Low-temperature Orthorhombic Phase Research Articles

Bandgap Pressure Coefficient of a CH3NH3PbI3 Thin Film Perovskite.

Recent scientific interest in examining the bandgap evolution of a MAPbI3 hybrid perovskite by applying hydrostatic pressure has mostly focused on a room-temperature tetragonal phase. In contrast, the pressure response of a low-temperature orthorhombic phase (OP) of MAPbI3 has not been explored and understood. In this research, we investigate for the first time how hydrostatic pressure alters the electronic landscape of the OP of MAPbI3. Pressure studies using photoluminescence combined with calculations within density functional theory at zero temperature allowed us to identify the main physical factors affecting the bandgap evolution of the OP of MAPbI3. The negative bandgap pressure coefficient was found to be strongly dependent on the temperature (α120K = -13.3 ± 0.1 meV/GPa, α80K = -29.8 ± 0.1 meV/GPa, and α40K = -36.3 ± 0.1 meV/GPa). Such dependence is related to the changes in the Pb-I bond length and geometry in the unit cell as the atomic configuration approaches the phase transition as well as the increasing phonon contribution to octahedral tilting as the temperature increases.

Structural effects of Mn-substitution in β-FeSe

In the present work the structural and microstructural properties of Mn substituted β-FeSe are inspected by using high-resolution synchrotron X-ray powder diffraction data collected between 10 K and 150 K. It is demonstrated that the thermal increase of the structural strain in the tetragonal phase progressively breaks the 4-fold symmetry at the local scale, qualitatively tracing the development of anisotropy measured in some physical properties, generally ascribed to nematicity. This result points to a close relationship and interplay between structural strain and physical properties; with this notion, the structural strain can be considered as the origin of the unexpected anisotropy observed in some physical properties. Remarkably, incommensurate satellite peaks are observed in the diffraction pattern collected in the low-temperature orthorhombic phase field, likely related to the occurrence of a charge-density-wave instability.

Correlation-driven metal-insulator transition in unconventional magnetic metal superoxides

We have extensively studied the electronic and magnetic properties of alkali sodium superoxide (NaO2) in comparison with that of potassium superoxide (KO2) both at high and low temperatures, using first-principles electronic structure calculations. The unpaired electron donated by the alkali atoms Na and K to the O atoms, forming dimers in NaO2 and KO2 respectively, control their properties. In both cases, this unpaired electron is the cause of orbital fluctuations in the O-π* manifold. Both undergo several structural phase transitions to minimize orbital fluctuations. As a result, the O2− dimers undergo several rotational configurations. This leads to a complex linking of their orbital and spin degrees of freedom. Therefore, as the orientation of the O2− dimers changes, so do the magnetic properties that were found to be controlled by this unpaired electron. Simultaneously the alkali ion cages surrounding the O2− dimers change from square in the pyrite phase to rhombus and rectangle in the orthorhombic phase for NaO2 and square in the tetragonal phase to a parallelogram in the monoclinic phase for KO2 on the plane cutting through the dimers. The different degree of the lifting of degeneracy in the O-π* manifold is owing to an intrinsic difference in the electrostatic interaction between the K/Na cages and the O2− dimmers, as shown by the band structures of NaO2 in the low-temperature orthorhombic phase and that of KO2 in the monoclinic phase. This, along with electron correlation among the localized O-π* electrons, establishes complete orbital ordering (OO) and thereby metal-insulator transition (MIT) in both systems. Correlation-induced MIT also occurs upon K doping for Na in NaO2 but is predicted to take place at a temperature higher than that in pure NaO2. This makes MIT in Rb/Cs-doped NaO2 a possibility at higher temperatures.

Nonlinear terahertz control of the lead halide perovskite lattice

Lead halide perovskites (LHPs) have emerged as an excellent class of semiconductors for next-generation solar cells and optoelectronic devices. Tailoring physical properties by fine-tuning the lattice structures has been explored in these materials by chemical composition or morphology. Nevertheless, its dynamic counterpart, phonon-driven ultrafast material control, as contemporarily harnessed for oxide perovskites, has not yet been established. Here, we use intense THz electric fields to obtain direct lattice control via nonlinear excitation of coherent octahedral twist modes in hybrid CH3NH3PbBr3 and all-inorganic CsPbBr3 perovskites. These Raman-active phonons at 0.9 to 1.3 THz are found to govern the ultrafast THz-induced Kerr effect in the low-temperature orthorhombic phase and thus dominate the phonon-modulated polarizability with potential implications for dynamic charge carrier screening beyond the Fröhlich polaron. Our work opens the door to selective control of LHP's vibrational degrees of freedom governing phase transitions and dynamic disorder.

Anomalous magnetic entropy changes of bulk and powder Mn0.5Fe0.5-xNi1+xSi0.94Al0.06 intermetallic system

The multi-component MnTX (T = Ni, Co; X = p-block elements) intermetallic system have attracted intensive attention due to their tunable magneto-structural phase transition and associated giant magnetocaloric effect. Here, we report an experimental study on the magnetic, magnetocaloric, and electrical properties of alloys that belong to the MnTX family- Mn0.5Fe0.5-xNi1+xSi0.94Al0.06. For all values of x (0≤ x ≤0.10), the materials exhibited a first-order magneto-structural phase transition from a low-temperature ferromagnetic orthorhombic phase to a high-temperature paramagnetic hexagonal phase. The transition was accompanied by large magnetic entropy changes of up to –22 and −57 J kg−1K−1 at T = 322 K for field changes of 2 T and 5 T, respectively. Most noticeably, a sharp jump in electrical resistivity was observed at T = 320 K for x = 0.1, which confirmed that the first-order phase transition preserved the mechanical stability of the material. The alloy with x = 0.1 was further crushed into powder and the magnetic properties were compared to its bulk counterpart. The enhanced mechanical stability and nearly zero magnetic hysteresis loss along with the fact that the system is constituted of cheap, non-toxic elements make this system worthy of consideration for near-room temperature magnetic refrigeration applications.

Structural, magnetic and magnetocaloric properties of hexagonal MnCoGe-based thin films

The characteristic size of the magnetocaloric materials in magnetic refrigerators, thermomagnetic regenerators and energy harvesters is usually down to micrometer and nanometer length scales, which provides the largest possible contact surface area to the heat transfer liquid. Therefore, it is pivotal to investigate the structural, magnetic and magnetocaloric properties of magnetocaloric materials at micrometer and nanometer length scales. In the present study, we have successfully fabricated MnCoGe-based thin films using the pulsed laser deposition (PLD) technique. The as-deposited film shows a dense surface, homogeneous chemical distribution and a preferred orientation along the c-axis of the hexagonal Ni2In-type structure. In-situ temperature-dependent X-ray diffraction measurements on the as-deposited films reveal an irreversible structural degradation above 300 °C due to the considerable evaporation of the Mn atoms, which causes the formation of microscale pores in the films. The thermomagnetic behavior of the film annealed at 250 °C displays a reversible second-order ferromagnetic transition of the hexagonal phase at Curie temperature TC = 275 K, and a first-order hexagonal-orthorhombic structural transition below 200 K. The decoupling of the magnetic and structural transitions brings reversible magnetocaloric effect with the entropy change ΔS values of 2.5 J/(kgK) and the refrigeration capacity reaching 89 J/kg in Δμ0H = 5 T, which are comparable to other reported magnetocaloric thin films. Moreover, antiferromagnetic coupling in the low-temperature orthorhombic phase was observed, which has rarely been reported in the MnCoGe-based bulk alloys. The different magnetic ground states between the thin film and the bulk alloys may be originated from the strain effect imposed by the Al2O3 substrate. Consequently, this work not only provides insights into the structural, magnetic and magnetocaloric properties of the MnCoGe-based alloys at a nanometer length scale, but also offers a new idea for tailoring the multifunctional properties for this material family.

Specific Features of Phase Formation and Properties of Compounds La2W1 + xO6 + 3x (x ~ 0; 0.11–0.22)

The hexagonal solid solution La2W1 + xO6 + 3x (x ~ 0.11) and the orthorhombic low-temperature phase β-La2WO6 (La2W1 + xO6 + 3x (x = 0)) are synthesized by the methods of preliminary mechanical activation of oxides followed by high-temperature synthesis at 1400°С, 4 h. In addition, by using the method of crystallization from solution, a single crystal of the La2W1 + xO6 + 3x (x ~ 0.22) composition isostructural to La2W1 + xO6 + 3x (x ~ 0.11) is grown. Both ceramics and the single crystal are studied by the methods of Raman spectroscopy, thermal analysis, and thermogravimetry. Their conductivity is studied by impedance spectroscopy in dry and humid air. The hexagonal single crystal La2W1 + xO6 + 3x (x ~ 0.22) demonstrates strong luminescence in the IR region, in contrast to the hexagonal ceramics La2W1 + xO6 + 3x (x ~ 0.11) and the β-La2WO6 ceramics with the orthorhombic structure. The polycrystalline La2W1 + xO6 + 3x (x ~ 0.11) ceramics is found to be more stable under redox conditions as compared with the single crystal. The conductivity of the hexagonal single crystal La2W1 + xO6 + 3x (x ~ 0.22) is of the oxygen-ionic nature and lower than the conductivity of ceramics La2W1 + xO6 + 3x (x ~ 0.11) due to its perfect structure. The contribution of the protonic component of conductivity is absent for both the hexagonal solid solution La2W1 + xO6 + 3x (x ~ 0.11) and the single crystal La2W1 + xO6 + 3x (x ~ 0.22), their conductivity being of the purely ionic nature with the close values of activation energy (0.89 and 1.08 eV, respectively). The synthesized β-La2WO6 ceramics demonstrates a small contribution of the protonic conductivity in humid air equal to ~1 × 10–6 S/cm at 600°С, which is close to the conductivity of the earlier studied strontium-doped solid solution La1.96Sr0.04WO6 – δ based on β-La2WO6.

Luminescence and Dielectric Switchable Properties of a 1D (1,1,1-Trimethylhydrazinium)PbI3 Hybrid Perovskitoid.

The synthesis and investigation of the physicochemical properties of a novel one-dimensional (1D) hybrid organic-inorganic perovskitoid templated by the 1,1,1-trimethylhydrazinium (Me3Hy+) cation are reported. (Me3Hy)[PbI3] crystallizes in the hexagonal P63/m symmetry and undergoes two phase transitions (PTs) during heating (cooling) at 322 (320) and 207 (202) K. X-ray diffraction data and temperature-dependent vibrational studies show that the second-order PT to the high-temperature hexagonal P63/mmc phase is associated with a weak change in entropy and is related to weak structural changes and different confinement of cations in the available space. The second PT to the low-temperature orthorhombic Pbca phase that corresponds to the high change in entropy and dielectric switching is associated with an ordering of the trimethylhydrazinium cations, re-arrangement and strengthening of hydrogen bonds, and slightly shifted lead-iodide octahedral chains. The high-pressure Raman data revealed two additional PTs, one between 2.8 and 3.2 GPa, related to the symmetry decrease, ordering of the cations, and inorganic chain distortion, and the other in the 6.4-6.8 GPa range related to the partial and reversible amorphization. Optical studies revealed that (Me3Hy)[PbI3] has a wide band gap (3.20 eV) and emits reddish-orange excitonic emission at low temperatures with an activation energy of 65 meV.

High-pressure investigations in CH3NH3PbX3 ( X = I, Br, and Cl): Suppression of ion migration and stabilization of low-temperature structure

Hybrid organic-inorganic halide perovskites represent a promising next-generation photovoltaic material with drawbacks for structure stability and composition concerns. Demonstrations of ion migration and molecular dynamics suggest room for structural contraction and subsequent property adjustments. Here, we deploy dielectric and infrared spectroscopy under external pressure to probe the full structural phase diagram and dielectric response of methylammonium lead halide perovskites ${\mathrm{CH}}_{3}{\mathrm{NH}}_{3}\mathrm{Pb}{X}_{3}$ ($X\phantom{\rule{4pt}{0ex}}=$ I, Br, or Cl). Ion migration can be fully suppressed by pressure beyond 4 GPa. The low-temperature orthorhombic phase transition can be gradually enhanced and stabilized at ambient conditions with increasing pressure. A slow relaxation mode, presumably the motion of the ${\mathrm{CH}}_{3}{\mathrm{NH}}_{3}^{+}$ cation, is observed at lower pressure and is absent in the orthorhombic phase for every halide.

Crystal structure thermal evolution and novel orthorhombic phase of methylammonium lead bromide, CH3NH3PbBr3

Methylammonium (MA) lead trihalide perovskites, CH3NH3PbX3 (X = I, Br, Cl), have emerged as a new class of light-absorbing materials for photovoltaic applications, reaching efficiencies of 23% when implemented in solar cell heterojunctions. In particular, MAPbBr3 is a promising member with a large bandgap that gives rise to a high open circuit voltage. Here we present a structural study from neutron diffraction (ND) data of an undeuterated MAPbBr3 specimen, carried out to follow its crystallographic behaviour in the 2–298 K temperature range. Besides the known crystallographic phases, i.e. the high-temperature Pmoverline{3}m cubic structure, the intermediate I4/mcm tetragonal symmetry and the low-temperature Pnma orthorhombic phase, we additionally identified, from a detailed sequential ND analysis, a novel intermediate phase within the 148.5–154.0 K temperature range as an orthorhombic Imma structure, early associated with a coexistence of phases. Moreover, our ND data allowed us to unveil the configuration of the organic MA units and their complete localization within the mentioned temperature range, thus improving the crystallographic description of this compound. The evolution with temperature of the H-bonds between the organic molecule and the inorganic cage is also followed. A deep knowledge of the crystal structure and, in particular, the MA conformation inside the perovskite cage seems essential to establish structure–property correlations that may drive further improvements.

Giant room temperature magnetocaloric response in a (MnNiSi)1−x(FeNiGa)x system

The coincidence of magnetic and structural transitions near room temperature is observed in (MnNiSi)1−x(FeNiGa)x (x = 0.16 and 0.17) systems, which leads to a coupled magnetostructural transition (MST) from a high-temperature paramagnetic Ni2In-type hexagonal phase to a low-temperature ferromagnetic TiNiSi-type orthorhombic phase associated with a substantial change in magnetization and a large change in structural unit cell volume, and thus, across MST, a giant magnetocaloric effect is obtained in these systems. The alloys with x = 0.16 and 0.17 are observed to show a giant isothermal magnetic entropy change (ΔSM) of about −26.2 and −63.2 J kg−1 K−1, accompanied with a large relative cooling power of about 220.1 and 264.5 J/kg, respectively, due to a magnetic field change (μ0ΔH) of 5 T only. Moreover, the material with x = 0.16 and 0.17 shows a large temperature average magnetic entropy change of about −21.64 and −34.4 J kg−1 K−1 over a temperature span of 10 K due to μ0ΔH ∼ 5 T. Thus, these low-cost materials with giant magnetocaloric responses are highly suitable to be used as magnetic refrigerants for room temperature solid-state-based cooling technology.

Refined GFN1-xTB Parameters for Engineering Phase-Stable CsPbX3 Perovskites

Halide perovskites are a promising class of materials for optoelectronic applications due to their excellent optoelectronic performance. However, they suffer from several dynamical degradation problems, the characterization of which is challenging. Atomic scale simulations can provide valuable insights; however, the high computational cost of first-principles methods such as DFT makes it difficult to model dynamical processes in large perovskite systems. In this work, we present refined GFN1-xTB parameters for the accurate description of structural and dynamical properties of CsPbBr3, CsPbI3, and CsPb(I1–xBrx)3. Our molecular dynamics simulations show that the phase stability is strongly correlated to the displacement of ions in the perovskites. In the low temperature orthorhombic phase, directional movement of the Cs cations increases their distance to the surrounding halides, initiating a transition to the nonperovskite phase. However, once enough thermal energy is available, the increased Cs–halide distance can be compensated by increased halide fluctuations, resulting in a transition to the phase-stable tetragonal or cubic phases. Furthermore, we find the mixing of halides increases halide displacement over a significant range of temperatures, resulting in lower phase transition temperatures and therefore improved phase stability.

Influence of the Halide Ion on the A-Site Dynamics in FAPbX3 (X = Br and Cl)

The optoelectronic properties and ultimately photovoltaic performance of hybrid lead halide perovskites are inherently related to the dynamics of the organic cations. Here, we report the dynamics of the formamidinium (FA) cation in FAPbX3 perovskites for chloride and bromide varieties, as studied by neutron spectroscopy. Elastic fixed window scans showed the onset of the reorientational motion of FA cations in FAPbCl3 to occur at a considerably higher temperature compared to that in FAPbBr3. In addition, we observed two distinct dynamical transitions only in the chloride system, suggesting a significant variation in the reorientational motions of the FA cation with temperature. Quasielastic neutron scattering data analysis of FAPbCl3 showed that in the low-temperature orthorhombic phase, FA cations undergo twofold jump reorientations about the C–H axis, which evolve into an isotropic rotation in the intermediate tetragonal and high-temperature cubic phases. Comparing the results with those from FAPbBr3 reveals that the time scale and barrier to reorientation and the geometry of the reorientational motion of the FA cations are significantly different for the two halides. We note that this dependence of the dynamic properties of the A-site cation on the halide is unique to the FA series; the geometry of methylammonium (MA) cation dynamics in MAPbX3 is known to be insensitive to different halides.

Charge-Ordering and Structural Transition in the New Organic Conductor δ'-(BEDT-TTF)2CF3CF2SO3.

We report structural, transport, and optical properties and electronic structure calculations of the δ′-(BEDT-TTF)2CF3CF2SO3 (BEDT-TTF = bis(ethylenedithio)tetrathiafulvalene) organic conductor that has been synthesized by electrocrystallization. Electronic structure calculations demonstrate the quasi-one-dimensional Fermi surfaces of the compound, while the optical spectra are characteristic for a dimer–Mott insulator. The single-crystal X-ray diffraction measurements reveal the structural phase transition at 200 K from the ambient-temperature monoclinic P21/m phase to the low-temperature orthorhombic Pca21 phase, while the resistivity measurements clearly show the first order semiconductor-semiconductor transition at the same temperature. This transition is accompanied by charge-ordering as it is confirmed by splitting of charge-sensitive vibrational modes observed in the Raman and infrared spectra. The horizontal stripe charge-order pattern is suggested based on the crystal structure, band structure calculations, and optical spectra.

Challenges and Applications to Operando and In Situ TEM Imaging and Spectroscopic Capabilities in a Cryogenic Temperature Range.

ConspectusIn this Account, we describe the challenges and promising applications of transmission electron microscopy (TEM) imaging and spectroscopy at cryogenic temperatures. Our work focuses on two areas of application: the delay of electron-beam-induced degradation and following low-temperature phenomena in a continuous and variable temperature range. For the former, we present a study of LiMn1.5Ni0.5O4 lithium ion battery cathode material that undergoes electron beam-induced degradation when studied at room temperature by TEM. Cryogenic imaging reveals the true structure of LiMn1.5Ni0.5O4 nanoparticles in their discharged state. Improved stability under electron beam irradiation was confirmed by following the evolution of the O K-edge fine structure by electron energy-loss spectroscopy. Our results demonstrate that the effect of radiation damage on discharged LiMn1.5Ni0.5O4 was previously underestimated and that atomic-resolution imaging at cryogenic temperature has a potential to be generalized to most of the Li-based materials and beyond. For the latter, we present two studies in the imaging of low-temperature phenomena on the local scale, namely, the evolution of ferroelectric and ferromagnetic domains walls, in BaTiO3 and Y3Fe5O12 systems, respectively, in a continuous and variable temperature range. Continuous imaging of the phase transition in BaTiO3, a prototypical ferroelectric system, from the low-temperature orthorhombic phase continuously up to the centrosymmetric high-temperature phase is shown to be possible inside a TEM. Similarly, the propagation of domain walls in Y3Fe5O12, a magnetic insulator, is studied from ∼120 to ∼400 K and combined with the application of a magnetic field and electrical current pulses to mimic the operando conditions as in domain wall memory and logic devices for information technology. Such studies are promising for studying the pinning of the ferroelectric and magnetic domains versus temperature, spin-polarized current, and externally applied magnetic field to better manipulate the domain walls. The capability of combining operando TEM stimuli such as current, voltage, and/or magnetic field with in situ TEM imaging in a continuous cryogenic temperature range will allow the uncovering of fundamental phenomena on the nanometer scale. These studies were made possible using a MEMS-based TEM holder that allowed an electron-transparent sample to be transferred and electrically contacted on a MEMS chip. The six-contact double-tilt holder allows the alignment of the specimen into its zone axis while simultaneously using four electrical contacts to regulate the temperature and two contacts to apply the electrical stimuli, i.e., operando TEM imaging. This Account leads to the demonstration of (i) the high-resolution imaging and spectroscopy of nanoparticles oriented in the desired [110] zone-axis direction at cryogenic temperatures to mitigate the electron beam degradation, (ii) imaging of low-temperature transitions with accurate and continuous control of the temperature that allowed single-frame observation of the presence of both the orthorhombic and tetragonal phases in the BaTiO3 system, and (iii) magnetic domain wall propagation as a function of temperature, magnetic field, and current pulses (100 ns with a 100 kHz repetition rate) in the Y3Fe5O12 system.

Low-temperature phase transition and magnetic properties of K3YbSi2O7

The new ambient-temperature hexagonal (space group P63 /mmc) polymorph of tripotassium ytterbium(III) disilicate (β-K3YbSi2O7) has been synthesized by the high-temperature flux method and subsequently structurally characterized. In the course of the temperature-dependent single-crystal diffraction experiments, a phase transformation of β-K3YbSi2O7 to a novel low-temperature orthorhombic phase (β′-K3YbSi2O7, space group Cmcm) has been observed at about 210 K. β-K3YbSi2O7 is isostructural with K3ErSi2O7, whereas β′-K3YbSi2O7 adopts a new type of structure. Both compounds can be built up from a regular alternation of layers of two types, which are parallel to the (001) plane. In the octahedral layer, YbO6 octahedra are isolated and linked by K1O6+3 polyhedra. The second, slightly thicker sorosilicate layer is formed by a combination of Si2O7 dimers and K2O6+3 polyhedra. The boundary between the layers is a pseudo-kagome oxide sheet based on 3.6.3.6 meshes. The phase transition is due to a tilt of the two SiO4 tetrahedra forming a single dimer which induces a decrease of the Si—O—Si angle between bridging Si—O bonds from 180° (dictated by symmetry in space group P63/mmc) to ≃164°. Magnetic characterization indicates that K3YbSi2O7 remains paramagnetic down to 2 K, showing no apparent influence of the phase transformation on its magnetic properties. Analysis of the magnetization data revealed the positions of the three lowest crystal field levels of the Yb3+ cations, as well as the corresponding projections of their angular momentum on the direction of the magnetic field.

Antiferromagnetic ordering and possible lattice response to dynamic uranium valence in U3O8

Determining the correct electronic structure of ${\mathrm{U}}_{3}{\mathrm{O}}_{8}$ remains a formidable experimental and theoretical challenge. In the low-temperature phase, two crystallographic U sites are separated into a distinct 2U(V)$+1\mathrm{U}$(VI) oxidation configuration. At low temperatures, the U(V) sites form a distorted honeycomb lattice, but the U(VI) sit on a triangular sublattice, suggesting potential for magnetic frustration effects. The spin configuration of the unpaired $f$ electrons on the U(V) sites is likely antiferromagnetic (AFM) from susceptibility measurements, but this has not been confirmed. Here, we present a neutron scattering investigation of the structure and dynamics of ${\mathrm{U}}_{3}{\mathrm{O}}_{8}$ from 1.7 to 600 K. We confirm static AFM ordering onset at between 22 and 25 K, which is present down to at least 1.7 K with AFM peaks corresponding to [0.5 1 1] and [0.5 2 2] in the orthorhombic phase. These measurements rule out static AFM order along the $a$ axis of the $Amm2$ phase, a configuration previously suggested by theory. Above 100 K a quasielastic scattering channel opens that we speculate arises from a lattice relaxation response to thermally activated electron hopping. This term does not conform to a magnetic form factor, so it is not related to spin relaxations. If correct, this mechanism stabilizes a continuous valence transition from $2\mathrm{U}(\mathrm{V})+1\mathrm{U}(\mathrm{VI})$ in the low-temperature ($T<600$ K) orthorhombic phase to the hexagonal phase that contains only one degenerate U site, wherein the U valence can be dynamically stabilized between $\mathrm{U}(\mathrm{V})\ensuremath{\leftrightarrow}\mathrm{U}(\mathrm{VI})$ by phonon-assisted electron hopping.

Thermoelectric materials with crystal-amorphicity duality induced by large atomic size mismatch

Summary Discovering novel materials and attaining higher performance are the eternal pursuit of thermoelectric materials research. Here, we report a material series, (Cu1−xAgx)2(Te1−ySy) (0.16 ≤ x ≤ 0.24, 0.16 ≤ y ≤ 0.24), which adopts a complex orthorhombic structure differing from any known crystal structure of (Cu/Ag)2(S/Te). This material series is featured by the crystal-amorphicity duality induced by the large anionic size mismatch: a crystalline sublattice of highly size-mismatched anions Te/S coexists with an amorphous-like sublattice of cations Cu/Ag. In the context of structure-property correlation, the crystal-amorphicity duality gave rise to not only interesting electrical properties but also exceptionally low lattice thermal conductivities from 300 to 1,000 K. A state-of-the-art figure of merit zT of 2.0 is obtained in the x = y = 0.22 sample at 1,000 K. These results give insights into crystal-amorphicity duality as a paradigm-shifting materials design approach to develop high-performance thermoelectric materials.

Martensitic transition assisted modification of the antiferromagnetic ordering in Ge-site doped MnNiGe alloys

The magnetic equiatomic alloys of the nominal compositions ${\mathrm{MnNiGe}}_{0.9}{\mathrm{Al}}_{0.1}$ and ${\mathrm{MnNiGe}}_{0.928}{\mathrm{Ga}}_{0.072}$ have been investigated through the neutron powder diffraction (NPD) technique. Both these alloys undergo a martensitic phase transition from the high-temperature hexagonal (${\mathrm{Ni}}_{2}\mathrm{In}$ type, with space group $P{6}_{3}/mmc$) austenite phase to the low-temperature orthorhombic (TiNiSi type, with space group $Pnma$) martensite phase. Our NPD analysis confirmed an incommensurate antiferromagnetic ordering of the low-temperature martensitic phase with helical modulation. The incommensurate propagation vector $\mathbf{k}$ is found to be (0.2065(1),0,0) and (0.2088(4),0,0) for Al and Ga doped alloys, respectively, at 1.5 K and shows a monotonic increase with increasing sample temperature. On the other hand, a commensurate antiferromagnetic ordering was observed for the high-temperature austenite phase. The incomplete martensitic transition allowed both these incommensurate and commensurate antiferromagnetic structures to coexist down to the lowest temperature of measurement (1.5 K).

Synthesis and structural characterization of orthorhombic Cu3–δ Sb (δ ≈ 0.1) and hexagonal Cu3Sb1–xInx (x ≈ 0.2) phases

Abstract Cu3Sb is a known copper-rich phase in the Cu–Sb binary phase diagram. It is reported to be dimorphic, with a low-temperature form adopting the orthorhombic Cu3Ti structure type (space group Pmmn, No. 59). The high-temperature form crystallizes in the cubic space group F m 3 ‾ m $Fm‾{3}m$ (No. 225), and is isostructural with BiF3. Neither polymorph has been carefully characterized to date, with both structures being assigned to the respective structure type, but never refined. With this study, we provide structural evidence, based on single-crystal and powder X-ray diffraction data that the low-temperature orthorhombic phase exists with a significant amount of defects on one of the Cu-sites. As a result, its composition is not Cu3Sb, but rather Cu3–δ Sb (δ = 0.13(1)). The cubic form could not be accessed as a part of this study, but another Cu-rich phase, Cu3Sb≈0.8In≈0.2, was also identified. It adopts the hexagonal Ni3Sn structure type (space group P63/mmc, No. 194) and represents an In-substituted variant of a hitherto unknown structural modification of Cu3Sb. Whether the latter can exist as a binary phase, or what is the minimum amount of In inclusions needed to stabilize it remains to be determined. Measurements of the thermopower of Cu3–δ Sb (δ = 0.13(1)) were conducted in the range of 300–600 K and demonstrated a maximum value of ca. 50 μV/K at 600 K, indicative of a p-type transport mechanism. Electrical resistivity measurements for the same sample confirmed that it exhibits metallic-like behavior, with a room temperature value of 0.43 mΩ cm. Electronic structure calculations show the absence of a band gap. Thermal analysis was utilized to ascertain the congruent melting of both phases.