Crystal Structure Modifications in Organic Charge-transfer Salt – k-[(BEDT-TTF)_1-x(BEDT-STF)_x]₂Cu₂(CN)₃

Petr Doležal

Matematicko-fyzikální fakulta UK, Ke Karlovu 5, 121 16 Praha 2

k-[(BEDT-TTF)_1-x(BEDT-STF)_x]₂Cu₂(CN)₃series are 2D organic salts realizing a paradigmatic Mott-metal insulator transition. The localization of conduction electrons is driven by strong electronic correlations which can be tuned by hydrostatic pressure or by substitution with BEDT-STF cations as in the present study. The parent k-(BEDT-TTF)₂Cu₂(CN)₃compound remains without magnetic ordering down to the lowest temperatures. To fully understand its magnetic properties it is necessary to assess its crystal structure in detail. The (BEDT-TTF) cations are separated by the layers of Cu₂(CN)₃anions within in the bc plane, see figure below. If one looks on the cations along a crystallographic axis it is possible to see the triangular arrangement of (BEDT-TTF)₂ dimers, see panel (b) below. The triangular lattice is close to ideal, t/t’ = 0.83, providing ideal conditions for geometrical frustration [1]. Each dimer is carrying one electron with spin S = ½. The interaction between spins is antiferromagnetic [2]. These all are necessary conditions for the formation of quantum spin liquid (QSL) sate and also the reason why this compound was the prime QSL candidate. The observation of T* = 6 K anomaly in thermal expansion brought the question about the proper ground state, which seems to be rather a valence bond solid (VBS) than QSL [3]. This leads to the more general question whether geometrical frustration on its own can fully suppress antiferromagnetic order and whether the systems can truly remain stable towards magneto-structural instabilities, like VBS phases, down to absolute zero.

Our presented contribution is focused on low temperature X-ray diffraction study of a single crystalline sample k-(BEDT-TTF)₂Cu₂(CN)₃, showing pronounced anomaly of lattice parameters around T* = 6 K. The structural results are supplemented by a microscopic study using nuclear magnetic resonance (NMR).

This work was supported by the Czech Science Foundation under Grant No. 23-06810O.

Crystal Structure Modifications in Organic Charge-transfer Salt – k-[(BEDT-TTF)_1-x(BEDT-STF)_x]₂Cu₂(CN)₃

Petr Doležal

Matematicko-fyzikální fakulta UK, Ke Karlovu 5, 121 16 Praha 2

1. H. C. Kandpal, I. Opahle, Y.-Z. Zhang et al., Phys. Rev. Lett. 103 (2009) 67004.

2. Y. Shimizu, K. Miyagawa, K. Kanoda et al., Phys. Rev. Lett. 91 (2003) 107001.

3. B. Miksch, A. Pustogow, M. J. Rahim et al., Science 372 (2021) 276-279.

4. A. Pustogow, Solids 3 (2022) 93-110.

Figure: Crystal structure of k-(BEDT-TTF)₂Cu₂(CN)₃2D organic salt. a) View along b axis. b) The arrangement of dimers within bc plane. Taken from [4].

Crystal Structure Modifications in Organic Charge-transfer Salt – k-[(BEDT-TTF)1-x(BEDT-STF)x]2Cu2(CN)3

Petr Doležal

Matematicko-fyzikální fakulta UK, Ke Karlovu 5, 121 16 Praha 2

1. H. C. Kandpal, I. Opahle, Y.-Z. Zhang et al., Phys. Rev. Lett. 103 (2009) 67004.

2. Y. Shimizu, K. Miyagawa, K. Kanoda et al., Phys. Rev. Lett. 91 (2003) 107001.

3. B. Miksch, A. Pustogow, M. J. Rahim et al., Science 372 (2021) 276-279.

4. A. Pustogow, Solids 3 (2022) 93-110.

Figure: Crystal structure of k-(BEDT-TTF)2Cu2(CN)3 2D organic salt. a) View along b axis. b) The arrangement of dimers within bc plane. Taken from [4].

Crystal Structure Modifications in Organic Charge-transfer Salt – k-[(BEDT-TTF)_1-x(BEDT-STF)_x]₂Cu₂(CN)₃

Figure: Crystal structure of k-(BEDT-TTF)₂Cu₂(CN)₃2D organic salt. a) View along b axis. b) The arrangement of dimers within bc plane. Taken from [4].