Evidence for charge localization in the ferromagnetic phase of La1-xCaxMnO3 from high real-space-resolution x-ray diffraction

High real-space-resolution atomic pair distribution functions of La1-xCaxMnO3 (x = 0.12, 0.25, and 0.33) have been measured using high-energy x-ray powder diffraction to study the size and shape of the MnO6 octahedron as a function of temperature and doping. In the paramagnetic insulating phase we find evidence for three distinct bond lengths which we: ascribe to Mn4+-O, Mn3+-O-short, and Mn3+-O-long bonds, respectively. In the ferromagnetic metallic (FM) phase, for x = 0.33 and T = 20 K, we find a single Mn-O bond length; however, as the metal-insulator transition is approached either by increasing Tor decreasing x, intensity progressively appears around r = 2.15 and in the region 1.85-1.9 Angstrom suggesting the appearance of Mn3+-O-long bonds and short Mn4+-O bonds. This is strong evidence that charge localized and delocalized phases coexist close to the metal-insulator transition in the FM phase.