First-principles studies of phase stability and crystal structures in Li-Zn mixed-metal borohydrides

We address the problem of finding mixed-metal borohydrides with favorable thermodynamics and illustrate the approach using the example of LiZn ₂(BH₄)₅. Using density functional theory (DFT), along with the grand-canonical linear programming method (GCLP), we examine the experimentally and computationally proposed crystal structures and the finite-temperature thermodynamics of dehydrogenation for the quaternary hydride LiZn₂(BH₄)₅. We find the following: (i) For LiZn₂(BH₄)₅, DFT calculations of the experimental crystal structures reveal that the structure from the neutron diffraction experiments of Ravnsbæk is more stable [by 24 kJ/(mol f.u.)] than that based on a previous x-ray study. (ii) Our DFT calculations show that when using the neutron-diffraction structure of LiZn₂(BH ₄)₅, the recently theoretically predicted LiZn(BH ₄)₃ compound is unstable with respect to the decomposition into LiZn₂(BH₄)₅+LiBH₄. (iii) GCLP calculations show that even though LiZn₂(BH₄)₅ is a combination of weakly [Zn(BH₄)₂] and strongly (LiBH₄) bound borohydrides, its decomposition is not intermediate between the two individual borohydrides. Rather, we find that the decomposition of LiZn₂(BH₄)₅ is divided into a weakly exothermic step [LiZn₂(BH₄)₅→ 2Zn+15LiBH₄+25Li₂B₁₂H₁₂+365H ₂] and three strong endothermic steps (12LiBH₄→ 10LiH+Li₂B₁₂H₁₂+13H₂; Zn+LiH→LiZn+12H₂; 2Zn+Li₂B₁₂H ₁₂→2LiZn+12B+6H₂). DFT-calculated ΔHZPET=0K values for the first three LiZn₂(BH₄)₅ decomposition steps are -19, +37, +74 kJ/(mol H₂), respectively. The behavior of LiZn₂(BH₄)₅ shows that mixed-metal borohydrides formed by mixing borohydrides of high and low thermodynamics stabilities do not necessarily have an intermediate decomposition tendency. Our results suggest the correct strategy to find intermediate decomposition in mixed-metal borohydrides is to search for stable mixed-metal products such as ternary metal borides.