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.