Effects of X-Ray Irradiation and Barium Deficiency on Grain Boundary States in BaTiO3:Ta Ceramics

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The effects of x-irradiation and Ba deficiency in BaTiO3:Ta ceramics were investigated in conjunction with the mechanism of an abrupt increase in electrical resistivity at the Curie temperature $T_{\text{c}}$, which was called the mode-I positive temperature coefficient of resistivity (PTCR). X-irradiation decreases the resistivity and the potential barrier height at grain boundaries in the rhombohedral and cubic specimens, but increases the ESR intensity of a singlet signal at $g=2.005$. On the other hand, introducing Ba deficiency increases the room-temperature resistivity $\rho_{\text{RT}}$ and the magnitude of resistivity jump $\rho_{\text{mode-I}}$ at $T_{\text{c}}$, but both $\rho_{\text{RT}}$ and $\rho_{\text{mode-I}}$ become to decrease when Ba deficiency exceeds 0.03 at%. In highly Ba-deficient specimens, the singlet ESR signal also decreases with increasing Ba deficiency. For explaining the irradiation effects, we proposed two models. One is the hole-recombination model, in which trapped-electrons at VBa-associated acceptor levels recombine with the holes created by irradiation. The other is the electron-detrapping model for the electrons trapped by acceptor levels. The explanation of the effects of x-irradiation and Ba deficiency was attempted by assuming VBa-associated centers (VBa, VBa–VO, VBa–F and VBa–F+). We conclude that predominant acceptor states at grain boundaries are VBa–VO-type defects. It is considered that the mode-I PTCR originats from critical changes in the density of states (DOS) of VBa–VO and the Fermi energy at tetragonal-to-cubic transition. This type of trap activation may lead to the formation of VBa–F+ and VBa–F from VBa–VO.
2004-02-15

Effects of X-Ray Irradiation and Barium Deficiency on Grain Boundary States in BaTiO3:Ta Ceramics

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