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6 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2024

Origin of charge density wave in topological semimetals SrAl 4 and EuAl 4

Topological semimetals in BaAl 4 -type structure show many interesting behaviors, such as charge density wave (CDW) in SrAl 4 and EuAl 4 , but not the isostructural and isovalent BaAl 4 , SrGa 4 , and BaGa 4 . Here using Wannier functions based on density functional theory, we calculate the susceptibility functions with millions of k-points to reach the small q-vector and study the origin and driving force behind the CDW. Our comparative study reveals that the origin of the CDW in SrAl 4 and EuAl 4 is the strong electron-phonon coupling interaction for the transverse acoustic mode at small q-vector along the Γ-Z direction besides the maximum of the real part of the susceptibility function from the nested Fermi surfaces of the Dirac-like bands, which explains well the absence of CDW in the other closely related compounds in a good agreement with experiment. We also connect the different CDW behaviors in the Al compounds to the macroscopic elastic properties.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on SrAl(OF2)2 by Materials Project

SrAl(OF2)2 crystallizes in the monoclinic P2_1/c space group. The structure is three-dimensional. Sr is bonded in a 8-coordinate geometry to two equivalent O and six F atoms. There are one shorter (2.69 Å) and one longer (2.71 Å) Sr–O bond lengths. There are a spread of Sr–F bond distances ranging from 2.44–2.67 Å. Al is bonded in a 6-coordinate geometry to two equivalent O and four F atoms. There are one shorter (1.96 Å) and one longer (2.09 Å) Al–O bond lengths. There are a spread of Al–F bond distances ranging from 1.73–1.83 Å. There are two inequivalent O sites. In the first O site, O is bonded in a 1-coordinate geometry to two equivalent Al and one O atom. The O–O bond length is 1.44 Å. In the second O site, O is bonded in a bent 150 degrees geometry to two equivalent Sr atoms. There are four inequivalent F sites. In the first F site, F is bonded in a distorted bent 150 degrees geometry to one Sr and one Al atom. In the second F site, F is bonded in a distorted trigonal planar geometry to two equivalent Sr and one Al atom. In the third F site, F is bonded in a distorted bent 120 degrees geometry to one Sr and one Al atom. In the fourth F site, F is bonded in a 1-coordinate geometry to two equivalent Sr and one Al atom.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on SrAl(OF2)2 by Materials Project

SrAl(OF2)2 crystallizes in the monoclinic C2/c space group. The structure is three-dimensional. Sr is bonded in a 9-coordinate geometry to two equivalent O and seven F atoms. There are one shorter (2.76 Å) and one longer (2.84 Å) Sr–O bond lengths. There are a spread of Sr–F bond distances ranging from 2.46–2.70 Å. Al is bonded to two equivalent O and four F atoms to form edge-sharing AlO2F4 octahedra. There is one shorter (1.95 Å) and one longer (1.98 Å) Al–O bond length. There are a spread of Al–F bond distances ranging from 1.75–1.82 Å. There are two inequivalent O sites. In the first O site, O is bonded in a bent 120 degrees geometry to two equivalent Al atoms. In the second O site, O is bonded in a bent 150 degrees geometry to two equivalent Sr atoms. There are four inequivalent F sites. In the first F site, F is bonded in a 1-coordinate geometry to two equivalent Sr and one Al atom. In the second F site, F is bonded in a distorted bent 150 degrees geometry to one Sr and one Al atom. In the third F site, F is bonded in a 1-coordinate geometry to two equivalent Sr and one Al atom. In the fourth F site, F is bonded in a 1-coordinate geometry to two equivalent Sr and one Al atom.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on SrAl by Materials Project

AlSr is delta Molybdenum Boride-like structured and crystallizes in the cubic P2_13 space group. The structure is three-dimensional. there are four inequivalent Sr sites. In the first Sr site, Sr is bonded in a 7-coordinate geometry to seven Al atoms. There are a spread of Sr–Al bond distances ranging from 3.36–3.61 Å. In the second Sr site, Sr is bonded in a 1-coordinate geometry to seven Al atoms. There are a spread of Sr–Al bond distances ranging from 3.11–3.64 Å. In the third Sr site, Sr is bonded in a 6-coordinate geometry to six Al atoms. There are three shorter (3.43 Å) and three longer (3.44 Å) Sr–Al bond lengths. In the fourth Sr site, Sr is bonded in a 6-coordinate geometry to six Al atoms. There are a spread of Sr–Al bond distances ranging from 3.31–3.74 Å. There are four inequivalent Al sites. In the first Al site, Al is bonded in a 7-coordinate geometry to seven Sr atoms. In the second Al site, Al is bonded in a 9-coordinate geometry to six Sr and three equivalent Al atoms. All Al–Al bond lengths are 2.71 Å. In the third Al site, Al is bonded in a 9-coordinate geometry to seven Sr and two equivalent Al atoms. Both Al–Al bond lengths are 2.85 Å. In the fourth Al site, Al is bonded in a 9-coordinate geometry to six Sr and three Al atoms. Both Al–Al bond lengths are 2.73 Å.

36 MATERIALS SCIENCE↗
DOE OSTI2020

A phytophotonic approach to enhanced photosynthesis

Photosynthesis is the dominant biotic carbon sink on earth and hence presents an opportunity for enhanced sequestration of CO 2 . If the average net carbon fixation efficiency of terrestrial plants could be increased by 3.3%, all anthropogenic CO 2 accumulating in the atmosphere could instead be reduced and incorporated into terrestrial biomass. Plants make inefficient use of the overly abundant sunlight available to them, a result of having evolved to be competitive and survive highly dynamic environmental conditions rather than maximize photosynthetic productivity. We explore herein a phytophotonic approach to enhanced photosynthesis, whereby sunlight is redistributed by means of luminescent or persistent luminescent (PersL) materials. Phytophotonics has potential at varied scales, ranging from photobioreactors to greenhouses all the way to crops in the field, the latter having the potential to impact planetary CO 2 levels. The approach is three-fold: a spectral redistribution to relieve high-light-stress at the top surface of leaves and increasingly drive photosynthesis deeper in leaves and canopies; a minute-scale temporal redistribution to bridge periods of intermittent shade and reduce shock associated with variable light conditions; and a multiple-hour temporal redistribution to shift a fraction of high-intensity midday lighting to evening hours. Based on simulations of photoluminescent materials and light quality experiments with a model algal system, it is shown that while lengthening daylight hours will require significant improvements in PersL materials, the other two approaches show more immediate promise. We demonstrate a means of concentrating PersL light from SrAl 2 O 4 :Eu,Dy, approaching levels needed to effectively bridge periods of natural shade, and outline the scientific questions and technical hurdles remaining to realize the benefits of the proposed spectral shift.

59 BASIC BIOLOGICAL SCIENCES↗