Structural phase stability ofB2andB32intermetallic compounds

Abstract

For each of the compounds CsAu, LiAu, LiB, LiAl, LiGa, LiIn, LiTl, LiZn, LiCd, LiHg, NaIn, and NaTl the difference is calculated between the total energy of the compounds in the B2 (CsCl) and B32 (NaTl) crystal structures. The calculations are carried out over a pressure (P) range large enough to predict pressure-induced phase transitions. The structural differences in free energy at P=0 agree in sign with the observation of stable structures. The bonding of CsAu and LiAu appear to be quite different: CsAu is ionic and LiAu metallic. In the present theory where only these two structures are compared, CsAu has, at P=0, the B2 structure, but is predicted to undergo a transition to B32 at P≊45 kbar. LiAu is, according to the calculations, not stable at P=0 in the CsCl structure; it requires an external pressure >145 kbar to exist in that structure. Trends of structural energy differences for the I-II and I-III compounds are analyzed by the examination of charge distributions, partial pressures, and their relation to constituent-atom ‘‘sizes.’’ The I-II and I-III compounds which at P=0 have the NaTl structure are predicted to undergo a structural phase transition at moderately high pressure (∼200 kbar for LiAl). The bonding characteristics are examined by calculation of (nonspherical) charge distributions. Further analysis is supported by ‘‘frozen-potential’’ energy-difference calculations. The (hypothetical) LiB compound is the only one among the Zintl phases studied here which is semiconducting. It is also the case in which the covalent bonding is most pronounced. A few results of calculations for other compounds with structures related to those of the main topic of the work are also presented. These are for the pnictides, ${\mathrm{Li}}_3$Sb and ${\mathrm{Cs}}_3$Sb, which, in spite of expected similarities, show substantial differences as far as charge distributions are concerned.