Incommensurate lattice distortion in the high temperature tetragonal phase of La_2-x(Sr,Ba)_xCuO₄

We report incommensurate diffuse (ICD) scattering appearing in the high-temperature-tetragonal (HTT) phase of La_2-x(Sr,Ba) _xCuO₄ with 0.07 ≤ x ≤ 0.20 observed by the neutron diffraction technique. For all compositions, a sharp superlattice peak of the low-temperature-orthorhombic (LTO) structure is replaced by a pair of ICD peaks with the modulation vector parallel to the CuO₆ octahedral tilting direction, that is, the diagonal Cu-Cu direction of the CuO₂ plane, above the LTO-HTT transition temperature T_s. The temperature dependences of the incommensurability δ for all samples scale approximately as T/T_s, while those of the integrated intensity of the ICD peaks scale as (T - T_s)^-1. These observations together with absence of ICD peaks in the non-superconducting x = 0.05 sample evince a universal incommensurate lattice instability of hole-doped 214 cuprates in the superconducting regime.