Thermal-induced Ion Migration,structure Variation And Optical/thermal Responses In Rare Earth Ion Doped Optical Functional Materials

Ion migration in solids is an important physical and chemical process,which is not only a common phenomenon in many electrical materials,but also may occur in high power laser amplification and high power laser display materials.The heat accumulation produced by high power laser in the device not only causes the local temperature increase of the material,but also may damage the critical component of materials.The phenomenon of thermal-driven composition ion migration may be an alert prior to irreversible damage to the materials,and it also has a very important effect on the heat transport properties of materials.The related research in the field of optical functional materials is relatively scarce.In addition,the physical picture of the connection between the changes in microstructure and optical properties caused by thermal-induced ion migration is still unclear.Therefore,it is of great significance for the dynamic monitoring technology and performance improvement of optical functional materials to further study the thermal-induced ion migration behavior and the response of the related structures to the light/thermal properties of the wide band gap optical functional materials.Based on the previous work of our research team,we utilized up-conversion(UC)luminescence of Yb³⁺-Er³⁺to respond thermal-induced Li⁺migration behavior in this paper.The relationship between the change of microstructure and the emission of Er³⁺was further interpreted by the ion migration distance,optical response distance,the ion migration path and the second-order phase transition caused by ion migration.Afterwards,we attempted to further investigate the effect of the thermal-activated migration of O^2-on the evolution of structure of the incommensurate modulated phase in CaGd₂(Mo O₄)₄:Yb³⁺,Er³⁺materials of more complex structure by using the UC luminescence of rare earth ions.The main research results of this paper are as follows:(1)CaTiO₃:Li⁺,Yb³⁺,Er³⁺was synthesized through the traditional high temperature solid-state reaction method.Variable temperature X-ray diffraction(XRD)and variable temperature heat capacity data show that the second-order phase transition occurs at?125?.The results of temperature-dependent UC emission spectra,temperature-dependent AC impedance spectroscope and temperature-dependent thermal conductivity can demonstrate that Li⁺migration occurs at?125?,which indicates that the UC luminescence of Er³⁺can respond to the thermal-induced Li⁺migration.We utilized the classical ion conductance model to estimate the jumping distance of Li⁺.The jumping distance d is?4.9?.We analyzed that Li⁺would be in the second coordination shell of Er³⁺after ion migration.First-principles calculations were used to simulate the possible defect types and Li⁺migration paths in CaTiO₃:Li⁺,Er³⁺.The relationship between Er³⁺luminescence and the second-order phase transition induced by Li⁺migration is interpreted by means of configurational entropy(the number of microstates).(2)A series of CaGd₂(Mo/WO₄)₄:Yb³⁺,Er³⁺samples were prepared by high temperature solid-state reaction method.XRD results at room temperature indicate that incommensurate modulated phase may exist in CaGd₂(Mo O₄)₄:Yb³⁺,Er³⁺powder sample,but not in corresponding ceramic sample and CaGd₂(WO₄)₄:Yb³⁺,Er³⁺powder sample.In the UC emission spectra of CaGd₂(Mo O₄)₄:Yb³⁺,Er³⁺,the plot of the logarithm of the red-green luminescence intensity ratio versus reciprocal of absolute temperature show that there is a break point at?250?.The results of temperature-dependent XRD and temperature-dependent Raman spectra can demonstrate that thermal-activated migration of O^2-occurs in the range of?250?.This process leads to the breaking of the bridging oxygen bond in Mo-O-Mo and variation of incommensurate modulated structure.More evidence is needed for further research.