MOLECULAR GLASSES: NMR AND DIELECTRIC SUSCEPTIBILITY MEASUREMENTS

Keywords:quadrupolar glass orientational glass susceptibility fluctuation-dissipation.

АНАЛИЗ ЭКСПЕРИМЕНТАЛЬНЫХ ПОЛОС ПОГЛОЩЕНИЯ ЯМР ПОЛИКРИСТАЛЛОВ С ИСПОЛЬЗОВАНИЕМ X-ФУНКЦИИ ХЕЙМАНА, PEROVSKITE SOLID SOLUTIONS — A MONTE CARLO STUDY OF THE DEEP EARTH ANALOGUE (K, Na)MgF3, ...

We review the properties of simple diatomic molecular glasses as explored by nuclear magnetic resonance techniques and measurements of the dielectric susceptibility. <...> We focus on the behavior of classical molecular rotors formed by solid N2—Ar mixtures and discuss the time dependent behavior in terms of replica symmetry breaking theories. <...> Of all the different types of glasses: fragile glasses [ 1 ], spin glasses [ 2 ], orientational glasses [ 3—6], and quadrupolar glasses [ 7, 8 ], the simple molecular glasses [ 9 ] provide a particularly straightforward case where the characteristic glass features of frustration and disorder can be formulated clearly and realized experimentally without ambiguity. <...> In this review we will discuss the properties of molecular glasses formed by solid mixtures of diatomic molecules and isoelectronic diluents, and in particular solid ortho-para H2 and solid N2—Ar alloys and analyze the characteristic hysteresis in terms of a generalized fluctuation-dissipation theorem introduced by Cugliandolo and Kurchan [ 10 ]. <...> Long-range orientational ordering of the molecular axes of pure diatomic molecular solids occur in several cases — ortho-H2, N2, O2 … at low temperatures. <...> Note that for hydrogen the orbital angular momentum J is a good quantum number with J = 1 for ortho-H2 molecules at low temperatures and J = 0 for spherical para-H2 molecules. <...> Ortho-H2 and N2 molecules possess an electric quadrupole moment as well as an interatomic axis both of which are free to fluctuate with respect to the molecules fixed center of mass in the lattice structure at high temperatures. <...> The frustration for the interacting quadrupoles in the f.c.c. structure is geometrical in nature and arises because of the anisotropy of the quadrupole-quadrupole interactions. <...> Illustration of a quadrupolar glass by ellipsoids of different shapes and sizes with respect to the alignment axes Wi. <...> For solid-ortho-para hydrogen mixtures the critical concentration is 55 % and for solid N2—Ar mixtures it is 77 %. <...> Fig. 2 shows the phase diagram for the orientational ordering in solid ortho-para H2 mixtures. <...> Below the critical concentration one does not observe long range order but only local ordering in the random electrical fields produced by the surrounding molecules. <...> On cooling samples in this low concentration region NMR studies observed a fairly rapid freezing of both the molecular orientations and also the quadrupole moments <...>

• Похожие документы из РУАЭСТ
• |
• Похожие документы из Руконт