Self-consistent Phonon Approach Research Articles

Compressive strain-induced enhancement of thermoelectric performance in lead-free halide double perovskites K2SnX6 (X = I, Br, Cl)

Exploring thermoelectric materials with high performance and low cost is of great importance in mitigating environmental and energy challenges. Here, we provide an atomistic insight into strain-induced enhancement of thermoelectric performance in potassium-based halide double perovskite K2SnX6 (X = I, Br, Cl) using first-principles calculations. To get reliable predictions for transport properties, we adopt advanced methods such as self-energy relaxation time approximation for electron transport and unified theory for lattice transport in combination with self-consistent phonon approach. Our calculations highlight a promising thermoelectric figure of merit ZT over 1.01 in K2SnI6 when applying a compressive strain of −6%, being tenfold larger than those in the uncompressed compounds, suggesting that compressing is an effective way to enhance the thermoelectric performance of halide double perovskites.

Anharmonic lattice dynamics and structural phase transition of SnTe monolayer from first principles

In this work we investigate the role of quartic anharmonicity on the lattice- and thermo-dynamic properties of rectangular () and square (β) phases of two-dimensional (2D) SnTe monolayer (ML) by using self-consistent phonon (SCP) theory, based on the first-principles calculations. For both phases, as compared to the usual harmonic approximation (HA), the renormalized phonon frequency at the optical modes (4–10) is found to be increased upon the inclusion of quartic anharmonicity via the SCP method, where the effects of cubic anharmonicity are neglected. At the experimentally observed transition temperature (T c = 270 K), the difference in the vibrational free-energy between the square and rectangular phases of SnTe ML, calculated by using the anharmonic SCP correction is found to be much closer to the structural energy gain as compared to that obtained by using only the quasi-harmonic contribution. This validates the significance of SCP approach over the HA to explain the lattice dynamics properties and predict the T c for SnTe ML and similar 2D compounds. The calculated lattice thermal conductivity of square SnTe ML (e.g. 10.67 W m−1K−1 at 300 K) is higher than that of the rectangular SnTe ML (e.g. 6.72 W m−1K−1 at 300 K) due to the relatively higher corresponding thermodynamic parameters: specific heat capacity, group velocity, and phonon lifetime obtained for the square SnTe ML. Particularly, the low energy phonon modes are found to transport most of the heat in the system and, hence, played major role to the total lattice thermal conductivity.

Phase Stability, Strong Four-Phonon Scattering, and Low Lattice Thermal Conductivity in Superatom-Based Superionic Conductor Na3OBH4.

Superatom-based superionic conductors are of current interest due to their promising applications in solid-state electrolytes for rechargeable batteries. However, much less attention has been paid to their thermal properties, which are vital for safety and performance. Motivated by the recent synthesis of superatom-based superionic conductor Na3OBH4 consisting of superhalogen cluster BH4, we systematically investigate its lattice dynamics and thermal conductivity using the density functional theory combined with a self-consistent phonon approach. We reveal the bonding hierarchy features by studying the electron localization function and potential energy surface and further unveil the rattling effect of the BH4 superatom, which introduces strong quartic anharmonicity and induces soft phonon modes in low temperatures by assisting Na displacements, thus calling for the necessity of quartic renormalization and four-phonon scattering in calculating the lattice thermal conductivity. We find that the contribution of four-phonon processes to the lattice thermal conductivity increases from 13 to 32% when the temperature rises from 200 to 400 K. At room temperature (300 K), the phonon scattering phase space is enlarged by 133% due to the four-phonon interactions, and the lattice thermal conductivity is evaluated to be 5.34 W/mK, reduced by 24% as compared with a value of 6.99 W/mK involving three-phonon scattering only. These findings provide a better understanding of the lattice stability and thermal transport properties of superionic conductor Na3OBH4, shedding light on the role of strong quartic anharmonicity played in superatom-based materials.

Experimental and theoretical evidence of the temperature-induced wurtzite to rocksalt phase transition in GaN under high pressure

The $p\text{\ensuremath{-}}T$ conditions of the solid-solid phase transition from the wurtzite to rocksalt structure in GaN are determined both experimentally and by ab initio calculations. Experimental evaluation was based on the x-ray absorption measurements in a laser-heated diamond anvil cell. At 300 K, the transition was observed near 47 GPa. At lower pressures, the wurtzite to rocksalt transition has been induced by high temperature: 1420 K at 42 GPa and about 2100 K at 37 GPa. Thus the slope of the wurtzite-rocksalt borderline could be evaluated as negative and nonlinear. On the part of the theory, the $p\text{\ensuremath{-}}T$ borderline was determined from a comparative analysis of the temperature dependences of the Gibbs potential of the wurtzite and rocksalt structures for different pressure. The Gibbs potentials were calculated within the quasiharmonic approximation and the self-consistent phonon approach. The results obtained with the self-consistent phonon approach show that the inclusion of the anharmonic phonon effects is indispensable to obtain a very good agreement with the experimental data. Possible consequences of the observed anharmonicity for the still unknown melting behavior of GaN are discussed. In particular, it is suggested that the melting temperature of the rocksalt-GaN, at pressure around 37 GPa, is not much higher than 2100 K.

Direct-space-self-consistent-phonon treatment of monolayer structures and dynamics.

Computations, which would have been intractable just a few years ago, are now possible on desktop workstations. Such is the case for the application of the Self-Consistent-Phonon (SCP) approximation to large monolayer clusters on structured surfaces, combining a SCP approach to the system dynamics with a random walk approach to finding the optimum positions of the adsorbed atoms. This combination of techniques enables the investigation of the stability, structure, and dynamics of incommensurate adsorbed monolayers at low temperatures. We refer to this approach as the Direct-Space-Self-Consistent-Phonon framework. We present the application of this framework to the study of rare-gas and molecular hydrogen adsorbates on the graphite basal-plane surface and (for xenon) the Pt(111) surface. The largest cluster size consists of 4096 particles, a system that is large enough to examine incommensurate phases without significant adverse boundary effects. The existence of "pseudo-gaps" in the phonon spectrum of nearly commensurate monolayers is demonstrated, and the implication of such "pseudo-gaps" for the determination of the location of any commensurate ↔ incommensurate phase transition is explored. The stability of striped incommensurate structures vs hexagonal incommensurate structures is examined. The inherent difficulties of using this approach for the highly quantum monolayer solids is shown to generate some particular problems. Nevertheless, we demonstrate that this approach to the stability, structure, and dynamics of quantum monolayer solids is a very useful tool in the theorist's arsenal. By implication, this approach should also be useful in the study of adsorption on graphene and carbon nanotubes at low temperatures.

Interfacial thermal transport with strong system-bath coupling: A phonon delocalization effect

We study the effect of system-bath coupling strength on quantum thermal transport through the interface of two weakly coupled anharmonic molecular chains using quantum self-consistent phonon approach. The heat current shows a resonant to bi-resonant transition due to the variations in the interfacial coupling and temperature, which is attributed to the delocalization of phonon modes. Delocalization occurs only in the strong system-bath coupling regime and we utilize it to model a thermal rectifier whose ratio can be non-monotonically tuned not only with the intrinsic system parameters but also with the external temperature.

The self-consistent ab initio lattice dynamical method

We describe a method for calculating temperature dependent phonon spectra self consistently from first principles. The method combines concepts from Born’s self-consistent phonon approach with ab initio calculations of accurate interatomic forces in a supercell. Test calculations on the high temperature bcc phase of Ti, Zr, Hf, Sc and Y, as representative examples, reproduce the observed high temperature phonon frequencies with good accuracy. By use of an embedded atom potential we demonstrate the method’s relevance in calculating approximate critical temperatures of solid–solid phase transitions for the hcp to bcc transition in Zr.

Entropy Driven Stabilization of Energetically Unstable Crystal Structures Explained from First Principles Theory

Conventional methods to calculate the thermodynamics of crystals evaluate the harmonic phonon spectra and therefore do not work in frequent and important situations where the crystal structure is unstable in the harmonic approximation, such as the body-centered cubic (bcc) crystal structure when it appears as a high-temperature phase of many metals. A method for calculating temperature dependent phonon spectra self-consistently from first principles has been developed to address this issue. The method combines concepts from Born's interatomic self-consistent phonon approach with first principles calculations of accurate interatomic forces in a supercell. The method has been tested on the high-temperature bcc phase of Ti, Zr, and Hf, as representative examples, and is found to reproduce the observed high-temperature phonon frequencies with good accuracy.

Vibrational broadening of x-ray emission spectra: A first-principles study on diamond

We present a general theoretical scheme that allows an accurate account of the vibrational broadening of core-level line shapes in crystals, using state-of-the-art electronic-structure techniques within the Franck-Condon approximation. Electronic and vibrational degrees of freedom are consistently treated using ab initio density-functional theory. Neglecting the core-hole lifetime, anharmonic effects are treated within a self-consistent phonon approach. Finite core-hole lifetime effects are simulated by an independent-boson model, which, however, we have been able to apply only when lattice distortions are treated in the harmonic approximation. We apply the theory to the diamond 1s core-level emission spectrum. In the limit of infinite core-hole lifetime, we find that (i) the Stokes shift is about 3 eV, (ii) the harmonic approximation overestimates the Stokes shift by \ensuremath{\sim}1.4 eV, and (iii) phonon broadening is about 2 eV. A comparison with experiment reveals that the phonon bandwidth is comparable to the core-hole lifetime, which we estimate \ensuremath{\Gamma}\ensuremath{\approx}110 meV.

Self-consistent phonon approaches for the hydrogen bond chain

The hydrogen bonded ammonia chain model is studied by means of the standard self-consistent phonon approach and two modified versions. The effective crystal constant, force constant, free energy, and ratio of the first-order free energy to the zero order as a function of temperature are numerically obtained with the three approaches, and then compared. The standard approach gives the lowest free energy. Violation of the convexity is found in one of the modified approaches near the temperature which is regarded as the melting temperature.

Calculation of the temperature dependence of the negative thermal expansion of polyethylene chains by means of the self-consistent harmonic approximation

A parameter-free model based on molecular mechanics with zero Kelvin potentials is developed to describe quantitatively the shortening of the unit cell of polyethylene crystals in chain direction with increasing temperature. Based on the well-known conception that an increase of the amplitudes of the torsional vibrations will cause the thermal shrinkage, we introduce a self-consistent phonon approach to calculate the temperature dependence of the angular displacement correlation function taking into account the anharmonicity of the potential. By means of these functions, we calculate the mean value of the length of a representative chain segment and compare the results with those obtained from x-ray diffraction measurements. © 1996 John Wiley & Sons, Inc.

Premelting base pair opening probability and drug binding constant of a daunomycin-poly d(GCAT).poly d(ATGC) complex

We calculate room temperature thermal fluctuational base pair opening probability of a daunomycin-poly d(GCAT).poly d(ATGC) complex. This system is constructed at an atomic level of detail based on x-ray analysis of a crystal structure. The base pair opening probabilities are calculated from a modified self-consistent phonon approach of anharmonic lattice dynamics theory. We find that daunomycin binding substantially enhances the thermal stability of one of the base pairs adjacent the drug because of strong hydrogen bonding between the drug and the base. The possible effect of this enhanced stability on the drug inhibition of DNA transcription and replication is discussed. We also calculate the probability of drug dissociation from the helix based on the selfconsistent calculation of the probability of the disruption of drug-base H-bonds and the unstacking probability of the drug. The calculations can be used to determine the equilibrium drug binding constant which is found to be in good agreement with observations on similar daunomycin-DNA systems.

Modified self-consistent phonon calculation of the dependence of DNA melting temperature on guanine-cytosine content.

We have modified the self-consistent phonon approach to treat DNA double helices modeled with a large repeating unit (N base pairs) so that melting behaviors of DNA polymers of various base-pair sequences can be studied. The melting temperatures of DNA polymers of different G-C--to--A-T ratios were calculated (G, C, A, and T denote guanine, cytosine, adenine, and thymine, respectively). It is shown that a DNA polymer with a relatively higher content of A-T melts at a lower temperature than DNA with a higher content of G-C. The melting temperature increases linearly when the G-C content in a DNA polymer increases. This is in agreement with both experimental observations and theoretical analysis.

Criterion for DNA melting in the mean-field modified self-consistent phonon theory.

We have examined the validity of the first-order-perturbation method in calculating eigenfunctions and the criterion for helix melting of mean-field polymers in the modified self-consistent phonon approach (MSPA) theory. It is found that the instability in the self-consistent solution is due to the breakdown of the first-order perturbation. The instability as a criterion for helix melting is therefore techniquely inappropriate. However, the breakdown of the perturbation is due to facts that are directly related to the onset of softening. Previously predicted melting temperatures for various sequence DNA polymers may still represent good estimates to the actual melting temperatures. An alternative criterion is required to define the melting temperature of the polymer DNA double helix in the MSPA theory.

Defect-mediated hydrogen-bond melting in B-DNA polymers.

This paper explores the melting temperatures of the hydrogen bonds that hold together the two strands of DNA. We choose as samples both homopolymers and copolymers of DNA. First, we investigate the melting temperatures of hydrogen bonds throughout the polymer via a mean-field approach. Then a defect is introduced by cutting the hydrogen bonds in one unit cell at room temperature. We study the propagation of this defect to neighboring cells with rise in temperature. The calculation is a modified self-consistent phonon approach. The agreement between the calculated melting temperatures and observed melting temperatures is excellent for defected melting. The defected melting is an approximation to initiation-site melting.

Self-consistent phonon approach to thermal vibrations in model small clusters.

Thermal atomic vibrations in small clusters have been studied theoretically using the self-consistent phonon method. We have calculated the free energy for model clusters in which constituent atoms interact with each other through a pairwise potential and are vibrating with a frequency ${\ensuremath{\omega}}_{n}$ for the nth atom. Thermal softening and thermal expansion are investigated as a function of temperature for different structures of cluster sizes, N=7, 9, and 13. A structure with pentagonal caps is shown to be more stable vibrationally than a section of cubic structure of the same size. The instability temperature in the present model clusters is discussed in the context of melting in small clusters.

Hydrogen-bond melting in B-DNA copolymers in a mean-field self-consistent phonon approach.

We explore hydrogen-bond melting in B-DNA for the copolymers poly(dGC)-poly(dGC), poly(dAC)-poly(dGT), poly(dAG)-poly(dCT), and poly(dAT)-poly(dAT). Here dGC refers to a repeating sequence of a guanine base followed by a cytosine base on one strand. The melting is explored through a mean-field self-consistent phonon theory based on the Green's-function method. By our calculations, the melting temperatures are 385, 366, 357, and 325 K for the four helices, respectively. The onset of melting, in helices with both A-T and G-C base pairs, is either in the A-T pair or in the G-C pair, depending on the effective near-neighbor interaction.