The role of low-lying optical phonons in lattice thermal conductance of rare-earth pyrochlores: A first-principle study

Abstract

Rare-earth pyrochlores, commonly exhibiting anomalously low lattice thermal conductivities, are considered as promising topcoat materials for thermal barrier coatings. However the structural origin underlying their low thermal conductivities remain unclear. In the present study, we investigated the phonon properties of two groups of RE pyrochlores, Ln2Zr2O7 (Ln = La, Nd, Sm, Gd) and Gd2T2O7 (T = Zr, Hf, Sn, Pb) employing density functional theory and quasi harmonic approximation.Through the relaxation time approximation (RTA) with Debye model, the thermal conductivities of those RE pyrochlores were predicted, showing good agreement with experimental measurements. The low thermal conductivities of RE pyrochlores were shown to largely come from the interference between the low-lying optical branches and acoustic branches. The structural origin underlying the low-lying optical branches was then clarified and the competition between scattering processes in transverse and longitude acoustic branches was discussed.

Impact of isovalent and aliovalent substitution on the mechanical and thermal properties of Gd2Zr2O7

“…The elastic constants reported by Lan et al . employing GGA method are generally smaller than our LDA results, while they also found that the results of Gd 2 Zr 2 O 7 and Sm 2 Zr 2 O 7 are very similar to each other 36 . We find that the mechanical stability criteria, i.e., (C 11 -C 12 ) > 0, C 44  > 0, and (C 11  + 2C 12 ) > 0 1 , are satisfied for all compounds, indicating that Sm-substituted Gd 2 Zr 2 O 7 compounds are mechanically stable.…”

First-principles calculation and experimental investigation of lattice dynamics in the rare-earth pyrochloresR2Ti2O7(R=Ruminy1, Valdez2, Wehinger3 et al. 2016Phys. Rev. B336480View full textAdd to dashboardCite

“…However, according to the phonon calculations presented in Ref. [22], pyrochlores in which the B-site cation is a member of group IV, such as Sn, Ge, or Pb, have significantly different phonon frequencies. In comparison with the partial phonon densities of states calculated for a subset of zirconate and hafnate pyrochlores [22], we find that the phonon band structures of (3+,4+) pyrochlores have essentially identical features, with adjustments that can be classified generally in two ways, as illustrated in Fig.…”

“…[22], pyrochlores in which the B-site cation is a member of group IV, such as Sn, Ge, or Pb, have significantly different phonon frequencies. In comparison with the partial phonon densities of states calculated for a subset of zirconate and hafnate pyrochlores [22], we find that the phonon band structures of (3+,4+) pyrochlores have essentially identical features, with adjustments that can be classified generally in two ways, as illustrated in Fig. 11: First, the larger ionic radii of both A and B ions lead to an expansion of the unit cell, which reduces the frequencies of phonon vibrations across the entire phonon spectrum, especially of the phonon modes dominated by the light oxygen ions.…”

“…In the pyrochlore titanates, available calculations of the phonon spectrum are limited to the point, where the energies and symmetries of phonons have previously been predicted [16,17]. Experimentally, the lattice dynamics of both titanate and zirconate pyrochlores were measured using the Q = 0 sensitive Raman scattering [18][19][20] and infrared absorption techniques [21], but neither could confirm the existence of the low-lying optical phonon modes that were thought to be responsible for the low thermal conductivity of pyrochlore materials at elevated temperatures [22].…”

“…3, and appear in the phonon DOS, where they contribute substantially to the first peak, centered at 8 meV, while the longitudinal acoustic phonon branches reach 14 meV at the Brillouin zone boundaries. Recently, low-lying optical modes were identified in different rare-earth pyrochlore materials, mainly zirconates, and it was established that their interference with the acoustic phonon modes could suppress the lattice thermal conductivity [22].…”