Strong reduction of lattice effects in mixed-valence manganites related to crystal symmetry

Calorimetric, thermal expansion, resistivity, and magnetization measurements in La2/3(Ca_1-xSrₓ)1/3MnO₃ samples evidence the existence of a critical tolerance factor t that determines the relevance of lattice effects in mixed-valence manganites. Samples with $x<0.15$ (i.e., with $t<0.92)$ exhibit a first-order magnetic transition at the Curie temperature TC as well as anomalous volume and magnetic entropy changes, high volume sensitivity to magnetic fields and high magnetoresistance at TC; whereas those with $t>0.92$ do not. Samples of the former group show orthorhombic structures, whereas the other are rhombohedral. Taking into account that a rhombohedral symmetry forbids a static long-range-cooperative Jahn-Teller distortion, these results are taken as a probe of the strong reduction of lattice effects beyond this critical value of the tolerance factor. A new phase boundary in the phase diagram of manganese perovskites, that separates not only crystallographic phases, but also samples in which lattice effects must be taken into account, is proposed.