COMMENT ON TILTING OF THE CUO6 OCTAHEDRA IN LA1.83-XEU0.17SRXCUO4 AS SEEN BY EU-151 MOSSBAUER-SPECTROSCOPY

We discuss the different characteristic time scales of the x-ray absor ption fine structure (XAFS), tau(X), and Mossbauer spectroscopy (MS), tau(MS), techniques for structural determination at small-length scale s. We show that XAFS and MS can measure different local atomic arrange ments in La1.83-xEu0.17SrxCuO4 provided the local structural units are dynamical. Combined knowledge from these complementary techniques is used to determine that the local tilts of the CuO6 octahedra do not va nish with temperature but are dynamic, with the oxygen atoms at the ap ex positions [O(2)] hopping among local minima of the energy's surface . Evidence is found for a crossover between different time domains in which the hopping time, tau(0), changes from tau(0) < tau(MS) to tau(0 ) > tau(MS). We conclude that the Mossbauer results [Phys. Rev. B 54, R800 (1996)], contrary to the authors' conclusions, do not contradict our XAFS results [Phys. Rev. Lett. 76, 439 (1996)].