Landau theory

About: Landau theory is a research topic. Over the lifetime, 2882 publications have been published within this topic receiving 57078 citations.

Landau theory of the orthorhombic Fddd phase.

Abstract: Numerical self-consistent-field theory calculations by Tyler and Morse [Phys. Rev. Lett. 94, 208302 (2005)] predict a stable orthorhombic network phase with space group $Fddd$ in very weakly segregated diblock copolymer melts. Here, we examine the predicted stability of this phase within a simple Landau theory of weakly ordered crystals, and within a straightforward extension of Leibler's theory of weakly segregated diblock copolymer melts. An $Fddd$ structure with a ratio of unit cell parameters $(a:b:c)=(1:2:2\sqrt{3})$ is found to compete very closely with the hexagonal $(H)$ and lamellar $(L)$ phases along the predicted $H\text{\ensuremath{-}}L$ phase boundary, and to be stable within a very narrow range of parameters around this metastable boundary.

On the phase transitions in ammonium hexafluoroaluminate(III).

Abstract: The heat capacities of (NH4)3[AlF6] were measured between 11 and 300 K with an adiabatic calorimeter. Two anomalies were found at (193.0±0.3) K and (220.79±0.05) K. The enthalpy and entropy of the phase transitions are (790±150) J mol−1 and (4.2±0.8) J K−1 mol−1 for the former, and (4030±150) J mol−1 and (18.5±0.7) J K−1 mol−1 for the latter, respectively. The total anomalous entropy, (22.7±1.5) J K−1 mol−1, is in agreement with R In 16=23.05 J K−1 mol−1 predicted by a model involving orientational disorder of the hexafluoroaluminate(III) and the ammonium ions. Discontinuities in fluorine T1 and T1ρ at 221 K by pulse NMR measurement also support this model. The potential barrier height of the hindered rotation of the NH4+ ion was estimated to be 7 kJ mol−1 from the heat capacity data. The intermediate phase undercooled and coexisted with the low temperature phase in a limited range of temperature. The upper phase transition is of the first order but the anomalous heat capacity follows the Landau theory of...

Ferroelastic Phase Transition in (NH4)3H(SeO4)2

Abstract: Optical observation of twin structures, X-ray examinations and the measurement of the dielectric constant of (NH 4 ) 3 H(SeO 4 ) 2 were carried out in reference to the phase transition at 29°C ( T c ). It was found that (NH 4 ) 3 H(SeO 4 ) 2 undergoes the improper ferro-elastic phase transition at T c . The crystal is determined to be monoclinic withthe space group A 2/ a below T c and trigonal with the space group R \bar3 m above T c . The crystal has spontaneous strain ( e 22 - e 11 )/2=-3.0×10 -3 and e 13 =3.3×10 -3 at room temperature. It was clarified by both observation of domains and symmetry consideration that the crystal exhibits twin structures with two types of domain walls and three kinds of domain orientations at room temperature. The behaviour of the dielectric constant observed near T c is well explained by the Landau theory.