  We report incommensurate diffuse (ICD) scattering appearing in the
high-temperature-tetragonal (HTT) phase of La$_{2-x}$(Sr,Ba)$_{x}$CuO$_{4}$
with $0.07 \leq x \leq 0.20$ 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 CuO$_6$ octahedral tilting direction, that is, the diagonal
Cu-Cu direction of the CuO$_2$ plane, above the LTO-HTT transition temperature
$T_s$. The temperature dependences of the incommensurability $\delta$ 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.
