Synchrotron X-ray Diffraction Study of BaFe2As2 and CaFe2As2 at High Pressures up to 56 GPa: Ambient and Low-Temperatures Down to 33 K

We report high pressure powder synchrotron x-ray diffraction studies on MFe2As2 (M=Ba, Ca) over a range of temperatures and pressures up to about 56 GPa using a membrane diamond anvil cell. A phase transition to a collapsed tetragonal phase is observed in both compounds upon compression. However, at 300 (33) K in the Ba-compound the transition occurs at 26 (29) GPa, which is a much higher pressure than 1.7 (0.3) GPa at 300 (40) K in the Ca-compound, due to its larger volume. It is important to note that the transition in both compounds occurs when they are compressed to almost the same value of the unit cell volume and attain similar ct/at ratios. We also show that the FeAs4 tetrahedra are much less compressible and more distorted in the collapsed tetragonal phase than their nearly regular shape in the ambient pressure phase. We present a detailed analysis of the pressure dependence of the structures as well as equation of states in these important BaFe2As2 and CaFe2As2 compounds.Comment: 26 pages, 12 figure

Pressure dependence of the low- temperature crystal structure and phase transition behaviour of CaFeAsF and SrFeAsF: A synchrotron x-ray diffraction study

We report systematic investigation of high pressure crystal structures and structural phase transition upto 46 GPa in CaFeAsF and 40 GPa in SrFeAsF at 40 K using powder synchrotron x-ray diffraction experiments and Rietveld analysis of the diffraction data. We find that CaFeAsF undergoes orthorhombic to monoclinic phase transition at Pc = 13.7 GPa while increasing pressure. SrFeAsF exhibits coexistence of orthorhombic and monoclinic phases over a large pressure range from 9 to 39 GPa. The coexistence of the two phases indicates that the transition is of first order in nature. Unlike in the 122 compounds (BaFe2As2 & CaFe2As2) we do not find any collapse tetragonal transition. The transition to a lower symmetry phase (orthorhombic to monoclinic) in 1111 compounds under pressure is in contrast with the transition to a high symmetry phase (orthorhombic to tetragonal) in 122 type compounds. On heating from 40 K at high pressure, CaFeAsF undergoes monoclinic to tetragonal phase transition around 25 GPa and 200 K. Further, it does not show any post-tetragonal phase transition and remains in the tetragonal phase upto 25 GPa at 300 K. The dPc/dT is found to be positive for the CaFeAsF & CaFe2As2, however the same was not found in case of BaFe2As2. We discuss observations of structural evolution in the context of superconductivity in these and other Fe-based compounds. It appears that the closeness of the Fe-As-Fe bond angle to its ideal tetrahedral value of 109.470 might be associated with occurrence of superconductivity at low temperature.Comment: 23 pages, 11 Figure

Neutron and synchrotron X-ray powder study of copper(II) chloride complex with deuterated 1-ethyltetrazole

The structure of the copper(II) chloride complex with deuterated 1-ethyltetrazole has been investigated in the temperature range of 2-290 K using neutron and synchrotron X-ray powder diffraction. The compound was found to exhibit structural transformation at ca 180 K, without change of space group and main structural motif. At higher temperatures, the complex reveals positional disorder of the ethyl group, whereas no disorder is observed at lower temperatures. Temperature dependence of the lattice parameters, obtained from synchrotron X-ray data, showed main lattice changes at the transformation, explained by structural features of the complex. From the magnetic measurements, the effect of the disorder on paramagnetic behaviour of the compound was found. Detailed structural data of the compound at 2 and 290 K, obtained from neutron powder diffraction data, are reported. © by Oldenbourg Wissenschaftsverlag, München

Verwey-Type Charge Ordering and Site-Selective Mott Transition in Fe4O5under Pressure

The metal-insulator transition driven by electronic correlations is one of the most fundamental concepts in condensed matter. In mixed-valence compounds, this transition is often accompanied by charge ordering (CO), resulting in the emergence of complex phases and unusual behaviors. The famous example is the archetypal mixed-valence mineral magnetite, Fe3O4, exhibiting a complex charge-ordering below the Verwey transition, whose nature has been a subject of long-time debates. In our study, using high-resolution X-ray diffraction supplemented by resistance measurements and DFT+DMFT calculations, the electronic, magnetic, and structural properties of recently synthesized mixed-valence Fe4O5are investigated under pressure to ∼100 GPa. Our calculations, consistent with experiment, reveal that at ambient conditions Fe4O5is a narrow-gap insulator characterized by the original Verwey-type CO. Under pressure Fe4O5undergoes a series of electronic and magnetic-state transitions with an unusual compressional behavior above ∼50 GPa. A site-dependent collapse of local magnetic moments is followed by the site-selective insulator-to-metal transition at ∼84 GPa, occurring at the octahedral Fe sites. This phase transition is accompanied by a 2+ to 3+ valence change of the prismatic Fe ions and collapse of CO. We provide a microscopic explanation of the complex charge ordering in Fe4O5which "unifies" it with the behavior of two archetypal examples of charge- or bond-ordered materials, magnetite and rare-earth nickelates (RNiO3). We find that at low temperatures the Verwey-type CO competes with the "trimeron"/"dimeron" charge ordered states, allowing for pressure/temperature tuning of charge ordering. Summing up the available data, we present the pressure-temperature phase diagram of Fe4O5 © 2022 American Chemical Society. All rights reserved.EAR-1634415; National Science Foundation, NSF: EAR-1606856; U.S. Department of Energy, USDOE: DE-FG02-94ER14466; Office of Science, SC; Argonne National Laboratory, ANL: DE-AC02-06CH11357; Deutsche Forschungsgemeinschaft, DFG: OV-110/3-2; Russian Foundation for Basic Research, РФФИ: 20-42-660027; Israel Science Foundation, ISF: 1552/18, 1748/20; Russian Science Foundation, RSF: 19-72-30043; 122021000039-4We thank L. S. Dubrovinsky, I. A. Abrikosov, and V. Prakapenka for their interest in this research and B. Lavina for fruitful discussions about in situ DAC synthesis. We are grateful to M. Hanfland for the assistance in using beamline ID-15B of ESRF, Grenoble, France. Portions of this work were performed at GeoSoilEnviroCARS (The University of Chicago, Sector 13), Advanced Photon Source (APS), Argonne National Laboratory. GeoSoilEnviroCARS is supported by the National Science Foundation-Earth Sciences (Grant EAR-1634415) and Department of Energy-GeoSciences (Grant DE-FG02-94ER14466). This research used resources of the Advanced Photon Source, a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE-AC02-06CH11357. Use of the COMPRES-GSECARS gas loading system was supported by COMPRES under NSF Cooperative Agreement EAR-1606856 and by GSECARS through NSF Grant EAR-1634415 and DOE Grant DE-FG02-94ER14466.The work was partly supported by the Israel Science Foundation (Grants No. 1552/18 and 1748/20) and the Deutsche Forschungsgemeinschaft Grant No. OV-110/3-2. The theoretical analysis was supported by Russian Foundation for the Basic Research (Project No. 20-42-660027). The DFT calculations were supported by the state assignment of Minobrnauki of Russia (Theme “Electron” No. 122021000039-4). The DFT+DMFT calculations were supported by the Russian Science Foundation (Project No. 19-72-30043)

Structure and lattice dynamics of copper- and silver-based superionic conducting chalcogenides

The high-temperature crystal structure and lattice dynamics of superionic silver- and copper-based chalcogenides have been investigated by powder diffraction and inelastic neutron scattering. The obtained issues provide the experimental background for understanding of correlations between microscopic phenomena and macroscopic properties (ionic conductivity, thermodynamics, and thermal expansion). Moreover, these experimental results are an experimental base for further experimental (total scattering) and computational (molecular dynamics simulations) investigations of cation diffusion in superionic silver- and copper-based chalcogenides

INFLUENCE OF FERTILIZERS ON ACCUMULATION OF HEAVY METAL IN SOIL AND PHYTO MASS OF GRAIN CROPS

It’s common knowledge that heavy metal (HM) can enter the soil together with fertilizers which contaminates agricultural landscapes. The purpose of the research is to study influence of fertilizers on accumulation and migration of Cd, Pb ,Zn, Cu, Co, Mn in soil and plants. The research was conducted during field experiments in chernozem (blacksoil). We studied two levels of plant fertilizing: the first one without fertilizers and the second one with fertilizing of norms of NPK. The fertilizers were distributed in the following way: spring wheat (Triticum vulgare) – N60P60K60; spring barley (Hordeum vulgare) and oats (Avena sativa) – N45P45K20; millet (Panicum miliaceum) – N30P40K40; peas (Pisum sativum) – N10P60K60; buckwheat (Fagopyrum esculentum) – N45P60K60.v We found out that fertilizing raises the amount of Сd, Рb Zn, Сu, Со and Мn on 10-36% in soil and increases mobility of Zn, Си, Со and Мn on 25%. The increase of fertilizing of spring wheat, barley, oats, millet and buckwheat reduces whole volume of HM migration in phyto mass on 5-30%.  But it also stimulates Pb migration in the plants of spring wheat, migration of Сd, Zn and Сu into bio mass of oats and barley, and migration of Cd and Mn into peas. The main portion of accumulated elements is stored in a root system of a plant, Zn and Cu are able to transport into inflorescence. The volume of HM accumulation in phyto mass of fertilized plants doesn’t exceed TLV

High-temperature structural evolution of caesium and rubidium triiodoplumbates

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