  We show that the quasi-skutterudite superconductor Sr_3Ir_4Sn_{13} undergoes
a structural transition from a simple cubic parent structure, the I-phase, to a
superlattice variant, the I'-phase, which has a lattice parameter twice that of
the high temperature phase. We argue that the superlattice distortion is
associated with a charge density wave transition of the conduction electron
system and demonstrate that the superlattice transition temperature T* can be
suppressed to zero by combining chemical and physical pressure. This enables
the first comprehensive investigation of a superlattice quantum phase
transition and its interplay with superconductivity in a cubic charge density
wave system.
