Materials Science and Engineering B104 (2003) 150–155 Resistivity and IR reflectivity of calcium-doped neodymium nickelates M. Zaghrioui ∗ , N. Poirot, I. Monot-Laffez Laboratoire d’Electrodynamique des Matériaux Avancés, CNRS-UMR 6157, CEA, CS 2903, 3 place Jean Jaurès, IUT de Blois 41029 Blois cedex, France Abstract Structural, electrical and IR reflectivity have been investigated for hole doped Nd 2-x Ca x NiO 4 + δ . X-ray diffraction has shown that the or- thorhombic 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 Nd 1.67 Ca 0.33 NiO 4 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. © 2003 Elsevier B.V. All rights reserved. Keywords: Nickelates; Stripes; Charge ordering; IR reflectivity; Doping effects 1. Introduction R 2-x A x NiO 4 (A is alkaline cation = Sr, Ca, Ba) com- pounds have caused a large interest these last years. On one hand, these compounds present many similarities with the high temperature superconducting system R 2-x A x CuO 4 [1], although they are not superconductors, what offer a way to explore the mechanism in copper oxides. On the other hand, they offer a rich variety of physics such a charge ordering, magnetic transitions and electrical properties [2–5]. R 2-x A x NiO 4 have a K 2 NiF 4 -type structure and they are charge transfer antiferromagnetic insulators for x = 0 [6]. Several groups have reported on preparation, single crystal and ceramics, and physical properties of La 2-x Sr x NiO 4 + δ [7–9]. The structural, electrical and magnetic properties are very sensitive to the excess of oxygen and alkaline dop- ing. These compounds present a charge and spin ordering (stripes) for certain rates of doping, like La 1.67 Sr 0.33 NiO 4 [3,4,10–12] and La 2 NiO 4.11 [4,9]. This phenomenon is also observed in supracondutors cuprates (LaSrCuO 4 ) [1]. This is consists to an ordering of holes on lines separate antifer- romagnetic domains. These lines are along [110] axis and they are static in nickelates [13]. Whereas in cuprates they are along [100] and [010] axes and they are dynamic [14]. ∗ Corresponding author. These differences tend to explain insulate behaviour and su- perconductivity in these compounds. According to above, all investigations are focused on lan- thanum nickelates. A little is concerns neodymium nicke- lates [15]. We were thus interested in these last compounds, in order to understand the influence of the calcium and to investigate the existence of stripes in these compositions. We report in this paper the synthesis by citrate method of Nd 2-x Ca x NiO 4 + δ (0.05≤x ≤ 0.50), its X-ray diffraction analysis and variation of lattice parameters versus the dop- ing at room temperature. The electrical conductivity mea- surements were carried out from 77 to 300 K, and the data were fitted using several conduction models. IR reflectivity at room temperature is reported and the optical conductiv- ity deduced from a double damping model with adding a plasma contribution. 2. Experimental details Nd 2-x Ca x NiO 4 + δ polycrystalline ceramics were pre- pared by citrate method (polyacrylamid gel) [16] and by annealed at high temperature. Initial materials were spherical Ni (99.9% Chempur), Nd 2 O 3 (99.9% Chempur) and CaCO 3 (99.9% Chempur). The neodymium oxide was annealed at 1200 ◦ C for 12 h 0921-5107/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0921-5107(03)00188-0