Study On The Doping Control Of Sr₃Sn₂O₇ Phase Transition

Ferroelectric materials,characterized by the reversible spontaneous polarization,can be applied to various functional devices in the fields of storage,sensing,detection and solid-state refrigeration.In recent years,in order to achieve the good high-frequency performance and low energy consumption of devices,researchers have been trying to find qualified multiferroics.Hybrid improper ferroelectricity?HIF?is a candidate to realize multiferroics because it avoids the contradiction of electronic configuration of traditional ferroelectrics.In recent years,a number of HIF materials have been discovered in Ruddlesden-Popper perovskite materials,which are expected to open up new fields for the research of multiferroics.This thesis is based on Sr₃Sn₂O₇.We study the structure evolution,phase transition and Raman spectrum of this material by Ba doping and Ca doping.The ceramics of Sr_3-xBa_xSn₂O₇?x=0,0.1,0.2,0.3,0.6,0.8,1?were synthesized by solid-state reaction method.Through the refinement of XRD data,we can get the layered perovskite structure of the material,which is stacked by two layers of perovskite and one layer of rock salt in the direction of[001].The lattice constants of Sr_3-xBa_xSn₂O₇increase with increasing of Ba doping.With increasing of the Ba²⁺concentration from x=0 to x=0.2,the phase transition temperature reduces by 73 K.By studying the Raman spectra of these materials,we found that the phase transition is strongly related to the tilting and rotation of Sn O₆octahedra.The doping of Ba significantly aggravates the anharmonic effect and increases the Raman activity of the vibration mode A₂^?5?,and reduces the phonon lifetime and the Raman activity of the vibration mode A₁^?17?.The ceramics of Sr_3-xCa_xSn₂O₇?x=0,0.1,0.2,0.3,0.6,0.8,1?was also synthesized by solid-state sintering.It can be seen from the structural refinement of XRD data that with the doping concentration from x=0 to x=1,the lattice parameters of Sr_3-xCa_xSn₂O₇ gradually increased,and the inclination of Sn O₆ octahedron along the c-axis gradually strengthened.With the doping concentration from x=0 to x=0.3,the phase transition temperature of Sr_3-xCa_xSn₂O₇ increased by 183.3 K.There is a linear relationship between the phase transition temperature and the tolerance factor.With the increase of the tolerance factor,the phase transition temperature is decreasing.Sr₃Sn₂O₇ undergoes a phase transition at 407.9 K,and its space group changes from A2₁am to Pnab.Near the phase transition temperature,the Raman shift of the phonon mode A₁^?2?of Sr3-x Cax Sn2O7 changes abruptly with the change of temperature,and the phonon mode A₂^?2?disappears gradually.In this paper,the effect of Ba and Ca doping on Sr₃Sn₂O₇ phase transition temperature is revealed,which is helpful for the application of Sr₃Sn₂O_7.The vibrational mode of the phonon involved in the Sr₃Sn₂O₇ phase transition process is determined by the study of Raman spectrum,which makes the microscopic process of the phase transition clearer.This study can be used for reference in the study of similar materials in Hybrid improper ferroelectric system.