  In this review, I present the electronic properties of the beta-pyrochlore
oxide CsW2O6 and other related materials. At 215 K, CsW2O6 exhibits an
electronic phase transition to a nonmagnetic insulating state, which exhibits a
complex self-organization of 5d electrons. In this phase transition, various
factors, such as geometrical frustration of the pyrochlore structure,
moderately strong electron correlation, Jahn-Teller-like distortion, phase
transition to the insulating state, and formation of chemical bonds in solids,
which are powerful driving forces for achieving a wide variety of electronic
properties in transition metal oxides, play important roles. In CsW2O6, the
interplay of these factors has led to the emergence of an electronic phase
transition that preserves the cubic symmetry, three-dimensional nesting of
Fermi surfaces, a possible charge order with a fractional valence satisfying
the Anderson condition, and an equilateral-triangular trimer formation by a
three-centered-two-electron bond. In addition to the introduction of these
unique features of CsW2O6, each of them has been compared to those of other
materials to provide an overview of the electronic properties of a wide variety
of related materials, which can contribute to a complete understanding of the
vast and infinite electronic phenomena in transition metal oxides.
