Metastable Kitaev Magnets

Faranak Bahrami¹, Mykola Abramchuk¹, Oleg Lebedev², Fazel Tafti¹

Affiliations

¹ Department of Physics, Boston College, Chestnut Hill, MA 02467, USA.
² Laboratoire CRISMAT, ENSICAEN-CNRS UMR6508, 14050 Caen, France.

PMID: 35164130
PMCID: PMC8840360
DOI: 10.3390/molecules27030871

Metastable Kitaev Magnets

Faranak Bahrami et al. Molecules. 2022.

. 2022 Jan 27;27(3):871.

doi: 10.3390/molecules27030871.

Authors

Faranak Bahrami¹, Mykola Abramchuk¹, Oleg Lebedev², Fazel Tafti¹

Affiliations

¹ Department of Physics, Boston College, Chestnut Hill, MA 02467, USA.
² Laboratoire CRISMAT, ENSICAEN-CNRS UMR6508, 14050 Caen, France.

PMID: 35164130
PMCID: PMC8840360
DOI: 10.3390/molecules27030871

Abstract

Nearly two decades ago, Alexei Kitaev proposed a model for spin-1/2 particles with bond-directional interactions on a two-dimensional honeycomb lattice which had the potential to host a quantum spin-liquid ground state. This work initiated numerous investigations to design and synthesize materials that would physically realize the Kitaev Hamiltonian. The first generation of such materials, such as Na2IrO3, α-Li2IrO3, and α-RuCl3, revealed the presence of non-Kitaev interactions such as the Heisenberg and off-diagonal exchange. Both physical pressure and chemical doping were used to tune the relative strength of the Kitaev and competing interactions; however, little progress was made towards achieving a purely Kitaev system. Here, we review the recent breakthrough in modifying Kitaev magnets via topochemical methods that has led to the second generation of Kitaev materials. We show how structural modifications due to the topotactic exchange reactions can alter the magnetic interactions in favor of a quantum spin-liquid phase.

Keywords: magnetism; metastable; topochemical.

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Conflict of interest statement

The authors declare no conflict of interest.

Figures

**Figure 1**
(a) The bond angle ( $ϕ$ ) between edge-shared octahedral units plays a significant role in tuning the magnetic interactions. (b) Edge-sharing octahedral units create a honeycomb structure in Kitaev magnets such as $α$ -Li $_{2}$ IrO $_{3}$ and Na $_{2}$ IrO $_{3}$ . (c) Interplay between CEF and SOC creates the isospin-1/2 state in the Kitaev magnets.

**Figure 2**
Synthesis of the second-generation Kitaev magnets from the first-generation materials through (a) partial and (b) complete exchange reactions. Both generations have honeycomb layers. The topochemical change of interlayer coordination from octahedral to linear modifies the intra-layer Ir-O-Ir bond angles due to the change of oxygen positions.

**Figure 3**
(a) After each heat cycle, the powder X-ray pattern of $α$ -Li $_{2}$ IrO $_{3}$ shows more pronounced peaks, especially between 20 and 30 degrees where the honeycomb Bragg peaks appear. The number of times each sample has been reheated is shown on the right above its respective pattern. (b) The X-ray patterns of two second-generation Kitaev magnets, H $_{3}$ LiIr $_{2}$ O $_{6}$ (green) and Ag $_{3}$ LiIr $_{2}$ O $_{6}$ (gray data, reproduced from [28]). The inset shows the asymmetric broadening of the honeycomb Bragg peaks in Ag $_{3}$ LiIr $_{2}$ O $_{6}$ due to stacking faults. In H $_{3}$ LiIr $_{2}$ O $_{6}$ , the honeycomb peaks are hardly discernible due to high structural disorder.

**Figure 4**
HAADF-TEM images from (a) $α$ -Li $_{2}$ IrO $_{3}$ and (b) Ag $_{3}$ LiIr $_{2}$ O $_{6}$ . The images show an abundance of stacking faults in Ag $_{3}$ LiIr $_{2}$ O $_{6}$ unlike $α$ -Li $_{2}$ IrO $_{3}$ , due to the weaker interlayer bonding in the former. The electron diffraction patterns are presented as insets and reveal less streaking in $α$ -Li $_{2}$ IrO $_{3}$ due to fewer stacking faults compared to Ag $_{3}$ LiIr $_{2}$ O $_{6}$ .

**Figure 5**
Exchange paths for (a) K, (b) J, and (c) $Γ$ terms in Equation (4). The d and p orbitals are painted in blue and red, respectively. The numbers show the hopping sequence in the perturbation.

**Figure 6**
(a) Heat capacity ( $C / T$ ) plotted as a function of temperature below 30 K for the first-generation Kitaev magnet $α$ -Li $_{2}$ IrO $_{3}$ and its second-generation derivatives Ag $_{3}$ LiIr $_{2}$ O $_{6}$ and H $_{3}$ LiIr $_{2}$ O $_{6}$ . The data for $α$ -Li $_{2}$ IrO $_{3}$ and Ag $_{3}$ LiIr $_{2}$ O $_{6}$ are reproduced from Refs. [2,28]. (b) A similar comparison is made between Na $_{2}$ IrO $_{3}$ (first generation) and Cu $_{2}$ IrO $_{3}$ (second generation). The data are reproduced from Ref. [24].

**Figure 7**
(a) Magnetic susceptibility ( $χ$ ) plotted as a function of temperature below 30 K for the first-generation Kitaev magnet $α$ -Li $_{2}$ IrO $_{3}$ and its second-generation derivatives Ag $_{3}$ LiIr $_{2}$ O $_{6}$ and H $_{3}$ LiIr $_{2}$ O $_{6}$ . The data for $α$ -Li $_{2}$ IrO $_{3}$ and Ag $_{3}$ LiIr $_{2}$ O $_{6}$ are reproduced from Refs. [19,28] (The y range for $α$ -Li $_{2}$ IrO $_{3}$ is from 4.8 to 5.3). (b) A similar comparison is made between Na $_{2}$ IrO $_{3}$ (first generation) and Cu $_{2}$ IrO $_{3}$ (second generation). The data for Na $_{2}$ IrO $_{3}$ and Cu $_{2}$ IrO $_{3}$ are reproduced from Refs. [2,24].

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References

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Metastable Kitaev Magnets

Affiliations

Metastable Kitaev Magnets

Authors

Affiliations

Abstract

Conflict of interest statement

Figures

References

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