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

Abstract

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(BH 4 ) 3 , NaZn 2 (BH 4 ) 5 , LiZn(BH 4 ) 3 and LiZn 2 (BH 4 ) 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 NaZn 2 (BH 4 ) 5 and NaZn(BH 4 ) 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(BH 4 ) 3 with negative mixing energy and find that the experimental structure of NaZn 2 (BH 4 ) 5 and the PEGS obtained structure of NaZn(BH 4 ) 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(BH 4 ) 3 . As a consequence of this predicted low-energy compound,

Chloride substitution induced by mechano-chemical reactions between NaBH 4 and transition metal chlorides

“…The combination of two metals is expected to decrease the desorption temperatures compared to the more stable alkali or alkaline earth metal borohydride precursor [3,4]. Similar suggestions were obtained from computational screenings of mixed metal borohydrides based on density functional theory (DFT) [5,6]. In [5], stable mixed borohydrides with promising decomposition temperatures were predicted to have averaged metal electronegativities in the 1.3-1.6 range, and favor M M (BH 4 ) 2−5 combinations with M = Li, Na, K, and M = Zn, Ni, Co.…”

Structure and diffusion in liquid complex hydrides viaab initiomolecular dynamics

“…coarser fast Fourier transform grids) was used to improve the speed of the calculations, requiring a higher energy cutoff than the default to avoid simultaneously reducing the accuracy of the interatomic force calculations. DFT has been successfully applied to a wide variety of solid-state hydrides and hydrogen storage reactions 6,9,[17][18][19][20][35][36][37][38][39][40][41][42][43][44][45][46][47][48][49] , however, there are far fewer studies of liquid complex hydrides 21,50 . To study the bulk liquid behavior of the four complex hydrides under a range of thermal conditions, we perform canonical (NVT) Born-Oppenheimer AIMD simulations at 300 K, 500 K and 1000 K. For each compound and temperature, we perform three runs beginning from the solid structure with varied initial velocities to sample a greater portion of phase space (i.e.…”

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

“…Compounds with LiZn(BH 4 ) 3 and LiZn 2 (BH 4 ) 5 stoichiometries were proposed by previous theoretical and experimental reports, as described in the introduction 13,14,16,18 . Utilizing DFT, we relax the two experimentally proposed structures (PXD and PND) for LiZn 2 (BH 4 ) 5 (i.e.…”

“…Following this work,Černý et al 15 performed a combination of in situ synchrotron powder diffraction and energy minimization (DFT) study on LiZn 2 (BH 4 ) 5 . The authors 15 also proposed a new compound LiZn(BH 4 ) 3 , though LiZn(BH 4 ) 3 is not observed experimentally at ambient pressure and in the temperature range of 100-400 K. Subsequently, motivated by these theoretical and experimental predictions, Aidhy et al performed a phase stability study 16 of Li-Zn mixed-metal borohydride compounds, using a combination of DFT and a recently developed crystal structure prediction method -the prototype electrostatic ground state (PEGS) method 17 . This theoretical DFT+PEGS work successfully predicted a low-energy LiZn(BH 4 ) 3 phase, which has a lower DFT energy than the two-phase mixture of Zn(BH 4 ) 2 and LiZn 2 (BH 4 ) 5 in the previously mentioned PXD structure.…”

“…This theoretical DFT+PEGS work successfully predicted a low-energy LiZn(BH 4 ) 3 phase, which has a lower DFT energy than the two-phase mixture of Zn(BH 4 ) 2 and LiZn 2 (BH 4 ) 5 in the previously mentioned PXD structure. Aidhy et al 16 only reported static DFT energies at T=0K, without considering vibrational contributions. They 16 found surprisingly, the LiZn 2 (BH 4 ) 5 phase obtained in the X-ray work 14 was ∼2 kJ/(mol cation) above the tie line between LiZn(BH) 3 and Zn(BH 4 ) 2 , indicating either that: (i) the observed LiZn 2 (BH 4 ) 5 phase is not stable , or (ii) the previously proposed PXD crystal structure of LiZn 2 (BH 4 ) 5 is incorrect and hence the DFT energy (based on this structure) is artificially too high, leading to an incorrect prediction of its instability.…”