Non percolative nature of the metal-insulator transition and persistence of local Jahn-Teller distortions in the rhombohedral regime of La$_{1-x}$Ca$_{x}$MnO$_{3}$

Abstract

Evolution of the average and local crystal structure of Ca-doped LaMnO$_{3}$ has been studied across the metal to insulator (MI) and the orthorhombic to rhombohedral (OR) structural phase transitions over a broad temperature range for two Ca concentrations ($x = 0.18; 0.22$). Combined Rietveld and high real space resolution atomic pair distribution function (PDF) analysis of neutron total scattering data was carried out with aims of exploring the possibility of nanoscale phase separation (PS) in relation to MI transition, and charting the evolution of local Jahn-Teller (JT) distortion of MnO$_{6}$ octahedra across the OR transition at T$_{S}$ $\sim$ 720 K. The study utilized explicit two-phase PDF structural modeling, revealing that away from TMI there is no evidence for nanoscale phase coexistence. The local JT-distortions disappear abruptly upon crossing into the metallic regime both with doping and temperature, with only small temperature-independent signature of quenched disorder being observable at low temperature as compared to CaMnO$_{3}$. The results hence do not support the percolative scenario for the MI transition in La$_{1-x}$Ca$_{x}$MnO$_{3}$ based on PS, and question its ubiquity in the manganites. In contrast to LaMnO3 that exhibits long range orbital correlations and sizeable octahedral distortions at low temperature, the doped samples with compositions straddling the MI boundary exhibit correlations (in the insulating regime) limited to only $\sim$ 1 nm with observably smaller distortions. In $x = 0.22$ sample local JT-distortions are found to persist across the OR transition and deep into the R-phase (up to $\sim$ 1050 K) where they are crystallographically prohibited. Their magnitude and subnanometer spatial extent remain unchanged.