Sites occupation and thermodynamic properties of the TiCr_2-xMn_x-H₂ (0≤x≤1) system: Statistical thermodynamics analysis

Pressure-composition isotherms of the ternary (pseudobinary) Laves phases TiCr_2-xMn_x-H₂(D₂) (0≤x≤1) systems were measured over a wide range of temperatures and pressures (up to 1000 atm H₂). The lower composition hydride phase, TiB₂H_̃3 (where B=Cr+Mn), is formed for all these compound series (i.e. for 0≤x≤1) with a similar stability (i.e. independent of x). On the other hand, the high-composition hydride phase, TiB₂H_̃4, is apparent only for lower concentrations of manganese substitution (x<0.75). A generalized statistical thermodynamics treatment, which considers the possible simultaneous occupation of two interstitial sites (namely the Ti₂B₂ and the TiB₃ tetrahedral sites) was applied, leading to calculated isotherms with two plateau regions. The fit of the calculated isotherms to the experimental ones yielded the microscopic energy-related parameters utilized in the statistical thermodynamics models (i.e. the H-lattice and the H-H nearest neighbors interaction parameters) as well as the two sites occupation fraction and its hydrogen composition variation. It turned out that an almost step-like sequential occupation of the above two sites is indicated by the above analysis, consistent with experimental structural data in the literature. A correlation between the interstitial sites volume and the pairwise H-H interaction parameter was found, with an opposite trend displayed for the two types of sites. The disappearance of the higher composition hydride phase at higher manganese composition is accounted for by the above correlations.