Compositional control of electrical transport properties in the new series of defect thiospinels, Ga1-xGexV4S8-δ (0≤x≤1)

A new series of non-stoichiometric sulfides Ga1-xGexV4S8-delta (0 <= x <= 1; delta <= 0.23) has been synthesized at high temperatures by heating stoichiometric mixtures of the elements in sealed quartz tubes. The samples have been characterized by powder X-ray diffraction, SQUID magnetometry and electrical transport-property measurements. Structural analysis reveals that a solid solution is formed throughout this composition range, whilst thermogravimetric data reveal sulfur deficiency of up to 2.9% in the quaternary phases. Magnetic measurements suggest that the ferromagnetic behavior of the end-member phase GaV4S8 is retained at x not equal 0.7; samples in this composition range showing a marked increase in magnetization at low temperatures. By contrast Ga0.25Ge0.75V4S8-delta appears to undergo antiferromagnetic ordering at ca. 15 K. All materials with x#1 are n-type semiconductors whose resistivity falls by almost six orders of magnitude with decreasing Ga content, whilst the end-member phase GeV4S8-delta is a p-type semiconductor. The results demonstrate that the physical properties are determined principally by the degree of electron filling of narrow-band states arising from intracluster V-V interactions. (C) 2009 Elsevier Inc. All rights reserved.