Metal–insulator transition and small-to-large polaron crossover in $\hbox {La}_{2}\hbox {NiO}_{4+\delta }/\hbox {BaTiO}_{3}$ composites

Abstract

Structure and electrical resistivity of $(1 - x)\hbox {La}_{2}\hbox {NiO}_{4+\delta }/x\hbox {BaTiO}_{3}$ composites ($x = 0.05, 0.1, 0.2, 0.3, 0.5$) produced by combining the sol–gel and ceramic sintering methods have been investigated. Among the samples sintered at temperature $1300^{\circ }\hbox {C}$ for 16 h, the metal–insulator transition (MIT) temperature of $x = 0.1$ sample, which is $T_{\mathrm{MI}} = 700~\hbox {K}$, is lower than the MIT temperature (800 K) of the pristine $\hbox {La}_{2}\hbox {NiO}_{4+\delta }$ (LNO) perovskite. Reduction of resistivity of $x \le 0.3$ composite is mainly due to decrease of the scattering of electrical carriers by composite large grain boundaries and the structural change of the LNO component. The temperature dependence of the resistivity of $(1 - x)\hbox {La}_{2}\hbox {NiO}_{4+\delta }/x\hbox {BaTiO}_{3}$ composites is well-explained by a two conducting component model consisting of small polarons (SP) and large polarons (LP). A crossover between SP and LP with increasing temperature is described by the probability volume fraction function f for SP and $1 - f$ for LP, which are equal to 1/2 at transition temperature $T_{\mathrm{MI}}$. The observed lowest resistivity $\rho =11~\hbox {m}\Omega$ cm for $x = 0.1$ sample corresponds to the lowest SP thermal activation energy $E_{\mathrm{a}}$, the contributions of residual and phonon resistivities at $T_{\mathrm{MI}}$. The MIT of these composites satisfies approximately the Mott criterion.