Formation of Orthorhombic and Multiferroic Hexagonal Phases from an Undercooled RMnO_3 (R = Rare-Earth Element) Melt Using a Containerless Technique

摘要

Containerless solidification of the undercooled RMnO_3 melt was carried out to study the formation of orthorhombic and hexagonal phases under controlled oxygen partial pressure. The 0-RMnO_3 (R = La to Gd) perovskite with a space group of Pbnm and A-RMnO_3 (R = Dy to Lu) with a space group of P6_3cm were solidified from an undercooled melt at Po_2 = 10~5 Pa. The RMnO_3 samples showed the undercooling of 230-380 K at Po_2 = 10~5 Pa. The surface morphologies changed from rough spherical to a smooth faceted planes with a decreasing ionic radius from La (0.1032 nm) to Lu (0.0861 nm). X-ray diffractometry and scanning electron microscopy results showed the existence of o-RMnO_3 and A-RMnO_3. Thermodynamic stabilities of the as-solidified phases were studied at 10~5 Pa; as a result, the hexagonal phases changed to orthorhombic for the DyMnO_3 and HoMn0_3 systems after annealing at 1673 K. Based on the thermogravimetric analysis results, oxygen deficiency, bulk composition, and tolerance factor were calculated.