Resistivity and IR reflectivity of calcium-doped neodymium nickelates

Structural, electrical and IR reflectivity have been investigated for hole doped Nd2−xCaxNiO4+δ. X-ray diffraction has shown that the orthorhombic strain decreases when x increases from 0.05 to 0.33; above the strain increases with x. The dc resistivity indicates a semi-conducting behaviour, which can be described using two conduction modes: a variable-range hopping (VRH) mode at low temperature and a thermically activated mode at high temperature. The temperature of change of conduction mode is function of doping rate. Besides, the thermal resistance of Nd1.67Ca0.33NiO4 compound presents an incident around 200 K, which is attributed to a charge ordering. IR reflectivity does not show any difference in phonon region. But, it exhibits a Mott-Hubbard gap and a difference on plasma frequency depending on x.