  Polycrystalline Nd$_{1-x}$Eu$_{x}$NiO$_3$ ($0 \leq x \leq 0.5$) 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 \AA{} 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 $T_{\rm MI}\sim 193$ 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 $T_{\rm MI}$. 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$_{3}$.
