Etude ab initio des Spectres des Phonons et les Propriétés Thermodynamiques des Semi-Conducteurs

Abstract

Wehavestudiedthestructural,dynamicalandthermodynamicalpropertiesofYttrium Pnictides YX(X=N,P,AsandSb)inbothphasesB1(NaCl)andB2(CsCl).Weuse¯rst- principles calculations,bymeansofthepseudopotentialmethodusingthegeneralized gradientapproximation(GGA).Wecalculatedstructuralparameters;thelatticeconstant are obtainedbyminimizingtheenergyasfunctionofvolumeandbulkmodulusandits derivativeforYX(X=N,P,AsandSb)compoundsinbothphasesB1andB2.Theyarein goodagreementwiththepublisheddata.wahaveobtainedthetransitionpressure Pt from B1 toB2whentheenthalpyareequalanditisabout171,72,75and34GPaforYN,YP, YAsandYSbcompoundsrespectively.Weevaluatedtheelasticconstants C11,C12 et C44 whichsatisfythemechanicalstabilitiescriteria.DensityFunctionalPerturbationTheory (DFPT) isappliedtodeterminethephonondispersionrelationsalongthehighsymmetry lines withcorrespondingphonondensityofstates.InB1phase,thefrequenciesarepositive valuesthatshowthestabilityofthisstructure.InB2phase,thefrequenciesarenegative valuesthatshowthatthisstructureisunstableat0GPa.Byusingthecalculatedphonon frequencies, weobtainedthethermodynamicproperties;constant-volumespeci¯cheat Cv whichincreasewithtemperatureandapproachesthelimitingDulong-Petit,theentropy S increasewithtemperatureatdi®erentpressures.