Low-dimensional materials with chain-like (one-dimensional) or layered (two-dimensional) structures are of significant interest due to their anisotropic electrical, optical, and thermal properties. One material with a chain-like structure, BaTiS3 (BTS), was recently shown to possess giant in-plane optical anisotropy and glass-like thermal conductivity. To understand the origin of these effects, it is necessary to fully characterize the optical, thermal, and electronic anisotropy of BTS. To this end, BTS crystals with different orientations (a- and c-axis orientations) were grown by chemical vapor transport. X-ray absorption spectroscopy was used to characterize the local structure and electronic anisotropy of BTS. Fourier transform infrared reflection/transmission spectra show a large in-plane optical anisotropy in the a-oriented crystals, while the c-axis oriented crystals were nearly isotropic in-plane. BTS platelet crystals are promising uniaxial materials for infrared optics with their optic axis parallel to the c-axis. The thermal conductivity measurements revealed a thermal anisotropy of ∼4.5 between the c- and a-axis. Time-domain Brillouin scattering showed that the longitudinal sound speed along the two axes is nearly the same, suggesting that the thermal anisotropy is a result of different phonon scattering rates.
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The absorption indicatrix as an empirical model to describe anisotropy in X-ray absorption spectra of pyroxenes
Abstract Anisotropic absorption in crystals is routinely observed in many spectroscopic methods and is recognized in visible light optics as pleochroism in crystalline materials. As with other spectrosco-pies, anisotropy in Fe K-edge X-ray absorption spectroscopy (XAS) can serve both as an indicator of the general structural arrangement and as a conundrum in quantifying the proportions of absorbers in crystals. In materials containing multiple absorbers, observed anisotropies can typically be represented by a linear relationship between measured spectroscopic peak intensities and relative absorber concentrations. In this study, oriented XAS analysis of pyroxenes demonstrates that the macroscopic theory that describes visible light absorption anisotropy of triaxially anisotropic materials can also be applied to X-ray absorption in pyroxenes, as long as the orientation and magnitude of the characteristic absorption vectors are known for each energy. Oriented single-crystal XAS analysis of pyroxenes also shows that the measured magnitude of characteristic absorption axes at a given orientation is energy-dependent and cannot be reproduced by linear combination of intermediate spectra. Although the macroscopic model describes a majority of the anisotropy, there is distinct discordance between the observed and interpolated spectra in the pre-edge between 7109 and 7115 eV, which is marked by spikes in RMSE/mean intensity ratio. Absorption indicatrices for samples analyzed in the visible and at X-ray wavelengths are modeled with a three-dimensional (3D) pedal surface, which functions as an empirical way of interpolating between the observed absorption data. This surface only requires a maximum of three coefficients, and results from the summation of 3D lemniscates. An absorption indicatrix model can be used to characterize anisotropic absorption in crystals and provides a way of comparing XAS spectra from randomly oriented crystals, such as those from polished sections, to a database of characterized crystals.
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- NSF-PAR ID:
- 10325062
- Date Published:
- Journal Name:
- American Mineralogist
- Volume:
- 107
- Issue:
- 4
- ISSN:
- 0003-004X
- Page Range / eLocation ID:
- 654 to 663
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract Topological materials discovery has emerged as an important frontier in condensed matter physics. While theoretical classification frameworks have been used to identify thousands of candidate topological materials, experimental determination of materials’ topology often poses significant technical challenges. X‐ray absorption spectroscopy (XAS) is a widely used materials characterization technique sensitive to atoms’ local symmetry and chemical bonding, which are intimately linked to band topology by the theory of topological quantum chemistry (TQC). Moreover, as a local structural probe, XAS is known to have high quantitative agreement between experiment and calculation, suggesting that insights from computational spectra can effectively inform experiments. In this work, computed X‐ray absorption near‐edge structure (XANES) spectra of more than 10 000 inorganic materials to train a neural network (NN) classifier that predicts topological class directly from XANES signatures, achieving
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Cation site occupation is an important determinant of materials properties, especially in a complex system with multiple cations such as in ternary spinels. Many methods for extracting the cation site information have been explored in the past, including analysis of spectra obtained through K-edge X-ray absorption spectroscopy (XAS). In this work, we measure the effectiveness of X-ray emission spectroscopy (XES) for determining the cation site occupation. As a test system we use spinel phase Co x Mn 3−x O 4 nanoparticles contaminated with CoO phases because Co and Mn can occupy all cation sites and the impurity simulates typical products of oxide syntheses. We take advantage of the spin and oxidation state sensitive Kβ 1,3 peak obtained using XES and demonstrate that XES is a powerful and reliable technique for determining site occupation in ternary spinel systems. Comparison between the extended X-ray absorption fine structure (EXAFS) and XES techniques reveals that XES provides not only the site occupation information as EXAFS, but also additional information on the oxidation states of the cations at each site. We show that the error for EXAFS can be as high as 35% which makes the results obtained ambiguous for certain stoichiometries, whereas for XES, the error determined is consistently smaller than 10%. Thus, we conclude that XES is a superior and a far more accurate method than XAS in extracting cation site occupation in spinel crystal structures.more » « less
- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.2138/am-2021-7950
