On the Conical Point Crossing: The Case of the Orthorhombic Lattice

Abstract

It is shown that the conical point crossing, known to appear at the K point in the Brilloiun zone of the hexagonal lattice, may appear also in those of the orthorhombic and the monoclinic lattices. The feature of the conical point crossing in the orthorhombic lattice induced by the ferroelastic phase transition is concretely studied. Generally in the lattices with the symmetry lower than hexagonal, the cone becomes the elliptic cone, and the conical k-point shifts to a general position inside the Brillouin zone. The existence of the conical point crossing in the orthorhombic and the monoclinic lattices can be proved using some models, but not solely by symmetry considerations.