Insights from experiment and ab initio calculations into the glasslike transition in the molecular conductor $\ensuremath{\kappa}\ensuremath{-}{(\mathrm{BEDT}\ensuremath{-}\mathrm{TTF})}_{2}{\mathrm{Hg}(\mathrm{SCN})}_{2}\mathrm{Cl}$

Insights from experiment and ab initio calculations into the glasslike transition in the molecular conductor $κ - {(BEDT - TTF)}_{2} {Hg (SCN)}_{2} Cl$

Elena Gati, Stephen M. Winter, John A. Schlueter, Harald Schubert, Jens Müller, and Michael Lang

Phys. Rev. B 97, 075115 – Published 9 February 2018

Abstract

We present high-resolution measurements of the relative length change as a function of temperature of the organic charge-transfer salt $κ - {(BEDT - TTF)}_{2} {Hg (SCN)}_{2} Cl$ . We identify anomalous features at $T_{g} \approx 63$ K which can be assigned to a kinetic glasslike ordering transition. By determining the activation energy $E_{A}$ , this glasslike transition can be related to conformational degrees of freedom of the ethylene endgroups of the organic building block BEDT-TTF. As opposed to other $κ$ -(BEDT-TTF) $_{2} X$ salts, we identify a peculiar ethylene endgroup ordering in the present material in which only one of the two crystallographically inequivalent ethylene endgroups is subject to glasslike ordering. This experimental finding is fully consistent with our predictions from ab initio calculations from which we estimate the energy differences $Δ E$ and the activation energies $E_{A}$ between different conformations. The present results indicate that the specific interaction between the ethylene endgroups and the nearby anion layers leads to different energetics of the inequivalent ethylene endgroups, as evidenced by different ratios $E_{A} / Δ E$ . We infer that the ratio $E_{A} / Δ E$ is a suitable parameter to identify the tendency of ethylene endgroups toward glasslike freezing.

Received 24 October 2017
Revised 20 December 2017

DOI:https://doi.org/10.1103/PhysRevB.97.075115

Physics Subject Headings (PhySH)

Glass transition Thermal expansion

Molecular solids Strongly correlated systems

Density functional theory

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Elena Gati¹, Stephen M. Winter^1,2, John A. Schlueter^3,4, Harald Schubert¹, Jens Müller¹, and Michael Lang¹

¹Physikalisches Institut, J.W. Goethe-Universität Frankfurt(M), SFB/TR49, D-60438 Frankfurt(M), Germany
²Institut für Theoretische Physik, J.W. Goethe-Universität Frankfurt(M), SFB/TR49, D-60438 Frankfurt(M), Germany
³Division of Materials Research, National Science Foundation, Arlington, Virginia 22230, USA
⁴Materials Science Division, Argonne National Laboratory, Argonne, Illinois 60439, USA

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Vol. 97, Iss. 7 — 15 February 2018

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Figure 1
Structure of $κ - {(BEDT-TTF)}_{2} {Hg (SCN)}_{2} Cl$ : (a) alter- nating layers of (BEDT- ${TTF)}_{2}^{+}$ and (Hg( ${SCN)}_{2} Cl$ ) $^{-}$ along the out-of-plane $a$ axis. C7/C8 and C9/C10 refer to the carbon atoms of the two crystallographically inequivalent ethylene endgroups in the present material which we label with inner and outer EEGs, respectively; (b) polymeric anion layer (Hg( ${SCN)}_{2} Cl$ ) $^{-}$ viewed along the in-plane $b$ and $c$ axes, indicating the dominant chainlike character along the $c$ axis; (c) orientational degrees of freedom of the ethylene endgroups (EEGs) of the BEDT-TTF molecule. $E$ refers to the eclipsed configuration, $S$ to the staggered configuration.
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Figure 2
(a) Schematic two-level system in which two states with energy difference $2 Δ E$ are separated by an activation barrier $E_{A}$ ; (b) view on BEDT-TTF molecules in three different ethylene endgroup (EEG) conformations, which can be realized in $κ - {(BEDT-TTF)}_{2} {Hg (SCN)}_{2} Cl$ . The majority conformation is a staggered conformation, labeled as $S$ (1). The configuration $E$ (1) [ $E$ (2)] is obtained from $S$ (1) by changing the orientation of the inner EEGs containing carbon atoms C7 and C8 (outer EEGs containing carbon atoms C9 and C10); (c) computed energy values $2 Δ E$ and $E_{A}$ for the processes $S$ (1) $\leftrightarrow E$ (1) and $S$ (1) $\leftrightarrow E$ (2) in $κ - {(BEDT-TTF)}_{2} {Hg (SCN)}_{2} Cl$ . The green background color indicates that our calculations predict a glasslike transition for this process, the red background color indicates that no glasslike transition is predicted.
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Figure 3
Preferred ethylene endgroup (EEG) conformation [staggered $S$ (1)] relative to the nearby anion layer for $κ - {(BEDT-TTF)}_{2} {Hg (SCN)}_{2} Cl$ , viewed within the $a^{'} c$ plane (a) and the $a b$ plane (b). $a^{'}$ indicates a small rotation of the $a$ axis around the $c$ axis. Green and brown dashed lines indicate close contacts of the outer EEGs (labeled with C9 and C10) to the Cl and (SCN) ligands in the nearby anion layer. Orange lines indicate close contacts of the inner EEGs (labeled with C7 and C8) to the (SCN) ligands in the nearby anion layer.
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Figure 4
Relative length change $Δ L_{i} (T) / L_{i}$ normalized to the reference temperature $T_{0} = 200$ K (a) and thermal expansion coefficient $α_{i} (T) = L_{i}^{- 1} d L_{i} / d T$ (b) of the organic charge-transfer salt $κ - {(BEDT - TTF)}_{2} {Hg (SCN)}_{2} Cl$ upon slow warming ( $q_{h} = + 1.5 K$ /h) after slow cooling ( $q_{c} = - 3 K$ /h). Red line corresponds to data taken along the out-of-plane $i = a$ axis, green and blue line to data along the in-plane $i = b$ and $c$ axis, respectively. The anomalous kinks in $Δ L_{i} (T) / L_{i}$ and the steplike features in $α_{i} (T)$ at $T_{g} \approx 63$ K can be assigned to signatures of the glasslike transition (see main text for a detailed discussion). The jumps in $Δ L_{i} / L_{i}$ and the divergent behavior in $α_{i} (T)$ at $T_{MI} \approx 30 K$ are related to a charge-order metal-insulator transition the investigation of which is not in the focus of the present study and will be discussed in detail elsewhere [49].
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Figure 5
Thermal expansion coefficient $α_{c} (T)$ of $κ - {(BEDT - TTF)}_{2} {Hg (SCN)}_{2} Cl$ measured along the in-plane $c$ axis around $T_{g} \approx 63 K$ : (a) hysteresis between data taken upon slow warming ( $q_{h} = + 1.5 K$ /h, red closed circles) and data taken upon slow cooling ( $q_{c} = - 1.2 K$ /h, blue open circles); (b) data taken upon slow warming ( $q_{h} = + 1.5 K$ /h) after different cooling procedures: red closed circles represent data after slow cooling ( $q_{c} = - 3$ K/h), black open circles represent data taken after fast cooling ( $q_{c} = - 20.7$ K/h).
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Figure 6
Cooling-rate dependence of the glasslike transition temperature $T_{g} (| q |)$ and determination of the activation energy $E_{A}$ : (a) Thermal expansion coefficient $α_{c} (T)$ of $κ - {(BEDT - TTF)}_{2} {Hg (SCN)}_{2} Cl$ measured along the in-plane $c$ axis upon cooling using different cooling rates $- (1.2 \pm 0.05) K / h \leq q_{c} \leq - (20.7 \pm 0.3) K$ /h; (b) Arrhenius plot of $T_{g}^{- 1}$ vs. $| q |$ , yielding an activation energy of $E_{A} = (2800 \pm 300) K$ (for details of the analysis, see main text).
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Physical Review B

covering condensed matter and materials physics

Insights from experiment and ab initio calculations into the glasslike transition in the molecular conductor $κ - {(BEDT - TTF)}_{2} {Hg (SCN)}_{2} Cl$

Elena Gati, Stephen M. Winter, John A. Schlueter, Harald Schubert, Jens Müller, and Michael Lang

Phys. Rev. B 97, 075115 – Published 9 February 2018

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Physical Review B

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Insights from experiment and ab initio calculations into the glasslike transition in the molecular conductor κ−(BEDT−TTF)2Hg(SCN)2Cl

Elena Gati, Stephen M. Winter, John A. Schlueter, Harald Schubert, Jens Müller, and Michael Lang

Phys. Rev. B 97, 075115 – Published 9 February 2018

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Insights from experiment and ab initio calculations into the glasslike transition in the molecular conductor $κ - {(BEDT - TTF)}_{2} {Hg (SCN)}_{2} Cl$