Role of octahedral tilting induced acoustic softening on limiting thermal transport in SrSnO3

Abstract

Octahedral tilting is a fundamental structural distortion in perovskites, governing key phenomena such as lattice stabilizing, soft phonon dynamics, group-theoretical analysis, phase transitions, ferroelectricity, and even for tunable electronic band gap. However, its influence on lattice thermal conductivity (kL) remains poorly understood. In the archetypal perovskite SrTiO3, tilting in the low-temperature tetragonal phase is known to enhance kL by suppressing specific phonon scattering channels around 200 cm-1. Here, we investigate the thermal transport in strontium stannate (SrSnO3), another perovskite oxide that undergoes temperature-driven phase transitions, and reveal a completely opposite effect. Through a systematic study across its orthorhombic, tetragonal, and cubic phases, we demonstrate that octahedral tilting in the tetragonal phase of SrSnO3 anomalously triggers acoustic phonon softening. This softening manifests as reduced frequencies and group velocities in low-frequency (<3 THz) acoustic modes, creating a large decrease for heat transport, particularly along the c-axis. Consequently, kL is significantly suppressed, decreasing from 7.48 W m-1 K-1 to 6.06 W m-1 K-1 as the tilting angle increases by a mere 1 degree. These findings identify tilting-induced acoustic softening as a pivotal mechanism for limiting and controlling anisotropic thermal transport in SrSnO3, presenting a stark contrast to the established behavior in SrTiO3.

Figures (6)

• Figure 1: Crystal structure of (a) orthorhombic phase, (b) tetragonal phase, and (c) cubic phase of SrSnO$_3$ viewed along the $c$-axis, showing the arrangement of Sr (green), Sn (gray), and O (red) atoms in a perovskite lattice. (d) Phonon dispersion curves of the orthorhombic, tetragonal phase, and cubic phase of SrSnO$_3$, with high-symmetry points labeled along the Brillouin zone path ($\Gamma$--Z--T--Y--S--R--U--X--$\Gamma$--Y). (e) Lattice thermal conductivity with 3ph and 4ph interactions as a function of temperature. (f) Comparison of the c-axis thermal conductivity of the orthorhombic phase with experimental values considering grain boundary scattering and vacancy scattering zhang2023temperature.
• Figure 2: (a) Renormalized phonon dispersion curves of all three SSO phases (orthorhombic, tetragonal, and cubic) as a function of temperature, with high-symmetry points labeled along the Brillouin zone path ($\Gamma$--Z--T--Y--S--R--U--X--$\Gamma$--Y). The temperatures ranging from 100 K to 500 K for orthorhombic phase, 1100 K to 1300 K for tetragonal phase, and 1400 K to 1600 K for cubic phase. (b) Brillouin zone path for the phonon dispersion. (c) Phonon scattering rate of the orthorhombic phase as a function of temperature from 100 K to 500 K. (d) Lattice thermal conductivity ($\kappa_L$) of SrSnO$_3$ as a function of temperature (100--1600 K).
• Figure 3: (a) Phonon dispersion curves at 1200 K along high-symmetry directions (X, $\Gamma$, Z) in the tetragonal phase. (b) Phonon scattering rates as a function of frequency at 1000 K, comparing orthorhombic (red squares) and tetragonal (blue circles) phases. (c) Phonon scattering rates as a function of frequency at 1400 K, comparing cubic (purple squares) and tetragonal (blue circles) phases. (d) Spectral $\kappa_{p, SCPH}^{3, 4ph}$ at 1000 K for different structural phases and orientations: orthorhombic along the $c$-axis (solid red line), orthorhombic along the $b$-axis (dashed red line), tetragonal along the $c$-axis (solid blue line), and tetragonal along the $b$-axis (dashed blue line).
• Figure 4: (a) Crystal structure of the perovskite material at 1200 K, illustrating the arrangement of atoms in a tetragonal phase with tilting angle of $\alpha = 12.998^\circ$ and blue arrows denoting the rotational directions of the octahedron. The structure shows the unit cell with red spheres representing O atoms, green spheres representing Sr atoms, and gray spheres representing the Sn atoms. (b) $\kappa_{p, SCPH}^{3ph}$ along the $a$-axis and $c$-axis as a function of the angular deviation $\Delta\alpha$ (in degrees) at 1200 K.
• Figure 5: (a)Spectral $\kappa_{p, SCPH}^{3ph}$ at 1200 K for tetragonal phase and $c$-axis orientations with $\Delta\alpha$ from $-0.5^\circ$ to $0.5^\circ$. (b) Tilting angle dependent phonon dispersion curves along high-symmetry paths in the Brillouin zone ($\Gamma$, Z and X), with acoustic (purple to blue) and optical (gray) branches distinguished.