Investigation of the local Mn-O distortions in the concentration-induced Metal/Insulator transition materials, La$_{1-x}$ Ca$_{x}$MnO$_{3}$ (x= 0.16-0.22) using EXAFS.

The pseudo-cubic manganites La$_{1-x}$ Ca$_{x}$MnO$_{3}$ (LCMO) are CMR (Colossal Magnetoresistance) materials for x between approximately 0.2 and 0.5. They have a temperature driven Metal/Insulator (MI) transition at the ferromagnetic transition temperature T$_{c}$ which is caused by Jahn-Teller-like distortions which form around Mn sites as T increases through T$_{c}$. Near x=0.2 the MI is driven by concentration. To investigate the concentration and temperature dependent local distortions for low doped powder samples (LCMO x = 16{\%}, 18{\%}, 20{\%}, 22{\%}), we carried out EXAFS (Extended X-ray Absorption Fine Structure) experiment at SSRL (Stanford Synchrotron Radiation Laboratory). The results of our preliminary analysis show the following: (1) a sharp, step increase in $\sigma ^{2}$ near T$_{c}$ ($\sigma $ is the width of the Mn-O pair distribution function (PDF)), (2) the magnitude of the step decreases rapidly with decreasing x, (3) the value of $\sigma ^{2}$ at low T is close to the value for zero-point motion for x = 0.22, but increases substantially for lower x, indicating that in the magnetized state a large J-T distortions remain at low T for low x. Also the magnetization at low T is small for lower x, indicating that not all of the sample is ferromagnetic.