Influence of a strontium-enriched intergranular cluster network on the electrical conductivity of In$_2$O$_3$-SrO ceramics

A metastable hexagonal $R$-phase is revealed in polycrystalline In$_2$O$_3$-SrO samples, which has the form of a network made up of mesoscopic clusters (60–180 $\mathring{\mathrm{A}}$ in size). The clusters arise from strontium-enriched regions near grain boundaries in the main cubic structure of indium oxide. It is shown that annealing in oxygen at $T_a\ge$ 300$^\circ$C saturates dangling bonds between the $R$-phase and the matrix and makes the system metastable. This state shows up in the presence of (i) solitary diffuse maxima from the $R$-phase imposed on Debye lines from the main phase in the X-ray diffraction pattern and (ii) the electron cyclotron resonance (ECR) line with $g$ = 1.875. In addition, the sample in this state acquires a high resistivity ($\rho\sim 10^6 \Omega$ cm). Relaxation at $T\le$ 300$^\circ$C after annealing at $T_a>$ 300$^\circ$C disrupts bonds between the strontium-enriched clusters of the $R$-phase and the indium oxide matrix. This causes spatial separation of the clusters, disruption of their coherent bonds with the matrix structure, and escape of excess oxygen from the sample along grain boundaries. As a result, a new stable state forms, which is characterized by (i) a series of diffuse maxima from the $R$-phase imposed on lines assigned to the main phase, (ii) the presence of the ECR line with $g$ = 2 with the line with $g$ = 1.875 retained, and (iii) the transition of the sample to a low-resistivity state ($\rho\sim 100 \Omega$ cm).