Thermoelectric power ofCdCr2−xGaxSe4p-type spinel semiconductors

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The electrical conductivity of the as-grown single crystals with the general chemical formula $\mathrm{Cd}{\mathrm{Cr}}_{2\ensuremath{-}x}{\mathrm{Ga}}_{x}{\mathrm{Se}}_{4}$ (where $x=0.00$, 0.015, 0.02, 0.04, and 0.06) has been measured in the ⟨001⟩ direction and in temperature range from $80\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}310\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. The thermoelectric power measurements were carried out in the temperature range from $80\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}350\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ for single crystals with $x=0.00$, 0.015, and 0.02 and at room temperature for the remaining samples. These investigations showed that all single crystals of the series are the $p$-type semiconductors. Calculations of the vacancy model parameters and the magnetic exchange integrals revealed that structural defects play a significant role in the electrical transport processes and that the strong magnetic competition between the double-exchange and the Kramers-Anderson superexchange interactions takes place. For the single crystal with the lowest gallium content $(x=0.015)$ a steplike structure of the electrical conductivity (SLS EC) has been discovered. The thermoelectric power analysis made for single crystals with $x=0.00$, 0.015, and 0.02 revealed that the phonon drag and diffusion components of thermopower play an essential role in the Seebeck phenomenon. These effects are interpreted within a framework of the quantum band model and the Debye theory of the specific heat of solid state taking into account both a cluster formation and the structure defects.