The effect of Ti on the wetting of CaF2 substrate by In-Ti and Ga-Ti alloys.: Ab-initio consideration

CaF2 is a thermodynamically stable, non-reactive compound, displaying a relatively high contact angle with pure liquid metal melts. A remarkable decrease of this contact angle takes place when small amounts of Ti are added to liquid In (a decrease from 125 to 20 degrees) or to liquid Ga (from 118 to 60 degrees). Thermodynamic analysis indicates that this feature cannot be attributed to chemical reactions between the substrate and the melt. It was suggested that the reason for this behavior may be a preferential titanium adsorption from the liquid In-Ti or Ga-Ti solution at the substrate surface. In order to understand the nature of the In-Ti or Ga-Ti bonding in the vicinity of the CaF2 surface, the adsorption energy of 0.5 monolayer of In and Ga was computed for three different surface conditions: (i) clean CaF2(111), (ii) CaF2(111) with In or Ga adatoms, and (iii) CaF2(111) with Ti adatoms. The differences in adsorption energies for these configurations are related to the electron charge redistribution in the vicinity of the interface, and to the density of states of the electronic subsystems. It was found that the adsorption energy of In or Ga increases due to the lateral interactions with the adatoms. According to the analysis, a strong lateral interaction exists between Ti adatoms and Me, while relatively weak interaction exists between Me and Me adatoms. The difference of the lateral interactions was considered in order to explain the improvement of the wetting of CaF2 substrate by Ti alloying of In and Ga.