The High Pressure Science and Engineering Center meets every Wednesday in the BPB conference room at 11:30 AM. We typically have a scientific talk, reports on students’ attendance of conferences and workshops and general discussions. We welcome anyone interested in learning about high pressure science and about HiPSEC to attend our meetings. Please e-mail firstname.lastname@example.org if you wish to receive the seminar alert.
Wednesday, December 13, 11:30 am, BPB 217
Structural analysis under High Pressure using X-ray Absorption Spectroscopy
X-ray absorption spectroscopy (XAS) is a very powerful technique for probing coordination chemistry and structural disorder. XAS is an element selective spectroscopic technique that can be used to probe both electronic and structural characteristics of materials. XAS can be broken down into 2 primary regions. The first is called the X-ray Absorption Near Edge Structure (XANES), which is primarily sensitive to the electronic configuration and oxidation state of the absorbing atom. The second region is called the Extended X-ray Absorption Fine Structure (EXAFS), this is the region that is sensitive to coordination and structural disorder. This talk will focus on two different methods of analysis and interpretation of the EXAFS spectrum, and how it relates to high-pressure studies. The first method is fitting a measured EXAFS spectrum to the EXAFS equation using a combination of scattering potentials unique to the atomic configuration of your sample. The second method is based on generating disorder in your structure via running an Evolutionary Algorithm to randomly displace atoms in a super cell, and then using a Reverse Monte Carlo method to compare an EXAFS spectrum generated from the disordered structure to the experimental spectrum.
Wednesday, November 29, 11:30 am, BPB 217
Single-molecule Magnets: “A Lemon Worth Squeezing”
Dr. Christos Lampropoulos
University of North Florida
Single-molecule magnets (SMMs) are a class of molecular magnetic materials, where the magnetic properties are intrinsic to each molecule. Their core structure resembles mixed-valent metal oxides, while their molecular envelope can be altered at will with chemistry. They are monodisperse, crystalline, and can be made soluble in a wide range of solvents. SMMs are mesoscopic systems, as the classical property of magnetization hysteresis coexists with several quantum effects, such as quantum tunneling of the magnetization, quantum phase interference and others. The first SMM reported is a mixed-valent Mn (III/IV) dodecanuclear cluster, which has served as the “Drosophila” of molecular magnetism. It can be easily synthesized in one step from cheap starting materials, it crystallizes in a symmetric space group, and its physical properties are well understood. We have been investigating ways to link members of the Mn12 family of SMMs into dimensional arrays (1D, 2D, and 3D), as well as oligomers. Multidimensional Mn12 networks can be thought of as SMM-based metal organic frameworks (SMM-MOFs), which could have potential applications in gas storage, chemical separations, and spintronics. Oligomeric SMMs on the other hand have potential applications in quantum gates. Investigation of SMMs under pressure has been limited to a handful of reports on monomeric SMMs, and we intend to expand this to polymeric SMM-based coordination polymers. In this talk our synthetic work will be discussed, the structures of the synthesized coordination polymers, the magnetic and spectroscopic properties of the resulting materials, as well as evidence on the usefulness of pressure studies to answer important scientific questions.
Wednesday, November 15, 11:30 am, BPB 217
High pressure/ high temperature synthesis of novel technetium nitride compounds
Recent advances in high pressure/high temperature techniques has led to the synthesis of many new materials with interesting properties. In particular, the exploration of transition metal nitrides has opened up possibilities of new superhard, superconducting and magnetic materials. The goal of this project is to harness the unique properties that light atom compounds can exhibit (high hardness, high melting point, chemical stability, etc.) to create candidate wasteforms for technetium for its long-term storage. Little work has been done experimentally with technetium; however, there are many computational papers that suggest array of different technetium nitride compounds with varying stoichiometries and structures. High pressure/high temperature experiments have been performed with elemental technetium and nitrogen. Starting materials were compressed and laser heated with in situ diffraction at APS sector 16-IDB. Preliminary analysis of the data will be discussed.
Wednesday, November 8, 11:30 am, BPB 217
Structure Predictions for Calcium Oxide at High Pressures-Theory and Experiment
Calcium and oxygen are amongst the most abundant elements in the Earth’s interior. CaO is a widely used compound formed from these elements and is very stable at ambient pressures. Although the crystal structure and chemical composition of CaO seems to be simple and well understood, metastable or stable Ca-O compounds with unexpected stoichiometries most likely exist at high pressures. In my past work, ﬁrst-principles density functional theory calculations and ab intio evolutionary simulations were used to predict high pressure Ca-O structures. I showed that under pressures as low as 10 GPa and as high as 150 GPa that the stability of the Ca-O system changes and new materials emerge with diﬀerent stable or metastable structures. The high pressure phase diagram for these compounds was determined along with the density of states calculations, and stoichiometry plots.
Guided by these theoretical simulations, experimental work has begun involving calcium oxide and a chemical variation, CaO2 which is a peroxide, as a precursor route to accessing oxygen-rich Ca-O stoichiometries. We present a number of different pressure-temperature conditions using the laser heated diamond anvil cell. This prevents new challenges as the theoretical predictions that were the original basis of this work, were based off the dioxide version of CaO2. New calculations have been done and predictions have been made involving this peroxide, which will be compared against experimental results gathered in the last few months. Another primary goal of this work was to explore the various possibilities in the disproportionation of the compound, which is invaluable information for the next phase of this project. The work done thus far has primarily been done to lay the groundwork for the coming larger project within the next year.
Wednesday, October 25, 11:30 am, BPB 217
P-P bonding in metallic GaP
Barbara Lavina and Eunja Kim
We report on the experimental and theoretical characterization of a novel GaP polymorph formed at 43 GPa and 1300 K. The phase adopts the crystal structure with Pearson notation oS24 first observed in InSb. DFT calculations show that oS24 is more stable than other candidate structures, including oS8, the phase hitherto considered the stable polymorph of the semiconductor in this pressure range. Here we show that the emergence of the oS24 structure is attributable to the differentiation of phosphorous atoms in those forming P-P dimers and those coordinated by Ga exclusively. The complex bonding explains the symmetry lowering with respect to what is generally expected for semiconductors high-pressure polymorphs. The metallization of GaP does not occur via a uniform change of the nature of its bonds but through the formation of an anisotropic phase containing different bond types.
Wednesday, September 27, 11:30 am, BPB 217
In-situ experimental study of high pressure ices and liquid thermodynamic properties, implication for water-rich planets structure, evolution and habitability
The presence of several phases of deep high-pressure ices in large icy moons hydrosphere has often been pointed as a major limitation for the habitability of an uppermost ocean. As they are gravitationally stable bellow liquid H2O, they are thought to act as a chemical barrier between the rocky bed and the ocean. Solutes, including salt species such as NaCl and MgSO4, have been suggested inside icy world oceans from remote sensing, magnetic field measurements and chondritic material alteration models. As a matter of facts the pressures and temperatures inside these hydrospheres are very different from the one found in Earth aqueous environments, so most of our current thermodynamic databases do not cover the range of conditions relevant for modeling realistically large icy worlds interiors.
Recent experimental results from our team and others have shown that the presence of solutes, and more particularly salts (Na-Mg-SO4-Cl ionic species), in equilibrium with high pressure ices have large effects on the stability, buoyancy and chemistry of all the phases present at these extreme conditions.
Effects currently being investigated by our research group, and that will be presented, covers ices melting curve depressions, brines density and chemical incorporation effects on the crystallographic structure of HP ice polymorphs.
We will also see the new planetary evolution scenarios suggested by these new material and thermodynamic properties and how this could suggest the existence of new habitable environments in large icy worlds, even when high pressure ices dominate the total volume of the hydrosphere.
Wednesday, September 6, 11:30 am, BPB 217
Application of laser technology for static and dynamic compression experiments
A laser differs from other sources of light in that it emits light coherently. The coherence allows a laser to be focused to a tight spot and also to produce short pulses of light. Such a laser enabling to confine energy is extensively applicable for high-pressure experiments. One of the most useful tool using the laser for the high-pressure experiments is a laser-heated diamond anvil cell (LHDAC). A recent study using this method has achieved to produce the conditions equivalent to the center of the Earth (377 GPa and 5700 K) (Tateno et al., Science, 2010). However, such high P-T conditions can be generated in only opaque materials such as metals because of the light source. We have developed a both-sided CO2-laser heating system to generate extremely high P-T conditions in not only opaque but also transparent materials (Kimura et al., J. Chem. Phys. 2014) and succeeded to determine the melting temperatures of MgO up to ~50 GPa. Our experimental study revealed that MgO has the highest melting temperature in the lower mantle minerals (~6000 K at ~50 GPa) (Kimura et al., Nat. Commun., 2017).
The laser with a temporal coherence can be applied for dynamic compression. A laser-driven shock compression is the most effective method to produce the highest pressure more than 1 TPa. However, a rapid increase in shock temperature with increasing pressure is difficult to suppress since shock conditions are constrained by Rankine-Hugoniot equations. We have developed a diamond anvil cell for a laser-driven shock experiments (Kimura et al., Phys. Plasmas 2010) and demonstrated that off-Hugoniot states where the shock temperatures are lower than that on the principal Hugoniot curve can be produced by coupling the shock loading with the static compression (Kimura et al., J. Chem. Phys. 2015). This fact proposes that this compression technique opens up the opportunity to experimentally access planetary interiors including Uranus, Neptune, Jupiter, and Saturn. I introduce these experimental tools developed by using the laser technology.
Wednesday, August 30, 11:30 am, BPB 217
Studies of HgF2 and XeF2 at extreme conditions
Recent theoretical works have proposed forming mercury fluoride compounds past a +2 oxidation states using DACs. An important but difficult problem in this experiment, would be to provide Mercury Fluoride with an abundance of fluorine. To create excess fluorine in situ, we are studying the decomposition of XeF2 under hard X-rays. In this talk, I will discuss recent results from Argonne.
Wednesday, August 23, 11:30 am, BPB 217
Investigations of thermoelectric material FeSb2 using powder x-ray diffraction and nuclear resonant inelastic x-ray scattering (NRIXS)
Performance of thermoelectric materials is dependent on crystal structure and phonon behavior. Iron diantimonide, FeSb2, has recently gained interest as a thermoelectric material after discovery of a colossal Seebeck coefficient, suggested to be the result of phonon-dragging mechanism within the lattice. For these experiments, Fe-57 enriched FeSb2 was synthesized via solid state reaction. Nuclear resonant inelastic x-ray scattering (NRIXS) measurements conducted at the Sector 16 ID-D beamline of the Advanced Photon Source allow us to study the contribution of iron nuclei to the phonon density of states (PDOS) for this material. NRIXS measurements were performed up to 40 GPa in order to study the PDOS evolution with pressure. These measurements are complemented by a high pressure powder x-ray diffraction study performed up to approximately 60 GPa. Nuclear forward scattering (NFS) measurements were performed to 52 GPa.
Wednesday, August 16, 11:30 am, BPB 217
Topics in Technetium Oxide Chemistry
Our recent reevaluation of molecular oxide Tc2O7, including a new single crystal structure study and electron-impact mass spectroscopy (EI-MS). Differences in the EI-MS indicates Tc2O7 should behave differently compared to Re2O7 in the gas phase. Motivated by this, we investigated the molecular and electronic structures of both the isolated molecules and solid-state structures of the group 7 heptoxides using density functional theory. Bonding analysis of the isolated molecules of each species show the presence of unusual three-center covalent bond between the metal atoms and the bridging oxygen. Differences in the covalent character of this bond explains the observed differences in gas phase molecular structures. The solid-state simulations correctly predict the known crystal structures within a few percent. Analysis of the bonding bands in the solid-state also show the presence of the three-center covalent bond. Homologue studies of Tc2O7 show why a linear molecular configuration is preferred in the solid-state. Homologue and compression studies of Re2O7show why a polymeric structure is adopted instead of a molecular one like Mn and Tc. For decades it has been know that the oxidation of Tc (mainly from TcO2) can result in a red oily, liquid compound. It was proposed that the red product was formed by the hydrolysis of Tc2O7 to HTcO4 followed by the reduction of HTcO4. We investigated several hydrated species of Tc2O7 and HTcO4 with DFT/TD-DFT and show that these compounds do not exhibit the characteristic UV-Vis adsorption band around 510 nm. Electron-Ion mass spectrometry measurements of Tc2O7 indicated several other lower oxidation state candidate stoichiometries. We explored several molecular arrangements of these candidate stoichiometries, and have discovered a bonding motif that closely reproduces the experimentally observed adsorption at 510 nm. In addition, we explored dimer configurations that suggest why the red compound exists as an oily liquid and has not yet been crystallized.
Wednesday, August 9, 11:30 am, BPB 217
Dynamic Viscosity in Metals
Most of us don’t think about viscosity except to realize that honey has a lot of it and water not so much. However, understanding viscosity is key to understanding high temperature, high strain-rate processes. Unfortunately, it is very difficult to predict theoretically and measure experimentally. We present two methods for simulating viscosity and two methods for measuring it experimentally.
Wednesday, July 26, 11:30 am, BPB 217
Synthesis, Elastic, and Electronic Properties of Transition Metal Boride
Transition-metals borides (TMBs) have attracted significant interests because of their broad range of industrial applications such as abrasive, refractory, corrosion-resistant and energy storage, superconductivity, electrode materials, crucibles, and ingot molds for precision metallurgy. Syntheses of new hard borides and understanding their mechanism of deformation under stress remain to be an exciting and active area of research. In this talk, I will discuss some recent progresses in synthesizing borides with high hardness and bulk modulus under high pressure using a D-DIA-type large volume press (LVP) at UNLV
Wednesday, July 19, 11:30 am, BPB 217
Measurement of the Energy and High-Pressure Dependence of X-ray-induced Decomposition of Crystalline Strontium Oxalate
We report measurements of the X-ray induced decomposition of crystalline strontium oxalate (SrC2O4) as function of energy and high-pressure in two separate experiments. SrC2O4 at ambient conditions was irradiated with monochromatic synchrotron X-rays ranging in energy from 15 to 28 keV. A broad resonance of the decomposition yield was observed with a clear maximum when irradiating with ~20 keV X-rays and ambient pressure. Little or no decomposition was observed at 15 keV which is below the Sr K-shell energy of 16.12 keV suggesting that excitation of core-electrons may play an important role in the destabilization of the C2O42- anion. A second experiment was performed to investigate the high-pressure dependence of the X-ray induced decomposition of strontium oxalate at fixed energy. SrC2O4 was compressed in diamond anvil cell (DAC) in the pressure range from 0 to 7.6 GPa with 1 GPa increments and irradiated in situ with 20 keV X-rays. A marked pressure dependence of the decomposition yield of SrC2O4 was observed with a decomposition yield maximum around 1 GPa, suggesting that different crystal structures of the material play an important role in the decomposition process. We suspect that this is due to a phase transition observed near this pressure
Wednesday, July 12, 11:30 am, BPB 217
Thermal motion in technetium heptoxide, Tc2O7
The binary heptoxides of group 7 transition metal form a series of unique structures. Two of the three crystallizing as molecular solids with two metal centers bridged by a single oxygen. Molecular metal oxides stable under ambient conditions are rare and offer unique insight into the chemistry and physics of materials. The XRD structure of Tc2O7 is characterized by centrosymmetric linear molecules with a single bridging oxygen. Using variable temperature SCXRD we have found anomalous thermal expansion behavior between room temperature and 100 K that presents as negative thermal bond expansion. In coordination with the structural study molecular dynamics calculations have been carried out. The thermal motion of the Tc2O7 is described using the combined experimental and theoretical tool.
Monday, July 10, 10:00 am, BPB 217
Instrumentation and Measurement of Thermoelectric and Structural Properties of Binary Chalcogenides and Half-Heusler Alloys at Extreme Conditions Using a Paris-Edinburgh Press
Understanding the high-pressure behavior of transport properties has been a driving force in the study of materials under extreme conditions for well over a century being pioneered by P.W. Bridgman in the early 20th century. Research dedicated to the study of these properties leads to a variety of important applications: exploration of insulator to semi-conductor to metal structural and electronic phase transitions, correlation of structural phase transitions and the electronic properties along phase boundaries, testing validity of theoretical models, understanding the effects of chemical pressure, among a slew of other applications. This work has designed and developed a specialized sample cell assembly for use with a Paris-Edinburgh press capable of performing high-pressure and high-temperature (HP-HT) electrical resistance, Seebeck coefficient, thermal conductivity measurements alongside energy-dispersive X-ray diffraction and X-ray radiography imaging up to 6 GPa and 500°C to fully characterize the electrical, thermal, and structural properties of materials simultaneously at extreme conditions. This system has been installed at Argonne National Laboratory at the Advanced Photon Source at the Sector 16 BM-B beamline of the High-Pressure Collaborative Access Team and is now available to general users as a measurement technique. Application of this system has been applied to thermoelectric materials: PbTe, SnTe, TiCoSb, and TiNiSn. Thermoelectric materials provide a valuable means of converting waste heat into useful electrical energy and studying their HP-HT properties allows a better understanding and identification of greater efficiency through tuning of transport properties. The detailed discussion of the design and development of this system alongside the important results on the thermoelectric materials mentioned will be presented in this dissertation.
Wednesday, July 5, 11:30 am, BPB 217
The CALYPSO Structure Prediction Method and Its Application
Prediction of stable structure, based only on the knowledge of the chemical composition, is a central problem of physical, chemical, and materials sciences. It is extremely difficult as it basically involves in classifying a huge number of energy minima on the multi-dimensional potential energy surfaces. Solving this problem would open new ways for understanding the behaviors of materials, where experiments are difficult. The recently developed CALYPSO (http://www.calypso.cn) method made an important progress in solving it, enabling efficient and reliable prediction/determination the structures including the zero dimensional (0D) nano-clusters or molecules, two dimensional (2D) layered materials and three dimensional (3D) crystals. In this talk, I will give a very brief overview of CALYPSO structure prediction method and present several examples of its application. In particular, I will discuss the application of this method to explore the high pressure phase diagrams of carbon dioxide and iron oxide hydroxide. Our research includes crystal structure searches for high pressure phases, and detailed analysis of the structural, dynamical, and electronic properties of potentially stable phases. The outcomes of our work is to obtain a fundamental understanding of the high pressure behavior of carbon dioxide (iron oxide hydroxide), and of the formation mechanisms and structural changes involved and provide vital input for experimental studies.
Wednesday, June 6, 11:30 am, BPB 217
Characterizing strain heterogeneities in polycrystalline quartz deformed under high PT conditions
Rheological studies of rocks and minerals allow researchers to study the grain-scale deformation mechanisms that govern large-scale geologic processes from mountain building to mantle mixing. Deforming rock samples with high pressure and temperature apparatuses similar to the Griggs piston cylinder apparatus allows us to simulate deformation at depth. However, many apparatuses are limited to “cook-and-look” analysis and require modeling techniques to determine the evolution of deformation patterns found in experimental samples. A previous study used two-dimensional (2D) plane strain finite element (FE) models to analyze the development of stress patterns in polycrystalline rocks. The study suggested rhythmic patterns in deformed rocks develop as a result of stress percolating through the elastically and plastically disordered system. 2D plane strain simulations are a convenient tool for modeling the long-range stress patterns of rocks because they do not require the processing power of a supercomputer. This study will assess the utility of the method by comparing the micro strain patterns of experimentally deformed samples with strain patterns in 2D FE models. Experimental micro strain will be measured using digital image correlation (DIC) of the sample before and after deformation. Electron channeling contrast imaging (ECCI) in conjunction with electron backscattered diffraction (EBSD) will be used to image and characterize plastic deformation in the sample using a field emission scanning electron microscope (FE-SEM). The 2D plane strain geometry of the FE models will be recreated experimentally by deforming slabs of polycrystalline quartz secured between two alumina half-cylinders in a Griggs apparatus. Optimal models with exact grain orientations and grain boundaries of the starting material will be generated using EBSD crystallographic orientation maps. This study can provide experimental evidence of stress percolation and help quantify the influence of grain-scale mechanisms and grain interaction on the bulk rheology of earth materials.
Wednesday, May 31, 11:30 am, BPB 217
Advanced synthesis of pHEMA-TiO2 hybrid materials by high-pressure induced polymerization methods and analysis of their optical properties
The specific functional properties of organic-inorganic hybrid materials depend on their microscopic structure as well as the nature of the interface between the organic and inorganic components. Many routes exist to fabricate hybrid materials. One of the most prominent is the incorporation of inorganic building blocks in organic polymers in order to combine the advantages of organic polymers with those of the inorganic component. The development of applications for these hybrid materials is often limited by their mechanical behavior. While an increase of the inorganic component concentration can improve the hybrid’s functional properties, it can simultaneously leads to a degradation of the mechanical properties by limiting the extent of the polymer network.
In this presentation I will discuss a new high pressure (HP) approach for the fabrication of nanoparticulate pHEMA-TiO2 (pHEMA = poly-(2-Hydroxyethyl)methacrylate) hybrid materials with high concentration of inorganic part and stable mechanical properties. First, I will present the spontaneous polymerization of HEMA under static pressure (up to 1.6 GPa). I will also show that HP-induced polymerization can be considerably accelerated if laser irradiation (488 or 355nm) is applied. Second, I will discuss a new high pressure rump (HPR) process originally developed for the super-fast polymerization of HEMA and later successfully applied for the fabrication of organic-inorganic hybrid materials. Finally, I will speak about optical properties of pHEMA-TiO2 hybrid materials synthesized by HPR process, studied by pump-probe photodarkening experiments.
Wednesday, May 24, 11:30 am, BPB 217
Molecular dynamics simulations of mechanical and dynamical properties of carbon nanomaterials
Molecular dynamics simulation is a classical computer simulation method that describes inter-atomic interactions via empirical potential functions. When applied appropriately, it can handle larger number of atoms than first-principles calculations and is much faster & efficient in providing valuable insights into some novel phenomena. In this presentation, I will briefly discuss some basics of molecular dynamics simulations including its pros and cons. I will then present some examples of using this method to study mechanical properties of nanomaterials and their dynamics. For example, using the famous AIREBO potential, molecular dynamics simulations can be used to predict the critical buckling behavior of carbon nanotubes. Using the same potential, we proposed a self-assembly approach to design an all-carbon core-shell nanostructure, which is the central component of a novel photovoltaic device, experiment results showed that such approach can notably improve the power conversion efficiency due to increased contact between carbon nanotubes and C60 molecues.
Wednesday, May 17, 11:30 am, BPB 217
Looking into the normal state of a High Critical Temperature Superconductor: High pressure, high magnetic field Fermiology studies of YBCO
Our team’s expertise lies in the measurements of magnetization and Fermi surfaces of strongly correlated electronic systems, under the extreme conditions of high pressures , low temperatures and high magnetic fields . I will describe how those measurements are made, and how those extremes are reached. I will then discuss our most recent data on the archetypal High Temperature Superconductor YBCO.
The pnictide, cuprate and molecular conductor families exhibit similar phase diagrams, leading to a great deal of interest in a common mechanism for a “universal phase diagram”. The typical ingredients for such phase diagrams include an antiferromagnetic phase, a superconducting dome, and possibly one, or several quantum critical points (QCP) [3,4]. The interplay between these various ingredients, in particular the origin of the superconducting dome, has been one of the hottest topics in condensed matter physics for the past 30 years, and is still heavily studied and debated. Chemical doping is one traditional way to look at such materials, however thermodynamic variables such as magnetic field or hydrostatic pressure have proven to be powerful tools to explore this phase diagram, with very strong magnetic fields being used to suppress the superconducting dome, allowing one to investigate the QCP.
Our group performed high pressure Fermi surface measurements (Shubnikov-de Haas effect) of YBCO6.5 (p=0.1) at He-3 temperatures in pulsed fields to 70 T and static fields of 45 T, and pressures of 25 GPa using plastic and metal diamond anvil cells (DACs), respectively. These cells are coupled with an LC tank circuit based on a tunnel diode oscillator. The small coil that makes up the inductor of this LC circuit and resides in the high pressure volume of the DAC senses changes in sample resistivity (or magnetism in insulators) due to variations in temperature, pressure or magnetic field. Our Fermiology studies clearly show a strongly diverging effective mass at 4.5 GPa that is associated with a local maximum in frequency and critical superconducting temperature. The high critical field Hc2 in this material limits our study in the low pressure range to pressures below 7 GPa in the 45 T hybrid magnet, but by increasing pressure to 25 GPa we are able to once again see quantum oscillations and find that the orbital frequency has increased from 550 T at ambient pressure to 690 T. Pulsed field high pressure studies are currently planned to shed light on the region between 7 and 25 GPa. This now allows us to use pressure to develop a B-P-T phase diagram that will permit a more complete picture of HTS to be pursued and answer how CDWs and the pseudogap play a role in superconductivity.
 D. Graf et al., High Pressure Research 31(4), 533 (2011)
 The National High Magnetic Field Laboratory’s website has a lot of tutorials and information about magnetism and magnets. A good introduction to high magnetic fields can be found at :
 S. Badoux, et al., Nature, 531, 210 (2016).
 B. Ramshaw, et al., Science, 348, 317 (2015).
Wednesday, Apr 26, 11:30 am, BPB 217
Calorimetry Study of the Phase Diagrams of EuNi2Ge2 and Eu2Ni3Ge5Under Pressure
Dr. S. Esakki Muthu
In this talk I will present the phase diagrams of EuNi2Ge2 and Eu2Ni3Ge5studied by ac calorimetry under pressure using a diamond anvil cell. In EuNi2Ge2 the antiferromagnetic transition exists up to 1.5 GPa. The sudden disappearance of magnetic order around 2 GPa is confirmed, consistent with the probable occurrence of a first order valence transition near that pressure. The ac calorimetry results on Eu2Ni3Ge5 clearly show 2 antiferromagnetic transitions, and suggest that magnetic order persists up to higher pressure than previously expected. At high pressure, where heavy fermion behavior has previously been reported, the Néel temperature is decreasing, and magnetic order is expected to disappear at an extrapolated pressure of 12-14 GPa. A semi-quantitative analysis of the pressure dependence of the specific heat does not show any large changes, but is compatible with a moderate enhancement of g. The similarities with the phase diagrams of Yb and Ce heavy fermion systems will be discussed.
Wednesday, Apr 19, 11:30 am, BPB 217
Laser annealing after a kinetically hindered phase transition in the pyrochlore La2Sn2O7
There is interest in identifying materials for use in radioactive waste applications and studying their structural properties. Studies has shown that pyrochlore (A2B2O7) compounds might be promising candidates for such purposes. In an attempt to obtain a broader understanding of the structural stability of these compounds, we have investigated the pyrochlore La2Sn2O7 under extreme conditions: high temperature isobaric and high-pressure isothermal study. Both quasi-hydrostatic and non-hydrostatic isothermal high pressure compression runs were undertaken to reveal alternative structural transformations and studied using synchrotron angle-dispersive X-ray diffraction and Raman scattering techniques, supported by ab-initio random structure searching (AIRSS). Compression using He as a quasi-hydrostatic pressure transmitting medium permitted the observation of a new high pressure phase via a kinetically hindered first-order phase transition that begins at 49 GPa and finally goes to completion at 61 GPa. Alternatively, the non-hydrostatic compression, using no pressure-transmitting medium, revealed the emergence of a phase transition at 46 GPa. However, it appears that this process occurs via a stress-induced pathway and in fact transforms into an amorphized phase by 67 GPa. Annealing the amorphized phase, through laser heating, at 70 GPa allows us to crystallize the high-pressure phase which is recoverable to ambient conditions. Our AIRSS calculations reveal three competing metastable structures, close in energy at 70 GPa.
Wednesday, Mar 22, 11:30 am, BPB 217
Unexpected Structural and Electronic Behavior in Sn3N4
Group 14 nitrides have been a topic of recent scientific focus due to their interesting mechanical and electronic properties. My talk will describe our research into the behavior of the nitrogen-rich nitride Sn3N4 under high pressure-temperature conditions. Using in-house experimental optical techniques and in situ X-ray diffraction, coupled with laser heating in the diamond anvil cell we have observed exotic electronic behavior with pressure. The band gap in this optoelectronic material reveals an opening from 0.5 to 3 eV with pressure between 0 and 130 GPa, after which the band gap slowly closes. A component of this talk will be focused on the possible mechanisms for this phenomenon. The structure of Sn3N4 at these extreme pressures (measured up to 230 GPa) has not yet been solved, and I will discuss the methods and difficulties faced in identifying the new phases predicted by ab initio random structure searching (AIRSS).
Wednesday, Mar 15, 11:30 am, BPB 217
Quantum Critical Heavy Fermions
High pressure techniques can be used to tune heavy fermion materials to a quantum critical point. Presented here is the basic theory behind this. The Kondo effect, the Anderson model, the Kondo lattice and the RKKY interaction are discussed qualitatively. Selected publications from experimental work performed at HiPSEC that have demonstrated this as well as potential future work are also discussed.
Wednesday, Mar 1, 11:30 am, BPB 217
Shock Recovery of Bismuth
Between 0 and 10 GPa there are five different bismuth phases. High-pressure bismuth (Bi) phases have been examined in static compression experiments; however, none could be recovered to ambient conditions. Here we report Bi-III recovery (stable above 3 GPa) to ambient conditions from a shock compression experiment to 5.7 GPa. Bi-III was identified by synchrotron micro-diffraction and backscatter electron imaging. Our work shows shock-compression provides a tool for recovering high-pressure phases that otherwise elude decompression. DOE/NV/25946–3140
Wednesday, Feb 8, 11:30 am, BPB 217
Recreating planetary cores in the laboratory
Of the different planets in our solar system, our giant icy neighbors Neptune and Uranus are extremely fascinating, Uranus being the more peculiar of the two. Though Uranus is almost 2 billion kilometers closer to the sun than Neptune, its average surface temperature can dip as low as 50 K, actually making it colder than Neptune. The primary belief for this is that the core of Uranus, which is believed to be made of diamond, silicates, iron, and nickel, has shed most of its energy; cooling down to a point where it no longer radiates much heat. Despite having a cold core, Uranus has a very active and unique magnetic field. Unlike Earth, whose magnetic field is driven by a very hot active core, it is believed that Uranus’s unique magnetic field is actually driven by mixtures of super ionic and metallic molecular compounds. It has been measured that the primary components of its mantle are water, ammonia, and methane, which have all been predicted to show superionic properties at the conditions present within the mantle. There have recently been efforts in attempting to recreate the conditions necessary to verify these predicted phases, primarily in the area of shock compression. This is difficult as the region of P-T space that these superionic phases exist cannot be reached by traditional shock compression. In a traditional shock experiment the path the hugoniot takes overshoots the targeted region due to high amounts of entropy generated, so very high temperatures are reached along with very high pressures. One way around this is by compressing the sample to a specific density prior to shocking in order to take off hugoniot shock paths, reaching the pressures necessary at much lower temperatures. I will discuss some details of how these pre-compressed experiments are performed as well as how Velocity Interferometry for Any Reflector (VISAR) is used to interpret the results.
Wednesday, Jan 18, 11:30 am, BPB 217
Adventures with 5d orbitals at high pressure
Advanced Photon Source, Argonne National Laboratory
While first-row (3d) transition metal (TM) oxides continue to provide a rich playground for studies of electron correlations, recent focus has shifted to third-row (5d) TM oxides in the search for novel quantum states. The sizable spin-orbit interaction in heavy 5d ions, coupled with reduced on-site Coulomb interactions as a result of the large spatial extent of 5d orbitals, create unique experimental and theoretical opportunities for discovery of new electronic phases of matter.
We have studied some of the consequences of enhanced S-O coupling and spatial extent of 5d orbitals on the electronic structure and magnetic (exchange) interactions in a novel “iridate” magnetic insulator, Sr2IrO4. The high-brilliance, penetration power, and polarization/energy tunability of synchrotron radiation enable the use of x-ray absorption spectroscopy (including circular dichroism) and resonant magnetic scattering techniques in the diamond anvil cell providing exquisite sensitivity to the evolution of electronic correlations at high pressure. Among other findings, we discovered that pressure leads to frustration of exchange interactions in the square lattice of entangled spin-orbital Iridium moments and emergence of quantum paramagnetism, possibly a quantum spin liquid phase.
Higher brilliance, 4th generation synchrotron light sources now in development around the globe will bring unprecedented opportunities for studies of electronic/magnetic order at the limit of static high-pressure generation technologies.