  Epitaxial La3/4Ca1/4MnO3/MgO(100) (LCMO) thin films show unusual rhombohedral
(R-3c) structure with a new perovskite superstructure due to unique ordering of
La and Ca at the A-site positions. Very sharp insulator-metal and
para-ferromagnetic phase transitions at temperatures up to TMI ~ TC=295 K were
observed. The ordered films were electronically homogeneous down to 1 nm scale
as revealed by scanning tunnelling microscopy/spectroscopy. In contrast,
orthorhombic and A-site disordered LCMO demonstrate broadened phase transitions
as well as mesoscopic phase separation for T<<TC. The unique La/Ca ordering
suppresses cation mismatch stress within one super-cell, a~1.55 nm, enhancing
electronic homogeneity. Phase separation scenario seems not to be a unique
mechanism for CMR as very large CMR=500 % was also observed in A-site ordered
films.
