This review summarizes experimental evidence for the freezing of reorienting moments in solids. The moments may be of dipolar or quadrupolar nature, or both; they belong to one of the constituents of a mixed-crystal solid. Extensive results are reported for the following systems: KC1 doped with hydroxyl, potassium tantalate doped with Li, Na and Nb, alkali halide cyanides and alkali-alkali cyanides, rubidium ammonium dihydrogen phosphate, solid ortho-para hydrogen and argon-nitrogen mixtures. These have clearly glass-like properties. In other systems, results are limited to one or two methods hinting at glass formation; some of those are also reported. Clustering phenomena and the slow-down of reorientations at the freezing temperature are observed in susceptibility measurements and by local probing on nuclear spins. The modulation of the structure by cluster formation is revealed by diffraction experiments. These phenomena are confronted with model predictions and numerical simulations. l , 2. IntroductionWe apply the term 'orientational glass' to a solid consisting of a regular lattice some of whose sites are occupied by constituents containing a dipole or quadrupole moment. These moments have orientational degrees o f freedom; they interact with one another and, below some freezing temperature Tf, their motion slows and they freeze into a configuration devoid of long-range order. A typical example is KC1 doped with OH molecules that replace C1 at random sites. At ! 0 K the dipoles associated with the OH molecules reorient rapidly, whereas at 0-1 K their orientations are static on experimental time scales, yet there is no macroscopic (spontaneous) polarization. Early investigations of this orientational-glass prototype (of the subspecies 'dipole glass') were performed by Kfinzig et al. (1964). Brout (1965)considered its short-range order and pointed out the analogy with what was to become the spin-glass prototype, Cu doped with Mn, where Mn has a spin of ~ and therefore contains magnetic degrees of freedom.The use of the word 'glass' suggests a similarity with what we call ~canonical glasses' (the prototype of which is fused silica, SiO2), in order to distinguish them from orientational and spin glasses. There are indeed orientational degrees of freedom in SiO2, in addition to rapid freezing of the moments and a low-temperature configuration devoid of order. Owing to the lack of an underlying lattice, however, it is impossible to associate moments at site i with interactions between moments at sites i and j, which is a pre-requisite for modelling a glass in terms of a Hamiltonian.Spin-glass models are based almost entirely on the existence of a Hamiltonian containing interactions between spins and their coupling to a magnetic field. The interactions chosen to model spin glasses are, in general, totally unspecific for spins. They are usually probabilistic distributions o f interaction strengths with a variance Var~j much larger than their average ~j and with both quantities independent of i j. The picture of spi...

The hysteresis and kinetics of capillary condensation of N2 and Ar in linear mesopores, produced by etching of Si wafers, have been studied for different pore shapes, including the ink bottle geometry. Pore blocking has been observed in the solid state of the pore fillings, but not in the liquid state. We conclude that individual local geometries such as the pore mouth, a blind end, or a single constriction have no effect on the shape of sorption isotherms, that the pore space should be regarded as a statistical ensemble of pore segments with a lot of quenched disorder.

The optical birefringence of rodlike nematogens (7CB, 8CB), imbibed in parallel silica channels with 10 nm diameter and 300 microm length, is measured and compared to the thermotropic bulk behavior. The orientational order of the confined liquid crystals, quantified by the uniaxial nematic ordering parameter, evolves continuously between paranematic and nematic states, in contrast to the discontinuous isotropic-to-nematic bulk phase transitions. A Landau-de Gennes model reveals that the strength of the orientational ordering fields, imposed by the silica walls, is beyond a critical threshold, that separates discontinuous from continuous paranematic-to-nematic behavior. Quenched disorder effects, attributable to wall irregularities, leave the transition temperatures affected only marginally, despite the strong ordering fields in the channels.

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Ar condensed into a porous glass matrix has been investigated by simultaneous measurements of adsorptiondesorption isotherms and x-ray diffraction patterns as function of the pore filling above and below the melting point. The chemical-potential-temperature phase diagram has been established. It is consistent with a firstorder phase transition between the adsorbate state and the capillary condensed state, above and below the melting temperature. The adsorbate and the capillary condensed state can also be distinguished in the diffraction patterns. A consistent picture of the structure and the thermodynamics is obtained.