VOLUME 88, NUMBER 11 PHYSICAL REVIEW LETTERS 18 MARCH 2002 Chemical Isomerism as a Key to Explore Free-Energy Landscapes in Disordered Matter C. Talón, 1 F. J. Bermejo, 2,3 C. Cabrillo, 2 G. J. Cuello, 4 M. A. González, 4 J. W. Richardson, Jr., 5 A. Criado, 6 M. A. Ramos, 1 S. Vieira, 1 F. L. Cumbrera, 7 and L. M. González 7 1 Departamento Física Materia Condensada, C-III, Universidad Autónoma de Madrid, E-28049 Cantoblanco, Spain 2 Departamento Electricidad y Electrónica, Facultad de Ciencias UPV/EHU, P.O. Box 644, E-48080 Leioa, Spain 3 Consejo Superior de Investigaciones Científicas, Serrano 123, E-28006 Madrid, Spain 4 Institut Laue Langevin, B.P. 156, F-38042 Grenoble Cedex, France 5 IPNS Division, Argonne National Laboratory, Argonne, Illinois 60439 6 Departamento Física Materia Condensada, Fac. Ciencias, P.O. Box 1065, E-41080 Seville, Spain 7 Departamento Física, Facultad de Ciencias, Universidad de Extremadura, E-06071 Badajoz, Spain (Received 28 September 2001; published 1 March 2002) The effects of a minor chemical modification on the microscopic structure of a material in its glass and crystal phases are investigated by the concurrent use of neutron diffraction and computer simulation. Significant changes in short-, intermediate-, and long-range order are found, resulting from the change in molecular structure. These differences are explainable by a shift in the balance between directional and excluded-volume interactions. DOI: 10.1103/PhysRevLett.88.115506 PACS numbers: 61.43.Fs, 61.12. –q The nature of intermediate-range-order (IRO) in amor- phous matter, that is the existence of spatial regularities at distances beyond those separating nearest neighbors, remains to be fully understood. Within a glass (or liquid), structural arrangements at short scales result from chemi- cal and topological details of its constituent particles and in fact, significant progress has been achieved in our understanding of atomic arrangements in materials such as chalcogenide [1], oxide [2], or molecular glasses [3]. How- ever, the mechanisms responsible for ordering at distances which are several times that of a characteristic unit forming the glass await clarification. Here we report on how minor chemical details such as a change in the position of a func- tional group within the same structural unit leads to signifi- cant changes in structure at scales well beyond those involved in short-range packing. More specifically, we have conducted neutron diffraction (ND) studies on the glass and crystal structures of the two isomers of fully deuterated propyl alcohol (CD 3 CD 2 CD 2 OD and CD 3 CDODCD 3 referred to as 1-Pr and 2-Pr hereafter) which differ by the location of the OD group. Such an isomeric change rather than altering properties such as van der Waals volumes (124.8 and 127.7 Å 3 , respec- tively) and electric dipole moments (about 1.66 debye) to any significant extent, leads to a change in the over- all molecular “shape.” The latter translates into rather different macroscopic properties such as the crystal melting points (T m  148 and 185 K, for 1-Pr and 2-Pr, respectively), glass-transition temperatures (T g  98 and 115 K), and liquid densities which differ by some 2% [4] at room temperature. Our first measurements of the S Q static structure factors of both glasses and crystals were performed using the D4c diffractometer at the ILL (Grenoble). Crystal structure determination was achieved using the GPPD powder diffractometer at IPNS (Argonne). Although studies mostly dealing with the liquid have appeared [5], up to the authors’ knowledge the crystal and glass structures of 1-Pr and 2-Pr are not known. The samples were prepared in situ starting from a deep quench into liquid nitrogen of room-temperature liq- uids and subsequent placement into a precooled cryostat. Data treatment and analysis followed standard routes for correction of sample normalization [6]. While measure- ment of the glass structure factors was straightforward, those for the crystals were complicated by the disparate crystallization kinetics of both isomers. As noticed previ- ously [7], 2-Pr crystallizes spontaneously at temperatures somewhat below T m (achieved here at 130 K) while complete crystallization of 1-Pr requires annealing over several hours at temperatures within a narrow range about 135 K. The SQ’s are shown in Fig. 1(a) and the result- ing Dr  total static pair correlation functions are given in Fig. 2(a). Most of the diffuse patterns in SQ’s for momentum transfers above Q  6 Å 21 are attributable to details pertaining to the molecular form factors. Such correlations are suitably taken care of by subtracting from SQ an intramolecular contribution f 1 Q [8], f 1 Q  2 M12 X i 1 M X j i 11 b i b j sinQd ij  Qd ij exp2g ij Q 2 2 , (1) where b i stands for coherent scattering lengths of the i th nuclei. The sum runs over i j atomic pairs which are separated by equilibrium distances d ij and the g ij are the mean-square amplitudes of atomic vibrations. The intermolecular structure factors D m Q  SQ 2 f 1 Q resulting after fits to the large-Q part of SQ of f 1 Q’s estimated from molecular force-field models contain information about orientational and center-of-mass cor- relations and are shown in Fig. 1(b). A first remark concerns the close match between the positions Q p of the 115506-1 0031-9007 02 88(11) 115506(4)$20.00 © 2002 The American Physical Society 115506-1