Quasi-2D spin-Peierls transition due to interstitial anionic electrons in K(NH₃)₂

In an article published in Science Bulletina Chinese team of scientists predicts a new electrode K(NH₃)₂, with interstitial electrons distributed among cages formed by six ammonia molecules and forming a quasi-2D triangular lattice. They revealed that this material undergoes a spin-Peierls phase transition under moderate pressure.

This study was led by Prof. Jian Sun (National Laboratory of Solid State Microstructures, School of Physics and Collaborative Innovation Center of Advanced Microstructures, Nanjing University). The team used their proprietary crystal structure prediction software MAGUS and performed first-principles calculations to determine the crystal structures of the moderate-pressure potassium ammonia compound, which has long been recognized as an electrode at ambient conditions.

Electron-phonon interactions and electron-electron correlations represent two crucial facets in condensed matter physics. In a model system of a half-filled spin-1/2 anti-ferromagnetic chain, the lattice dimerization induced by electron-nucleus interaction can be intensified by Coulomb repulsion in situ, resulting in a spin-Peierls state. However, in two dimensions, the real material that exhibits such phenomena has never been found.

On the other hand, electrodes are materials in which non-bonded electrons occupy crystal voids and exhibit anionic behavior (IAEs). It is well known that the correlations between spin-polarized IAEs and their coupling with neighboring nuclei could induce more interesting quantum phenomena.

However, to date, very few works have investigated the interactions between the correlated IAEs and phonons. One of the main reasons is the large number of atoms in organic electrodes, where most antiferromagnetic IAEs emerge.

The team identified that the R-3m K(NH₃)₂ achieves thermodynamic stability at about 2 GPa, using a rhombohedral primitive cell, and the ammonia molecules are on both sides of potassium layers.

Some of the valence electrons are distributed in cavities between the layers surrounded by six hydrogen atoms, forming interstitial anionic electrons. The band exceeding the Fermi level is mainly attributed to these IAEs, which exist as isolated entities with bridging ammonia molecules.

The researchers also examined the pressure effects on the phonon and electronic properties. The van-Hove singularities (VHSs) are brought to the Fermi level under higher pressure, which induces the Peierls-type instability and dimerized structure. These VHSs also contribute a step-like density of states, improving electron correlations and causing magnetic instability. The magnetic ground state is found to be zigzag-type anti-ferromagnetism, which can be described by the Heisenberg model with modulated nearest adjacent magnetic interactions.

More importantly, first-principles calculations show that magnetic instability and Peierls instability not only coexist, but also positively interact, creating a spin-Peierls transition scenario unprecedented in realistic 2D material, especially when it comes to IAEs.

“It is very intriguing to reveal such abundant physical phenomena in realistic material. The interactions between correlated IAEs and phonons can provide inspiration for the exploration of magnetic interactions, structural deformations and charge density waves,” says Jian.