Ab initioinvestigation of the lattice dynamics and thermophysical properties of BCC vanadium and niobium

In the present work, we have performed the phonon dispersion calculations of body-centered cubic vanadium (V) and niobium (Nb) with the supercell approach using different supercell size. Using DFT method, the calculated phonon spectra of V and Nb are found to be in a good agreement with the available experimental data. Our calculated results show a 'dip'-like feature in the longitudinal acoustic phonon mode along the Γ-H high symmetric path for both transition metals in the case of supercell size4×4×4. However, in supercell size2×2×2and3×3×3, the 'dip'-like feature is not clearly visible. In addition to this, thermodynamical properties are also computed, which compare well with the experimental data. Apart from this, the phonon lifetime due to electron-phonon interactions (τephph) and phonon-phonon interactions (PPIs) (τphph) are calculated. The effect of PPIs is studied by computing the average phonon lifetime for all acoustic branches. The value ofτephphof V (Nb) is found to be 23.16 (24.70)×10-15s at 100 K, which gets decreased to 1.51 (1.85)×10-15s at 1000 K. Theτphphof V (Nb) is found to be 8.59 (18.09)×10-12and 0.83 (1.76)×10-12s at 100 and 1000 K, respectively. Nextly, the lattice thermal conductivity is computed using linearized phonon Boltzmann equation. The present work suggests that studying the variation of phonon dispersion with supercell size is crucial for understanding the phonon properties of solids accurately.

Beyond uniformity: Exploring the heterogeneous and dynamic nature of the microtubule lattice

A fair amount of research on microtubules since their discovery in 1963 has focused on their dynamic tips. In contrast, the microtubule lattice was long believed to be highly regular and static, and consequently received far less attention. Yet, as it turned out, the microtubule lattice is neither as regular, nor as static as previously believed: structural studies uncovered the remarkable wealth of different conformations the lattice can accommodate. In the last decade, the microtubule lattice was shown to be labile and to spontaneously undergo renovation, a phenomenon that is intimately linked to structural defects and was called "microtubule self-repair". Following this breakthrough discovery, further recent research provided a deeper understanding of the lattice self-repair mechanism, which we review here. Instrumental to these discoveries were in vitro microtubule reconstitution assays, in which microtubules are grown from the minimal components required for their dynamics. In this review, we propose a shift from the term "lattice self-repair" to "lattice dynamics", since this phenomenon is an inherent property of microtubules and can happen without microtubule damage. We focus on how in vitro microtubule reconstitution assays helped us learn (1) which types of structural variations microtubules display, (2) how these structural variations influence lattice dynamics and microtubule damage caused by mechanical stress, (3) how lattice dynamics impact tip dynamics, and (4) how microtubule-associated proteins (MAPs) can play a role in structuring the lattice. Finally, we discuss the unanswered questions about lattice dynamics and how technical advances will help us tackle these questions.

Reciprocal space temperature-dependent phonons method from ab-initio dynamics

We present a robust reciprocal-space implementation of the temperature-dependent effective potential method, our implementation can scale easily to large cell and long sampling time. It is interoperable with standard ab-initio molecular dynamics and with Langevin dynamics. We prove that both sampling methods can be efficient and accurate if a thermostat is used to control temperature and dynamics parameters are used to optimize the sampling efficiency.
By way of example, we apply it to study anharmonic phonon renormalization in weakly and strongly anharmonic materials, reproducing the temperature effect on phonon frequencies, crossing of phase transition, and stabilization of high-temperature phases.

Vacancy and phonon dispersion properties of Be, Co, Hf, Mg, and Re by modified embedded atom method potentials

The modified embedded atom method (MEAM) potentials improved by Jin et al. (Appl. Phys. A120 (2015), p. 189) were applied to calculate the mono- and bi-vacancy properties as well as the phonon dispersions for hexagonal close-packed (HCP) metals Be, Co, Hf, Mg, and Re. We expressed the formulas for calculating the mono- and bi-vacancy properties by the molecular static (MS) method based on the MEAM potentials for HCP metals. The lattice dynamics (LD) method and the MEAM potentials were adopted to calculate the phonon dispersion properties. The calculation results show better agreement with the experimental data than the previous calculations by using the unimproved embedded atom model.

Lattice dynamics calculations and high-pressure Raman spectra of the ZnMoO4

We report here the analysis of vibrational properties of the ZnMoO4 by using theoretical and experimental approaches, well as results of high pressure experiments in this system. The analysis of the lattice dynamics calculations through the classical rigid ion model, was applied to determine the mode assignment in the triclinic phase of the ZnMoO₄. Additionally, the experimental high-pressure Raman spectra of the ZnMoO₄ were carried out from 0 GPa up to 6.83 GPa to shed light on the structural stability of this system. The pressure-dependent studies showed that this crystal undergoes a first order phase transition at around 1.05 GPa. The Raman spectrum analysis of the new phase shows a significant change in the number of modes for the spectral range of 20-1000 cm^-1. The instability of this phase occurs due to the decrease of the MoO bond lengths in the high-pressure phase, connected with tilting and/or rotations of the MoO₄ tetrahedra leading to a disorder at the MoO₄ sites. The second and third phase transformations were observed, respectively, at about 2.9 GPa and 4.77 GPa, with strong evidences, in the Raman spectra, of crystal symmetry change. The principal component analysis (PCA) and the hierarchical cluster analysis (HCA) were used in order to infer the intervals of pressure where the different phases do exist. Discussion about the number of non equivalent sites for Mo ions and the kind of coordination for molybdenum atoms is also furnished.

Lattice dynamical investigations for Raman and infrared frequencies of Te doped Bi1-xTa1-xTe2xO4; 0<or=x<or=0.2

We have applied short range force constant model and used normal coordinate analysis based on G-F matrix method in investigating for Raman and infrared modes in Te doped Bi1-xTa1-xTe2xO4; 0<or=x<or=0.2 in its orthorhombic phase. The zone centre phonons calculations is made by using six stretching and seven bending force constants in its orthorhombic phase having space group Pnna with four formula units. All zone centre modes have been assigned in this calculation for the first time. The calculated Raman and infrared wave numbers agree satisfactorily with the available experimental results. The significant contribution of each force constant towards calculated Raman and infrared wave numbers has also been made through investigating the potential energy distribution. The Te-atom is present in equal amount at Bi and Ta site in this compound. The role of tellurium atom is exhibiting one mode behaviour in the ternary compound Bi1-xTa1-xTe2xO4 (0<or=x<or=0.2).

Lattice vibrations of AVO4 crystals (A=Lu, Yb, Dy, Tb, Ce)

A short range force constant model has been applied using normal coordinates to investigate the Raman and the infrared wavenumbers in rare earth AVO4 compounds (A=Lu, Yb, Dy, Tb, Ce) having space group I41/a and symmetry C4h(6). The calculation of zone center phonons has been made by using four stretching and five bending force constants. The calculated Raman and infrared wavenumbers are in very good agreement with the observed ones. The present calculations interpret reasonably the mode assignment of 779 cm(-1) as Eg mode and 853 cm(-1) as Ag mode in case of LuVO4, which were assigned differently in earlier observation. The potential energy distribution has also been investigated for determining the significance of contribution from each force constant toward the Raman and the infrared wavenumbers.