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"Simon, J"
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Dynamic crystallography reveals spontaneous anisotropy in cubic GeTe
Cubic energy materials such as thermoelectrics or hybrid perovskite materials are often understood to be highly disordered1,2. In GeTe and related IV–VI compounds, this is thought to provide the low thermal conductivities needed for thermoelectric applications1. Since conventional crystallography cannot distinguish between static disorder and atomic motions, we develop the energy-resolved variable-shutter pair distribution function technique. This collects structural snapshots with varying exposure times, on timescales relevant for atomic motions. In disagreement with previous interpretations3–5, we find the time-averaged structure of GeTe to be crystalline at all temperatures, but with anisotropic anharmonic dynamics at higher temperatures that resemble static disorder at fast shutter speeds, with correlated ferroelectric fluctuations along the c direction. We show that this anisotropy naturally emerges from a Ginzburg–Landau model that couples polarization fluctuations through long-range elastic interactions6. By accessing time-dependent atomic correlations in energy materials, we resolve the long-standing disagreement between local and average structure probes1,7–9 and show that spontaneous anisotropy is ubiquitous in cubic IV–VI materials.Cubic materials such as GeTe have low lattice thermal conductivity, thought to arise from a non-cubic local structural transition. Here, using a variable-shutter pair distribution function method, the average structure is shown to be crystalline but with anisotropic dynamics at higher temperatures.
Journal Article
Enhancement of the superconducting transition temperature of FeSe by intercalation of a molecular spacer layer
Altering the composition of the spacer layers present in iron-based superconductors is one strategy for increasing the temperature below which they superconduct. Now, intercalating FeSe with molecular spacer layers is also shown to enhance the superconducting transition temperature.
The discovery of high-temperature superconductivity in a layered iron arsenide
1
has led to an intensive search to optimize the superconducting properties of iron-based superconductors by changing the chemical composition of the spacer layer between adjacent anionic iron arsenide layers
2
,
3
,
4
,
5
,
6
,
7
. Superconductivity has been found in iron arsenides with cationic spacer layers consisting of metal ions (for example, Li
+
, Na
+
, K
+
, Ba
2+
) or PbO- or perovskite-type oxide layers, and also in Fe
1.01
Se (ref.
8
) with neutral layers similar in structure to those found in the iron arsenides and no spacer layer. Here we demonstrate the synthesis of Li
x
(NH
2
)
y
(NH
3
)
1−
y
Fe
2
Se
2
(
x
~0.6;
y
~0.2), with lithium ions, lithium amide and ammonia acting as the spacer layer between FeSe layers, which exhibits superconductivity at 43(1) K, higher than in any FeSe-derived compound reported so far. We have determined the crystal structure using neutron powder diffraction and used magnetometry and muon-spin rotation data to determine the superconducting properties. This new synthetic route opens up the possibility of further exploitation of related molecular intercalations in this and other systems to greatly optimize the superconducting properties in this family.
Journal Article
Genetic code expansion in stable cell lines enables encoded chromatin modification
Genetically encoded unnatural amino acids provide powerful strategies for modulating the molecular functions of proteins in mammalian cells. However, this approach has not been coupled to genome-wide measurements, because efficient incorporation of unnatural amino acids is limited to transient expression settings that lead to very heterogeneous expression. We demonstrate that stable integration of the
Methanosarcina mazei
pyrrolysyl-tRNA synthetase (PylRS)/tRNA
Pyl
CUA
pair (and its derivatives) into the mammalian genome enables efficient, homogeneous incorporation of unnatural amino acids into target proteins in diverse mammalian cells, and we reveal the distinct transcriptional responses of embryonic stem cells and mouse embryonic fibroblasts to amber codon suppression. Genetically encoding
N
-ɛ-acetyl-lysine in place of six lysine residues in histone H3 enables deposition of pre-acetylated histones into cellular chromatin, via a pathway that is orthogonal to enzymatic modification. After synthetically encoding lysine-acetylation at natural modification sites, we determined the consequences of acetylation at specific amino acids in histones for gene expression.
Stable integration of genes that facilitate the incorporation of unnatural amino acids into the amber stop codon in genes of interest allows targeted integration of acetyl-lysine into histone H3.3 and the investigation of its effects in mouse embryonic stem cells.
Journal Article
The rise of the X-ray atomic pair distribution function method: a series of fortunate events
The atomic pair distribution function (PDF) technique is a powerful approach to gain quantitative insight into the structure of materials where the structural coherence extends only over a few nanometres. In this paper, I focus on PDF from synchrotron X-rays and describe what is the PDF and where it came from, as well as key moments on the journey that have contributed to its enormous recent growth and expanding impact in materials science today. Synchrotron X-ray sources played a starring role in this story. This article is part of the theme issue ‘Fifty years of synchrotron science: achievements and opportunities’.
Journal Article
Unlocking the potential of lignocellulosic biomass through plant science
1366 I. 1366 II. 1367 III. 1368 IV. 1368 V. 1369 VI. 1370 VII. 1372 VIII. 1372 IX. 1376 X. 1377 1377 References 1377 SUMMARY: The aim of producing sustainable liquid biofuels and chemicals from lignocellulosic biomass remains high on the sustainability agenda, but is challenged by the costs of producing fermentable sugars from these materials. Sugars from plant biomass can be fermented to alcohols or even alkanes, creating a liquid fuel in which carbon released on combustion is balanced by its photosynthetic capture. Large amounts of sugar are present in the woody, nonfood parts of crops and could be used for fuel production without compromising global food security. However, the sugar in woody biomass is locked up in the complex and recalcitrant lignocellulosic plant cell wall, making it difficult and expensive to extract. In this paper, we review what is known about the major polymeric components of woody plant biomass, with an emphasis on the molecular interactions that contribute to its recalcitrance to enzymatic digestion. In addition, we review the extensive research that has been carried out in order to understand and reduce lignocellulose recalcitrance and enable more cost‐effective production of fuel from woody plant biomass.
Journal Article