Theoretical Calculation On The Bandgap And Optical Properties Of Mn Doped CaTiO₃

As a representative perovskite structure,CaTiO₃ has excellent dielectric properties,photocatalytic properties,biocompatibility and so on,so it has a good application prospect in the fields of photocatalysis,medicine,sensors and so on.At room temperature,CaTiO₃ has a wide band gap of 3.7 eV,so its utilization of sunlight is limited,which limits its application in photoelectric field.Doping is often necessary to improve the photoelectric properties of CaTiO₃.Mn ions are generally considered as transition metal ions with Jahn teller effect.When Mn ion with Jahn teller effect is introduced into CaTiO₃,due to the effect of crystal field,the band gap of matrix material will be significantly reduced due to the effect of crystal field,and the decrease of crystal structure symmetry caused by this ion is also likely to induce ferroelectric properties.In this paper,we first study the occupation of Mn in CaTiO₃ lattice based on thermodynamic calculation software,and point out the doping site for the following first principles calculation;then we use the first principles calculation to study the influence of Mn doping content on the band gap and optical properties of CaTiO₃.The software Thermo-Calc is used in the thermodynamic calculation.By calculating the single point equilibrium of CaTi_1-xMn_xO₃(x=0,12.5%,25%,50%)at different temperatures,it is found that Mn can only replace Ti ion to form a stable phase with single perovskite structure.Obviously,this result indicates the doping sites for the first principles calculation.After that,the first principle calculation of CaTi_1-xMn_xO₃(x=0,12.5%,25%,50%)is carried out by CASTEP module in Materials Studio.The results show that the band gap width decreases monotonically with the increase of Mn doping content.The calculated band gap of CaTiO₃ is 2.442 e V without doping,and decreases to 0.519 eV when x=50%.In addition,with the increase of Mn doping content,the absorption coefficient of the system increases monotonously in the visible light range.When x=12.5%and 25%,the absorption coefficient of the material system in the visible light range is 10⁴ cm^-1;when x=50%,the absorption coefficient of the material system in the visible light range is up to 10⁵ cm^-1 order.In addition,the calculation results also show that Mn doping is beneficial to the appearance of ferroelectric properties.When the doping content is x=50%,it is impossible to determine whether Mn occupies the ordered or disordered sites in CaTiO₃.The above doping calculation is equivalent to that of the disordered occupation of Mn in CaTiO₃.In order to further determine whether Mn may occupy an ordered position in CaTiO₃,the double perovskite structure of Ca₂TiMnO₆ was constructed,and the total energy of Ca₂TiMnO₆ and CaTi_0.5Mn_0.5O₃ were compared.It is found that there is almost no difference between them.Therefore,the exact position of Mn in CaTiO₃ still needs further experimental study.In addition,the band structure and optical properties of Ca₂TiMnO₆ were calculated.The results show that the band gap of Ca₂TiMnO₆ is 0.8 eV,which is slightly larger than that of CaTi_0.5Mn_0.5O₃,while the absorption coefficient of Ca₂TiMnO₆ in visible light range is smaller than that of CaTi_0.5Mn_0.5O₃.