The thermoelectric effect enables direct and reversible conversion between thermal and electrical energy, and provides a viable route for power generation from waste heat The efficiency of thermoelectric materials is dictated by the dimensionless figure of merit, ZT (where Z is the figure of merit and T is absolute temperature), which governs the Carnot efficiency for heat conversion Enhancements above the generally high threshold value of 25 have important implications for commercial deployment, especially for compounds free of Pb and Te Here we report an unprecedented ZT of 26 ± 03 at 923 K, realized in SnSe single crystals measured along the b axis of the room-temperature orthorhombic unit cell This material also shows a high ZT of 23 ± 03 along the c axis but a significantly reduced ZT of 08 ± 02 along the a axis We attribute the remarkably high ZT along the b axis to the intrinsically ultralow lattice thermal conductivity in SnSe The layered structure of SnSe derives from a distorted rock-salt structure, and features anomalously high Gruneisen parameters, which reflect the anharmonic and anisotropic bonding We attribute the exceptionally low lattice thermal conductivity (023 ± 003 W m(-1) K(-1) at 973 K) in SnSe to the anharmonicity These findings highlight alternative strategies to nanostructuring for achieving high thermoelectric performance

Ultralow thermal conductivity and high thermoelectric figure of merit in SnSe crystals

Strong Closed-Shell Interactions in Inorganic Chemistry.

Recent studies have revealed that single-layer transition-metal oxides and dichalcogenides (MX2) might offer properties superior to those of graphene. So far, only very few MX2 compounds have been synthesized as suspended single layers, and some of them have been exfoliated as thin sheets. Using first-principles structure optimization and phonon calculations based on density functional theory, we predict that, out of 88 different combinations of MX2 compounds, several of them can be stable in free-standing, single-layer honeycomb-like structures. These materials have two-dimensional hexagonal lattices and have top-view appearances as if they consisted of either honeycombs or centered honeycombs. However, their bonding is different from that of graphene; they can be viewed as a positively charged plane of transition-metal atoms sandwiched between two planes of negatively charged oxygen or chalcogen atoms. Electron correlation in transition-metal oxides was treated by including Coulomb repulsion through LDA...

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Stable, Single-Layer MX2 Transition-Metal Oxides and Dichalcogenides in a Honeycomb-Like Structure

Understanding elementary excitations and their couplings in condensed matter systems is critical for developing better energy-conversion devices. In thermoelectric materials, the heat-to-electricity conversion efficiency is directly improved by suppressing the propagation of phonon quasiparticles responsible for macroscopic thermal transport. The current record material for thermoelectric conversion efficiency, SnSe, has an ultralow thermal conductivity, but the mechanism behind the strong phonon scattering remains largely unknown. From inelastic neutron scattering measurements and first-principles simulations, we mapped the four-dimensional phonon dispersion surfaces of SnSe, and found the origin of the ionic-potential anharmonicity responsible for the unique properties of SnSe. We show that the giant phonon scattering arises from an unstable electronic structure, with orbital interactions leading to a ferroelectric-like lattice instability. The present results provide a microscopic picture connecting electronic structure and phonon anharmonicity in SnSe, and offers new insights on how electron–phonon and phonon–phonon interactions may lead to the realization of ultralow thermal conductivity. Tin selenide is at present the best thermoelectric conversion material. Neutron scattering results and ab initio simulations show that the large phonon scattering is due to the development of a lattice instability driven by orbital interactions.

Orbitally driven giant phonon anharmonicity in SnSe

Iron disulfide for solar energy conversion

Neutron diffraction study of the structural phase transition in SnS and SnSe

High pressure X-ray diffraction experiments have been performed on PbS, PbSe and PbTe up to 300 kbar with synchrotron radiation. PbS, PbSe and PbTe undergo pressure-induced first-order structural phase transition from NaCl-type (B1) phase to an intermediate phase at about 22, 45 and 60 kbar, respectively. Further structural phase transitions from the intermediate phase to the CsCl-type (B2) phase have been observed in PbS, PbSe and PbTe at about 215, 160 and 130 kbar, respectively. The crystal structures of the intermediate phases of these compounds are discussed.

High pressure X-ray diffraction study on the structural phase transitions in PbS, PbSe and PbTe with synchrotron radiation

2014 The crystal structure of SnS has been investigated as a function of temperature by elastic neutron scattering from 293 K to 1 000 K. SnS undergoes a second order displacive phase transition from the 03B1-phase (B16, Pbnm) to the high temperature 03B2-phase (B33, Cmcm) at 887 K. The mean square displacement of the Sn atom which is isotropic at room temperature becomes increasingly anisotropic and anharmonic near Tc. Energy dispersive Xray diffraction experiments have been performed on IV-VI semiconductors PbS, PbSe, PbTe, GeS, GeSe, SnS and SnSe as a function of hydrostatic pressure to 340 kbar. PbS, PbSe and PbTe undergo transition from the NaCl type (B1) phase to the orthorhombic phases (B33 or B16) at about 22, 45 and 60 kbar, respectively. The orthorhombic phases of PbS and PbSe have been found to be of TlI type (B33, Cmcm) and that in PbTe has been found to be GeS type (B16, Pbnm). PbS, PbSe and PbTe undergo further phase transitions to the cubic CsCl type (B2) phase at about 215, 160 and 130 kbar, respectively. The orthorhombic IV-VI compounds GeS, GeSe, SnS and SnSe do not undergo any phase transition up to 340 kbar. Revue Phys. Appl. 19 (1984) 807-813 SEPTEMBRE 1984,

https://hal.archives-ouvertes.fr/jpa-00245267/document

Temperature and pressure induced phase transition in IV-VI compounds

High pressure X-ray diffraction study on p-FeS2, m-FeS2 and MnS2 to 340 kbar: A possible high spin-low spin transition in MnS2

Neutron-scattering techniques have been used to study the magnetic structure and spin dynamics of the Pr and Cu spins in ${\mathrm{Pr}}_{2}$${\mathrm{CuO}}_{4}$. In the ordered state the Cu spin-wave velocity c has been determined to be 0.85\ifmmode\pm\else\textpm\fi{}0.08 eV \AA{}, which corresponds to an in-plane nearest-neighbor exchange constant J=130\ifmmode\pm\else\textpm\fi{}13 meV. A spin-wave gap of \ensuremath{\sim}5 meV has been observed, corresponding to a reduced anisotropy constant ${\mathrm{\ensuremath{\alpha}}}_{\mathrm{\ensuremath{\parallel}}}$=(J-${\mathit{J}}^{\mathit{x}\mathit{y}}$)/J of \ensuremath{\sim}2\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}4}$. In the paramagnetic regime the evolution of the Cu spin-correlation length with temperature is adequately described by the renormalized classical theory for the quantum nonlinear sigma model. For the Pr ions, significant dispersion is observed for the first excited-state crystal-field level, directly demonstrating that there are Pr-Pr exchange interactions both within the a-b plane as well as along the c axis. These interactions along the c axis must be mediated through the CuO planes which are also involved in superconductivity in these cuprate materials. A singlet-doublet magnetic exciton model, with Pr-Pr Heisenberg exchange terms as large as \ensuremath{\sim}0.8 meV, provides a good quantitative description of the measured dispersion relations. The temperature dependence of the magnetic excitons also can be qualitatively understood with this theory if the exchange terms are modified by a temperature-dependent renormalization factor.The zero-field ordered moment at low temperatures for the Cu is determined to be 0.40\ifmmode\pm\else\textpm\fi{}0.02${\mathrm{\ensuremath{\mu}}}_{\mathit{B}}$, in good agreement with results reported by other groups. However, field-dependent diffraction measurements suggest that the correct Cu spin structure is the noncollinear one, where spins in adjacent layers along the c axis are orthogonal, rather than the collinear structure assumed by other groups. This noncollinearity is also reflected in the configuration of the small induced moments (0.08\ifmmode\pm\else\textpm\fi{}0.005${\mathrm{\ensuremath{\mu}}}_{\mathit{B}}$) that develop at low temperatures on the Pr ions. The magnetic-field--temperature phase diagram for the case of an applied field along the [11\ifmmode\bar\else\textasciimacron\fi{}0] direction reveals that the spin-rotation energy increases rapidly with decreasing temperature from \ensuremath{\sim}200 K down to 4.5 K.

T. Chattopadhyay

Papers

Neutron diffraction study of the structural phase transition in SnS and SnSe

High pressure X-ray diffraction study on the structural phase transitions in PbS, PbSe and PbTe with synchrotron radiation

Temperature and pressure induced phase transition in IV-VI compounds

High pressure X-ray diffraction study on p-FeS2, m-FeS2 and MnS2 to 340 kbar: A possible high spin-low spin transition in MnS2

Magnetic structure and spin dynamics of the Pr and Cu in Pr2CuO4.