263 research outputs found
Nanoscale clusters in the high performance thermoelectric AgPbmSbTem+2
The local structure of the AgPbmSbTem+2 series of thermoelectric materials
has been studied using the atomic pair distribution function (PDF) method.
Three candidate-models were attempted for the structure of this class of
materials using either a one-phase or a two-phase modeling procedure. Combining
modeling the PDF with HRTEM data we show that AgPbmSbTem+2 contains nanoscale
inclusions with composition close to AgPb3SbTe5 randomly embedded in a PbTe
matrix.Comment: 7 pages, 5 figures, 2 tables, submitted to PR
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A new camera for high-resolution infrared imaging of works of art
A new camera – SIRIS (scanning infrared imaging system) – developed at the National Gallery in London allows high-resolution images to be made in the near infrared region (900–1700 nm). The camera is based on a commercially available 320 × 256 pixel indium gallium arsenide area array sensor. This relatively small sensor is moved across the focal plane of the camera using two orthogonal translation stages to give images of c. 5000 × 5000 pixels. The main advantages of the SIRIS camera over scanning infrared devices or sequential image capture and mosaic assembly are its comparative portability and rapid image acquisition – making a 5000 × 5000 pixel image takes less than 20 minutes. The SIRIS camera can operate at a range of resolutions; from around 2.5 pixels per millimetre over an area of up to 2 × 2 m to 10 pixels per millimetre when examining an area measuring 0.5 × 0.5 m. The development of the mechanical, optical and electronic components of the camera, including the design of a new lens, is described. The software used to control image capture and to assemble the individual frames into a seamless mosaic image is mentioned. The camera was designed primarily to examine underdrawings in paintings; preliminary results from test targets and paintings imaged in situ are presented and the quality of the images compared with those from other cameras currently used for this application
Nanoscale alpha-structural domains in the phonon-glass thermoelectric material beta-Zn4Sb3
A study of the local atomic structure of the promising thermoelectric material beta-Zn4Sb3, using atomic pair distribution function (PDF) analysis of x-ray- and neutron-diffraction data, suggests that the material is nanostructured. The local structure of the beta phase closely resembles that of the low-temperature alpha phase. The alpha structure contains ordered zinc interstitial atoms which are not long range ordered in the beta phase. A rough estimate of the domain size from a visual inspection of the PDF is <~10 nm. It is probable that the nanoscale domains found in this study play an important role in the exceptionally low thermal conductivity of beta-Zn4Sb3
Unconventional order-disorder phase transition in improper ferroelectric hexagonal manganites
The improper ferroelectricity in YMnO and other related multiferroic
hexagonal manganites are known to cause topologically protected ferroelectric
domains that give rise to rich and diverse physical phenomena. The local
structure and structural coherence across the ferroelectric transition,
however, were previously not well understood. Here we reveal the evolution of
the local structure with temperature in YMnO using neutron total scattering
techniques, and interpret them with the help of first-principles calculations.
The results show that, at room temperature, the local and average structures
are consistent with the established ferroelectric symmetry. On
heating, both local and average structural analyses show striking anomalies
from K up to the Curie temperature consistent with increasing
fluctuations of the order parameter angle. These fluctuations result in an
unusual local symmetry lowering into a \textit{continuum of structures} on
heating. This local symmetry breaking persists into the high-symmetry non-polar
phase, constituting an unconventional type of order-disorder transition.Comment: 10 pages, 5 figure
Neutron diffraction evidence of microscopic charge inhomogeneities in the CuO2 plane of superconducting La2-xSrxCuO4 (0 < x <0.30)
We present local structural evidence supporting the presence of charge
inhomogeneities in the CuO2 planes of underdoped La2-xSrxCuO4. High-resolution
atomic pair distribution functions have been obtained from neutron powder
diffraction data over the range of doping 0 < x < 0.30 at 10 K. Despite the
average structure getting less orthorhombic we see a broadening of the in-plane
Cu-O bond distribution as a function of doping up to optimal doping. Thereafter
the peak abruptly sharpens. Complementary evidence is also evident from the
observation of octahedral tilt disorder in the PDF at higher atomic separation.
This suggests a crossover from a charge inhomogeneous state at and below
optimal doping to a homogeneous charge state above optimal doping. The strong
response of the local structure to the charge-state implies a strong
electron-lattice coupling in these materials.Comment: 4 pages, 3 figures, submitted to Physical Review Letters (27-th of
June 1999) resubmitted to Phys. Rev. Lett. (8th of March 2000
Local atomic structure and discommensurations in the charge density wave of CeTe3
The local structure of CeTe3 in the incommensurate charge density wave
(IC-CDW) state has been obtained using atomic pair distribution function (PDF)
analysis of x-ray diffraction data. Local atomic distortions in the Te-nets due
to the CDW are larger than observed crystallographically, resulting in distinct
short and long Te-Te bonds. Observation of different distortion amplitudes in
the local and average structures are explained by the discommensurated nature
of the CDW since the PDF is sensitive to the local displacements within the
commensurate regions whereas the crystallographic result averages over many
discommensurated domains. The result is supported by STM data. This is the
first quantitative local structural study within the commensurate domains in an
IC-CDW system.Comment: 4 pages, 4 figure
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