Helen E. A. Brand

Human occupation of northern Australia by 65,000 years ago

The time of arrival of people in Australia is an unresolved question. It is relevant to debates about when modern humans first dispersed out of Africa and when their descendants incorporated genetic material from Neanderthals, Denisovans and possibly other hominins. Humans have also been implicated in the extinction of Australia’s megafauna. Here we report the results of new excavations conducted at Madjedbebe, a rock shelter in northern Australia. Artefacts in primary depositional context are concentrated in three dense bands, with the stratigraphic integrity of the deposit demonstrated by artefact refits and by optical dating and other analyses of the sediments. Human occupation began around 65,000 years ago, with a distinctive stone tool assemblage including grinding stones, ground ochres, reflective additives and ground-edge hatchet heads. This evidence sets a new minimum age for the arrival of humans in Australia, the dispersal of modern humans out of Africa, and the subsequent interactions of modern humans with Neanderthals and Denisovans.

Structural evolution of high energy density V3+/V4+ mixed valent Na3V2O2x(PO4)2F3−2x (x = 0.8) sodium vanadium fluorophosphate using in situ synchrotron X-ray powder diffraction

Sodium-ion batteries have become good candidates for energy storage technology. For this purpose it is crucial to search for and optimize new electrode and electrolyte materials. Sodium vanadium fluorophosphates are considered promising cathodes but further studies are required to elucidate their electrochemical and structural behavior. Therefore, this work focuses on the time-resolved in situ synchrotron X-ray powder diffraction study of Na3V2O2x(PO4)2F3−2x (x = 0.8) while electrochemically cycling. Reaction mechanism evolution, lattice parameters and sodium evolution, and the maximum possible sodium extraction under the applied electrochemical constraints, are some of the features that have been determined for both a fresh and an offline pre-cycled cell. The reaction mechanism evolution undergoes a solid solution reaction with a two-phase region for the first lower-potential plateau while a predominantly solid solution behavior is observed for the second higher-potential plateau. Lattice and volume evolution is clearly dependent on the Na insertion/extraction mechanism, the sodium occupancy and distribution amongst the two crystallographic sites, and the electrochemical cycling history. The comparison between the fresh and the pre-cycled cell shows that there is a Na site preference depending on the cell and history and that Na swaps from one site to the other during cycling. This suggests sodium site occupancy and mobility in the tunnels is interchangeable and fluid, a favorable characteristic for a cathode in a sodium-ion battery.

A comprehensive picture of the current rate dependence of the structural evolution of P2-Na2/3Fe2/3Mn1/3O2

Cathodes that feature a layered structure are attractive reversible sodium hosts for ambient temperature sodium-ion batteries which may meet the demands for large-scale energy storage devices. However, crystallographic data on these electrodes are limited to equilibrium or quasi-equilibrium information. Here we report the current-dependent structural evolution of the P2-Na2/3Fe2/3Mn1/3O2 electrode during charge/discharge at different current rates. The structural evolution is highly dependent on the current rate used, e.g., there is significant disorder in the layered structure near the charged state at slower rates and following the cessation of high-current rate cycling. At moderate and high rates this disordered structure does not appear. In addition, at the slower rates the disordered structure persists during subsequent discharge. In all rates examined, we show the presence of an additional two-phase region that has not been observed before, where both phases maintain P63/mmc symmetry but with varying sodium contents. Notably, most of the charge at each current rate is transferred via P2 (P63/mmc) phases with varying sodium contents. This illustrates that the high-rate performance of these electrodes is in part due to the preservation of the P2 structure and the disordered phases appear predominantly at lower rates. Such current-dependent structural information is critical to understand how electrodes function in batteries which can be used to develop optimised charge/discharge routines and better materials.

XPS and NEXAFS study of fluorine modified TiO2 nano-ovoids reveals dependence of Ti3+ surface population on the modifying agent

Crystalline titanium dioxide was synthesised under mild conditions by the thermal degradation of peroxotitanic acid in the presence of a number of fluoride-containing surface modifying agents (NH4F, NH4BF4, NH4PF6, NBu4F, NBu4BF4, NBu4PF6). The resulting materials were characterised by PXRD, SEM, HRTEM, XPS and NEXAFS. Particle phase, size, and surface area were noticeably affected by the choice of surface modifying agent. Both the cation and anion comprising the modifying agent affect the surface Ti3+ population of the materials, with two apparent trends observed: F− > BF4− > PF6− and NBu4+ > NH4+. All materials displayed evidence of fluorine doping on their surfaces, although no evidence of bulk doping was observed.

Structural evolution of mixed valent (V3+/V4+) and V4+ sodium vanadium fluorophosphates as cathodes in sodium-ion batteries: comparisons, overcharging and mid-term cycling

Sodium vanadium fluorophosphates belonging to the Na3V2O2x(PO4)2F3−2x family of compounds have recently shown very good electrochemical performance versus Na/Na+ providing high working voltages (3.6 and 4.1 V) and good specific capacity values. In this work the electrochemical behaviour and structural evolution of two compositions, Na3V2O1.6(PO4)2F1.4 (V3.8+) and Na3V2O2(PO4)2F (V4+), are detailed using time-resolved in situ synchrotron X-ray powder diffraction. For the first time in sodium-ion batteries the effects of overcharging and mid-term cycling are analyzed using this technique. Differences in the composition of both materials lead to different combinations of biphasic and single-phase reaction mechanisms while charging up to 4.3 V and overcharging up to 4.8 V. Moreover, the analysis of particle size broadening of both samples reveals the higher stress suffered by the V4+ compared to the more disordered V3.8+ sample. The more “flexible” structure of the V3.8+ sample allows for maximum sodium extraction when overcharging up to 4.8 V while in the case of the V4+ sample no evidence is shown of more sodium extraction between 4.3 V and 4.8 V. Furthermore, the analysis of both materials after 10 cycles shows the appearance of secondary phases due to the degradation of the material or the battery itself (e.g. electrolyte degradation). This study shows examples of the possible degradation mechanisms (and phases) while overcharging and mid-term cycling which is in turn crucial to making better electrodes, either based on these materials or generally in cathodes for sodium-ion batteries.

Ancient micrometeorites suggestive of an oxygen-rich Archaean upper atmosphere

It is widely accepted that Earth’s early atmosphere contained less than 0.001 per cent of the present-day atmospheric oxygen (O2) level, until the Great Oxidation Event resulted in a major rise in O2 concentration about 2.4 billion years ago. There are multiple lines of evidence for low O2 concentrations on early Earth, but all previous observations relate to the composition of the lower atmosphere in the Archaean era; to date no method has been developed to sample the Archaean upper atmosphere. We have extracted fossil micrometeorites from limestone sedimentary rock that had accumulated slowly 2.7 billion years ago before being preserved in Australia’s Pilbara region. We propose that these micrometeorites formed when sand-sized particles entered Earth’s atmosphere and melted at altitudes of about 75 to 90 kilometres (given an atmospheric density similar to that of today). Here we show that the FeNi metal in the resulting cosmic spherules was oxidized while molten, and quench-crystallized to form spheres of interlocking dendritic crystals primarily of magnetite (Fe3O4), with wüstite (FeO)+metal preserved in a few particles. Our model of atmospheric micrometeorite oxidation suggests that Archaean upper-atmosphere oxygen concentrations may have been close to those of the present-day Earth, and that the ratio of oxygen to carbon monoxide was sufficiently high to prevent noticeable inhibition of oxidation by carbon monoxide. The anomalous sulfur isotope (Δ33S) signature of pyrite (FeS2) in seafloor sediments from this period, which requires an anoxic surface environment, implies that there may have been minimal mixing between the upper and lower atmosphere during the Archaean.

Structural evolution of NASICON-type Li1+xAlxGe2−x(PO4)3 using in situ synchrotron X-ray powder diffraction

Fast Li-ion conducting Li1+xAlxGe2−x(PO4)3 or LAGP ceramics are the most commonly used anode-protecting membranes in new generation Li–air batteries. The electrochemical properties of this solid membrane (electrolyte) are highly dependent on the purity of the phase and the actual amount of Al incorporated into the structure which often deviates from the synthetic inputs for different annealing conditions. Hence, optimizing the annealing temperature range is of great importance to achieve desirable phases and therefore optimized properties. Here in situ synchrotron X-ray diffraction is carried out during the synthesis of LAGP. Starting with ball-milled and calcined LAGP glass powders we observe the structural evolution during the glass to ceramic transition. Sequential Rietveld refinements show that the dominant Al-poor LGP phase transforms into an Al-incorporated LAGP structure at temperatures higher than 800 °C. The c lattice parameter is found to be highly dependent on the temperature and also the amount of Al incorporated into the structure. The relationship between the c lattice parameter and Al concentration in LAGP is evaluated and the correlation can be used to allow the estimation of Al doping. Thus this work allows the lattice parameter to “fingerprint” the dopant concentration.

Moisture exposed layered oxide electrodes as Na-ion battery cathodes

Mn-rich layered oxides of P2 Na2/3Mn0.8Fe0.1Ti0.1O2 have been shown to exhibit a remarkably stable electrochemical performance even after exposure to moisture for extended periods of time. Here, a detailed investigation of the electrochemical performance of pristine, protonated, and hydrated electrodes is reported. Neutron powder diffraction and 23Na NMR are employed in order to correlate the overall electrochemical performance of each electrode with that of the as-synthesized crystal structure. The effects of proton and water (or OH) moieties on the Na+ layers are discussed based on the electrochemical performance of each phase. The complete structural evolution of the protonated and pristine P2 Na2/3Mn0.8Fe0.1Ti0.1O2 electrodes during charge/discharge is determined via in situ synchrotron X-ray diffraction. The protonated phase at the potential cut-offs (1.5–4.2 and 2–4 V) and the applied currents used shows a predominantly solid-solution reaction with little evidence of a secondary phase while the pristine phase shows the formation of secondary phases and typically better electrochemical capacities. Therefore, the formation of the secondary phase, in part, enhances capacity in this system. Thus moisture exposure (and subsequent treatment) of generally P2 electrodes can lead to significantly different structural evolution during charge/discharge reactions and hence observed capacities.

Structure and thermal expansion of sulfuric acid octahydrate

Synchrotron X-ray powder diffraction has been used to structurally characterize crystallization products from 37.8 and 40.5 wt% aqueous sulfuric acid solutions. It is confirmed that, despite speculation in the literature, the structure that predominately crystallized from these solutions is sulfuric acid octahydrate (SAO). The existence of an uncharacterized phase is also noted. It was found that existing models proposed for the crystal structure of SAO did not satisfactorily fit to the data acquired here, and hence a new structure solution was sought. It is reported here that the structure of SAO is contained within a unit cell with I2 symmetry with a = 7.44247 (11), b = 7.4450 (1), c = 26.1168 (3) A, β = 125.0428 (7)°, V = 1184.78 (3) A3 at 80 K. Data were collected at temperatures between 80 and 198 K, which enabled determination of the thermal expansion of SAO.

The first Australian synchrotron powder diffraction analysis of pigment from a Wandjina motif in the Kimberley, Western Australia

Abstract We report the identification of minerals in stratified paint layers from a Wandjina motif in the central Kimberley region, Western Australia, via synchrotron powder diffraction. Interpreting our findings with reference to previous pigment characterisations of Wandjina motifs, we outline the potential of this method for rock art investigations. We particularly highlight the implications of successful major and minor phase identification in very small (~3 µg) pigment samples. The results of this pilot study show that crystallographic data is critical in helping to separate environmental/cultural signatures from post-depositional processes within anthropogenically applied pigments. In Wandjina rock art, crystallography facilitates the examination of the cultural context of rock art production within an assemblage ethnographically known to have undergone regular, ritual repainting.