Emergence of glassy-like dynamics in an orientationally ordered phase

M. Romanini, Ph. Negrier, J. Ll. Tamarit, S. Capaccioli, M. Barrio, L. C. Pardo, and D. Mondieig

Phys. Rev. B 85, 134201 – Published 4 April 2012

Abstract

The dynamics of a simple rigid pseudoglobular molecule (2-adamantanone) has been studied by means of dielectric spectroscopy and examined under the constraints imposed by the space group of the crystal structure determined by x-ray powder diffraction. The low-temperature monoclinic structure of 2-adamantanone, with one molecule per asymmetric unit ( $Z^{'} = 1$ ), displays a statistical intrinsic disorder, concerning the site occupancy of the oxygen atom along three different sites. Such a physically identifiable disorder gives rise to large-angle molecular rotations which inherently lead to time-average fluctuations of the molecular dipole, thus contributing to the dielectric susceptibility. The dielectric spectra for the low-temperature “ordered” phase displays a universal feature of glassy-like materials, i.e., coexistence of α- and β-relaxation processes. The former is clearly identified with the strongly restricted reorientational motions within the long-range “ordered” crystalline lattice. The latter, never observed before in fully translationally and highly orientationally ordered phases, displays all the properties of an original Johari-Goldstein β-relaxation, in spite of the strong character of this glass-like phase. These findings can be explained according to the coupling model, applied to such “ordered” phases.

Received 27 September 2011

DOI:https://doi.org/10.1103/PhysRevB.85.134201

Authors & Affiliations

M. Romanini¹, Ph. Negrier², J. Ll. Tamarit^1,*, S. Capaccioli³, M. Barrio¹, L. C. Pardo¹, and D. Mondieig²

¹Grup de Caracterització de Materials, Departament de Física i Enginyeria Nuclear, ETSEIB, Diagonal 647, Universitat Politècnica de Catalunya, 08028 Barcelona, Catalonia, Spain
²Laboratoire Onde et Matière d’Aquitaine, UMR 5798 au CNRS-Université Bordeaux I, 351 cours de la Libération, 33405 Talence Cedex, France
³Dipartimento di Fisica, Università di Pisa, Largo B. Pontecorvo 3, I-56127 Pisa, Italy

^*jose.luis.tamarit@upc.edu

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Vol. 85, Iss. 13 — 1 April 2012

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Images

Figure 1
Experimental (red circles) and calculated (black line) diffraction patterns along with the difference profile (blue “a” line) of monoclinic OO phase II at 190 K. Vertical bars indicate the calculated Bragg peak positions. Green “b” line corresponds to the face-centered-cubic OD phase I at 210 K. Inset shows the cell volume versus temperature in the OO phase II. Dashed and dotted lines are linear regression to data above and below $T_{g}$ . Vertical dashed line shows $T_{g}$ = 130 K.Reuse & Permissions
Figure 2
Left panel: Picture of the (00l) crystallographic phase of OO phase II with oxygen atoms (red) within their three occupational sites. Red (darker gray in printed version) labeled atoms correspond to the oxygen atoms within their three sites of fractional occupancies 0.25 (O1), 0.25 (O2), and 0.50 (O3). Right panel: The same projection with 70 $%$ of the van der Waals radii of the atoms.Reuse & Permissions
Figure 3
Dielectric loss spectra of phase II of 2-adamantanone for several temperatures. The solid lines are examples of the fits using Havriliak-Negami and Cole-Cole functions for the α- and β-processes, respectively.Reuse & Permissions
Figure 4
Relaxation times as a function of reciprocal of temperature for the different relaxation processes in the OD phase I and in the OO phase II. Phase I: Dashed lines show $τ_{C}$ $_{3}$ , 2π/3-rotations of the C=O dipolar axis between the $〈 001 〉$ cubic lattice directions; dotted lines show $τ_{π / 2}$ , π/2 jump rotations of the molecules around the C=O axis; data are from NMR (Ref. 35) (dark green color) and from IQNS (Ref. 34) (light green color); $α_{I}$ process (equivalent to rotations of the C=O dipolar axis) from dielectric spectroscopy from Ref. 23 (red solid up triangles) and from this work (blue open up triangles). Phase II: α-relaxation times are represented by open and closed inverted triangles, respectively, from this work and Ref. 23 (blue and red symbols, respectively), while β-relaxation times are shown as blue-white inverted triangles. Lines are linear regressions to data. Gray asterisks are the predicted β-relaxation times calculated according to the coupling model (see text). Inset: $β_{KWW}$ (upper) and Kirkwood factor $g_{K}$ (lower) of phases I and II as a function of temperature.Reuse & Permissions
Figure 5
Standard deviation of distribution of logarithmic relaxation times, $σ_{\ln τ}$ , as a function proportional to reciprocal temperature obtained by means of the procedure described in Ref. 51 through the fitting parameters obtained from experimental values (filled circles). Continuous red line would correspond to a constant distribution of activation energy ( $σ_{E}$ ) barriers while dotted blue line represents the fit of the obtained values. Inset: Distribution of activation energy, $σ_{E}$ , obtained by means of $σ_{E}$ = RT $σ_{\ln τ}$ as a function of temperature.Reuse & Permissions

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