The role of low-lying optical phonons in lattice thermal conductance of rare-earth pyrochlores: A first-principle study (original) (raw)
Related papers
Influence of Er substitution for La on the thermal conductivity of (La1−xErx)2Zr2O7 pyrochlores
Materials Research Bulletin, 2015
The substitution of Er 3+ for La 3+ effectively reduces the thermal conductivity of La 2 Zr 2 O 7. With the increasing Er 3+ content, the thermal conductivity significantly decreases at lower temperatures. At higher temperatures, however, the thermal conductivity of the (La 1-x Er x) 2 Zr 2 O 7 becomes similar, indicating that the thermal conductivity is less sensitive to composition variation. Highlights ► • The substitution of Er 3+ for La 3+ reduces the thermal conductivity of La 2 Zr 2 O 7. ► • The thermal conductivity reveals a nearly temperature-independent behavior. ► • A novel phonon scattering source (rattlers) was found in this study. ► • The rattlers are more efficient in scattering low frequency phonons. ► • The present study also provides a novel way to reduce the thermal conductivity.
Thermal conductivity of rare earth zirconate pyrochlore from first principles
The quasi-harmonic approximation combined with first-principles calculations are employed to study the thermal conductivities of rare earth zirconate. The thermal conductivities are estimated using Slack's model. Good agreement between theory and experiments is observed at high temperature. The anisotropy of minimum thermal conductivity of Ln 2 Zr 2 O 7 pyrochlore is discussed based on Clarke's model. The thermal conductivities are plotted in three-dimensional space and the projections at (0 0 1) and (1 1 0) crystal planes are also illustrated.
Thermal Conductivity of the Rare-Earth Strontium Aluminates
Journal of the American Ceramic Society, 2010
The thermal conductivity of a series of complex aluminates, RE 2 SrAl 2 O 7 , with different rare-earth (RE) ions, has been measured up to 10001C. There is a strong dependence on the atomic number of the RE ion, ranging from an approximately 1/T dependence for the lanthanum strontium aluminate to an almost temperature-independent behavior of the dysprosium strontium aluminate. The latter conductivity is comparable with that of yttria-stabilized zirconia, the current material of choice for thermal barrier coatings. The temperature dependence of the thermal conductivities of all the aluminates studied can be fit to a standard phonon-phonon scattering model, modified to account for a minimum phonon mean free path, in which the difference in behavior is attributed to increased phonon-phonon scattering with the atomic mass of the RE ion. Although a satisfactory parametric fit is obtained, the model does not take into account either the detailed layer structure of the aluminates, consisting of alternating rock-salt and perovskite layers in a natural superlattice structure, or the site preferences of the RE ion. This suggests that further model development is warranted.
ES Energy & Environment, 2022
Knowledge of lattice dynamics is essential for understanding the physical properties of materials and optimizing the performance for applications. Alkali halides (MX, M = Na, K, Rb, and Cs; X = Cl, Br, and I) have been widely recognized for their simple crystal structures and low thermal conductivities. At room temperature, the thermal conductivity (κ) of RbBr is nearly twice as large as that of RbCl and RbI, while the thermal conductivities of three compounds in CsX halides are comparable. These thermal conductivity trends with increased atomic mass are significantly different from common sense that the thermal conductivity conventionally decreases with the increasing atomic mass and decreasing electronegativity difference. However, little attention has been paid to the microscopic mechanism of these anomalous thermal conductions in RbX and CsX. Here, we report a systematic investigation of the thermal transport properties in alkali halides by the Boltzmann transport equation based on first-principles calculations. The results show that the anomalous thermal conductivity trends of RbX and CsX mainly attribute to the disparity of optical phonons in each compound. The more dispersive the optical phonons, the greater their contribution to the thermal conductivity, and thus the thermal conductivities of compounds with heavier atoms are enhanced. The disparity of optical phonons originates from the mass difference in the compound. Our work offers deeper insight into the unusual phonon thermal transport phenomenon in alkali halides and provides a fundamental reference for rooting the thermal conductivities in related functional materials and finding novel materials with thermal conduction beyond conventional cognition.
Thermal Conductivity and Phonon Engineering in Low-Dimensional Structures
1998
The use of first principles methods based on density functional theory to investigate novel thermoelectric materials is illustrated for several empty and filled skutterudite compounds, including CoSb 3 , C0P3, La(Fe,Co) 4 Sbi 2 and La(Fe,Co)P 12 . Band structures and their relationship to transport properties especially as regards optimization of thermoelectric properties is discussed. Phonon models constructed from calculations and existing experimental data for CoSb 3 are presented. These have been extended to the filled skutterudites, particularly LaFe 4 Sbi2 using additional first principles calculations to fix the La related parameters in the model. This model allows an interpretation of neutron scattering data as well as an understanding of the low frequency phonon modes that transport heat in these compounds.
Anomalous Lattice Thermal Conductivity in Rocksalt IIAVIA Compounds
Materials with an intrinsic (ultra)low lattice thermal conductivity (k L) are critically important for the development of efficient energy conversion devices. In the present work, we have investigated microscopic origins of low k L behavior in BaO, BaS and MgTe by exploring lattice dynamics and phonon transport of 16 iso-structural MX (Mg, Ca, Sr, Ba and X = O, S, Se and Te) compounds in the rocksalt (NaCl)-type structure by comparing their lattice transport properties with the champion thermoeletric isostructural material, PbTe. Anomalous trends are observed for k L in MX compounds except the MgX series in contrast to the expected trend from their atomic mass. The underlying mechanisms for such low k L behavior in relatively low atomic mass systems namely BaO, BaS and MgTe compounds are thoroughly analyzed. We propose the following dominant factors that might be responsible for low k L behavior in these materials: 1) softening of transverse acoustic (TA) phonon modes despite low atomic mass, 2) low lying optic (LLO) phonon modes fall deep into acoustic mode region which enhances overlap between longitudinal acoustic (LA) and LLO phonon modes which increases scattering phase space, 3) short phonon lifetimes and high scattering rates, 4) relatively high density (ρ) and large Grüneisen parameter. Moreover, tensile strain also causes a further reduction in k L for BaO, BaS and MgTe through phonon softening and near ferroelectric instability. Our comprehensive study on 16 binary MX compounds might provide a pathway for designing (ultra)low k L materials even with simple crystal systems through phonon engineering.
arXiv (Cornell University), 2019
High-entropy ceramics generally exhibit reduced thermal conductivity, but little is known about what controls this suppression and which descriptor can predict it. Herein, 18 medium-and high-entropy pyrochlores were synthesized to measure their thermal conductivity and Young's modulus. Up to 35% reductions in thermal conductivity were achieved with retained moduli, thereby attaining insulative yet stiff properties for potential thermal barrier coating applications. Notably, the measured thermal conductivity correlates well with a modified size disorder parameter *. Thus, this * is suggested as a useful descriptor for designing thermallyinsulative medium-and high-entropy ceramics (broadly defined as "compositionally-complex ceramics").
Analysis of Phonon Heat Conductivity of Semiconductors
The various inadequacies of Callaway's phenomenological model of lattice thermal conductivity has been critically analyzed and the model is repaired in a modified form in which the systematic replacement of life time by line widths amicably resolves the various issues. The involvement of various scattering events in the heat transport, e.g., boundary scattering, impurity scattering, anharmonic phonon scattering, resonance scattering and interference scattering has been addressed in the new framework with the help of quantum dynamical many body theory. The technological importance of Ge is well known and hence it becomes significantto investigate the thermal behavior of it in details as its electrical properties are temperature dependent. Further, the CdTe also shows its vital importance in the fabrication of infrared optical windows and photo voltaic solar cells. The phonon heat conductivity of Ge and CdTe in the temperature range 3.3-298 K and 1.780-239.260 K based on the modified Callaway model have been analyzed and excellent agreements between theory and experiments are reported. The present formulation is found to be well justified and can be successfully applied for the calculations of thermal conductivity of several other crystalline solids.
First-principles calculation and experimental investigation of lattice dynamics in the rare-earth pyrochlores<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML">mml:mrowmml:msubmml:miRmml:mn2mml:msubmml:miTimml:mn2<mml:m...
Physical review, 2016
We present a model of the lattice dynamics of the rare earth titanate pyrochlores R2Ti2O7 (R = Tb, Dy, Ho), which are important materials in the study of frustrated magnetism. The phonon modes are obtained by density functional calculations, and these predictions are verified by comparison with scattering experiments. Single crystal inelastic neutron scattering is used to measure acoustic phonons along high symmetry directions for R = Tb, Ho; single crystal inelastic x-ray scattering is used to measure numerous optical modes throughout the Brillouin zone for R = Ho; and powder inelastic neutron scattering is used to estimate the phonon density of states for R = Tb, Dy, Ho. Good agreement between the calculations and all measurements is obtained, allowing confident assignment of the energies and symmetries of the phonons in these materials under ambient conditions. The knowledge of the phonon spectrum is important for understanding spin-lattice interactions, and can be expected to be transferred readily to other members of the series to guide the search for unconventional magnetic excitations.