First-order transition at T-C in the orthomanganites

The orthomanganites L(0.7)A(0.3)MnO(3), L=rare-earth or Y and A=alkaline earth, exhibit a first-order transition at a ferromagnetic Curie temperature T-C, where T-C drops with decreasing tolerance factor t from a high near 350 K at t=0.98 to a T-C*approximate to 60 K at t less than or equal to 0.96. The paramagnetic phase is a polaronic conductor; the ferromagnetic phase contains mobile electrons in extended states for larger tolerance factors t>t(c)=0.96, but remains polaronic for t<t(c). The temperature dependence of the thermopower is used to explore the polaron-extended state transition for five nominal compositions (La1-xNdx)(0.7)Ca0.3MnO3 designed to have tolerance factors near the critical value t(c). Comparison of the temperature dependence of the thermopower, alpha(T), as a result of chemical pressure, i.e., t<1, with that for hydrostatic pressure reveals an anomalously large compressibility of the Mn-O bonds, which is characteristic of a double-well potential at a crossover from an electronic state of lower mean kinetic energy to one of greater kinetic energy. It also reveals a trapping of mobile holes above T-C that increases with decreasing T-C. In the orthomanganites, the crossover is from polaronic-e to extended-o* electronic conduction at T-C, a crossover that is responsible for the ''colossal'' negative magnetoresistance observed in the orthomanganites.