Effect of Nb doping on epsilon negative behaviour of Sr 2 MnO 4

Solid-state ceramic route was used to synthesize certain Nb doped compounds of the system Sr 2 Mn 1- x Nb x O 4 ( x = 0.1, 0.2, 0.3, 0.4, 0.5) in air. Out of these, the x = 0.4, 0.5 compositions were found to consist of an impurity of Sr 8 Nb 4 O 18 , which elucidates a solubility limit of Nb in the Sr 2 MnO 4 . Permittivity and AC conductivity studies with respect to frequency and temperature exhibited negative permittivity in the entire range of the measurement (30–600 °C) for x = 0.1, 0.2 compositions, whereas in the case of x = 0.3, 0.4, and 0.5 compositions, the same behaviour was seen above 80, 180, and 230 °C, respectively. The plasmonic oscillations of charge carriers may be responsible for the negative permittivity behaviour, which is analysed by the Drude–Lorentz model. Compositional and temperature dependent sign of the permittivity of the system Sr 2 Mn 1- x Nb x O 4 makes this system promising materials for switching action in the wide frequency range and temperature range.