METAL INSULATOR TRANSITIONS IN PURE AND ALUMINUM DOPED VANADIUM-SESQUIOXIDE
Abstract
Extensive electrical and thermoelectric measurements on electrical transitions in (V(,1-x)Al(,x))(,2)O(,3) have been undertaken. Electrical properties of (V(,1-x)Al(,x))(,2)O(,3) are similar to (V(,1-x)Cr(,x))(,2)O(,3). An increase in Al doping (0 (LESSTHEQ) x (LESSTHEQ) 0.0158) raises the temperature of low temperature transition and elevates the resistivity of metallic phase. The high temperature transition is characterized by series of abrupt jumps in resistivity. Thermoelectric power measurements indicates that the high temperature phase may be characterized by a gap which opens up and gradually closes as the temperature is raised; on the other hand Al- or Cr-doping narrows the 3d bands of the parent material by deleting states from the host band. Special equipment utilizing low drift, low noise, high input impedance instrumentation amplifiers have been constructed, to measure the Seebeck coefficient ((alpha)) of low temperature antiferromagnetic and insulating phase of Al- and Cr-doped V(,2)O(,3). Two types of behavior have been noted: (a) for 0 (LESSTHEQ) x < 0.018 (V(,1-x)M(,x))(,2)O(,3) alloys (where M (TBOND) Al or Cr) show an abrupt jump from small positive values (3 to 9 (mu)V/k) to large positive values in the vicinity of (300 to 800 (mu)V/k) at the Neel temperature and then a gradual decrease of (alpha) towards zero. (b) Alloys with the compositions 0.018 < x < 0.1 show a jump from large positive values of 300 to 600 (mu)V/K towards negative values at the Neel temperature and then a gradual variation of (alpha) with temperature. These measurements rule out the overlap model for the metallic phase of V(,2)O(,3). Heat capacity C(,p), and enthalpy (DELTA)H measurements have been carried out for the high temperature transition in Cr- and Al-doped V(,2)O(,3). H values fall in the range of 40 to 100 calories/mole for 0 (LESSTHEQ) x < 0.0158 for Al- or Cr-V(,2)O(,3). The transition entropy (DELTA)S brings out the crucial role of lattice in the high temperature transition. A phase diagram for (V(,1-x)Al(,x))(,2)O(,3) has been presented for the first time. Electrical measurements have been carried out on high purity, stoichometric single crystals of MnO and CoO in the temperature range 300 to 550 K. These measurements substantiate the view that MnO and CoO are small-polaron conductors. Preliminary measurements on two forms of cinnabar ((alpha)-HgS) show that they are n type semiconductors.
Degree
Ph.D.
Subject Area
Chemistry
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