掺锰BaTiO_3PTCR陶瓷的电子顺磁共振研究

ELECTRON PARAMAGNETIC RESONANCE OF Mn-DOPED BaTiO_3 PTCR CERAMICS

利用电子顺磁共振 (EPR)方法对Mn掺杂的BaTiO3PTCR陶瓷在 12 0～ 45 0K的温度范围内进行了研究 .研究结果表明 :高自旋Mn2 + 离子的 6重谱线非常明显 ,表征谱线位置的g因子为 2 .0 0 99,Mn2 + 离子信号强弱与BaTiO3 晶体结构密切相关 .Mn2 + 离子信号的温度分布呈U型 ,即在三方相和立方相时 ,信号强度较强 ,而在四方相和斜方相时信号很弱 .这是因为在四方相和斜方相中 ,由于BaTiO3 晶体结构的扭曲和畸变 ,导致Mn2 + 和VO 空位缔合生成Mn2 + -VO 复合缺陷 .未成对电子的消失以及复合缺陷Mn2 + -VO 的生成是Mn2 + 在四方相和斜方相( 2 0 0～ 380K)消失的主因 .实验中没有发现Mn3 + 和Mn4+ 的信号 ,而与实验的环境条件一致 .掺Mn后样品Curie点的漂移可能是Mn进入晶格的一个判据 .此外 ,由于试样的n(Ba) /n(Ti) =1,因而没有发现钡空位VBa及其它的复合缺陷 (如 :VBa-Mn2 + ,Mn3 + -VO,La3 + -Mn3 + 等 ) .Mn2 +

Paramagnetic centers in La_doped BaTiO 3 [ n (Ba)∶ n (Ti)=1] ceramics added with MnO 2 were measured at 120-450 K by electron paramagnetic resonance (EPR) for investigating the effect of the centers on the positive temperature coefficient of resistivity (PTCR). EPR detected one signal only: an anisotropically broad singlet signal at g =2.009 9, a sextet signal due to Mn 2+ . And Mn 2+ ion signal was distributed with temperatures in an U_shape, that is, two signals arised in the rhombohedral and cubic phases, but are sharply weakened in the tetragonal and orthorhombic phases. Meanwhile, the Mn 2+ -V O defect pair is formed in these two phases. It is unable to detect Mn 3+ and Mn 4+ by EPR. The former requires lower temperature (<10 K) and the later exists in the samples treated at higher temperature (～1 575 K) than used herein. The shift of Curie temperature was evidenced by the site occupancy of Mn in the BaTiO 3 lattice. It is concluded that the number of electron traps (such as Mn 2+ ) increase the tetragonal to cubic transition, leading to the great improvement in the PTCR effect.