  Excitonic absorption, reflection and photoluminescence spectra of mixed
Zn(P_{1-x}As_{x})_{2}crystals over the full range of x (0 < x < 1) and
Zn_{1-x}Cd_{x}P_{2} crystals at 0 < x < 0.05 have been studied at low
temperatures (1.8 K). The decrease of the energy gap in Zn(P_{1-x}As_{x})_{2}
at the increase of x occurs slightly sublinearly. The rydbergs of excitonic
series in this crystals decrease as well, and the dependences Ry(x) for all
series are strongly superlinear at small x. In Zn_{1-x}Cd_{x}P_{2} crystals the
energy gap and rydbergs decrease at the increase of x (at 0 < x < 0.05) as
well. The dependences of E_{g} and Ry on x are considerably stronger in
Zn(P_{1-x}As_{x})_{2} than in Zn_{1-x}Cd_{x}P_{2}. At the increase of x the
half-width of excitonic absorption lines increases monotonically in both type
crystals that is evidence of the increasing role of fluctuations of crystal
potential.
