Spin reorientation in TlFe1.6Se2 with complete vacancy ordering

The relationship between vacancy ordering and magnetism in TlFe1.6Se2 has been investigated via single crystal neutron diffraction, nuclear forward scattering, and transmission electron microscopy. The examination of chemically and structurally homogenous crystals allows the true ground state to be revealed, which is characterized by Fe moments lying in the ab-plane below 100K. This is in sharp contrast to crystals containing regions of order and disorder, where a competition between c-axis and ab-plane orientations of the moments is observed. The properties of partially-disordered TlFe1.6Se2 are therefore not associated with solely the ordered or disordered regions. This contrasts the viewpoint that phase separation results in independent physical properties in intercalated iron selenides, suggesting a coupling between ordered and disordered regions may play an important role in the superconducting analogues.Comment: Minor changes; updated references and funding acknowledgemen

Lattice Dynamics in the FeSb₃ Skutterudite

Thin films of FeSb3 were characterized by electronic transport, magnetometry, x-ray diffraction, 57Fe and 121Sb nuclear inelastic scattering, and 57Fe Mössbauer spectroscopy. Resistivity and magnetometry measurements reveal semiconducting behavior with a 16.3(4) meV band gap and an effective paramagnetic moment of 0.57(6) B, respectively. A systematic comparison of the lattice dynamics with CoSb3 and EuFe 4Sb12 reveals that the Fe4Sb12 framework is softer than the Co4Sb12 framework, and that the observed softening and the associated lowering of the lattice thermal conductivity in the RFe4Sb12 filled skutterudites are not only related to the filler but also to the Fe4Sb12 framework

Software for the frontiers of quantum chemistry:An overview of developments in the Q-Chem 5 package

This article summarizes technical advances contained in the fifth major release of the Q-Chem quantum chemistry program package, covering developments since 2015. A comprehensive library of exchange–correlation functionals, along with a suite of correlated many-body methods, continues to be a hallmark of the Q-Chem software. The many-body methods include novel variants of both coupled-cluster and configuration-interaction approaches along with methods based on the algebraic diagrammatic construction and variational reduced density-matrix methods. Methods highlighted in Q-Chem 5 include a suite of tools for modeling core-level spectroscopy, methods for describing metastable resonances, methods for computing vibronic spectra, the nuclear–electronic orbital method, and several different energy decomposition analysis techniques. High-performance capabilities including multithreaded parallelism and support for calculations on graphics processing units are described. Q-Chem boasts a community of well over 100 active academic developers, and the continuing evolution of the software is supported by an “open teamware” model and an increasingly modular design

Hard-X-Ray Spectroscopy with a Spectrographic Approach

Hard-x-ray spectroscopy relies on a suite of modern techniques for studies of vibrational, electronic, andmagnetic excitations in condensed matter. At present, the energy resolution of these techniques can beimproved only by decreasing the spectral window of the involved optics—monochromators and analyzers—thereby sacrificing the intensity. Here,we demonstrate hard-x-ray spectroscopy with greatly improved energyresolution without narrowing the spectral window by adapting principles of spectrographic imaging tothe hard-x-ray regime. Similar to Newton’s classical prism, the hard-x-ray spectrograph disperses different“colors”—i.e., energies—of x-ray photons in space. Then, selecting each energy component with a slit ensureshigh energy resolution, whereas measuring x-ray spectra with all components of a broad spectral windowkeeps the intensity. We employ the principles of spectrographic imaging for phonon spectroscopy. Here thenew approach revealed anomalous soft atomic dynamics in α-iron, a phenomenon which was not previouslyreported in the literature.We argue that hard-x-ray spectrographic imaging also could be a path to discoveringnew physics in studies of electronic and magnetic excitations

On a hyperfine interaction in ε-Fe

We explore alternative ways to Mössbauer spectroscopy such as nuclear forward scattering of synchrotron radiation, and synchrotron radiation perturbed angular correlation spectroscopy to reveal the elusive and long-sought hyperfine interactions in ε-Fe. We indicate that synchrotron radiation perturbed angular correlation spectroscopy is the most viable method

Experimental and Ab Initio Study of Cu2SnS3Cu_{2}SnS_{3} (CTS) Polymorphs for Thermoelectric Applications

Cu$_2$SnS$_3$ (CTS) is a medium-temperature, ecofriendly, p-type thermoelectric material known for phonon-glass-electron-crystal characteristic. In the present work, ordered and disordered CTS samples were prepared from elemental powders, and their electronic and vibrational properties were systematically investigated by experimental methods and ab initio calculations. The disordered CTS polymorph presents a higher power factor, PF ∼ 1.5 μW/K$^2$ cm, than the ordered and stable phase, PF ∼ 0.5 μW/K$^2$ cm, above 700 K, as an effect of a smaller band gap and higher carrier concentration. Most importantly, the disordered CTS shows an ultralow thermal conductivity, k ∼ 0.4–0.2 W/m K, as compared to ordered, k ∼ 1.0–0.4W/m K, in the temperature range of 323–723 K. The combined effect of a higher PF and lower k results in a higher figure of merit, zT ∼ 0.5 at 723 K, obtained for disordered CTS without resorting to chemical alloying. It turns out that structural disorder contributes to the suppression of thermal conductivity. While group velocity of acoustic phonons, as shown both by experiments and ab initio calculations, is similar in the two polymorphs, a strong anharmonicity characterizes the disordered CTS, resulting in the presence of low-lying optical modes acting as traps for heat transmission. Density functional theory/density functional perturbation theory simulations and nuclear inelastic scattering combined with high-resolution diffraction studies of the lattice parameters reveal details of phonon–phonon interactions in CTS with unprecedented effectiveness

Sound Velocities in FeSi at Lower Mantle Conditions and the Origin of Ultralow-Velocity Zones

The origin of ultralow-velocity zones (ULVZs) remains an open question despite recent advances in mineral physics and seismology. Here, we examine the hypothesis that FeSi formed from core-mantle chemical reactions is a plausible source of ULVZs at the core-mantle boundary (CMB). The sound velocities of B2-structured FeSi were measured up to 115(2) GPa and 1600(200) K by nuclear inelastic scattering (NIS) in laser-heated diamond anvil cells (LH-DACs). Within uncertainties, the sound velocities of B2-FeSi display negligible anharmonicity, hence validating the extrapolation of velocity-density relations (Birch's law) to P-T conditions of the CMB. The sound velocities of B2-FeSi are significantly lower compared to other candidate phases in a lowermost mantle assemblage, and the Preliminary Reference Earth Model at CMB conditions. Less than 8.4 vol% of FeSi in the aggregate is thus sufficient to explain both the velocity decrements and the high density anomaly observed in a wide range of ULVZs.ISSN:0094-8276ISSN:1944-800

Application of nuclear inelastic scattering spectroscopy to the frequency scale calibration of ab initio calculated phonon density of states of quasi-one-dimensional ternary iron chalcogenide RbFeSe2

This study aims to examine the applicability of nuclear inelastic scattering (NIS) and conventional Mössbauer spectroscopy for calibration of the frequency scale of ab initio calculated phonon density of states (PDOS) of iron ternary chalcogenides. NIS measurements are carried out on the quasi-one-dimensional ternary chalcogenide RbFeSe2 to obtain the partial PDOS of the iron atoms in the compound. We compare the experimental PDOS with our previous results on vibrational properties of RbFeSe2 obtained with density functional theory (DFT) ab initio calculations, conventional Mössbauer, and infra-red spectroscopies. The experimental PDOS measured by NIS is collated with the ab initio calculated one. The frequency correction factor for the ab initio results is determined as 1.077, in good agreement with value of 1.08 obtained previously from the temperature dependence of the Lamb–Mössbauer factor of the iron atoms in RbFeSe2. We conclude that nuclear inelastic scattering and temperature dependence of the Lamb–Mössbauer factor in conventional Mössbauer spectroscopy can be equally applied for evaluation of the frequency correction factor for ab initio calculated phonon density of iron of ternary chalcogenides