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The semiconductor-insulator phase transition of the single-layer manganiteLa0.5Sr1.5MnO4 has been studied by means of high resolution synchrotron x-raypowder diffraction and resonant x-ray scattering at the Mn K edge. We concludethat a concomitant structural transition from tetragonal I4/mmm to orthorhombicCmcm phases drives this electronic transition. A detailed symmetry-modeanalysis reveals that condensation of three soft modes -Delta_2(B2u), X1+(B2u)and X1+(A)- acting on the oxygen atoms accounts for the structuraltransformation. The Delta_2 mode leads to a pseudo Jahn-Teller distortion (inthe orthorhombic bc-plane only) on one Mn site (Mn1) whereas the two X1+ modesproduce an overall contraction of the other Mn site (Mn2) and expansion of theMn1 one. The X1+ modes are responsible for the tetragonal superlattice(1/2,1/2,0)-type reflections in agreement with a checkerboard ordering of twodifferent Mn sites. A strong enhancement of the scattered intensity has beenobserved for these superlattice reflections close to the Mn K edge, which couldbe ascribed to some degree of charge disproportion between the two Mn sites ofabout 0.15 electrons. We also found that the local geometrical anisotropy ofthe Mn1 atoms and its ordering originated by the condensed Delta_2 mode aloneperfectly explains the resonant scattering of forbidden (1/4,1/4,0)-typereflections without invoking any orbital ordering.

Structural changes at the semiconductor-insulator phase transition in the single layered La0.5Sr1.5MnO4 perovskite