- University of Cambridge, Department of Earth Sciences, Faculty Memberadd
- Geochemistry, Biomineralisation, Synchrotron Radiation, Neutron Diffraction, Extreme Conditions (Physics), Geophysics, and 17 moreMicrostructural Evolution, Mechanical Behavior Of Materials, Phase Transformations, Materials Characterization, X-ray diffraction and Rietveld Method applied areas of physics, chemistry, geology, medical and engineering and materials science., Mineralogy, Thermodynamics, Clay Minerals, Infrared spectroscopy, Disordered Systems, Dielectric Spectroscopy, Earth Sciences, Perovskites, High Pressure, Ionic Conductivity, Quartz, Laser-Matter interactions, and Ferroic Physicsedit
- Dean of the College of Science and President's Chair in Earth Sciences (Asian School of the Environment & School of M... moreDean of the College of Science and President's Chair in Earth Sciences (Asian School of the Environment & School of Materials Science and Engineering), NTU Singapore.
Emeritus Professor in Mineral Physics, Department of Earth Sciences, University of Cambridge (and former Head of Department)
Emeritus Fellow, Jesus College, University of Cambridgeedit - Ekhard Saljeedit
Phase transformations occur in most types of materials, including ceramics, metals, polymers, diverse organic and inorganic compounds, minerals, and even crystalline viruses. They have been studied in almost all branches of science, but... more
Phase transformations occur in most types of materials, including ceramics, metals, polymers, diverse organic and inorganic compounds, minerals, and even crystalline viruses. They have been studied in almost all branches of science, but particularly in physics, chemistry, engineering, materials science and earth sciences. In some cases the objective has been to produce materials in which phase transformations are suppressed, to preserve the structural integrity of some engineering product, for example, while in other cases the objective is to maximise the effects of a transformation, so as to enhance properties such as superconductivity, for example. A long tradition of studying transformation processes in minerals has evolved from the need to understand the physical and thermodynamic properties of minerals in the bulk earth and in the natural environment at its surface. The processes of interest have included magnetism, ferroelasticity, ferroelectricity, atomic ordering, radiation damage, polymorphism, amorphisation and many others-in fact there are very few minerals which show no influence of transformation processes in the critical range of pressures and temperatures relevant to the earth. As in all other areas of science, an intense effort has been made to tum qualitative understanding into quantitative description and prediction via the simultaneous development of theory, experiments and simulations. In the last few years rather fast progress has been made in this context, largely through an interdisciplinary effort, and it seemed to us to be timely to produce a review volume for the benefit of the wider scientific community which summarises the current state of the art. The selection of transformation processes covered here is by no means comprehensive, but represents a coherent view of some of the most important processes which occur specifically in minerals.
Research Interests: Geology, Geochemistry, Mineralogy, Chemistry, Crystallization, and 15 moreVolatiles in Mantle Processes, Quartz, fluid Inclusions, Diamonds, Mantle metasomatism, Melt Inclusions, High Temperature, Diamond, Infrared spectra, IR Spectroscopy, Ambient Temperature, Sub, Fluid Inclusion, Mantle Fluids, and FT IR
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A stable 6-fold coordinated AuF4 compound with anomalous Au4+ is calculated to be stable under compression.
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... As early as 1983, Nord and co-workers had employed neutron diffraction for the study of NiFe cation distributions in the olivine-related phosphate sarcopside [30, 31]. ... [52] of a typicalhigh-pressure phengitic mica of com-position... more
... As early as 1983, Nord and co-workers had employed neutron diffraction for the study of NiFe cation distributions in the olivine-related phosphate sarcopside [30, 31]. ... [52] of a typicalhigh-pressure phengitic mica of com-position K0.9[Mg0.58Al1.43][Si3.57Al0.43]O10 ...
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Research Interests: Geology and Geochemistry
NMR DataThis research data supports “Melt-Quenched Glasses of Metal-Organic Frameworks” which has been published in “Journal of the American Chemical Society".http://dx.doi.org/10.1021/jacs.5b13220This work was supported by the EPSRC... more
NMR DataThis research data supports “Melt-Quenched Glasses of Metal-Organic Frameworks” which has been published in “Journal of the American Chemical Society".http://dx.doi.org/10.1021/jacs.5b13220This work was supported by the EPSRC [grant number EP/M00869X/1]
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Metal halide perovskites (MHPs) are of great interest for optoelectronics because of their high quantum efficiency in solar cells and light-emitting devices. However, exploring an effective strategy to further improve their optical... more
Metal halide perovskites (MHPs) are of great interest for optoelectronics because of their high quantum efficiency in solar cells and light-emitting devices. However, exploring an effective strategy to further improve their optical activities remains a considerable challenge. Here, we report that nanocrystals (NCs) of the initially nonfluorescent zero-dimensional (0D) cesium lead halide perovskite CsPbBr exhibit a distinct emission under a high pressure of 3.01 GPa. Subsequently, the emission intensity of CsPbBr NCs experiences a significant increase upon further compression. Joint experimental and theoretical analyses indicate that such pressure-induced emission (PIE) may be ascribed to the enhanced optical activity and the increased binding energy of self-trapped excitons upon compression. This phenomenon is a result of the large distortion of [PbBr] octahedral motifs resulting from a structural phase transition. Our findings demonstrate that high pressure can be a robust tool to ...
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Pressure quenching of optical emission largely limits the potential application of many materials in optical pressure-sensing devices, since emission intensity is crucially connected to performance. Boosting visible-light emission at high... more
Pressure quenching of optical emission largely limits the potential application of many materials in optical pressure-sensing devices, since emission intensity is crucially connected to performance. Boosting visible-light emission at high pressure is, therefore, an important goal. Here, we demonstrate that the emission of CdSe nanocrystals (NCs) can be enhanced by more than an order of magnitude by compression. The brightest emission can be achieved at pressures corresponding to the phase transitions in different sized CdSe NCs. Very bright blue emission can be obtained by exploiting the increase in band gap with increasing pressure. First-principles calculations indicate that the interaction between the capping oleic acid (OA) layer and the CdSe core is strengthened with increased Hirshfeld charge at high pressure. The effective surface reconstruction associated with the removal of surface-related trap states is highly responsible for the pressure-induced emission enhancement of th...
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In the past decade, the group V-VI compounds have been widely investigated due to their excellent properties and applications. It is now accepted that diverse stoichiometry can yield new compounds with unanticipated properties, uncovering... more
In the past decade, the group V-VI compounds have been widely investigated due to their excellent properties and applications. It is now accepted that diverse stoichiometry can yield new compounds with unanticipated properties, uncovering potentially new physicochemical mechanisms. However, in this group, aside from the conventional AB-type, no other energetically stable stoichiometry has been reported yet. Here, we report that BiS is unstable and decomposes into stoichiometric BiS and BiS with different Bi valence states upon compression. Encouragingly, we successfully synthesized the predicted BiS phase and thus, confirmed its existence. Our current calculations reveal that the found BiS phase is a semimetal, associated with the increased concentration of nonmetallic S. The present results represent the first counterintuitive stable stoichiometry of group V-VI and provide a good example in designing and synthesizing new compounds under compression.
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High resoln. synchrotron x-ray powder diffraction, in conjunction with magic angle spinning 29Si NMR and electron diffraction, were used to det. structures of synthetic analogs based on the 3 dimensional silicate framework of the natural... more
High resoln. synchrotron x-ray powder diffraction, in conjunction with magic angle spinning 29Si NMR and electron diffraction, were used to det. structures of synthetic analogs based on the 3 dimensional silicate framework of the natural mineral leucite (KAlSi2O6). These structures were refined by the Rietveld method. The structure detns. of dry and hydrothermal synthesized K2MgSi5O12 analogs illustrate the techniques used, and the structural differences between analogs are discussed in terms of cation sizes and synthetic conditions. [on SciFinder(R)]
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Polymeric nitrogen, stabilized by compressing pure molecular nitrogen, has yet to be recovered to ambient conditions, precluding its application as a high-energy density material. Here we suggest a route for synthesis of a tetragonal... more
Polymeric nitrogen, stabilized by compressing pure molecular nitrogen, has yet to be recovered to ambient conditions, precluding its application as a high-energy density material. Here we suggest a route for synthesis of a tetragonal polymeric nitrogen, denoted t-N, via He-N compounds at high pressures. Using first-principles calculations with structure searching, we predict a class of nitrides with stoichiometry HeN that are energetically stable (relative to a mixture of solid He and N) above 8.5 GPa. At high pressure, HeN comprises a polymeric channel-like nitrogen framework filled with linearly arranged helium atoms. The nitrogen framework persists to ambient pressure on decompression after removal of helium, forming pure polymeric nitrogen, t-N. t-N is dynamically and mechanically stable at ambient pressure with an estimated energy density of ~11.31 kJ/g, marking it out as a remarkable high-energy density material. This expands the known polymeric forms of nitrogen and indicates...
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Research Interests: Geology, Geochemistry, Chemistry, Carbonate, Vaterite, and 4 moreCalcite, Aragonite, Fluorine, and Calcium Carbonate
The paragenesis of carbonates in the Cold Bokkeveld CM chondrite is determined from a detailed petrographic, chemical, spectroscopic, and isotopic study of nine associations of carbonates (aragonite, calcite, and dolomite) with other... more
The paragenesis of carbonates in the Cold Bokkeveld CM chondrite is determined from a detailed petrographic, chemical, spectroscopic, and isotopic study of nine associations of carbonates (aragonite, calcite, and dolomite) with other secondary minerals that occur within the meteorite. Our study reveals the existence of carbonates displaying petrographic features that are distinct from those of type 1 and type 2 carbonates commonly observed in CM2 meteorites. These include carbonates interstitial to octahedral magnetite crystals, for which a new designation of “type 1c” is suggested. The O isotopic values of dolomite (δ18O ranging from +21.1 to +25.8‰ and Δ17O from −4.9 to −4.0‰) are similar to those measured in dolomites from other CM chondrites. The presence of complex carbonates with a CaCO3 core and Mg‐enriched rim implies several generations of fluids and/or their evolving composition on the CM parent body(ies). Petrographic characteristics indicate at least six stages of potent...
Research Interests: Geology, Geochemistry, Mineralogy, Chemistry, GE, and 6 moreCarbonate, Dolomite, Petrography, Calcite, Aragonite, and Meteorite
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With "easy" ores becoming diminished, extractive industries are now shifting towards difficult deposits. In the future, gold-arsenic-bearing refractory ores will represent a prime example of the type of ores that may become more... more
With "easy" ores becoming diminished, extractive industries are now shifting towards difficult deposits. In the future, gold-arsenic-bearing refractory ores will represent a prime example of the type of ores that may become more typical sourse for global gold recovery operations. Mining and beneficiation of As-bearing ores inevitably produce As-bearing wastes, which exacerbate any natural As mobilization. The mobility of As is governed by the interplay of redox reactions, adsorption/desorption, ion exchange, precipitation/dissolution, and biotransformation. The dominant processes depend on local biogeochemical conditions of the media, such as pH, Eh, chemical composition, as well as the presence and intensity of biological activity. This article provides an overview of current research on arsenic contamination of the environment caused by mineralization, mining and extraction of gold on the example of specific gold deposits.
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In this work, we demonstrate a remarkable change of chemical trend of Iron under high pressure that is of great importance for understanding the distribution of elements in the Earth's mantle and core. Using first principles crystal... more
In this work, we demonstrate a remarkable change of chemical trend of Iron under high pressure that is of great importance for understanding the distribution of elements in the Earth's mantle and core. Using first principles crystal structure search method, we conduct a systematic study of the propensity of p block elements to chemically bind with iron under high pressures ranging from ambient conditions to that of Earth's core. We show that under increasing pressure, iron tends to reverse its chemical nature, changing from an electron donor (reductant) to an electron acceptor, and oxidizes p-block elements in many compounds. Such reverse chemistry has a significant impact on the stoichiometries, bond types and strengths, structures and properties of iron compounds under deep planetary conditions.
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Metal-organic frameworks (MOFs) are microporous materials with huge potential as host structures for chemical processes, including retention, catalytic reaction, or separation of guest molecules. Structural collapse at high-pressure, and... more
Metal-organic frameworks (MOFs) are microporous materials with huge potential as host structures for chemical processes, including retention, catalytic reaction, or separation of guest molecules. Structural collapse at high-pressure, and unusual behaviours at elevated temperatures, such as melting and transitions to liquid states, have recently been observed in the family. Here, we show that the effect of the application of simultaneous high-pressure and -temperature on a MOF can be understood in terms of silicate analogues, with crystalline, amorphous and liquid states occurring across the pressure - temperature phase diagram. The response of ZIF-62, the MOF on which we focus, to simultaneous pressure and temperature reveals a complex behaviour with distinct high- and low- density amorphous phases occurring over different regions of the pressure-temperature space. In-situ powder X-ray diffraction, Raman spectroscopy and optical microscopy reveal that the stability of the liquid MOF...
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Electrocaloric (EC) materials show promise in eco-friendly solid-state refrigeration and integrable on-chip thermal management. While direct measurement of EC thin-films still remains challenging, a generic theoretical framework for... more
Electrocaloric (EC) materials show promise in eco-friendly solid-state refrigeration and integrable on-chip thermal management. While direct measurement of EC thin-films still remains challenging, a generic theoretical framework for quantifying the cooling properties of rich EC materials including normal-, relaxor-, organic- and anti-ferroelectrics is imperative for exploiting new flexible and room-temperature cooling alternatives. Here, we present a versatile theory that combines Master equation with Maxwell relations and analytically relates the macroscopic cooling responses in EC materials with the intrinsic diffuseness of phase transitions and correlation characteristics. Under increased electric fields, both EC entropy and adiabatic temperature changes increase quadratically initially, followed by further linear growth and eventual gradual saturation. The upper bound of entropy change (∆Smax) is limited by distinct correlation volumes (V cr ) and transition diffuseness. The lin...
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Research Interests: Geology, Geochemistry, Chemistry, Infrared spectroscopy, In situ, and 4 morePhonon, Infrared, Sub, and Phlogopite
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The calcite tests of foraminifera lie in marine sediments for thousands to millions of years, before being analyzed to generate trace element and isotope paleoproxy records. These sediments constitute a distinct physio‐chemical... more
The calcite tests of foraminifera lie in marine sediments for thousands to millions of years, before being analyzed to generate trace element and isotope paleoproxy records. These sediments constitute a distinct physio‐chemical environment from the conditions in which the tests formed. Storage in sediments can modify the trace element and isotopic content of foraminiferal calcite through diagenetic alteration, which has the potential to confound their paleoceanographic interpretation. A previous study of Globorotalia tumida from the Ontong Java Plateau, western equatorial Pacific, found that preferential dissolution of higher‐Mg chamber calcite and the preservation of a low‐Mg crust on the tests significantly reduced whole‐test Mg/Ca and Sr/Ca. Here we revisit specimens with a combination of synchrotron X‐ray computed tomography (sXCT) and electron probe microanalyses to reevaluate the nature of their diagenetic alteration. The dissolution of higher‐Mg calcite with depth was directl...