because of their interesting physical and electronic properties. Some of these materials are high Tc ferroelectrics, while others are quasi-1D metals which show metal-semiconductor transition at low temperatures [1,2,3]. The latter is discussed in terms of a Peierls transition and charge-density wave, but their occurrence has not been definitely established yet. The basic structure of these compounds ca be derived from an ideal perovskite by stacking layers of corner-sharing BO6 octahedra. Consecutive slabs of octahedral, the thickness of which is determined by n, are displaced with respect to each other by about one-half the body diagonal of the octahedron and are separated by an additional layer of oxygen. In this study, crystals of pentacalcium pentaniobium heptadecaoxide (Ca5Nbn5O17) ( n = 5) were grown by floating-zone melting and the crystal structure at room temperature was determined by single-crystal x-ray diffraction with synchroton radiation using a CCD area detector. The compound crystallized in P21/c with a = 7.735(1) Å, b = 5.4851(5) Å, c = 32.171(17) Å β = 96.83(3) o. structure refinement indicates that the crystal is twinned. The coordination of Nb atoms reveals that the NbO6 octahedra in the middle of the slabs are the least distorted while those at the borders are the ones which are most deformed. Computed valences for Nb5+ also suggest that the extra electron in the 4d orbital of Nb are most likely found in the middle of the octahedral slabs

Structures of perovskite-related layered AnBnO3n+2

Perovskite-related oxidic compounds AnBnO3n+2 (where A = Ca, La or Sr and B = Ti or Nb) with n = 4, 4.5, 5, 6, and 7 have been the subject of much research, because of their one-dimensional metallic behavior which has been attributed to Peierls transition and charge density wave [1,2]. Their structures are derived from the ABO3 perovskite-type structure with additional layers of oxygen separating the slabs of BO6 octahedra which are parallel to the (110) planes. The width of the slab is determined by the oxygen content and is given directly by the parameter n. In this study, single crystals of several of these compounds were prepared by floating-zone melting [1] and their crystal structures were determined at ambient conditions by single crystal X-ray diffraction with synchrotron radiation using a CCD area detector. For some of these compounds superstructures were observed while some could be described by modulated structures. Particular attention is given to the distortions of the BO6 octahedra and their variations across the width of the slabs, the different environments of the A cations, and the chemical ordering of these cations

Stability at high pressure, elastic behavior and pressure-induced structural evolution of "Al(5)BO(9)", a mullite-type ceramic material

Elastic behavior and pressure-induced structural evolution of synthetic boron-mullite "Al5BO9" (a = 5.678(2) \uc5, b = 15.015(4) \uc5 and c = 7.700(3) \uc5, space group Cmc21, Z = 4) were investigated up to 7.4 GPa by in situ single-crystal X-ray diffraction with a diamond anvil cell under hydrostatic conditions. No phase transition or anomalous compressional behavior occurred within the investigated P range. Fitting the P-V data with a truncated second-order (in energy) Birch-Murnaghan Equation-of-State (BM-EoS), using the data weighted by the uncertainties in P and V, we obtained: V0 = 656.4(3) \uc53 and KT0 = 165(7) GPa (\u3b2V0 = 0.0061(3) GPa-1). The evolution of the Eulerian finite strain versus normalized stress (fE-FE plot) leads to an almost horizontal trend, showing that a truncated second-order BM-EoS is appropriate to describe the elastic behavior of "Al5BO9" within the investigated P range. The weighted linear regression through the data points gives: FE(0) = 159(11) GPa. Axial compressibility coefficients yielded: \u3b2a = 1.4(2) 7 10-3 GPa-1, \u3b2b = 3.4(4) 7 10-3 GPa-1, and \u3b2c = 1.7(3) 7 10-3 GPa-1 (\u3b2a:\u3b2b:\u3b2c = 1:2.43:1.21). The highest compressibilities observed in this study within (100) can be ascribed to the presence of voids represented by five-membered rings of polyhedra: Al1-Al3-Al4-Al1-Al3, which allow accommodating the effect of pressure by polyhedral tilting. Polyhedral tilting around the voids also explains the higher compressibility along [010] than along [001]. The stiffer crystallographic direction observed here might be controlled by the infinite chains of edge-sharing octahedra running along [100], which act as "pillars", making the structure less compressible along the a-axis than along the b- and c-axis. Along [100], compression can only be accommodated by deformation of the edge-sharing octahedra (and/or by compression of the Al-O bond lengths), as no polyhedral tilting can occur. In addition, a comparative elastic analysis among the mullite-type materials is carried out

On the labyrinthine world of arsenites : a single-crystal neutron and X-ray diffraction study of cafarsite

The crystal chemistry of a cafarsite sample from the fengitic orthogneisses of the Mt. Leone-Arbola nappe (Lower Penninic), forming the central body of Mount Cervandone and cropping out both in Switzerland and Italy (Alpe Devero area, Verbano-Cusio-Ossola province), was investigated by electron microprobe analysis in wavelength-dispersive mode (EPMA-WDS), single-crystal Raman spectroscopy, and single-crystal X-ray and neutron diffraction at 293 K. The sample of cafarsite of this study was found experimentally to be anhydrous and the chemical formula obtained on the basis of the EPMA-WDS data and structural refinements is the following: Ca1,Ca2 (Ca15.56Na0.44)Σ16 Fe1 (Na0.53Fe2+ 0.17REE0.30)Σ1.00 Mn1,Ti,Fe2 (Ti7.46Fe3+ 4.47Fe2+ 3.20Mn2+ 0.85Al0.11) Σ16.11 As1,As2,As3 (AsO3)28 F F, with the general chemical formula Ca16(Na,Fe2+,REE)(Ti, Fe3+,Fe2+,Mn2+,Al)16(AsO3)28F [or Ca16(Na,Fe2+,REE)(Ti,Fe3+,Al)12(Fe2+,Mn)4(AsO3)28F]. Our experimental findings show that fluorine, which was unconsidered in the previous studies, is a key element. The anhydrous nature of this sample is also confirmed by its Raman spectrum, which does not show any evidence of active bands ascribable to the O-H stretching region. The X-ray and neutron structure refinements provide a structure model that is partially in agreement with the previous experimental findings. The space group (i.e. Pn3) and the unit-cell constant [i.e. 15.9507(4) Å] are conform to the literature data, but the structure of cafarsite, here refined, contains the following building units: three independent AsO3 groups (trigonal pyramids), one CaO6F polyhedron, one CaO8 polyhedron, two independent (Ti,Fe)O6 octahedra, one (Na,Fe,REE)O8 polyhedron, and one (Mn,Fe)O6 octahedron. Connections among polyhedra are mainly due to edge- or vertex-sharing; the AsO3 groups are not connected to each other

Structure of perovskite-related layered AnBnO3n+2

Perovskite-related oxidic compounds AnBnO3n+2 (where A = Ca, La or Sr and B = Ti or Nb) with n = 4, 4.5, 5, 6, and 7 have been the subject of much research, because of their one-dimensional metallic behavior which has been attributed to Peierls transition and charge density wave [1,2]. Their structures are derived from the ABO3 perovskite-type structure with additional layers of oxygen separating the slabs of BO6 octahedra which are parallel to the (110) planes. The width of the slab is determined by the oxygen content and is given directly by the parameter n. In this study, single crystals of several of these compounds were prepared by floating-zone melting [1] and their crystal structures were determined at ambient conditions by single crystal X-ray diffraction with synchrotron radiation using a CCD area detector. For some of these compounds superstructures were observed while some could be described by modulated structures. Particular attention is given to the distortions of the BO6 octahedra and their variations across the width of the slabs, the different environments of the A cations, and the chemical ordering of these cations