The outstanding thermoelectric performance of GeTe has attracted significant attention in the research community in recent years. However, many of the underlying physical mechanisms that contribute to GeTe's exceptionally high figure of merit (zT) remain not fully understood. In this study, an Sb-Bi codoped GeTe single crystal (Ge0.86Sb0.08Bi0.06)Te with an ultrahigh zT of 2.7 at 700 K and a record high device zT of 1.41 in the temperature range of 300-773 K was synthesized and investigated. The ultrahigh zT is attributed to the extremely low lattice thermal conductivity induced by strong electron-phonon (EP) interactions as revealed by the experimentally observed Kohn anomaly, through inelastic neutron scattering (INS) measurements. First-principles calculations further demonstrate that the remarkable EP interaction arises from the Fermi surface nesting featured in a one-dimensional (double-walled) topology. Our finding unravels the ultrahigh-zT mechanism in GeTe-based materials, serving as an inspiring guide toward high thermoelectric performance.

It is important to study the properties of high quality single crystal in order to resolve the issue of an interesting material in which certain debatable fundamental properties exist. However, it is unfortunate that a sizable single crystal for experimental measurements is not always available. NbN is one of the examples; it has attracted scientific and engineering interest due to its diverse physical properties and a variety of structural phases. Until now superconductivity is only observed in cubic delta- and tetragonal gamma-NbN but not in hexagonal epsilon-NbN. Recently, Zou et al. reported the observation of superconductivity with T-c similar to 11.6 K in a hexagonal epsilon-NbN based on the measurement on a multiphase powder specimen. In order to resolve the issue, the work used the electron backscattering diffraction technique to characterize phases of micron-size NbN crystals from commercial powders and measure their transport properties. Our results unambiguously confirm that the hexagonal epsilon-NbN phase is not superconducting.

A record high zT of 2.2 at 740 K is reported in Ge0.92Sb0.08Te single crystals, with an optimal hole carrier concentration ≈4 × 1020 cm−3 that simultaneously maximizes the power factor (PF) ≈56 µW cm−1 K−2 and minimizes the thermal conductivity ≈1.9 Wm−1 K−1. In addition to the presence of herringbone domains and stacking faults, the Ge0.92Sb0.08Te exhibits significant modification to phonon dispersion with an extra phonon excitation around ≈5–6 meV at Γ point of the Brillouin zone as confirmed through inelastic neutron scattering (INS) measurements. Density functional theory (DFT) confirmed this phonon excitation, and predicted another higher energy phonon excitation ≈12–13 meV at W point. These phonon excitations collectively increase the number of phonon decay channels leading to softening of phonon frequencies such that a three‐phonon process is dominant in Ge0.92Sb0.08Te, in contrast to a dominant four‐phonon process in pristine GeTe, highlighting the importance of phonon engineering approaches to improving thermoelectric (TE) performance.

In this work, the role of Cu dopants in the development of superconductivity in Bi2Se3 is investigated. A series of CuxBi2Se3 (x = 0-0.3) crystals is grown using the Bridgman method and electrochemical techniques. Based on the observable lattice increases along the c axis, the existence of Cu atoms intercalated in the van der Waals (vdW) gaps in Bi2Se3 is verified by x-ray diffraction, scanning electron microsopy-energy dispersive spectroscopy, and transmission electron microscopy analysis. Furthermore, the chemical state of the Cu is found to be zero valence by characterization of the x-ray photoelectron and Auger electron spectra. The absence of Cu1+ and Cu2+ in the electron energy-loss spectroscopy near-edge fine structure is confirmed as well. Meanwhile, our Raman data also show the same result: the intercalation of Cu in the vdW gap which weakens the binding of the quintuple layers in the Cu0.1Bi2Se3 crystal. The electric resistance and magnetic susceptibility show a superconducting transition near 3-3.4 K in the series of Cu-doped Bi2Se3. A sharp superconducting transition with the highest value of T-C = 3.4K and the largest magnetic shielding fraction of 84% is observed in the optimized 10% Cu-doped Bi2Se3. These results imply that the formation of superconducting quasiparticles is not related to the charge transfer of Cu, but is supported by the internal stress of Cu intercalated in the van der Waals gap. The superconducting transition observed in the specific heat implies that the superconductivity in CuxBi2Se3 is unconventional.

Single-crystalline SnSe has attracted much attention because of its record high figure-of-merit ZT ≈ 2.6; however, this high ZT has been associated with the low mass density of samples which leaves the intrinsic ZT of fully dense pristine SnSe in question. To this end, we prepared high-quality fully dense SnSe single crystals and performed detailed structural, electrical, and thermal transport measurements over a wide temperature range along the major crystallographic directions. Our single crystals were fully dense and of high purity as confirmed via high statistics 119Sn Mössbauer spectroscopy that revealed <0.35 at. % Sn(IV) in pristine SnSe. The temperature-dependent heat capacity (C p) provided evidence for the displacive second-order phase transition from Pnma to Cmcm phase at T c ≈ 800 K and a small but finite Sommerfeld coefficient γ0 which implied the presence of a finite Fermi surface. Interestingly, despite its strongly temperature-dependent band gap inferred from density functional theory calculations, SnSe behaves like a low-carrier-concentration multiband metal below 600 K, above which it exhibits a semiconducting behavior. Notably, our high-quality single-crystalline SnSe exhibits a thermoelectric figure-of-merit ZT ∼1.0, ∼0.8, and ∼0.25 at 850 K along the b, c, and a directions, respectively.

Recently, the anisotropic single crystalline SnSe has gained tremendous interest as a promising thermoelectric material. The elastic constants of such anisotropic crystals are notoriously difficult to measure yet play a crucial role in many thermodynamic properties. We report for the first time the nine independent elastic constants of its stiffness tensor as measured by resonant ultrasound spectroscopy. Our experimental values of the elastic constants are in good agreement with those reported by the density functional theory of SnSe, except for C12. The Voigt–Reuss–Hill method was used to determine the isotropic polycrystalline elastic moduli from the measured elastic constants of SnSe, which were found to be in agreement with theoretical values. Notably, the heat capacity of single crystalline SnSe deduced from our measured elastic moduli is in excellent agreement with the room temperature value of heat capacity determined from thermal transport measurements.

Narrow-band greem-emitting phosphor beta-SiA-lON:Eu has been widely used in advanced wide-gamut backlighting devices. However, the origins for unusual sharp lines in photoluminescence emission at room temperature and tunable narrow-band-emission tailored by reducing Al-O in beta-SiAlON:Eu are still unclear. Here, the presence of sharp-line fine structure in the emission spectra of beta-SiAlON:Eu is mainly due to purely electronic transitions (zero phonon lines), and their vibronic repetitions resulting from the multimicroenvironment around Eu2+ ions that has been revealed by relative emission intensity of sharp line depends on excitation wavelength and monotonously increasing decay time. The specific features of the Eu2+ occupying interstitial sites indicate that the effect of crystal field strength can be neglected. Therefore, the enhanced rigidity and higher ordering structure of beta-SiAlON:Eu with decreasing the substitution of Si-N by Al-O become the main factors in decreasing electron lattice coupling and reducing inhomogeneous broadening, favoring the blue shift and narrow of the emission band, the enhanced thermal stability, as well as the charge state of Eu2+. Our results provide new insights for explaining the reason for narrow-band-emission in beta-SiAlON:Eu, which will deliver an impetus for the exploration of phosphors with narrow band and ordering structure.

Superconductivity variations deduced from the x-ray diffraction and the magnetic and heat-capacity measurements in the pseudoternary Sr(Pd${}_{1\ensuremath{-}x}{T}_{x}$)${}_{2}$Ge${}_{2}$ layer system [Pd($4{d}^{8}$), $T$ $=$ Co($3{d}^{7}$), Ni($3{d}^{8}$), or Rh($4{d}^{7}$); $0\ensuremath{\leqslant}x\ensuremath{\leqslant}1$] are reported. For the BaFe${}_{2}$As${}_{2}$-type tetragonal structure, the degenerate $n{d}^{7}$ or $n{d}^{8}$ orbitals of transition metal $T$ are split by $c$-axis squeezed $T$Ge${}_{4}$ tetrahedral crystal field in the $T$-Ge layer. For the isoelectronic Sr(Pd${}_{1\ensuremath{-}x}$Ni${}_{x}$)${}_{2}$Ge${}_{2}$ system, the superconducting transition temperature ${T}_{c}$ decreases monotonically from 3.12 K for $4d$-band SrPd${}_{2}$Ge${}_{2}$ to 0.92 K for $3d$-band SrNi${}_{2}$Ge${}_{2}$, where major contributions of conduction electrons are from the half filled dispersive three-dimensional (3D)-like upper-lying $n{d}_{xz,yz}$ bands. For the Sr(Pd${}_{1\ensuremath{-}x}$Rh${}_{x}$)${}_{2}$Ge${}_{2}$ system, ${T}_{c}$ decreases to 2.40 K with 25$%$ of $4{d}^{7}$ Rh substitution. For the Sr(Pd${}_{1\ensuremath{-}x}$Co${}_{x}$)${}_{2}$Ge${}_{2}$ system, ${T}_{c}$ decreases sharply to 2.58 K with only 3$%$ of $3{d}^{7}$ Co substitution. No superconductivity is expected for SrRh${}_{2}$Ge${}_{2}$ and SrCo${}_{2}$Ge${}_{2}$ with lower density of states in ${d}_{xz,yz}$ bands due to down shift of Fermi energy ${E}_{F}$ by one less electron per transition metal. The lower ${T}_{c}$ of the present electron-overdoped ($n{d}^{7}$ or $n{d}^{8}$) compound is due to dispersive 3D-like $n{d}_{xz,yz}$ conduction bands with weak electron correlation, in comparison with the less-electron-doped ($3{d}^{6.1}$) 22-K superconductor BaFe${}_{1.8}$Co${}_{0.2}$As${}_{2}$ or the hole-doped ($3{d}^{5.9}$) 38-K superconductor Ba${}_{0.6}$K${}_{0.4}$Fe${}_{2}$As${}_{2}$ where electron contribution is from less dispersive 2D-like lower-lying $3{d}_{xy}$ conduction band with stronger electron correlation.

We have successfully synthesized three quasi-two-dimensional geometrically frustrated magnetic compounds (alpha-MCr2O4, M = Ca, Sr, Ba) using the spark-plasma-sintering technique. All these members of the alpha-MCr2O4 family consist of the stacking planar triangular lattices of Cr3+ spins (S = 3/2), separated by nonmagnetic alkaline-earth ions. Their corresponding magnetic susceptibility, specific heat, dielectric permittivity, and ferroelectric polarization are systematically investigated. A long-range magnetic ordering arises below the Neel temperature (around 40 K) in each member of the alpha-MCr2O4 family, which changes to the quasi-12 degrees. proper-screw-type helical spin structure at low temperature. A very small but confirmed spontaneous electric polarization emerges concomitantly with this magnetic ordering. The direction of electric polarization is found within the basal triangular plane. The multiferroicity in alpha-MCr2O4 can not be explained within the frameworks of the magnetic exchange striction or the inverse Dzyaloshinskii-Moriya interaction. The observed results are more compatible with the newly proposed Arima mechanism that is associated with the d-p hybridization between the ligand and transition-metal ions, modified by the spin-orbit coupling. The evolution of multiferroic properties with the increasing interplanar spacing (as M changes from Ca to Ba) reveals the importance of interlayer interaction in this new family of frustrated magnetic systems.

A new multiferroic material, CuBr(2) , is reported for the first time. CuBr(2) has not only a high transition temperature (close to liquid nitrogen temperature) but also low dielectric loss and strong magnetoelectric coupling. These findings reveal the importance of anion effects, in the search for the high temperature multiferroics materials among these low-dimensional spin systems.