Showing papers on "Phase transition published in 1976"

Investigation of phase transitions of lipids and lipid mixtures by sensitivity differential scanning calorimetry.

Abstract: High sensitivity differential scanning calorimetry is applied to the study of the thermotropic behavior of mixtures of synthetic phospholipids in multilamellar aqueous suspensions. The systems dimyristoylphosphatidylcholine dipalmitoylphosphatidylcholine, and dimyristoylphosphatidylethanolamine-distearoylphosphatidylcholine, although definitely nonideal, exhibit essentially complete miscibility in both gel and liquid crystalline states, while the system dilauroylphosphatidylcholine-distearoylphosphatidylcholine is monotectic with lateral phase separation in the gel state. Comparison of the observed transition curves with theoretical curves calculated from the calorimetrically determined phase diagrams supports a literal interpretation of the phase diagrams.

Introduction to Liquid Crystals

Abstract: 1 Liquid Crystal Mesophases.- 1. Mesophases.- 1.1 Disordered Crystal Mesophases.- 1.2 Ordered Fluid Mesophases.- 2. Types of Liquid Crystals.- 2.1 Thermotropic Liquid Crystals.- 2.2 Lyotropic Liquid Crystals.- 3. Classification According to Molecular Order.- 3.1 Nematic Order.- 3.2 Cholesteric Order.- 3.3 Smectic Order.- 4. Polymorphism in Thermotropic Liquid Crystals.- 5. Molecular Structure of Thermotropic Mesogens.- 6. Properties of Ordered Fluid Mesophases.- 2 Structure-Property Relationships in Thermotropic Organic Liquid Crystals.- 1. Introduction.- 2. Organic Mesophases.- 3. General Structural Features of Mesogens.- 4. Effects of Structure on Mesophase Thermal Stability.- 5. Homologous Series.- 6. Materials for Device Applications.- 7. Summary.- 3 Introduction to the Molecular Theory of Nematic Liquid Crystals.- 1. Introduction.- 2. Symmetry and the Order Parameter.- 3. The Molecular Potential.- 4. The Orientational Distribution function.- 5. Thermodynamics of the Nematic Phase.- 6. Fluctuations at Tc.- 4 Generalized Mean Field Theory of Nematic Liquid Crystals.- 1. Introduction.- 2. The Pair Interaction Potential.- 3. The Mean Field Approximation.- 4. Statistical Thermodynamics.- 5. Nature of the Parameters UL.- 6. The Need for Higher Order Terms in V1.- 5 Hard Rod Model of the Nematic-Isotropic Phase Transition.- 1. Introduction.- 2. Derivation of Onsager Equations.- 3. Solution of Onsager Equations in a Simplified Case.- 6 Nematic Order: The Long Range Orientational Distribution Function.- 1. Introduction.- 2. The Orientational Distribution function.- 3. Macroscopic Definition of Nematic Order.- 4. Relationship Between Microscopic and Macroscopic Order Parameters.- 5. Experimental Measurements.- 5.1 Measurements of ?P2(cos ?)? Based on Macroscopic Anisotropies.- 5.2 Measurements of ?P2(cos ?)? Based on Microscopic Anisotropies.- 6. Experimental Data.- 7 Introduction to the Molecular Theory of Smectic-A Liquid Crystals.- 1. Introduction.- 2. Symmetry, Structure and Order Parameters.- 3. Phase Diagrams.- 4. The Molecular Potential.- 5. Statistical Thermodynamics.- 6. Numerical Results.- 7. Improved Theory.- 8. The Possibility of Second-Order Transitions.- 8 Introduction to the Elastic Continuum Theory of Liquid Crystals.- 1. Introduction.- 2. The Fundamental Equation of the Continuum Theory of Liquid Crystals.- 3. Applications of the Elastic Continuum Theory.- 3.1 Twisted Nematic Cell.- 3.2 Magnetic Coherence Length.- 3.3 Freedericksz Transition.- 3.4 Field-Induced Cholesteric-Nematic Transition.- 4. Concluding Remarks.- 9 Electrohydrodynamic Instabilities in Nematic Liquid Crystals.- 1. Introduction.- 2. Nature of the Instability and the Balance of Forces.- 3. Dielectric Response.- 4. Hydrodynamic Effects.- 5. The Boundary Value Problem in the Conduction Regime.- 6. The Torque Balance Equation.- 7. Numerical Results and Comparison with Experiment.- 8. Range of Applicability.- 10 The Landau-de Gennes Theory of Liquid Crystal Phase Transitions.- 1. Introduction.- 2. Derivation of the Fundamental Equations of the Landau-de Gennes Theory.- 2.1 The Partition function.- 2.2 The Landau Expansion.- 2.3 Generalization of the Landau Expansion to Liquid Crystals.- 3. Thermodynamic Properties of Liquid Crystal Phase Transitions.- 4. Fluctuation Phenomena.- 4.1 Homophase Fluctuations in the Isotropic Phase.- 4.2 Heterophase Fluctuations.- 5. Observation of Fluctuations Using Light Scattering.- 6. Magnetic Birefringence and the Paranematic Susceptibility.- Appendix A.- Appendix B.- 11 Introduction to the Optical Properties of Cholesteric and Chiral Nematic Liquid Crystals.- 1. Introduction.- 2. Maxwell's Equations.- 3. Discussion.- 4. Conclusion.- Appendix A.- Appendix B.- 12 Liquid-Crystal Displays-Packaging and Surface Treatments.- 1. Introduction.- 2. Packaging.- 3. Electrodes.- 4. Surface Orientation.- 5. Influence of Packaging on Surface Orientation.- 6. Summary.- 13 Pressure Effects in Sealed Liquid-Crystal Cells.- 1. Introduction.- 2. Effect of Temperature Change.- 3. Effect of Glass Thickness.- 4. The Case of a Rigid Container.- 14 Liquid-Crystal Displays-Electro-optic Effects and Addressing Techniques.- 1. Introduction.- 2. Electro-optic Phenomena.- 2.1 Field-Induced Birefringence.- 2.2 Twisted Nematic Effect.- 2.3 Guest-Host Effect.- 2.4 Cholesteric-to-Nematic Transition.- 2.5 Dynamic Scattering.- 2.6 Storage Mode.- 2.7 Transient Response.- 3. Display-Related Parameters.- 3.1 Display Life.- 3.2 Temperature Dependence.- 4. Addressing Techniques.- 4.1 Matrix Addressing.- 4.2 Beam Scanning.- 5. Summary.- 15 Liquid-Crystal Optical Waveguides.- 1. Introduction.- 2. Guided Optical Waves.- 3. Phase Matching and Coupling.- 4. Scattering.- 5. Liquid Crystal Waveguides.- 6. Conclusions.- 16 The Electro-optic Transfer Function in Nematic Liquids.- 1. Introduction.- 2. Geometrical Considerations in Optical Measurements.- 3. Field Effects-Negative Dielectric Anisotropy.- 4. Field Effects-Positive Dielectric Anisotropy.- 5. Hydrodynamic Effects-Diffraction by Domains.- 6. Dynamic Scattering.- 7. Photoconductor Control.- 17 Electrochemistry in Nematic Liquid-Crystal Solvents.- 1. Introduction.- 2. Equilibrium Properties of Bulk Solutions.- 3. Electrochemical Reactions.- 18 Lyotropic Liquid Crystals and Biological Membranes: The Crucial Role of Water.- 1. Introduction.- 2. Lyotropic Liquid Crystals.- 2.1 Constituents of Lyotropics.- 2.2 Micelles.- 2.3 Structure of Lyotropics.- 3. Biological Membranes.- 3.1 Constituents of Membranes.- 3.2 Structures of Membranes.- 4. Interaction of Amphiphilic Compounds with Water.- 4.1 Solubility of Hydrocarbons in Water.- 4.2 Solubility of Ionized Species in Water.- 4.3 Aggregation of Amphiphilic Compounds.- 5. Conclusion.

Theory of discommensurations and the commensurate-incommensurate charge-density-wave phase transition

Abstract: The lowest-energy state of the incommensurate charge density wave (CDW) near the lock-in transition is found to be a distorted plane wave. An exact solution is found in the weak-coupling limit and the lock-in phase transition is continuous. A new defect is found in the commensurate CDW, a discommensuration, in which the phase of the CDW slips by $\frac{2\ensuremath{\pi}}{3}$ relative to the perfectly locked in CDW. The lock-in phase transition is interpreted as a defect melting transition with a finite density of discommensurations in the incommensurate state.

Infrared Bounds, Phase Transitions and Continuous Symmetry Breaking

Abstract: We present a new method for rigorously proving the existence of phase transitions. In particular, we prove that phase transitions occur in (φ·φ) 3 2 quantum field theories and classical, isotropic Heisenberg models in 3 or more dimensions. The central element of the proof is that for fixed ferromagnetic nearest neighbor coupling, the absolutely continuous part of the two point function ink space is bounded by 0(k−2). When applicable, our results can be fairly accurate numerically. For example, our lower bounds on the critical temperature in the three dimensional Ising (resp. classical Heisenberg) model agrees with that obtained by high temperature expansions to within 14% (resp. a factor of 9%).

A calorimetric and fluorescent probe study of the gel-liquid crystalline phase transition in small, single-lamellar dipalmitoylphosphatidylcholine vesicles.

Abstract: The results of a calorimetric and fluorescent probe study of the thermotropic behavior of various types of dispersions of dipalmitolphosphatidylcholine bilayer vesicles are reported. Bangham-type, multilamellar vesicles exhibit tow distinct phase transitions at 34.6 and 41.2 degrees C. On the other hand, single-lamellar spherical vesicles appear to exhibit a single transition at 37 degrees C. The single-lamellar vesicles are thermodynamically unstable below 27 degrees C and slowly transform into a multilamellar structure with a single phase transition of 41.2 degrees C. These transformed structures resemble, but are not identical with, Bangham-type vesicles. An experimentally testable thermodynamic and kinetic model based upon these results is developed.

Application of the renormalization group to phase transitions in disordered systems

Abstract: The critical behavior of spin systems with quenched disorder is studied by renormalization-group methods. For the randomly dilute $m$-vector model, the $n=0$ limit is used to construct a translationally invariant effective Hamiltonian which describes the original disordered system. This Hamiltonian is analyzed in the $\ensuremath{\epsilon}$ expansion to order ${\ensuremath{\epsilon}}^{2}$. Sharp second-order phase transitions with exponents which do not depend continuously on impurity concentration are predicted. For $mg{m}_{c}\ensuremath{\equiv}4\ensuremath{-}4\ensuremath{\epsilon}+O({\ensuremath{\epsilon}}^{2})$ the isotropic $m$-component fixed point, which characterizes the critical behavior of the pure system, is stable. For $ml{m}_{c}$, a new random fixed point becomes stable. The exponents corresponding to this fixed point are $\ensuremath{\eta}=[\frac{(5{m}^{2}\ensuremath{-}8m)}{256{(m\ensuremath{-}1)}^{2}}]{\ensuremath{\epsilon}}^{2}+O({\ensuremath{\epsilon}}^{3})$, $\ensuremath{ u}=\frac{1}{2}+[\frac{3m}{32(m\ensuremath{-}1)}]\ensuremath{\epsilon}+[\frac{m(127{m}^{2}\ensuremath{-}572m\ensuremath{-}32)}{4096{(m\ensuremath{-}1)}^{3}}]{\ensuremath{\epsilon}}^{2}+O({\ensuremath{\epsilon}}^{3})$ for $m\ensuremath{ e}1$, and $\ensuremath{\eta}=\ensuremath{-}\frac{\ensuremath{\epsilon}}{106}+O({\ensuremath{\epsilon}}^{\frac{3}{2}})$, $\ensuremath{ u}=\frac{1}{2}+\frac{{(\frac{6\ensuremath{\epsilon}}{53})}^{\frac{1}{2}}}{4}+O(\ensuremath{\epsilon})$ for $m=1$. More general random systems are qualitatively discussed from the effective-Hamiltonian viewpoint.

Lowering of Dimensionality in Phase Transitions with Random Fields

Abstract: We prove that to all orders in perturbation expansion, the critical exponents of a phase transition in a $d$-dimensional ($4ldl6$) system with short-range exchange and a random quenched field are the same as those of a ($d\ensuremath{-}2$)-dimensional pure system. Heuristic arguments are given to discuss both this result and the random-field Ising model for $2ldl6$.

Phase transitions in the Gaussian core system

Abstract: Some aspects of phase transition behavior have been studied for a classical system of particles which interact in pairs via repelling Gaussian potentials. In a specific low‐temperature, low‐density limit, this model acts as though it were composed of rigid spheres. Study of lattice sums suggests that at T=0, a fcc crystal stable at low density transforms under compression to a bcc crystal. It is shown that sufficient compression at any fixed T≳0 always produces a fluid phase. Under suitable temperature and density conditions, the model can exhibit positive, negative, or vanishing volumes of melting.

A second harmonic analyzer for the detection of non-centrosymmetry

Abstract: The existence of optical second harmonic generation has been shown to be a highly reliable and sensitive physical test for the detection of crystalline non-centrosymmetry. A second harmonic analyzer has been constructed which can resolve space group ambiguities arising from Friedel's Law with a confidence level greater than 99%. The system has been optimized for use with powdered crystalline samples so as to obviate the need for large single crystals and thus facilitates rapid determination of crystalline non-centrosymmetry. The present analyzer can routinely detect second harmonic generation at levels 1/1000 of that generated in a quartz standard, this is about an order of magnitude increase over previously reported systems. Data are reported on several materials including dibenzyldisulfide, and [(C6H5)3P]3CuBF4. The detection of structural phase transitions with the second harmonic analyzer is reported for BaTiO3, colemanite and phenanthrene. Second harmonic generation in the `cubic' phase of BaTiO3 promises to be a powerful tool for determining the dynamics of the ferroelectric phase transition. It is the most direct method for establishing the existence or nonexistence of microscopic polar regions well above the Curie point in a nominally centrosymmetric phase.

Phase transition in the two-dimensional Coulomb gas, and the interfacial roughening transition

Abstract: The behavior of the interface in the three-dimensional (3-D) Ising and related lattice systems is modeled using a 2-D array of columns of varying heights We show that the "roughening" transition (the transition from a localized to a delocalized interface) is directly related to the metal-insulator transition in a 2-D Coulomb gas Implications of this relationship are discussed

Temperature dependence of 1,6-diphenyl-1,3,5-hexatriene fluorescence in phophoslipid artificial membranes.

Abstract: The fluorescence polarization of 1,6-diphenyl-1,3,5-hexatriene in phospholipid vesicles is a function of the physical state of the lipid. Below the phase transition, the polarization approaches the theoretical maximum for total immobilization while above the phase transition the fluorescence becomes nearly completely depolarized. The discontinuity in the temperature dependence of polarization occurs within a temperature range under 5 degrees C in the case of pure phospholipids, but for mixed phospholipids occurs over a temperature range greater than 20 degrees C. From these data, phase diagrams describing the gel-sol equilibrium can be constructed; the phase diagrams correspond well with those described in the literature which were constructed using spin-label probes or from x-ray diffraction patterns. The marked change in polarization at the phase transition may be related to the packing of the probe molecule into the lipid bilayer: fluorescence measurements on oriented bilayers indicate that below the phase transition the long axis of the probe is oriented perpendicular to the plane of the membrane while above the transition the probe is oriented randomly relative to the plane of the membrane.

Phase Transition in Surface-Aligned Nematic Films

Abstract: Properties of the nematic-isotropic phase transition in surface-aligned nematic films are calculated using the Landau-de Gennes theory. It is shown that there is a critical thickness ${D}^{c}$ of the film below which the transition from the nematic phase to the isotropic phase becomes continuous. At the critical thickness the transition is second order and exhibits critical behavior. For $N\ensuremath{-}[e\ensuremath{-}\mathrm{methoxybenzylidine}]p\ensuremath{-}\mathrm{butylaniline}$, ${D}^{c}$ is calculated to be about 1000 \AA{}, and the critical temperature is 0.17 K above the bulk first-order transition temperature. Magnetic and dimensional effects are discussed.

Theory of Lipid Monolayer and Bilayer Phase Transitions" Effect of Headgroup Interactions

Abstract: Headgroup and soft core interactions are added to a lipid monolayer-bilayer model and the surface pressure-area phase diagrams are calculated. The results show that quite small headgroup interactions can have biologically significant effects on the transition temperature and the phase diagram. In particular, the difference in transition temperatures of lecithins and phosphatidyl ethanolamines is easy to reproduce in the model. The phosphatidic acid systems seem to require weak transient hydrogen bonding which is also conjectured to play a role in most of the lipid systems. By a simple surface free energy argument it is shown that monolayers under a surface pressure of 50 dynes/cm should behave as bilayers, in agreement with experiment. Although the headgroup interactions are biologically very significant, in fundamental studies of the main phase transition in lipids they are secondary in importance to the hydrocarbon chain interactions (including the excluded volume interaction, the rotational isomerism, and the attractive van der Waals interaction).

Evidence for phase boundary lipid. Permeability of Tempo-choline into dimyristoylphosphatidylcholine vesicles at the phase transition.

Abstract: The existence of distinct regions of mismatch in molecular packing at the interfaces of the fluid and ordered domains during the phase transition of dimyristoylphosphatidylcholine vesicles has been demonstrated by measuring the temperature dependence of the permeability to a spin-label cation and comparing this with a statistical mechanical calculation of the fraction of interfacial lipid. The kinetics of uptake and release of the 2,2,6,6-tetramethylpiperidinyl-1-oxycholine (Tempo-choline) spin label by single-bilayer dimyristoylphosphatidylcholine vesicles were measured using electron spin resonance spectroscopy to quantitate the amount of spin label present within the vesicles after removal of the external spin-label by ascorbate at 0 degrees C. Both the uptake and release experiments show that the Tempo-choline permeability peaks to a sharp maximum at the lipid-phase transition, the vesicles being almost impermeable to Tempo-choline below the transition and having a much reduced permeability above. The temperature profile of the permeability is in reasonable quantitative agreement with calculations of the fraction of interfacial boundary lipid from the Zimm and Bragg theory of cooperative transitions, which use independent spin-label measurements of the degree of transition to determine the cooperativity parameter. The relatively high intrinsic permeability of the interfacial regions (P approximately 0.2-1.0 X 10(-8) cm/s) is attributed to the mismatch in molecular packing of the lipid molecules at the ordered-fluid boundaries, which could have important implications not only for permeability in natural membranes (e.g., in transmitter release), but also for the function of membrane-bound enzymes and transport proteins.

Theoretical model of superconductivity and the martensitic transformation in A 15 compounds

Abstract: The effect of the martensitic transition on superconductivity in the $A15$ compounds is examined theoretically. The BCS pairing potential is added to the Peierls potential in the Gorkov model of the martensitic transition. We find that the two potentials compete to open an energy gap on the common portion of Fermi surface and that one phase transition depresses the other. It is found that the ${T}_{c}=17$ K in ${\mathrm{V}}_{3}$Si includes a 0.3-K reduction due to this competition and that superconductivity arrests the development of tetragonality below 17 K. It is predicted that at high pressure (\ensuremath{\sim} 25 kbar) the martensitic transition will be precluded.

Landau-Ginzburg mean-field theory for the nematic to smectic-C and nematic to smectic-A phase transitions

Abstract: A Landau-Ginzburg free energy is presented that yields a nematic to smectic-$A$ (NA) transition, a nematic to smectic-$C$ (NC) transition, and a Lifshitz point (at the boundary between the NA and NC transitions). Fluctuation enhancements of the Frank elastic constants are calculated. For the NC transition, all three elastic constants ${K}_{1}$, ${K}_{2}$, and ${K}_{3}$ diverge as ${\ensuremath{\xi}}^{2}$, where $\ensuremath{\xi}$ is the correlation length for fluctuations of the smectic order parameter. At the Lifshitz point, ${K}_{1}$ and ${K}_{2}$ diverge as $\mathrm{ln}\ensuremath{\xi}$ whereas ${K}_{3}$ diverges as $\ensuremath{\xi}$.

Modified electron-gas study of the stability, elastic properties, and high-pressure behavior of MgO and CaO crystals

Abstract: A theoretical study of the crystal structure and bonding in the simple inorganic oxide crystals magnesium oxide and calcium oxide is reported. An a priori method based on an electron-gas approximation is applied to an ionic description of these crystals. The quantities calculated are the equilibrium interionic separation, the elastic constants and their pressure derivatives, the transition pressures and volume changes in high-pressure-induced phase transitions, and the thermodynamic stability of the crystal structures. Comparison with available experimental data shows good agreement, without any adjustable parameters in the theory. We conclude that the ionic description of these crystals is rather accurate for energy calculations, provided that a suitably stabilized oxide ion is used in the theory.

Quantum effects in an n-component vector model for structural phase transitions

Abstract: The influence of quantum effects on the critical properties of an $n$-component vector model for structural phase transitions is explored. It is shown for $n=1,2, \mathrm{and} \ensuremath{\infty}$ that these effects can suppress the occurrence of a phase transition for all dimensions $d$. This turns out to be of particular relevance for systems where the classical transition temperature almost vanishes (${T}_{c}=0$ represents the displacive limit). We also discuss the critical properties at this limit for $n=\ensuremath{\infty}$ and show that the critical exponents change discontinously.

Interactions between anesthetics and lipid mixtures. Normal alcohols.

Abstract: The effects of normal alcohols up to 1-dodecanol on phase transitions in phosphatidylcholines and phosphatidylethanolamines have been studied using chlorophyll a as fluorescent probe. With the phosphatidylcholines, alcohols up to octanol cause a lowering of the transition temperature, and a broadening of the transition, whereas for dipalmitoylphosphatidylethanolamine, only a lowering of the transition is observed. The lowering of the phase transition temperature in dipalmitoylphosphatidylcholine by butanol and hexanol is close to that expected for ideal behavior, but the behavior of the longer chain alcohols becomes less ideal. The effects of these alcohols on mixtures of lipids have been studied, and they illustrate the care necessary if these plots of temperatures of onset and completion of gel phase formation are to be called "phase diagrams". The effect of 1 -octanol on mixtures of lipids is to increase the proportion of lipid present in the lipid-crystalline state. In contrast, 1-decanol causes an increase in the phase transition temperature for dimyristoylphosphatidylcholine, although it lowers the transition temperature for dipalmitoylphosphatidylcholine, and 1 -dodecanol raises the transition temperature for both of these phosphatidylcholines, although it lowers that for dipalmitoylphosphatidylethanolamine. Dodecanol appears to behave in these lipid bilayer membranes as a lipid with a phase transition temperature of ca. 55 degrees C. Anesthesia is discussed as a phenomenon of liquidus extension: alcohols up to 1 -octanol increase the proportion of lipid in the liquidus state and result in anesthesia, whereas the longer alcohols do not, and result in catalepsy.

Phase Transitions in Long-Range Ferromagnetic Chains

Abstract: The critical behavior of one-dimensional ferromagnetic chains with isotropic long-range interactions between $n$-component spins is investigated near the critical range of ${r}^{\ensuremath{-}2}$ Different formulations of the renormalization group are used for $ng1$ and $n=1$ It is shown that the behavior is quite different in the two cases, with very close analogies to the short-range problem near two dimensions Critical exponents are calculated to first order

A van der Waals-type equation of state for fluids with associating molecules.

Abstract: The basic assumptions of van der Waals theory are contained in two well-known concepts: excluded volume (repulsive forces) and a homogeneous, isotropic field potential (attractive forces). We have superimposed on these, one more well-known concept: the existence of dimers, trimers, etc., at chemical equilibrium. With reasonable simplifying assumptions, we obtain a closed-form equation of state, applicable to all fluid densities, and potentially useful for fluids containing strongly polar or hydrogen-bonded molecules. At all temperatures and at high densities, the equation of state suggests a phase transition where, because of extensive association, a new (solid-like) phase is in equilibrium with a “normal” fluid phase. The boundary between these phases has no critical point.

Critical behavior of the electrical resistivity in magnetic systems

Abstract: The effect of critical fluctuations on the resistivity near magnetic and order-disorder phase transitions is discussed. It is shown that in all magnetic and electronic systems the asymptotic high-momentum spin correlation functions dominate the temperature dependence of the resistivity sufficiently close to the transition. The dependence on the parameters of the system of the critical behavior and the way in which this behavior is approached is discussed in detail. In particular, the importance of the transition from a classical to a proper critical behavior, for the interpretation of experimental results, is emphasized, and recent renormalization-group results for the form of the correlation functions are utilized. For semiconductors the effects of fluctuations on the band gap are also calculated. It is suggested that the Fisher-Langer relation between the temperature derivative of the resistivity and the specific heat should be valid over a considerable temperature range outside the critical region. Theoretical predictions are compared with available experimental results on the resistivity and band gaps. It is shown that these results can all be understood, at least qualitatively, in the Born approximation by using only the most general known properties of the system.

Spin-reorientation transitions in rare-earth magnets

Abstract: This review treats the little-studied magnetic phase transitions of the order-order, spin-reorientation type, in which a rotation of the magnetization vector (or of die antiferromagnetism vector) with respect to the crystallographic axes occurs upon change of temperature, magnetic field, or external pressure. It presents theoretical ideas about magnetic phase transitions of the spin-reorientation type; it considers the effect of domain structure on spin-reorientation phase transitions, aad critical fluctuations during such transitions. Taking as an example rare-earth magnets (orthoferrites, iron-garnets, and rare-earth metals and intermetallic compounds), it considers the experimental data on various types of spin-reorientation transitions and on the anomalies of physical properties at such transitions.