Hexagonal versus perovskite phase of manganiteRMnO3(R=Y,Ho,Er,Tm,Yb,Lu)

Abstract

The floating-zone method and high-pressure synthesis have been used to obtain the hexagonal and the perovskite $R\mathrm{Mn}{\mathrm{O}}_3$ $(R=\mathrm{Y},\mathrm{Ho},\mathrm{Er},\mathrm{Tm},\mathrm{Yb},\mathrm{Lu})$ compounds. We have refined the crystal structure and characterized the compounds with measurements of magnetic susceptibility $\chi \left(T\right)$ and thermal conductivity $\kappa \left(T\right)$. The systematic change of $\kappa \left(T\right)$ below $T_N$ found in all members of the hexagonal $R\mathrm{Mn}{\mathrm{O}}_3$ family shows that some spin-independent bond-length fluctuation plays an important role in the suppression of $\kappa \left(T\right)$ below $T_N$ as well as in the paramagnetic phase. The responsible soft vibrational mode is identified. In contrast, the perovskite $R\mathrm{Mn}{\mathrm{O}}_3$ shows a phonon-like $\kappa \left(T\right)$ below room temperature, but with an anomalously large critical scattering at $T_N$. A phase diagram of transition temperatures versus the $R^{3+}$-ion radius for both hexagonal and perovskite phases is also given.