Effects Of Morphology And Size On The High Pressure Structural Phase Transition Of Manganese Oxides

Manganese oxides have shown unique physical and chemical properties in catalytic,battery,magnetism and adsorption technology,so they have important research significance and application prospect.Compared with bulk materials,manganese oxide nanomaterials have more excellent properties,which has attracted much attention in the field of materials research in recent years.As a new means of material research,high pressure technology has been widely used in the study of physical and chemical properties of materials.The purpose of high pressure research is mainly to discover new properties and new behaviors of materials,which will help us to design and synthesize new materials with better properties,which has important value in the design and synthesis of materials.In addition,due to many new behaviors of nanocrystals under high pressure,the study of nanomaterials under high pressure has received extensive attention.And the study of the high pressure structural stability and phase transition of nanomaterials is of great scientific significance.Because it is a crucial factor in the design,synthesis and application of materials.Although high pressure studies on nanomaterials have been widely concerned,the phase transformation behavior related to their morphology is still unclear.And the properties of manganese oxide nanomaterials are closely related to their morphology,nanosize and crystal structure.Therefore,studying the behaviors of manganese oxide nanomaterials with different morphologies under high pressure will help to understand the influence of morphology and size on the high pressure behaviors and properties of materials.In this paper,the structural transformation of Mn₃O₄nanorods and MnO₂nanorods under high pressure was studied by means of high pressure X-ray diffraction,in-situ Raman spectroscopy and transmission electron microscopy.The influence of material morphology and size on its high pressure behavior was investigated.The main conclusions of the experiment are as follows:1.The high pressure behaviors of Mn₃O₄ nanorods have been studied for the first time.It is found that Mn₃O₄nanorods undergo two phase transitions upon compression,from initial phase to orthogonal Ca Ti₂O₄-type structure at 15.3 GPa,and then to marokite-like phase at 18.9 GPa.Upon decompression,the high pressure marokite-like phase can be retained to ambient pressure conditions.At the same time,compared with Mn₃O₄bulk and nanoparticles,we found that Mn₃O₄nanorods are obviously different from that of its bulk materials and nanoparticles.Upon compression,the phase transition sequence of Mn₃O₄nanorods is similar to the nanoparticles,while the decompression behavior is consistent with the bulk Mn₃O₄.We think this may be related to the nanoscale effect and the unique one-dimensional morphology of nanorods.In addition,both the growth orientation and the suppressed Jahn-Teller distortion of the Mn₃O₄nanorods are crucial factors for their high pressure behaviors.2.The high pressure behaviors of MnO₂ nanorods were investigated.The results of synchrotron radiation X-ray diffraction show that when the pressure reaches 11.0GPa,the diffraction peak starts broadening and splitting.There is a transformation from tetragonal phase to orthogonal Ca Cl₂-type structure.The splitting of the diffraction peak may be caused by the distortion of the tetragonal phase.Compared with the bulk materials previously reported,MnO₂nanorods have higher phase transition pressure.In addition,the volume changes continuously with the pressure,indicating that the structural phase transition belongs to the second order phase transition,which is consistent with previous studies.The Birch-Murnaghan equation of state was used to fit the volume-pressure function,and the volume modulus was obtained as 226 GPa.Compared with the previously reported 328 GPa(bulk MnO₂),our results show a significant difference.And the phase transformation pressure of MnO₂was only 0.3 GPa in the early studies on its bulk materials,while our study on MnO₂nanorods found that the phase transformation pressure increased to 11.0 GPa.It is further confirmed that the morphology and size have a significant effect on the high pressure behaviors of manganese oxides.