Interplay between the Reorientational Dynamics of the B3H8 - Anion and the Structure in KB3H8.

X. Chen,M. S. Andersson,T. R. Jensen,W. Zhou,U. Häussermann,J. B. Grinderslev,M. Karlsson,X.-M. Chen,T. J. Udovic

doi:10.1021/acs.jpcc.0c10186

Abstract

The structure and reorientational dynamics of KB3H8 were studied by using quasielastic and inelastic neutron scattering, Raman spectroscopy, first-principles calculations, differential scanning calorimetry, and in situ synchrotron radiation powder X-ray diffraction. The results reveal the existence of a previously unknown polymorph in between the α′- and β-polymorphs. Furthermore, it was found that the [B3H8]− anion undergoes different reorientational motions in the three polymorphs α, α′, and β. In α-KB3H8, the [B3H8]− anion performs 3-fold rotations in the plane created by the three boron atoms, which changes to a 2-fold rotation around the C2 symmetry axis of the [B3H8]− anion upon transitioning to α′-KB3H8. After transitioning to β-KB3H8, the [B3H8]− anion performs 4-fold rotations in the plane created by the three boron atoms, which indicates that the local structure of β-KB3H8 deviates from the global cubic NaCl-type structure. The results also indicate that the high reorientational mobility of the [B3H8]− anion facilitates the K+ cation conductivity, since the 2-orders-of-magnitude increase in the anion reorientational mobility observed between 297 and 311 K coincides with a large increase in K+ conductivity.

Highlights

Over the past decades, metal borohydrides have undergone a remarkable transformation from a small family of compounds, primarily investigated for their hydrogen-storage properties, to a family of materials with a wide range of interesting properties, such as superionic conductivity, luminescence, and magnetism.[1−3] The wide array of properties found in metal borohydrides is linked to the flexible structural and compositional nature of these compounds
The results show that the [B3H8]− anion performs 3-fold rotations in the plane created by the three boron atoms of the anion in the α-polymorph, which changes to a 2
KB3H8 has been investigated by using differential scanning calorimetry, Raman spectroscopy, in situ synchrotron radiation powder X-ray diffraction, inelastic neutron scattering, firstprinciples calculations, and quasielastic neutron scattering

Summary

Introduction

Metal borohydrides have undergone a remarkable transformation from a small family of compounds, primarily investigated for their hydrogen-storage properties, to a family of materials with a wide range of interesting properties, such as superionic conductivity, luminescence, and magnetism.[1−3] The wide array of properties found in metal borohydrides is linked to the flexible structural and compositional nature of these compounds. In a recent X-ray diffraction study of the temperature-induced structural evolution of KB3H8, it was found that KB3H8 exhibits three polymorphs (α-KB3H8, α′-KB3H8, and β-KB3H8).[10] In particular, in the temperature interval 233−288 K, Bragg reflections of the monoclinic α-polymorph start merging, and at 288 K, it undergoes a second-order polymorphic transition to the orthorhombic α′-polymorph. The study showed that the [B3H8]− anion is in an ordered state in the lower-temperature αand α′-polymorphs, while transforming to an orientationally disordered state in the high-temperature β-polymorph (see Figure 1).[10] The anion disorder suggests that it might be undergoing rapid reorientations similar to what has been observed in other metal borohydrides such as MBH4 (M = Li, Na, K, and NH4) M2B12H12

Methods

Results

Conclusion