Dielectric study of the ferroelectric phase transition of KH2PO4

We report high precision measurements of the dielectric constant of Potassium Dihydrogen Phosphate as a function of temperature and electric field. Landau model parameters are evaluated and the biasing field E_c which drives the transition second order is determined.     

Anomalous light scattering at the ferroelectric phase transition of KH2PO4

The temperature-dependent scattered light intensity accompanying the ferroelectric phase transition of KH₂PO₄ is presented as an isofrequency-temperature scan. The temperature dependence of the data is shown to be better fit to the coupled harmonic oscillator model than to the single overdamped oscillator. This mode of data presentation also displays the anomalous light scattering near the phase transition.     

On the phase transition in KH2PO4

The dynamical theory of the phase transition in KDP recently developed by Kobayashi (J. Phys. Soc. Japan24 (1968), 497) is modified by taking into account the interaction of the a-polarized transverse acoustic phonon both with the proton tunnelling mode and the transverse optic phonon polarized along thec-axis. The coupled modes of this three-component system are found in the paraelectric phase within the framework of the RPA approximation. The expressions for the sound velocity in the short-circuited (E = 0) crystal and the static susceptibility at constant stress are derived and well known thermodynamic formulae are obtained if the re-normalized elastic constantc ₆₆ ^P and the piezoelectric constanta ₃₆ are introduced. It is shown that these so-called true constants should in principle exhibit anomalies near the proton ordering temperature. Also the role of the depolarizing energy in the phase transition of KDP is discussed. The shift of the phase transition temperature of a small insulated particle of KDP is shown to be only a few degrees. Finally, it is explained why the domain structure of a short-circuited or insulated crystal should be essentially the same.The author thanks Dr. J. Fousek and Dr. V.Janovec of the Institute of Physics in Prague for many valuable discussions of this problem.     

Comment on “dielectric study of the ferroelectric transition of KH2PO4”

We reanalyze a.c. dielectric data taken by Eberhard and Horn for KH₂PO₄ near T_c, and obtain a much lower value of the critical bias field, near 300 V/cm in accord with results found by other workers.     

Transient optical absorption of hole polarons in ADP (NH4H2PO4) and KDP (KH2PO4) crystals

A study of transient optical absorption of the ADP (NH₄H₂PO₄) and KDP (KH₂PO₄) nonlinear crystals in the visible and UV spectral regions is reported. Measurements made by absorption optical spectroscopy with nanosecond-time resolution established that the transient optical absorption (TOA) of these crystals originates from optical transitions in the hole A and B radicals and the optical-density relaxation kinetics is rate-controlled by interdefect tunneling recombination, which involves these hole centers and the electronic H⁰ centers representing neutral hydrogen atoms. At 290 K, hole polarons and the H⁰ centers undergo thermally stimulated migration, which is not accompanied by carrier ejection into the conduction or valence band. The slow components of the TOA kinetics with characteristic times from a few tens of milliseconds to a few seconds can be assigned to diffusion-controlled annihilation of hydrogen vacancies associated with impurity or structural defects.     

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.     

Phase mixing in the critical region of the ferroelectric phase transition in KD2PO4

A novel neutron high resolution double crystal arrangement has been used to measure changes in d-spacing, d-spread and mosaicity through the first order ferroelectric phase transition of DKDP. Coexistence of both para and ferroelectric phases was directly observed in the range T_c to T_c — 3 K. No hysteresis could be detected within the precision of the temperature measurement (10^-3K). The origin of the overshoot effect has been elucidated.     

The pH effects on the growth rate of KDP (KH2PO4) crystal by investigating Raman active lattice modes

We report on the dependence of the pH value on the growth rates of KDP single crystals. Extensive experimental work has been carried out in order to find the optimum pH ranges for growing KDP single crystals with suitable sizes and high optical quality. Different techniques including micro‐Raman back‐scattering spectroscopy, UV/vis/IR transmission spectroscopy and X‐ray diffraction have been employed for this investigation. Deuterated substituted single crystals of KDP and DKDP also have been grown for the investigation of growth rates and Raman active mode identification purposes. The molecular vibration modes of the grown crystals, including internal modes of PO₄ tetrahedrons molecular vibrations, external modes of optical phonons and hydrogen bonding modes have been determined exactly by micro‐Raman back‐scattering spectroscopy. The best pH values of the solution for the KDP crystal growth with reasonably higher growth rates from aqueous solutions that have been supersaturated ata temperature range of 30–50 °C have been found to be in the pH range of 3.2–5.4. Copyright © 2007 John Wiley & Sons, Ltd.     

Propagation of longitudinal ultrasonic waves in RbH2PO4 near its ferroelectric phase transition temperature

Ferroelectric phase transition in RbH₂PO₄ has been investigated using propagation of longitudinal acoustic waves along the polar axis near the transition temperature. The velocity of this mode is continuous across the transition temperature. Velocity data in the ferroelectric phase are analyzed in terms of coupled soft modeacoustic mode model of Pytte to obtain the temperature dependence of the soft mode frequency. The attenuation data in the ferroelectric phase show power law dependence. It follows scaling behaviour of the type predicted by Kawasaki from the mode-mode coupling theory and the dynamical scaling.     

High-pressure neutron diffraction studies of KH2PO4-type phase transitions

Abstract

High-pressure neutron-diffraction studies have been carried out on KH₂PO₄, KD₂PO₄, H₂C₄O₄ and PbHPO₄, using single-crystal samples. The results show that changes in the H-ordering temperature, T_C, with pressure (or deuteration) in these systems can be largely attributed to the variation of the H-site separation, δ, with pressure (or deuteration).     

Microscopic fluorescence imaging of bulk defect clusters in KH(2)PO(4) crystals

A microscopic fluorescence imaging system is used to detect optically active centers located inside a transparent dielectric crystal. Defect centers in the bulk of KH(2)PO(4) crystals are imaged based on their near-infrared emission following photoexcitation. The spatial resolution of the system is 1mum in the image plane and 25mum in depth. The experimental results indicate the presence of a large number of optically active defect clusters in different KH(2)PO(4) crystals, whereas the concentration of these clusters depends on the crystal sector and growth method.     

Emergence of supersteps on KH2PO4 crystal surfaces

In situ AFM investigation of growth on the [100] face of KH2PO4 in the presence of Al(III) and other trivalent metals reveals the emergence of a new type of morphological feature-the superstep. Supersteps, or step bunches consisting of 50-1500 elementary steps, are responsible for growth at all supersaturations and exhibit behavior not predicted by accepted models. The step velocity of the superstep is greater than that of single atomic steps and increases with step height. The steepness of the step riser reaches a limiting value of only 11.8 degrees.     

Vibrational study of the superprotonic phase transition in the mixed crystal Rb2(HSeO4)(H2PO4)

Raman spectra (10–1200 cm⁻¹) of polycrystalline samples of Rb₂(HSeO₄)(H₂PO₄) were studied at temperatures ranging from 300 to 423 K. An assignment of most of the observed bands is proposed. The first‐order phase transition previously detected at 382 K was characterized as: This superionic‐protonic transition is believed to be governed by librations of the HSe/PO₄²⁻ ion and the A OH (A = Se, P) stretching mode. It corresponds to the weakening of  Se(P) O H˙˙˙ H O Se(P) hydrogen bonds and to the melting of the proton sublattice into a quasi‐liquid state in which the protons and the HSe/PO₄²⁻ ions contribute to the unusually high conductivity. The activation energy that was determined from the plot Δν_1/2 versus temperature for the ν (A OH) band has the same order of magnitude as that determined from conductivity measurements. Copyright © 2006 John Wiley & Sons, Ltd.     

Order-disorder transition in Na3Bi(PO4)2

Na₃Bi(PO₄)₂ exhibits several phase transitions at about 575, 820 and 905°C. The structure was determined at ambient temperature (α-form) and above the first transition (β-form). The α-form cell is monoclinic with a = 19.86(1), b = 5.353(6), c = 13.96(3) Å, β = 110.64(7)°, Z = 8, space group P2₁/ _c ; the structure was solved from 3769 independent reflections to an R value, calculated on intensities, of 0.069. The β-form cell is orthorhombic with a = 18.71(3), b = 7.18(2), c = 5.429(7) Å, Z = 4, space group Pnam; the structure was solved to an R value, calculated on structure factors, of 0.055 using intensities of 858 unique reflections measured on a single crystal at 650°C. Both structures are related to that of glaserite. At high temperature, one of the PO₄ tetrahedra is statistically disordered over two positions related by the m-mirror. Below the transition, ordering of this ion leads to a unit cell of lower symmetry. At the transition, two individuals grow on the two sides of the m-mirror which disappears; thus, at ambient temperature, the crystals are systematically twinned. Above the second transition, the unit cell is hexagonal.