Vibrational anisotropy of <i>δ</i>-(Al,Fe)OOH single crystals as probed by nuclear resonant inelastic X-ray scattering

The formation of high-pressure oxyhydroxide phases spanned by the components AlOOH–FeOOH–MgSiO2(OH)2 in experiments suggests their capability to retain hydrogen in Earth's lower mantle. Understanding the vibrational properties of high-pressure phases provides the basis for assessing their thermal properties, which are required to compute phase diagrams and physical properties. Vibrational properties can be highly anisotropic, in particular for materials with crystal structures of low symmetry that contain directed structural groups or components. We used nuclear resonant inelastic X-ray scattering (NRIXS) to probe lattice vibrations that involve motions of 57Fe atoms in δ-(Al0.87Fe0.13)OOH single crystals. From the recorded single-crystal NRIXS spectra, we calculated projections of the partial phonon density of states along different crystallographic directions. To describe the anisotropy of central vibrational properties, we define and derive tensors for the partial phonon density of states, the Lamb–Mössbauer factor, the mean kinetic energy per vibrational mode, and the mean force constant of 57Fe atoms. We further show how the anisotropy of the Lamb–Mössbauer factor can be translated into anisotropic displacement parameters for 57Fe atoms and relate our findings on vibrational anisotropy to the crystal structure of δ-(Al,Fe)OOH. As a potential application of single-crystal NRIXS at high pressures, we discuss the evaluation of anisotropic thermal stresses in the context of elastic geobarometry for mineral inclusions. Our results on single crystals of δ-(Al,Fe)OOH demonstrate the sensitivity of NRIXS to vibrational anisotropy and provide an in-depth description of the vibrational behavior of Fe3+ cations in a crystal structure that may motivate future applications of NRIXS to study anisotropic vibrational properties of minerals.

Bonding Heterogeneity in Mixed-Anion Compounds Realizes Ultralow Lattice Thermal Conductivity

<p><a>Crystalline materials with intrinsically low lattice thermal conductivity (</a><i>κ</i>_lat) pave the way towards high performance in various energy applications, including thermoelectrics. Here we demonstrate a strategy to realize ultralow <i>κ</i>_lat using mixed-anion compounds. Our calculations reveal that locally distorted structures in chalcohalides MnPnS₂Cl (Pn = Sb, Bi) derives a bonding heterogeneity, which in turn causes a peak splitting of the phonon density of states. This splitting induces a large amount of scattering phase space. Consequently, <i>κ</i>_lat of MnPnS₂Cl is significantly lower than that of a single-anion sulfide CuTaS₃ with a similar crystal structure. Experimental <i>κ</i>_lat of MnPnS₂Cl takes an ultralow value of about 0.5 W m⁻¹ K⁻¹ at 300 K. Our findings will encourage the exploration of thermal transport in mixed-anion compounds, which remain a vast unexplored space, especially regarding unexpectedly low <i>κ</i>_lat in lightweight materials derived from the bonding heterogeneity.</p>

Bonding Heterogeneity in Mixed-Anion Compounds Realizes Ultralow Lattice Thermal Conductivity

<p><a>Crystalline materials with intrinsically low lattice thermal conductivity (</a><i>κ</i>_lat) pave the way towards high performance in various energy applications, including thermoelectrics. Here we demonstrate a strategy to realize ultralow <i>κ</i>_lat using mixed-anion compounds. Our calculations reveal that locally distorted structures in chalcohalides MnPnS₂Cl (Pn = Sb, Bi) derives a bonding heterogeneity, which in turn causes a peak splitting of the phonon density of states. This splitting induces a large amount of scattering phase space. Consequently, <i>κ</i>_lat of MnPnS₂Cl is significantly lower than that of a single-anion sulfide CuTaS₃ with a similar crystal structure. Experimental <i>κ</i>_lat of MnPnS₂Cl takes an ultralow value of about 0.5 W m⁻¹ K⁻¹ at 300 K. Our findings will encourage the exploration of thermal transport in mixed-anion compounds, which remain a vast unexplored space, especially regarding unexpectedly low <i>κ</i>_lat in lightweight materials derived from the bonding heterogeneity.</p>

Cooper Pairs Distribution function for bcc Niobium under pressure from first-principles

In this paper, we report Cooper Pairs Distribution function $${D}_{cp}(\omega ,{T}_{c})$$ D cp ( ω , T c ) for bcc Niobium under pressure. This function reveals information about the superconductor state through the determination of the spectral regions for Cooper-pairs formation. $${D}_{cp}(\omega ,{T}_{c})$$ D cp ( ω , T c ) is built from the well-established Eliashberg spectral function and phonon density of states, calculated by first-principles. $${D}_{cp}(\omega ,{T}_{c})$$ D cp ( ω , T c ) for Nb suggests that the low-frequency vibration region $$\left(\omega <6 \,{\text{meV}}\right)$$ ω < 6 meV is where Cooper-pairs are possible. From $${D}_{cp}(\omega ,{T}_{c})$$ D cp ( ω , T c ) , it is possible to obtain the $${N}_{cp}$$ N cp parameter, which is proportional to the total number of Cooper-Pairs formed at a temperature $${T}_{c}$$ T c . The $${N}_{cp}$$ N cp parameter allows an approach to the understanding of the Nb $${T}_{c}$$ T c anomalies, measured around 5 and 50 GPa.