Anisotropic Lattice Expansion Research Articles

High-temperature tribological behavior of high-entropy sublattice oxide, nitride, and diboride coatings

The tribological performance of sputtered (Al,Cr,Nb,Ta,Ti)N, ▪ , and ▪ coatings on steel substrates was compared against TiN in dry ball-on-disk and scratch tests in ambient air at 20°C, 400°C and 700°C. The (Al,Cr,Nb,Ta,Ti)N and TiN perform similar in all tests, with (Al,Cr,Nb,Ta,Ti)N showing higher oxidation and abrasion resistance. The adhesion of (Al,Cr,Nb,Ta,Ti)N is superior to TiN at 20°C, but worse at elevated temperature due to an earlier onset of recovery processes that increase the mismatch in coefficient of thermal expansion with the substrate. The ▪ is the most abrasion resistant coating at room temperature owing to its high hardness, but suffers from oxidation in hot air. Scratch tests yield a similar adhesion strength to TiN at 20°C, but the anisotropic lattice expansion and shrinkage of the hexagonal structure at elevated temperatures lead to early delamination in the scratch test. The ▪ also adheres poorly on the steel, resulting in quick delamination during all tests.

Magnetoelastic Coupling Evidence by Anisotropic Crossed Thermal Expansion in Magnetocaloric RSrCoFeO6 (R = Sm, Eu) Double Perovskites.

Double perovskite oxides, characterized by their tunable magnetic properties and robust interconnection between the lattice and magnetic degrees of freedom, present an enticing foundation for advanced magnetic refrigeration materials. Herein, we delve into the influence of rare-earth elements on RSrCoFeO6 (R = Sm, Eu) disordered double perovskites by examining their structural, electronic, magnetic, and magnetocaloric properties. Temperature-dependent synchrotron X-ray diffraction analysis confirmed the stability of the orthorhombic phase (Pnma) across a wide temperature range. X-ray photoemission spectroscopy revealed that both Sm and Eu are in the 3+ state, whereas multiple states for Co2+/3+ and Fe3+/4+ are identified. The magnetic investigation and magnetocaloric effect (MCE) analysis brought to light the presence of a long-range antiferromagnetic (AFM) order with a second-order phase transition (SOPT) in both samples. The maximum magnetic entropy change ΔSMmax was approximately 0.9 J/kg K for both samples at applied field 0-7 T, manifesting prominently above Neel temperatures TN ≈ 93 K (Sm) and 84 K (Eu). Nevertheless, different relative cooling powers (RCP) of 112.6 J/kg (Sm) and 95.5 J/kg (Eu) were observed. A detailed analysis of the temperature-dependent lattice parameters shed light on a distinct magnetocaloric effect across the magnetic transition temperature, unveiling an anisotropic thermal expansion [αV = 1.41 × 10-5 K-1 (Sm) and αV = 1.54 × 10-5 K-1 (Eu)] wherein the thermal expansion axial ratio αbSm/αbEu = 0.61 became lower with increasing temperature, which suggests that the Eu sample experiences a greater thermal expansion in the b-axis direction. At the atomic bonding level, the evidence for magnetoelastic coupling around the magnetic transition temperatures TN was found through the anomalies along the average Co/Fe-O bond distance, formal valence, octahedral distortion, as well as an anisotropic lattice expansion.

Investigating the role of dispersion corrections and anharmonic effects on the phase transition in SrZrS3: A systematic analysis from AIMD free energy calculations.

A thermally driven needle-like (NL) to distorted perovskite (DP) phase transition in SrZrS3 was investigated by means of abinitio free energy calculations accelerated by machine learning. As a first step, a systematic screening of the methods to include long-range interactions in semilocal density functional theory Perdew-Burke-Ernzerhof calculations was performed. Out of the ten correction schemes tested, the Tkatchenko-Scheffler method with iterative Hirshfeld partitioning method was found to yield the best match between calculated and experimental lattice geometries, while predicting the correct order of stability of NL and DP phases at zero temperature. This method was then used in free energy calculations, performed using several approaches, so as to determine the effect of various anharmonicity contributions, such as the anisotropic thermal lattice expansion or the thermally induced internal structure changes, on the phase transition temperature (TNP→DP). Accounting for the full anharmonicity by combining the NPT molecular dynamics data with thermodynamic integration with harmonic reference provided our best estimate of TNL→DP = 867K. Although this result is ∼150K lower than the experimental value, it still provides an improvement by nearly 300K compared to the previous theoretical report by Koocher et al. [Inorg. Chem. 62, 11134-11141 (2023)].

Interstitial nitrogen-modified Y2Fe16SiNy compounds towards enhanced high-frequency magnetic properties

The rapid development of information technology urgently requires high-frequency soft magnetic materials with excellent electromagnetic performance. Herein, we synthesized the disordered dual-phase Y2Fe16Si intermetallic compound and introduced nitrogen atoms into its interstitial crystal sites by the gas-solid reaction, remarkably improving its magnetic and electromagnetic properties. The significant magneto-volume effect generated by introduced nitrogen atoms increases the compound's magnetization and Curie temperature, leading to increased permeability of Y2Fe16Si-paraffin composites. Furthermore, the synergistic effect of the magneto-volume and the chemical bonding change the compound's electric polarization by charge neutralization, dramatically decreasing the permittivity of Y2Fe16Si-paraffin composites upon nitrogenation. The decreased permittivity and increased permeability enhance the impedance matching and magnetic loss capability of the composites. Besides, the introduction of interstitial nitrogen gives rise to a higher ratio of the out-of-plane to in-plane anisotropy field of Y2Fe16Si due to anisotropic lattice expansion, which combines with enhanced magnetization to raise the Snoek limit and operating frequency of the composites. Consequently, Y2Fe16SiNy-paraffin composites exhibit the maximum effective absorption bandwidth (EAB) of 5.84 GHz in a thickness of 1.2 mm, and the minimum reflection loss (RLmin) of –50.52 dB in a thickness of 1.5 mm covering the Ku-band, demonstrating strong absorption, broad EAB with thin thickness and high operating frequency. This work shows the superiority of the interstitial atom effect in the high-frequency electromagnetic field, and the effect is applicable and scalable for rare-earth Fe-based intermetallic compounds to develop excellent microwave absorbers.

Crystal structure, microstructure, and mechanical properties of heat-treated oyster shells

We investigated the crystal structure and mechanical properties of oyster shells subjected to heat treatment under increasing temperature conditions. The shell contained folia and chalky layers. The folia layer comprised two CaCO3 phases: 72.3% calcite and 27.7% aragonite. The lattice parameters of the calcite and aragonite present in the folia layer did not correspond to those of the synthesized sample. The anisotropic lattice expansion was observed in calcite and aragonite in the folia layer during heat-treatment. The chalky layer has also the anisotropic lattice expansion, but the expansion was disappeared at 573 K. The microhardness (HV value) of the folia layer decreased rapidly from 122 to 11 HV at temperatures 573–673 K owing to the phase transformation from aragonite to calcite in this temperature range. The microhardness of the chalky layer at RT was 125 HV, which decreased to 15 HV at 373 K. Crack propagation with increasing temperature was investigated using a micro-Vickers apparatus. In the folia layer, cracks were produced inside the prism, and they propagated along the lamellar structure. The cracks initiated and propagated along the organic biopolymer interlayers in a zigzag manner. No cracks were observed in the chalky layers of the heat-treated samples. The toughness of the chalky layer was superior to that of the folia layer. From our results, we can conclude that oyster shells comprise two types of materials with different mechanical properties.

Rational design, crystal structure, and frustrated magnetism of the Ge-containing YbFe2O4-type layered oxides In2Zn3-xCoxGeO8 (0 ≤ x ≤ 3).

YbFe2O4-type layered oxides have attracted tremendous interest because the unique crystal comprises two distinct geometrically frustrated triangular cation-sublattices. Herein, a series of YbFe2O4-type materials In2Zn3-xCoxGeO8 (0 ≤ x ≤ 3) were rationally designed and experimentally synthesized for the first time. The crystal structures of In2Zn3-xCoxGeO8 were investigated comprehensively by Rietveld refinements against high-resolution monochromatic Cu Kα1 XRD data. Zn2+, Co2+, and Ge4+ cations are distributed randomly on the [MO]2 bilayer and possess a trigonal bipyramid (TBP) coordination geometry. Because Co2+ has an unpaired electron in the dz2 orbital and a larger electronegativity than Zn2+, Co2+-to-Zn2+ equivalent substitution in In2Zn3-xCoxGeO8 results in more compact MO5-TBPs, which is the origin of anisotropic lattice expansion and contraction along the a and c axes, respectively. The Co2+ moments in the [MO]2 bilayer are strongly AFM coupled and geometrically frustrated, therefore resulting in a spin-glass magnetic transition at around Tg = 20 K for In2ZnCo2GeO8, while a long-range AFM ordering is established for In2Co3GeO8 with a Néel temperature of 53 K, attributed to the significantly enhanced AFM interactions and increased In3+/Co2+ anti-site disordering, as compared to those in In2ZnCo2GeO8.

Electrodeposition of Rhenium Sandwich Structures and the Superconducting Transition Behavior

Superconductors with enhanced superconducting critical temperatures (Tc) above the boiling point of liquid helium (4.2 K) are of interest for interconnections in superconducting quantum computing systems to avoid ohmic heating[1]. Crystalline Re in bulk is a type-I superconductor with a Tc of about 1.7K[2]. An elevated Tc up to about 6K in electrodeposited amorphous Re has been reported[3]. In addition, various elevation in Tc has also been reported for Re film under shear strain[4] or doped with foreign elements such as Tungsten (W), Osmium (Os), and Carbon (C)[5, 6].This work aims to test a hypothesis that the Tc of electrodeposited Re films can be tuned using compressive and tensile strains on the Re films generated with thermal expansions difference. Figure 1 shows a diagram of the strains on Re film sandwiched between metals with different coefficients of thermal expansions (CTEs) when the temperature is lowered. For example, Cu has a higher CTE than Re and Cr has a slightly lower CTE than Re. Such sandwich structures are prepared at room temperature using electrochemical deposition. The DC resistance measurements at cryogenic temperatures confirmed that the superconducting transition for the Re films sandwiched between Cu occurs at a lower temperature than the Re film alone. On the other hand, when Cr is used, the sandwiched Re structure shows a slightly enhanced superconducting critical temperature. The effect of thermal annealing i.e., the recrystallization of electrodeposited Re films at an elevated temperature, and electrochemical behavior of Re, Cr, and Cu electrodeposition will be discussed in the talk. References Radenbaugh, R., Refrigeration for superconductors. Proceedings of the IEEE, 2004. 92(10): p. 1719-1734.Hulm, J.K., Superconductivity of Pure Metallic Rhenium. Physical Review, 1954. 94(5): p. 1390-1391.Pappas, D.P., et al., Enhanced superconducting transition temperature in electroplated rhenium. Applied Physics Letters, 2018. 112(18): p. 182601.Mito, M., et al., Large enhancement of superconducting transition temperature in single-element superconducting rhenium by shear strain. Scientific Reports, 2016. 6(1): p. 36337.Chu, C.W., W.L. McMillan, and H.L. Luo, Superconductivity of Re-Os, Re-Ru, Ru-Os, and Re-W hcp Alloy Systems and Slightly Doped Re. Physical Review B, 1971. 3(11): p. 3757-3762.Zhu, Q., et al., Anisotropic lattice expansion and enhancement of superconductivity induced by interstitial carbon doping in Rhenium. Journal of Alloys and Compounds, 2021. 878: p. 160290. Figure 1

Phase transformation of hydride-phase Nd2Co7 with superlattice structure

We investigated the phase stability of Nd2Co7 with a superlattice structure, its hydrogenation properties, and phase transformation during hydrogen absorption–desorption process. The results showed that Ce2Ni7-type and Gd2Co7-type structures coexisted in the annealed sample up to 1273 ​K. The Gd2Co7-type phase disappeared at temperatures above 1323 ​K. We successfully synthesized a largely single-phase Ce2Ni7-type Nd2Co7 when the alloy was annealed at 1373 ​K, followed by quenching in ice water. The phase transformation follows the order: hexagonal Nd2Co7 (space group P63/mmc) → hexagonal Nd2Co7H2.7 (space group P63/mmc) → orthorhombic Nd2Co7H8.1 (space group Pbcn), upon hydrogen absorption. Anisotropic lattice expansion along the c axis was observed in Nd2Co7H2.7. The unit cell of Nd2Co7H8.1 underwent isotropic expansion along the a, b, and c axes. However, the volume expansions of the MgZn2-type cell, CaCu5-type cell I, and CaCu5-type cell II were uneven at 36.0%, 8.9%, and 13.0%, respectively.

Thermal Enhanced Resistive Switching Performance of <100>‐oriented Perovskite [(TZ‐H)2(PbBr4)]n with High Working Temperature: a Triazolium/(PbBr4)n2n– Interfacial Interaction Insight

AbstractDespite the great progress has been made on device parameters and working mechanism of perovskite‐based memristors, thermal instability under extreme conditions limits their performance, and thermal effect on their resistive switching (RS) characteristics remains unclear. Herein, from the viewpoint of organic/inorganic interfacial interaction in a novel 2D <100>‐oriented perovskite [(TZ‐H)2(PbBr4)]n (TZ = 1H‐1,2,4‐triazole), thermal effect on the RS performance of perovskite‐based memristor is investigated. This FTO/[(TZ‐H)2(PbBr4)]n/Ag memristor can exhibit high thermal tolerance with a working temperature of 170 °C, and the best RS characteristics can be achieved at 140 °C. Mechanistic studies are executed based on X‐ray single structural analysis, powder X‐ray diffraction, UV–Vis, and fluorescence. Before the occurrence of phase change below 140 °C (α‐phase), anisotropic lattice expansion illustrated by the weaker inter‐layer NH···Br hydrogen bond, longer layer–layer distances, more distorted PbBr6 octrahedra, larger optical gap, and quenched fluorescence can be beneficial for the trap‐filled limited process. After phase transformation into β‐phase, the breakage of layer–layer interaction and looser layer–layer packing will inhibit the halogen migration, resulting in more space charge limited conduction characters. The unique thermal enhanced RS performance with status monitored by fluorescent chromism can provide a new paradigm for the development of new memristors with highly environmental tolerance.

Oxygen-Deficient (Ln,Sr)2NiO4- δ Nickelates for Oxygen Electrodes of Solid Oxide Fuel and Electrolysis Cells: Anisotropic Thermochemical Expansion and Thermomechanical Constraints

Perovskite-like and perovskite-related oxides based on variable-valence transition metals may exhibit substantial changes in oxygen content in the course of temperature cycling or under variation of oxygen chemical potential at elevated temperatures. This gives rise to a chemical contribution to the thermal expansion (chemical expansion/contraction or chemical strain) originating mainly from the accompanying changes in the average oxidation state of transition metal cations and, consequently, their ionic radii. Excessive dimensional changes caused by the chemical expansion impose constraints on the applicability of functional oxides at elevated temperatures and under variable redox conditions (due to thermomechanical instability or incompatibility with other materials) and complicate the processing of oxide ceramics at high temperatures. Another unfavorable phenomenon is microcracking caused by a strongly anisotropic expansion of crystal lattice, with effects on the microstructural and mechanical integrity and electrical and electrochemical properties. The present work is a representative study case that focuses on layered Ln2NiO4-based ceramic materials for oxygen electrodes of solid oxide fuel/electrolysis cells (SOFC/SOEC).Ln2NiO4+δ–based oxides (Ln = La, Pr, Nd) with perovskite-related K2NiF4-type structure demonstrate high oxygen diffusivity and surface exchange kinetics in combination with comparatively high electronic conductivity and are considered, therefore, as attractive mixed ionic-electronic conductors for oxygen electrodes of high-temperature SOFC/SOEC [1]. Acceptor-type substitution of Ln3+ by Sr2+ in Ln2NiO4+ δ is compensated by comparatively high oxygen deficiency in Sr-rich Ln2-xSrxNiO4-δ phases at elevated temperatures [2,3]. The transition from oxygen excess to oxygen deficiency is expected to be accompanied by a change in the ionic transport mechanism from prevailing interstitial oxygen diffusion in rock-salt layers to oxygen vacancy diffusion in perovskite-type layers.Ceramic powders of Ln2-xSrxNiO4-δ (Ln = La, Nd, Pr, x = 1.0-1.6) were prepared by the Pechini method with final calcination steps at 1150-1200°C in oxygen. Thermogravimetric studies confirmed that acceptor-type substitution by strontium is compensated by the formation of oxygen vacancies and electron-holes and progressively increases high-temperature oxygen nonstoichiometry (Fig.1A), which reaches as high as δ = 0.36-0.40 for x = 1.6 at 900°C in air. High-temperature XRD studies demonstrated that Ln2-xSrxNiO4-δ nickelates exhibit strongly anisotropic lattice expansion interrelated with oxygen deficiency changes on heating (Fig.1B). Highly anisotropic expansion induces microcracking phenomenon, which, in turn, results in a strong hysteresis in dimensional changes (Fig.1C) and also in variations of electrical conductivity (Fig.1D) on cycling in air for the ceramic samples sintered at 1200-1300°C.An appropriate approach to minimize or neglect the undesirable effects of anisotropic expansion and microcracking is to preserve the grain size in Ln2-xSrxNiO4-δ ceramics or porous electrodes at ~1 micron or submicron level. In particular, spark plasma sintering with subsequent careful oxidation treatment was adopted to avoid grain growth and to produce mechanically stable ceramics with smooth reversible thermochemical expansion and enhanced electrical properties. Acknowledgments: This work was supported by the FCT/MCTES/FEDER (projects CARBOSTEAM (POCI-01-0145-FEDER-032295) and CICECO-Aveiro Institute of Materials (UIDB/50011/2020, UIDP/50011/2020 & LA/P/0006/2020), and PhD grant SFRH/BD/138773/2018).

Evidence for a coupled magnetic-crystallographic transition in La0.9Ce0.1Fe12B6

Competition between antiferromagnetic (AFM), paramagnetic (PM), and ferromagnetic (FM) states in ${\mathrm{La}}_{0.9}{\mathrm{Ce}}_{0.1}{\mathrm{Fe}}_{12}{\mathrm{B}}_{6}$ compound is investigated by means of temperature- and magnetic field-dependent x-ray diffraction, magnetization, linear thermal expansion, and magnetostriction experiments. It is shown that both AFM and PM phases get converted into the FM phase via a first-order metamagnetic transition, which is accompanied by a huge forced-volume magnetostriction $\mathrm{\ensuremath{\Delta}}V/V(25\phantom{\rule{0.16em}{0ex}}\mathrm{K},\phantom{\rule{0.16em}{0ex}}6\phantom{\rule{0.16em}{0ex}}\mathrm{T})=1.15%$. X-ray powder diffraction reveals a magnetic field-induced crystallographic phase transition from a $R\overline{3}m$ rhombohedral (AFM, PM) to a $C2/m$ monoclinic (FM) structure. A peculiarly anisotropic lattice expansion as well as giant negative thermal expansion with a volumetric thermal expansion coefficient ${\ensuremath{\alpha}}_{V}=\ensuremath{-}193\ifmmode\times\else\texttimes\fi{}{10}^{--6}\phantom{\rule{0.16em}{0ex}}{\mathrm{K}}^{--1}$ are observed. These findings point to the significance of magnetoelastic effects in this metamagnet and illustrate the strength of the coupling between lattice and spin degrees of freedom in the ${\mathrm{La}}_{0.9}{\mathrm{Ce}}_{0.1}{\mathrm{Fe}}_{12}{\mathrm{B}}_{6}$ intermetallic compound.

Anisotropic lattice expansion and enhancement of superconductivity induced by interstitial carbon doping in Rhenium

We report the effect of carbon doping on the structural and superconducting properties of elemental Re studied in a series of polycrystalline ReCx samples with x up to 0.10. Structural analyses indicate that the C atoms occupy the octahedral interstitial sites in the hexagonal close-packed Re lattice. This leads to an expansion of unit cell mainly along the a-axis and a shift of the binding energies of Re towards higher values. Furthermore, these ReCx samples are found to be weakly coupled, fully gapped, type-II superconductors. With increasing C content x, both Tc and zero temperature upper critical field Bc2(0) increase monotonically. At x = 0.10, Tc and Bc2(0) reach 2.65 K and 1.18 T, respectively, which are more than 50% and 60 times larger than the corresponding values of pure Re. Combined with theoretical calculations, we show that the enhancement in Tc induced by C doping is due to the increase in both the density of states at the Fermi level and electron-phonon coupling. Our results suggest that the introduction of interstitial atoms provides a viable way to tune the superconductivity in transition metal elements.

Controlling Magnetic Anisotropy in a Zero-Dimensional S = 1 Magnet Using Isotropic Cation Substitution.

The [Zn1–xNix(HF2)(pyz)2]SbF6 (x = 0.2; pyz = pyrazine) solid solution exhibits a zero-field splitting (D) that is 22% larger [D = 16.2(2) K (11.3(2) cm–1)] than that observed in the x = 1 material [D = 13.3(1) K (9.2(1) cm–1)]. The substantial change in D is accomplished by an anisotropic lattice expansion in the MN4 (M = Zn or Ni) plane, wherein the increased concentration of isotropic Zn(II) ions induces a nonlinear variation in M-F and M-N bond lengths. In this, we exploit the relative donor atom hardness, where M-F and M-N form strong ionic and weak coordinate covalent bonds, respectively, the latter being more sensitive to substitution of Ni by the slightly larger Zn(II) ion. In this way, we are able to tune the single-ion anisotropy of a magnetic lattice site by Zn-substitution on nearby sites. This effect has possible applications in the field of single-ion magnets and the design of other molecule-based magnetic systems.

Anisotropic thermal lattice expansion and crystallographic structure of strontium aluminide within Al-10Sr alloy as measured by in-situ neutron diffraction

The aluminium strontium master alloy Al-10Sr has been investigated by in-situ neutron diffraction upon a heating-cooling cycle, revealing composition, crystallographic structure, lattice evolution and linear thermal expansion coefficients. Expansion of the Al matrix between [23.5 … 26.7]·10−6 K−1 depends on temperature and fits well to the literature values, extrapolating to higher temperature at 800 K. Thermal expansion is highly anisotropic for tetragonal Al4Sr by a factor of 1.86 with values of 20.8 and 11.1·10−6 K−1 in a and c-axis. The even larger discrepancy to the Al matrix is prone to residual intergranular phase stresses explaining the brittleness of such composite material. Upon first heating, recovery of the initially plastically deformed material is observed until 600 K and 700 K, for Al4Sr and Al. Rietveld analysis refines the 4e Wyckoff positions of the I 4/m m m crystal structure to z = 0.39 revealing that local tetrahedrons are regular while local hexagons are stretched, in contrast to literature. Its lattice parameters report to aI = 4.44240(48) Å, cI = 11.0836(15) Å at 300 K. Furthermore, the manuscript demonstrates full technical analysis of the neutron data. Findings feed into data bases and an outlook for improving mechanical properties of Al4Sr composites is given.

Synthesis, characterization and thermal behaviour of solid phases in the quasi-ternary system Mg(SCN)2-H2O-THF.

Mg(SCN)2·4H2O can be converted into previously unknown compounds Mg(SCN)2·(4 - x) H2O·xTHF with x = 0, 2 and 4 by multiple recrystallization in tetrahydrofuran (THF). The phases were characterized by infrared spectroscopy (IR), thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC), and their crystal structures were solved from X-ray powder diffraction (XRPD) data. In the crystal structures isolated Mg(NCS)2(H2O)4-x(THF)x units form layered motifs. The thermal behavior of Mg(SCN)2·4H2O and Mg(SCN)2·4THF was investigated by temperature dependent in situ XRPD, where Mg(SCN)2·4THF was found to acquire a room temperature (α-form) and high temperature modification (β-form). The phase transformation is associated with an order-disorder transition of the THF molecules and with a reversion of the stacking order of the layered motifs. Further heating eventually leads to the formation of Mg(SCN)2·2THF. There thiocyanate related sulfur atoms fill the voids in the coordination sphere of magnesium, which leads to the formation of one dimensional electroneutral ∞[Mg(NCS)2/2(SCN)2/2(THF)2] chains. All investigated Mg(SCN)2·(4 - x) H2O·xTHF phases exhibit a remarkable anisotropic thermal expansion, and Mg(SCN)2·4H2O and Mg(SCN)2·2THF were found to show both positive and negative thermal expansion coefficients.

Investigation of the itinerant metamagnetic system Hf0.75Ta0.25Fe2 under extreme conditions of pressure or magnetic field

The magnetic and structural properties of Hf0.75Ta0.25Fe2 have been investigated under the duplicate conditions, i.e., high magnetic fields and high pressures, by combining magnetization measurements up to 26 T, neutron powder diffraction experiments up to 14.5 T, and angle dispersive synchrotron x-ray diffraction up to 10 GPa. The intermetallic compound Hf0.75Ta0.25Fe2 exhibits an antiferromagnetic (AFM) ground state below the Néel temperature TN = 342 K. In the AFM structure, the Fe-6h magnetic moments (1.02 μΒ at 2 K) align along the c axis while Fe atoms at 2a sites are not magnetically ordered. We further show that the AFM ground state gets transformed into a ferromagnetic (FM) state via a field-induced metamagnetic phase transition. Magnetic-field dependent neutron diffractograms demonstrate that FM phase sets in with a change of the easy magnetization direction from axial to basal-plane and an extremely anisotropic lattice expansion. The unit cell expands drastically in the basal plane upon increasing the applied magnetic field, while its dimension along c-axis shrinks slightly; resulting in a huge positive volume magnetostriction ΔV/V = 0.78%. The application of hydrostatic pressure leads to the continuous decrease of the cell parameters. The shrinkage of the hexagonal cell is anisotropic with a larger compression along the six-fold symmetry axis c. A bulk modulus of K0 = 247 GPa has been deduced from the pressure-volume relationship.

On the Dependence of Ionic Transport on Crystal Orientation in Nasicon-Type Solid Electrolytes

The dependence of ionic transport on crystal orientations in NaSICON-type solid electroytes is studied on flux-grown M3Sc2(PO4)3 (M = Na, Ag) single crystals with well-defined facets. Herein, we provide the first impedance spectroscopy study to characterize ion conduction along different crystallographic orientations in this important class of materials for electrochemical energy storage systems. Moreover, we used single crystal X-ray diffraction, differential scanning calorimetry, 23Na NMR spin-lattice relaxation measurements, and ab initio molecular dynamics simulations to study the interplay of structure and ion transport taking place at different length scales. We conclude that the phase behavior in NaSICON-type materials is strongly linked to ion diffusion. At room temperature, ionic conductivity is slightly anisotropic along the crystallographic orientations [001] and [100]. The slightly different activation energies are related to diffusion bottlenecks solely changing along [001]. This change is caused by anisotropic thermal lattice expansion. With increasing temperature, ion transport increasingly becomes isotropic finally resulting in an order-disorder phase transition from C2/c to R−3c. This phase transition is associated with a clear change in activation energy solely along [001]; it can be traced back to the increasing jump distance along this crystal orientation with temperature. Astonishingly, changing the ionic charge carrier, i.e., when going from Na+ to Ag+, shifts the phase transition temperature by 140 K toward lower temperature. The Arrhenius behaviour remains, however, similar. This finding is related to the higher mobility of Ag+ in the NaSICON framework leading to isotropic ion diffusion at much lower temperatures. Overall, flux-grown M3Sc2(PO4)3 allowed us to show that ionic transport parameters and phase stability sensitively depend on crystal chemistry.

Hydride formation in Ti6Al4V: An in situ synchrotron X-ray diffraction study

Hydrogen uptake causes hydride formation, lattice strains and other crystallographic changes in Ti6Al4V. To monitor these changes and understand them better, we developed an in situ hydrogen-charging setup for transmission X-ray diffraction, and employed it at a high-energy synchrotron X-ray source. We observed anisotropic lattice expansion by H ingress, as well as hydride formation induced microstrain evolution in both phases. The hydride phase was also crystallographically characterized, including its orientation relationship with the neighboring phases, employing electron backscattered diffraction analysis. Further applications of the presented methodology to investigate H-microstructure interactions are also discussed.

Structural features of solid-solid phase transitions and lattice dynamics in U3O8

Triuranium octoxide $({\mathrm{U}}_{3}{\mathrm{O}}_{8})$ undergoes an orthorhombic to hexagonal structural phase transition near ${T}_{s}=305{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$, and a separate nonstructural phase transition at ${T}_{c}=210{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$. The later transition has previously been associated with temperature-induced fluctuations in the uranium oxidation state. A discontinuity in the slope of electrical conductivity versus temperature measurement at $210{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$ has supported this idea. The orthorhombic phase has three crystallographic sites in two distinct oxidation configurations [2 U(V) and 1 U(VI)], whereas the hexagonal phase has one distinct uranium site. High-resolution x-ray diffraction measurements eliminate the possibility of superlattice Bragg reflections to less than 0.2 ${\mathrm{e}}^{\ensuremath{-}}$ scattering power and ${\mathrm{U}}_{3}{\mathrm{O}}_{8}$ is not metallic; consequently, the presence of oxidation fluctuations is required for charge balancing. Interestingly, the order-to-disorder transition occurs at a much lower temperature than the structural transition. Using temperature-dependent x-ray diffraction and Raman spectroscopy, we show anisotropic lattice expansion in the in-plane $b$ and $c$ lattice constants. A specific discontinuity in the temperature derivatives of the $b$ and $c$ lattice constants are the first reported structural signatures of the order-to-disorder transition, suggestive of a change in local U--O coordination. Phonon frequencies of ${\mathrm{U}}_{3}{\mathrm{O}}_{8}$ measured by Raman spectroscopy show significant temperature-dependent dynamics. Redshifting of several modes between 40 and $300{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$ cannot be explained by unit cell expansion alone because the unit cell volume decreases in this region. Instead, we show that phonon frequencies are highly correlated with the anisotropic lattice expansion/contraction along specific crystallographic directions.

On the dependence of ionic transport on crystal orientation in NaSICON-type solid electrolytes

The dependence of ionic transport on crystal orientations in NaSICON-type solid electrolytes is studied on flux-grown M3Sc2(PO4)3 (M = Na, Ag) single crystals with well-defined facets. Herein, we provide the first impedance spectroscopy study to characterize ion conduction along different crystallographic orientations in this important class of materials for electrochemical energy storage systems. Moreover, we used single crystal x-ray diffraction, differential scanning calorimetry, 23Na NMR spin-lattice relaxation measurements, and ab initio molecular dynamics simulations to study the interplay of structure and ion transport taking place at different length scales. We conclude that the phase behavior in NaSICON-type materials is strongly linked to ion diffusion. At room temperature, ionic conductivity is slightly anisotropic along the crystallographic orientations [001] and [100]. The slightly different activation energies are related to diffusion bottlenecks solely changing along [001]. This change is caused by anisotropic thermal lattice expansion. With increasing temperature, ion transport increasingly becomes isotropic finally resulting in an order-disorder phase transition from C2/c to R−3c. This phase transition is associated with a clear change in activation energy solely along [001]; it can be traced back to the increasing jump distance along this crystal orientation with temperature. Astonishingly, changing the ionic charge carrier, i.e. when going from Na+ to Ag+, shifts the phase transition temperature by 140 K towards lower temperature. The Arrhenius behavior remains, however, similar. This finding is related to the higher mobility of Ag+ in the NaSICON framework leading to isotropic ion diffusion at much lower temperatures. Overall, flux-grown M3Sc2(PO4)3 allowed us to show that ionic transport parameters and phase stability sensitively depend on crystal chemistry.