JOURNAL OF MATERIALS SCIENCE LETTERS 4 (1985) 1421-1424 On the electrical transport in nickel telluride Y. P. YADAVA, R. A. SINGH Department of Physics, University of Gorakhpur, Gorakhpur 273001, India The increasing utility of the various character- istics of the transition metal alloys and com- pounds in electronic and energy conversion devices have drawn considerable attention from various researchers towards this class of materials. The electrical transport properties of nickel telluride (NiTe), which is an important intermetallic compound of the 3d transition metal chalcogenide series, have been studied by various workers [1-4], but not in much detail. Its magnetic [5 7], thermodynamic [8, 9] and crystallographic [5, 10] properties have been investigated in detail by various workers. Nickel telluride is an antiferromagnetic com- pound with a nickel-arsenide-type crystal struc- ture. In a crystal of NiTe, Ni 2+ ions occupy a simple hexagonal lattice and Te 2- ions occupy a close-packed hexagonal lattice, both lattices interpenetrating each other. The crystal parameters of NiTe are a = 0.3390nm, c = 0.5344nm and c/a = 1.350. In order to under- stand the electrical transport at higher tem- perature, we have measured the electrical con- ductivity, thermoelectric power and dielectric constant of nickel telluride in the temperature range 300 to 1000 K and the results have been analysed in the light of various conduction mechanisms. Polycrystalline nickel telluride (purity 99.99%) was obtained from the Alfa division of Ventron Corporation, USA, and has been used as-received. Owing to lack of crystal growing facilities in our laboratory, we have used pressed pellets of NiTe for our various measurements. The pellets were made with the help of a hand- operated hydraulic press and a suitable die. The pellet, on which final measurements were carried out, was made at a pressure of 6.5 x 106gcm -z and had a diameter of 1.13 cm, and thickness of 0.82cm. Before measurements were made, the pellet was annealed at a temperature of 700 ° C for 18 h in a tightly closed platinum crucible in order to avoid oxidation at higher temperature. The annealing of materials is very necessary to obtain good and reliable results. True electrical conductivity and thermoelectric data are almost impossible on an unannealed sample [11]. In order to ensure better electrical contact, both the surfaces of the pellet, which were in contact with the platinum electrodes, were painted with silver paint. The electrical conductivity and thermoelectric power were measured with a digital multimeter PM 2522/90 Philips (India), with an accuracy better than ± 0.25% and _ 0.20% for resistance and e.m.f, measurements, respectively. The dielectric constant was determined by measuring the capacity of a parallel-plate condenser with the pellet as dielectric. The capacity was rec- orded using a universal LCR Bridge 901-I, (Ruttonsha Simpson, India) which operates at an internal frequency of 1 kHz with an accuracy _ 1.25% of the reading ___ 0.25% at full-scale range. For the measurement of thermoelectric power, a temperature difference of AT = 20°C was produced across the pellet with the help of a microfurnace fitted with the sample holder assembly, and temperatures of both the surfaces of the pellet were recorded by chromel alumel thermocouples attached to the platinum elec- trodes. Each reading was taken after attaining thermal equilibrium (about 2.5h). In all the measurements, the two-electrode method was employed and mica sheets were used for insu- lation. The details regarding sample holder assembly and measuring techniques have been given elsewhere [12]. Electrical conductivity, a, of the polycrystal- line nickel telluride was measured in the tem- perature range 300 to 1000K, on the pressed pellet using the two-electrode method. We plotted log~r against reciprocal temperature (103/T) and the plot is shown in Fig. 1. The plot has two different slopes in the temperature ranges 300 to 450 and 450 to 1000 K, respectively. As shown in Fig. 1, the electrical conductivity of 0261-8028/85 $03.00 + . 12 © 1985 Chapman and Hall Ltd. 1 421
