  The effects of the cooperative Jahn-Teller effect on the crystal structure
and the stability of the charge ordered (CO) state were studied by measurements
of powder X-ray diffraction, resistivity, and ultrasound for Pr1-xCaxMnO3
(0.5<=x<=0.875). Powder X-ray diffraction revealed a change of the crystal
structure from tetragonally compressed to tetragonally elongated orthorhombic
between x=0.75 and x=0.8 in the CO state, resulting from the crossover of the
cooperative Jahn-Teller vibration mode from Q2 to Q3. The relative stiffening
of the ultrasound (DeltaV/V) reflecting the magnitude of the cooperative
Jahn-Teller lattice distortion in the CO state increases with increasing x from
0.5 to 0.625, reaching the largest and being almost x-independence for
0.625<x<0.8, and drops steeply with further increase of x. Coincident with the
variation of the DeltaV/V with x, the stability of the CO state reflected by
the magnetoresistance effect increases with increasing x from 0.5 to 0.625,
reaching the most stable for 0.625<x<0.825, and becomes unstable with further
increase of x. These features demonstrate that the cooperative Jahn-Teller
lattice distortion is one of the key ingredients in understanding the essential
physics of the CO state in manganites.
