Abstract
A model and an approximate theory is presented for conduction in solids which have a charge-ordered low-temperature phase and show a first-order phase transition of 'semiconductor-semimetal' type at a characteristic temperature Tnu (e.g. Fe3O4). The structure and the conduction mechanisms of the low-temperature phase are analysed. It is shown that the phase transition is of first order; an approximate treatment of the transition is proposed for obtaining a simple relation between Tnu and the correlation parameter. The structure of the high-temperature phase is discussed and the main features of the conduction are considered. The behaviour and mechanisms of the steady and frequency-dependent conductivity, generally speaking, are essentially different at temperatures higher and lower than Tnu .

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