Low-temperature transmission electron microscopy and x-ray diffraction (XRD) studies have been carried out on pellet and powder samples of ${\text{Pr}}_{0.5}{\text{Ca}}_{0.5}{\text{MnO}}_3$. These studies have revealed appearance of a different type of charge-orbital ordered (COO) phase, resulting due to flipping of $e_g$ orbitals from $d_{3x^2-r^2}/d_{3y^2-r^2}$ to $d_{3x^2-r^2}/d_{3z^2-r^2}$ configuration. This orbital flip results in a changeover of the COO superlattice-ordering vector from (1/2,0,0) to (1/4,1/2,1/4) in the $Pnma$ phase. This COO phase coexists with the conventional COO phase. Low-temperature XRD studies show that the COO phase appears only in pellet sample and not in the corresponding powder sample. The powder sample shows only conventional COO phase. Volume fractions of conventional and the other type COO phases in pellet sample of ${\text{Pr}}_{0.5}{\text{Ca}}_{0.5}{\text{MnO}}_3$ is estimated to be $\sim 55\%$ and 45%, respectively. The occurrence of orbital flip has been attributed to local strain building up in the pellet sample. The strain builds up during cooling because manganite has anisotropic thermal expansion coefficients.

• Received 18 April 2008

DOI:https://doi.org/10.1103/PhysRevB.78.174106