Anomalously high capacitance of β-rhombohedral boron induced by structural defects

Abstract

Capacitance and dielectric loss tangent measurements have been carried out for β-rhombohedral boron within the frequency range 102 – 104 Hz and temperature interval 77 – 373 K. Different specimens were studied: high-purity macrocrystals, vapor-liquid-grown faceted crystals, zone-melted single crystals, and crucible-melted polycrystals. Within the whole ranges of measurements, the capacitance of all samples was 10–60 times larger than their geometric capacitance. Temperature dependences of the capacitance reveal two-level step-like behavior. Location of levels is independent from the applied electric field frequency, but at higher frequencies, the capacitance steps shift to lower temperatures. For all frequencies, an abrupt rise in capacitance takes place at one and same temperature ∼ 290 K. At low temperature, dielectric losses are negligible, but they increase towards the room temperature. At ∼ 290 K (i.e., simultaneously with the abrupt rise in capacitance) their value abruptly falls down. Dielectric properties of β-rhombohedral boron are discussed on the basis of generalized barrier model of the heterogeneous semiconductor within its three-layer version. Planar defects like twins and stacking faults characteristic for real crystals are assumed to produce local elastic stresses sufficient for the local lowering in conductivity due to the piezoresistive effect. Consequently, layers adjacent to the planar defects should represent low-conducting inclusions in the relatively high-conducting matrix. In addition, the lesser-conducting barrier layers should appear at their boundaries. Effect at ∼ 290 K seems to be related with a symmetry-restoring phase transition driven by the occupation patterns of certain atomic sites in β-rhombohedral crystalline boron lattice.