The High Pressure Synthesis And Physical Properties Of Doped HoMnO₃

RMnO₃ have much exciting physical properties, for example, magnetocaloric effect, multiferroic and colossal magnetoresistance, which make them become a hot research field. For the rare earth element with smalle r ionic radius, such as Ho, Er, Tm, Yb, Lu and Y, their manganites crystallize into hexagonal structures by solid state reaction method at ambient pressure, so it is important to obtain the orthorhombic perovskite manganites and study their physical properties. For the orthorhombic HoMnO₃, it is usually an antiferromagnetic insulator. Both the ferroelectric Curie temperature TC and antiferromagnetic Néel temperature TN of HoMnO₃ with orthorhombic structure are far away from room temperature, which results in that it is difficult to apply this material. Therefore, we study how structure and physical properties of the material could be influenced by doping Ca at A site or Ga at Bsite, which is used to change the physical properties of the material.In this paper, the hexagonal manganites RMnO₃（R=Ho-Lu, Y） systems transform into the orthorhombic systems. The X-ray diffraction experiments indicate that with decreasing ionic radius, the tolerance factor, cell parameters and cell volume gradually decreases, but there is no obvious regularity in the changes in bond lengths and angles.The orthorhombic Ho1-x Cax MnO₃ was synthetized by using the solid state reaction method. With increasing Ca content, the tolera nce factor increases, and cell volume gradually decreases. However, there is no obvious regularity in the changes in bond lengths and angles. Both the antiferromagnetic N éel temperature TN and magnetic susceptibility gradually decrease in the sample. Analyzing resistivity for Ho1-xCaxMnO₃ as a function of the temperature, we found the resistivity of samples still decrease up to 0.3, but the resistivity increases when x=0.5. By studying microscopic mechanism of the conduction, the conductive model of Ho1-xCaxMnO₃（x=0.1-0.3） is variable range hopping conductivity, and that of Ho0.5Ca0.5MnO₃ is thermal activation.The orthorhombic HoMn1-x Gax O3 was synthetized by using the high temperature and pressure method. But, when the Ga content is larger than 0.3, the impurities of Ho₃Ga₅O₁₂ emergence in the sample. With increasing Ga content, the crystal structure of the sample is more stable, and both cell parameters（a, c） and cell volume（V） gradually decreases, but cell parameters b gradually increases. In terms of bond Mn-O lengths and O-Mn-O angles, there is no obvious regularity.