Free charge localization and effective dielectric permittivity in oxides

Abstract

This review will deal with several types of free charge localization in oxides and their consequences on the effective dielectric spectra of such materials. The first one is the polaronic localization at the unit cell scale on residual impurities in ferroelectric networks. The second one is the collective localization of free charge at macroscopic interfaces like surfaces, electrodes and grain boundaries in ceramics. Polarons have been observed in many oxide perovskites mostly when cations having several stable electronic configurations are present. In manganites, the density of such polarons is so high as to drive a net lattice of interacting polarons. On the other hand, in ferroelectric materials like BaTiO 3 and LiNbO 3 , the density of polarons is usually very small but they can influence strongly the macroscopic conductivity. The contribution of such polarons to the dielectric spectra of ferroelectric materials is described. Even residual impurities as for example Iron can induce well-defined anomalies at very low temperatures. This is mostly resulting from the interaction between localized polarons and the highly polarizable ferroelectric network in which they are embedded. The case of such residual polarons in SrTiO 3 will be described in more detail, emphasizing the quantum polaron state at liquid helium temperatures. Recently, several nonferroelectric oxides have been shown to display giant effective dielectric permittivity. It is first shown that the frequency/temperature behavior of such parameters is very similar in very different compounds (donor-doped BaTiO 3 , CaCu 3 Ti 4 O 12 , LuFe 2 O 4 , Li-doped NiO, etc.). This similarity calls for a common origin of the giant dielectric permittivity in these compounds. A space charge localization at macroscopic interfaces can be the key for such extremely high dielectric permittivity.

Dielectric and magnetic properties of (Bi1-xLaxFeO3)0.5(PbTiO3)0.5 ceramics prepared by high energy mechanochemical technique

“…For the ceramics with La content x=0.5 one can observe in addition the Curie point anomaly at~500 K. The hightemperature dielectric anomaly was related by us to the behavior of oxygen vacancies and divalent vacancies at the Asite introduced by La 3+ doping [38,43], whereas the lowtemperature anomaly is due to compositional disorder in highly polarizable lattice [43]. Frequency dependences of dielectric losses excluded the relaxation of space charge polarization below~225 K in the ceramics.…”

“…In the vicinity of 400 K at lower frequencies one can observe a trace of dielectric anomaly that increases with the La-doping and at x≥ 0.3 transforms into a frequency dependent anomaly. We would like to ascribe this anomaly to the presence of oxygen vacancies and divalent vacancies at the A-site introduced by La 3+ doping [38,43]. For the (Bi 1-x La x FeO 3 ) 0.5 (PbTiO 3 ) 0.5 ceramics with La content x=0.5 one can observe the Curie point anomaly at~500 K.…”

“…Double ionized oxygen and A-site vacancies or even oxygen ions which start to diffuse at temperatures higher than 500 K [38] can contribute to the electric transport in our ceramics at high temperatures. It should be noted that the activation energy of oxygen vacancies in YFeO 3 was found to be very similar (1.08 eV) [48].…”

“…(Bi 1-x La x FeO 3 ) 0.5 (PbTiO 3 ) 0.5 ceramics is a complex In external electric field E space charge is accumulated at the grain boundaries resulting in space charge polarization the effect of which is apparent at low frequencies and high temperatures [38]. As dielectric losses are related to energy dissipated in the system we observe a decrease in tanδ with increasing frequency because the reorientation of the space charge ceases to follow the frequency of the applied electric field E. To define frequency and temperature window in which the space charge polarization effect prevails in (Bi 1-x La x FeO 3 ) 0.5 (PbTiO 3 ) 0.5 ceramics the frequency dependences of tanδ at various temperatures are shown in Fig.…”

Localized polarons and conductive charge carriers: Understanding CaCu3Ti4O12 over a broad temperature range

“…In addition to the above analysis, we also need to consider the high-temperature dielectric response induced by the conductivity due to the thermally activated non-localized conductive charge carriers 43 , which is accounted for by a new term (the last term in the equation below)…”