  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.
