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1. The Crystal Structure of Al4SiC4 Revisited

Author

Ong, Chin Shen, primary, Donzel-Gargand, Olivier, additional, Berastegui, Pedro, additional, Cedervall, Johan, additional, Bayrak Pehlivan, Ilknur, additional, Hervoches, Charles, additional, Beran, Premysl, additional, Edvinsson, Tomas, additional, Eriksson, Olle, additional, and Jansson, Ulf, additional

Published
2024
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6. Structural studies of Aurivillius phase ferroelectrics and related materials

Author

Hervoches, Charles H. and Lightfoot, Phil

Subjects
537, QC596.5H4, Ferroelectricity
Abstract

The research carried out for this thesis has concentrated on Aurivillius bismuth oxide materials and the closely related Sillen and Bipox bismuth oxyhalides. X-ray and neutron powder diffraction techniques have been used to characterise precisely their structure and revealed several important features. The unexpected presence of a double phase transition scheme has been discovered for the Aurivillius phases Sr0.85Bi2.1Ta2O9 and SrBi4Ti4O15, both materials present the particularity of having an even number of perovskite layers n (n = 2 and n = 4 for Sr0.85Bi2.1Ta2O9 and SrBi4Ti4O15 respectively). The first transition from orthorhombic space group A21am to Amam, occurs at Tc (375deg.C and 550deg.C for Sr0.85Bi2.1Ta2O9 and SrBi4Ti4O15 respectively). The second transition from orthorhombic Amam to tetragonal I4/mmm, occurs at higher temperature (550deg.C and 650deg.C for Sr0.85Bi2.1Ta2O9 and SrBi4Ti4O15 respectively). These phase transitions have been understood in terms of specific octahedral tilt and displacive modes. In contrast, the n = 2 phase SrBi2Nb2O9 and the n = 3 phase Bi4Ti3O12 are shown to undergo a single-step transition from orthorhombic space group A21am to tetragonal I4/mmm at Tc = 440deg.C for SrBi2Nb2O9, and from orthorhombic space group B2cb to tetragonal I4/mmm at Tc = 675deg.C for Bi4Ti3O12. The atomic disorder of the Bi and A cations in the fluorite and perovskite sites of the Aurivillius phases has been demonstrated and thoroughly studied for the solid solutions Bi4-xSrxTi3-xNbxO12 and Bi4-xLaxTi3O12, and "size-matching" between the fluorite and the perovskite site has been found to be the key to understanding this disorder phenomenon. In the Sillen family, we extended the members of the Bi2MO4Cl group, where M = Lanthanide to the three following compounds: Bi2ErO4Cl, Bi2YbO4Cl, and Bi2LuO4Cl. We have demonstrated that in the Bi2MO4Cl group, the ionic radius of M must be greater than that of Sc3+ (r (Sc3+) = 0.87 Å for CN = 8). Another new compound with a novel ordering scheme for this family has been studied, viz. Bi5TeO8.5I2, crystallising in the orthorhombic space group Cmm2. Te4+ is shown to adopt only one of the three available M sites, thus inducing a polar structure. In the Bipox family, the crystal structures of two materials exhibiting ferroelectric properties, Bi4NbO8Cl and Bi4TaO8Cl have been determined to be orthorhombic, space group P21cn.

Published
2002

8. TbMgNi4-xCox-(H,D)2 System. II : Correlation between Structural and Magnetic Properties

Author

Paul-Boncour, Valerie, Beran, Premysl, Hervoches, Charles, Shtender, Vitalii, Paul-Boncour, Valerie, Beran, Premysl, Hervoches, Charles, and Shtender, Vitalii

Abstract

The magnetic properties of TbMgNi4-xCox intermetallic compounds and selected hydrides and deuterides of this system have been studied by various techniques, including magnetic measurements, in situ X-ray and neutron powder diffraction. The intermetallic compounds crystallize in a SnMgCu4-type structure and magnetically order below a Curie temperature (T-C), which increases exponentially with the Co content. This can be due to the ordering of the Co sublattice. On the other hand, the insertion of D or H in TbMgNiCo3 strongly decreases T-C. The X-ray diffraction measurements versus temperature reveal cell volume minima at T-C for the compounds with x = 1-3 without any hints of the structure change. The analysis of the neutron diffraction patterns for the intermetallics with x = 2 and 3 indicates a slightly canted ferrimagnetic structure below T-C. The Tb moments refined at 16 K are 4.1(2) (mu B)/Tb for x = 2, and 6.2(1) mu B/Tb for x = 3, which are smaller than the free ion value (9.5 mu B/Tb). This reduction can be due to the influence of temperature but also reveals the crystal field effect. As Ni and Co occupy statistically the same Wyckoff site, an average Ni/Co moment was refined, leading to 1.7(2) (mu B)/atom for x = 2 and 1.8(1) mu B/atom for x = 3 at 16 K. This moment is slightly canted compared to the Tb moment.

Published
2023
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13. Residual Stress Distribution Analysis in Advanced Materials by Neutron Diffraction: The Case of Spherical Storage Tank Butt Weld

Author

Hervoches Charles, Vrána Miroslav, Mikula Pavol, Mráz Ľuboš, Baksa Peter, and Baláž Milan

Subjects
Engineering (General). Civil engineering (General), TA1-2040
Abstract

The two-axis diffractometer SPN-100 at the Center of Accelerators and Nuclear Analytical Methods (CANAM) in Řež, is a neutron diffraction instrument dedicated to residual stress analyses in polycrystalline materials, it has recently been equipped with a new two-dimensional position sensitive detector (2D-PSD) and a six-axis robotic arm for sample positioning. Recent results of strain/stress distribution measurements with the upgraded tools are presented. The measured materials are made of C-Mn unalloyed steel used for spherical storage tank. Welding-induced residual stresses have direct influence on ductility and toughness of both weld metal and heat affected zone of the weld area, which affect the service life time of structures. Normal, transversal and longitudinal components of the stresses obtained in the vicinity of the butt weld from a spherical storage tank were measured by neutron diffraction at CANAM.

Published
2019
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22. Characterization of the Microstructure, Local Macro-Texture and Residual Stress Field of Commercially Pure Titanium Grade 2 Prepared by CONFORM ECAP

Author

Németh, Gergely, Horváth, Klaudia, Hervoches, Charles, Cejpek, Petr, Palán, Jan, Duchek, Michal, and Máthis, Kristián

Subjects
lcsh:TN1-997, stress, strain, CONFORM, ECAP, EBSD, titanium, texture, residual, lcsh:Mining engineering. Metallurgy
Abstract

The paper investigated the residual strain and stress distribution, microstructure, and macro-texture along the transverse direction of commercially pure titanium grade 2 samples prepared by the CONFORM ECAP technique. This method belongs to the severe plastic deformation methods, hence, it could be assumed that residual stress fields would be present in the work-pieces. Residual stresses cannot be directly measured, thus, neutron diffraction measurements, Electron back-scatter diffraction (EBSD) investigations, and local X-ray macro-texture measurements were performed in different regions of the sample to determine the data for the residual stress calculation. The calculation was based on the modified Kr&ouml, ner model. Neutron diffraction strain scans and residual stress calculations revealed that symmetrical residual strain and stress gradients with compression character were present in the axial and hoop direction after one and two passes. Asymmetric distribution of the residual strains and stresses remained after the third pass of the CONFORM ECAP. EBSD investigations showed that after the first pass, significant grain refinement occurred, however, further passes did not cause any dramatic grain refinement. X-ray texture measurements revealed that local macro-texture was dependent on the number of passes of the CONFORM ECAP and on the investigated area in the samples.

Published
2018

23. Assessment of Retained Austenite in Fine Grained Inductive Heat Treated Spring Steel

Author

Olina, Anna, Píška, Miroslav, Petrenec, Martin, Hervoches, Charles, Beran, Přemysl, Pechoušek, Jiří, Král, Petr, Olina, Anna, Píška, Miroslav, Petrenec, Martin, Hervoches, Charles, Beran, Přemysl, Pechoušek, Jiří, and Král, Petr

Abstract

Advanced thermomechanical hot rolling is becoming a widely used technology for the production of fine-grained spring steel. Different rapid phase transformations during the inductive heat treatment of such steel causes the inhomogeneous mixture of martensitic, bainitic, and austenitic phases that affects the service properties of the steel. An important task is to assess the amount of retained austenite and its distribution over the cross-section of the inductive quenched and tempered wire in order to evaluate the mechanical properties of the material. Three different analytical methods were used for the comparative quantitative assessment of the amount of retained austenite in both the core and rim areas of the sample cross-section: neutron diffraction—for the bulk of the material, Mössbauer spectroscopy—for measurement in a surface layer, and the metallographic investigations carried by the EBSD. The methods confirmed the excessive amount of retained austenite in the core area that could negatively affect the plasticity of the material., Pokročilé termomechanické válcování za tepla se stává široce používanou výrobou jemnozrnné pružinové oceli. Různé rychlosti fázové transformace během induktivního tepelného zpracování oceli způsobuje nehomogenní směs martenzitických, bainitických a austenitických fází, které ovlivňují provozní vlastnosti oceli. Důležitým úkolem je zhodnotit obsah zadrženého austenitu a jeho distribuci v průřezu indukčně kaleného a temperovaného drátu za účelem vyhodnocení mechanických vlastností materiálu. Tři různé analytické metody byly použity pro srovnávací kvantitativní hodnocení množství zadrženého austenitu v obou oblastech jádra a obvodu průřezu vzorku: pomocí pomalých neutronů - pro většinu materiálu, Mössbauerovy spektroskopie pro měření v povrchové vrstvě a metalografické vyšetření metodou EBSD. Metody potvrdily nadměrné množství zadrženého austenitu v jádru oblasti, která by mohla negativně ovlivnit plasticitu materiálu.

Published
2019

24. High-Entropy NASICON Phosphates (Na3M2(PO4)3and NaMPO4Ox, M = Ti, V, Mn, Cr, and Zr) for Sodium Electrochemistry

Author

Wu, Bing, Hou, Guorong, Kovalska, Evgeniya, Mazanek, Vlastimil, Marvan, Petr, Liao, Liping, Dekanovsky, Lukas, Sedmidubsky, David, Marek, Ivo, Hervoches, Charles, and Sofer, Zdenek

Abstract

High-entropy materials, with complex compositions and unique cocktail characteristics, have recently drawn significant attention. Additionally, a family of sodium super ion conductors (NASICONs)-structured phosphates in energy storage areas shows a comprehensive application for traditional alkaline ion batteries and, in particular, solid-state electrolytes. However, there is no precedent in fabricating this kind of NASICON-type high-entropy phase. Here, we report the successful fabrication of two well-crystallized high-entropy phosphates, namely, Na3(Ti0.2V0.2Mn0.2Cr0.2Zr0.2)2(PO4)3(HE-N3M2P3) and Na(Ti0.2V0.2Mn0.2Cr0.2Zr0.2)2PO4Ox(HE-NMP). The prepared materials in which the transition metals (TMs) of Ti, V, Mn, Cr, and Zr occupy the same 12c Wykoff position can form a structure analogous to R3̅cNa3V2(PO4)3that is carefully determined by X-ray diffraction, neutron diffraction, and transmission electron microscopy. Further, their performance for sodium ion batteries and sodium-based solid-state electrolytes was evaluated. The HE-N3M2P3might exhibit a promising electrochemical performance for sodium storage in terms of its structure resembling that of Na3V2(PO4)3. Meanwhile, the HE-NMP shows considerable electrochemical activity with numerous broad redox ranges during extraction and insertion of Na+, related to the coexistence of several TM elements. The evaluated temperature-dependent ionic conductivity for HE-NMP solid electrolyte varies from 10–6to 10–5S cm–1from room temperature to 398.15 K, offering high potential for energy storage applications as a new high-entropy system.

Published
2022
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29. Ni4Nb1.8W0.1Ti0.1O9: Stabilization of the I-Type Ni4Nb2O9Structure and First Magnetic Study of This Corundum-like Compound

Author

Beran, Premysl, Huvé, Marielle, Maignan, Antoine, Hervoches, Charles, Fauth, François, and Martin, Christine

Abstract

For the first time, we report on the structural and magnetic properties of a polycrystalline sample of Ni4Nb2O9from I-type (Fdd2), obtained by the partial cosubstitution of Nb5+by Ti4+and W6+. The crystal structure is investigated by combining synchrotron X-ray, neutron, and electron diffraction at room temperature. This I-type structure is derived from the corundum-like Ni4Nb2O9II-type (Pbcn) and is noncentrosymmetric and polar. The Ni-lattice is composed of the stacking of distorted honeycomb layers with double zigzag ribbons 60° disoriented from each other in two successive double layers. The connection between layers is ensured by the sharing of octahedra faces building (Nb,W,Ti)2O9, Ni2O9, and Ni3O12units. This study shows how the disruption of the Nb2O6units by smaller d0cations impacts the structure, significantly modifying the Ni network and, thus, the magnetic properties. The latter were studied by dc- and ac-magnetic susceptibility, and the magnetic structure was solved by neutron powder diffraction. A ferrimagnetic behavior occurs below 68 K, followed by a re-entrant spin-glass-like behavior below ≈50 K.

Published
2024
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35. Complete structural model for lanthanum tungstate: a chemically stable high temperature proton conductor by means of intrinsic defects

Author

Magrasó, Anna, primary, Polfus, Jonathan M., additional, Frontera, Carlos, additional, Canales-Vázquez, Jesús, additional, Kalland, Liv-Elisif, additional, Hervoches, Charles H., additional, Erdal, Skjalg, additional, Hancke, Ragnhild, additional, Islam, M. Saiful, additional, Norby, Truls, additional, and Haugsrud, Reidar, additional

Published
2012
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36. Crystal structure, magnetism, and thermoelectric properties of Nd1−xSrxFeO3−δ(0.1 ≤ x≤ 0.9)

Author

Nakatsugawa, Hiroshi, Kamatani, Yudai, Okamoto, Yoichi, and Hervoches, Charles H.

Abstract

Polycrystalline Nd1−xSrxFeO3−δ(0.1 ≤ x≤ 0.9) samples exhibit a single-phase perovskite-type crystal structure and G-type antiferromagnetism with small ferromagnetic order. Assuming that the Fe site is in the mixed-valence state, the spin state of Fe changes from a low-spin (LS) or intermediate spin Fe3+-dominant state at x≤ 0.5 to a LS Fe4+dominant state at x≥ 0.6. This strongly suggests that the charge carriers change from holes to electrons. In fact, at temperatures less than 500 K, these samples show a p-type Seebeck coefficient for 0.1 ≤ x≤ 0.5 and an n-type Seebeck coefficient for 0.6 ≤ x≤ 0.9. However, for 0.6 ≤ x≤ 0.9, Fe4+is reduced to Fe3+because of an oxygen deficiency in the 500–600 K temperature range. In particular, Nd0.7Sr0.3FeO2.99(1)shows the largest p-type ZT= 0.025 at 765 K, whereas compositions with high n-type ZTcould not be identified.

Published
2023
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43. Variable temperature neutron diffraction study of crystal structure and transport pathways in oxide ion conductors Bi12.5Ln1.5ReO24.5 (Ln=Lu, Er).

Author

Hervoches, Charles H. and Greaves, Colin

Subjects
*BISMUTH compounds, *NEUTRON diffraction, *CRYSTAL structure, *NEUTRON transport theory, *IONIC conductivity, *EFFECT of temperature on metals
Abstract

Samples of highly conducting Bi12.5Lu1.5ReO24.5 and Bi12.5Er1.5ReO24.5 have been studied by neutron powder diffraction at room temperature for both phases and at 25°C≤T≤500°C in the case of Bi12.5Er1.5ReO24.5. Both materials crystallize in the cubic δ-Bi2O3 related system, space group Fm-3m. Changes in the oxygen sublattice at 25°C≤T≤500°C have been investigated by the Rietveld and maximum entropy methods. [ABSTRACT FROM AUTHOR]

Published
2014
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45. Variable temperature neutron diffraction study of Bi3ReO8 oxide ion conductor

Author

Hervoches, Charles H. and Greaves, Colin

Subjects
*NEUTRON diffraction, *OXIDES, *CRYSTAL structure, *CONDUCTIVITY of electrolytes, *HIGH temperatures, *PHASE transitions, *DIFFUSION
Abstract

Abstract: The crystal structure of fluorite-related Bi3ReO8 containing isolated ReO4 tetrahedra has been studied by powder X-ray and neutron diffraction at elevated temperatures. The phase is cubic and undergoes a phase transition between T ~150°C and 250°C passing from space group P213 at 25°C to I213 at higher temperature. The material is an oxide ion conductor and presents a sharp increase in conductivity at T ~450°C, with σ ~8×10−3 Scm−1 above 500°C. Two different long range oxide ion diffusion mechanisms involving vacancy and interstitial defects are proposed. Rotation of isolated ReO4 tetrahedra seem to facilitate the oxide ion conduction. [Copyright &y& Elsevier]

Published
2012
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46. Crystal structure and magnetic properties of the solid-solution phase Ca3Co2− v Sc v O6

Author

Hervoches, Charles H., Fredenborg, Vivian Miksch, Kjekshus, Arne, Fjellvåg, Helmer, and Hauback, Bjørn C.

Subjects
*MAGNETIC properties, *PRISMS, *CHAINS, *ATOMS
Abstract

Abstract: The two crystallographically non-equivalent Co atoms of the quasi-one-dimensional crystal structure of Ca3Co2O6 form chains with alternating, face-sharing polyhedra of Co2O6 trigonal prisms and Co1O6 octahedra. This compound forms a substitutional solid-solution phase with Sc, in which the Sc atoms enter the Co2 sublattice exclusively. The homogeneity range of Ca3Co2− v Sc v O6 (more specifically Ca3Co1Co21− v Sc v O6) extends up to v≈0.55. The crystal structure belongs to space group R3¯c with lattice parameters (in hexagonal setting): 9.0846(3)⩽a⩽9.1300(2)Å and 10.3885(4)⩽c⩽10.4677(4)Å. The magnetic moment decreases rapidly with increasing amount of the non-magnetic Sc solute in the lattice. [Copyright &y& Elsevier]

Published
2007
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47. Structural Behavior of the Four-Layer Aurivillius-Phase Ferroelectrics SrBi4Ti4O15 and Bi5Ti3FeO15

Author

Hervoches, Charles H., Snedden, Alan, Riggs, Richard, Kilcoyne, Susan H., Manuel, Pascal, and Lightfoot, Philip

Subjects
*FERROELASTIC crystals, *CRYSTALLOGRAPHY, *NEUTRON diffraction
Abstract

Rietveld refinement of powder neutron diffraction data has been used to study the crystal structures of the four-layer Aurivillius-phase ferroelectrics Bi5Ti3FeO15 (at 25°C) and SrBi4Ti4O15 (at a series of temperatures up to 800°C). At 25°C both materials adopt the polar orthorhombic space group A21am, in common with two-layer analogues such as SrBi2Ta2O9. At temperatures well above the ferroelectric Curie temperature (i.e., at temperatures of 650°C and above, with Tc∼550°C) SrBi4Ti4O15 transforms to the centrosymmetric tetragonal space group I4/mmm. However, there is good evidence from the raw diffraction data of a very subtle intermediate paraelectric orthorhombic phase, of Amam symmetry, in the region 550>650°C. The distortion in the ferroelectric phase can be traced to displacements of the cations in the A site of the perovskite block, with cooperative tilting of the BO6 octahedra. The nature of the octahedral tilt system, cation disorder at the perovskite A and B sites, and the phase transition sequence in SrBi4Ti4O15, which parallels that found in SrBi2Ta2O9, are discussed. [ABSTRACT FROM AUTHOR]

Published
2002
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