  The Jahn-Teller effect in the charge-ordered (CO) state for La1-xCaxMnO3
(0.5<x<0.87) was studied by measuring the low-temperature powder x-ray
diffraction, internal friction, and shear modulus. We find that the
electron-lattice interaction with the static Jahn-Teller distortion is the
strongest near x=0.75 in the CO state. It was particularly observed that a
crossover of the Jahn-Teller vibration mode from Q2 to Q3 near x=0.75 induces
crossovers of the crystal structure from tetragonally compressed to
tetragonally elongated orthorhombic, and of the magnetic structure from CE-type
to C-type near x=0.75. The experimental results give strong evidence that the
Jahn-Teller effect not only plays a key role in stabilizing the CO state, but
also determines the magnetic and crystal structures in the CO state for
La1-xCaxMnO3.
