An 17O nuclear quadrupole resonance study of hydrogen atom motion in KH2PO4

The nuclear quadrupole resonance of ¹⁷O naturally abundant in powdered KH₂PO₄ is measured in its ferro-electric and para-electric phases. Fine structure is revealed due to the magnetic dipolar interaction of the oxygen nucleus and the proton in a neighbouring hydrogen atom.     

17O NQR study of the ferroelectric phase transition in PbHPO4

The temperature dependence of the ¹⁷O NQR spectra of the O-H---O bonded oxygens in PbHPO₄ have been studied using a proton-¹⁷O double resonance technique. The results show that the protons move between two off-centre sites above T_c and freeze into one of the possible equilibrium sites below T_c.     

Raman scattering of high-temperature phase transition in KH2PO4

Raman scattering was applied to study the high-temperature phase transition (near 175°C) in KH₂PO₄. Drastic temperature-dependent changes were observed to take place in the normal modes of B₁ symmetry between 1000–3400 cm^?1. The disintegration of the dominant broad feature near 2500 cm^?1 when temperature rises beyond 150°C suggests that the alteration of the hydrogen-bond network is closely connected with this high-temperature phase transition.     

Raman spectral of low-lying longitudinal mode of KH2PO4 and KD2PO4 in ferroelectric phase

The origin of the low-lying modes of KH₂PO₄ and KD₂PO₄ in the ferroelectric phase has been clarified by the z(xy)z? Raman scattering experiments. The “S-mode”, which has been usually assigned to the proton tunneling mode in KH₂PO₄ at T<T_c, has been found in the z(xy)z? spectrum of KD₂PO₄ in contrast to the x(xy)y spectrum. It has been found that the frequency of the “S-mode” of KD₂PO₄ is higher than that of KH₂PO₄. These results have shown that the “S-mode” is far from the proton tunneling mode nor the proton/deuteron mode at all. From the present Raman spectroscopy, it is concluded that the “S-mode” is assigned to the libration mode of the PO₄ tertahedrons.     

A shell model for the H-bonded ferroelectric KH2PO4

A shell model for KH₂PO₄ (KDP), the prototype compound of the family of H-bonded ferroelectric materials, has been constructed by adjusting the interaction parameters to first-principles calculations. Structural properties, energy barriers, phonons, and the relative stability between the ferroelectric (FE) phase and a relevant antiferroelectric metastable structure associated to domain walls, compare very favorably to available first-principles and experimental data. Molecular dynamics simulations show that the model behaves satisfactorily within the FE phase. This model will be used to study the elusive structure of the paraelectric (PE) phase and the nature of the FE–PE phase transition.     

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.     

Oscillatory behaviour of ultrasonic attenuation in KH2PO4 near the phase transition temperature

Oscillations in ultrasonic attenuation have been observed in KH₂PO₄ near the ferroelectric phase transition temperature for longitudinal ultrasonic wave propagation along the polarization axis. Oscillations are present both above and below the transition temperature; the periods of oscillations are different in the two phases for the same ultrasonic frequency and the same ∣T ? T_c∣. The temperature dependence of the oscillations shows that they are due to the phase transition.     

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.     

Positron annihilation in KH2PO4

A positron annihilation study of KH₂PO₄ provides new evidence of a high temperature phase transition at 447 K. The results suggest the formation of a long lived positron or positronium state which could be associated with bond defects.     

The antiferroelectric phase transition in TlH2PO4 and TlD2PO4

It is shown by X-ray diffraction measurements that the transitions at 230 K in TlH₂PO₄ and at ≈ 350 K in TlD₂PO₄ are structurally equivalent. In both cases, on cooling through the transition, new reflections appear at points ($\text{h +}\text{1}\text{2}$, $\text{k +}\text{1}\text{2}$, l) referred to the primitive monoclinic unit cell of the high-temperature phase. In TlH₂PO₄ some distinct but broadened scattering remains at the ($\text{h +}\text{1}\text{2}$, $\text{k +}\text{1}\text{2}$, l) points up to 10 K, or more, above the transition temperature.     

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.     

Sensitive detection of 17O pure quadrupole resonance by nuclear triple resonance method

The pure quadrupole resonance of naturally abundant ¹⁷O was sensitively detected in some solids by the newly developed nuclear triple resonance method.     

Low-frequency Raman spectra of paraelectric KH2PO4

The spectra recently published by Tominaga and Urabe are shown to be in good qualitative agreement with the current coupled-mode models of the KDP transition. The need for a careful quantitative analysis of this type of data over a broad frequency range is discussed.     

The transverse dielectric properties of KH2PO4

We have measured the transverse dielectric constant of KH₂PO₄ crystal from liquid N₂ to room temperatures. Extending the Blinc-de Gennes' model in conjunction with a cluster approximation leads to a new independent determination of the Slater-Takagi parameters and improves significantly the fit of the model to the data.     

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.     

Impurities detection by optical techniques in KH2PO4 crystals

In this paper we examine how optical techniques can be used for impurities (or defects) detection and identification in KH₂PO₄ (KDP) components. This is important in so far as some of these defects are responsible for a much weaker than expected Laser-Induced Damage Threshold (LIDT) in these materials, i.e. for a weaker resistance to a laser shot. KDP materials are investigated by photothermal deflection, fluorescence and photoexcitation with the aim of localizing and identifying the laser-induced damage precursors. The rapidly grown KDP crystals are shown to be heterogeneous from the absorption, fluorescence and composition point of view. Impurities concentrations are measured directly by Inductively Coupled Plasma Atomic Emission Spectrometry (ICP-AES) and tentatively correlated to some optical characteristics and to the LIDT of KDP materials.     

Detection and characterization of absorption heterogeneities in KH2PO4 crystals

The SOCRATE facility is used for the detection of heterogeneous absorption in large and rapidly grown KH₂PO₄ (KDP) crystals. The different regions are subsequently characterized by spectrophotometry and laser-damage measurement. In accordance with previous results from the literature we found that, by using the SOCRATE damage measurement set-up with a large beam (300 ∗ 600 μm²), the absorption level does not play a major role in the damage mechanism at 355 nm. However, by testing the damage resistance of the same KDP component with a focused beam (9 μm diameter) at 355 nm, we highlight for the first time significantly different damage resistances between highly and weakly absorbing regions from a KDP crystal. Indeed, with a focused beam, we demonstrate that the laser-damage threshold may be lower in some sectors cut in the pyramidal part than in sectors from the prismatic part. This means that the absorbing defects at high concentration are not the predominant damage precursors at 355 nm.     

Low-frequency models in KH2PO4-type crystals

The phonon responses along the three crystallographic orientations of KH₂PO₄, 90% deuterated K(H_1–xD_x)₂ PO₄ and NH₄H₂PO₄, deduced from the best fit of the factorised form of the dielectric function to the infrared reflectivity spectra, are compared. Results evidence the activity of a low-frequency heavily damped extra-mode in both directions parallel and perpendicular to the c axis, due to the coupling of an internal mode with the inter-site protonic motion and external models.     

Far infrared reflectivity spectra of a KH2PO4 single crystal

Conclusion The experimental results presented in this paper show that two heavily damped modes, due to proton tunneling, are active in the two phases of KDP: the well-known soft mode which attains a hard-core frequency at the transition temperature and a low frequency mode polarized in a direction perpendicular to the c axis. These modes are responsible for the temperature variation of the low frequency dielectric permittivity in the two crystallographic directions. Due to the size of the samples we do not obtain a single domain crystal, and it was impossible to distinguish the B₁ and B₂ modes. The 200–300 cm^–1 spectral range is very sensitive to the orientation of the incident wave, and certainly corresponds to the hydrogen bonds vibrations.     

A dynamic study of the quasi-one-dimensional hydrogen-bonded ferroelectric crystal CsH2PO4

The dynamics of the strong-anisotropic Ising model in a transverse field is used with the purpose to explain the dielectric critical slowing down observed experimentally in the quasi-one-dimensional hydrogen-bonded ferroelectric crystal CsH₂PO₄. A good agreement with the experimental data of the temperature dependence of the dielectric constant and the relaxation time is obtained.