183 research outputs found
Spontaneous Octahedral Tilting in the Cubic Inorganic Caesium Halide Perovskites CsSnX and CsPbX (X = F, Cl, Br, I)
The local crystal structures of many perovskite-structured materials deviate
from the average space group symmetry. We demonstrate, from lattice-dynamics
calculations based on quantum chemical force constants, that all the
caesium-lead and caesium-tin halide perovskites exhibit vibrational
instabilities associated with octahedral titling in their high-temperature
cubic phase. Anharmonic double-well potentials are found for zone-boundary
phonon modes in all compounds with barriers ranging from 108 to 512 meV. The
well depth is correlated with the tolerance factor and the chemistry of the
composition, but is not proportional to the imaginary harmonic phonon
frequency. We provide quantitative insights into the thermodynamic driving
forces and distinguish between dynamic and static disorder based on the
potential-energy landscape. A positive band gap deformation (spectral
blueshift) accompanies the structural distortion, with implications for
understanding the performance of these materials in applications areas
including solar cells and light-emitting diodes
Direct Observation of Dynamic Symmetry Breaking above Room Temperature in Methylammonium Lead Iodide Perovskite
Lead halide perovskites such as methylammonium lead triiodide (MAPI) have
outstanding optical and electronic properties for photovoltaic applications,
yet a full understanding of how this solution processable material works so
well is currently missing. Previous research has revealed that MAPI possesses
multiple forms of static disorder regardless of preparation method, which is
surprising in light of its excellent performance. Using high energy resolution
inelastic X-ray (HERIX) scattering, we measure phonon dispersions in MAPI and
find direct evidence for another form of disorder in single crystals: large
amplitude anharmonic zone-edge rotational instabilities of the PbI_6 octahedra
that persist to room temperature and above, left over from structural phase
transitions that take place tens to hundreds of degrees below. Phonon
calculations show that the orientations of the methylammonium couple strongly
and cooperatively to these modes. The result is a non-centrosymmetric,
instantaneous local structure, which we observe in atomic pair distribution
function (PDF) measurements. This local symmetry breaking is unobservable by
Bragg diffraction, but can explain key material properties such as the
structural phase sequence, ultra low thermal transport, and large minority
charge carrier lifetimes despite moderate carrier mobility.Comment: 30 pages, 11 figure
Y<sub>2</sub>Ti<sub>2</sub>O<sub>5</sub>S<sub>2</sub> – a promising n-type oxysulphide for thermoelectric applications
Thermoelectric materials offer an unambiguous solution to the ever-increasing global demand for energy by harnessing the Seebeck effect to convert waste heat to electrical energy. Mixed-anion materials are ideal candidate thermoelectric materials due to their thermal stability and potential for “phonon-glass, electron-crystal” behaviour. In this study, we use density-functional theory (DFT) calculations to investigate Y2Ti2O5S2, a cation-deficient Ruddlesden-Popper system, as a potential thermoelectric. We use hybrid DFT to calculate the electronic structure and band alignment, which indicate a preference for n-type doping with highly anisotropic in-plane and the out-of-plane charge-carrier mobilities as a result of the anisotropy in the crystal structure. We compute phonon spectra and calculate the lattice thermal conductivity within the single-mode relaxation-time approximation using lifetimes obtained by considering three-phonon interactions. We also calculate the transport properties using the momentum relaxation-time approximation to solve the electronic Boltzmann transport equations. The predicted transport properties and lattice thermal conductivity suggest a maximum in-plane ZT of 1.18 at 1000 K with a carrier concentration of 2.37 × 1020 cm−3. Finally, we discuss further the origins of the low lattice thermal conductivity, in particular exploring the possibility of nanostructuring to lower the phonon mean free path, reduce the thermal conductivity, and further enhance the ZT. Given the experimentally-evidenced high thermal stability and the favourable band alignment found in this work, Y2Ti2O5S2 has the potential to be a promising high-temperature n-type thermoelectric
Estimation of semiconductor-like pigment concentrations in paint mixtures and their differentiation from paint layers using first-derivative reflectance spectra.
Identification of the techniques employed by artists, e.g. mixing and layering of paints, if used together with information about their colour palette and style, can help to attribute works of art with more confidence. In this study, we show how the pigment composition in binary paint mixtures can be quantified using optical-reflectance spectroscopy, by analysis of the peak features corresponding to colour-transition edges in the first-derivative spectra. This technique is found to be more robust than a number of other spectral-analysis methods, which can suffer due to shifts in the transition edges in mixed paints compared to those observed in spectra of pure ones. Our method also provides a means of distinguishing paint mixtures from layering in some cases. The spectroscopy also shows the presence of multiple electronic transitions, accessible within a narrow energy range, to be a common feature of many coloured pigments, which electronic-structure calculations attribute to shallow band edges. We also demonstrate the successful application of the reflectance-analysis technique to painted areas on a selection of medieval illuminated manuscripts.ARP is indebted to St. John’s College, Cambridge for providing a scholarship to fund this study, and to ASD Inc. (through the Alexander Goetz programme) and Analytik UK Ltd. for the loan of a Fieldspec 4 spectroradiometer for the completion of this work. JMS is indebted to Trinity College, Cambridge for provision of an Internal Graduate Studentship, and to the UK Engineering and Physical Sciences Research Council (EPSRC) for support under grant no. EP/K004956/1. The computational modelling was performed on the UK national HPC facility (Archer), accessed through the Materials Chemistry Consortium, which is funded through EPSRC grant no. EP/L000202.This is the final version of the article. It first appeared from Elsevier via https://doi.org/10.1016/j.talanta.2016.03.05
Lattice dynamics and vibrational spectra of the orthorhombic, tetragonal and cubic phases of methylammonium lead iodide
The hybrid halide perovskite CH3NH3PbI3 exhibits a complex structural
behaviour, with successive transitions between orthorhombic, tetragonal and
cubic polymorphs at ca. 165 K and 327 K. Herein we report first-principles
lattice dynamics (phonon spectrum) for each phase of CH3NH3PbI3. The
equilibrium structures compare well to solutions of temperature-dependent
powder neutron diffraction. By following the normal modes we calculate infrared
and Raman intensities of the vibrations, and compare them to the measurement of
a single crystal where the Raman laser is controlled to avoid degradation of
the sample. Despite a clear separation in energy between low frequency modes
associated with the inorganic PbI3 network and high-frequency modes of the
organic CH3NH3+ cation, significant coupling between them is found, which
emphasises the interplay between molecular orientation and the corner-sharing
octahedral networks in the structural transformations. Soft modes are found at
the boundary of the Brillouin zone of the cubic phase, consistent with
displacive instabilities and anharmonicity involving tilting of the PbI6
octahedra around room temperature.Comment: 9 pages, 4 figure
Phonon anharmonicity, lifetimes, and thermal transport in CH<sub>3</sub>NH<sub>3</sub>PbI<sub>3</sub> from many-body perturbation theory
Lattice vibrations in CHNHPbI are strongly interacting, with
double well instabilities present at the Brillouin zone boundary. Analysis
within a first-principles lattice dynamics framework reveals anharmonic
potentials with short phonon quasi-particle lifetimes and mean-free paths. The
phonon behaviour is distinct from the inorganic semiconductors GaAs and CdTe
where three-phonon interaction strengths are three orders of magnitude smaller.
The implications for the applications of hybrid halide perovskites arising from
thermal conductivity, band-gap deformation, and charge-carrier scattering
through electron-phonon coupling, are presented
An integration of attachment theory and reinforcement sensitivity theory
This thesis examined how relationship experiences shape people\u27s sensitivity to detect threat and reward in romantic relationships and substance use scenarios. Findings indicated that anxious individuals experienced difficulty in distinguishing between threat and reward. In contrast, avoidant individuals were quick to detect threat either fleeing or confronting the problem aggressively
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