Phonon Dynamics and Multipolar Isomorphic Transition in β-Pyrochlore KOs<sub>2</sub>O<sub>6</sub>

Hattori, Kazumasa; Tsunetsugu, Hirokazu

doi:10.1143/jpsj.80.023714

Cited by 8 publications

(18 citation statements)

References 28 publications

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“…This may be the case with the osmates analogue, KOs2O6, for which zero-temperature calculations have revealed a highly anharmonic potential with a flat bottom. [37][38][39][40] …”

Section: B Novel Rattling K Dynamics and Thermal Conductivitymentioning

confidence: 99%

Novel K rattling: A new route to thermoelectric materials?

Shoko

Kearley

Peterson

et al. 2014

Journal of Applied Physics

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We have performed ab initio molecular dynamics (MD) simulations to study the alkali-metal dynamics in the Al-doped (KAl0.33W1.67O6 and RbAl0.33W1.67O6) and undoped (KW2O6 and RbW2O6) defect pyrochlore tungstates. The K atoms exhibit novel rattling dynamics in both the doped and undoped tungstates while the Rb atoms do not. The KAl0.33W1.67O6 experimental thermal conductivity curve shows an unusual depression between ~ 50 K and ~ 250 K, coinciding with two crossovers in the K dynamics: the first at ~ 50 K, from oscillatory to diffusive, and the second at ~ 250 K, from diffusive back to oscillatory. We found that the low-temperature crossover is a result of the system transitioning below the activation energy of the diffusive dynamics whereas the high-temperature crossover is driven by a complex reconstruction of the local potential around the K atoms due to the cage dynamics.This leads to a hardening of the K potential with increasing temperature. This unusual reconstruction of the potential may have important implications for the interpretation of finite-temperature dynamics based on zero-temperature potentials in similar materials. The key result is that the novel K rattling, involving local diffusion, leads to a significant reduction in the thermal conductivity. We suggest that this may open a new route in the phonon engineering of cage compounds for thermoelectric materials where the rattlers are specifically selected to reduce the lattice thermal conductivity by the mechanism of local diffusion. a)

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“…This may be the case with the osmates analogue, KOs2O6, for which zero-temperature calculations have revealed a highly anharmonic potential with a flat bottom. [37][38][39][40] …”

Section: B Novel Rattling K Dynamics and Thermal Conductivitymentioning

confidence: 99%

Novel K rattling: A new route to thermoelectric materials?

Shoko

Kearley

Peterson

et al. 2014

Journal of Applied Physics

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“…In this Section, we calculate the alkali-metal RMSDs for each pyrochlore at the three temperatures, 20 K, 100 K, and 300 K, and these values can be compared directly to the experimental data summarized by Hattori and Tsunetsugu [11]. We also calculate the potential of mean force (PMF) for the alkali metals at the different temperatures from the formula [39]:…”

Section: Root-mean-square Displacements (Rmsds) and Potential Of Meanmentioning

confidence: 99%

Temperature dependence of alkali-metal rattling dynamics in theβ-pyrochlores, AOs₂O₆(A = K, Rb, Cs), from MD simulation

Shoko

Peterson²,

Kearley³

2014

J. Phys.: Condens. Matter

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We investigate the temperature response of the alkali-metal rattling modes in β-pyrochlores, AOs2O6 (A = K, Rb, Cs), from the results of ab initio molecular dynamics (MD) simulations performed at 20 K, 100 K and 300 K. Our results show that the temperature response of the T1u mode is clearly different from that of the T2g mode for all three pyrochlores. In this regard, two features are of particular note for both K and Rb; (1) the T1u mode exhibits a distinctly stronger softening response with decreasing temperature compared to the T2g mode, and (2) the T1u mode becomes stronger and sharper with decreasing temperature. These two findings suggest that the T1u mode is significantly more anharmonic and sensitive to the cage dynamics than the T2g mode. Examination of the local potentials around the alkali-metal atoms reveals that K has the flattest and most anharmonic potential at all temperatures while Cs exhibits the narrowest potential. The temperature dependence of the local potentials reveals that, for K, the potential at a higher temperature is not a simple extrapolation to higher energy of that at a lower temperature. Instead, we find significant reconstruction of the potential at different temperatures. Finally, we explore the temperature response of the coupling between the alkali metals and find a complex temperature dependence which suggests that the origin of the coupling may be more complex than a pure Coulomb interaction. We also find an unexpected increase in the static disorder of the system at low temperatures for the K and Rb pyrochlores.

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“…Hattori and Tsunetsugu have investigated a realistic model including three dimensional anharmonic phonons with tetrahedral symmetry and have confirmed that T c is strongly enhanced with increasing the third-order anharmonicity of the potential. 18,19 Chang et al have discussed the superconductivity by using the strong-coupling approach in the anharmonic phonon model including fourth-order terms. 20 The present authors have revealed the anharmonicity-controlled strong-coupling tendency for superconductivity induced by rattling from the analysis of the anharmonic Holstein model in the strongcoupling theory.…”

Section: Introductionmentioning

confidence: 99%

Anomalous Isotope Effect in Rattling-Induced Superconductor

Oshiba

Hotta

2012

J. Phys. Soc. Jpn.

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To clarify that the Cooper pair in -pyrochlore oxides is mediated by the anharmonic oscillation of a guest atom, i.e., rattling, we propose an experiment to detect an anomalous isotope effect. In the formula T c / M À , where T c is the superconducting transition temperature and M denotes the mass of the oscillator, it is found that the exponent increases with the increase in anharmonicity of the potential for the guest atom. We predict that becomes larger than 1/2 in rattling-induced superconductors, in sharp contrast to ¼ 1=2 for weak-coupling superconductivity due to harmonic phonons and < 1=2 for strong-coupling superconductivity with the inclusion of the effect of Coulomb interaction.

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Phonon Dynamics and Multipolar Isomorphic Transition in β-Pyrochlore KOs₂O₆

Cited by 8 publications

References 28 publications

Novel K rattling: A new route to thermoelectric materials?

Novel K rattling: A new route to thermoelectric materials?

Temperature dependence of alkali-metal rattling dynamics in theβ-pyrochlores, AOs₂O₆(A = K, Rb, Cs), from MD simulation

Anomalous Isotope Effect in Rattling-Induced Superconductor

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Phonon Dynamics and Multipolar Isomorphic Transition in β-Pyrochlore KOs2O6

Cited by 8 publications

References 28 publications

Novel K rattling: A new route to thermoelectric materials?

Novel K rattling: A new route to thermoelectric materials?

Temperature dependence of alkali-metal rattling dynamics in theβ-pyrochlores, AOs2O6(A = K, Rb, Cs), from MD simulation

Anomalous Isotope Effect in Rattling-Induced Superconductor

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Product

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Phonon Dynamics and Multipolar Isomorphic Transition in β-Pyrochlore KOs₂O₆

Temperature dependence of alkali-metal rattling dynamics in theβ-pyrochlores, AOs₂O₆(A = K, Rb, Cs), from MD simulation