Lead halide perovskites such as methylammonium lead triiodide (CH 3 NH 3 PbI 3 ) have outstanding optical and electronic properties for photovoltaic applications, yet a full understanding of how this solution processable material works so well is currently missing. Previous

Compressional wave velocity-density (V P − ρ) relations of candidate Fe alloys at relevant pressure-temperature conditions of the Earth's core are critically needed to evaluate the composition, seismic signatures, and geodynamics of the planet's remotest region. Specifically, comparison between seismic V P − ρ profiles of the core and candidate Fe alloys provides first-order information on the amount and type of potential light elements-including H, C, O, Si, and/or S-needed to compensate the density deficit of the core. To address this issue, here we have surveyed and analyzed the literature results in conjunction with newly measured V P − ρ results of hexagonal closest-packed (hcp) Fe and hcp-Fe 0.85 Si 0.15 alloy using in situ highenergy resolution inelastic X-ray scattering and X-ray diffraction. The nature of the Fe-Si alloy where Si is readily soluble in Fe represents an ideal solid-solution case to better understand the lightelement alloying effects. Our results show that high temperature significantly decreases the V P of hcp-Fe at high pressures, and the Fe-Si alloy exhibits similar high-pressure V P − ρ behavior to hcp-Fe via a constant density offset. These V P − ρ data at a given temperature can be better described by an empirical power-law function with a concave behavior at higher densities than with a linear approximation. Our new datasets, together with literature results, allow us to build new V P − ρ models of Fe alloys in order to determine the chemical composition of the core. Our models show that the V P − ρ profile of Fe with 8 wt % Si at 6,000 K matches well with the Preliminary Reference Earth Model of the inner core.compressional-wave velocity | high pressure-temperature E nigmatic properties of the Earth's inner core have recently been discovered including differential super-rotation (1), seismic anisotropies (2-4), and fine-scale seismic heterogeneities (5, 6). Deciphering these observations requires solid knowledge about the composition of the Earth's inner core and, therefore, the elasticity of candidate Fe alloys (7-16). Since F. Birch pointed out in the 1950s that Earth's core is too dense if composed of Fe or Fe-Ni alloy alone (13), a number of candidate major light elements, including oxygen (O), silicon (Si), sulfur (S), carbon (C), and hydrogen (H), have been suggested via cosmochemical, geochemical, and geophysical evidence (17). To ascertain the identity and exact amount of light elements needed in the Earth's inner core, one key piece of information lies in the comparison of the seismic V P − ρ profiles with reliable laboratory measurements of these properties for candidate Fe alloys. Potential Fe-light element alloy must have V P − ρ profiles that match seismic models such as the Preliminary Reference Earth Model and AK135 (7,8). Thus, this requires precise experimental results describing the V P − ρ relationships of Fe alloys at pressure-temperature (P-T) conditions relevant to the Earth's core. To address this issue, here, we present new experimental measurements on the V...

We report high-resolution inelastic x-ray measurements of the soft phonon mode in the chargedensity-wave compound TiSe2. We observe a complete softening of a transverse optic phonon at the L point, i.e. q = (0.5, 0, 0.5), at T ≈ TCDW . Renormalized phonon energies are observed over a large wavevector range (0.3, 0, 0.5) ≤ q ≤ (0.5, 0, 0.5). Detailed ab-initio calculations for the electronic and lattice dynamical properties of TiSe2 are in quantitative agreement with experimental frequencies for the phonon branch involving the soft mode. The observed broad range of renormalized phonon frequencies is directly related to a broad peak in the electronic susceptibility stabilizing the chargedensity-wave ordered state. Our analysis demonstrates that a conventional electron-phonon coupling mechanism can explain a structural instability and the charge-density-wave order in TiSe2 although other mechanisms might further boost the transition temperature.PACS numbers: 71.45. Lr, 63.20.kd, 63.20.dd, 63.20.dk The origin of charge-density-wave (CDW ) order, i.e., a periodic modulation of the electronic density, is a long-standing problem relevant to a number of important issues in condensed matter physics, such as the role of stripes in cuprates [1] and charge fluctuations in the colossal magnetoresistive manganites [2]. Chan and Heine derived the criterion for a stable CDW phase with a modulation wavevector q as [3]where η q is the electron-phonon coupling (EPC) matrix element associated with a mode at an unrenormalized energy of ω bare , χ q is the dielectric response of the conduction electrons, and U q and V q are their Coulomb and exchange interactions. Static CDW order typically is taken as a result of a divergent electronic susceptibility χ q due to nesting, i.e. parallel sheets of the Fermi surface (FS) separated by twice the Fermi wavevector 2k f . Electron-phonon coupling (EPC) is required to stabilize the structural distortion and, hence, an acoustic phonon mode at the CDW wavevector q CDW = 2k f softens to zero energy at the transition temperature T CDW [3,4]. However, when electronic probes reported only small and not well nested Fermi surfaces this scenario has been discarded for the prototypical CDW compound TiSe 2 [5][6][7].Alternative scenarios such as indirect or band-type Jahn-Teller effects [6][7][8] and, most prominently, exciton formation [5,9,10] are discussed in the theoretical as well as experimental literature. More recently, van Wezel et al. have invoked a model including exciton formation as well as EPC [11] and have shown that it can explain data from angle-resolved photoemission spectroscopy (ARPES) [12], formerly taken as evidence of an excitonic insulating phase in TiSe 2 [9]. Determining the origin of CDW formation in TiSe 2 is all the more important with respect to the nature of superconductivity, which emerges both as function of pressure [13] and Cu intercalation [14]. In particular, pressure induced superconductivity is expected to be closely linked to the nature of the parent CDW state.I...

Inelastic neutron measurements of the high temperature lattice excitations in NaI show that in thermal equilibrium at 555 K an intrinsic mode, localized in three dimensions, occurs at a single frequency near the center of the spectral phonon gap, polarized along [111]. At higher temperatures the intrinsic localized mode gains intensity. Higher energy inelastic neutron and x-ray scattering measurements on a room temperature NaI crystal indicate that the creation energy of the ground state of the intrinsic localized mode is 299 meV.

Intrinsically low lattice thermal conductivity (κlat) in superionic conductors is of great interest for energy conversion applications in thermoelectrics. Yet, the complex atomic dynamics leading to superionicity and ultralow thermal conductivity remain poorly understood. Here, we report a comprehensive study of the lattice dynamics and superionic diffusion in AgCrSe2 from energy- and momentum-resolved neutron and X-ray scattering techniques, combined with first-principles calculations. Our results settle unresolved questions about the lattice dynamics and thermal conduction mechanism in AgCrSe2. We find that the heat-carrying long-wavelength transverse acoustic (TA) phonons coexist with the ultrafast diffusion of Ag ions in the superionic phase, while the short-wavelength nondispersive TA phonons break down. Strong scattering of phonon quasiparticles by anharmonicity and Ag disorder are the origin of intrinsically low κlat. The breakdown of short-wavelength TA phonons is directly related to the Ag diffusion, with the vibrational spectral weight associated to Ag oscillations evolving into stochastic decaying fluctuations. Furthermore, the origin of fast ionic diffusion is shown to arise from extended flat basins in the energy landscape and collective hopping behavior facilitated by strong repulsion between Ag ions. These results provide fundamental insights into the complex atomic dynamics of superionic conductors.