Showing papers on "Charge ordering published in 1973"

Some Problems about Polarons in Transition Metal Compounds

Abstract: It has been suggested by several authors that in Fe3O4 and the vanadium bronzes (e.g. Na x V2O5) the current carriers are small or intermediate polarons, heavy enough to “crystallize” at low temperatures. This hypothesis is examined in detail, the interaction between polarons and the effect of a high concentration on the polaron’s energy and mass being discussed. In Fe3O4 it is suggested that there is no contradiction between this model and the “band” model of Cullen and Callen [1]. In Na x V2O5 for most values of x which are not a simple fraction of the number of sites, the condensed phase must be disordered (a “ polaron glass”) and the high linear specific heat at low temperatures is related to the disorder. The blue bronze K0.3MoO3 has electrical properties similar to F3O4, though the conductivity is higher and the low magnetic susceptibilities suggest polaron pairing. The relationship of the sharp transition to the Neel point in some “amorphous antiferromagnetics”, and to the Wigner transition is discussed.

Dopant Effects upon the Verwey Transition in Fe3O4

Abstract: Additional adiabatic heat capacity calorimetry has confirmed the bifurcated nature of the Verwey transition in pure and doped Fe3O4 samples. Two sharp peaks occur at 113.3 and 118.8 K for a 99.99% pure ceramic sample. Data on a hydrothermal sample confirmed the presence of both peaks. However, the presence of 0.2 weight per cent of Mn(II) raised them by 3.7 and 4.1 K, respectively. Large single‐crystals of naturally occurring magnetite revealed only a broad transition with a high temperature shoulder. The total transitional enthalpy for the pure ceramic sample was 155 cal mol−1 or a value about 10 ± 1% higher than that for the two doped samples. Observation of the positive shift of transition temperatures for the Mn(II)‐doped (hydrothermal) sample appears contradictory not only to the trend reported in a recent study (in which magnetic sensing was used) but in particular to that observed for a Zn(II)‐doped sample. Calorimetric sensing of the transition temperature appears to be more reproducible than eith...