arXiv subjects

A. P. Moina

Publications and source records attributed to A. P. Moina.

At least 19 recordsLinked to original sources

arXiv2025

Phase transitions and polarization switching in quasi-one-dimensional organic ferroelectrics of phenyltetrazole family

Pseudospin models are proposed for description of the phase transitions, dielectric characteristics, and polarization switching in two crystals of the phenyltetrazole family. One of them, APHTZ, is a canted ferroelectric, whereas the other, MPHTZ is a simple antiferroelectric. In APHTZ the electric field, applied perpendicularly to the axis of spontaneous polarization, flips the polarization in one of the two sublattices, effectively rotating the non-zero net polarization by 90$^\circ$ and switching the system between two different ferroelectric configurations. The temperature-electric field phase diagrams are constructed. The diagram topology appears to be typical for the Ising-like antiferroelectric systems.

cond-mat.mtrl-sci↗

arXiv2024

Ising model in the Rényi statistics: the finite size effects

The Rényi statistics is applied for a description of finite size effects in the 1D Ising model. We calculate the internal energy of the spin chain and the system temperature using the Rényi distribution and postulate them to be equal to their counterparts, obtained in the microcanonical ensemble. It allows us to self-consistently derive the Rényi $q$-index and the Lagrange parameter $T$ to relate them to the physically observed system temperature $T_{\rm ph}$, and to show that the entropic phase transitions are possible in a broad temperature domain. We have also studied the temperature dependence of the internal energy $U(T_{\rm ph})$ at constant $q$ and an influence of the size related effects on the system thermodynamics.

cond-mat.stat-mech↗

arXiv2023

Electric field induced polarization rotation in squaric acid crystals revisited

Using the previously developed model we revisit the problem of the electric field induced polarization rotation in antiferroelectric crystals of squaric acid. We test an alternative set of the model parameters, according to which the dipole moments associated with the H$_2$C$_4$O$_4$ groups are assumed to be parallel to the diagonals of the $ac$ plane. The $T$-$E$ phase diagrams and the polarization curves $P(E)$ for the fields directed along the $a$ axis and along one of the diagonals are considered. Comparison of the theoretical results with the newly published experimental data confirm the validity of the model. The calculations reveal no apparent advantage of the new set of the parameters over the previously used set.

cond-mat.mtrl-sci↗

arXiv2022

Negative/positive electrocaloric effect in antiferroelectric squaric acid

Using the previously developed model we study the electrocaloric effect (ECE) in antiferroelectric crystals of squaric acid. At low temperatures, the polarization reorientation by the electric field applied along the crystallographic $a$ axis of these crystals is predicted to be a two-stage process, with one of the sublattice polarizations switched twice, by 90$^\circ$ each time. The $T$-$E$ landscapes of the model polarization and entropy are explored. The ECE, characterized by the introduced electric Grüneisen parameter, is found to be negative in the antiferroelectric phase. In the intermediate ferrielectric phase with perpendicular sublattice polarizations it is positive at low fields, but becomes slightly negative below the transition to the ferroelectric phase. In the ferroelectric phase the ECE is positive at all temperatures and fields. Maximal EC temperature shift magnitude is predicted to be just less than -3 K at 200 kV/cm. The supercritical behavior of the Grüneisen parameter in the crossover region between two bicritical end points is explored.

cond-mat.mtrl-sci↗

arXiv2021

Influence of external field direction on polarization rotation in antiferroelectric squaric acid H$_2$C$_4$O$_4$

Using the previously developed model we explore the processes of polarization rotation in antiferroelectric crystals of squaric acid by the electric fields directed arbitrarily within the $ac$ plane. Except for some particular directions of the field, the two-step polarization reorientation at low temperatures is predicted: first, to the noncollinear phase with perpendicular sublattice polarizations and then to the collinear ferroelectric phase. However, when the field is directed along the axis of spontaneous sublattice polarizations, the intermediate noncollinear phase is absent; when the field is at 45$^\circ$ to this axis, the field for transition to the ferroelectric phase tends to infinity. The ground state proton configurations and the directions of the sublattice polarization vectors are determined for all field orientations. The $T$-$E$ phase diagrams are constructed for the fields directed along the diagonals of the $ac$ plane and for the above discussed particular directions of the field.

cond-mat.mtrl-sci↗

arXiv2020

Polarization rotation by external electric field in two-dimensional antiferroelectric squaric acid, H$_2$C$_4$O$_4$

A pseudospin model for description of the influence of the electric field, confined to the plane of sublattice polarization, on the two-dimensional squaric acid antiferroelectrics is developed. The system behavior is analyzed in terms of the parameters of ferroelectric and antiferroelectric ordering, as well as the non-collinearity angle $θ$, which is the angle between sublattice polarizations. The temperature-electric field $T-E_1$ phase diagram is constructed. Most of the field-induced transitions are found to be associated with polarization rotation. The observed phases include the collinear ferroelectric and non-collinear ferrielectric with almost antiparallel and perpendicular polarizations of the sublattices, respectively. The diagram also contains two regions, where the non-collinearity angle varies continuously between, nominally, 0 and 90$^\circ$ and between 90$^\circ$ and 180$^\circ$. The first and second order transition lines, supercritical lines, critical end points, and critical points on the $T-E_1$ phase diagram are detected.

cond-mat.mtrl-sci↗

arXiv2020

Effects of diagonal strains and H-bond geometry in antiferroelectric squaric acid crystals

The proton ordering model of the phase transition and physical properties of antiferroelectric crystals of squaric acid is modified by taking into account the influence of diagonal lattice strains and of the local geometry of hydrogen bonds, namely of the distance $δ$ between the H-sites on a bond. Thermal expansion, the spontaneous strain $\varepsilon_1-\varepsilon_3$, and specific heat of squaric acid are well described by the proposed model. However, a consistent description of hydrostatic pressure influence on the transition temperature is possible only with further modifications of the model.

cond-mat.mtrl-sci↗

arXiv2015

Piezocaloric and multicaloric effect in the KH2PO4 type ferroelectrics

Using the proton ordering model modified by taking into account the dependence of the dipole moments on the order parameter [Vdovych et al, 2014], we explore the piezocaloric and multicaloric effects in the KH$_{2}$PO$_{4}$ type ferroelectrics, caused by the shear stress $σ_6$ and longitudinal electric field $E_3$. The multicaloric effect is shown to be stronger than either electrocaloric or piezocaloric effects, especially at temperatures far from the phase transition.

cond-mat.mtrl-sci↗

arXiv2015

Electrocaloric effect in KH$_2$PO$_4$ family crystals

The proton ordering model for the KH$_{2}$PO$_{4}$ type ferroelectrics is modified by taking into account the dependence of the effective dipole moments on the proton ordering parameter. Within the four-particle cluster approximation we calculate the crystal polarization and explore the electrocaloric effect. Smearing of the ferroelectric phase transition by a longitudinal electric field is described. A good agreement with experiment is obtained.

cond-mat.mtrl-sci↗

arXiv2014

Electrocaloric effect in KH$_2$PO$_4$

The proton ordering model for the KH$_{2}$PO$_{4}$ type ferroelectrics is modified by taking into account non-linear effects, namely, the dependence of the effective dipole moments on the proton ordering parameter. Within the four-particle cluster approximation we calculate the crystal polarization, longitudinal dielectric permittivity, specific heat, and explore the electrocaloric effect. Smearing of the ferroelectric phase transition by the longitudinal electric field is described. A good agreement with experiment is obtained.

cond-mat.mtrl-sci↗

arXiv2013

Transverse Piezoelectric Resonance in KH$_2$PO$_4$ Type Crystals

Within the framework of proton model with taking into account the piezoelectric interaction with the shear strain $\varepsilon_4$, a dynamic dielectric response of KH$_2$PO$_4$ family crystals to the electric field perpendicular to the axis of spontaneous polarizaiton is considered. Piezoelectric resonance frequencies of rectangular thin plates of the crystals cut in the (100) plane (0$^\circ$ X-cut) are calculated.

cond-mat.mtrl-sci↗

arXiv2013

Defect-mediated relaxation and non-linear susceptibilities of Rochelle salt

The deformable pseudospin Mitsui model is modified in order to take into account interactions of the ordering dipoles of Rochelle salt with dipoles, associated with switchable crystal defects. Using the Glauber-type kinetics of the ordering and defect pseudospins, we calculate the linear, second, and third order dynamic susceptibilities and piezoelectric coefficients of the system. The defect-assisted dispersion of the dynamic characteristics below 1 kHz is described. Behavior of the linear and non-linear susceptibilities close to T_C1,2 is also satisfactorily described by the presented model.

cond-mat.mtrl-sci↗

arXiv2013

Piezoelectric Resonance in KH2PO4 Type Crystals Revisited

Within the framework of proton ordering model with taking into account the piezoelectric interaction with the shear strain $\varepsilon_6$, a dynamic dielectric response of KD2PO4 type ferroelectrics is considered. An expression for the piezoelectric resonance frequencies of the rectangular thin plate of the crystal cut in the (001) plane is obtained.

cond-mat.mtrl-sci↗

arXiv2012

Major physical characteristics of Rochelle salt: the role of thermal strains

We compare the results for the related to the shear strain ε_4 physical characteristics of Rochelle salt obtained within the recently developed modified two-sublattice Mitsui model that takes into account the strain ε_4 and the diagonal components of the strain tensor ε_1, ε_2, ε_3 with the results of the previous modification of the Mitsui model with the strain ε_4 only. Within the framework of the model with the diagonal (thermal expansion) strains, we also reexamine the effects of the longitudinal electric field E_1 on the dielectric properties of Rochelle salt.

cond-mat.mtrl-sci↗

arXiv2012

Piezoelectric resonance in Rochelle salt: the contribution of diagonal strains

Within the framework of two-sublattice Mitsui model with taking into account the shear strain $\varepsilon_4$ and the diagonal strains $\varepsilon_2$ and $\varepsilon_3$, a dynamic dielectric response of Rochelle salt X-cuts is considered. Experimentally observed phenomena of crystal clamping by high frequency electric field, piezoelectric resonance and microwave dispersion are described. It is shown that the lowest resonant frequency is always associated with the $\varepsilon_4$ shear mode

cond-mat.mtrl-sci↗

arXiv2011

Mitsui model with diagonal strains: A unified description of external pressure effect and thermal expansion of Rochelle salt NaKC_4H_4O_6\cdot4H_2O

We elaborate a modification of the deformable two-sublattice Mitsui model of [Levitskii R.R. et al., Phys. Rev. B. 2003, 67, 174112] and [Levitskii R.R. et al., Condens. Matter Phys., 2005, 8, 881] that consistently takes into account diagonal components of the strain tensor, arising either due to external pressures or due to thermal expansion. We calculate the related to those strains thermal, piezoelectric, and elastic characteristics of the system. Using the developed fitting procedure, a set of the model parameters is found for the case of Rochelle salt crystals, providing a satisfactory agreement with the available experimental data for the hydrostatic and uniaxial pressure dependences of the Curie temperatures, temperature dependences of spontaneous diagonal strains, linear thermal expansion coefficients, elastic constants c_ij^E and c_i4^E, piezoelectric coefficients d_1i and g_1i (i=1,2,3). The hydrostatic pressure variation of dielectric permittivity is described using a derived expression for the permittivity of a partially clamped crystal. The dipole moments and the asymmetry parameter of Rochelle salt are found to increase with hydrostatic pressure.

cond-mat.mtrl-sci↗

arXiv2005

Influence of external factors on dielectric permittivity of Rochelle salt: humidity, annealing, stresses, electric field

The present work contains results of experimental investigation of external factors, such as dessicating/wetting, thermal annealing, uniaxial and hydrostatic pressures on dielectric permittivity of Rochelle salt crystals. The obtained results are compared with available literature data. A conslusion is made that the dispersion of experimental data can be attributed to internal polar point defects in crystals and to influence of storage conditions. The obtained results are analyzed within the phenomenological Landau approach.

cond-mat.mtrl-sci↗

arXiv2000

Hydrostatic pressure influence on dielectric permittivity of KH2PO4 and KD2PO4 in the piezoelectric resonance region

We measured transverse and longitudinal dielectric permittivities of KDP and DKDP in the piezoelectric resonance frequency region under the hydrostatic pressure. The transverse permittivity decreases with pressure in the paraelectric phase and increases in the ferroelectric phase. The pressure dependence of the transverse permittivity of deuterated DKDP is well described by the presented theory. The longitudinal permittivity of a pure KDP exhibits several peaks in the vicinity of the transition point. Upon increasing hydrostatic pressure, the peaks get closer and may merge.

cond-mat.mtrl-sci↗