  Since the discovery of the Verwey transition in magnetite, transition metal
compounds with pyrochlore structures have been intensively studied as a
platform for realizing remarkable electronic phase transitions. We report the
discovery of a unique phase transition that preserves the cubic symmetry of the
beta-pyrochlore oxide CsW$_2$O$_6$, where each of W 5d electrons are confined
in regular-triangle W3 trimers. This trimer formation is an unprecedented
self-organization of d electrons, which can be resolved into a charge order
satisfying the Anderson condition in a nontrivial way, orbital order caused by
the distortion of WO6 octahedra, and the formation of a spin-singlet pair in a
regular-triangle trimer. Electronic instability due to the unusual
three-dimensional nesting of Fermi surfaces and the localized nature of the 5d
electrons characteristic of the pyrochlore oxides were found to play important
roles in this unique charge-orbital-spin coupled phenomenon.
