In this NATO-sponsored Advanced Research Workshop we succeeded in bringing together approximately forty scientists working in the three main areas of structurally incommensurate materials: incommensurate crystals (primarily ferroelectric insulators), incommensurate liquid crystals, and metallic quasi-crystals. Although these three classes of materials are quite distinct, the commonality of the physics of the origin and descrip­ tion of these incommensurate structures is striking and evident in these proceedings. A measure of the success of this conference was the degree to which interaction among the three subgroups occurred; this was facili­ tated by approximately equal amounts of theory and experiment in the papers presented. We thank the University of Colorado for providing pleasant housing and conference facilities at a modest cost, and we are especially grate­ ful to Ann Underwood, who retyped all the manuscripts into camera-ready form. J. F. Scott Boulder, Colorado N. A. Clark v CONTENTS PART I: INCOMMENSURATE CRYSTALS A. Theory A PHENOMENOLOGICAL THEORY OF THE TRANSITION SEQUENCE INCLUDING AN INCOMMENSURATE (COMMENSURATE) PHASE SANDWICHED BY REENTRANT COMMENSURATE (INCOMMENSURATE) PHASE - Yoshihiro Ishibashi . . . . . 1 DAUPHINE-TWIN DOMAIN CONFIGURATIONS IN QUARTZ AND ALUMINUM PHOSPHATE - M. B. Wa lker . . . . . . . . . . 9 ELASTIC AND INELASTIC SCATTERING FROM QUASI-PERIODIC STRUCTURES - T. Janssen and R. Currat . . . . . 19 ARE EXOTIC CONSEQUENCES OF INCOMMENSURABILITY IN SOLIDS EXPERIMENTALLY OBSERVABLE? - J. B. Sokoloff. 35 B. Theory - Numerical l1ethods THE APPLICATION OF AXIAL ISING MODELS TO THE DESCRIPTION OF MODULATED ORDER - Julia Yeomans . . 45 TWO-DIMENSIONAL MODELS OF COMMENSURATE-INCOMMENSURATE PHASE TRANSITIONS - Palll D. Beale . . .

Inhalt

I: Incommensurate Crystals.- A. Theory.- A Phenomenological Theory of the Transition Sequence Including an Incommensurate (Commensurate) Phase Sandwiched by Reentrant Commensurate (Incommensurate) Phase.- Dauphiné-Twin Domain Configurations in Quartz and Aluminum Phosphate.- Elastic and Inelastic Scattering from Quasi-Periodic Structures.- Are Exotic Consequences of Incommensurability in Solids Experimentally Observable?.- B. Theory - Numerical Methods.- The Application of Axial Ising Models to the Description of Modulated Order.- Two-Dimensional Models of Commensurate-Incommensurate Phase Transitions.- Growth Kinetics in a Frustrated System: The Quenched Axial Next-Nearest-Neighbor Ising Model.- C. Experiment.- Electron Microscopy of Incommensurate Structures.- Effect of Transverse Electric Fields on the Incommensurate Phase of NaNO2.- Light Scattering from Incommensurate Insulators: Mainly BaMnF4.- Brillouin Scattering Study of Incommensurate Crystals.- Experimental Results Related to the Normal-Incommensurate Phase Transition in A2MX4 Compounds.- Some Aspects of Raman Scattering from A2BX4 Compounds.- Intrinsic Defects in Insulating Incommensurate Crystals.- Thermal Hysteresis, Solitons and Domain Walls.- Thermal Memory and Phase Conjugation Experiments in Incommensurate Barium Sodium Niobate.- II: Incommensurate Liquid Crystals.- A. Theory.- Phasons in Quasi-Crystals and Incommensurate Liquid Crystals.- The Frustrated Spin-Gas Theory of Multiply Reentrant Liquid Crystals.- Macroscopic Description of Ferroelectric Chiral Smectic C* Liquid Crystals.- On the Molecular Theory of Smectic-A Liquid Crystals.- Order Electricity at Smectic Liquid Crystal Interfaces.- Macroscopic Variables in Commensurate and Incommensurate Condensed Phases, Quasicrystals and Phasmids.- How the Smectic a Phase Adapts to Two Incommensurate Periods in Asymmetric Liquid Crystalline Systems.- B. Experiment.- An Incommensurate Smectic a Phase.- The Nematic and Smectic-A1 Phases In DB7NO2: High Resolution X-Ray Study and Synthesis.- X-Ray Diffraction by Incommensurate Liquid Crystals.- Anomalous Heat Capacity Associated with the Incommensurate SmA Phase in Db7OCn + 80Cb.- Thermal Conductivity Studies and Free-Standing Liquid-Crystal Film Calorimetry as Two Applications of the AC Calorimetric Technique.- III: Incommensurate Quasi-Crystals.- Quasiperiodic Patterns with Icosahedral Symmetry.- Frustration and Order in Rapidly Cooled Metals.- Local Atomic Environments in the Manganese-Aluminum Icosahedral Phase.- IV: Two-Dimensional and Layered Systems.- X-Ray Scattering from Two-Dimensional Liquids Modulated by a Periodic Host: Theory, Simulation and Experiment.- Artificially Structured Incommensurate Materials.