  In the scenario of Landau phase transition theory in condensed matter
physics, any thermal dynamic phase transition must be subject to some kind of
broken symmetries, that are relative to its spin, charge, orbital and lattice.
Here we report a rare phase transition at Tp ~120 K or 140 K in layered
materials LnCu4-{\delta}P2 (Ln=Eu, Sr) driven by a subtle structural-distortion
without any broken symmetry on charge, spin and lattice. The variations of the
lattice parameters, ({\Delta}Lc/Lc) ~ 0.013% or 0.062%, verified by thermal
expansion, is much less than that for a typical crystalline phase transition
(~0.5-1%), but the significant anomaly in heat capacity provides clear evidence
of its intrinsic nature of thermodynamic transition.
