The Crystal Structure of Al4SiC4 Revisited.

Al4SiC4 is a ternary wide-band-gap semiconductor with a high strength-to-weight ratio and excellent oxidation resistance. It consists of slabs of Al4C3 separated by SiC layers with the space group of P63mc. The space group allows Si to occupy two different 2a Wykoff sites, with previous studies reporting that Si occupies only one of the two sites, giving it an ordered structure. Another hitherto unexplored possibility is that Si can be randomly distributed on both 2a sites. In this work, we revisit the published ordered crystal structure using experimental methods and density functional theory (DFT). Al4SiC4 was synthesized by high-temperature sintering at 1800 °C from a powder mixture of Al4C3 and SiC. Neutron diffraction confirmed that Al4SiC4 crystallized with the space group of P63mc, with diffraction patterns that could be fitted to both the ordered and the disordered structures. Scanning transmission electron microscopy, however, provided clear evidence supporting the latter, with DFT calculations further confirming that it is 0.16 eV lower in energy per Al4SiC4 formula unit than the former. TEM analysis revealed Al vacancies in some of the atomic layers that can introduce p-type doping and direct band gaps of 0.7 and 1.2 eV, agreeing with our optical measurements. Finally, we propose that although the calculated formation energy of the Al vacancies is high, the vacancies are stabilized by entropy effects at the high synthesis temperature. This indicates that the cooling procedure after high-temperature synthesis can be important in determining the vacancy content and the electronic properties of Al4SiC4.

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

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 (TC), 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 TC. The X-ray diffraction measurements versus temperature reveal cell volume minima at TC 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 TC. The Tb moments refined at 16 K are 4.1(2) µB/Tb for x = 2, and 6.2(1) µB/Tb for x = 3, which are smaller than the free ion value (9.5 µB/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) µB/atom for x = 2 and 1.8(1) µB/atom for x = 3 at 16 K. This moment is slightly canted compared to the Tb moment.

Dynamic Structure Evolution of Extensively Delithiated High Voltage Spinel Li1+xNi0.5Mn1.5O4 x < 1.5.

High voltage spinel is one of the most promising next-generation cobalt-free cathode materials for lithium ion battery applications. Besides the typically utilized compositional range of LixNi0.5Mn1.5O4 0 < x < 1 in the voltage window of 4.90-3.00 V, additional 1.5 mol of Li per formula unit can be introduced into the structure, in an extended voltage range to 1.50 V. Theoretically, this leads to significant increase of the specific energy from 690 to 1190 Wh/kg. However, utilization of the extended potential window leads to rapid capacity fading and voltage polarization that lack a comprehensive explanation. In this work, we conducted potentiostatic entropymetry, operando XRD and neutron diffraction on the ordered stoichiometric spinel LixNi0.5Mn1.5O4 within 0 < x < 2.5 in order to understand the dynamic structure evolution and correlate it with the voltage profile. During the two-phase reaction from cubic (x < 1) to tetragonal (x > 1) phase at ∼2.8 V, we identified the evolution of a second tetragonal phase with x > 2. The structural evaluation during the delithiation indicates the formation of an intermediate phase with cubic symmetry at a lithium content of x = 1.5. Evaluation of neutron diffraction data, with maximum entropy method, of the highly lithiated phase LixNi0.5Mn1.5O4 with 2 < x < 2.5 strongly suggests that lithium ions are located on octahedral 8a and tetrahedral 4a positions of the distorted tetragonal phase I41amd. Consequently, we were able to provide a conclusive explanation for the additional voltage step at 2.10 V, the sloping voltage profile below 1.80 V, and the additional voltage step at ∼3.80 V.

Revealing the Magnetic Structure and Properties of Mn(Co,Ge)2.

The atomic and magnetic structures of Mn(Co,Ge)2 are reported herein. The system crystallizes in the space group P63/mmc as a superstructure of the MgZn2-type structure. The system exhibits two magnetic transitions with associated magnetic structures, a ferromagnetic (FM) structure around room temperature, and an incommensurate structure at lower temperatures. The FM structure, occurring between 193 and 329 K, is found to be a member of the magnetic space group P63/mm'c'. The incommensurate structure found below 193 K is helical with propagation vector k = (0 0 0.0483). Crystallographic results are corroborated by magnetic measurements and ab initio calculations.

Stabilization of the Trigonal Langasite Structure in Ca3Ga2-2xZnxGe4+xO14 (0 ≤ x ≤ 1) with Partial Ordering of Three Isoelectronic Cations Characterized by a Multitechnique Approach.

Crystallization of oxide glasses rich in Zn2+, Ga3+, and Ge4+ is of interest for the synthesis of new transparent ceramics. In this context, we report the identification and detailed structural characterization of a new solid solution Ca3Ga2-2xZnxGe4+xO14 (0 ≤ x ≤ 1). These compounds adopt the trigonal langasite structure type, offering three possible crystallographic sites for the coordination of isoelectronic Zn2+, Ga3+, and Ge4+. We used neutron diffraction to determine distributions of Ga3+/Ge4+ and Zn2+/Ge4+ in the simpler end members Ca3Ga2Ge4O14 and Ca3ZnGe5O14, while for the complex intermediate member Ca3GaZn0.5Ge4.5O14, we used an original approach combining quantitative 2D analysis of atomic-resolution STEM-EDS maps with neutron diffraction. This revealed that, across the solid solution, the tetrahedral D sites remain fully occupied by Ge4+, while Zn2+, Ga3+, and the remaining Ge4+ are shared between octahedral B- and tetrahedral C sites in proportions that depend upon their relative ionic radii. The adoption of the trigonal langasite structure by glass-crystallized Ca3ZnGe5O14, a compound that was previously observed only in a distorted monoclinic langasite polymorph, is attributed to substantial disorder between Zn2+ and Ge4+ over the B and C sites. The quantitative 2D refinement of atomic-resolution STEM-EDS maps is applicable to a wide range of materials where multiple cations with poor scattering contrast are distributed over different crystallographic sites in a crystal structure.

Texture and Differential Stress Development in W/Ni-Co Composite after Rotary Swaging.

Knowledge of texture and residual stresses in tungsten heavy pseudoalloys is substantial for the microstructure optimization. These characteristics were determined in cold and warm rotary swaged W/NiCo composite with help of neutron diffraction. The results were discussed in view of the observed microstructure and mechanical properties. The investigated bars consisted of tungsten agglomerates (bcc lattice) surrounded by NiCo-based matrix (fcc lattice). No preferential crystallographic orientation was found in the as-sintered bar. A strong texture was formed in both the tungsten agglomerates (<101> fiber texture parallel to the swaging axis) and in the NiCo-based matrix (<111> fiber texture) after rotary swaging. Although usually of double-fiber texture, the <001> fiber of the fcc structures was nearly missing in the matrix. Further, the cold-swaged bar exhibited substantially stronger texture for both phases which corresponds to the higher measured ultimate tensile strength. The residual stress differences were employed for characterization of the stress state of the bars. The largest residual stress difference (≈400 MPa) was found at the center of the bar deformed at room temperature. The hoop stresses were non-symmetrical with respect to the swaging axis, which was likely caused by the elliptical cross section of the as-sintered bar.

Structure and Properties of High­Strength Ti Grade 4 Prepared by Severe Plastic Deformation and Subsequent Heat Treatment.

Severe plastic deformation represented by three passes in Conform SPD and subsequent rotary swaging was applied on Ti grade 4. This process caused extreme strengthening of material, accompanied by reduction of ductility. Mechanical properties of such material were then tuned by a suitable heat treatment. Measurements of in situ electrical resistance, in situ XRD and hardness indicated the appropriate temperature to be 450 °C for the heat treatment required to obtain desired mechanical properties. The optimal duration of annealing was stated to be 3 h. As was verified by neutron diffraction, SEM and TEM microstructure observation, the material underwent recrystallization during this heat treatment. That was documented by changes of the grain shape and evaluation of crystallite size, as well as of the reduction of internal stresses. In annealed state, the yield stress and ultimate tensile stress decreased form 1205 to 871 MPa and 1224 to 950 MPa, respectively, while the ductility increased from 7.8% to 25.1%. This study also shows that mechanical properties of Ti grade 4 processed by continual industrially applicable process (Conform SPD) are comparable with those obtained by ECAP.

Correlating Microstrain and Activated Slip Systems with Mechanical Properties within Rotary Swaged WNiCo Pseudoalloy.

Due to their superb mechanical properties and high specific mass, tungsten heavy alloys are used in demanding applications, such as kinetic penetrators, gyroscope rotors, or radiation shielding. However, their structure, consisting of hard tungsten particles embedded in a soft matrix, makes the deformation processing a challenging task. This study focused on the characterization of deformation behavior during thermomechanical processing of a WNiCo tungsten heavy alloy (THA) via the method of rotary swaging at various temperatures. Emphasis is given to microstrain development and determination of the activated slip systems and dislocation density via neutron diffraction. The analyses showed that the grains of the NiCo2W matrix refined significantly after the deformation treatments. The microstrain was higher in the cold swaged sample (44.2 × 10-4). Both the samples swaged at 20 °C and 900 °C exhibited the activation of edge dislocations with <111> {110} or <110> {111} slip systems, and/or screw dislocations with <110> slip system in the NiCo2W matrix. Dislocation densities were determined and the results were correlated with the final mechanical properties of the swaged bars.

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

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.

Critical Assessment of Techniques for the Description of the Phase Composition of Advanced High-Strength Steels.

The phase composition and portion of individual phases in advanced high-strength steels (AHSS) CP1000 and DP1000 was studied by complementary microscopic and diffraction techniques. CP1000 and DP1000 steel grades have a high strength-to-density ratio and they are used in many applications in the automotive industry. The microstructure of the CP1000 "complex phase" steel consists of ferrite, bainite, martensite and a small amount of retained austenite. DP1000 is a dual phase steel, which has a structure of a ferritic matrix with islands of martensite and a minor amount of retained austenite. The influence of selected etchants (Nital, LePera, Beraha I, Nital followed by metabisulfite, Nital followed by LePera, and Nital followed by Beraha I) on the microstructure image is described. X-ray diffraction, neutron diffraction and light optical, scanning and transmission electron microscopy were used in this work for advanced characterization of the microstructure and phase composition. The information provided by each technique is critically compared.

Transformation Pathway upon Heating of Metastable ß Titanium Alloy Ti-15Mo Investigated by Neutron Diffraction.

A transformation pathway during thermal treatment of metastable ß Ti-15Mo alloy was investigated by in situ neutron diffraction. The evolution of individual phases α , ß , and ω was investigated during linear heating with two heating rates of 1.9 ∘ C / min and 5 ∘ C / min and during aging at 450 ∘ C . The results showed that with a sufficient heating rate (5 ∘ C / min in this case), the ω phase dissolves before the α phase forms. On the other hand, for the slower heating rate of 1.9 ∘ C / min , a small temperature interval of the coexistence of the α and ω phases was detected. Volume fractions and lattice parameters of all phases were also determined.

Structural characterization of semi-heusler/light metal composites prepared by spark plasma sintering.

A composite of powders of semi-Heusler ferromagnetic shape memory and pure titanium was successfully prepared by spark plasma sintering at the temperature of 950 °C. Sintering resulted in the formation of small precipitates and intermetallic phases at the heterogeneous interfaces. Various complementary experimental methods were used to fully characterize the microstructure. Imaging methods including transmission and scanning electron microscopy with energy dispersive X-ray spectroscopy revealed a position and chemical composition of individual intermetallic phases and precipitates. The crystalline structure of the phases was examined by a joint refinement of X-ray and neutron diffraction patterns. It was found that Co38Ni33Al29 decomposes into the B2-(Co,Ni)Al matrix and A1-(Co,Ni,Al) particles during sintering, while Al, Co and Ni diffuse into Ti forming an eutectic two phase structure with C9-Ti2(Co,Ni) precipitates. Complicated interface intermetallic structure containing C9-Ti2(Co,Ni), B2-(Co,Ni)Ti and L21-(Co,Ni)(Al,Ti) was completely revealed. In addition, C9-Ti2(Co,Ni) and A1-(Co,Ni,Al) precipitates were investigated by an advanced method of small angle neutron scattering. This study proves that powder metallurgy followed by spark plasma sintering is an appropriate technique to prepare bulk composites from very dissimilar materials.

Nonlinear magnetoelectric effect in paraelectric state of Co4Nb2O9 single crystal.

We report the structural, magnetoelectric (ME), magnetic and electric control of magnetic properties in Co4Nb2O9 (CNO) single crystal. A detailed ME measurement reveals a nonlinear ME effect instead of a linear ME effect in CNO single crystal. By fitting the magnetization-electric field (M-E) curve, it can be found that the linear ([Formula: see text]) and quadratic (γ) coefficients equal to ~8.27 ps/m and ~-6.46 ps/MV for upper branch, as well as ~8.38 ps/m and ~6.75 ps/MV for the lower branch. More importantly, a pronounced response was observed under a small cooling magnetic field, which cannot even cause the spin flop. This suggests a magnetoelectric effect can occur at paraelectric state for CNO single crystal. Furthermore, we also found that the magnetization of every axis responds to electric field applied along a-axis, but fails to do so when the electric field is applied c-axis. Such findings supply a direct evidence to the magnetic structure and ME coupling mechanism indirectly reflected by our neutron experiment.

Neutron diffraction as a precise and reliable method for obtaining structural properties of bulk quantities of graphene.

Graphene based carbon materials have attracted a great deal of attention in the last decade; nowadays tons of graphene are produced yearly. However, there is lack of precise and reliable techniques for the determination of structural properties of graphene on the bulk scale. The analytical methods being routinely applied for graphene characterization, including TEM and AFM, can be only used for the study of scant amounts of graphene samples and do not give general information on the average number of layers and the structure of the prepared graphenes. On the other hand, diffraction methods can be advantageously used to obtain information on the average thickness of the produced graphene as well as on the average sheets lateral dimensions, without the necessity of sample dispersion in solvents. We present a study of the structural properties of graphene prepared by chemical and thermal reduction of graphite oxide, comparing SEM, STEM, AFM, Raman spectroscopy, BET, X-ray and neutron diffraction methods. Our study brings new deep insights into the basic structural properties of graphene in a bulk form. Given the importance of a suitable characterization technique on the bulk materials, we wish to highlight the importance of these diffraction techniques for accurate determination of the graphene thickness and lateral parameters.

Long range ordered magnetic and atomic structures of the quasicrystal approximant in the Tb-Au-Si system.

The atomic and magnetic structure of the 1/1 Tb(14)Au(70)Si(16) quasicrystal approximant has been solved by combining x-ray and neutron diffraction data. The atomic structure is classified as a Tsai-type 1/1 approximant with certain structural deviations from the prototype structures; there are additional atomic positions in the so-called cubic interstices as well as in the cluster centers. The magnetic property and neutron diffraction measurements indicate the magnetic structure to be ferrimagnetic-like below 9 K in contrast to the related Gd(14)Au(70)Si(16) structure that is reported to be purely ferromagnetic.

Layered and pillared metal carboxyethylphosphonate hybrid compounds.

A series of carboxyethylphosphonate hybrid materials has been prepared: Mn(II)(O3PCH2CH2COOH) *H2O (1), Mn(III)(OH)(O3PCH2CH2COOH)*H2O (2), Al3(III)(OH)3(O3PCH2CH2CO2)2 *3H2O (3) and Cr2(III)(OH)3(O3PCH2CH2CO2) *3H2O (4). Compounds 1 and 2 were synthesized from Mn(III)(CH3COO)3 *2H2O under hydrothermal, or refluxing treatments, respectively. The crystal structures of the manganese-bearing solids have been solved ab initio from laboratory X-ray powder diffraction data and refined by the Rietveld method. 1 crystallises in a orthorhombic cell and 2 in monoclinic symmetry. Both solids have inorganic 2D layered structures with the acid carboxylic groups pointing towards the interlayer space, and the layers linked only through hydrogen bonds. The inorganic layers of these compounds are formed by manganese atoms in distorted octahedral environments linked together by the phosphonate groups. The crystal structure of 3 has been solved ab initio from synchrotron X-ray powder diffraction data. This solid shows a pillared structure with the phosphonate and carboxylate groups cross-linking the inorganic layers. These layers contain chains of aluminium octahedra running parallel to each other. 4 is amorphous and the IR-UV-VIS spectra suggest a framework with Cr(III) cations in octahedral environments. Thermal, spectroscopic and magnetic data for manganese and chromium compounds as well as the structural details of these solids are discussed.