Electrical Properties and Electronic Configuration of the Monocarbide, Monophosphide, and Monosulfide of Plutonium

Abstract

The electrical properties of PuC, PuP, and PuS were measured over the temperature range of 300° to 1000°K. The monocarbide and monophosphide had resistivities of 257 and 752 μΩ·cm, respectively, at room temperature; these values did not change appreciably with temperature. Plutonium monocarbide is a p-type conductor with a low absolute thermoelectric power which is only slightly temperature dependent. The monophosphide is also a p-type conductor at room temperature, but it changes to an n-type conductor above 740°K. The quasimetallic conduction of PuC and PuP is essentially proportional to the mobility as indicated by thermoelectric power measurements. Plutonium monosulfide was found to be an intrinsic semiconductor from 425° to 700°K with an activation energy for conduction of 0.240 eV. Calculations show that above 700°K PuS becomes degenerate as the carrier concentration exceeds 1019 cm−3 and a gradual transition to metallic conductivity is indicated. A peak in the absolute thermoelectric power of 115 μV°K−1 was observed for this compound which is an n-type conductor over the temperature range studied. In these plutonium compounds and similar actinide compounds conduction appears to be governed by a systematic change of carrier concentration in the s+d valence band and scattering caused by s+d, f transitions. A split in the f states is proposed as the mechanism responsible for the semiconductor properties of PuS. A comparison was made of the compound unit cell volumes and metal radii for the rare-earth and actinide mononitrides, monophosphides, and monosulfides. The data indicate effective valences of four electrons per atom for the actinide mononitrides and between three and four electrons per atom for the actinide monophosphides and monosulfides. These plutonium compounds do not have much promise as thermoelectric materials because their figures of merit are all too low for acceptable power generation.