Anharmonic lattice dynamics of superionic lithium nitride

Superionic crystals reach an ionic conductivity comparable to liquid electrolytes following a superionic transition at high temperature. The physical mechanisms that lead to this behaviour remain poorly understood. It has been proposed that superionic transitions are accompanied by the breakdown of specific phonon modes linked to characteristic diffusion processes. Any changes in vibrational properties across the superionic transition may therefore provide insights into the underlying physics of this phenomenon. Here, we apply a combination of lattice dynamics and ab initio molecular dynamics to probe the vibrational properties of the archetypal superionic conductor Li3N. We assess harmonic, quasi-harmonic, and anharmonic descriptions of the phonons. The harmonic and quasi-harmonic models show no change in features across the superionic transition. The fully anharmonic model, however, exhibits a phonon breakdown for all modes above the superionic transition temperature. The implications for developing lattice-dynamics based descriptors for superionic conductors are discussed.

Solid to superionic transition in iron oxide-hydroxide

<p>At planetary interior conditions, water ice has been proved to enter a superionic phase recently since it was predicted about 30-year ago. Hydrogen in superionic water become liquid-like, and move freely within solid oxygen lattice. Under extreme pressure and temperature conditions of Earth’s deep mantle, the solid-superionic transition can also occur readily in the pyrite-type FeO<sub>2</sub>Hx, a candidate mineral in the lower mantle and probably also in other hydrous minerals. We find that when the pressure increases beyond 73 GPa at room temperature, symmetric hydroxyl bonds are softened and the H<sup>+</sup> (or proton) become diffusive within the vicinity of its crystallographic site. Increasing temperature under pressure, the diffusivity of hydrogen is extended beyond individual unit cell to cover the entire solid, and the electrical conductivity soars, indicating a transition to the superionic state which is characterized by freely-moving proton and solid FeO<sub>2</sub> lattice. The superionic hydrogen will dramatically change the geophysical picture of electrical conductivity and magnetism, as well as geochemical processes of hydrogen isotopic mixing and redox equilibria at local regions of Earth’s deep interiors.</p>

Anharmonic lattice dynamics of superionic lithium nitride

Superionic crystals reach an ionic conductivity comparable to liquid electrolytes following a superionic transition at high temperature. The physical mechanisms that lead to this behaviour remain poorly understood. It has...

Dynamic disorder phonon scattering mediated by Cu atomic hopping and diffusion in Cu3SbSe3

Cu3SbSe3 that exhibits distinct liquid-like sublattice due to the heterogeneous bonding environment has emerged as a promising low cost superionic semiconductor with intrinsic ultralow thermal conductivity. However, the relationship between atomic dynamics resulting in liquid-like diffusion and anomalous phonon transport properties remains poorly understood. Herein, combing ab initio molecular dynamics with temperature-dependent Raman measurements, we have performed a thorough investigation on the lattice dynamics of Cu3SbSe3. Superionic transition is unveiled for both structurally inequivalent Cu atoms at elevated temperatures, while the Se-formed tetrahedral framework can simultaneously maintain. An intermediate state of Cu3SbSe3 through the mixture of quasi-1D/2D Cu nearest-neighbor vacancy hopping is discovered below the superionic transition temperature. Our results also manifest that phonons predominately involved with Cu contributions along diffusion channels have been strongly scattered during the superionic transition, whereas the liquid-like diffusion of Cu is too slow to completely breakdown the propagation of all transverse phonon modes. The insight provided by this work into the atomic dynamics and phonon scattering relationship may pave the way for further phonon engineering of Cu3SbSe3 and related superionic materials.

Характеристики Li-=SUP=-+-=/SUP=--ионной проводимости кристаллов Li-=SUB=-3-=/SUB=-R-=SUB=-2-=/SUB=-(PO-=SUB=-4-=/SUB=-)-=SUB=-3-=/SUB=- (R=Fe,Sc) в суперионном состоянии

The characteristics of Li+-ion conductivity σdc of structural γ modifications of Li_3R_2(PO_4)_3 compounds (R = Fe, Sc) existing in the superionic state have been investigated by impedance spectroscopy. The type of structural framework [R_2P_3O_12]_∞^3- affects the σdc value and the σdc activation enthalpy in these compounds. The ion transport activation enthalpy in γ-Li_3R_2(PO_4)_3 (Δ H _σ = 0.31 ± 0.03 eV) is lower than in γ-Li_3Fe_2(PO_4)_3 (Δ H _σ = 0.36 ± 0.03 eV). The conductivity of γ-Li_3Fe_2(PO_4)_3 (σ_dc = 0.02 S/cm at 573 K) is twice as high as that of γ-Li_3R_2(PO_4)_3. A decrease in temperature causes a structural transformation of Li_3R_2(PO_4)_3 from the superionic γ modification (space group Pcan ) through the intermediate metastable β modification (space group P 2_1/ n ) into the “dielectric” α modification (space group P 2_1/ n ). Upon cooling, σdc for both phosphates decreases by a factor of about 100 at the superionic TSIC transition. In Li_3Fe_2(PO_4)_3 σdc gradually decreases in the temperature range T _SIC = 430–540 K, whereas in Li_3R_2(PO_4)_3 σdc undergoes a jump at T _SIC = 540 ± 25 K. Possible crystallochemical factors responsible for the difference in the σdc and Δ H _σ values and the thermodynamics and kinetics of the superionic transition for Li_3R_2(PO_4)_3 are discussed.