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.     

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.     

Second-harmonic scattering by domains in RbH2PO4 ferroelectrics

Second-harmonic scattering (SHS) of light by laminar ferroelectric domains of the orthorhombic phase of RbH₂PO₄ below T_c=147 K is studied in detail. CCD images of the far-field SHS pattern reveal up to six off-axis distinct intensity maxima in a plane normal to the ferroelectric axis c. These scattering maxima are interpreted on the basis of the quasi-phase matching (QPM) conditions for the three sets of polarizations allowed by the nonvanishing elements of the second-order nonlinear susceptibility tensor. On this assumption scattering angles of the three SHS modes calculated from crystal optics are shown to fit well the experimental data for any incidence of the pump beam normal to c. Scattering intensities of the two most intense modes are also measured versus the component of the wave vector perpendicular to the modulated structure. A stochastic model giving the intensity as a function of the density probability of domain widths is developed and is shown to give a good fit of the scattered intensity. Moreover, it permits the evaluation of the mean and dispersion of the ferroelectric domain widths, which are found to be, respectively, 2.6 and 0.7 μm in the crystal under study. This is consistent with the domain size of the ferroelastic counterpart revealed by polarization microscopy.     

17O quadrupole resonance study of the ferroelectric phase transition in KH2PO4

The temperature dependence of the ¹⁷O NQR spectra in KH₂PO₄ has been measured using a proton-¹⁷O double resonance technique in the laboratory frame. The spectra are consistent with a model where the protons rapidly move above T_c between the two equilibrium sites in the O-H--O bonds, whereas the motion freezes in below T_c.     

Electronic effect and elastic properties of RbH2PO4 crystals

We have studied the dielectric, elastic piezoelectric properties, and the electrocaloric effect of a single crystal of RbH₂PO₄. If RbDP undergoes a first order transition, the temperature range must be small and a small field can change it to a second order one.     

On asymmetric propagation on transverse acoustic waves near the structural phase transition in RbH3(SeO3)2

Recently ultrasonic measurements of the RbH₃(SeO₃)₂ crystal have shown that a transverse acoustic wave propagating along the modulation axis of the incommensurate structure (c-axis) and polarized along the polar axis (b-axis) exhibits greater temperature anomalies of its velocity and damping than a wave propagating along the b-axis and polarized along the c-axis. The possible explanation of this experimental result is given on the basis of the Landau theory.     

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.     

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.     

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.     

Infrared spectra of paraelectric and ferroelectric CsH2PO4

ir spectra are reported for polycrystalline CsH₂PO₄ above and below the transformation temperature, 153 K. Three absorption bands at 1760 cm^-1 2350 cm^-1 and 2800 cm^-1 are assigned to -OHO- vibrations. Whilst the 2350 cm^-1 band remains essentially unaltered, the 1760 cm^-1 band splits into multiplets and the 2800 cm^-1 band shifts to lower frequency on cooling below 153 K. A broad band centred at 3240 cm^-1 is found to exist in the ferroelectric phase and is associated with the -OHO- mode involving the H(2) proton ordered below the transition.     

EPR study of the low temperature ferroelectric phase transition in Cu doped Rb2ZnCl4 single crystals

Temperature dependent EPR measurements on copper doped Rb₂ZnCl₄ single crystals allowed us to evidence and study the P2₁cn↔C1c1 structural phase transition that takes place in this compound at 74.6 K. From the two types of Cu²⁺ centers localized at different anionic sites, called Cu²⁺(I) and Cu²⁺(II), which are formed in this compound, only the Cu²⁺(II) centers exhibit observable changes in their EPR spectra, attributable to the symmetry lowering. The observed changes have been related to the soft-mode responsible for the structural phase transition.     

Investigation of the ferroelectronic phase transition of CsH2PO4 by the pyroelectric methods

Pyroelectric methods are used for estimation of the polarization of the CDP crystal. The peak value of the pyroelectric coefficient is observed at 154.5 K. The phase of CDP above T_c up to ca. 230 K is revealed to be a polar one with polarization fluctuations in this temperature region.     

Dielectric properties of CsH2PO4 and CsD2PO4

Orthorhombic CsH₂PO₄ undergoes a ferroelectric transition at T_c = ?119.5°C, whereas the ferroelectric transition temperature in isomorphous CsD₂PO₄ is T_c = ?5.55°C. The transitions are of first order in both cases. The rather large isotope effect demonstrates the importance of the OHO bonds in the transition mechanism.     

Thermoreflectance of single-crystal HgCr2Se4 near the ferromagnetic-to-paramagnetic phase transition

The thermoreflectance of the ferromagnetic semiconductor HgCr₂Se₄ (T_c = 106 K) has been measured between 45 and 340 K in the photon energy range from 1.7 to 2.8 eV. The spectral line shape and the signal intensity behave critically near the magnetic phase transition. The optical transitions and the effect of magnetic ordering are discussed.     

Coexistence of ferroelectric and antiferroelectric microregions in the paraelectric phase of NH4H2PO4 (ADP)

Ammonium dihydrogen phosphate NH₄H₂PO₄ (ADP) is an antiferroelectric (AFE) compound belonging to the KDP-type family of hydrogen-bonded ferroelectrics. Recent ab initio results have shown that the optimization of the N–H–O bridges in ADP leads to the stabilization of the AFE state over a FE one. However, electron spin probe measurements have suggested that microregions of both phases may coexist above the critical antiferroelectric–paraelectric transition temperature. We have performed first principles studies of the energetics and relative stability of different AFE and FE defects embedded in a paraelectric (PE) matrix of ADP. Our analysis indicates that FE and AFE clusters are stable and may coexist in the PE phase, thus confirming the above suggestion.     

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.     

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.     

Antiferroelectricity in TID2PO4

Polarization versus electric field double hysteresis loops along the b-axis of TID₂PO₄ were observed. From this fact and the permittivity versus temperature characteristic, the low temperature phase of TID₂PO₄ was confirmed to be antiferroelectric.     

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.