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

We use a combination of first-principles density functional theory (DFT) calculations and the recently developed prototype electrostatic ground state (PEGS) method to predict low-energy crystal structures and study phase stability of Li-Zn and Na-Zn mixed-metal borohydride compounds [i.e., NaZn(BH4)(3), NaZn2(BH4)(5), LiZn(BH4)(3), and LiZn2(BH4)(5)]. We find the following: (i) DFT + PEGS successfully predicts low-energy structures in these mixed-metal borohydride systems. (ii) DFT calculations show negative mixing energies in both the Li-Zn and Na-Zn borohydride systems, consistent with the observation of mixed-metal ordering in these systems. (iii) Our DFT calculations of the recently reported experimental crystal structures of NaZn2(BH4)(5) and NaZn(BH4)(3) show that the former has a negative mixing energy, while the latter has a positive mixing energy. (iv) Using the PEGS approach, we predict a new crystal structure of NaZn(BH4)(3) with negative mixing energy and find that the experimental structure of NaZn2(BH4)(5) and the PEGS obtained structure of NaZn(BH4)(3) lie on the ground state convex hull. (v) In the Li-Zn borohydride system, we have used the PEGS + DFT approach to predict a stable crystal structure of new, previously unobserved stoichiometry, LiZn(BH4)(3). As a consequence of this predicted low-energy compound, DFT calculations of the experimentally reported structure of LiZn2(BH4)(5) show that it is unstable with respect to decomposition into LiZn(BH4)(3) + Zn(BH4)(2). (vi) In addition, we also elucidate the ground state crystal structure of NaBH4, and confirm that reorientation of (BH4) units is the driving force behind the order-disorder phase transition in NaBH4. (vii) Finally, we predict a new low-energy crystal structure of Zn(BH4)(2), and illustrate its similarities with the crystal structure of Mg(BH4)(2).