Design Principles for Rotational Cluster Anion [BH4]− Promote Superionic Conductivity in Sodium-Rich Antiperovskite Na3OBH4

Abstract

The ionic conductivity is determined by the activation energies and the jump attempt frequencies of carrier migrations between adjacent sites. However, few works in the past have considered the influence of the attempt frequency. In this work, the attempt frequencies of carrier migrations in Na3OBH4 and Na3OBr are calculated using density functional theory and written as the contribution of each element using frequency analysis, which can be used to calculate the conductivity of a carrier in combination with the defect concentration, migration barrier of carrier migration in equilibrium. The results show that the conductivity of sodium cations can be greatly improved by substituting Br− with the [BH4]− tetrahedral anion in an antiperovskite structure due to two reasons: (1) the paddle-wheel mechanism (PWM) of the [BH4]− anionic groups can effectively reduce the activation energy and (2) the [BH4]− anionic groups with a high vibration frequency undoubtedly improve the carrier jump attempt frequency by the buffering effect. The quasi-harmonic approximation (QHA) is used to describe the isobaric behavior of Na3OBH4 and Na3OBr, and the results show that carriers have a higher jump attempt frequency in the zero-pressure compared with the constant volume. The ionic conductivity (σ) in Na3OBr and Na3OBH4 are obtained from the Arrhenius equation. The results show that the calculated sodium ionic conductivity of Na3OBH4 is 3–4 orders of magnitude higher than that of Na3OBr at room temperature. The calculated sodium ionic conductivity is lower than the experimental measurement, and the discrepancy is attributed to the overestimation of the calculated migration barrier and the underestimation of the defect concentration.