Asel Sartbaeva

Li + ion motion in quartz and β-eucryptite studied by dielectric spectroscopy and atomistic simulations

Li+ motion in quartz and β-eucryptite (LiAlSiO4) is investigated by dielectric spectroscopy and classical simulations. Simulations are performed using a combination of traditional energy-minimization (using the GULP code) and a mechanical approach derived from Rigid Unit modelling, implemented in the GASP code. Comparison of the simulation approaches shows that geometrical modelling of cooperative framework motions can be combined with electrostatic and Buckingham interactions to give meaningful results for motion of interstitial ions in quartz frameworks. The experimental results can be accounted for on the basis of Li+ motion in the presence of Al substitutional defects.

A neutron diffraction and Rietveld analysis of cooperative Li motion in beta-eucryptite

Li + mobility and structural changes in the aluminosilicate beta-eucryptite have been studied by high-resolution powder neutron diffraction as a function of temperature from 573 to 873 K. A Li split-atom model was used to describe the disorder of Li + ions in the structure. Analysis of the data shows two anomalies around 698 and 758 K. The first anomaly is attributed to an Mpoint zone boundary transition reported i nt he literature, which is connected to the appearance of the incommensurate structure. The second anomaly is attributed to a displacive phase transition of the framework and doubling in the a cell parameter.

Template-Based Geometric Simulation of Flexible Frameworks

Specialised modelling and simulation methods implementing simplified physical models are valuable generators of insight. Template-based geometric simulation is a specialised method for modelling flexible framework structures made up of rigid units. We review the background, development and implementation of the method, and its applications to the study of framework materials such as zeolites and perovskites. The “flexibility window” property of zeolite frameworks is a particularly significant discovery made using geometric simulation. Software implementing geometric simulation of framework materials, “GASP”, is freely available to researchers.

Ionic diffusion in quartz studied by transport measurements, SIMS and atomistic simulations

Ionic diffusion in the quartz–β-eucryptite system is studied by DC transport measurements, SIMS and atomistic simulations. Transport data show a large transient increase in ionic current at the α–β phase transition of quartz (the Hedvall effect). The SIMS data indicate two diffusion processes, one involving rapid Li + motion and the other involving penetration of Al and Li atoms into quartz at the phase transition. Atomistic simulations explain why the fine microstructure of twin domain walls in quartz near the transition does not hinder Li + diffusion. (Some figures in this article are in colour only in the electronic version)

Neutron Compton scattering investigation of sodium hydride: From bulk material to encapsulated nanoparticulates in amorphous silica gel

In this study we utilize neutron Compton scattering (NCS) to determine differences in nuclear momentum distributions in NaH, both as bulk material and encapsulated as nanoscale particles (from 20 to 50 nm in diameter) within an amorphous silica-gel matrix (SiGNaH). In addition, elemental Na dispersed in such a matrix is also studied (SiGNa). Data treatment and fitting of experimental spectra yields comparison of the nuclear Compton profiles and radial momentum distributions for the proton in both bulk NaH and nanoscale SiGNaH, with resultant proton kinetic energies being in agreement with previous inelastic neutron studies of bulk NaH. Slight differences in proton radial momentum distributions for bulk and nanoscale systems are witnessed and discussed. The technique of stoichiometric-fixing is applied to the backscattering spectra of each system in order to examine changes in the Na profile width, and NCS is shown to be sensitive to the chemical environment change of this heavier nucleus. Examination of the Si and O profile widths in the gel samples also supports this method.

GASP: software for geometric simulations of flexibility in polyhedral and molecular framework structures

Template-based geometric simulation is a specialised method for modelling flexible framework structures made up of rigid units using a simplified, localised physical model. The strengths of the method are its ability to handle large all-atom structural models rapidly and at minimal computational expense, and to provide insights into the links between local bonding and steric geometry and global flexibility. We review the implementation of geometric simulation in the ‘GASP’ software, and its application to the study of materials including zeolites, perovskites and metal–organic frameworks. The latest version (5) of GASP has significant improvements and extensions, in particular an improved algorithm for relaxation of atomic positions, and the capacity to handle both polyhedral and molecular structural units. GASP is freely available to researchers.

Influence of alkali metal cations on the formation of zeolites under hydrothermal conditions with no organic structure directing agents

Alkali metal cations play an important role in directing formation of zeolite frameworks in the absence of organic structure directing agents. The interplay between Na and Cs cations in directing formation of zeolite RHO is the main focus of this study. Depending on the ratio of Na and Cs in the precursor gel, with no organic structure directing agents, several different zeolites form during synthesis. This indicates that pure zeolite RHO can only be obtained at a very specific Cs to Na ratio. More generally, this may shed light on the bigger question of how zeolite pores form in the absence of organic structure directing agents.

Local probes show that framework modification in zeolites occurs on ammonium exchange without calcination

Framework modification involving a change in the Al environment from tetrahedral to octahedral is frequently reported in ammonium-exchanged zeolites after calcination, steaming, acid wash and other harsh treatments. We characterized Na-Y and partially exchanged NH4-Y zeolites by spectroscopic and diffraction methods in order to determine if NH4 exchange itself causes framework modification. High-resolution MAS SS NMR data show that even without calcination, some Al atoms change their local environment from tetrahedral to octahedral. PDF analysis corroborates this change and shows an increase in the aluminium coordination number. TEM images show no significant change in morphology of the crystallites, and no major changes in crystallinity are detected by X-ray diffraction. A similar result is obtained on Na-A zeolite suggesting that such increase in the coordination number of Al atoms during the exchange is a common feature in zeolites. Local probes, NMR and PDF analysis show changes in the local environments of Al atoms in the framework, which are usually not possible to see in the average structure.

Microwave synthesis of LTN framework zeolite with no organic structure directing agents

We present a new microwave synthesis method of Linde Type N zeolite without organic structure directing agents (OSDAs). Linde Type N zeolite (framework type LTN) is one of the most complex zeolite structures with a very large unit cell. It has been synthesised previously using hydrothermal methods. We also report syntheses of zeolite A with LTA framework without OSDAs, by hydrothermal methods at low temperatures (40–60 °C). The as-synthesised zeolites were characterised using X-ray powder diffraction, solid state nuclear magnetic resonance, and scanning electron microscopy. LTA and LTN frameworks are obtained at lower synthesis temperatures, while sodalite (SOD framework) is produced at higher temperatures, confirming the crystallization of denser zeolites at higher temperatures.

Intrinsic flexibility of porous materials; theory, modelling and the flexibility window of the EMT zeolite framework

Framework materials possess intrinsic flexibility which can be investigated using geometric simulation. We review framework flexibility properties in energy materials and present novel results on the flexibility window of the EMT zeolite framework containing 18-crown-6 ether as a structure directing agent (SDA).