Structure And Electric Properties Of Donor-acceptor Co-doped TiO₂ Ceramics

Rutile TiO2 is classic incipient ferroelectrics with manifold excellent performances.Rutile TiO2 modified by donor-acceptor ions show the electrical properties such as ferroelectricity and colossal permittivity have attracted increasing interests in recent years.However,the formation mechanism of different type defects is still unclear,and the influences on electric properties affected by the ceramics structure still need to be studied systematically.Based on the thesis,Nb5+(Ta5+)-Al3+,Ta5+-La3+,and Ta5+-Al3+-La3+ were used as donor and acceptor ions to explore the formation mechanism and influence factors of donor-acceptor pair.Moreover,the micro mechanism of ferroelectricity in binary oxide induced by ionic pair and the relationship between synthesis and ionic pair formation have been investigated.And the dielectric strength of the colossal permittivity ceramic is greatly improved by using triple donor-acceptor ions co-doped TiO2 ceramics.The X-ray diffraction analysis and the first principle calculation results show that Nb5+-Al3+ and Ta5+-Al3+ can form ionic pair along the[001]direction in the TiO2 lattice due to the proper ionic radius.It can be found that Nb5+-Al3+ and Ta5+-Al3+ co-doped TiO2 ceramics exhibit ferroelectricity and piezoelectricity at room temperature,although TiO2 has central symmetry.The saturation polarization of Nb5+-Al3+ co-doped TiO2 ceramics is 2.8 ?C/cm2 and the piezoelectric modulus is 4 pm/V under electric field of 80 kV/cm at room temperature,when the doping content is 2 mol.%.The mechanism of the ferroelectricity of co-doped TiO2 is as followed:on the one hand,local lattice distortion induced by ionic pair destroys the local symmetry of the crystal.On the other hand,local electric field provided by diploe of ionic pair leads the reverse displacement of anions and cations,which causes the local polarization in co-doped TiO2 ceramics.The effect of preparation process on the formation of ionic pair has been studied.The structure and electrical properties of Nb5+-Al3+ co-doped TiO2 ceramics prepared by different sintering time and aging temperature with two doping content were investigated.The results show that long sintering time can destroy the Nb5+-Al3+ pairs,which causes the co-doped ceramics with 2 mol.%doping content loss the ferroelectricity.But the as-sintered ceramics exhibit ferroelectricity again after aging at 900 ?,which facilitates the formation of Nb5+-Al3+Nb5+-A13+pairs.For co-doped ceramics with 8 mol.%doping content,the increasing concentration of carriers induced by long sintering time cause higher dielectric constant and dielectric loss.The isolated Nb5+ and Al3+ ions tend to form Nb5+-Al3+pairs when as-sintered ceramic was aged at 900 ?.The formation of Nb5+-Al3+pairs reduce the concentration of carriers,which reduces the dielectric loss of co-doped ceramic by four order of magnitude.The colossal permittivity co-doped TiO2 ceramics show excellent frequency stability.In order to investigate the effect of ionic radius on the formation of ionic pair,the structure and electrical properties of Ta5+-La3+ co-doped TiO2 ceramics with larger ionic radius donor-acceptor ions were studied.The X-ray diffraction analysis and the first principle calculation results show that Ta5+ and La3+ ions tend to form defect complexes rather than ionic pair in one unit cell.Ta5+ ions form Ta25+Ti3+Ti4+ defect complex and La3+ ions form defect complex La23+VoTi3+with oxygen vacancy.The electrons trapped by defect complexes cause the co-doped TiO2 ceramics exhibit colossal permittivity and low dielectric loss in a wide frequency/temperature ranges.When the Ta5+-La3+ doping content is larger than 0.2 mol.%,the co-doped ceramic exhibits frequency independent colossal permittivity(>2.5 x 104)as well as a low dielectric loss(<0.07)over a very board temperature range from 25 to 400 ?.Based on different type of defects formed by donor-acceptor ions with different radius,Ta5+,Al3+,and La3+ ions were used as donor and acceptor to explore the effects of La3+ contents on the structure and electrical properties of co-doped ceramics.It can be found that Ta5+ and Al3+ ions form Ta5+-Al3+ pair along[001]direction without La3+ ions,the co-doped ceramic has ferroelectricity.When La3+ ions are doped into TiO2,Al3+ ions tend to form La3+Al3+VoTi3+ defect complex with La3+ ions rather than ionic pair,Ta5+-Al3+ pair are destroyed.The co-doped ceramic has colossal permittivity(>104)and a low dielectric loss(<0.2)over a wide board frequency/temperature ranges when the content ratio of Al3+ and La3+ is 3:1.Meanwhile,the current density of co-doped ceramic is reduced by four order of magnitude.The dielectric strength is improved significantly.