Structure, phase transitions and ionic conductivity of K_3NdSi_6O_(15)·xH_2O. I. α-K_3NdSi_6O_(15)·2H_2O and its polymorphs

Hydrothermally grown crystals of α-K_3NdSi_6O_(15)·2H_2O, potassium neodymium silicate, have been studied by single-crystal X-ray methods. The compound crystallizes in space group Pbam, contains four formula units per unit cell and has lattice constants a = 16.008 (2), b = 15.004 (2) and c = 7.2794 (7) Å, giving a calculated density of 2.683 Mg m^(−3). Refinement was carried out with 2161 independent structure factors to a residual, R(F), of 0.0528 [wR(F^2) = 0.1562] using anisotropic temperature factors for all atoms other than those associated with water molecules. The structure is based on highly corrugated (Si_2O_5^(2−))_∞ layers which can be generated by the condensation of xonotlite-like ribbons, which can, in turn, be generated by the condensation of wollastonite-like chains. The silicate layers are connected by Nd octahedra to form a three-dimensional framework. Potassium ions and water molecules are located in interstitial sites within this framework, in particular, within channels that extend along [001]. Aging of as-grown crystals at room temperature for periods of six months or more results in an ordering phenomenon that causes the length of the c axis to double. In addition, two phase transitions were found to occur upon heating. The high-temperature transformations, investigated by differential scanning calorimetry, thermal gravimetric analysis and high-temperature X-ray diffraction, are reversible, suggesting displacive transformations in which the layers remain intact. Conductivity measurements along all three crystallographic axes showed the conductivity to be greatest along [001] and further suggest that the channels present in the room-temperature structure are preserved at high temperatures so as to serve as pathways for easy ion transport. Ion-exchange experiments revealed that silver can readily be incorporated into the structure

Structure, phase transitions and ionic conductivity of K_3NdSi_6O_(15)·xH_2O. II. Structure of β-K_3NdSi_6O_(15)

Hydrothermally grown crystals of β-K_3NdSi_6O_(15), potassium neodymium silicate, have been studied by single-crystal X-ray methods. Under appropriate conditions, the compound crystallizes in space group Bb2_1m and has lattice constants a = 14.370 (2), b = 15.518 (2) and c = 14.265 (2) Å. There are 30 atom sites in the asymmetric unit of the basic structure. With eight formula units per unit cell, the calculated density is 2.798 Mg m^(-3). Refinement was carried out to a residual, wR(F^2), of 0.1177 [R(F) = 0.0416] using anisotropic temperature factors for all atoms. The structure is based on (Si_2O_5^(2-))∞ layers, connected by Nd polyhedra to form a three-dimensional framework. Potassium ion sites, some of which are only partially occupied, are located within channels that run between the silicate layers. The silica-neodymia framework of β-K_3NdSi_6O_(15), in particular the linkages formed between the silicate layers and Nd polyhedra, bears some similarities to that of the essentially isocompositional phase α-K_3NdSi_6O_(15)·2H_2O. In both, the silicate layers are corrugated so as to accommodate a simple cubic array of NdO_6 octahedra with lattice constant 7.5 Å. Furthermore, the Si_2O_5 layers in β-K_3NdSi_6O_(15) are topologically identical to those of the mineral sazhinite, Na_2HCeSi_6O_(15). Although β-K_3NdSi_6O_(15) and sazhinite are not isostructural, the structures of each can be described as slight distortions of a high-symmetry parent structure with space group Pbmm

Structure, phase transitions and ionic conductivity of K_3NdSi_6O_(15)·xH_2O. I. α-K_3NdSi_6O_(15)·2H_2O and its polymorphs

Hydrothermally grown crystals of α-K_3NdSi_6O_(15)·2H_2O, potassium neodymium silicate, have been studied by single-crystal X-ray methods. The compound crystallizes in space group Pbam, contains four formula units per unit cell and has lattice constants a = 16.008 (2), b = 15.004 (2) and c = 7.2794 (7) Å, giving a calculated density of 2.683 Mg m^(−3). Refinement was carried out with 2161 independent structure factors to a residual, R(F), of 0.0528 [wR(F^2) = 0.1562] using anisotropic temperature factors for all atoms other than those associated with water molecules. The structure is based on highly corrugated (Si_2O_5^(2−))_∞ layers which can be generated by the condensation of xonotlite-like ribbons, which can, in turn, be generated by the condensation of wollastonite-like chains. The silicate layers are connected by Nd octahedra to form a three-dimensional framework. Potassium ions and water molecules are located in interstitial sites within this framework, in particular, within channels that extend along [001]. Aging of as-grown crystals at room temperature for periods of six months or more results in an ordering phenomenon that causes the length of the c axis to double. In addition, two phase transitions were found to occur upon heating. The high-temperature transformations, investigated by differential scanning calorimetry, thermal gravimetric analysis and high-temperature X-ray diffraction, are reversible, suggesting displacive transformations in which the layers remain intact. Conductivity measurements along all three crystallographic axes showed the conductivity to be greatest along [001] and further suggest that the channels present in the room-temperature structure are preserved at high temperatures so as to serve as pathways for easy ion transport. Ion-exchange experiments revealed that silver can readily be incorporated into the structure

Contribution to the crystal chemistry of lead-antimony sulfosalts: Systematic Pb-versus-Sb crossed substitution in the plagionite homologous series, Pb2N-1(Pb1-xSbx)2(Sb1-xPbx)2Sb6S13+2N

The plagionite homologous series contains four well-defined members with the general formula Pb1+2NSb8S13+2N: fülöppite (N = 1), plagionite (N = 2), heteromorphite (N = 3), and semseyite (N = 4). The crystal structure of several natural and synthetic samples of fülöppite, plagionite, and semseyite have been refined through single-crystal X-ray diffraction, confirming the systematic Pb-versus-Sb crossed substitution observed previously in semseyite and fülöppite. This crossed substitution takes place mainly in two adjacent cation sites in the middle of the constitutive SnS-type layer. The substitution coefficient x appears variable, even for a given species, with the highest values observed in synthetic fülöppite samples. The developed structural formula of the plagionite homologues can be given as Pb2N-1(Pb1-xSbx)2(Sb1-xPbx)2Sb6S13+2N. In the studied samples, x varies between ∼ 0.10 and 0.40. In the ribbons within the SnS-type layer, (Pb=Sb) mixing can be considered the result of the combination, in a variable ratio, of two cation sequences, i.e. (Sb-Sb-Sb)-Pb-Sb-(..), major in plagionite and semseyite, and (Sb-Sb-Sb)-Sb-Pb-(..), major in fülöppite and, probably, in heteromorphite. The published crystal structure of synthetic "Pb-free fülöppite"is revised according to this approach. It would correspond to a Na derivative, with a proposed structural formula of (Na0.5Sb0.5)(Na0.2Sb0.8)2(Na0.3Sb0.7)2Sb6S15, ideally Na1.5Sb9.5S15. In fülöppite, increasing x induces a flattening of the unit cell along c, with a slight volume decrease. Such a general Pb-versus-Sb crossed substitution would attenuate steric distortions in the middle of the SnS-type layer of the plagionite homologous series. Crystallization kinetics seem the main physical factor that controls such an isochemical substitution

Mechanical behavior of polycrystalline ceramics: Brittle fracture of SiC-Si3N4 materials

The first study area involved magnesium oxide and the role of anion impurities, while the second area was directed toward slow crack growth in silicon nitride-silicon carbide ceramics. The oxide program involved development of fabrication techniques for anion doped materials and evaluation of the role of these anions in the hot pressing response, grain boundary diffusion of nickel doped material, grain boundary microhardness, and grain growth

Oxidation mechanism in metal nanoclusters: Zn nanoclusters to ZnO hollow nanoclusters

Zn nanoclusters (NCs) are deposited by Low-energy cluster beam deposition technique. The mechanism of oxidation is studied by analysing their compositional and morphological evolution over a long span of time (three years) due to exposure to ambient atmosphere. It is concluded that the mechanism proceeds in two steps. In the first step, the shell of ZnO forms over Zn NCs rapidly up to certain limiting thickness: with in few days -- depending upon the size -- Zn NCs are converted to Zn-ZnO (core-shell), Zn-void-ZnO, or hollow ZnO type NCs. Bigger than ~15 nm become Zn-ZnO (core-shell) type: among them, NCs above ~25 nm could able to retain their initial geometrical shapes (namely triangular, hexagonal, rectangular and rhombohedral), but ~25 to 15 nm size NCs become irregular or distorted geometrical shapes. NCs between ~15 to 5 nm become Zn-void-ZnO type, and smaller than ~5 nm become ZnO hollow sphere type i.e. ZnO hollow NCs. In the second step, all Zn-void-ZnO and Zn-ZnO (core-shell) structures are converted to hollow ZnO NCs in a slow and gradual process, and the mechanism of conversion proceeds through expansion in size by incorporating ZnO monomers inside the shell. The observed oxidation behaviour of NCs is compared with theory of Cabrera - Mott on low-temperature oxidation of metal.Comment: 9 pages, 8 figure