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  • Author: Giuseppe Cruciani x
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Abstract

A synthetic strontium isomorph (Sr2ZnSi2O7) of the hardystonite structure (Ca2ZnSi2O7) was investigated with X-ray powder diffraction. The average structure (S.G. P-421 m), a = 8.0007(1) Å, c = 5.1722(1) Å, was refined by Rietveld method and a RF2 = 0.0216 was achieved using 394 independent reflections. The results show that Sr-hardystonite has a melilite-type structure with no incommensurate modulations. Its geometrical and bonding features are consistent with trends defined by the other so-called normal and modulated melilite structures.

Abstract

Gmelinite is a natural zeolite whose framework can be described as a parallel stacking of double six rings of tetrahedra in the ABAB sequence. Its space group is P63/mmc with a = 13.76 and c = 10.08 Å. This study describes the topological transformations of its Na-form |Na6.98K0.27Ca0.15(H2O22.43)| [Al7.41Si16.55O48]-GME, which occur when heating in air above 300 °C. Ex situ X-ray single-crystal analysis showed that gmelinite-Na transforms into a new structure with an AFI-type topology at about 300 °C. Its space group is P6/mcc with a = 13.80 and c = 8.50 Å. In situ X-ray powder diffraction patterns highlighted that, in the approximate 330–390 °C temperature range, GME → AFI transformation goes through a new intermediate phase whose topology differs from both GME and AFI. This phase transforms over the space of a few minutes into the AFI-type phase. This new “transient” phase is characterized by the presence of framework tetrahedra, which are only three-connected. Based on real time synchrotron powder diffraction data, the “transient” phase was modeled in space group P31c with a = 13.97 and c = 9.19 Å. Its crystal structure can be seen as an intermediate step between the GME and AFI crystal structures. The existence of this intermediate metastable phase could be due to the ~2 Å difference in the c parameter between the GME and AFI phases. The c parameter value in the “transient” metastable phase, which is roughly intermediate between the c value in GME and AFI, suggests that the “transient” phase exists as a way of avoiding the abrupt collapse of the GME structure along z direction during the GME-AFI topological transformation. The transformation of a natural gmelinite-Na in a material with AFI topology shows that it is possible to obtain Al-rich AFI materials whose properties are of particular importance in evaluating their potential as catalysts and adsorbents.

Abstract

The structural relaxation around the Co2+ ion along the gahnite (ZnAl2O4)-Co-aluminate (CoAl2O4) join was investigated by a combined X-ray diffraction (XRD) and electronic absorption spectroscopy (EAS) approach. Monophasic spinel samples (Zn1-yCoyAl2O4 with y = 0, 0.25, 0.5, 0.75, and 1 apfu) were obtained through solid-state reaction (1300 °C with slow cooling). The cobalt incorporation induces a linear increase of the unit-cell parameter (a) accompanied by an increasing inversion parameter (up to 0.07) so that the Co2+ for Al3+ substitution in the octahedral site is, at a first approximation, the cause of the lattice expansion. However, a careful consideration of T-O distances highlights the role played by an enhanced covalence degree of Zn-O bonds. The optical spectra are characterized by the occurrence of electronic transitions of Co2+ in tetrahedral coordination affected by a strong spin-orbit coupling, causing a threefold splitting of spin-allowed bands. Further complications stem from mixing of quadruplet and doublet states (leading to a consistent intensity gain of spin-forbidden bands) and vibronic effects (producing intense sidebands). Crystal field strength goes from 4187 to 4131 cm-1 with increasing cobalt amount, while the Racah B parameter is in the 744-751 cm-1 range (C ∼3375 cm-1). To achieve a reliable estimation of the local Co-O distance, the tetrahedral distance evolution was recast to eliminate the effects of the inversion degree. By this way, a relaxation coefficient as low as ε = 0.47 was obtained, i.e., significantly smaller than literature data for other spinel systems. The gahnite-Co-aluminate join seems to be constrained by the strong preference of Zn2+ for the tetrahedral site in which its enhanced covalency can be exerted, limiting the cation exchange between tetrahedral and octahedral sites as well as the lattice flexibility.

Abstract

(Al1-xCrx)3+Nb5+O4 (with 0 ≤ x ≤ 0.5) compounds have been investigated through the combination of X-ray powder diffraction (XRPD) and electronic absorption spectroscopy (EAS). In spite of the natural occurrence of AlTaO4, the lack of a mineral with composition AlNbO4 contrasts with the strong geochemical affinity between Nb and Ta elements. Rietveld refinements of XRPD data showed that the effective coordination numbers of the two non-equivalent octahedral sites (M1 and M2) in the AlNbO4 structure are much lower than expected, especially the one mainly occupied by Nb. This is in agreement with the very low crystal field strength values (10Dq) found by EAS for Cr3+ replacing Al at site M2. These findings imply that an unfavorable bonding situation occurs for Nb, Al, and Cr ions in the AlNbO4 structure, which can be regarded as substantially strained compared to AlTaO4, thus explaining the lack of a natural AlNbO4 isomorph. The observed long local Cr-O distances (low 10Dq) reveal that the AlNbO4 lattice is not relaxed as a consequence of the Cr-Al substitution (the relaxation coefficient ε is close to zero) and the AlNbO4 structure appears to follow the Vegard’s law. This is due to the fact that the Cr3+ for Al3+ substitution, for the limited range of solid solution (up to 0.2 apfu at site M2), does not induce any additional octahedral strain in a lattice already significantly strained.

Abstract

The continuous structural transformation of tetragonal analcime (Na15.87Al15.20Si32.64O96·16.3H2O) upon dehydration was studied, using Rietveld structure analysis of temperature- resolved powder diffraction data collected using synchrotron radiation. The variation of the a-c axis length difference and normalized intensity of the (200) reflection as a function of temperature suggest that tetragonal analcime evolves toward a cubic structure at high temperature. The removal of water was accompanied by a spreading of the initial Na sites into many positions bonded to the framework O atoms. The migration of H2O molecules through the [111] channels during dehydration caused the six-member ring apertures to open as widely as possible: this was accompanied by a twisting of the tetragonal prism, constituting the analcime framework, which led to an opposite tilting of tetrahedra connecting the prisms. These modifications induced by water diffusion are not energetically favored because they would increase the elastic energy of the system, and require a substantial thermal activation energy. The analcime framework reached a maximum distortion at about 650 K, the temperature of complete water loss, then underwent a relaxation process during which the T-O-T angles were restored to the starting value. The relative variation of cell volume associated with the opening of wide six-member ring channels during water migration, and then due to the framework relaxation process after complete dehydration, provides an explanation of the ‘‘negative thermal expansion’’ (i.e., volume contraction) effect in dehydrated analcime, which is complementary to that based on the Rigid Unit Modes theory.

Abstract

The crystal structure of albite from Stintino, Sardinia, Italy, was refined at different degrees of disorder to determine the (Si-Al) order-disorder process. Eight X-ray structure refinements were performed on single-crystal data collected at room temperature, after heating at 1050-1090 °C for 3 to 12 days. Average long-range order coefficients S between 0.93 and 0.24 were obtained for different samples. The results indicate that in the (Si-Al) disordering process Al enriches the T1m site more than T2o and T2m sites. This trend continues until both T1o and T1m are occupied by 30% of Al, and T2o and T2m by 20% of Al. No evidence of complete disorder in T1 and T2 sites has been experimentally found to date. The (Si-Al) disordering process induces a clockwise rotation of the four-membered rings of tetrahedra parallel to the (100) plane. An inverse linear relationship between the isotropic equivalent displacement parameter of the Na atom, Beq(Na), and S is interpreted as positional disorder of Na. The variations in the Na-O bond lengths with increasing disorder are explained by the related variations in the bond strengths of tetrahedral cations.

Abstract

Titanium-rich biotites from graphite-bearing metapelitic xenoliths, equilibrated at ca. 850 °C and 7 kbar in the presence of a granitic melt, have been studied through complete chemical analysis and single-crystal XRD refinements. The chemical study combines EMP analyses, hydrogen determination by both SIMS and C-H-N elemental analysis, and Mössbauer spectroscopy. Biotites in the analyzed xenoliths have TiO2 contents ranging from 4.5 to 4.9 wt% and an XFe of 0.67. Their F and Cl contents are negligible, and Fe3+/Fetot ranges from 10 to 16%. The H2O content of the biotites ranges from 2.4 to 2.8 wt%, and a small fraction of H is accommodated in the lattice as NH4. Based on these full chemical analyses, the calculated OH occupancy is 1.26 to 1.30 groups per formula unit, more than one third less than the stoichiometric value.

The entrance of Ti in the octahedral site of biotite is consistent with the Ti-oxy exchange, whereas Ti-Tschermak or Ti-vacancy substitutions play a very minor role. The Fe3+-oxy exchange cannot account for the observed OH deficiency.

From single-crystal XRD, biotites belong to the 1M polytype and contain variable amounts of stacking faults. The c cell parameter, K-O4 and <K-O>outer distances provide an independent estimate of the OH content, which agrees with SIMS determinations. The linear relationship between VITi4+ and the bond-length distortion of the cis-M2 octahedron reveals the partitioning of Ti4+ in M2, and the Ti4+ off-center shift toward O4 supports the occurrence of the Ti-oxy exchange. The ordering of Ti4+ over two non-equivalent M2 sites, which would be favored energetically, is in agreement with the evidence for a third octahedral site suggested by Mössbauer spectroscopy. The biotite dehydrogenation combined with the partitioning of Ti4+ in M2 and the low thermal expansion of Ti4+- containing octahedra, are the keys to understanding the thermal stabilization of Ti substitution in biotites.

Abstract

The crystal structure of ten different Y, REE (REE: rare earth elements) perovskite-type aluminates, doped with chromium, has been investigated by means of synchrotron powder diffraction in order to assess the crystallographic modifications related to varying the lanthanide ions. The symmetry change from orthorhombic (Y, Yb—Sm) to rhombohedral (Nd and Pr) has been confirmed. The anomalous behaviour of the unit cell parameters (in the Pnma setting, b and c contract while a expands as a function of the ‘lanthanide contraction’) has been clarified on the basis of changes in coordination number of the site containing the Y and lanthanide ion: this varies from approximately eight-fold in Yb-, Er-, Ho-, Y-, and Dy-orthoaluminates to nine-fold (or larger) in Gd-, Eu-, and Sm-orthoaluminates to twelve-fold (as in the archetypal cubic structure) in rhombohedral perovskites (Nd and Pr). The coordination number appears about two units greater in orthoaluminates with respect to the corresponding orthoferrites and an inverse correlation between the average bond distance of the Y, REE site and that of the octahedral Al (and Cr) hosting site has been observed.

Abstract

The isovalent substitution of chromium for aluminium at octahedral site along the Na(Al1-xCrx)P2O7 pyrophosphate solid solution has been investigated by means of the combined application of X-ray powder diffraction (XRPD) and electronic absorption spectroscopy (EAS). In agreement with the structural refinements, deconvolution of the optical spectra revealed a progressive decreasing of the crystal field strength parameter 10Dq moving toward the NaCrP2O7 end-member, meaning that the local chromium–oxygen bond distance increased along the join with the amount of chromium. The calculated structural relaxation coefficient around the substituent ion Cr3+ was ε=0.97 (i.e., very close to that predicted by the Hard Spheres model). A detailed comparison with compact crystal structures (i.e., perovskite, garnet, spinel, clinopyroxene, and corundum) as well as with other pyrophosphate compounds with II-NaMP2O7 structure-type (with M=Al, Cr, Fe, V, Ti, Mo and In) highlighted that compounds belonging to the Na(Al1-xCrx)P2O7 solid solution – characterized by a network connectivity which is basically a framework topology – undergo a structural relaxation that is confined within the first shell (i.e., occurs at the octahedral site), while P2O7 dimers act as rigid units.

Abstract

The Co2+ ion in fourfold coordination provides d-d electronic transitions with the strongest optical density among oxides and silicates. For this reason, it is widely used in pigments and dyes to get blue shades detectable down to a very low cobalt concentration. Such a low-detection limit turns the Co2+ ion into a suitable probe to disclose the local ligand environment in a wide range of materials by means of optical spectroscopy. Even if extensively studied in organometallic complexes, an in-depth investigation of optical properties of Co2+ in tetrahedral coordination into oxidic structures is limited to some case-study in minerals and synthetic analogs (spinel, zincite, gahnite, willemite, calcium cobalt selenite). The present study represents an attempt to outline crystal structural (long-range metal-oxygen distances, O-T-O bond angles, and distortion parameters by XRD) and optical parameters (10Dq, Racah B and C, band splitting by EAS) in 13 samples of oxides and silicates providing a wide set of different local fourfold coordination around Co2+ added as a dopant. Subtle variations of crystal field strength and interelectronic repulsion can be appreciated in gahnite, Ca-Sr-hardystonite, Ca-Sr-Baåkermanite, willemite, Ba2MgSi2O7 melilite-related (where Co2+ substitutes Mg2+ or Zn2+ by 0.25-0.3 apfu) as well as in gehlenite and fresnoite (where Co2+ substitutes Al3+ and Ti4+, respectively, by 0.2 apfu due to charge mismatch). Results are compared with literature data about hibonite, spinel s.s., staurolite, yttrium garnets, and zincite. Spectral interpretation is not straightforward owing to the occurrence of different Co2+ bands: spin-allowed and spin-forbidden electronic transitions, two- or threefold split due to both lowering of point symmetry at the tetrahedron and spin-orbit coupling plus presumably vibronic transitions. Optical spectra vary significantly even for apparently small changes in the long-range CoO4 arrangement as measured by XRD. The expected relationship between 10Dq and the mean Co-O distance is fulfilled, but the accommodation into small AlO4 sites in gehlenite (YAG and hibonite) implies a significant structural relaxation around the Co2+ ion. The threefold splitting of the spin-allowed 4T1(F) and 4T1(P) bands can be related to the angular distortion of the CoO4 tetrahedra. Overall, changes of spectral features of tetrahedrally coordinated Co2+ can be attributed to different local arrangement of ligands with an effect correlated to the second nearest neighbors by the bond valence theory. This was disclosed contrasting 10Dq with the ratio of the observed and ideal bond valence sum for the polyhedra sharing oxygen with the Co-centered tetrahedron.