Crystal structure, chemical expansion and phase stability of HoMnO3 at high temperature

Anisotropic thermal and chemical expansion of hexagonal HoMnO3 was investigated by high temperature X-ray diffraction in inert (N-2) and oxidizing (air) atmospheres up to 1623 K. A second order structural phase transition directly from P6(3)cm to P6(3)/mmc was found at 1298 +/- 4 K in N-2 atmosphere, and 1318 +/- 4 K in air. For the low temperature polymorph P6(3)cm the contraction of the c-axis was more rapid in inert than in oxidizing atmosphere. The c-axis of the P6(3)/mmc polymorph of HoMnO3 displayed anomalously high expansion above 1400 K, which is discussed in relation to chemical expansion caused by point defects. The a-axis expanded stronger in inert than oxidizing atmosphere. Anisotropic chemical and thermal expansion of the P6(3)cm phase of YMnO3 in N-2, air and O-2 atmospheres was found to be qualitatively similar to that of HoMnO3. Decomposition of hexagonal HoMnO3 by two different processes occurs in oxidizing atmosphere above similar to 1200 K followed by nucleation and growth of the perovskite polymorph of HoMnO3. A rapid, reconstructive transition from the perovskite back to the hexagonal polymorph was observed in situ at 1623 K upon reduction of the partial pressure of oxygen. A phase stability diagram of the hexagonal and orthorhombic polymorphs is proposed. Finally, distinctly non-linear electrical conductivity was observed for both HoMnO3 and YMnO3 in oxidizing atmosphere between 555 and 630 K, and shown to be associated with excess oxygen. (C) 2012 Elsevier Inc. All rights reserved.