Simple molecular systems at extreme conditions

This thesis project has focussed on the experimental study of simple molecular systems at extreme conditions. High-pressure and high-temperature techniques have been used in combination with Raman spectroscopy and X-ray diffraction diagnostics to characterise three simple molecular systems which are unified by the inclusion of nitrogen as a constituent element. The N2 molecule contains the only triple-bond amongst the elemental diatomics and is considered a model system for exploring the changes in structure and bonding induced by tuning pressure and temperature conditions. As such the nitrogen phase-diagram is a focus-point in current extreme conditions research and nitrogen has been found to exhibit a high-degree of polymorphism not observed in other simple molecular systems such as hydrogen or oxygen. Understanding molecular mixtures of nitrogen with other simple molecules at extreme conditions is significant to many scientific fields varying from chemistry to astronomy. The first system presented is the binary mixture of nitrogen and xenon which was studied as a function of pressure. The study constitutes the first comprehensive study of the xenon-nitrogen system at high-pressures. A new van der Waals compound was observed which underwent a phase transition at 14 GPa and was stable up to at least 180 GPa and 3000 K, conditions where pure nitrogen becomes amorphous. Optical measurements suggested possible metallization of the new compound around 120 GPa. The second system presented is the binary mixture of nitrogen and hydrogen which was studied both as a function of pressure and composition. Two known nitrogen-hydrogen structures were confirmed and a pressure-temperature path-dependent formation of hydrazine or ammonia was discovered. Additionally, one mixture was compressed to 242 GPa, the highest pressure investigated in the nitrogen-hydrogen system. The third system presented is the elemental nitrogen phase known as i-nitrogen, an elusive high-temperature polymorph which has hitherto eluded structure determination and proved challenging to access. i-nitrogen was successfully characterised as having an extraordinarily large unit cell containing 48 N2 molecules, making it the most complex molecular nitrogen structure to be determined unambiguously

Saturated gain spectrum of VECSELs determined by transient measurement of lasing onset

We describe time-resolved measurements of the evolution of the spectrum of radiation emitted by an optically-pumped continuous-wave InGaAs-GaAs quantum well laser, recorded as lasing builds up from noise to steady state. We extract a fitting parameter corresponding to the gain dispersion of the parabolic spectrum equal to ?79 ± 30 fs2 and ?36 ± 6 fs2 for a resonant and anti-resonant structure, respectively. Furthermore the recorded evolution of the spectrum allows for the calculation of an effective FWHM gain bandwidth for each structure, of 11 nm and 18 nm, respectively

Editorial: Frontiers in psychodynamic neuroscience

ISSN:1662-516

Pressure-Induced Phase Transition Versus Amorphization in Hybrid Methylammonium Lead Bromide Perovskite

The crystal structure of CH3NH3PbBr3 perovskite has been investigated under high-pressure by synchrotron-based powder X-ray diffraction. We found that after the previously reported phase transitions in CH3NH3PbBr3 (Pm-3m->Im-3->Pmn21), which occur below 2 GPa, there is a third transition to a crystalline phase at 4.6 GPa. This transition is reported here for the first time contradicting previous studies which reported amorphization of CH3NH3PbBr3 between 2.3 and 4.6 GPa. Our X-ray diffraction measurements show that CH3NH3PbBr3 remains crystalline up to 7.6 GPa, the highest pressure covered by experiments. The new high-pressure phase is also described by the space group Pmn21, but the transition involves abrupt changes in the unit-cell parameters and a 3% decrease of the unit-cell volume. Our conclusions are confirmed by optical-absorption experiments and visual observations and by the fact that changes induced by pressure up to 10 GPa are reversible. The optical studies also allow for the determination of the pressure dependence of the band-gap energy which is discussed using the structural information obtained from X-ray diffraction.Comment: 15 pages, 4 figure

Dynamical torque in CoxFe3–xO4 nanocube thin films characterized by femtosecond magneto-optics : a π-shift control of the magnetization precession

RT and PA would like to thank the Nuffield Foundation (ref. URB40673), the Physics Trust, and the Student Council of the School of Physics and Astronomy at St Andrews to support RT’s research internships. IB and PA acknowledge support from the Engineering and Physical Sciences Research Council (EPSRC, ref. EP/H010033/1).For spintronic devices excited by a sudden magnetic or optical perturbation, the torque acting on the magnetization plays a key role in its precession and damping. However the torque itself can be a dynamical quantity via the time dependent anisotropies of the system. A challenging problem for applications is then to disentangle the relative importance of various sources of anisotropies in the dynamical torque, such as the dipolar field, the crystal structure or the shape of the particular interacting magnetic nanostructures. Here, we take advantage of a range of colloidal cobalt ferrite nano-cubes assembled in 2D thin films under controlled magnetic fields to demonstrate that the phase φprec of the precession carries a strong signature of the dynamical anisotropies. Performing femtosecond magneto-optics, we show that φprec displays a π-shift for a particular angle θH of an external static magnetic field H. θH is controlled with the cobalt concentration, the laser intensity as well as the inter-particles interactions. Importantly it is shown that the shape anisotropy, which strongly departs from the one of equivalent bulk thin films or individual non-interacting nanoparticles, reveals the essential role played by the interparticles collective effects. This work shows the reliability of a non-invasive optical approach to characterize the dynamical torque in high density magnetic recording media made of organized and interacting nanoparticles.Publisher PDFPeer reviewe

Pressure-induced phase transition and band gap decrease in semiconducting β-Cu2V2O7

The understanding of the interplay between crystal structure and electronic structure in semiconductor materials is of great importance due to their potential technological applications. Pressure is an ideal external control parameter to tune the crystal structures of semiconductor materials in order to investigate their emergent piezo-electrical and optical properties. Accordingly, we investigate here the high-pressure behavior of the semiconducting antiferromagnetic material β-Cu2V2O7, finding it undergoes a pressure-induced phase transition to γ-Cu2V2O7 below 4000 atm. The pressure-induced structural and electronic evolutions are investigated by single-crystal X-ray diffraction, absorption spectroscopy and ab initio density functional theory calculations. β-Cu2V2O7 has previously been suggested as a promising photocatalyst for water splitting. Now, these new results suggest that β-Cu2V2O7 could also be of interest with regards to barocaloric effects, due to the low phase -transition pressure, in particular because it is a multiferroic material. Moreover, the phase transition involves an electronic band gap decrease of approximately 0.2 eV (from 1.93 to 1.75 eV) and a large structural volume collapse of approximately 7%.The authors acknowledge financial support from the Spanish Research Agency (AEI) and Spanish Ministry of Science and Investigation (MCIN) under projects PID2019106383GBC41/ C43/C44 (DOI: 10.13039/501100011033), and projects PGC2018-101464−B-I00 and PGC2018-097520-A-I00 (cofinanced by EU FEDER funds). The authors acknowledge financial support from the MALTA Consolider Team network, under project RED2018-102612-T. R.T. acknowledges funding from the Spanish Ministry of economy and competitiveness (MINECO) via the Juan de la Cierva Formación program (FJC2018-036185-I). J.G.P. thanks the Servicios Generales de Apoyo a la Investigación (SEGAI) at the University of La Laguna. A.L. and D.E. would like to thank the Generalitat Valenciana for the Ph.D. fellowship GRISOLIAP/2019/025, and the authors would also like to thank them for funding under the Grant Prometeo/2018/123 (EFIMAT). The authors also thank ALBA synchrotron light source for funded experiment under proposal numbers 2020074389 and 2020074398 at the MSPD-BL04 beamline

Pressure-induced band-gap increase in hydrated Ca(IO3)2 [Jamieson Award Introduction and Lecture]

The search for the next generation of nonlinear optical (NLO) materials has led to the synthesis and characterization of many new metal iodates [1,2]. However, finding an ideal NLO crystal with a high laser damage threshold, excellent thermal stability, a wide transparency window, and a large second-harmonic generation (SHG) response remains a challenge. This work is inspired by our previous high-pressure studies on metal iodates and the finding of the general inverse correlation between the bandgap of metal iodates (which only contain non-transition or closed-shell metal) and the distance of average I-O covalent bonds [3]. In this study [4], we report on the first instance of a metal iodate, hydrated Ca(IO3)2, with an opened bandgap achieved by applying external pressure. The bandgap increased from 4.52 eV to 4.92 eV without saturation at increasing pressure (Figure 1). This is explained by: ) molecular orbital (MO) diagrams established from theoretical calculations of the density of state (DOS) and crystal orbital overlap population (COOP), and) the pressure dependence of the bond distance of the primary, short, strong I-O covalent bonds and secondary, long, weak I···O halogen bonds, as obtained from powder X-ray diffraction measurements. In addition, two reversible isostructural phase transitions were observed in the pressure range of 6.6–8.0 and 13.0–15.5 GPa, characterized by nonlinear changes in the bandgap energy, crystal lattice parameters, and the occurrence of extra peaks and peak splitting in the Raman spectra

Pressure-induced band-gap energy increase in a metal iodate

[EN] A wide band gap is one of the essential requirements for metal iodates to be used as nonlinear optical materials. Usually, the band gap of these materials decreases under the application of pressure. Herein, we introduce a case in which the band-gap energy of a hydrated metal iodate, namely Ca(IO3)2 center dot H2O, has been successfully increased, from 4.52 to 4.92 eV, by applying external pressure without showing signs of saturation upon increasing pres-sure. The pressure-induced nonlinear band-gap opening correlates with the pressure-induced shortening of the I-O bond distances, as obtained from x-ray diffraction measurements. In addition, two pressure-induced isostructural phase transitions are observed in the pressure regions of 6.6-8.0 and 13.0-15.5 GPa. These two isostructural phase transitions cause a nonlinear pressure-induced evolution of the band-gap energy and crystal lattice parameters, as well as the occurrence of several extra peaks and peak splitting in Raman spectra.This study was supported by the MALTA ConsoliderTeam network (Project No. RED2018-102612-T), financed by MINECO/AEI/0.13039/501100003329,the I+D+i Project No.PID2019-106383GB-41/42 financed by MCIN/AEI/10.13039/501100011033, as well as by the Projects No. PROMETEO CIPROM/2021/075 (GREEN-MAT) and No. MFA/2022/007 financed by GeneralitatValenciana. This study forms part of the Advanced Materials programme and was supported by MCIN with funding from European Union Next Generation EU (PRTR-C17.I1) and by Generalitat Valenciana. A.L. and D.E. thank the Generalitat Valenciana for Ph.D. Fellowship No. GRISOLIAP/2019/025.R.T. and D.E. thank the Generalitat Valenciana for Postdoctoral Fellowship No.CIAPOS/2021/20.The authors also thank ALBA synchrotron light source for the experiment funded under Proposal No. AV-2021095390at the MSPD-BL04 beamline. J.S. and K.V. would like to acknowledge IIT Hyderabad for providing computational facilities. J.S. would like to acknowledge CSIR for his Ph.D.fellowship. G.V. would like to acknowledge the Institute of Eminence, University of Hyderabad (UoH-IoE-RC3-21-046)for funding and CMSD, University of Hyderabad, forproviding computational facilities. The authors also thank the Tirant supercomputer (Universitat de Valencia) for providing computational resources.Liang, A.; Shi, L.; Turnbull, R.; Manjón, F.; Ibáñez, J.; Popescu, C.; Jasmin, M.... (2022). Pressure-induced band-gap energy increase in a metal iodate. PHYSICAL REVIEW B-CONDENSED MATTER. 106(23):1-9. https://doi.org/10.1103/PhysRevB.106.235203191062

Pressure-induced band-gap energy increase in a metal iodate

A wide band gap is one of the essential requirements for metal iodates to be used as nonlinear optical materials. Usually, the band gap of these materials decreases under the application of pressure. Herein, we introduce a case in which the band-gap energy of a hydrated metal iodate, namely Ca(IO3)2 center dot H2O, has been successfully increased, from 4.52 to 4.92 eV, by applying external pressure without showing signs of saturation upon increasing pres-sure. The pressure-induced nonlinear band-gap opening correlates with the pressure-induced shortening of the I-O bond distances, as obtained from x-ray diffraction measurements. In addition, two pressure-induced isostructural phase transitions are observed in the pressure regions of 6.6-8.0 and 13.0-15.5 GPa. These two isostructural phase transitions cause a nonlinear pressure-induced evolution of the band-gap energy and crystal lattice parameters, as well as the occurrence of several extra peaks and peak splitting in Raman spectra.This study was supported by project MALTA Consolider Team network (RED2018‐102612‐T), financed by MINECO/AEI/10.13039/501100003329, I+D+i project PID2019‐106383GB‐41/42 financed by MCIN/AEI/10.13039/501100011033; as well as by projects PROMETEO CIPROM/2021/075 (GREENMAT) and MFA/2022/007 financed by Generalitat Valenciana. A.L. and D.E. thank the Generalitat Valenciana for the Ph.D. Fellowship No. GRISOLIAP/2019/025. R.T. and D.E. thank the Generalitat Valenciana for the postdoctoral Fellowship No. CIAPOS/2021/20. The authors also thank ALBA synchrotron light source for funded experiment under proposal number AV-2021095390 at the MSPD-BL04 beamline