Emergence of superconductivity in BaNi ₂(Ge _1-xP _x) ₂ at a structural instability

The physical properties and structural evolution of the 122-type solid solution BaNi ₂(Ge _1-xP _x) ₂ are reported. The in-plane X-X (X = Ge _1-xP _x) dimer formation present in the end member BaNi ₂Ge ₂, which results in a structural transition to orthorhombic symmetry, is completely suppressed to zero temperature on P substitution near x=0.7, and a dome-shaped superconducting phase with a maximum T _c=2.9 K emerges. Clear indications of phonon softening and enhanced electron-phonon coupling are observed at the composition of the structural instability. Our findings show that dimer breaking offers possibilities as a tuning parameter of superconductivity.