Crossover from Large to Small Polarons across the Metal-Insulator Transition in Manganites

We report Mn K-edge extended x-ray absorption fine structure spectra on La0.75Ca0.25MnO3 up to high momentum transfer across the metal-insulator sM-Id transition. The data show compelling evidence for (i) large or intermediate Jahn-Teller polarons (IJTP), characterized by an anomalous longer Mn-O bond sDR ­ 0.09 Åd in the metallic phase sT , 170 Kd, and (ii) appearance of small JT polarons (SJTP) at T . 170 K, characterized by a longer Mn-O bond sDR ­ 0.21 Åd, which coexist with the IJTP above the M-I transition and has equal probability in the temperature range of colossal magnetoresistance.