First-order transition atTCin the orthomanganites

The orthomanganites $L_{0.7}{A}_{0.3}\mathrm{Mn}{\mathrm{O}}_3$, $L=\mathrm{r}\mathrm{a}\mathrm{r}\mathrm{e}-\mathrm{e}\mathrm{a}\mathrm{r}\mathrm{t}\mathrm{h}\mathrm{o}\mathrm{r}\mathrm{Y}$ and $A=\mathrm{alkaline}\mathrm{earth}$, 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 $t=0.98\mathrm{}$ to a $T_C^{*}\approx 60$ K at $t<~0.96$. The paramagnetic phase is a polaronic conductor; the ferromagnetic phase contains mobile electrons in extended states for larger tolerance factors $t>\mathrm{}{t}_c=0.96\mathrm{}$, but remains polaronic for $t<\mathrm{}{t}_c$. The temperature dependence of the thermopower is used to explore the polaron-extended state transition for five nominal compositions ${\left({\mathrm{La}}_{1-x}{\mathrm{Nd}}_x\right)}_{0.7}{\mathrm{Ca}}_{0.3}\mathrm{Mn}{\mathrm{O}}_3$ designed to have tolerance factors near the critical value $t_c$. Comparison of the temperature dependence of the thermopower, $\alpha \mathrm{}\left(T\right)\mathrm{}$, as a result of chemical pressure, i.e., $t<\mathrm{}1\mathrm{}$, 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-${\sigma }^{*}$ electronic conduction at $T_C$, a crossover that is responsible for the "colossal" negative magnetoresistance observed in the orthomanganites.