  In two-dimensional materials, structure difference induces the difference in
phonon dispersions, leading to the anisotropy of in-plane thermal transport.
Here, we report an exceptional case in layered PdSe2, where the bonding, force
constants, and lattice constants are nearly-equal along the in-plane
crystallographic axis directions. The phonon dispersions show significant
differences between the Gamma-X and Gamma-Y directions, leading to the
anisotropy of in-plane thermal conductivity with a ratio up to 1.8. Such
anisotropy is not only unexpected in equilaterally structured (in-plane)
materials but also comparable to the record in the non-equilaterally structured
material reported to date. By combining inelastic X-ray scattering and
first-principles calculations, we attribute such anisotropy to the low-energy
phonons along Gamma-X, in particular, their lower group velocities and
"avoided-crossing" behavior. The different bucking structures between a-
(zigzag-type) and b-axis (flat-type) are mainly responsible for the unique
phonon dynamics properties of PdSe2. The present results illustrate the unusual
thermal conduction mechanism of the equilaterally structured materials and
provide valuable insights on thermal management in electronic devices.
