  The recently discovered layered kagome metals AV3Sb5 (A=K, Rb, and Cs)
attract intensive interest due to their intertwining with superconductivity,
charge-density-wave state, and nontrivial band topology. In this work, we show
by first-principles calculations and symmetry arguments that unconventional
type-II Dirac nodal line fermions close to the fermi level are present in
another latest class of experimentally synthesized kagome compounds AV6Sb6
(A=K, Rb, and Cs). These compounds possess a unique kagome (V3Sb)2 bilayer that
dominates their electronic and topological properties, instead of the kagome
V3Sb monolayer in AV3Sb5. Crystal symmetry guarantees that the type-II Dirac
nodal lines with quantized Berry phase lie in reflection-invariant planes of
the Brillouin zone. We further reveal that the type-II Dirac nodal lines remain
nearly intact in the presence of spin-orbital coupling and can be categorized
as a Z2 classification. The findings establish AV6Sb6 as a class of new
fascinating prototypes, which will extend the knowledge of interplay between
unconventionally topological fermions and exotic quantum ordered states in
kagome systems.
