Dielectric signatures of lattice instabilities at 32K and 245K in La₂₋ySryMO₄₊ₓ (M=Cu,Ni) Cuprates and Nickelates

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New dielectric transitions are observed at common temperatures 32K and 245K, in isostructural La₂CuO₄₊ₓ and La₅/₃Sr₁/₃NiO₄, that are signatures of local lattice (octahedral) instabilities. The present dielectric transitions reveal new aspects of the phase diagram of the perovskite cuprates and nickelates. They suggest that competition and coexistence of superconductivity with dielectricity occurs that is analogous to that between superconductivity and anti-ferromagnetism. These results also indicate that inhomogeneous electronic states, such as charge stripes and oxygen ordering, are strongly connected to underlying lattice instabilities.