Theoretical study of the H/D isotope effect on phase transition of hydrogen-bonded organic conductor κ-H₃(Cat-EDT-TTF)₂

Abstract

κ-H(Cat-EDT-TTF) (H-TTF) is a hydrogen-bonded π-electron system which was found to reveal C2/c symmetry at 50-293 K, while its isotopologue, κ-D(Cat-EDT-TTF) (D-TTF), showed the phase transition at 185 K from C2/c to P1[combining macron]. To elucidate the origin of such a difference, we calculated the potential energy curves (PECs) for the hydrogen transfer along the H-bonds in these conductors. We found that both the π-stacking and the hydrogen nuclear quantum effect drastically affected the hydrogen transfer energy. By taking account of both effects, we obtained a symmetric single-well effective PEC for H-TTF, which indicated that the hydrogen was always located at the center of the H-bond. By contrast, the effective PEC of D-TTF was a low-barrier double-well, indicating that the position of the H-bonded deuterium would change according to the temperature. We concluded that the π-stacking and the nuclear quantum effect were the key factors for the appearance of phase transition only in D-TTF.

Quantum-disordered state of magnetic and electric dipoles in an organic Mott system

“…In contrast to D-Cat, the hydrogen atoms in H-Cat do not localize down to low temperatures [5]. This is inconsistent with the fact [25] that the potential energy curve of the hydrogen bonds calculated for an isolated supramolecule has a double minimum potential with a large energy barrier of ∼ 800 K (see Figs 1b and f), which should localize the hydrogen atoms in H- The π-dimers form a slightly anisotropic triangular lattice with S = 1/2 spins [3] (magenta arrows). e, Spin and charge structures of a 2D π-electron layer in D-Cat below the phase transition temperature Tc of 185 K. Charge disproportionation associated with deuterium localization leads to a non-magnetic ground state below Tc.…”

“…In sharp contrast, the energy curve in D-Cat retains a double-well structure above Tc = 185 K (dashed line), leading to the deuterium localization at Tc (blue solid line).Cat at low temperatures. Recent theoretical calculations [6,25] have pointed out that the potential energy curve has a single-well structure and its bottoms become very shallow and anharmonic (see Fig. 1f) owing to a manybody effect arising from the network of hydrogen bonds and π-electrons.…”

Role of Hydrogen in the Spin-Orbital-Entangled Quantum Liquid Candidate H3LiIr2O6Lü1, Winter2, Valentí3 2018Phys. Rev. Lett.553440View full textAdd to dashboardCite

“…For example, while the O-H-O bonds in HCrO 2 remain symmetric, DCrO 2 undergoes a structural transition at T c ∼ 320 K, occasioned by the formation of asymmetric bonds in the bulk [51,52]. Similar effects [53,54] are also observed in the deuterated organic spin-liquid candidate κ-D 3 -(Cat-EDT-TTF) 2 (T c ∼ 185 K). We therefore computed the deuterium vibrational energies for D 3 LiIr 2 O 6 including a O-O distance of 2.54Å (see Fig.…”

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

“…− -type strong hydrogen-bond as shown in Figure 9(a) [80][81][82][83]. H-bonded hydrogen is located at the central position between two oxygen atoms and these H-bonds result in the generation of holes (+0.5) on both Cat-EDT-TTF molecuels.…”