Papers by Pamela Whitfield
Journal of The Electrochemical Society, Dec 1, 2020
The physiochemical properties of lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) in adiponitr... more The physiochemical properties of lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) in adiponitrile (ADN) electrolytes were explored as a function of concentration. The phase diagram and ionic conductivity plots show a distinct relationship between the eutectic composition of the electrolyte and the concentration of maximum ionic conductivity in the 25 °C isotherm. We propose a structure-based explanation for the variation of electrolyte ionic conductivity with LiTFSI concentration, where the eutectic concentration is a transitionary region at which the structure changes from solvated contact ion pairs to extended units of [Liz(ADN)xTFSIy]z−y aggregates. It is found through diffusion coefficient measurements using pulsed-field gradient (PFG) NMR that both D L i / D T F S I and D L i / D A D N increase with concentration until 2.9 M, where after Li+ becomes the fastest diffusing species, suggesting that ion hopping becomes the dominant transport mechanism for Li+. Variable diffusion-time (Δ) PFG NMR is used to track this evolution of the ion transport mechanism. A differentiation in Li+ transport between the micro and bulk levels that increases with concentration was observed. It is proposed that ion hopping within [Liz(ADN)xTFSIy]z−y aggregates dominates the micro-scale, while the bulk-scale is governed by vehicular transport. Lastly, we demonstrate that LiTFSI in ADN is a suitable electrolyte system for use in Li-O2 cells.
Acta Crystallographica Section A, Aug 29, 2010
The complex and often non-linear structural response of framework structures to changes in pressu... more The complex and often non-linear structural response of framework structures to changes in pressure, temperature, and especially composition (e.g. the plateau effect), is due to the delicate balance of forces between the framework components of strongly-bonded polyhedra and the interactions between the framework and extra-framework species. For example, we have previously shown [1] that whether the tilts of the octahedra in perovskites increase or decrease with increasing pressure depends on the relative strength of the bonding between the framework and the extra-framework cation compared to the strength of the cation-oxygen bonds within the octahedra. Feldspars are far more structurally complex than perovskites and have previously resisted the challenge of quantifying their behaviour in this way. However, Megaw (1974) showed [2] that the rigid-unit deformations of the tetrahedral framework of feldspars can be decomposed in to four tilts of the four tetrahedra that comprise the 4-rings that lie parallel to (010). Of these four tilts Megaw showed that only two, an outward tilt of the T2 tetrahedra and a wrinkle of the 4-ring, change significantly between different alkali feldspars. Analysis of the high-pressure and high-temperature data now available, from both experiment and DFT calculations, confirms that these are the dominant tilt mechanisms. In particular, we have found that changes in the wrinkle tilt are responsible for changes in the length of the feldspar crankshaft, and are thus responsible for 70% of the volume change of alkali feldspars with P, T, or composition.
Powder diffraction, Mar 1, 2024
Meeting abstracts, 2006
not Available.
Powder Diffraction, Nov 17, 2014
In many ways, studies of materials and minerals by powder-diffraction techniques are complementar... more In many ways, studies of materials and minerals by powder-diffraction techniques are complementary, with techniques honed in one field equally applicable to the other. Many of the example techniques described within this paper were developed for analysis of functional materials and subsequently applied to minerals. However, in a couple of cases, the study of new minerals was the initiation into techniques later used in materials-based studies. Hopefully they will show that the study of new minerals structures can provide opportunities to add new methodologies and approaches to future problems. In keeping with the Australian X-ray Analytical Association many of the examples have an Australian connection, the materials ranging from organics to battery materials.
Powder Diffraction, Mar 22, 2023
Dimethyl carbonate (DMC) is an important industrial solvent but is additionally a common componen... more Dimethyl carbonate (DMC) is an important industrial solvent but is additionally a common component of liquid lithium-ion battery electrolytes. Pure DMC has a melting point of 277 K, so encountering solidification under outdoor climatic conditions is very likely in many locations around the globe. Even eutectic, ethylene carbonate:dimethyl carbonate commercial LiPF6 salt electrolyte formulations can start to solidify at temperatures around 260 K with obvious consequences for their performance. No structures for crystalline DMC are currently available which could be a hindrance for in situ battery studies at reduced temperatures. A time-of-flight neutron powder diffraction study of the phase behavior and crystal structures of deuterated DMC was undertaken to help fill this knowledge gap. Three different orthorhombic crystalline phases were found with a previously unreported low-temperature phase transition around 50–55 K. The progression of Pbca → Pbcm → Ibam space groups follow a sequence of group–subgroup relationships with the final Ibam structure being disordered around the central carbon atom.
Powder Diffraction, Jun 1, 2005
The presence of multiple, neighbouring transition metals presents a challenge for powder diffract... more The presence of multiple, neighbouring transition metals presents a challenge for powder diffraction when trying to locate individual elements in a structure. Combining the information in X-ray and neutron data in simultaneous refinements has historically been a powerful tool. However, increasingly complex compositions require more information and element contrast than two datasets can provide. This paper presents results demonstrating the application of resonant powder diffraction as additional datasets in simultaneous X-ray and neutron Rietveld refinements. Final results are presented for a simple layered R-3m structure with four cations, and preliminary results for a more complex monoclinic C2/m structure, again with four different cations. The more complex structure presents challenges with respect to the occupational constraints across sites with different multiplicities whilst allowing the lithium-to-transition metal ratio to refine.
Powder Diffraction, Jun 9, 2016
Powder Diffraction, Dec 1, 2010
The crystal structure of the mineral strontiodresserite, (Sr,Ca)Al 2 (CO 3) 2 (OH) 4 •H 2 O, from... more The crystal structure of the mineral strontiodresserite, (Sr,Ca)Al 2 (CO 3) 2 (OH) 4 •H 2 O, from the Francon Quarry, Montreal, Quebec, Canada, has been solved from laboratory powder diffraction data using a combination of charge-flipping and simulated annealing methods. The structure is orthorhombic in space group Pnma with a = 16.0990(7), b = 5.6133(3) and c = 9.1804(4) Å (Z = 4) and the framework of the mineral is isostructural with that of dundasite. The strontium has a coordination number of 9 and the carbonate anions form a bridge between the SrO 9 polyhedra and AlO 6 octahedra. The water molecule lies in a channel that runs parallel to the b-axis. An ordered network of hydrogen atoms could be uniquely determined from crystal-chemical principles in the channels of strontiodresserite. Ab initio DFT energy minimization of the whole structure gave results in full agreement with X-ray refinement results for non-hydrogen atoms. The stability of this model (as well as that of the corresponding model of dundasite) in the proposed Pnma space group was tested by DFT optimization in space group P1 of random small distortions of this structure. This test confirms that both minerals are isostructural, including their hydrogen-bond networks.
Mineralogical Magazine, Oct 1, 2008
Angastonite, ideally CaMgAl2(PO4)2(OH)4-7H2O, is a newly defined mineral from the Penrice marble ... more Angastonite, ideally CaMgAl2(PO4)2(OH)4-7H2O, is a newly defined mineral from the Penrice marble quarry, South Australia. The mineral occurs as snow-white crusts and coatings up to ∼1 mm thick associated with minyulite, perhamite, crandallite and apatite-(CaF). The streak is white, the lustre is pearly and the estimated hardness is 2 on the Mohs scale. Angastonite forms platy crystals with the forms {010} (prominent), {101}, {101} and {100} (rare), and also occurs as replacements of an unknown pre-existing mineral. There is one cleavage direction on ﹛010} and no twinning has been observed. Angastonite is triclinic, P1̄, with a = 13.303(1) Å, b = 27.020(2) Å, c = 6.1070(7) Å α = 89.64(1)°, β = 83.44(1)°, γ = 80.444(8)°, V = 2150.5(4) Å3, with Z = 6. The mineral is optically biaxial (+), with refractive indices of α = 1.566(2), β = 1.572(2) and γ 1.584(2) and with 2Vmeas = 70(2)° and 2Vcalc = 71°. Orientation: X ≈ a, Y ≈ b, Z ≈ c; with crystals showing parallel extinction and no axial dispersion. Dmeas is 2.47 g/cm3, whilst Dcalc is 2.332 g/cm3. The strongest four powder-diffraction lines [d in Å, (I/I°), hkl] are: 13.38, (100), 020; 11.05, (25), 11̄0; 5.73, (23), 101, 230 and 111; 8.01, (21), 130. Angastonite is likely to be related to the montgomeryite-group members and have a similar crystal structure, based on slabs of phosphate tetrahedra and Al octahedra.
Thin Solid Films, Feb 1, 2003
SrFe Co O (ys0.0, 0.25, 0.50, 0.75, 0.90 and 1.0) thin films on sapphire substrates were prepared... more SrFe Co O (ys0.0, 0.25, 0.50, 0.75, 0.90 and 1.0) thin films on sapphire substrates were prepared by laser deposition, and y 1yy x characterized by elemental analysis using Rutherford backscattering spectroscopy, particle-induced X-ray emission and inductively coupled plasma-atomic emission spectroscopy, X-ray photoelectron spectroscopy, X-ray diffraction and scanning electron microscopy. In order to determine the relative stability of the films at 500 8C, the films were subjected to both thermochemical oxidizing and reducing treatments in 100% O and 2% H yargon gas mixtures, respectively. Thermochemical oxidizing treatments 2 2 of the SrFe Co O films resulted in dark-colored oxygen-rich films. Thermochemical reduction resulted in transparent yellowy 1 yy x brown films for the more iron-rich films, but not for the Co-rich films, which remained dark-colored. Fe-rich SrFe Co O (ys y 1yy x 0.50, 0.75 and 1.0) films exhibited the greatest degree of structural variation, resulting from the cubic perovskite™brownmillerite phase change upon reduction. Films with higher Co substitution showed smaller variations in the unit cell parameters. The SrCoO film was unstable under reducing conditions, with a Sr-enriched carbonate layer forming at the interface after exposure x to ambient atmosphere. XRD measurements conducted in situ at 500 8C in both air and nitrogen gases confirmed that reversible structural changes occur in the film solely as a consequence of changing the surrounding gas composition, with the largest changes in lattice spacing occurring for the SrFe Co O and SrFe Co O films.
Journal of Applied Physics, Oct 23, 2017
This work is an experimental study of intrinsic point defects in off-stoichiometric kesterite typ... more This work is an experimental study of intrinsic point defects in off-stoichiometric kesterite type CZTSe by means of neutron powder diffraction. We revealed the existence of copper vacancies (VCu), various cation anti site defects (CuZn, ZnCu, ZnSn, SnZn and CuZn) as well as interstitials (Cui, Zni) in a wide range of off-stoichiometric polycrystalline powder samples synthesized by solid state reaction. The results show, that the point defects present in off-stoichiometric CZTSe agree with the off-stoichiometry type model, assuming certain cation substitutions accounting for charge balance. Additional to the known off-stoichiometry types A to H new types (I to L) have been introduced. For the very first time a correlation between the chemical composition of the CZTSe kesterite type phase and the occurring intrinsic point defects is presented. Additional to the offstoichiometry type specific defects Cu/Zn disorder is always present in the CZTSe phase. In Cupoor/Zn-rich CZTSe, a composition considered as the one that delivers the best photovoltaic performance, mainly copper vacancies, ZnCu and ZnSn anti sites are present. Also this compositional region shows the lowest degree of Cu/Zn disorder.
Journal of Power Sources, Dec 1, 2007
Solid ionic conductors with good conductivity at room temperature and mechanical flexibility are ... more Solid ionic conductors with good conductivity at room temperature and mechanical flexibility are very attractive candidates for application as electrolytes for secondary lithium batteries. Plastic crystal electrolytes formed by doping succinonitrile with lithium salts can potentially meet these requirements. In this study, succinonitrile doped with various lithium salts were characterized by differential scanning calorimetry, powder X-ray diffraction, conductivity measurements and in electrochemical studies. The much better conductivities and electrochemical performance of certain lithium salts in comparison to the others studied has been found to be correlated to their propensity to form crystalline adducts with succinonitrile at low molar concentrations. Of the lithium salts studied, only two which did not form crystalline adducts at low concentrations: lithium bistrifluoromethanesulphonylimide (Li[CF 3 SO 2 ] 2 N) and lithium tetrafluorborate (LiBF 4) had high enough conductivities at ambient temperature for evaluation in lithium cells. Contrary to prior predictions, both showed reasonable capacities, high coulombic efficiencies, and good capacity retention even with metallic lithium as the anode.
Journal of The Electrochemical Society, 2002
We have investigated the electronic and atomic structure of a manganese-chromium-based layered ox... more We have investigated the electronic and atomic structure of a manganese-chromium-based layered oxide material Li͓Li 0.2 Cr 0.4 Mn 0.4 ͔O 2 during electrochemical cycling using in situ X-ray absorption spectroscopy. Our results indicate that charge compensation in the cathode material is achieved by the oxidation/reduction of octahedral Cr͑III͒ ions to tetrahedral Cr͑VI͒ ions during delithiation/lithiation. Manganese ions are present predominantly in the Mn͑IV͒ oxidation state and do not appear to actively participate in the charge compensation process. To accommodate the large changes in coordination symmetry of the Cr͑III͒ and Cr͑VI͒ ions, the chromium ions have to move between the regular octahedral sites in the R3 m-like lattice to interstitial tetrahedral sites during the charge/discharge process. The highly reversible ͑at least after the first charge͒ three-electron oxidation/ reductions and the easy mobility of the chromium between octahedral and tetrahedral sites are very unusual and interesting. Equally interesting is the fact that chromium is the active metal undergoing oxidation/reduction rather than manganese. Our results also suggest that in the local scale manganese and chromium ions are not evenly distributed in the as-prepared material, but are present in separate domains of Mn and Cr-rich regions.
Powder Diffraction, Jun 1, 2005
Meeting abstracts, 2008
not Available.
Review of Scientific Instruments, Jun 1, 1998
The Polaris high intensity, medium resolution, time-of-flight neutron diffractometer at ISIS has ... more The Polaris high intensity, medium resolution, time-of-flight neutron diffractometer at ISIS has been used in a real-time study to observe phase evolution in rods of melt-cast Bi 1.6 Pb 0.4 Sr 2 Ca 3 Cu 4 O y in situ as they were recrystallized by passage of a dc current under a series of controlled atmospheres. Together with simultaneous measurement of current and resistance to facilitate control over sample heating, the experiment required visual monitoring of the sample during neutron diffraction data collection. This article describes the apparatus constructed to carry out these experiments, along with the necessary modifications made to Polaris and its control systems, and presents results which demonstrate the effectiveness of the technique. The results presented show that the initial appearance of the Bi-2201 phase occurred before hot zone formation. On formation of the hot zone, development of Bi-2212 took place at the expense of Bi-2201. Finally, passage of the zone saw the Bi-2212 redissolve and Bi-2201 reform.
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Papers by Pamela Whitfield