  Structural features of the charge/orbital ordering (CO/OO) in single-layered
manganites Pr1-xCa1+xMnO4 have been investigated systematically by transmission
electron microscopy. Analyses of electron diffraction patterns as well as
dark-field images have revealed that the CO/OO shows a striking asymmetric
behavior as the hole doping x deviates from x = 0.5. The modulation wavenumber
linearly decreases with increasing x in the over-hole-doped (x > 0.5) crystals,
while much less dependent on x in the under-hole-doped (x < 0.5) crystals. A
temperature-induced incommensurate-commensurate crossover is observed in 0.35 <
x < 0.5 and x = 0.65. The correlation length of CO/OO in x = 0.3 was proven to
become shorter than that in x > 0.3.
