High Pressure-induced Structural Phase Transition In NbSe₂ And NaNbO₃/NaTaO₃

There are many kinds of niobium and tantalum materials, which are rich in structure and physical and chemical properties. In this paper, we mainly study the NbSe₂ and NaNbO₃, NaTaO₃ two kinds of compounds.NbSe₂ is a kind of crystal in transition metal dichalcogenides, which has typical two-dimensional structure and abundant physical properties. Because of its layered structure and high bond energy,the molecular bond can be Van der Waals’ force and the superconducting transition temperature Tc is 7.2K. Therefore, it has been in-depth study in the solar cell, photocatalyst, solid lubricant and low temperature superconducting field. All along, the study of NbSe₂ under high pressure, only concerned about the pressure on the critical temperature TC, upper critical field HC2（T） and the resistance coefficient and other aspects of the impact. At present, there are still many problems about the crystal structure of NbSe₂ under high pressure. In view of the above situation, we carried out the research on the structure of NbSe₂ and the characteristic vibration of the molecule under high pressure, and got the following results: the structure of NbSe₂ was characterized by XRD and Raman measurement. The study shows that the NbSe₂ crystal belongs to the hexagonal structure at ambient condition with lattice constants: a=6.88 ?,b=6.88 ?,c=12.482 ?, V=511.7?3. By means of Raman spectroscopy to identify the Raman vibration modes, thosepeak width centers are located at 225cm-1, 233cm-1 corresponding to the Se-Se stretching vibrational mode（A1g） and Nb-Se stretching vibrational mode（E12g）, respectively. By using high pressure X-radiation diffraction, high pressure Raman spectroscopy and high pressure XAFS system, the high pressure structure phase transition of NbSe₂ is studied. It was found that the Raman spectra show an obvious change at 2.9GPa, Se-Se stretching vibrational II modesplitted and there occurs a new Raman peak. It shows the occurrence of pressure induced phase transition in NbSe₂.However, high pressure XRD results show that the NbSe₂ maintains the hexagonal structure up to the highest pressure. XAFS experimental results further proved that the structure do not change obviously at high pressure. Therefore, we suggest that an isostructure phase transition maybe takes place at 2.9 GPa in NbSe₂. This is of great significance in understanding the structural transformation and the electrical properties of NbSe₂ under high pressure.Alkali metal niobate is a kind of important perovskite material, as the representative of this system,NaNbO₃ and NaTaO₃ are typical perovskite structures. NaNbO₃ possesses the physical properties of ferroelectric, antiferroelectric, paraelectric and piezoelectric. NaTaO₃ has good photocatalytic properties. Crystalline phase transitions of NaNbO₃ and NaTaO₃ are complex, the current research is mainly focused on the orthorhombic NaNbO₃, but the typical cubic structure needs to be studied furtherly. In addition, the high pressure study of cubic NaTaO₃ has not been reported so far either. We synthesized the nanometer sized cubic NaNbO₃/NaTaO₃ materials with uniform size and good morphology, and studied the crystal structure under high pressure, obtained the following research results: by using high pressure Xradiation diffraction system, the high pressure structure phase transitions of NaNbO₃ and NaTaO₃ were studied. The results show that NaTaO₃ and NaNbO₃ maintain the cubic structure, and no phase transformations are observed. By Birch-Murnagha equation of state, the bulk modulus of cubic phase NaNbO₃ and NaTaO₃ were obtained, which are 123（2.7）GPa and 80（2.4）GPa, respectively. NaNbO₃ has a lower compressibality and a higher bulk modulus than those of NaTaO₃. Compared with the orthorhombic NaNbO₃, the cubic has a lower bulk modulus. These works provide an important experimental basis for the further understanding of the structure of cubic NaTaO₃ and NaNbO₃.