Electronic compensation mechanism

In (2.13) and (2.14) the charge of the atiovalent" Zn dopant is compensated by an ionic defect (V") and an electronic defect (h ), respectively. To illustrate the difference between these ionic and electronic compensation mechanisms in more detail, consider Ti-doped Fe203. When subtracting the ionic compensation reaction from the electronic one, we obtain... 

Fig. 4.47. Schematic diagram ofthe compensation mechanism to prevent charging ofthe sample surface when primary energetic ions and electrons are used.

They are usually joined along the 110 plane of the lattice of the face-centered salt crystal, although we have not shown them this way (The 100 plane is illustrated in the diagram). Note that each vacancy has captured an electron in response to the charge-compensation mechanism which is operative for all defect reactions. In this case, it is the anion which is affected whereas in the "F-center", it was the cation which was affected (see equation 3.2.8. given above). These associated, negatively-charged, vacancies have quite different absorption properties than that of the F-center. 

We shall consider in details how the above described approach can be applied to Cr ion doped in KMgF3 crystal, at perfect octahedral site symmetry [35]. After doping, Cr substitutes for Mg " " ions at the center of an octahedron formed by six fluorine ions. The Cr " — F distance is 1.995 A [36]. We do not discuss here the charge compensating mechanisms required to maintain electrical neutrality of the samples, but, instead, focus on the electronic and optical properties of the [CrFe] " units. The DFT-based treatment of the defects related to the doping and their impact on the JT effects was given in [37]. 

It seems, therefore, that the recent characterization of the phase compensation and structural properties of tin-antimony oxides may readily be correlated with the Mossbauer determination of cationic oxidation states and lattice distortion, especially in materials containing a high concentration of antimony. The charge compensation mechanism, however, remains an intriguing aspect of this material and the nonlocalization of electron density may well be considered as a feature of potential catalytic relevance. In this respect a technique which is well suited to the study of the dependence of catalytic performance on the presence of any spin free species and semiconducting properties is ESR. 

D. Lu, X. Sun, M. Toda Electron Spin Resonance Investigations and Compensation Mechanism of Europium-Doped Barium Titanate Ceramics Japanese Journal of Applied Physics Vol. 45, No. 11, 2006, pp. 8782-8788... 

At high PO2, the compensation mechanism switches from electronic to cation vacancy compensation (or self compensation), where the donor charge is compensated by the formation of Sr vacancies [98, 99]. This is accompanied by the formation of a secondary Sr-rich phase. The nature of this phase is not fully determined and is suggested to be either an Sr +i Ti 03 +i phase within the matrix [100] or a separate SrO phase [101]. A separate phase is denoted below simply for clarity ... 

This alternate charge compensation mechanism removes electronic carriers and thus greatly decreases the electronic conductivity of the material [98]. Indeed, the very high conductivities reported for La-doped STO are only created upon sintering at high temperatures under very reducing conditions [92, 93, 96, 102]. 

The results of the mechanical properties can be explained on the basis of morphology. The scanning electron micrographs (SEM) of fractured samples of biocomposites at 40 phr loading are shown in figure. 3. It can be seen that all the bionanofillers are well dispersed into polymer matrix without much agglomeration. This is due to the better compatibility between the modified polysaccharides nanoparticles and the NR matrix (Fig. 4A and B). While in case of unmodified polysaccharides nanoparticles the reduction in size compensates for the hydrophilic nature (Fig. 3C and D). In case of CB composites (Fig. 3E) relatively coarse, two-phase morphology is seen. 

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