On the phase transition in foreign phase inclusions in CsH2AsO4, KD2PO4, and KH2PO4 crystals

The low-frequency internal friction Q ^?1 and the shear modulus G in a paraelectric phase of CsH₂AsO₄, KD₂PO₄, and KH₂PO₄ ferroelectrics were studied using a reversed torsion pendulum method. Anomalies in the Q ^?1(T) and G(T) dependences were observed above the Curie temperatures of these crystals, at temperatures 308, 253, and 293 K, respectively. The anomalies were associated with a first-order phase transition $(\bar 42m \to mm2)$ occurring in the foreign phase inclusions.     

Displacive and order-disorder behavior of KDP-type ferroelectrics on the local scale

The modified strong dipole-proton coupling (MSDPC) model, which predicted several static and dynamic dielectric properties of KH₂PO₄ or KDP-type ferroelectrics, was used to investigate the properties of these crystals on the local scale. Results calculated by molecular dynamics (MD) simulation show that both order-disorder and displacive characteristics of one PO₄ dipole are present in KDP and KD₂PO₄ (DKDP). These results correlate with experimental data from NMR and neutron scattering studies of local properties.     

On the transverse electrical susceptibilities of KDP type crystals

Using statistical Green's function technique exact expression for the transverse electrical susceptibility (x_$\text{a}$) for KDP type crsytals has been derived with the total pseudo-spin lattice coupled mode Hamiltonian. Very good fittings of the experimental data of x_$\text{a}$ for KH₂PO₄, RbH₂PO₄ and KH₂AsO₄ with the derived expression have been obtained with a single set of Blinc-de Geenes (BG) parameters being different for different salts. The temperature dependence of the E-mode character along the transverse $\text{a}$-axis has also been studied theoretically and compared with the recent experimental results available for KH₂PO₄ only.     

Ultrasonic propagation near the ferroelectric phase transition temperature in RbH2PO4

Ferroelectric phase transition in RbH₂PO₄ is studied by the propagation of longitudinal ultrasonic waves along the polar axis. The velocity data are analyzed to obtain the temperature dependence of the soft mode frequency in the ferroelectric phase. Unlike KH₂PO₄ and KD₂PO₄, the attenuation in RbH₂PO₄ does not exhibit a relaxation type behaviour.     

Far-infrared reflectivity spectra of cesium dihydrogen arsenate,in para- and ferroelectric phases

The temperature dependence of the infrared (IR) reflectivity spectra of CsH₂AsO₄ is studied for the first time, for both polarizations parallel and perpendicular to the ferroelectric c-axis, in the para-and ferroelectric phase, in the frequency range 10–600 cm⁻¹. The results obtained are compared and discussed in relation to the well known IR reflectivity spectroscopic studies of KH₂PO₄.     

Intermediate-temperature polymorphic phase transition in KH2PO4: A synchrotron X-ray diffraction study

We have used synchrotron X-ray diffraction to investigate the structural and chemical changes undergone by polycrystalline KH₂PO₄ (KDP) upon heating within the 30-250 °C temperature interval. Our data show evidence of a polymorphic transition at T∼190 °C from the room-temperature tetragonal KDP phase to a new intermediate-temperature monoclinic KDP modification (spacegroup P2₁/m and lattice parameters a=7.590, b=6.209, c=4.530 Å, and β=107.36°). The monoclinic RDP polymorph remains stable upon further heating to 235 °C, and is isomorphic to its RbH₂PO₄ and CsH₂PO₄ counterparts.     

The infra-red spectra of some ferroelectric compounds with short hydrogen bonds

The infra-red spectra of KH₂PO₄, NH₄H₂PO₄, NaH₂PO₄, KH₂AsO₄, NH₂H₂AsO₄, Ag₂H₃IO₆ and (NH₄)₂H₃IO₆ and of their deuterated analogues have been recorded at room temperature and some of them also at low temperature in the ferroelectric phase. The interpretation of the particularly interesting spectral region between 3000 and 1500 cm^?1, containing several OH bands, has been made in terms of the tunnelling of the protons between two minima of potential energy. These were taken to be of equal depth in the non-ferroelectric phase and unsymmetrical in the ferroelectric form. A quantum-mechanical treatment of the vibrational problem of the latter type has been carried out and is presented in the Appendix. Good agreement has been found between the theoretically predicted energy levels and the experimental data.     

Light scattering spectra of polarization fluctuations and models of the phase transition in KDP type ferroelectrics

Abstract

Recent light scattering studies in KH₂PO₄ type hydrogen-bonded crystals are reviewed. Dynamical properties of these crystals have been discussed in terms of the coupled tunneling proton-optical phonon model up to date. Light scattering spectra in several GHz and cm^?1 regions with various scattering geometries observed in KH₂PO₄, KD₂PO₄ and their mixed crystals, however, revealed that all these spectra are well analysed in terms of a coupled polarization relaxation mode-acoustic phonon system. Since most of the spontaneous polarization is due to shifts of K, P and O ions, relaxation motions of H₂PO₄ dipoles arc concluded to be the origin of these low-frequency spectra. Models of the phase transition mechanism are discussed on the basis of these results and other recent experimental studies including the explanation of large isotope effects in these crystals.     

Paramagnetic resonance of the (Cr5+O4)H2 Halperin-Varma center in NH4H2AsO4 and KH2AsO4

The electron paramagnetic resonance (EPR) spectrum of Cr⁵⁺(3d ¹,S=1/2) in undeuterated and 58%-deuterated NH₄H₂AsO₄ was investigated. The EPRg-value tensors in the paraelectric and antiferroelectric phases are almost the same as those observed at high and low temperatures in KH₂PO₄ and KH₂AsO₄. This shows that the (Cr⁵⁺O₄)H₂ complex is the same in all crystals, i.e., a wave function ofd _x ²–y ² character coupled to two lateral protons which reorient among the four possible Slater configurations in the paraelectric phase. The remarkable isotope shift of the local dynamic reorientational slowing-down observed in KH₂PO₄ and KH₂AsO₄ in approximate proportion to the shift of the bulk ferroelectric transition temperaturesT _c ^F , and the antiferroelectricT _c ^AF of NH₄H₂AsO₄, is analyzed qualitatively. It is shown to result mainly from the isotope effect of the short-range interactionJ _sr via protons and deuterons for the impurity and for the bulk. Q-band data of the (Cr⁵⁺O₄)H₂ center in 75%-deuterated KH₂AsO₄ are also reported. An averaged high- and nonaveraged low-temperature EPR spectrum is observed in a temperature range of 250 to 290 K. The intensity ratio of the two follows an exp 2(T—T)/ law over four orders of magnitude atT=266 to 273 K and=5.3 to 6.1 K depending on the orientation of the external magnetic field. This novel result in motional narrowing is analyzed analytically to be compatible with a distributionP _A of ₀(T, T, ) of half width, in reorientation times withE=0.23 ±0.02 eV, , probably resulting from the statistical occupation of deuterium atoms among the O—–H–O bridges. Comparison with a theory of Binder definitely proves the extrinsic slowing-down and thus Halperin-Varma type character. In the range of temperatures investigated no local freeze-out has been detected.     

High-energy proton-irradiation effects on the charge transport and electric dipole moment of KH2PO4

We have studied the charge transport and dielectric properties of KH₂PO₄ (KDP) single crystals, irradiated by proton beams with high energies up to 20 MeV, by means of the temperature-dependent electrical conductivity and dielectric constant measurements. It is shown that the electrical conductivity and the electric dipole moment may be sensitively controlled by the proton irradiation energy and dose. The activation energy and the dipole moment were shown to be strongly correlated.     

Dipole ordering and ionic conductivity in NASICON-Type Na<Subscript>3</Subscript>Cr<Subscript>2</Subscript>(PO<Subscript>4</Subscript>)<Subscript>3</Subscript> structures

The aspects of structure, dipole ordering, and ionic conductivity of the Na₃Cr₂(PO₄)₃ crystal with the four polymorphic phases (α, α', β, and γ) have been investigated. The features of the α-Na₃Cr₂(PO₄)₃ crystal structure and its dipole ordering and relaxation polarization in the low-temperature α and α' phases have been refined. The occurrence of Na₃Cr₂(PO₄)₃ dipole ordering in the α and α' phases and high ionic conductivity in the β and γ phases is attributed to the structural changes in the rhombohedral [Me₂(PO₄)₃]^–3_3∞ crystal frame upon phase transformations α → α', α' → β, and β → γ. A model for explaining the dipole ordering and ionic conductivity phenomena in Na₃Cr₂(PO₄)₃ is proposed.     

EPR evidence for local pseudo freeze out of impurity dynamics in KH2PO4 type ferroelectrics and antiferroelectrics

The temperature dependence of the reorientation rates of the AsO₄^4? centers in KH₂AsO₄ and NH₄H₂AsO₄ as well as of the CrO₄^3? centers in KH₂PO₄ shows a strong deviation from an Arrhenius behavior which can be quantitatively described as a result of the local “pseudo freeze-out” of the dynamics of these centers and the associated polarization cloud far above the transition temperature T_C of the host crystal.     

Dynamical,scattering and dielectric properties of ferroelectric crystals

A unified treatment is given of various properties of ferroelectric crystals in their non-polar phases, typified by BaTiO₃, KH₂PO₄ and NaNO₂. Certain relations are established between measurable dielectric and scattering properties. Critical scattering is discussed in terms of lattice dynamics.     

On the drastic temperature broadening of hard mode Raman lines of ferroelectric KDP type crystals near Tc

The drastic broadening of a few A₁ Raman lines of ferroelectric KH₂PO₄(KDP) and KH₂AsO₄(KDA) crystals have been measured at various temperatures near T_c. The results in KDP are in fair agreement with two different simplified expressions of linewidths, which are based on pseudospin-phonon interaction arguments. No such agreement is found between the results and a simplified linewidth expression derived from a theory based on soft mode-hard mode interaction arguments. The linewidths and the reciprocal of the proton spin-lattice relaxation time in KDP exhibit a very similar temperature dependence below T_c but behave differently above T_c. This phenomena is presented in detail, and discussed.     

Transverse piezoelectric properties of KH2PO4 type crystals

It is shown that the anomalous temperature dependence of the transverse piozoelectric constant d₁₄ for KH₂PO₄ type crystals is the result of the antiferroelectric-like properties along the x-axis.     

Crystal stability and the theory of ferroelectricity part II. Piezoelectric crystals

In Part I the dielectric properties of cubic crystals with atoms in special positions were discussed, with particular reference to crystals which become unstable against a transverse optic mode of vibration of long wavelength. These considerations are now extended to crystals of general symmetry, and it is shown that the anomalous temperature dependence of certain dielectric, piezoelectric, and elastic constants in the paraelectric phase of ferroelectric crystals can be described and correlated in a very natural way in terms of crystal stability. A formula relating the dispersion frequency of the anomalous dielectric constant to the spontaneous polarization and the displacements of the atoms in a ferroelectric transition, previously derived for barium titanate, is generalized. The transitions in KH₂PO₄ and in NH₄H₂PO₄, and the ordering of the hydrogen atoms in these crystals, are discussed qualitatively in terms of the stability of normal modes of vibration.     

Temperature dependence of the far infrared reflectivity spectra of a KD2PO4

Complete data on far infrared reflectivity spectra of a KD₂PO₄ single crystal in the temperature range 7–300 K, from the millimeter wave up to 400 cm^–1, for all crystallographic orientations of the crystal, in the ferro- and paraelectric phase, are given for the first time. The temperature dependence of the phonon spectra is similar to that we have observed and published for KH₂PO₄.     

Anisotropic domain-wall dynamics in proton-irradiated KH2PO4

KH₂PO₄ single crystals have been studied by employing complex impedance measurements in view of the domain freezing effect. As a result, distinct behaviors of the anisotropic domain-wall dynamics in the activation energy of domain freezing and the Vogel–Fulcher temperature before and after proton irradiation have been identified in the anisotropic crystal structure.     

A Statistical Theory of Dipole Ordering in Solid Solutions of ADP- and KDP-Crystals with the H22 Structure

A theory of dipole ordering in solid solutions of isomorphic ferro- and antiferroelectrics with the H₂₂ structure is developed. Free energies and thermodynamic potentials of the systems are calculated. Equilibrium conditions are studied. The Curie temperatures for phase transitions are found. The K_1–x (NH₄)_xH₂PO₄, M_1–x M_xH₂PO₄ (M, M = K, Rb, Cs), and (NH₄)_1–x Tl_xH₂PO₄ are examined. The temperature dependence of the shear modulus and dielectric constant in the vicinity of the point of phase transition is estimated for dihydrophosphate potassium ammonate. The calculation results are compared with experiment.     

Features of the behavior of longitudinal static and dynamic dielectric, piezoelectric, and elastic characteristics of KH2PO4 and NH4H2PO4 crystals

Along with their electromechanical coupling coefficients, the longitudinal dielectric, piezoelectric and elastic characteristics in ferroelectric KH₂PO₄ and antiferroelectric NH₄H₂PO₄ crystals are calculated using a modified proton ordering model that considers piezoelectric coupling and the four-particle cluster approximation. The possibility of detecting piezoactivity in solid solutions of K_{1 − x} (NH₄)xH₂PO₄ is substantiated.