Metal-insulator transition in Nd${}_{1-x}$Eu${}_x$NiO${}_3$ compounds

Abstract

Polycrystalline Nd${}_{1-x}$Eu${}_x$NiO${}_3$ ($0\le x\le 0.5$) compounds were synthesized in order to investigate the character of the metal-insulator (MI) phase transition in this series. Samples were prepared through the sol-gel route and subjected to heat treatments at $\sim$1000 ${}^{\circ }$C under oxygen pressures as high as 80 bar. X-ray Diffraction (XRD) and Neutron Powder Diffraction (NPD), electrical resistivity $\rho (T)$, and Magnetization $M(T)$ measurements were performed on these compounds. The results of NPD and XRD indicated that the samples crystallize in an orthorhombic distorted perovskite structure, space group $Pbnm$. The analysis of the structural parameters revealed a sudden and small expansion of $\sim$0.2% of the unit cell volume when electronic localization occurs. This expansion was attributed to a small increase of $\sim$0.003 Å of the average Ni-O distance and a simultaneous decrease of $\sim$$-0.5^{\circ }$ of the Ni-O-Ni superexchange angle. The $\rho (T)$ measurements revealed a MI transition occurring at temperatures ranging from $T_{\mathrm{MI}}\sim 193$ to 336 K for samples with $x=0$ and 0.50, respectively. These measurements also show a large thermal hysteresis in NdNiO${}_3$ during heating and cooling processes suggesting a first-order character of the phase transition at $T_{\mathrm{MI}}$. The width of this thermal hysteresis was found to decrease appreciably for the sample Nd${}_{0.7}$Eu${}_{0.3}$NiO${}_3$. The results indicate that cation disorder associated with increasing substitution of Nd by Eu is responsible for changing the first order character of the transition in NdNiO${}_3$.

19 pages, 9 figures