New insights into the structural properties of κ-(BEDT-TTF)₂Ag₂(CN)₃spin liquid

Abstract

<jats:p>Here, the first accurate study is presented of the room-temperature and 100 K structures of one of the first organic spin liquids, κ-(BEDT-TTF)<jats:sub>2</jats:sub>Ag<jats:sub>2</jats:sub>(CN)<jats:sub>3</jats:sub>. It is shown that the monoclinic structure determined previously is only the average one. It is shown that the exact structure presents triclinic symmetry with two non-equivalent dimers in the unit cell. But surprisingly this does not lead to a sizeable charge disproportionation between dimers. The difference from the analogue compound κ-(BEDT-TTF)<jats:sub>2</jats:sub>Cu<jats:sub>2</jats:sub>(CN)<jats:sub>3</jats:sub> which also presents a spin liquid phase is discussed in detail. The data provided here show the importance of the anionic layer and in particular the transition metal position in the process of symmetry breaking. The possible impact of the symmetry breaking, albeit weak, on the spin-liquid mechanism and the influence of various disorders on the physical properties of this system is also discussed.</jats:p>

Molecular quantum materials: electronic phases and charge dynamics in two-dimensional organic solids

“…There is a broad consensus that static charge order is less than 1%. This is supported by the most recent x-ray diffraction measurements of κ-(BEDT-TTF) 2 Cu 2 (CN) 3 and κ-(BEDT-TTF) 2 Ag 2 (CN) 3 by Foury-Leylekian et al [71,72]. At the same time, the new data provide evidence for the symmetry breaking of the non-polar mean P2 1 /c structure thereby proving the presence of non-equivalent crystallographic sites and allowing the formation of electric dipoles.…”

“…An anomalous dielectric peak shows up as an ubiquitous property independent of the nature of the ground state in the spin sector [458,523,576,608,610,620] The relaxor-type dielectric response suggests a short-range ferroelectric-like order [576,621] and the existence of domains of low symmetry equivalent with the average high symmetry, in accord with density functional (DFT) calculations and structural refinements [71,72,523,556,620]. However, both spectroscopic [490,523,610,620] and structural measurements [71,72] consistently demonstrate the equally distributed charge in quantum spin liquid compounds. Moreover, the persistence of strong quantum fluctuations due to charge or dipolar fluctuations in the kHz-MHz frequency range is unlikely because no Barrett-like behavior of the dielectric constant [cf.…”

“…At low frequencies, oscillations occur in the double-well potential corresponding to a small charge disproportionation of opposite charge polarity; importantly, spatially extended domain walls connecting two respective domains are also present; they give rise to the observed dielectric response. The structural results of Foury-Leylekian et al support the relevancy of this theoretical consideration as they evidence a symmetry breaking; however the tiny inter-dimer charge imbalance is hardly above the resolution limit [71,72]. The extension to a larger number of randomly distributed domains, which lift off the inversion symmetry in the anionic layers, is straightforward, as identified by density-functional theory calculations of Lazić et al; the calculations find the ground state quasi-degenerate in energy and with reduced symmetry [523,556,620].…”

“…The space group is commonly solved in monoclinic P2 1 /c, in which all four BEDT-TTF molecules are equivalent [69,70]. However, the P2 1 /c is only the average structure, while the exact structure presents a triclinic symmetry with two non-equivalent crystallographic sites [71,72]. In the panel (d) to (f) the projection of one of the layers is shown.…”