Hexagonal versus perovskite phase of manganite RMnO3 (R=Y, Ho, Er, Tm, Yb, Lu)

The floating-zone method and high-pressure synthesis have been used to obtain the hexagonal and the perovskite RMnO3 (R=Y, Ho, Er, Tm,Yb, Lu) compounds. We have refined the crystal structure and characterized the compounds with measurements of magnetic susceptibility chi(T) and thermal conductivity kappa(T). The systematic change of kappa(T) below T-N found in all members of the hexagonal RMnO3 family shows that some spin-independent bond-length fluctuation plays an important role in the suppression of kappa(T) below T-N as well as in the paramagnetic phase. The responsible soft vibrational mode is identified. In contrast, the perovskite RMnO3 shows a phonon-like kappa(T) below room temperature, but with an anomalously large critical scattering at T-N. A phase diagram of transition temperatures versus the R3+-ion radius for both hexagonal and perovskite phases is also given.