Energy & functional materials: high pressure, high temperature synthesis & characterization

Resume : Some layered phyllosilicates (e.g. kaolinite or smectites) are used for a preparation of new functional materials by intercalation of suitable organic species into interlayer space of clays that can be followed by a further chemical modification (e.g. grafting) of the internal layer surface. Phyllosilicate minerals such as talc, pyrophyllite, lizardite, and kaolinite have expandable layered structure, in which non-bonding dispersive interactions and hydrogen bonds are responsible for layer stacking stabilization. It represents a serious problem for a correct prediction of their structure and properties by conventional DFT functionals. In this work, a performance of several DFT methods was validated on models of talc, pyrophyllite, lizardite, kaolinite

Resume : In this talk I will outline a new technique to measure and control the properties of quantum materials using in-situ tunable strain fields. I will use iron based superconductors as an example to demonstrate how symmetry breaking strains enable us to probe the electronic nematic fluctuations, which appears to be correlated with the optimal superconducting transition temperatures. I will also show how strain could sensitively alter the transport behaviors of ZrTe5, a Dirac semimetal candidate. I will then discuss how the effects of strain in the context of a topological phase transition in this material.

Resume : Thermoelectricity is the phenomenon of conversion temperature gradient into electric current and vice versa. This phenomena is becoming more important as it reduces the losses during energy conversion, mainly due to heat. The figure of merit (ZT) for thermoelectric materials is the quantity that reflects how efficient a material is. ZT is inversely proportional to the thermal conductivity. Which means that for a better thermoelectric material, low thermal conductivity is to be achieved. In the literature, the thermal conductivity of HgTe (mercury telluride) was calculated for the zincblende (low pressure) and cinnabar (high pressure) phases by solving the Boltzmann transport equation using second- and third-order force constants calculated from Density Functional Theory. The calculations for the zincblende phase have shown an almost five times higher lattice thermal conductivity than the experimental values [Journal of Applied Physics 117, 245101 (2015)]. To better understand the discrepancy and the phonon scattering processes in both Zincblende and cinnabar phases, our recently proposed residual analysis method [Physical Review B 95, 195203 (2017)] is used and we find an unusual forth-order phonon anharmonicity. Time domain normal mode analysis was conducted to achieve more accurate values for thermal conductivity and gain better understanding the abnormal phonon transport in thermoelectric material HgTe.

Resume : Phase stability of Ti, Mg and Al at hydrostatic pressures upto 210, 270 and 390 GPa, respectively, was investigated by means of first principles. The influence of pressure on the phase transition is analyzed based on electronic structure calculations. Reliability of exchange-correlation potential under pressures was examined. It was found that the PAW-GGA potential with p semi-core states in valence electrons to be the best potential for various phases of Ti under high pressures. For titanium, the δ phase has similar structure to the β phase after full relaxation indicating a phase transition from δ to β could be occurred through all pressures considered here. At zero pressure, the ω phase is the most stable phase followed by α, γ, and β (δ) phases, and the phase transition order is ω→γ ()→β () with increasing of pressure, however, at high pressures (above 106 GPa), the phase stability is in the order of , and . Mg only keep stable in its ground structure (hcp) below 52 GPa and will then transform to bcc structure, which will be the most stable phase upto 270 GPa. Al can occur in its ground structure (fcc) under the hydrostatic pressure up to 143 GPa and then will transform to hcp structure as the increasing of pressure, while the bcc structure of Al will become the most stable phase above the pressure of 324 GPa. The evaluated critical pressures for phase transition in Ti, Mg and Al were well consistent with other theoretical and experimental results, implying the validity of our calculation approaches. Electronic structures are studied to clarify the influence mechanisms of pressure on phase transition. It is found that the bonding states are broadened in energy range with increasing pressure and the values of total DOS at the Fermi energy are gradually decreased in Ti systems, particularly in the  and  phases, which may be the reason that the β (δ) phase becomes the most stable phase at high pressure. However, it was not shown obvious changes in electronic structures in Mg and Al with increasing of pressures. The change of relative number of s, p, and d electrons are studied in detail. A connection between the change in number of valence electrons and the phase stability of all three metals was observed.

Resume : Pressure dramatically alters a material’s physical and chemical properties over varying energy scales, adding a new dimension to energy-related, electronic, optical, mechanical, biological, and environmental applications. Halide perovskites are a class of versatile materials that have recently received extraordinary research attention as they exhibit remarkable structural, optical, and electronic properties applicable to a wide range of technologies needed for a sustainable energy future. In this talk, I will focus on how pressure can alter halide perovskites’ electronic landscapes, affect excited-state dynamics, and afford new structures not accessible through conventional syntheses. I will highlight two examples where pressure enables improved understanding of the origins of unusual phenomena and novel materials synthesis. (1) I will describe the first instance of pressure-induced room-temperature electrical conductivity arising in a two-dimensional Cu(II)-Cl organic-inorganic hybrid perovskite. (2) I will discuss drastic pressure-induced structural, electronic, and optical modifications to three-dimensional Pb-X (X = Br, I) hybrid perovskites and the first observation of metallization in (CH3NH3)PbI3.

Resume : Due the finite nature and the damaging greenhouse gas emissions of fossil fuel there is a constant search for advanced technologies that take advantage of substitute, clean energy resources. An example of said technology is thermoelectrics. Thermoelectrics allows a conversion of waste heat into electricity. Compared to other energy conversion approaches, thermoelectric efficiency still has a way to go and thus needs to be further improved. The performance of thermoelectric devices is assessed by the dimensionless figure of merit ZT of the material, defined as ZT =α2σT/, where α, σ, and T are the Seebeck coefficient, the electrical and thermal conductivities, and the absolute temperature, respectively. The thermal conductivity is a combination of thermal conductivity via electrons, κe, and via phonons, κl. The main difficulty in improvement of the efficiency of a thermoelectric device is due to the complex relation between σ, α and Improving the performance of thermoelectric materials is usually done either by improving the power factor, α2σ, or by applying phonon scattering methods in order to lower the thermal conductivity. Bismuth–telluride-based alloys are of great importance not only as the best thermoelectric materials with the maximal ZT values close to unity near room temperature, but also due to the potential for further performance improvement. Since these alloys are highly anisotropic, there is a direct relation between the orientation and the thermoelectric properties of the ingots. Hence, higher ZT values can be achieved by improving the orientation anisotropy of the ingots. In this study Bi2TexSex compositions were electronically optimized by various CHI3 doping levels, preferred alignment of the crystallographic orientation, and lattice thermal conductivity minimization. The synthesis route included rocking furnace melting, energetic ball milling and hot pressing under optimal conditions for enhancement of the thermoelectric figure of merit, ZT, at temperatures higher than 200oC, commonly applied in low temperature power generation applications. The transport properties in perpendicular to the pressing direction were examined.

Resume : Magneto-electric multiferroics are defined as a class of materials that combine the ferroelectric and the ferromagnetic properties in a single phase. [1] They are drawing considerable research interests not only for discovering various attractive fundamental physical phenomena but also for potential novel technological applications. However, either of the known Type I and Type II multiferroic materials has its own drawback as device materials. [2] Therefore, new materials with optimized properties useful for real applications are still to be found and are subject of active researches. Recently, the rather less explored doubly ordered perovskites AA’BB’O6 have attracted a growing interest [3-6]. The ordered AA’BB’O6 compounds exhibit a rock salt type ordering at the B-sites, and a layered ordering at the A-sites. The rock salt ordered B-sites are very common and have been observed in many A2BB’O6 compounds, while the latter layered ordering is very rare. The layered A-sites ordering is stabilized by the second-order Jahn-Teller distortions of the highly charged B’ cations [4], and the ordering in long range disappeared in the absence of the rock salt B-sites ordering [7]. Theoretical investigations predicted that Hybrid Improper Ferroelectricity (HIF) could be induced in this class of materials [8]. A polarization as large as ~16µC/cm2 was estimated from the structure of NaLaMnWO6 [6]. In addition, the compounds AA’BB’O6 could order with magnetic cations at the B-site (e.g. NaLaMnWO6 [5], NaLaCoWO6 [3]), making the magneto-electric coupling possible. However, real examples of this class materials are very limited so far. In this study, we synthesized a series of doubly ordered perovskites NaLnCoWO6 (Ln= Y, La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er and Yb) by solid state reaction, nine of which (Ln= Y, Sm, Eu, Gd, Tb, Dy, Ho, Er and Yb) are new phases prepared under high temperature and high pressure conditions. Their structural properties were investigated at room temperature by synchrotron X-ray powder diffraction and neutron powder diffraction. All of them crystallize in monoclinic structures, especially the nine new compounds have the polar space group P21 symmetry, as confirmed by second harmonic generation measurements. The P21 polar structures were decomposed and refined in terms of symmetry modes, demonstrating that the polar mode is induced by two non-polar modes in a manner of HIF. The amplitudes of these three major modes all increase with decreasing the Ln cation size. The spontaneous ferroelectric polarization is estimated from the neutron diffraction data of three compounds (Ln= Y, Tb, and Ho), and can be as large as ~20 µC/cm2. References: [1] Schmid, H. Ferroelectrics 1994, 162, 317-338. [2] Khomskii, D. Physics 2009, 2, 20. [3] Arillo, M. A. et al. J. Mater. Chem. 1997, 7(5), 801-806. [4] Knapp, M. C. et al. J. Solid State Chem. 2006, 179, 1076-1085. [5] King, G. et al. Chem. Mater. 2007, 19, 6451-6458. [6] Fukushima, T. et al. Phys. Chem. Chem. Phys. 2011, 13, 12186-12190. [7] King, G. et al. J. Mater. Chem. 2010, 20, 5785-5796. [8] Mulder, A. T. et al. Adv. Funct. Mater. 2013, 23, 4810-4820.

Resume : In this extensive study, the thermodynamic and diffusion properties of the liquid Ni-Al alloy are investigated over a wide temperature and concentration range by using molecular dynamics (MD) simulations. MD simulations are performed by using a reliable 2009 embedded-atom method (EAM) potential due to Purja Pun and Mishin. The results also permit the description of the temperature and concentration dependence of the thermodynamic factor for interdiffusion. In addition, the simulations permit analysis of the heat of transport in thermotransport. It is possible to estimate the contributions of Onsager off-diagonal terms for the temperature and concentration range under consideration. The results agree well with previous published experimental and simulation data where available. Keywords: Molecular Dynamics Simulation; Diffusion; Thermotransport; Ni-Al Alloys.

Resume : Polymorphism, which refers to the occurrence of multiple chemically identical but structurally distinct phases, is a critical phenomenon in materials science and condensed matter physics. Diamond and graphite are well-known examples. Recently, configuration disorder was compositionally engineered into single lattices, leading to the discovery of high-entropy alloys (HEAs)1,2. For these novel entropy-stabilized forms of crystalline matter with extremely high structural stability, is polymorphism still possible? Herein, by employing an in situ high-pressure synchrotron radiation X-ray diffraction (XRD) technique in a diamond anvil cell, we discovered an unprecedented polymorphic transition from fcc (face-centered-cubic)-to-hcp (hexagonal-close-packing) in the prototype CoCrFeMnNi HEA. The transition is irreversible, and our in situ high-temperature synchrotron radiation XRD experiments at different pressures of the retained hcp HEA unambiguously revealed that the fcc phase was a stable polymorph at high temperatures, while the hcp structure was thermodynamically more favorable at lower temperatures. As the pressure increased, the critical temperature for the hcp-to-fcc transformation also rose. Reference: 1. Cantor B, Chang ITH, Knight P, Vincent AJB. Microstructural development in equiatomic multicomponent alloys. Mater. Sci. Eng., A 375–377, 213-218 (2004) 2. Yeh JW, et al. Nanostructured High-Entropy Alloys with Multiple Principal Elements: Novel Alloy Design Concepts and Outcomes. Adv. Eng. Mater. 6, 299-303 (2004).

Resume : Inelastic x-ray scattering (IXS) methods have been gaining popularity due to unique insight they provide into electronic and local structure of materials. Thanks to their high penetrating power, inelastic scattering of hard x-ray photons allows in-situ and in-operando investigations of complex systems. In-situ IXS techniques at high pressure and temperature are particularly useful in geophysical and geochemical applications for studying local chemical and electronic structures, like coordination and spin states. Additionally, IXS is used to explore low energy collective excitations like phonons, magnons, and charge transfer processes and their spatial dependence. Furthermore, the well-defined non-resonant inelastic cross section helps unambiguously determine ground state symmetries of correlated systems.

Resume : Elemental rare-earth metals exhibit intriguing properties arising from strong correlations effects due to partially filled f-electron shells. In this talk I will discuss the low-temperature structural and magnetic properties of Dysprosium under high pressure. In particular, the anomalous thermal expansion of Dysprosium will be investigated using first-principle calculations with noncollinear magnetism. I also will discuss the pressure-induced volume collapse of Cerium at room temperature. Model Hamiltonian simulations including hybridization effects and atomic multiplet interactions will be discussed together with recent X-ray Raman measurements.

Resume : An overview is given of the use of high-energy X-ray spectroscopy, such as X-ray absorption and resonant inelastic X-ray scattering (RIXS), to study the changes in the electronic structure of transition-metal compounds under high pressure. For iridates, RIXS can probe changes in the magnitude of the spin-orbit interaction and the branching ratio in X-ray absorption is sensitive to the ground-state expectation value of the spin-orbit coupling. The ground-state expectation value of the spin-orbit interaction is a combination of the jeff=1/2 spins that describe the magnetic ground state plus the jeff=3/2 spins that couple to the eg states. Observed experimental changes in the expectation values are often stronger than expected from changes in the lattice parameters. Additionally, using V2O3 as an example, the sensitivity of X-ray spectroscopy to insulator-metal transitions is discussed.

Resume : The hierarchical porous structures with controlled conductivity were fabricated by nesting soft templates on hard templates such as fabric structure. In this way, microporous structures of 4-6 nm were formed on macroporous structures of 10-15 μm in diameter. By immersing porous PET fabrics into resol/F127, in conjunction with a high-temperature treatment for carbonization, both the soft and hard templates were decomposed, while the precursor was carbonized. This created a conductive nature with resistivity less than 3.5×10-3 Ωm in ambient condition. Due to the capillary pressure in the porous structure, the contact area between liquid and porous carbon material has been greatly increased, which leads to a high sensitivity of the porous carbon material when used for concentration detection of methanol or hydrogen peroxide (H2O2). This new design concept could prove highly-effective for electroanalytical biosensors.

Resume : CdS thin film is deposited by spray pyrolysis technique on heated glass substrate at 350°C. The physical properties of the film are characterized by several techniques in order to study their structural, surface, optical and electrical properties. It is observed from X-ray diffraction (XRD) analysis that the CdS film is mainly composed with hexagonal Wurtzite structure with a preferred grain orientation along (101) plane. From optical measurements, the average optical transmission is 80 % of CdS sample and the band gap value is found 2.4 eV. The Hall Effect electrical measurements show that the sample is n-type and the value of the electrical resistivity 2.5x102 (Ω.cm) is obtained.

Resume : Being transparent in а wide spectral region, oxygen-free CaF2 single crystals are used as optical materials in many applications. The presence of impurities and radiation-induced structural defects greatly affect the properties of CaF2. The processes of defect creation by swift ions in fluorite remain poorly understood. The samples were irradiated with 0.23-GeV Xe ions (DC-60, Astana), 2.38-GeV Bi ions (UNILAC, GSI, Darmstadt) or 100-keV protons (KIIA 500 kV implanter, Helsinki) at 295 K.The damage was analyzed via induced optical absorption (IOA, 1.4-10.5 eV) and its thermal annealing in a stepwise regime (up to 1100 K). Three dominant complex IOA bands peaked at ~2.2, ~6.5, ~9.8 eV were registered. Experiments with photo- and thermal influence on IOA confirmed a complex structure of the 2.2 eV band associated with different F-type aggregates and Ca-colloids previously studied by many authors in additively colored crystals. The main IOA annealing stage occurs at 470-580 K, in parallel with the dissociation of the most stable hole centers – trifluoride molecules (peaked at ~ 6.5 eV). Of special interest is the IOA band at 9.8 eV which can be induced exclusively by swift heavy ions. This band is tentatively ascribed to electronic excitations nearby complex structural defects. The joint action of shock waves and decay/recombination of radiation-induced electronic excitations is responsible for the creation of novel defects with complex structure under Xe or Bi irradiation.

Resume : Perylene bisimides (PBIs) are dyes and pigments known for combining high absorption and emission in the visible region with their thermal and photochemical stability. Non-conventional H-bonded aggregation driven by free-imide groups has been reported to promote alternative J-type aggregate formation in non-polar solvents. J-aggregates are highly desired thanks to their bathochromically shifted absorption and fluorescence (by exciton coupling), together with hyperchromicity and superradiance compared to the monomer. Herein we present the water soluble analogue PBI4 showing interesting aggregation in water and in solid state. Unlike hydrophobic precursors, PBI4 aggregates in water upon increasing temperature indicating an entropy-driven self-assembly. Temperature dependent Resonance Raman (RR) spectroscopy at different wavelengths was employed for the structural characterization of PBI4 in aqueous solution versus toluene and in aggregated thin films. In order to gain insights on the structure of the aggregate, theoretical calculations of the normal modes of PBI4 with the use of Gaussian were performed to correlate the varied vibrational fingerprint upon aggregation to specific structural changes. These assignments indicate a distortion of the perylene core upon aggregation, where the bonds along the perylene long axis lengthen and the ones perpendicular to that shorten, suggesting a head-to-tail arrangement. Complementary FT-IR measurements provides further information on the aggregation process.