Tensile Microstrain Fluctuations in the BaPbO Units in Superconducting BaPb_1-xBi_xO₃ by Scanning Dispersive Micro-XANES

BaPb1 xBixO3 (BPBO) bismuthate, showing high T-C superconductivity for 0.05 < x < 0.35, is an archetypal system for studying the complex inhomogeneity of perovskite lattice favoring the emergence of quantum coherence, called the superstripes phase. Local lattice fluctuations, detected by EXAFS; nanoscale stripes, detected by electron microscopy; and two competing crystalline structures, detected by diffraction, are known to characterize the superconducting phase. At nanoscale [BaBiO3] centered nanoscale units (BBO) coexist with BaPbO3 centered (BPO) units in the BPBO perovskite; therefore, we expect a tensile microstrain in BPO units due the misfit strain between the two different lattices. Here, we report the measurement of the spatial micro-fluctuations of the local tensile microstrain epsilon in the BaPO units in superconducting Ba(Pb-1 Bi-x(x))O-3 crystals with x(1) = 0.19 an x(2) = 0.28. We show here the feasibility of applying the scanning dispersive micro-X-ray absorption near edge structure (Sd mu XANES) technique, using focused synchrotron radiation, to probe the microscale spatial fluctuations of the microstrain in BPO units. This unconventional real-space Sd mu XANES microscopy at the Pb L-3 edge has been collected in the dispersive mode. Our experimental method allows us to measure either the local Bi chemical concentration x and the local lattice microstrain of local BBO and BPO units. The 5 x 5 micron-size spots from the focused X-ray beam allowed us to obtain maps of 1600 points covering an area of 200 x 200 microns. The mapping shows a substantial difference between the spatial fluctuations of the microstrain epsilon and the chemical inhomogeneity x. Moreover, we show the different relations epsilon(x) in samples with lower (x(1) = 0.19) and higher (x(2) = 0.28) doping respect to the optimum doping (x = 0.25).