Electrical properties and defect structure of cuprous chloride

Abstract

Measurements of electrical conductivity (σ), thermoelectric power (θ) and transport numbers in the temperature range 24 - 240°C have been made on CuCl in the pure state and after reaction with chlorine to varying extents. Attempts to measure the Hall Effect gave negative results for all samples. Data for pure CuCl confirm earlier reports that conduction is electronic (positive hole) at low temperatures and ionic (cation inter- stitials) at higher temperatures (above 160°C, for our samples). For the low-temperature range, the significance of various reported apparent activation energies of conduction (E[sub σ]) has been clarified in the present work in terms of an acceptor level at E[sub A] = 0.51 eV above the valance band (cation vacancy as trapping site). The purest samples show compensated behaviour, E[sub σ] = E[sub A]. Less pure samples show uncompensated behaviour, E being close to E[sub A]/2. % ionic conduction has been estimated in three ways: from the conductivity data, from classical gravimetric transport number measurements, and from the θ - T curve (which has not previously been interpreted, although measurements of θ have been reported by other workers). The behaviour of slightly-chlorinated CuCl (which conducts much better than pure CuCl) correlates with current interpretations of the conduction mechanism in NiO. Conduction is electronic (i.e. not ionic) at all temperatures (24 - 240°C). The θ - T curves have a pronounced maximum at about 100°C. They can be explained, and correlated with σ data, only on the basis of two conduction mechanisms in parallel, by holes in the valence band and electrons hopping at the acceptor level with an activation energy of migration E[sub µn] = 0.36 eV. At low temperatures, electron conduction predominates and E[sub σ] = E[sub µn]. Above 100 - 110°C, hole conduction predominates and E[sub σ] = E[sub µn] (as for the lower range in pure CuCl). An energy calculated from the rising portion of the θ - T curve correlates well with (E[sub A] - E[sub µn]), as theory predicts it should. CuCl reacted extensively with chlorine (20 - 65% conversion to CUCI₂) shows conduction phenomena believed to be those of the CuCl component of the heterogeneous solid, but with E[sub σ] = 0.88 eV (high T, > 50% conversion) and 0.15 eV (low T, > 20% conversion). Conduction is by holes-(contradicting an earlier suggestion from this laboratory that the higher value was for cation interstitials), and the higher value is assigned as another acceptor level, E[sub A] = 0.88 eV, probably for anion interstitials as hole traps. The lower value has not been fully explained. The origin of the two different trap depths, E[sub A] = 0.51 and 0.88 eV, is briefly considered, and it is suggested that the difference between them, 0.37 eV, may represent approximately the crystal field splitting of Cu 3d t2 orbitals from Cu 3de orbitals in the tetrahedral site symmetry of CuCl.