Phase transitions in perovskites near the tricritical point: an experimental study of KMn1–xCaxF3 and SrTiO3

2000 ◽  
Vol 64 (6) ◽  
pp. 971-982 ◽  
Author(s):  
M. C. Gallardo ◽  
F. J. Romero ◽  
S. A. Hayward ◽  
E. K. H. Salje ◽  
J. del Cerro
Keyword(s):  
Experimental Data ◽  
Experimental Study ◽  
Phase Transitions ◽  
Order Parameter ◽  
Tricritical Point ◽  
Mean Field ◽  
Calorimetric Data ◽  
X Ray ◽  
First Order ◽  
Field Behaviour

AbstractWe present experimental data for the Pm3m-I4/mcm phase transitions in the perovskite crystals KMn1-xCaxF3 and SrTiO3. Comparison of calorimetric data (latent heat and specific heat) with order parameter data (measured with X-ray rocking methods) indicates that these transitions follow mean-field behaviour, and may be described using Landau potentials where the free energy expansion includes terms up to Q6. This potential is characteristic of transitions close to the tricritical point. Comparison of the behaviour of SrTiO3 and KMnF3 indicates that KMnF3 is closer to the tricritical point; a small amount of substitution of Ca for Mn causes the transition to cross the tricritical point from first order to second order behaviour.

FIRST ORDER PHASE TRANSITIONS AND PHASE DIAGRAMS OF SYSTEMS WITH A TWO-COMPONENT ORDER PARAMETER

1994 ◽  
Vol 08 (24) ◽  
pp. 1493-1502
Author(s):  
EVGENIA J. BLAGOEVA
Keyword(s):  
Phase Transitions ◽  
Order Parameter ◽  
Mean Field ◽  
Internal Symmetry ◽  
Field Analysis ◽  
First Order ◽  
Specific Effects ◽  
Expansion Coefficients ◽  
First Order Phase ◽  
Component Order

A complete mean field analysis of the sixth order Landau free energy for a two-dimensional order parameter is presented. Special attention is paid to its application to first order phase transitions. The phase diagram for all the allowed values of the expansion coefficients is constructed and the specific effects arising from the internal symmetry of the order parameter are emphasized.

scholarly journals INFLUENCE OF PRESSURE ON SMECTIC A-NEMATIC PHASE TRANSITION

1999 ◽  
Vol 13 (09n10) ◽  
pp. 275-283 ◽  
Author(s):  
KISOR MUKHOPADHYAY ◽  
PRABIR K. MUKHERJEE
Keyword(s):  
Phase Transition ◽  
Phase Transitions ◽  
Phenomenological Model ◽  
Order Parameter ◽  
Nematic Phase ◽  
Tricritical Point ◽  
Second Order ◽  
Experimental Results ◽  
First Order ◽  
Smectic A

We propose a Landau-de Gennes phenomenological model to describe the pressure induced smectic A-nematic phase transition. The influence of pressure on smectic A-nematic phase transitions are discussed for varying coupling between orientational and translational order parameter with pressure. Increasing the pressure, the first order nematic-smectic A transition becomes second order at a tricritical point which agrees fairly well with available experimental results.

TEMPERATURE DEPENDENCE OF SURFACE TENSION OF LIQUID CRYSTALS AT THE PHASE TRANSITION

1995 ◽  
Vol 09 (03n04) ◽  
pp. 237-242 ◽  
Author(s):  
R. MOLDOVAN ◽  
M. TINTARU ◽  
T. BEICA ◽  
S. FRUNZA ◽  
D. N. STOENESCU
Keyword(s):  
Experimental Data ◽  
Phase Transition ◽  
Surface Tension ◽  
Free Energy ◽  
Surface Layer ◽  
Order Parameter ◽  
Unit Area ◽  
Negative Slope ◽  
First Order ◽  
First Order Transition

The surface tension is calculated as the excess of the free energy per unit area, due to the presence of a surface layer, using Landau–de Gennes expansions, in the hypothesis of a first order transition in the bulk and taking into account the dependence of the surface free energy from the surface tilt angle. The surface order parameter is calculated and surface-ordered phase above the phase transition temperature has been found. A variety of calculated surface tension versus temperature curves with a jump at the phase transition, with positive or negative slope, well describing the experimental data from literature, have been attained.

scholarly journals Thermal evolution of the crystal structure and phase transitions of KNbO 3

2018 ◽  
Vol 5 (6) ◽  
pp. 180368 ◽  
Author(s):  
S. L. Skjærvø ◽  
K. Høydalsvik ◽  
A. B. Blichfeld ◽  
M.-A. Einarsrud ◽  
T. Grande
Keyword(s):  
Crystal Structure ◽  
Phase Transitions ◽  
Thermal Evolution ◽  
Dynamical Instability ◽  
X Ray Diffraction ◽  
Two Phase ◽  
X Ray ◽  
Cell Parameters ◽  
First Order ◽  
Heating And Cooling

The thermal evolution of the crystal structure and phase transitions of KNbO 3 were investigated by high-temperature powder X-ray diffraction and Rietveld refinement of the diffraction data. Two phase transitions from orthorhombic ( Amm 2) to tetragonal ( P 4 mm ) and from tetragonal to cubic ( P m 3 ¯ m ) were confirmed, both on heating and cooling. Both phase transitions are first order based on the observed hysteresis. The mixed displacive and order–disorder nature of the tetragonal to cubic transition is argued based on symmetry and apparent divergence of the atomic positions from pseudo-cubic values. The transition between the orthorhombic and tetragonal phase shows no temperature-dependence for atomic positions and only thermal expansion of the unit cell parameters and is thus discussed in relation to a lattice dynamical instability.

scholarly journals Miscibility studies of two twist-bend nematic liquid crystal dimers with different average molecular curvatures. A comparison between experimental data and predictions of a Landau mean-field theory for the NTB–N phase transition

2016 ◽  
Vol 18 (6) ◽  
pp. 4394-4404 ◽  
Author(s):  
D. O. López ◽  
B. Robles-Hernández ◽  
J. Salud ◽  
M. R. de la Fuente ◽  
N. Sebastián ◽  
...  
Keyword(s):  
Field Theory ◽  
Experimental Data ◽  
Phase Transition ◽  
Liquid Crystal ◽  
Nematic Phase ◽  
Mean Field Theory ◽  
Mean Field ◽  
Landau Model ◽  
First Order ◽  
Miscibility Studies

We have developed a Landau model that predicts a first order twist-bend nematic–nematic phase transition.

scholarly journals Topological signature of first-order phase transitions in a mean-field model

2003 ◽  
Vol 62 (6) ◽  
pp. 775-781 ◽  
Author(s):  
L Angelani ◽  
L Casetti ◽  
M Pettini ◽  
G Ruocco ◽  
F Zamponi
Keyword(s):  
Phase Transitions ◽  
Field Model ◽  
Mean Field ◽  
Mean Field Model ◽  
First Order ◽  
First Order Phase

Structural studies of nematic and smectic phases

1983 ◽  
Vol 309 (1507) ◽  
pp. 145-153 ◽  
Keyword(s):  
Liquid Crystal ◽  
Mean Field ◽  
Structural Studies ◽  
X Ray ◽  
Smectic A ◽  
Smectic C ◽  
X Ray Scattering ◽  
Smectic Phases ◽  
Crystal Phases ◽  
Field Behaviour

The information about liquid crystal phases that can be obtained by light scattering and by high-resolution X-ray scattering is reviewed. Results for the nematic-smectic A transition suggest the de Gennes-McMillan model is correct, but adequate theoretical solutions to the model remain elusive. Recent results on the smectic A to smectic C transition are presented that show unambiguously that it exhibits classic mean-field behaviour and this is explained by a Ginzburg criterion argument. Some preliminary results of a study of a nematic-smectic A transition in a lyotropic material are given and indicate similarity to thermotropic materials.

OBSERVATION OF PHASE TRANSITIONS AT THE (100) SURFACE OF Al-3 at.% Ag

1995 ◽  
Vol 02 (02) ◽  
pp. 141-145 ◽  
Author(s):  
E. WETLI ◽  
M. HOCHSTRASSER ◽  
D. PESCIA ◽  
M. ERBUDAK
Keyword(s):  
Phase Transition ◽  
Phase Transitions ◽  
Photoelectron Spectroscopy ◽  
Short Range ◽  
Short Range Order ◽  
Elevated Temperatures ◽  
X Ray ◽  
First Order ◽  
First Order Phase Transition ◽  
First Order Phase

In the bulk binary alloy Al-3 at.% Ag , Ag 2 Al precipitates are formed below 410°C which are reversibly dissolved at elevated temperatures. We have followed this phase transition at a (100) surface as a function of temperature by monitoring the bandwidth of the Ag 4d states in X-ray photoelectron spectroscopy. Since the bandwidth measures the coordination number of the emitting atoms, it directly reveals the short-range order of the Ag atoms at the surface. The measurements show that the dissolution of the Ag -rich clusters starts at temperatures at least 100 K below the bulk transition, and the observed hysteresis behavior is indicative of a first-order phase transition at the surface.

A MEAN FIELD STUDY OF THE PHASE TRANSITIONS IN A CLASS OF SYSTEMS WITH COMPLEX ORDER PARAMETERS

1994 ◽  
Vol 08 (28) ◽  
pp. 3963-3986
Author(s):  
EVGENIA J. BLAGOEVA
Keyword(s):  
Phase Diagram ◽  
Phase Transitions ◽  
Order Parameter ◽  
Mean Field ◽  
Field Approximation ◽  
Mean Field Approximation ◽  
Unconventional Superconductors ◽  
Complex Order ◽  
The Mean ◽  
Expansion Coefficients

A generalized Landau free energy for a complex order parameter expanded up to sixth-order is investigated using group theoretical arguments and the mean-field approximation. Results for the phase transitions that occur are presented. The phase diagram for all allowed values of the expansion coefficients is constructed with an emphasis placed on the influence of the anisotropy in the order parameter space. The results can be used in discussions of unconventional superconductors and modulated structural and magnetic orderings.

scholarly journals Multi-mode excitation drives disorder during the ultrafast melting of a C4-symmetry-broken phase

2022 ◽  
Vol 13 (1) ◽  
Author(s):  
Daniel Perez-Salinas ◽  
Allan S. Johnson ◽  
Dharmalingam Prabhakaran ◽  
Simon Wall
Keyword(s):  
Phase Transitions ◽  
Order Parameter ◽  
Landau Theory ◽  
Mean Field ◽  
Atomic Scale ◽  
Time Dependent ◽  
Ginzburg Landau ◽  
Mode Excitation ◽  
The Mean ◽  
Multi Mode

AbstractSpontaneous C4-symmetry breaking phases are ubiquitous in layered quantum materials, and often compete with other phases such as superconductivity. Preferential suppression of the symmetry broken phases by light has been used to explain non-equilibrium light induced superconductivity, metallicity, and the creation of metastable states. Key to understanding how these phases emerge is understanding how C4 symmetry is restored. A leading approach is based on time-dependent Ginzburg-Landau theory, which explains the coherence response seen in many systems. However, we show that, for the case of the single layered manganite La0.5Sr1.5MnO4, the theory fails. Instead, we find an ultrafast inhomogeneous disordering transition in which the mean-field order parameter no longer reflects the atomic-scale state of the system. Our results suggest that disorder may be common to light-induced phase transitions, and methods beyond the mean-field are necessary for understanding and manipulating photoinduced phases.

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