Dependence of energy gap on<i>x</i>and<i>T</i>in<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mi mathvariant="normal">Zn</mml:mi></mml:mrow><mml:mrow><mml:mn>1</mml:mn><mml:mi mathvariant="normal">−</mml:mi><mml:mi mathvariant="normal">x</mml:mi></mml:mrow></mml:msub></mml:mrow></mml:math><mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mi mathvariant="normal">Mn</mml:mi></mml:mrow><mml:mrow><mml:mi mathvariant="normal">x</mml:mi></mml:mrow></mml:msub></mml:mrow></mml:math>Se: The role of exchange interaction

Photoluminescence and reflectivity measurements have been carried out on ${\mathrm{Zn}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Mn}}_{\mathrm{x}}$Se solid solutions in the complete range of crystal compositions, 0\ensuremath{\le}x\ensuremath{\le}0.55. The features of the photoluminescence and reflectivity data near the band edge enabled us to determine values of the fundamental band gap as a function of Mn molar fraction x and temperature (8 K\ensuremath{\le}T\ensuremath{\le}300 K). The energy gap in crystals with compositions in the vicinity of the zinc-blende--wurtzite structural transition (0.2\ensuremath{\lesssim}x\ensuremath{\lesssim}0.3) exhibited a scatter of values, probably associated with the presence of various polytypes (as evidenced by transmission-electron-microscopy measurements). The dependence of ${E}_{g}$ on x is anomalous for 0\ensuremath{\le}x\ensuremath{\le}0.2, showing a minimum in the ${E}_{g}$ vs x curve. A simple model (in terms of second-order perturbation theory in s-d and p-d interactions) relates this minimum to a maximum observed in the magnetic susceptibility as a function of x. For samples with x\ensuremath{\gtrsim}0.35, the ${E}_{g}$ dependence on T shows an onset of an additional blue shift as the temperature is lowered below \ensuremath{\sim}150 K. This onset, though not reproduced by our simple calculation, appears to be also related to the presence of s-d and p-d interactions.

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