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Triangular and hexagonal multilayer phosphorene quantum dots with armchair and zigzag terminations are investigated with the orthogonal tight-binding model. The effect of increasing the number of layers is revealed. The obtained results show that in a small size multilayer quantum dot, the edge states are as sensitive to the out-of-plane external electric fields as the edge states in a single layer dot to the in-plane external electric fields. The investigated optical absorption cross sections show that armchair phosphorene quantum dots have a regular behavior which should be useful for infrared detectors. In particular, it was found that in hexagonal armchair phosphorene dots, absorption peaks can be increased, decreased, or totally removed from the low-energy region depending on the orientation of the applied electric field. The effect of spurious doping can suppress the transitions < 0.4 eV, while the effect of the finite temperature is almost negligible. [ABSTRACT FROM AUTHOR]

Phase transitions in gadolinium potassium niobate GdK2Nb5O15 (GKN) ceramics have been investigated by x-ray diffraction, dielectric susceptibility, differential scanning calorimetry, and Raman scattering. The results of our complementary studies show that GKN exhibits two phase transitions at Tc1 = 238 °C and Tc2 = 375 °C attributed to the ferroelectric-antiferroelectric-paraelectric (FE-AFE-PE) phase transitions. According to the x-ray diffraction analysis, the FE and PE phases were refined in the polar P4bm and centrosymmetric P4/mbm space groups. For the intermediate phase, the structure is refined in the space group P4nc with doubling of the c unit cell parameter, which is compatible with an AFE phase. This result was confirmed by Raman spectroscopy since new low-frequency lines are activated in the AFE phase due to the Brillouin zone-folding effect, typical for the modulated phases. The presence of the AFE phase between the FE and PE phases and the complex nature of tetragonal tungsten bronze crystal structure can explain the large thermal hysteresis observed at the FE-AFE transition between heating and cooling cycle and the strong depression of the Curie-Weiss temperature T0. The semi-phenomenological Ising-like model based on the obtained experimental data is proposed to account for the observed FE-AFE-PE transition sequence. [ABSTRACT FROM AUTHOR]

In this paper the description of the dielectric and ferroelectric properties of BaTiO3 single crystals using the Landau phenomenological thermodynamic potential was addressed. Our results suggest that when using the sixth-power free energy expansion of the thermodynamic potential, remarkably different values of the fourth-power coefficient, β (the coefficient of Pi4 terms), are required to adequately reproduce the nonlinear dielectric behavior of the paraelectric phase and the electric field induced ferroelectric phase (Eb|[001]), respectively. In contrast, the eighth-power expansion with a common set of coefficients enables a good description for both phases at the same time. These features, together with the data available in literature, strongly attest to the necessity of the eighth-power terms in Landau thermodynamic potential of BaTiO3. In addition, the fourth-power coefficients, β and ξ (the coefficient of Pi2Pj2 terms), were evaluated from the nonlinear dielectric responses along [001], [011], and [111] orientations in the paraelectric phase. Appreciable temperature dependence was evidenced for both coefficients above TC. Further analysis on the linear dielectric response of the single domain crystal in the tetragonal phase demonstrated that temperature dependent anharmonic coefficients are also necessary for an adequate description of the dielectric behavior in the ferroelectric phase below TC. As a consequence, an eighth-power thermodynamic potential, with some of the anharmonic coefficients being temperature dependent, was proposed and compared with the existing potentials. In general the potential proposed in this work exhibits a higher quality in reproducing the dielectric and ferroelectric properties of this prototypic ferroelectric substance. [ABSTRACT FROM AUTHOR]

We have performed x-ray diffraction and Raman spectroscopy measurements in the temperature range of 300–873 K on a single phase epitaxially oriented BaTiO[sub 3] thin film grown by pulsed laser deposition on a single crystal MgO substrate. The θ–2θ room temperature diffraction measurements and asymmetric rocking curves indicate that the film is very weakly tetragonal with the c-axis parallel to the plane of the film. X-ray diffraction measurements up to high temperature reveal only a change in slope in the perpendicular to the plane lattice parameter around 450 K (in bulk T[sub c]=395 K) indicating that a diffuse-like of phase transition is taking place. Room temperature polarized Raman spectra show that the film is indeed tetragonal with C[sub 4v] symmetry and with the a-axis perpendicular to the film plane. Monitoring of the overdamped soft mode and the 308 cm[sup -1] mode confirms that the phase transition is taking place over a wide temperature range according to the x-ray results. The increase of the phase transition temperature is attributed to the stress effect induced by the substrate. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Polarized Raman spectra of the epitaxial Ba0.5Sr0.5TiO3 film, bi-color BaTiO3/Ba0.5Sr0.5TiO3 superlattice and tri-color BaTiO3/Ba0.5Sr0.5TiO3/SrTiO3 superlattice were studied in a broad temperature range of 80–700 K. Based on the temperature dependence of the polar modes we determined the phase transitions temperatures in the studied heterostructures. In the sub-THz frequency range of the spectra, we revealed the coexistence of the Debye-type central peak and soft mode in bi-color BaTiO3/Ba0.5Sr0.5TiO3 superlattice. [ABSTRACT FROM AUTHOR]

BaTiO3/SrTiO3superlattice has been grown on (001)MgO substrate by pulsed laser deposition. The unit cell parameters of BaTiO3and SrTiO3layers and modulation period have been determined using X-ray diffraction. It was found that BaTiO3and SrTiO3layers revealed the pseudo-tetragonal distortion with out-of-plane and in-plane polarization, respectively. Electrostatic interaction of the alternating layers leads to the polarization rotations and to the symmetry lowering of the superlattice to orthorhombic or monoclinic with partial depolarization of the Raman spectra. It was found that the phase transition to the paraelectric phase occurs at ∼480 K. [ABSTRACT FROM PUBLISHER]

The multicomponent Ising-like network model was developed to describe the ferroelectric phase transition in Tetragonal Tungsten Bronze (TTB) compounds. Using Monte Carlo simulation with Metropolis algorithm we reproduced the phase diagram of this model as a function of the temperature and exchange interaction and demonstrated it coherence with experimental data. [ABSTRACT FROM AUTHOR]

Extremely regular self-organized patterns of 90° ferroelastic domains have been reported in free-standing single crystal thin films of ferroelectric BaTiO3. Lukyanchuk et al . [Phys Rev B 79, 144111 (2009)] have recently shown that the domain size as a function of thickness for such free standing films can be well described assuming that the domains are due to stress caused by a surface tension layer that does not undergo the paraelectric–ferroelectric transition. From the starting point of Lukyanchuk’s model, it is shown here that the “universal” relationship between domain size and domain wall thickness previously observed in ferroelectrics, ferromagnets and multiferroics is also valid for ferroelastic domains. Further analysis of experimental data also shows that the domain wall thickness can vary considerably (an order of magnitude) from sample to sample even for the same material (BaTiO3), in spite of which the domain size scaling model is still valid, provided that the correct, sample dependent, domain wall thickness is used. [ABSTRACT FROM AUTHOR]

Extremely regular self-organized patterns of 90° ferroelastic domains have been reported in free-standing single crystal thin films of ferroelectric BaTiO3. Lukyanchuk et al . [Phys Rev B 79, 144111 (2009)] have recently shown that the domain size as a function of thickness for such free standing films can be well described assuming that the domains are due to stress caused by a surface tension layer that does not undergo the paraelectric–ferroelectric transition. From the starting point of Lukyanchuk’s model, it is shown here that the “universal” relationship between domain size and domain wall thickness previously observed in ferroelectrics, ferromagnets and multiferroics is also valid for ferroelastic domains. Further analysis of experimental data also shows that the domain wall thickness can vary considerably (an order of magnitude) from sample to sample even for the same material (BaTiO3), in spite of which the domain size scaling model is still valid, provided that the correct, sample dependent, domain wall thickness is used. [ABSTRACT FROM AUTHOR]

A new family of rare-earth ferroelectric Tetragonal Tungsten Bronze (TTB) - type compounds with general formula Pb2(1- x) K1+ xGdxNb5O15 was elaborated by solid state reaction. Structural and dielectric properties were investigated using X-ray diffraction and dielectric measurements. The symmetry of the ferroelectric phase strongly depends of the lead concentration. The ferroelectric phase of the compounds with high lead concentration (x < 0.35) was found to be described by the symmetry group Cm2m whereas the compounds with low lead concentration (x > 0.35) have the symmetry space group Pba2. The ferroelectric transition temperature was shown to decrease with the concentration of lead. [ABSTRACT FROM AUTHOR]

X-ray diffraction and Raman spectroscopy were used to examine the structure and the ferroelectric state of highly oriented barium titanate thin film deposited by the pulsed laser deposition on (001)MgO substrate. It is found that the film is weakly tetragonal and domains are a-axis oriented. The temperature dependencies of the out plane lattice parameter and the tetragonal Raman peak at 308 cm -1 show that the ferro-paraelectric phase transition occurs above the the bulk transition temperature and can be considered as diffuse. This behaviour is attributed to the stress effect. [ABSTRACT FROM AUTHOR]