Investigation of topological nodal line phonons in rhenium-based alkali metal oxides (AReO 4 ; A = Na, K, Rb) using first principle methods

Abstract

Topological phonons in spinless systems are gaining wide interest, as their symmetry-protected stability not only ensures persistence in crystalline materials but also opens avenues for novel applications in energy transport and quantum technologies. Motivated by this, we present a detailed first-principles investigation of the topological vibrational properties of scheelite-type alkali metal perrhenates AReO 4 (A = Na, K, Rb). All three materials adopt a body-centered tetragonal structure with the non-symmorphic space group I4 1 /a and exhibit dynamically stable phonon modes across the Brillouin zone. Our analysis uncovers a variety of topological phonon features, including symmetryprotected type-I nodal lines and quadratic nodal points, which arise due to twofold degeneracies along specific high-symmetry directions. Interestingly, KReO 4 uniquely displays an hourglass-like phonon dispersion in the frequency range of 2.6-3.3 THz along the Γ-X path, This feature likely arises from the potassium ion's intermediate size and bonding, which alter vibrational coupling and induce band inversion with nontrivial topological connectivity along the Γ-X path. To confirm the topological origin of the phonon crossings, surface phonon calculations were performed along selected orientations. The resulting spectral functions exhibit clear drumhead-like surface states confined within the projected nodal-line regions: 8.0-8.45 THz for NaReO 4 , 9.0-9.4 THz for KReO 4 , and 9.0-9.25 THz for RbReO 4 . These results highlight that A-site cation tuning can effectively control phonon topology and surface vibrational behavior in Re-based oxides. By adjusting ionic size, mass, and bonding, it directly influences vibrational coupling and band inversion, providing a simple, complementary route alongside strain, pressure, and B-site substitution for engineering topological features and guiding experimental design of oxides."