Pressure and isotope effects in the manganese-oxide perovskites

Measurements of the temperature dependence of the resistivity rho(T) and thermoelectric power alpha(T) under several hydrostatic pressures on O-18/O-16 isotope-exchanged (La1-xNdx)(0.7)Ca0.3MnO3 polycrystalline samples spanning the structural O'-O orthorhombic transition have demonstrated that the perovskite tolerance factor t increases with pressure, signaling an unusually compressible Mn-O bond. They have also indicated a change at the O'-O transition from static to dynamic cooperative Jahn-Teller deformations, from a second-order to a first-order magnetic transition, from Mn(IV) to two-Mn polarons in the paramagnetic region, from Mn(IV) polarons to a vibronic state below T-c, and a phase segregation in the O phase above T-c that traps out mobile polarons into ferromagnetic Mn(IV)-rich clusters within a Mn(IV)-poor matrix. Specific heat data show a transfer of spin entropy to configurational entropy on cooling through T-c in the O phase, and a single-crystal study of La1-xSrxMnO3, x = 0.12 and 0.15, has demonstrated a transition from polaronic to itinerant e electrons below T-c within the O phase. Magnetic-susceptibility measurements in low fields confirm the phase segregation above T-c and the existence of a ferromagnetic glass below T-c in the O phase. The intrinsic "colossal" magnetoresistance (CMR) is attributed to the growth to their percolation threshold of the ferromagnetic clusters existing above T-c, and the dramatic rise in T-c with increasing tolerance factor t in the O-orthorhombic phase to an increase in the density, and also the mobility, of the untrapped polarons above T-c.