Structural Phase Transition In Ferroelastic Lead Phosphate:High-temperature Light Scattering Studies

Environmental pollution is becoming more and more serious problem in the development of society and the consumption of energy.Multifarious material research can be a lot of a very effective way to solve the problem.Multifarious material as is a multi-functional material,with ferroelectric,ferromagnetic,ferroelastic.At the same time with two or two kinds of iron or more new materials.This performance produces a new basis for its inclusion of the characteristic coupling.Multifarious materials in the field of modern scientific research and practical practice to attract more and more attention and so far triggered a lot of research,published a number of research papers,is so far still maintain the most hot topic of the study one.In spite of this,it is very significant to develop the elastic properties of the ferroelastic material because of the difficult application of the ferroelastic of the multifarious material to the practical application.According to domestic and foreign reports from researchers at various research institutes,the phase transition of the ferroelastic material is the leading role in the study of the material.Lead phosphate,Pb₃?PO₄?₂,is a model improper ferroelastic material in the field of material and earth sciences.Thus it has been studied extensively by a various techniques in order to fully understand its phase transition behavior at high pressure and temperature.In this study,we have carried out high temperature Raman and Brillouin scattering study on the pure ferroelastic lead phosphate Pb₃?PO₄?₂ single crystals in VV and VH was scattering geometry up to 300?.The main work of the paper,the content layout,the results are as follows:?1?From the first chapter to the third chapter of these three chapters,this paper mainly explains the research background of multiferrous materials,and describes the research status of domestic and foreign research.The purpose,significance,classification and performance of this paper,free energy,hot melt,stress and strain and other theoretical basis,light scattering,crystal phase transformation and phase change classification for our scientific research to lay a certain foundation.?2?The Raman spectra show remarkable changes up to 182?,and then did not show clear temperature dependence behavior.The mode at 41cm^-1 at ambient temperature in VH geometry shows slight softening up to 182?,and disappeared above this temperature,while in VV geometry,this lowest mode did not fully disappear above 182?.The modes at 140 cm^-1,159 cm^-1 and 184 cm^-1 at ambient temperature are merged into one broad peak above 180?.The Raman modes at 386 and 413 cm^-1 also merged into one Raman mode above 185?.This results indicate that lead phosphate Pb₃?PO₄?₂ undergoes a first-order ferroelastic phase transition at T_c=182? from the high temperature?HT?trigonal?rhombohedral?phase(the space group R???m-D_3d⁵ to the ferroelastic low temperature?LT?monoclinic phase(the space group C2/C_2h⁶ caused by L-point instability in trigonal phase,which doubles the volume of the unit cell.Our result also shows the high temperature phase transition is very complicated.The high temperature behavior of the lowest Raman mode at 41 cm^-1 in VV geometry indicates that although the phase transition occurs at 182?,but some monoclinic micro-domains still exist above Tc.The broad nature of the Raman modes in the intermediate range at high temperature above the Tc indicates the existence of a disordered local structure in trigonal phase.?3?Brillouin spectroscopy was used to study the elastic properties and phase transition mechanism of ferroelastic Pb₃?PO₄?₂ at high temperature in backscattering geometry.One longitudinal acoustic?LA?and two transverse acoustic?TA?modes are observed at room temperature,consistent with the selection rule of monoclinic symmetry at the backward scattering geometry.The three acoustic phonons were measured in the temperature range of 25? to 300?.With increasing temperature,the phonon frequencies,line widths,and intensities show marked changes.At 181?,two TA modes are reduced to one.Then we observed only two acoustic phonons up to the 300? indicating a phase transition from a monoclinic structure to a higher symmetry phase.The observed LA and TA phonons above 180 ? are consistent with the selection rule of trigonal symmetry in this measured geometry.