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Ultralow thermal conductivity and fast ionic diffusion endow superionic materials with excellent performance both as thermoelectric converters and as solid-state electrolytes. Yet the correlation and interdependence between these two features remain unclear owing to a limited understanding of their complex atomic dynamics. Here we investigate ionic diffusion and lattice dynamics in argyrodite Ag8SnSe6using synchrotron X-ray and neutron scattering techniques along with machine-learned molecular dynamics. We identify a critical interplay of the vibrational dynamics of mobile Ag and a host framework that controls the overdamping of low-energy Ag-dominated phonons into a quasi-elastic response, enabling superionicity. Concomitantly, the persistence of long-wavelength transverse acoustic phonons across the superionic transition challenges a proposed ‘liquid-like thermal conduction’ picture. Rather, a striking thermal broadening of low-energy phonons, starting even below 50 K, reveals extreme phonon anharmonicity and weak bonding as underlying features of the potential energy surface responsible for the ultralow thermal conductivity (<0.5 W m−1K−1) and fast diffusion. Our results provide fundamental insights into the complex atomic dynamics in superionic materials for energy conversion and storage.

In the prevalent picture of ultrafast structural phase transitions, atomic motion occurs in a slowly varying potential energy surface adiabatically determined by fast electrons. However, this ignores non-conservative forces caused by electron–lattice collisions, which can substantially influence atomic motion. Most ultrafast techniques only probe the average structure and are less sensitive to random displacements and therefore do not detect the role played by non-conservative forces in phase transitions. Here we show that the lattice dynamics of the prototypical insulator–metal transition of vanadium dioxide cannot be described by potential energy alone. We use the sample temperature to control the preexisting lattice disorder before ultrafast photoexcitation across the phase transition and our ultrafast diffuse scattering experiments show that the fluctuations characteristic of rutile metal develop equally fast (120 fs) at initial temperatures of 100 and 300 K. This indicates that additional non-conservative forces are responsible for the increased lattice disorder. These results highlight the need for more sophisticated descriptions of ultrafast phenomena beyond the Born–Oppenheimer approximation as well as ultrafast probes of spatial fluctuations beyond the average unit cell measured by diffraction.

Superionic conductors possess liquid-like ionic diffusivity in the solid state, finding wide applicability from electrolytes in energy storage to materials for thermoelectric energy conversion. Type I superionic conductors (for example, AgI, Ag2Se and so on) are defined by a first-order transition to the superionic state and have so far been found exclusively in three-dimensional crystal structures. Here, we reveal a two-dimensional type I superionic conductor, α-KAg3Se2, by scattering techniques and complementary simulations. Quasi-elastic neutron scattering and ab initio molecular dynamics simulations confirm that the superionic Ag+ions are confined to subnanometre sheets, with the simulated local structure validated by experimental X-ray powder pair-distribution-function analysis. Finally, we demonstrate that the phase transition temperature can be controlled by chemical substitution of the alkali metal ions that compose the immobile charge-balancing layers. Our work thus extends the known classes of superionic conductors and will facilitate the design of new materials with tailored ionic conductivities and phase transitions.

Alloys between Mg3Sb2and Mg3Bi2have recently been shown to be exceptional thermoelectric materials due in part to their anomalously low thermal conductivity. In the present study, in situhigh-pressure synchrotron X-ray diffraction was used to investigate the structure and bonding in Mg3Sb2and Mg3Bi2at pressures up to 50 GPa. Our results confirm prior predictions of isotropic in-plane and out-of-plane compressibility but reveal large disparities between the bond strength of the two distinct Mg sites. Using single-crystal diffraction, we show that the octahedral Mg–Sb bonds are significantly more compressible than the tetrahedral Mg–Sb bonds in Mg3Sb2, which lends support to prior arguments that the weaker octahedral Mg bonds are responsible for the anomalous thermal properties of Mg3Sb2and Mg3Bi2. Further, we report the discovery of a displacive and reversible phase transition in both Mg3Sb2and Mg3Bi2above 7.8 and 4.0 GPa, respectively. The transition to the high-pressure structure involves a highly anisotropic volume collapse, in which the out-of-plane axis compresses significantly more than the in-plane axes. Single-crystal diffraction at high pressure was used to solve the monoclinic high-pressure structure (C2/m), which is a distorted variant of the ambient-pressure structure containing four unique Mg coordination environments.

Heat and charge transport properties of the metallic unconventional clathrate BaNi2P4, hosting Ba cations in oversized Ni8P16cages, are investigated. A novel method of single-crystal growth was developed, yielding 2–3 mm sized crystals of BaNi2P4. We also developed a setup to accurately measure thermal conductivity and electrical resistivity of the synthesized single crystals in a wide temperature range avoiding crystal remounting. BaNi2P4has a metallic temperature dependence of its electrical resistivity (decreasing with decreasing temperature) and manifests an unconventional T2power law for 50 K < T< 300 K; below 50 K, the power-law exponent increases gradually such that below 10 K the power law is T5, a predicted but extremely rarely experimentally observed dependence for peculiar electron–phonon interactions. Electronic band structure calculations, consistent with measurements of de Haas–van Alphen oscillations, show large band dispersions with significant contributions of Ba orbitals to states near the Fermi level, which is atypical for clathrates. The thermal properties of BaNi2P4were probed using a combination of variable-temperature single-crystal X-ray diffraction experiments, heat capacity measurements, first-principles phonon dispersion calculations, and inelastic neutron scattering measurements. BaNi2P4exhibits significant hybridization of the Ba-guest and Ni–P-framework vibrational modes, which may be enhanced via the detected split of the Ba position, which results in strong Ba-framework interactions.

AgSbSe2is a typical member of cubic I–V–VI2semiconductors, which are known for their extremely low lattice thermal conductivity (κl). However, the low electrical conductivity of AgSbSe2, below ∼10 S cm–1at room temperature, has hindered its thermoelectric performance. In this work, single-phase AgSbSe2bulk samples with much higher electrical conductivity were synthesized via self-propagating high-temperature synthesis (SHS) combined with spark plasma sintering (SPS) for the first time. Pb doping through the nonequilibrium process further increases the electrical conductivity to >100 S cm–1. Furthermore, continuously increased effective mass md*can be achieved upon Pb doping because of the multiple degenerate valence bands of AgSbSe2and the energy-filtering effect induced by in situ-formed nanodots. The simultaneous enhancement of both the electrical conductivity and Seebeck coefficient contributes to an unprecedentedly high average power factor of 6.75 μW cm–1K–2. Meanwhile, the introduced dense grain boundaries and point defects enhance the phonon scattering and consequently suppress κl, yielding a high ZTvalue of 1.2 at 723 K in AgSb0.94Pb0.06Se2. This study opens a new avenue for rapid, low-cost, large-scale production of AgSbSe2-based materials and demonstrates that Pb-doped AgSbSe2prepared via the SHS–SPS method is a promising candidate for thermoelectric applications.

Hexagonal iron sulfide exhibits a fascinating coexistence of metal–insulator, structural and magnetic transitions, reflecting an intimate interplay of its spin, phonon and charge degrees of freedom. Here, we show how a subtle competition of energetic and entropic free-energy components governs its thermodynamics and the sequence of phase transitions it undergoes upon cooling. By means of comprehensive neutron and X-ray scattering measurements, and supported by first-principles electronic structure simulations, we identify the critical role of the coupling between antiferromagnetic ordering and instabilities of anharmonic phonons in the metallic phase in driving the metal–insulator transition. The antiferromagnetic ordering enables the emergence of two zone-boundary soft phonons, whose coupling to a zone-centre mode drives the lattice distortion opening the electronic bandgap. Simultaneously, spin–lattice coupling opens a gap in the magnon spectrum that controls the entropy component of the metal–insulator transition free energy. These results reveal the importance of spin–phonon coupling to tune anharmonic effects, thus opening new avenues to design novel technologically important materials harbouring the metal–insulator transition and magnetoelectric behaviours.

Superionic crystals exhibit ionic mobilities comparable to liquids while maintaining a periodic crystalline lattice. The atomic dynamics leading to large ionic mobility have long been debated. A central question is whether phonon quasiparticles—which conduct heat in regular solids—survive in the superionic state, where a large fraction of the system exhibits liquid-like behaviour. Here we present the results of energy- and momentum-resolved scattering studies combined with first-principles calculations and show that in the superionic phase of CuCrSe2, long-wavelength acoustic phonons capable of heat conduction remain largely intact, whereas specific phonon quasiparticles dominated by the Cu ions break down as a result of anharmonicity and disorder. The weak bonding and large anharmonicity of the Cu sublattice are present already in the normal ordered state, resulting in low thermal conductivity even below the superionic transition. These results demonstrate that anharmonic phonon dynamics are at the origin of low thermal conductivity and superionicity in this class of materials.

Recent work on superionic conductors has demonstrated the influence of lattice dynamics and the softness of the lattice on ionic transport. When examining either the changes in the acoustic phonon spectrum or the whole phonon density of states, both a decreasing activation barrier of migration and a decreasing entropy of migration have been observed, highlighting that the paradigm of “the softer the lattice, the better” does not always hold true. However, both approaches to monitor the changing lattice dynamics probe different frequency ranges of the phonon spectrum, and thus, it is unclear if they are complementary. In this work, we investigate the lattice dynamics of the superionic conductor Na3PS4–xSexby probing the optical phonon modes and the acoustic phonon modes, as well as the phonon density of states via inelastic neutron scattering. Notably, Raman spectroscopy shows the evolution of multiple local symmetry reduced polyhedral species, which likely affect the local diffusion pathways. Meanwhile, density functional theory and the ionic transport data are used to compare the different approaches for assessing the lattice dynamics. This work shows that, while acoustic and inelastic methods may be used to experimentally assess the overall changing lattice stiffness, calculations of the average vibrational energies between the mobile ions and the anion framework are important to assess and computationally screen for ionic conductors.

Phonon scattering by point defects viamass differences and strain fluctuations could effectively reduce the lattice thermal conductivity. The atomic mass difference can be maximized by introducing the vacancies thus leading to a significant phonon scattering. Usually, the vacancies are introduced by tuning the stoichiometry or forming solid solution with certain compound that contains intrinsically high concentration of vacancies. In this work, we demonstrate that vacancies can be effectively induced by the self-compensation effect viachemical doping. Indium (In) vacancies in InSb were induced by Te-doping and a substantial reduction in thermal conductivity was observed. Room temperature lattice thermal conductivity of the melted and then hot-pressed InSb (without In vacancies) is ~ 14.5 W m−1K−1but only ~ 3.8 W m−1K−1for InSb0.96Te0.04(with In vacancies), a reduction of ~ 74%. The advantage of using this strategy for phonon engineering lies in the fact that a substantial reduction in thermal conductivity can be achieved even when the dopant concentration is rather low. Since the self-compensation effect is widely observed in different compounds, it indicates that the vacancy engineering strategy used here is also applicable to a variety of other materials to effectively reduce the lattice thermal conductivity.

The article reviews the music performance by Opéra de Québec, baritone Gregory Dahl, and tenor Steve Michaud at Grand Théâtre de Québec in Quebec on October 21, 2017.

The article reviews the music performance by soprano Sondra Radvanovsky and pianist Anthony Manoli at Grand Théâtre de Québec in Quebec on October 22, 2017.

The article previews flamenco music "Un dia cualquiera" and "Remembranzas" by guitarist Caroline Planté and singer Fernando Gallego at the Rialto Theater in Montreal, Quebec, on November 26, 2011 and at Le Gesù in Montreal in December 2, 2011.

The origin of the anomalous anharmonic lattice dynamics of lead telluride is investigated by molecular dynamics simulations with interatomic force constants (IFCs) obtained up to quartic terms from first principles. The calculations reproduce the asymmetry of the radial distribution function and the double peaks of transverse optical phonon previously observed with neutron diffraction and scattering experiments. They are identified to be due to the extremely large nearest-neighbor cubic IFCs in the [100] direction. The outstanding strength of the nearest-neighbor cubic IFCs, compared with the longer-range interactions, explains why the distortion in the radial distribution function is local.