Crystal Symmetry Research Articles

Lattice instability and magnetic phase transitions in strongly correlated MnAs

Using variable temperature x-ray total scattering in magnetic field, we study the interaction between lattice and magnetic degrees of freedom in MnAs, which loses its ferromagnetic order and hexagonal (‘H’) lattice symmetry at 318 K to recover the latter and become a true paramagnet when the temperature is increased to 400 K. Our results show that the 318 K transition is accompanied by highly anisotropic displacements of Mn atoms that appear as a lattice degree of freedom bridging the ‘H’ and orthorhombic phases of MnAs. This is a rare example of a lowering of an average crystal symmetry due to an increased displacive disorder emerging on heating. Our results also show that magnetic and lattice degrees of freedom appear coupled but not necessarily equivalent control variables for triggering phase transitions in strongly correlated systems in general and in particular in MnAs.

Nonlinear Plasmonic Meta‐Crystals with Coupling Induced Symmetry Breaking

AbstractSymmetry plays an important role in determining the amplitude and polarizations of nonlinear optical waves occurring in conventional crystals. While the symmetry of crystals is limited by the materials’ synthesis process, it can be easily controlled in photonic meta‐crystals. Both the linear and nonlinear optical properties of the meta‐crystals can be judiciously tailored by the local symmetry of the meta‐atoms. Here, it is demonstrated that the symmetry of the nonlinear meta‐crystals can be artificially manipulated by introducing the coupling between adjacent plasmonic meta‐atoms. By assembling the two groups of plasmonic meta‐atoms with inversion symmetry into a hexagonal boron nitride‐like lattice, the global inversion symmetry of the metasurface is broken. Second harmonic generations due to the coupling‐induced symmetry breaking are experimentally observed from the meta‐crystals. It is also found that the polarization of the second harmonic waves obey certain symmetry selection rules. The proposed methodology, which takes advantage of the coupling effects and the intrinsic nonlinearity of the plasmonic meta‐atoms, can be used to develop a variety of nonlinear nanophotonic devices with multiple functionalities.

Architected Piezoelectric Metamaterial With Designable Full Nonzero Piezoelectric Coefficients

Abstract Piezoelectric metamaterials have received extensive attention in the fields of robotics, nondestructive testing, energy harvesting, etc. Natural piezoelectric ceramics possess only five nonzero piezoelectric coefficients due to the crystal symmetry of ∞mm, which has limited the development of related devices. To obtain nonzero piezoelectric coefficients, previous studies mainly focus on assembling piezoelectric ceramic units or multiphase metamaterials. However, only part of the nonzero piezoelectric coefficients or locally piezoelectric electromechanical modes are achieved. Additionally, it still remains a challenge for manipulating the piezoelectric coefficients in a wide range. In this work, full nonzero piezoelectric coefficients are obtained by symmetry breaking in the architected piezoelectric metamaterial. The piezoelectric coefficients are designable over a wide range from positive to negative through manipulating the directions of each strut for the three-dimensional architected lattice. The architected metamaterials exhibit multiple positive/inverse piezoelectric modes, including normal and shear deformation. Finally, a smart gradient architected piezoelectric metamaterial is designed to take advantage of this feature, which can sense the position of the normal and shear force. This work paves the way for the manipulation of piezoelectric metamaterial in a wide range with designable full nonzero piezoelectric coefficients, thereby enabling application potential in the fields of smart sensing and actuation.

Oxide Coating Role on the Bulk Structural Stability of Active LiMn2O4 Cathodes.

The protective coating of the electrode materials is a known source of improvement of the cycling performances in battery devices. In the case of the LiMn2O4 cathodes, the coating with a thin alumina layer has been proven to show performance efficiency. However, the precise mechanism of its effect on the performance improvement of the electrodes is still not clear. In this work we investigate alumina-coating-induced effects on the structural dynamics of the active materials in correlation to the modified solid electrolyte interface dynamics. The local structures of coated and uncoated samples at different galvanostatic points are studied by both soft X-ray absorption measurements at the Mn L-edges and O K-edge (in total electron yield mode) and hard X-ray absorption at the Mn K-edge (in transmission mode). The different probing depths of the employed techniques allowed us to study the structural dynamics both at the surface and within the bulk of the active material. We demonstrate that the coating successfully hinders the Mn3+ disproportionation and, hence, the degradation of the active material. Side products (layered Li2MnO3 and MnO) and changes in the local crystal symmetry with formation of Li2Mn2O4 are observed in uncoated electrodes. The role of alumina coating on the stability of the passivation layer and its consequent effect on the structural stability of the bulk active materials is discussed.

1D Diisopropylammonium Lead Iodide Perovskite Shows Exceptional Optical Stability and Third‐Order Nonlinearity

AbstractLow‐dimensional lead‐halide perovskites show wide structural and compositional diversity. Multiple functionalities can be introduced in these materials for diverse linear and nonlinear optical applications. Nonlinear optical (NLO) processes depend on material polarizability, crystal symmetry, and processibility. The flexibility of choosing organic cations in lead‐halide perovskites provides an opportunity for enhanced NLO responses by enhancing the overall system polarizability. A highly polarizable diisopropylammonium [Dipa; ((CH3)2CH)2NH2+] is used as an A‐site cation to synthesize one‐dimensional (1D) lead‐iodide (DipaPbI3) perovskite single crystals and the third‐harmonic generation (THG) in the excitation wavelength range of 1200–1600 nm is explored. DipaPbI3 single crystals exhibit exceptionally efficient resonance‐enhanced THG, amazing optical stability, and a high polarization ratio due to enhanced bulk‐polarization. The THG efficiency (χ(3) ≈ 4 × 10−18 m2 V−2) and laser‐induced damage threshold (LIDT, 144 GW cm−2) are an order of magnitude higher than the reported perovskites explored as THG materials. Density functional theory calculations show a significantly higher polarizability for Dipa cation, asserting the influence of the cation polarizability on the NLO efficiency of the perovskite. The stability of these single crystals in ambient conditions for an extended period adds to their potential for commercial application in the near future.

Topological semimetal phases in a family of monolayer X3YZ6 (X=Nb,Ta, Y=Si,Ge,Sn, Z=S,Se,Te) with abundant nodal lines and nodes

The electronic and topological properties of single-layer X3YZ6 (X=Nb,Ta, Y=Si,Ge,Sn, Z=S,Se,Te) materials have been studied with the aid of first principles calculations. This kind of materials belong to topological semimetals (TMs) with abundant nodal lines and nodes. Considering their similar properties, we focus on the analysis of Ta3SnTe6 and Ta3SiSe6. The presence of spin–orbit coupling (SOC) leads to the transition from type-I nodal lines to Dirac points as well as the disappear of type-II Dirac points. The three-dimensional (3D) plot of the band structure reproduces vividly the characteristics of nodes and nodal lines. The appearance of the flat bands in (110) edge states further confirm their nontrivial topological properties. We also explore the relationship among different nodal lines (nodes), crystal symmetry and SOC. The type-I nodal lines are protected by Mz and Mỹ symmetry in the absence of SOC. Symmetry breaking leads to band splitting even in the presence of SOC. The single-layer X3YZ6 can be used as candidates for two-dimensional (2D) Dirac materials and provide a platform for further study of interesting physical phenomena.

Plant-extract-assisted green synthesis of silver nanoparticles using Macaranga Indica bark extract for antimicrobial and photocatalytic activity

A rapid and environment-friendly green synthesis of silver nanoparticles (AgNPs) using Macaranga Indica (M. Indica) plant extracts was explored in this study. The obtained AgNPs were characterized by UV–Vis, XRD, FTIR, and SEM for antimicrobial and photocatalytic activities. The typical surface plasmon resonance peak around 440 nm confirmed the formation of AgNPs. The particles were found to be very tiny in the range of 15–20 nm size and showed fcc crystal symmetry. The FT-IR study showed the presence of bio-molecules on the surface of AgNPs and acting as capping agents. The in vitro antibacterial screen of AgNPs showed the bio-capped AgNPs have a higher inhibitory action for E. coli and S. aureus followed by C.albicans. The degradation property of the AgNPs towards Rhodamine B (RhB)under different conditions revealed that the degradation efficiency of phytochemicals-coatedAgNPs on RhB was found to be high and around 88.8% within 70 min

Charge density wave-templated spin cycloid in topological semimetal NdSbxTe2−x−δ

Magnetic topological semimetals present open questions regarding the interplay of crystal symmetry, magnetism, band topology, and electron correlations. Ln${\mathrm{Sb}}_{x}{\mathrm{Te}}_{2\text{\ensuremath{-}}x\text{\ensuremath{-}}\ensuremath{\delta}}$ (Ln denotes Lanthanide) is a family of square-net-derived topological semimetals that allows compositional control of band filling, and access to different topological states via an evolving charge density wave (CDW) distortion. Previously studied Gd and Ce members containing a CDW have shown complex magnetic phase diagrams, which implied that spins localized on Ln interact with the CDW, but to this date no magnetic structures have been solved within the CDW regime of this family of compounds. Here, we report on the interplay of the CDW with magnetism in ${\mathrm{NdSb}}_{x}{\mathrm{Te}}_{2\text{\ensuremath{-}}x\text{\ensuremath{-}}\ensuremath{\delta}}$ by comparing the undistorted square net member ${\mathrm{NdSb}}_{0.94}{\mathrm{Te}}_{0.92}$ with the CDW-distorted phase ${\mathrm{NdSb}}_{0.48}{\mathrm{Te}}_{1.37}$, via single-crystal x-ray diffraction, magnetometry, heat capacity, and neutron powder diffraction. ${\mathrm{NdSb}}_{0.94}{\mathrm{Te}}_{0.92}$ is a collinear antiferromagnet with ${T}_{N}\ensuremath{\sim}2.7\phantom{\rule{4pt}{0ex}}\mathrm{K}$, where spins align antiparallel to each other, but parallel to the square net of the nuclear structure. ${\mathrm{NdSb}}_{0.48}{\mathrm{Te}}_{1.37}$ exhibits a nearly fivefold-modulated CDW (${q}_{\text{CDW}}=0.18$), isostructural to other Ln${\mathrm{Sb}}_{x}{\mathrm{Te}}_{2\text{\ensuremath{-}}x\text{\ensuremath{-}}\ensuremath{\delta}}$ at similar $x$. ${\mathrm{NdSb}}_{0.48}{\mathrm{Te}}_{1.37}$ displays more complex magnetism with ${T}_{N}=2.3\phantom{\rule{0.28em}{0ex}}\mathrm{K}$, additional metamagnetic transitions, and an elliptical cycloid magnetic structure with ${q}_{\text{mag}}=\ensuremath{-}0.41{\mathrm{b}}^{*}$. The magnitudes of ${q}_{\text{CDW}}$ and ${q}_{\text{mag}}$ exhibit an integer relationship, $1+2{q}_{\text{mag}}={q}_{\text{CDW}}$, implying a coupling between the CDW and magnetic structure. Given that the CDW is localized within the nonmagnetic distorted square net, we propose that conduction electrons ``template'' the spin modulation via the Ruderman-Kittel-Kasuya-Yosida interaction.

Eu3+-ion doped LnF3 NPs: Comparative study of the crystallographic, and photophysical properties

The thermal co-precipitation approach was employed for the synthesis of the Eu3+-ion doped YF3, LaF3, and GdF3 nanoparticles (NPs) and compared their photophysical properties. Ammonium fluorite was used as a fluorine source for the synthesis of metal fluorides at lower temperatures. Luminescent lanthanide fluoride (LnF3) NPs were characterized by different techniques containing X-ray diffraction (XRD), Thermogravimetric analysis (TGA), Fourier transform infrared (FTIR), FT-Raman, UV/Visible absorption spectra, and photoluminescence spectroscopic procedures to investigate the crystal structural, thermal, Raman effect, surface, optical, and luminescent properties. XRD study shows two types of crystal phase orthorhombic in YF3:Eu & GdF3:Eu and hexagonal in LaF3:Eu NPs. TGA, FTIR, and absorption spectra studies show the exterior adsorbed –OH molecules, which assist in the development of colloidal solution in aqueous media. Raman vibrations were strongly affected which were correlated with the XRD reflection planes. In comparative photo-physical properties, the excitation and emission spectra exhibited remarkable intensities of the Eu3+-ion transitions. Similarly, the emission spectra of all three samples exhibited the highest emission efficiency of the magnetically-dipole(5D0→7F2) transition in comparison to the electric-dipole 5D0→7F1 transition, which reflected the transformation of the crystal symmetry. Comparative analysis was presented to investigate the role of the host lattice on crystallinity, thermal stability, and luminescent properties. These findings are highly fruitful in designing photonic-based applications including optical biosensors, fingermark detection, security systems, etc.

Analogs of Rashba-Edelstein effect from density functional theory

Studies of structure-property relationships in spintronics are essential for the design of materials that can fill specific roles in devices. For example, materials with low symmetry allow unconventional configurations of charge-to-spin conversion, which can be used to generate efficient spin-orbit torques. Here, we explore the relationship between crystal symmetry and geometry of the Rashba-Edelstein effect (REE) that causes spin accumulation in response to an applied electric current. Based on a symmetry analysis performed for 230 crystallographic space groups, we identify classes of materials that can host conventional or collinear REE. Although transverse spin accumulation is commonly associated with the so-called ``Rashba materials'', we show that the presence of specific spin texture does not easily translate to the configuration of REE. More specifically, bulk crystals may simultaneously host different types of spin-orbit fields, depending on the crystallographic point group and the symmetry of the specific $k$ vector, which, averaged over the Brillouin zone, determine the direction and magnitude of the induced spin accumulation. To explore the connection between crystal symmetry, spin texture, and the magnitude of REE, we perform first-principles calculations for representative materials with different symmetries. We believe that our results will be helpful for further computational and experimental studies, as well as the design of spintronics devices.

Non-centrosymmetric topological phase probed by non-linear Hall effect

Abstract Non-centrosymmetric topological material has attracted intense attention due to its superior characteristics as compared with the centrosymmetric one, although probing the local quantum geometry in non-centrosymmetric topological material remains challenging. The non-linear Hall (NLH) effect provides an ideal tool to investigate the local quantum geometry. Here, we report a non-centrosymmetric topological phase in ZrTe5, probed by using the NLH effect. The angle-resolved and temperature-dependent NLH measurement reveals the inversion and ab-plane mirror symmetries breaking at <30 K, consistently with our theoretical calculation. Our findings identify a new non-centrosymmetric phase of ZrTe5 and provide a platform to probe and control local quantum geometry via crystal symmetries.

Growth mechanism and electronic properties of stacking mismatch boundaries in wurtzite III-nitride material

III-nitride materials including AlN, GaN, and InN are promising for semiconductor industry applications; however, the material growth process gives rise to high defect densities in the epilayer, which can affect the device performance. A systemical understanding of the defect physics is necessary for realistic applications. Among the defects in III-nitride materials, the stacking mismatch boundary (SMB) is a kind of extended defect generated due to the presence of a stacking fault, whose structure-function relationship is still not well understood. Here, we report on a first-principles investigation of the growth and electronic properties of the SMB in III-nitride materials. Based on the wurtzite crystal symmetry, it is found that the SMBs can be categorized into three basic types, depending on the terrace edge of the coalescent normal and stacking-fault regions on the (0001) surface, and the corresponding edge type is controllable by varying the chemical potential and initial nucleation size during the material growth process. Additionally, it is revealed that SMBs produce in-gap states in III-nitride materials with various properties, including itinerant magnetism with high Curie temperature and optical transition correlated with the experimentally observed sub-band-gap spectrum. It is worth noting that one type of SMB is a possible source of the yellow luminescence that is widely observed in GaN. Our findings add comprehensive insight into the SMB in III-nitride materials; the unique growth controllable property of an SMB is also a possible routine to broaden the applications of III-nitride materials.

Impact of cation modification on phonon-dressed exciton dynamics in a prototype two-dimensional hybrid organic-inorganic perovskite system.

Organic-cation engineering has recently proven effective in flexibly regulating two-dimensional hybrid organic-inorganic perovskites (2D HOIPs) to achieve a diversity of newly emerging applications. There have been many mechanistic studies based on the structural tunability of organic cations; nevertheless, those with an emphasis on the effect solely caused by the organic cations remain lacking. To this end, here we deliberately design a set of 2D HOIPs in which the inorganic layers are kept nearly intact upon cation modification, i.e., the precursor phenethylammonium lead iodide and its four derivatives with the phenyl group's para-position H being replaced by CH3, F, Cl, and Br. By means of femtosecond time-resolved transient absorption spectroscopy and temperature-dependent/time-resolved photoluminescence spectroscopy, we interrogate the subtle impact of cation modification on phonon dynamics, coherent phonon modes, phonon-dressed exciton dynamics, and excitonic emissions. A concerted trend for phonon lifetimes and exciton relaxation lifetimes regulated by cation modification is revealed, evidencing the existence of strong exciton-phonon coupling in this 2D HOIP system. The observed mass effect can be ascribed to the change in moment of inertia of organic cations. In addition, we observe an interesting interplay of exciton kinetics pertinent to population transfers between two emissive states, likely linked to the subtle variation in crystal symmetry induced by cation modification. The mechanistic insights gained from this work would be of value for the 2D HOIPs-based applications.

How does your gyroid grow? A mesoatomic perspective on supramolecular, soft matter network crystals

Supramolecular crystal phases of self-assembled soft matter share the symmetries of atomic and molecular crystals, but with vastly larger unit cells occupied by huge numbers of flexible molecules. Recent progress in understanding the formation of complex crystals of quasi-spherical soft matter domains derives from a mesoatomic analogy. In this analogy, the ultimate structure is broken down into micelle-like motifs, whose shape, size, and packing dictate thermodynamics. This begs a basic question for complex network phases, such as the double-gyroid: what are the mesoatomic ``building blocks'' of crystals with non-convex, polycontinuous, and inter-catenated domains? Here, the authors propose generic principles for divining the mesoatomic building blocks of network crystals and illustrate these principles for double-networks of diblock copolymers. They also propose and explore a minimal physical model for how mesoatomic shapes and packing can template a rich dynamics for the assembly and link formation of inter-catenating soft matter crystals.

Insights into electronic properties of strained two-dimensional semiconductors by out-of-plane bending

Strain engineering is an important strategy to modulate the electronic and optical properties of two-dimensional (2D) semiconductors. In experiments, an effective and feasible method to induce strains on 2D semiconductors is the out-of-plane bending. However, in contrast to the in-plane methods, it will generate a combined strain effect on 2D semiconductors, which deserves further explorations. In this work, we theoretically investigate the carrier transport-related electronic properties of arsenene, antimonene, phosphorene, and MoS2 under the out-of-plane bending. The bending effect can be disassembled into the in-plane and out-of-plane rolling strains. We find that the rolling always degrades the transport performance, while the in-plane strain could boost carrier mobilities by restraining the intervalley scattering. In other words, pursuing the maximum in-plane strain at the expense of minimum rolling should be the primary strategy to promote transports in 2D semiconductors through bending. Electrons in 2D semiconductors usually suffer from the serious intervalley scattering caused by optical phonons. The in-plane strain can break the crystal symmetry and separate nonequivalent energy valleys at band edges energetically, confining carrier transports at the Brillouin zone Γ point and eliminating the intervalley scattering. Investigation results show that the arsenene and antimonene are suitable for the bending technology, because of their small layer thicknesses which can relieve the rolling burden. Their electron and hole mobilities can be doubled simultaneously, compared with their unstrained 2D structures. From this study, the rules for the out-of-plane bending technology towards promoting transport abilities in 2D semiconductors are obtained.

A Double Perovskite BiFeMoO6: Microstructural, Optical, Dielectric and Transport Properties

In this communication, the synthesis (solid-state sintering) and characterization of a double perovskite BiFeMoO6 are reported. Analysis of X-ray diffraction (XRD) data provides monoclinic crystal symmetry with an average crystallite size of 85.6[Formula: see text]nm and lattice strain of 0.00078, respectively. The microstructural analysis of the sample was done using a scanning electron microscope (SEM) and the results show that grains are well-grown and distributed uniformly throughout the sample surface. The grains are visible clearly due to well-defined grain boundaries, and the effect on the mechanism of electrical ac conductivity was studied. The compositional purity of the sample was checked by energy dispersive X-ray (EDX) analysis spectrum which supports the presence of all constituent elements (Bi, Fe, Mo and O) in both weight and atomic percentages. The study of the Ultraviolet–Visible spectrum provides a bandgap energy of 1.8[Formula: see text]eV, suitable for photovoltaic applications. The measurements of the dielectric were used to confirm the existence of the Maxwell–Wagner type of dispersion. The study of impedance spectroscopy helps to understand the negative temperature coefficient of resistance (NTCR) character while the electrical modulus measurements claimed a non-Debye relaxation mechanism in the sample. The study of ac conductivity reveals the fact of thermally activated conduction mechanism in the sample. The presence of the semiconducting nature of the sample was checked from both Nyquist plots and Cole–Cole plots. The study of the resistance versus temperature reveals the fact of negative temperature coefficient (NTC) thermistor character and is suitable for some temperature sensor devices.

Nature of magnetic transitions and evidence for magnetoelastic coupling in the biskyrmion-host hexagonal compound MnNiGa

The hexagonal compound MnNiGa with Ni2In-type structure has emerged as an important biskyrmion-host system in which these textures are super-stable over a wide temperature range extending from 16 K to the Curie temperature TC ∼350 K under moderate fields. Both the intensity of the external magnetic field and the presence of magnetoelastic coupling are known to play key role in the stability of the skyrmionic textures. Here we present the results of the first comprehensive investigation of the order of the phase transitions and its field dependence along with magnetoelastic (i.e., spin-lattice) coupling using a combined magnetization and high-resolution synchrotron x-ray powder diffraction (SXRPD) study of MnNiGa. The temperature dependence of the magnetization M(T) exhibits a paramagnetic to collinear ferromagnetic (FM) transition at TC (∼347 ± 0.5 K) with thermal hysteresis and negative slope in the Arrott plots across the TC suggesting its first order character. The M(T) plot in the collinear FM phase exhibits an anomalously decreasing behaviour after reaching its peak value at 300 K followed by a step-like change below 210 K due to an additional spin reorientation transition (SRT) to a non-collinear FM phase with TSRT∼200 ± 1 K. The SRT also shows thermal hysteresis characteristic of a first order phase transition. Our field dependent studies reveal stabilization of the collinear FM phase to higher temperatures and destabilization of the SRT transition setting an upper limit on the magnetic field for the observation of biskyrmions in this system. The Rietveld analysis of the SXRPD data reveals anomalies in the unit cell parameters at TC and TSRT without any change in the crystal symmetry. The modelling of the temperature dependence of the unit cell volume in terms of the Debye-Grüneisen equation reveals significant deviation from phonon contributions and the presence of the quadratic spin-lattice coupling below the FM TC, in the precursor SRT regime as well as below TSRT. The present results on the magnetoelastic coupling in the hexagonal MnNiGa may provide the necessary insight towards understanding the extreme sensitivity of the magnetic skyrmionic textures to external stresses.

Competition between phonon-vacancy and four-phonon scattering in cubic boron arsenide by machine learning interatomic potential

Point defects can strongly suppress the thermal conductivity $\ensuremath{\kappa}$ of solid materials, which is crucial for a broad range of applications, such as thermal management of electronic devices and thermoelectrics. Understanding thermal transport in materials containing point defects often relies on atomistic simulations based on density functional theory (DFT) or empirical potentials (EPs). However, modeling thermal transport in defective materials using DFT is very computationally expensive or even prohibitive due to the breaking of crystal symmetry while EPs suffer from low accuracy. Recently, machine learning has been applied to the development of interatomic potentials, offering opportunities to model defective systems accurately and efficiently. Here, we present a Gaussian approximation potential (GAP) developed for crystalline cubic boron arsenide (c-BAs) with vacancies, which can achieve DFT-level accuracy in predicting its $\ensuremath{\kappa}$ and phonon transport properties at four orders of magnitude reduced computational cost, especially for phonon-vacancy and four-phonon scatterings. Particularly, we applied the GAP to investigate the effect of vacancies on the $\ensuremath{\kappa}$ of c-BAs by considering both three-phonon and four-phonon scattering. Special attention was paid to the competition between phonon-vacancy and four-phonon scattering, which tend to decrease and increase the temperature dependence of $\ensuremath{\kappa}$, respectively. Specifically, when the vacancy concentration is much lower than $0.07%$ $(2.56\ifmmode\times\else\texttimes\fi{}{10}^{19}\phantom{\rule{4pt}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3})$, four-phonon scattering plays stronger roles in determining the temperature dependence of $\ensuremath{\kappa}$. As the vacancy concentration increases to $0.07%$, the temperature dependence of $\ensuremath{\kappa}$ becomes close to that considering only three-phonon scattering, indicating the comparable effect of phonon-vacancy and four-phonon scattering. As the vacancy concentration further increases, the phonon-vacancy scattering becomes more dominant and pushes the $\ensuremath{\kappa}$ towards a temperature-independent behavior. Our work deepens the understanding of the phonon scattering landscape in c-BAs with vacancies and will be helpful for tailoring its thermal properties. Atomistic simulations combined with machine learning interatomic potentials are expected to be able to greatly advance the understanding of thermal transport in defective materials.

Influence of the Hydride Content on the Local Structure of a Perovskite Oxyhydride BaTiO3–xHx

The local structure of BaTiO3 perovskite has been investigated for studying phase transitions by several groups. While it has been shown that H– doping into BaTiO3 changes its long-range average crystal symmetry from tetragonal to cubic, factors contributing to the structural change have not been fully understood. In this work, we investigated the dependence of the local structure of BaTiO3–xHx on the doped H– amount by means of Raman and X-ray absorption fine-structure (XAFS) spectroscopy measurements. The x value of BaTiO3–xHx could vary in the range of 0–0.5 by changing the amount of the CaH2 precursor in the topochemical synthesis of BaTiO3–xHx. Raman spectroscopy indicated that the cubic phase is induced in the initial tetragonal phase when the doped H– amount was relatively large (x ≥ 0.2). On the other hand, XAFS measurements revealed that the symmetry of the TiO6 octahedral in BaTiO3–xHx is strongly affected by the doped H– amount. The local distortion of the Ti [111] displacement is relaxed with an increase of H–, causing the tetragonal-to-cubic structural change.

Pressure evolution of electronic and crystal structure of noncentrosymmetric EuCoGe3

We report on the pressure evolution of the electronic and crystal structures of the noncentrosymmetric antiferromagnet EuCoGe3. Using a diamond anvil cell, we performed high pressure fluorescence detected near-edge x-ray absorption spectroscopy at the Eu L3, Co K, and Ge K edges and synchrotron powder x-ray diffraction. In the Eu L3 spectrum, both divalent and trivalent Eu peaks are observed from the lowest pressure measurement (~2 GPa). By increasing pressure, the relative intensity of the trivalent Eu peak increases, and an average Eu valence continuously increases from 2.2 at 2 GPa to 2.31 at~50 GPa. On the other hand, no discernible changes are observed in the Co K and Ge K spectra as a function of pressure. With the increase in pressure, lattice parameters continuously decrease without changing I4mm symmetry. Our study revealed a robust divalent Eu state and an unchanged crystal symmetry of EuCoGe3 against pressure.