Metal-insulator transition in Nd1-xEuxNiO3 compounds.
Polycrystalline Nd1-xEuxNiO3 (0 <= x <= 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 similar to 1000 degrees 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 NPD and XRD results 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 similar to 0.2% of the unit cell volume when electronic localization occurs. This expansion was attributed to a small increase of similar to 0.003 angstrom of the average Ni-O distance and a simultaneous decrease of similar to -0.5 degrees of the Ni-O-Ni superexchange angle. The rho(T) measurements revealed a MI transition occurring at temperatures ranging from T-MI similar to 193 to 336 K for samples with x = 0 and 0.50, respectively. These measurements also show a large thermal hysteresis in NdNiO3 during heating and cooling processes, suggesting a first-order character of the phase transition at TMI. The width of this thermal hysteresis was found to decrease appreciably for the sample Nd0.7Eu0.3NiO3. 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 NdNiO3.