Nano scale phase coexistence and charge-ordering with 3dx2-y2 orbital-ordering in La0.25Sr0.75MnO3.01

The phase diagram of La0.25Sr0.75MnO3.01 has been investigated by performing temperature dependent powder x-ray diffraction, transmission electron microscopy (TEM), and resistivity and magnetization measurements. It undergoes competing structural phase-transitions from charge disordered cubic to charge-ordered (CO) tetragonal phase having compressed Jahn-Teller (JT) distorted octahedra (c/a <1) at ∼268 K followed by charge disordered but orbital order (CDO) I4/mcm twin tetragonal phase transition with elongated octahedra JT-distortion (c/a > 1) at ∼250 K. The three phases coexist from 250 to 210 K. The nano-scale (10-20 nm) coexistence of CO and twin tetragonal phases sustains down to 80 K, with twin tetragonal phase fraction increasing substantially on cooling below 210 K. In TEM diffraction data, the CO phase shows well defined super-lattice spots with ∼8x8-fold CO modulation along [110] type direction of the perovskite lattice and typical CO micro-structure with lattice fringes. The twin microstructure resulting in twin spots can also be observed for the elongated JT-distorted tetragonal phase. The compressed and elongated distortions of the octahedra for CO and twin tetragonal phases implies ferro orbital-ordering of 3dx2-y2 and 3d3z2-r2 orbitals in the ab-plane and along c-axis, respectively. During heating, the twin tetragonal to CO phase transformation was fully reversible with a wide hysteresis and the CO modulation was found to flip its direction from [110] to [101¯] & [011¯] in the same domain. Thus, our results reveal the nano-scale structural phase separation due to competitive nature of CO and twin phase over wide temperature range.