Dodecahedral Positions Research Articles

Thermoluminescence and optical absorption studies of natural grossular minerals

The optical absorption spectra of two samples of grossular have been measured at room temperature. An intense charge transfer band (UVCT) of iron extends to the visible and near infrared region. Some peaks associated to Fe 3 + ions in tetrahedral and octahedral positions have been identified and their energy levels were computed. Mn 2 + and Fe 2 + ions are responsible with some bands and probably these ions occupy dodecahedral positions. No change in the intensity of optical absorption spectra were found after gamma dose, but only the 505 nm band decreases with irradiation. The OH spectra, consisting of OH overtones at 2750 nm and asymmetric OH bands in the near infrared region were observed in the two samples. The heat treatment produces Fe 2 + → Fe 3 + and Mn 2 + → Mn 3 + by oxidation. This last was observed in sample II only. The thermally stimulated luminescence of both grossular samples has been investigated. Due to differences in iron and manganese concentration, not only a large difference has been observed in their optical absorption behavior, but also a striking difference in their thermoluminescent behavior. Actually, it is not clear whether other impurities such as Ti, Na and K that are present in quite different concentration in grossular I and II are also contributing to the thermoluminescenct properties of both samples.

Incorporation of Trivalent Cations in Synthetic Garnets A3B5O12 (A = Y, Lu−La, B = Al, Fe, Ga)

Static-lattice atomistic calculations have been used to study the solution energy for the incorporation of 13 foreign cations at 3 different lattice positions of 12 synthetic garnets. Trends have been obtained as a function of the ionic radius of the dopant cation, and the predictions about site preference have been compared with both literature and experimental data. The preferred substitution site is mainly determined by the ionic size and has been correctly predicted in all cases. Moreover, the energy difference between the preferred substitution site and the next favored site is relatively small in several cases, and hence the foreign ions can be inserted at two different positions by using the correct stoichiometry. A remarkably different behavior has been encountered for Al garnets, due to the smaller size of the unit cell. In particular, some cations, such as Fe3+ and Ga3+, can be inserted at the dodecahedral position usually occupied by the rare-earth ion. Despite the limitations of the static-lattice approach, the results of the present simulations help in the understanding of the defect chemistry of garnets, which is strongly responsible for the physicochemical properties (such as luminescence and ferrimagnetism) that make garnets interesting for technological applications. Such results lead to the possibility of tuning the optical and luminescence properties of garnets by the formation of different types of solid solutions.

Local atomic three-dimensional real-space structural analysis of icosahedral Mg–Zn–RE (RE = Y or Ho) alloys: strategy, method and models

The local structure of any condensed matter is accessible via an atomic pair distribution function (PDF) analysis. Here, the local atomic structure of the well-ordered face-centred icosahedral (fci)-Mg25Y11Zn64, fci-Ho9Mg26Zn65 and simple icosahedral Ho11Mg15Zn74 quasicrystals have been investigated by PDFs obtained from in-house X-ray and synchrotron powder data. Least-squares refinements using periodic 1/1 and 2/1 approximants as models for the local atomic structure have been performed. They reveal predominantly Frank–Kasper-type coordination polyhedra. The basic building block is a Bergman cluster of about 100 atoms of 14 Å diameter. RE8 cubes of edge length 5.4 Å are part of the cluster occupying pentagon dodecahedral positions. The cluster connection scheme follows in general that of Henley's canonical cell tiling. However, according to our results, overlapping clusters are likely to be present in the quasicrystalline structure. Thus interpenetration seems to be a general feature for clusters in quasicrystals, which complicates the ongoing discussion on their nature.

Effect of substitutional order on the relaxation of aluminum nuclei in Y3−x LuxAl5O12 mixed garnets

The spin-lattice relaxation time of 27Al nuclei residing in the octahedral and tetrahedral oxygen environment in Y3 −x LuxAl5O12 mixed aluminum-yttrium garnets was measured by the NMR method for 0≤x≤3. A maximum in the concentration dependence of the relaxation time was observed for both crystallographic positions at x=0.75; this maximum correlates with the minimum in the NMR linewidth, indicating the ordering obtained as the yttrium and lutetium ions occupy the dodecahedral positions in the garnet lattice.

Mie scattering and charge transfer phenomena as causes of the UV edge in the absorption spectra of natural and synthetic almandine garnets

The UV edge in the electronic absorption spectra of minerals, in many cases influencing their colour, is generally interpreted as the low-energy wing of very strong UV bands caused by ligand–metal charge transfer (CT) transitions (e.g. Burns 1993). However, Mie scattering theory shows that the presence of randomly distributed submicroscopic inclusions with narrow size distribution and a refractive index ni in a matrix with different refractive index nm may give rise to a λ-dependent, band-like scattering (e.g. Kortum 1969). Such scattering bands have so far not been considered as contributing to the UV edge. Single-crystal electronic absorption spectra of eight natural almandine-rich garnets (Alm60–Alm88), two synthetic almandine samples (Alm100), all of different colours, and synthetic spessartine were studied by means of a Zeiss microscope-spectrometer in the range 40 000–20 000 cm−1. Special techniques of spectral measurements with crossed analyzer and polarizer, which enable the registration of the scattering effect directly, were used as well. Four of the above garnets were also investigated using transmission electron microscopy. Different types of inclusions, from 10 to several 100 nm in size, were observed in the garnet matrices. They are abundant in cores of synthetic garnets, but very rare in most natural almandines studied. Electronic absorption spectra of the natural almandine garnets show largely varying UV edge position and, hence, intensity at a given wavenumber which correlates with the intensities of spin-forbidden dd bands of Fe3+ ions at 27 000 and 28 000 cm−1, superimposed on the long energy slope of the UV absorption. There are also positive correlations between Ti4+ and Fe3+ content, the latter recalculated on the basis of garnet stoichiometry, and UV edge intensity. Thus, the presence of Ti4+ and Fe3+ ions in octahedra, even in very low concentrations (0.0n at. pfu), leads to CT phenomena, that probably involve Fe2+ ions in edge-shared dodecahedral position and intensifies ligand- to-metal CT. The different colours of natural almandine garnets with similar Fe2+ contents studied here are caused by this effect. Consistent with the absence of inclusions in most natural garnets studied, λ-dependent scattering plays no role in their UV absorption. In contrast, in synthetic almandine and spessartine crystals, a different intensity of UV absorption was observed in inclusion-free rims and inclusion-enriched cores. Some of the latter demonstrate typical scattering patterns when measured at crossed polarizers.

Thermal decomposition of almandine garnet: Mössbauer study

The thermal decomposition of almandine garnet from Zoltye Vody, Ukraine, has been studied using57Fe Mossbauer spectroscopy. Room temperature Mossbauer spectrum of the initial powdered sample is characterised by one doublet corresponding to Fe2+ in dodecahedral position 24c. In the room temperature spectra of all heated almandine samples, a doublet corresponding to γ-Fe2O3 nanoparticles appeared. Depending on experimental conditions (heating temperature and time), the additional spectral lines of α-Fe2O3 and e-Fe2O3 were observed in Mossbauer spectra.

Valence and Localization of Praseodymium in Pr-Doped Zircon

The valence and localization of the praseodymium species in the zircon matrix of Pr-doped zircon powders obtained by calcination of the metal oxides in the presence of fluxes are analyzed through the use of X-ray diffraction and XAS (XANES and EXAFS) spectroscopy. It is concluded that the Pr cations present a tetravalent state and form a solid solution with the zircon lattice, in which they occupy the triangular dodecahedral positions of Zr. An enrichment of Pr(IV) ions in the outer layers of the pigment particles was also observed by XPS, indicating that they are heterogeneously distributed inside the zircon grains.

Fluorescence line narrowing Zeeman spectroscopy of Cr3+-doped Gd3Sc2Al3O12 garnet crystals. II. Calculation of the lineshape

For pt.I see ibid., vol.4, p.7307 (1992). The R1 line of Cr3+ in an antiferromagnetic garnet crystal Gd3Sc2Al3O12 (GSAG) is broadened inhomogeneously and homogeneously. Site-selective techniques, such as fluorescence line narrowing (FLN), can remove the inhomogeneous broadening created by strain and compositional disorder. The FLN spectrum then reflects the broadening due to processes including spin-spin exchange interaction between Cr3+ ions in octahedral positions and Gd3+ ions in dodecahedral positions in the GSAG unit cell. A magnetic field can remove the homogeneous broadening due to antiferromagnetic exchange. The R1 line is associated with the spin-forbidden 2E to 4A2 transition. In general, the selection rule on the R1 line breaks down through spin-orbit interaction on the Cr3+ ion. Simulations of the FLN spectra of Cr3+:GSAG as a function of both temperature and magnetic field have been used to estimate the magnitude of the spin-spin exchange coupling constant, J approximately=0.25-0.33 cm-1. Such simulations also show that for the 2E to 4A2 transition to antiferromagnetic GSAG the Gd3+ spin in the Cr3+-Gd3+ spin exchange system is conserved in the presence of a magnetic field (B>2T), whereas the selection rule breaks down in the absence of a magnetic field.

Reactions of heteroboranes. Synthesis of [2,2-(PPh3)2-1,2-SePtB10H10]·CH2CL2, its crystal and molecular structure and that of SeB11H11

The reaction (refluxing ethanol) of [Pt(PPh3)4] with closo-SeB11H11 affords two products. One, characterised spectroscopically, is [PtH(PPh3)3][SeB10H11]. The second product, isolated after recrystallisation from dichloromethane–ethanol solution is [2,2-(PPh3)2-1,2-SePtB10H10]·CH2Cl2(1). An X-ray diffraction study shows crystals of (1) to be triclinic, space group P, with two formula units in a cell of dimensions a= 12.837(3), b= 14.534(2), c= 11.898(3)A, α= 96.59(2), β= 107.31(2), γ= 81.89(l)°. Using 5 046 reflections, the structure was refined to R= 0.028. The dodecahedral SePtB10 cage has Se and Pt atoms occupying adjacent sites. The principal dimensions include the bond lengths Pt–Se 2.676(l), Se–B 2.098(9)–2.210(6), Pt–P 2.326(l) and 2.350(2), and Pt–B 2.231(8)–2.31 l(6)A and an interplanar angle of 80.0° between the plane containing the PtP2 unit and that containing the Se and Pt atoms and passing through the mid-point of the B(2)–B(3) bond opposite the Se atom. The crystal and molecular structure of SeB11H11 was studied by X-ray diffraction methods in an attempt to provide more information on Se–B bonding. Crystals of SeB11H11 are cubic, space group Pa3, with four molecules in a cell, a= 9.847(3)A. The structure was refined to R= 0.075 for 164 reflections. The molecule lies on a site with crystallographic symmetry; the electron density and refinement are consistent with scrambling of the Se atom and eleven B–H groups over the twelve dodecahedral positions. This precluded any precise determination of Se–B bond distances.

New garnet compounds with trivalent rare-earth ions on both dodecahedral and octahedral sites

This study deals with preparation of garnets of the type {Nd 3− y R y }[R x Ga 2− x ](Ga 3)O 12, in which R is Lu, Yb, Tm, Er, Ho, or Dy. It has been found that the placement of large rare-earth ions such as Nd 3+ on the dodecahedral sites opens up the lattice so that small rare-earth ions may enter the octahedral sites. For the case of R = Lu single-phase {Nd 3}[Lu 2]Ga 3O 12 (i.e., y = 0, x = 2) has been prepared. Calculations indicate that similar compounds of Yb and perhaps Tm have also been prepared, though impure. To prepare single-phase Yb and Tm compounds with x = 2, it is necessary simultaneously to place determined minimum amounts of R in the dodecahedral positions. This minimum required value of y increases with increasing radius of R. Many single-phase garnet solid solutions have also been prepared; the region in which pure garnets occur is indicated in a ternary diagram.