Pressure- and Composition-Induced Structural Quantum Phase Transition in the Cubic Superconductor<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mo stretchy="false">(</mml:mo><mml:mi>Sr</mml:mi><mml:mo>,</mml:mo><mml:mi>Ca</mml:mi><mml:msub><mml:mo stretchy="false">)</mml:mo><mml:mn>3</mml:mn></mml:msub><mml:msub><mml:mi>Ir</mml:mi><mml:mn>4</mml:mn></mml:msub><mml:msub><mml:mi>Sn</mml:mi><mml:mn>13</mml:mn></mml:msub></mml:math>

We show that the quasi-skutterudite superconductor Sr(3)Ir(4)Sn(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.