The temperature dependence of permittivity in MgO and Fe-MgO single crystals

Abstract

The permittivities (ε′) of undoped MgO and iron-doped Fe-MgO single crystals have been measured over the temperature range 20 to 650‡ C for frequencies between 500 Hz and 50 kHz. From 25 to around 200‡ C the temperature dependence of ε′ fits well with Havinga's formula and the value of [(ε′ − 1) (ε′ + 2)]⁻¹ (∂ε′/∂T) = 1.02 × 10⁻²K⁻¹ found for undoped MgO agrees closely with data published for lower temperature ranges; this increases considerably with the addition of iron, rising to 2.85×10⁻⁵K⁻¹ for MgO single crystals doped with 12 900 p.p.m. iron. Above 200‡ C the permittivity changes much more rapidly than the Havinga formula predicts, the variation being greater in iron-doped specimens. The frequency dependence ofε′ is also temperature-dependent; below 200‡ Cε′ follows [ε′(Ω) − ε′ _∞] ∫Ω ⁽ⁿ⁻¹⁾ withn=0.98±0.02 for all samples, but above 200‡ C the value ofε′ falls more rapidly with frequency than would be expected from this law. The effect is more pronounced for MgO with 12 900 p.p.m. iron. The results are discussed in terms of a contribution to the measured permittivity arising from temperature-enhanced conductivity.