We have calculated the pressure coefficients agr; of a few optical transitions in (001), (111), (110), and (201) GaP/InP ordered superlattices usingabinitiomethods. The equilibrium atomic geometries under hydrostatic pressure are obtained by direct minimization of the elastic enthalpy. We find that (i) the pressure coefficient of thelowestenergy transition is uniformly high, due to the Ggr;1ccharacter of the conductionhyphen;band minimum; (ii) the pressure coefficient of the transition to thesecondlowestconduction state at Ggr;macr; distinguishes the (111)hyphen;oriented (CuPt) superlattice (agr;=4.0 meV/kbar) from the remaining structures (agr;bartil;minus;2 meV/kbar). This is so because in CuPt we haveLfolding, while in the other structures we haveXfolding; (iii) the calculated pressures for the Ggr;rarr;Xcrossover are 45, 43, 12, and 16 kbar for the (001), (111), (110), and (201) superlattices, respectively. These trends reflect the zerohyphen;pressure Ggr;1cndash;X1cenergy separation and the Ggr;1cpressure coefficient of these structures. copy;1994 American Institute of Physics.
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