Application of Aerodynamic Levitator Laser Heating Technique : Microstructures of MgTi₂O₅ Crystal and Melt by in-situ Superhigh Temperature Raman Spectroscopy

The aerodynamic levitator laser (ADL) heating device coupled with picosecond time-gate Raman spectrometer was built. It breaks through the limitation of temperature and crucible material of conventional heating method. It greatly shields the interference of blackbody radiation on Raman signal under high temperature and extreme conditions by relying on the extremely short measurement cycle of picosecond pulsed laser. In-situ Raman spectra of MgTi2O5 melt with high melting points at superhigh temperatures (1 903, 1 953, 2 003 K) were measured for the first time. In-situ temperature-dependent Raman spectra of MgTi2O5 crystal before melting (1 673 K) were measured by coupling the third-generation intensified charge-coupled device (ICCD) detector and nanosecond pulsed laser. Raman spectra of the crystal broaden and redshift with increasing temperature from room temperature (RT) to 1 953 K, and the relative intensity decreases. A single broad envelop was observed when the temperature was increased to the melt (2 003 K). Indicating that the long-range ordered structure of the crystal has been destroyed and the microstructures in the system have changed essentially. The Raman spectrum of MgTi2O5 crystal at RT was calculated by density functional theory (DFT), and major vibration modes were thus assigned by comparing the calculated spectrum with the experimental one. The vibration peaks in the low wavenumber region (<350 cm(-1)) can be mainly attributed to the external lattice vibration modes. The peak at 485 cm i in the medium wavenumber region corresponds to Ti-O-Ti bending vibration, and the main characteristic peak at 648 and 787 cm (1) stand for O-Ti stretching vibration and O-Ti-O bending vibration in the TiO6 octahedron, respectively. A series of cluster models assumed in melt were constructed and simulated by the quantum chemistry ab initio calculation method. The characteristic Raman active vibration wavenumbers and their scattering cross section were obtained. After the experimental Raman spectra of melt were corrected by scattering cross section, the deconvolution of the molten Raman spectra was carried out, and the concentration distribution of various species was thus described quantitatively. Results show that there are TiO4 tetrahedral clusters (The relative mole fractions of respective species Q in different configurations are 54. 6% Q(0), 20. 1 %Q(1) 5. 0%Q(2) and 4. 8%Q(3) and Q is the titanium oxide tetrahedron with different bridge oxygen number i) and TiO6 octahedral clusters (Hexacoordinated titanium oxide octahedron, whose relative mole fraction is 14. 8% H-0) in MgTi2O5 melt. Ti4+ mainly exists in isolated tetrahedral structure Q(0) and dimer structure Q(1), and a small part exists in the form of isolated titanium oxide octahedral Ho. The isolated structure accounts for most of the composition of MgTi2O5 melt, which destroys the system's network connectivity and inhibits the glass forming ability. No solid-solid phase transitions were observed for MgTi2O5 crystal with the increasing temperature before melting. Above the melting point, the Ti-O polyhedron in the crystal changes from a single TiO6 species to the coexistence of both TiO4 and TiO6 species.