What is a tunnel? A comment on frequent imprecision when speaking about tunnels, at the example of two representative structures in the octahedral molecular sieve group

A paper by Su et al (2013)[1] is focused on the presentation of \b{eta}-MnO2 (pyrolusite) as a new high-capacity electrode for Na batteries, arguing that there is plenty of place in the tunnels to host sodium, which is clearly contrary to general knowledge. Similar imprecisions are found in other work so that it might be useful to shortly address the issue of cavities and tunnels in a general way. Here, two structures of the octahedral molecular sieve group (\b{eta}-MnO2 pyrolusite and {\alpha}-MnO2 hollandite) are taken as an example and analysed at some depth regarding their tunnel structure using advanced software. Tunnels and their occupation are of fundamental importance for the application of such materials as electrodes or in strongly correlated electron systems.Comment: 11 pages, 2 figures, 1 appendi

The dissolution of monosodium urate monohydrate crystals: formulation of a biocompatible buffer solution with potential use in the treatment of gouty arthropathies

The dissolving abilities (DAs) of several aqueous media for microcrystalline monosodium urate monohydrate (MSU, NaC5N4O3H3·H2O) have been investigated using UV spectrophotometry for quantitative analytical determinations and X-ray diffraction, scanning electron microscopy and polarized light optical microscopy to assess structural aspects. High DAs were found for a buffer labeled TMT which contains tris(hydroxymethyl)aminomethane (TRIS), tris(hydroxymethyl)aminomethane hydrochloride (TRIS·HCl), D-mannitol (MAN) and taurine (TAU) and gave DA30=1298(5) mg/L for synthetic MSU after 30 min incubation at 37°C and pH 7.4, most of the dissolution taking place within the first 5-10 min. Semiempirical molecular modelling techniques (ZINDO/1) show a favorable energy balance for the formation of a TRIS-urate-TRIS adduct which might explain the high DA values. Buffers containing linear or dendrimeric polyamines gave DA values which suggest that complex formation toward sodium cations is less important. An ex vivo MSU sample was found to have a significantly lower DA value (DA30=1124(5) mg/L in TMT) as well as a lower crystallinity than its synthetic counterpart, possibly related to the presence of a non-crystalline impurity such as endogenous proteins. Cytotoxicity tests based on the MTT assay were used to check the biocompatibility of the TMT buffer and showed only moderate cell mortality after 24 h contact with the buffer solution

Crystal structure and Al/Si-ordering of a synthetic nepheline

Nepheline with composition (□0.04K0.48Na0.48)Na3[Al0.99Si11.01O4]4 was synthesized at 1 000 °C/ 1 day from a mixture of the oxides in fused NaF. A small crystal fragment with minimum I(230) :I(320) was used for the collection of X-ray intensity data I(hkl) on an automatic single crystal diffractometer. After LP correction and data reduction in Laue clas., 6/m, a total of 749 structure amplitudes F0(hkl) has been obtained, with an internal consistency error of Ri =0.0075. Symbolic addition in space group P63/m gave unambiguously the positional parameters for all atoms except Ol, the axial oxygen on the triad. Refinement was carried out first in P63/m (R=0.072), and then in P63 (R=0.064) maintaining several geometric constraints in order to avoid correlations due to the pseudo-mirror plane perpendicular to c. The symmetry release is significant and allows for almost perfect Al/Si-ordering on all T-sites. The final model is nearly identical with that of Dollase (1970) for a natural nepheline, and all differences in parameters greater than 3σ can be explained by the different occupation of the big channel at 00z. Comparison of the Al/Si-ordering with that in other known nepheline structures allows the following conclusions : a) the ordering is almost perfect (with T2=T3=Si and T1=T4=Al) and collapses only for serious deviations from the ratio 0.9 ≤ Al/Si ≤ 1.0, b) within these limits, the degree of ordering does not depend upon conditions of crystal formation such as growth rate or thermal history, c) the T-O bond lengths for the 2 twofold (special) positions T1 an T2 show no systematic variation and are likely to be affected by the uncertainty in the orientation of these tetrahedra, as expressed by the splitting of O1 and high anisotropic temperature coefficients for O3 and O4 ; they are therefore of limited value as a criterion for Al/Si-ordering.Une néphéline de composition (□0.04K0.48Na0.48)Na3[Al0.99Si1.01O4]4 a été synthétisée à partir d'un mélange d'oxydes dans NaF fondu à 1 000 °C/1 jour. Pour mesurer les intensités avec un diffractomètre X automatique à monocristal, on a choisi un fragment cristallin présentant un rapport I(230):I(320) minimum. Après correction LP et réduction des données dans la classe de Laue 6/m, on a obtenu, au total, 749 facteurs de structures F0(hkl). L'addition symbolique dans le groupe spatial P63/m conduit sans ambiguïté aux coordonnées de tous les atomes sauf Ol, l'oxygène axial sur l'axe ternaire. L'affinement a été réalisé d'abord en P63/m (R=0,072), et ensuite en P63 (R=0,064) en appliquant diverses restrictions géométriques, afin d'éviter les corrélations causées par le pseudo-plan de symétrie perpendiculaire à c. La désymétrisation est significative et correspond à une distribution Al/Si ordonnée. Le modèle final est pratiquement identique à celui de Dollase (1970) pour une néphéline naturelle, et toutes les variations de paramètres supérieures à 30 peuvent être expliquées comme la conséquence de l'occupation différente des grands canaux situés à 00z. La comparaison de l'ordre Al/Si avec celui des autres structures de néphéline permet les conclusions suivantes : (a) l'ordre est presque parfait, avec T2=T3=Si et T1=T4=A1, et ne se détruit que dans le cas d'une importante déviation du rapport 0,9 ≤ Al/Si ≤ 1,0, (b) entre ces limites, le degré d'ordre ne dépend pas des conditions de croissance cristalline ni de l'histoire thermique, (c) les distances T-O pour les positions spéciales T1 et T2 ne montrent pas de variation systématique, et elles sont sujettes à l'incertitude sur l'orientation de ces tétraèdres, qui s'exprime par le dédoublement de O1 ainsi que par les coefficients de température élevés et anisotropes pour O3 et O4 ; leur valeur comme indicateur de l'ordre Al/Si est donc limitée.Gregorkiewitz Michael. Crystal structure and Al/Si-ordering of a synthetic nepheline. In: Bulletin de Minéralogie, volume 107, 3-4, 1984. Feldspaths et Feldspathoïdes

Sol-gel synthesis of microporous amorphous silica from purely inorganic precursors

Aqueous sodium silicate solutions and hydrochloric acid were used to prepare silica gels without any additives. In order to establish the optimum conditions for the preparation of microporous and monodisperse gels, the sol–gel reaction has been studied as a function of pH, concentration of silicate, time, and temperature. From 29Si NMR spectroscopy it was found that polycondensation sets on immediately after mixing the reagents, with a continuous increase of the fraction of 4-connected silicate tetrahedra (Q4) at the expense ofQ1andQ2. Polycondensation continues beyond the gelation point for 1 ≤ pH ≤ 7, but not for 7 ≤ pH ≤ 10. Two mechanisms were also observed for the gelation kinetics which is about second order with respect to [Si] for pH ≤ 7, whereas for pH > 7, the order increases rapidly with pH. Gels formed at pH > 7 had an electrostatic nature and redissolved in water, whereas for pH ≤ 7, gelation was irreversible as expected for a gel where individual colloid particles are linked by chemical (≡Si—O—Si≡) bonds. From nitrogen adsorption, it is found that the pore size depends mainly upon pH, the finest pores being obtained for pH 2–3 (BET surface area 650–700 m2/g, porosity 35%, pore radius 5–11 Å)

Alkali Cation Diffusion in the Channel System of Hexagonal Nepheline(K,Na,o)Na3[Al3(Al,Si)Si4O16]

Sodium-rich nepheline, (K0.47Na0.48o0.05)Na3[Al3.95Si4.05O16] (o = vacancy), has been synthesized from fused salts. Electron microprobe analyses and X-ray diffraction Rietveld refinement were used to ascertain the composition and the K:Na = 1:1 occupation, leaving 5% vacancies in the large channel||c. Guinier-Lenné powder X-ray diffraction from 100 to 895°C showed no phase transitions and yielded expansivities of a [Å] = 9.971(1) + 2.18(1)·10–4·T [°C] and c [Å] = 8.377(8) + 0.94(1)·10–4·T [°C]. Using polycrystalline pellets, impedance spectra (100–10^7 Hz) were registered at constant temperatures from 100 to 370°C. The dispersion of the permittivity, electric modulus, and impedance functions revealed three features with distinct Arrhenius behavior. Considering (i) anisotropic bulk diffusion within the grains, (ii) aggregation of these grains at random orientation in air, and (iii) intergrain and pellet-electrode contacts, models were developed to simulate the spectra. Agreement with the observed data is obtained for the following assignments: a high-frequency feature (Ea ~ 0.68 eV) corresponds to Na+ movement along the large channel, which is fast but occluded between K+ ions; a low-frequency electric modulus feature (Ea ~ 1.05 eV) corresponds to the movement of the blocking ion (K+) along the large channel; and a low-frequency impedance feature (Ea ~ 1.32 eV) corresponds to Na+ movement within the ab plane. Due to the presence of K+, the near dc conductivity ||c is only (ca. 10 times (at 231°C) higher than ||ab, in contrast with the high anisotropy (ca. 10^4) known for pure sodium nepheline. The accessible volume and its distribution within the framework were calculated for Na+ and K+ and shown to aid in understanding of the diffusion mechanism

Hollandite Nanoparticles for Chemical Sensing: Strain or Pseudosymmetry?

Parm

Preparation of silica-based microporous inorganic gas separation membranes

Fully inorganic silica-based membranes have been prepared using sol-gel procedures starting from sodium silicate solutions. An asymmetric membrane structure is obtained by multiple deposition of colloidal silica sols onto commercial ceramic supports. The microporous structure of the silica which forms the active top layer remains stable up to 873 K and is characterised by a mean pore diameter of 1.6 nm, a narrow pore size distribution and a porosity of 35%. The continuous silica top layer of the membrane was analysed by SEM and checked by gas permeation measurements. The permeabilities of CH4, N2 and CO2 were measured for membranes deposited onto zirconia supports as a function of temperature. The preliminary experimental results indicate reproducible and relatively high fluxes as well as a transport mechanism which basically follows Knudsen flow with contributions from surface and microporous diffusion