Preparation And Study On The Ion(Na⁺,Sm³⁺) Doping Modification Of Ruddlesden-Popper Structural Ca₃Ti₂O₇ Ceramics

Due to its most promise application potential in multifunctional devices,multiferroic materials have attracted extensive studies.However,except a few special cases there were few single phase multiferroics.In searching for new multiferroics.Ca₃Ti₂O₇ attracts much attention.Ca₃Ti₂O₇ belongs to perovskite oxide materials,it was widely investigated because of its rich physical properties.In this work,we prepared Ca₃Ti₂O₇ by a conventional solid-state method and detailed studies of structural,electrical and optical properties of the samples were investigated.Firstly,we chose Na⁺as a dopant and prepared layered perovskite Ca_3-xNa_xTi₂O₇(x=0,0.15)ceramics successfully by using a conventional solid reaction method.Ti-O bond lengths decrease and Ti-O₁-Ti bond angle increases after Na doping,which reflect that the Ti O₆ octahedral tilting behavior was modulated after Na doping.Our experiment confirmed the ferroelectricity of Ca₃Ti₂O₇.Meanwhile,there were 2 major discoveries in our investigation.The coercive field was greatly decreased from120k V/cm to 50k V/cm after Na doping and a diode-like effect were found in our samples.XPS analysis showed that oxygen vacancy concentration was effectively increased with Na doping while oxygen was a key factor for these 2 phenomena.Near to the sites of those Na-doped induced charged defects it is easier to nucleate domain and a smaller electric field is enough for the ceramic to switch its ferroelectric domain.The diode-like effect in the ceramics is most likely due to a p-n junction resulting from charge carriers rebalancing under the applied electric field.What's more,the leakage current was also increased owe to the oxygen vacancy concentration increasing.The optical band gap was decreased after Na doping as the first principle calculation show that Na-doped supercell exhibit a broad tail in valence band maximum,which may contribute to the decrease of the band gap.Secondly,we selected Sm³⁺ion as a dopant to substrate Ca²⁺cation and prepared Sm-doped Ca_3-xSm_xTi₂O₇(x=0,0.06,0.12),results show that diffraction peaks move from higher angles to lower angles,which demonstrate that Sm-doped Ca_3-xSm_xTi₂O₇(x=0,0.06,0.12)were synthesized successfully.The leakage current was decreased after Sm³⁺doping,XPS analysis show that the concentration of oxygen vacancy was decreased with the increasing of x,which may contribute to the leakage current density.Furthermore,to investigate the influence of oxygen vacancy on electronic structure,first principle calculations were also conducted to these samples,the calculation shows that oxygen vacancy could cause a decreased band-gap by creating an impurity levels.