Study of dielectric relaxation behavior in Nd₂CuO₄

The dielectric properties of the helium reduced Nd₂CuO₄ samples were studied by means of capacitance C and dissipation factor tan δ measurements with the test frequency f in the range from 20 Hz to 1 MHz and at temperature T between 5 K and 325 K. In addition to obtain high dielectric constant at room temperature and low frequencies, two frequency-dependent peaks in the tan δ curves and corresponding features in the real part dielectric constant ε′ curves were observed near 118 K and 255 K for f = 100 Hz. The peak temperatures observed in the tan ε curves at various test frequency f follow the relation f = f_ce^-W/kT, with f_c = 3.1 × 10¹³ (2.8 × 10¹⁴) Hz and the activation energy W = 0.282 (0.60) eV for the 118 K (255 K) peak for the sample annealed at 880°C. The low temperature activation is attributed to the excitations from the ground state of ⁴I_9/2 multiplets to the first excited states of the ⁴I_11/2 multiplets. This assignment is further confirmed by the fact that the activation energy is reduced to 0.245 eV for the sample reduced at 920°C. The high temperature activation is related to the intrinsic excitations of the CuO₂ layers, and its activation energy is unaffected by the reduction temperatures used in this study. Our results indicate that reducing treatment produces the effect of increasing oxygen vacancies, probably through the removal of the 0(2)'s (i.e., oxygens inbetween the Nd layers) in Nd₂CuO₄.