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.     

PLZST反铁电陶瓷电场诱导相变与相稳定性的研究

在三方铁电(FE)-四方反铁(AFE)的准同型相界附近制备了一系列组份为(Pb_0.97La_0.02)(Zr_1-x-ySn_yTi_x)O₃(x=0.09或0.1;0.16≤y≤0.38)的反铁电陶瓷.研究了Sn含量y对电场诱导AFE→FE相变电场E_c、反铁电双电滞回线损耗ΔE、以及温度诱导FE→AFE相变温度T_FE，AFE→顺电(PE)相变温度T_c的影响.沿AFE-FE相界Ti含量一定的条件下，E_c随着Sn含量y的增加而增大，ΔE减小，T_FE与T_c均降低.场诱相变的回线参量E_c，ΔE与相变温度T_FE和T_c相关联.在直流偏压下用原位X-射线衍射表征了相变时晶格结构的变化，结果表明，当电场达到AFE→FE相变临界场时，伴随相变的发生，晶格结构由四方相转变为三方相，晶胞体积增大. 关键词：     

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.     

Phase coexistence in proton glass

Abstract

Proton glasses are crystals of composition M_1?x(NW₄)_xW₂A0₄, where M = K,Rb,Cs, W = H,D, A = P, As. For x = 0 there is a ferroelectric (FE) transition, while for x-1 there is an antiferroelectric (AFE) transition. In both cases, the transition is from a paraelectric (PE) state of tetragonal structure with dynamically disordered hydrogen bonds to an ordered state of orthorhombic structure. For an intermediate x range there is no transition, but the hydrogen rearrangements slow down, and eventually display nonergodic behavior characteristic of glasses. We and others have shown from spontaneous polarization, dielectric permittivity, nuclear magnetic resonance, and neutron diffraction experiments that for smaller x there is coexistence of ferroelectric and paraelectric phases, and for larger × there is coexistence of antiferroelectric and paraelectric phases. We present a method for analytically describing this coexistence, and the degree to which this coexistence is spatial and/or temporal. We discuss also the experimental determination of these coexistence parameters.     

The double-phase transition of ErFe4Ge2. An XRPD study

High-quality powder XRD data of the compound ErFe₄Ge₂ collected in the ESRF beam line BM16, are presented for the entire magnetically ordered regime (T_N=44 K). The data analysis reveals the occurrence of a double symmetry breaking at the magnetic transition. This experiment has allowed us to distinguish between structural and magnetic satellites, both present in the neutron patterns, and to demonstrate the interdependence of structural and magnetic transitions. The high-temperature (HT) phase disproportionates by a first-order transition into two distinct phases: P4₂/mnm (T_c, T_N=44 K)→Cmmm (majority LT phase)+Pnnm (minority IT Phase) which coexist in proportions varying with temperature down to 4 K. The phase diagram comprises three temperature regions: (a) the HT range with T>T_N for the tetragonal P4₂/mnm phase; (b) the IT (intermediate temperature) range, 20 K<T<T_N, where the two phases coexist in strongly variable proportions and the Pnnm phase reaches its highest concentration (≈31%) around 30 K and (c) the LT (low temperature) range, 1.5–20 K, where the Cmmm phase is dominating (up to 95%). We suggests that this phenomenon is the result of competing magneto-elastic mechanisms involving the Er crystal field anisotropy, the Er–Er, Er–Fe and the Fe–Fe exchange interactions and their coupling with the lattice strains.     

Electromechanical-induced antiferroelectricben ferroelectric phase transition in PbLa(Zr,Sn,Ti)O3 ceramic

Antiferroelectric—ferroelectric (AFE—FE) phase transition in ceramic Pb_0.97La_0.02(Zr_0.75Sn_0.136Ti_0.114)O₃ (PLZST) was studied by dielectric spectroscopy as functions of frequency (10²—10⁵ Hz) and pressure (0—500 MPa) under a DC electric field. The hydrostatic pressure-dependent remnant polarization and dielectric constant were measured. The results show that remnant polarization of the metastable rhombohedral ferroelectric PLZST poled ceramic decreases sharply and depoles completely at phase transition under hydrostatic pressure. The dielectric constant undergoes an abrupt jump twice during a load and unload cycle under an electric field. The two abrupt jumps correspond to two phase transitions, FE—AFE and AFE—FE.     

Anisotropic dielectric properties of PbYb1/2Ta1/2O3 single crystals

PbYb_1/2Ta_1/2O₃ single crystals were obtained for the first time. They were grown by the flux method. The PbOPbF₂B₂O₃ system was used as a solvent. Dielectric investigations were carried out in 1 0 0_c, 1 1 0_c and 1 1 1_c pseudocubic directions. These studies pointed to anisotropy of dielectric properties. Frequency-independent ε′(T) and ε″(T) maxima related to the antiferroelectric–paraelectric (AFE—PE) phase transition are observed for all directions at 562 K. The frequency-dependent ε′(T) and ε″(T) maxima near 400 K related to the ferroelectric (FE)–AFE phase transition are observed only in 1 1 1_c direction. The hysteresis loops were observed in this direction only. These results point that ferroelectric relaxor properties appear only in 1 1 1_c direction. We propose to consider the ferroelectric phase as ferrielectric one.     

Dielectric properties and phase transitions of (Pb0.87La0.02Ba0.1)(Zr0.6Sn0.4−xTix)O3 ceramics with compositions near AFE/RFE phase boundary

The electrical properties and phase transition behavior of (Pb_0.87La_0.02Ba_0.1)(Zr_0.6Sn_0.4−xTi_x)O₃ solid solutions (PLBZST, 0.04≤x0.2) were investigated by the X-ray diffraction, permittivity, pyroelectric current, and P-E electric hysterisis loops. As the composition x increased from 0.04 to 0.2, the antiferroelectric ceramics (x≤0.07, AFE) with tetragonal phase changed to the ferroelectric relaxors (RFE, 0.09≤x). AFE ceramics showed a peculiar diffuse phase transition and dielectric relaxation at the low temperature (down to −100 °C) due to a frustration between AFE and FE state. With an increase in composition x, electrically field-induced AFE-FE switching field (E_AFE-FE) and AFE-paraelectric (PE) phase transition temperature (T_c) are depressed in the temperature (T)-Ti composition (x) phase diagram, a FE-AFE-PE triple phase point (T_tr) with the lowest transition temperature occurred at x=0.09. The pyroelectric currents under an application of various external electric field (E) were measured to identify a T-E phase diagram of the PLBZST compound.     

Low‐temperature Raman spectroscopic studies in NaNbO3

Raman spectroscopic measurements were carried out in the temperature range 10–300 K to understand the low‐temperature antiferroelectric (AFE)–ferroelectric (FE) phase transition in NaNbO₃. Several modes in the low wavenumber range were found to disappear, while some new modes appeared across the transition. The temperature dependence of mode wavenumbers suggests that, during cooling, the AFE–FE phase transition begins to occur at 180 K, while the reverse transition starts at 260 K during heating. During cooling, the two phases were found to coexist in the temperature range of 220–160 K. Upon heating, the FE phase is retained up to 240 K and both FE and AFE phases coexist in the temperature range 240–300 K. In contrast to the earlier reports, the present results suggest a different coexistence region and the reverse transition temperature. The reported relaxor‐type FE behaviour over a broad temperature is consistent with the observed coexistence of phases during cooling and heating cycles. Copyright © 2010 John Wiley & Sons, Ltd.     

The ν5Band of HClO4

The high-resolution infrared spectrum of perchloric acid has been observed in the 700–750 cm⁻¹region using the infrared beamline at the MAX-I electron storage ring in Lund, Sweden. The spectrum displays extensive rotational structure due to a typeaband and is assigned to ν₅, the HO–ClO₃stretch. Approximately 1100 transitions in H³⁵ClO₄and ca. 300 in H³⁷ClO₄have been fitted using single subband analysis, generating constants for transitions having the sameK. The origin of H³⁵ClO₄K= 3tseries is found to be 726.9971(4) cm⁻¹. Rotationally resolved infrared line positions are now available for the identification of HClO₄in the atmosphere, which may be produced by the heterogeneous oxidation of chlorine containing species in the stratosphere.     

23Na nuclear magnetic resonance study of the phase transition in ammonium rochelle salt,NaNH4C4H4O6 • 4H2O

The satellite NMR spectrum of ²³Na in ammonium Rochelle salt (NaNH₄C₄H₄O₆ ? 4H₂O) is investigated near the transition temperature. Each line in the paraelectric phase splits discontinuously into four lines at the transition temperature; this fact is compatible with the first-order phase transition and reveals the existence of the superstructure in the ferroelectric phase.     

Oxygen vacancy-induced aging of Mn-doped Ba0.8Sr0.2TiO3 ceramics in paraelectric and ferroelectric state

The aging properties of 0.01 mol% Mn-doped Ba_0.8Sr_0.2TiO₃ ceramics have been investigated from 30 °C to 400 °C at various frequencies. Decreases in ε′(T) of the aged sample compared to the fresh one around the tetragonal–cubic transition and in the regime of diffusion have been observed. The activation energy E_a = 1.25 eV obtained from the J–T loop at zero electric field indicates that oxygen vacancies dominate in the aging. The symmetry-conforming principle of point defects was employed to explain the time and temperature dependence of aging in the dielectric constant and double/constricted P–E loops of the samples aged in the paraelectric and ferroelectric state.     

Origin of antiferroelectricity in NH4H2PO4 from first principles

The low-temperature antiferroelectric (AFE) phase of NH4H2PO4 corresponds to H ordering in O-H-O bridges leading to H2PO4 group polarizations perpendicular to the tetragonal c axis and alternating in chains. We determine the microscopic origin of such order by means of first-principles calculations in the framework of the density functional theory. The formation of N-Hcdots, three dots, centeredO bridges with correlated charge transfers and NH4+ group distortions turn out to be essential in stabilizing the AFE configuration against a c-polarized ferroelectric (FE) phase, as well as other FE states polarized perpendicular to the c axis. These FE states lie only a few meV above the AFE phase, which explains the observation of FE-AFE phase coexistence near the AFE transition.     

Ion–ion interactions of LiPF6 and LiBF4 in propylene carbonate solutions

Self-diffusion coefficients of Li⁺ D_Li⁺, PF₆⁻ D_PF6⁻ and solvent propylene carbonate (PC) D_PC in LiPF₆−PC solutions were determined at 298 K by the pulse gradient spin echo (PGSE) NMR technique over the salt concentration range of 0.1–3.0 M (M = mol dm^– 3). The order of the diffusion coefficients was found to be D_Li⁺ < D_PF6⁻ < D_PC over the concentration range examined, and they were monotonically decreased with increasing the salt concentration. Haven ratio Λ/Λ_NMR, where Λ and Λ_NMR represent the ionic conductivity measured electrochemically and that estimated via the Nernst-Einstein equation using the diffusion coefficient, respectively, was evaluated as the measure of the ion–ion interaction in the LiPF₆–PC solutions. Though Λ/Λ_NMR values for LiPF₆-solutions decrease with increasing the salt concentration, they were greater than those for LiBF₄–PC solutions over the whole concentration range examined, which indicates that the ion pair formation ability of PF₆^– ion is weaker than that of the BF₄^– ion. The smaller value of the ionic conductivity for the highly concentrated LiPF₆–PC solution (above 2.0 M) than that of the LiBF₄-solutions can be attributed to the more rapidly increased viscosity relative to the LiBF₄-solution. Classic molecular dynamics (MD) simulations for the respective LiPF₆ and LiBF₄-solution of 0.5 and 1.0 M were also carried out based on the effective pair potentials. Diffusion coefficients, ionic conductivity and Haven ratio for these solutions were calculated from MD trajectories, and they qualitatively agree with those evaluated by experiments. Pair correlation functions g_LiO(r) (for Li⁺–O (PC) pair) and g_LiPF6(r) (for Li⁺–PF₆^– pair) or g_LiBF4(r) (for Li⁺–BF₄^– pair) revealed that the lithium ion weakly forms the contact ion pairs with PF₆^–, whilst strongly with BF₄^–, which supports the present experimental results. Moreover, the simulation results show that both anions in the contact ion pairs predominantly take the monodentate form, which is in contrast to the multidentate coordination predicted by ab initio calculation in gas phase.     

Elastooptic effect and sources of the spontaneous birefringence in Gd2(MoO4)3

It is shown that a shear stress applied to support the spontaneous shear in improper ferroelectric Gd₂(MoO₄)₃(GMO) reduces the birefringence Δn'₁₂ measured when viewing along the c-axis. The elastooptic coefficient p₆₆^E in the paraelectric phase has been determined including sign. Using these and known electrooptical data, the soft mode, electrooptic and elastooptic contributions to Δn′₁₂ of GMO are evaluated. The two latter contributions have opposite sign to that of the soft mode which plays a prevailing role.     

Crystal growth, spectroscopic and crystal field studies of [N(CH3)4]2MnCl4 and [N(CH3)4]2CoCl4 single crystals in the paraelectric phase

Single crystals [N(CH₃)₄]₂MnCl₄ and [N(CH₃)₄]₂CoCl₄ were grown by the slow evaporation technique from the super-saturated solutions. The samples obtained were undergone the X-ray and spectroscopic studies. Absorption spectra in the paraelectric phase at T=303 K have been recorded using the Shimadzu 160A double beam automatic scanning spectrophotometers. On the basis of the exchange charge model and Racah theory the crystal field parameters and Racah parameters have been calculated; all absorption bands for both crystals were given an assignment.     

Vibronic model for antiferroelectric phase transition in squaric acid, H2C4O4

The antiferroelectric phase transition in H₂C₄O₄ crystals is analyzed using the neutron diffraction and high resolution ¹³C, ¹⁷O NMR data, Hartree-Fock, Moller-Plesset and DFT calculations. Is shown that AFE phase transition is of mixed order-disorder and of displacive type represented within of the unified vibronic interaction model.     

Dynamic properties of P4O6S and P4O7: P spin–echo and P MAS–NMR investigations

The rotational dynamics of P₄O₆S and P₄O₇ in the solid state were studied by means of ³¹P NMR spectra of spinning and static powder samples in the temperature range of 153–295 K and 295–388 K, respectively. All spectra were simulated to confirm the type of the motion and to extract the time scales as a function of the temperature. Good agreement between experimental and theoretical data was obtained on the basis of a three-site jump model. For P₄O₆S, the activation energy and the pre-exponential factor derived from the lineshape simulations amount to 51(2) kJ/mol and 6(3)·10¹⁵ s⁻¹. For P₄O₇, the spectral analysis yields an activation energy of 67(1) kJ/mol and a pre-exponential factor of 6(2)·10¹⁴ s⁻¹. The dynamic behavior was checked independently by lineshape analyses under both MAS and static conditions. Activation energies are consistent within the errors for the lineshape analyses. Additionally, we have analyzed spin–lattice relaxation measurements, which show the correct trends for the activation energies.     

Magnetic resonances in (CH3NH3)2MnCl4

We have studied the magnetic excitations of (CH₃NH₃)₂MnCl₄ in the antiferromagnetic and in the spin-flop regimes by means of magnetic resonance in the millimeter range (60–100 GHz). Rather odd line shapes of the resonance absorption line for narrow lines are explained as interference effects between the resonant and the non-resonant circular wave in the sample. For the antiferromagnetic resonance (AFMR) and for the paramagnetic resonance (EPR, above the Néel temperature), we have also studied the line width as a function of temperature.     

Synthesis and transport properties of La2NiO4

La₂NiO₄ compounds were prepared by a modified sol–gel auto-combustion method, which is a low-temperature combustion synthesis procedure using microwave-assisted sol–gel as precursors. The high-temperature transport properties of the samples were investigated. The band structure, total density of states (DOS), and partial density of states (PDOS) of low-temperature orthorhombic (Bmab) phase and high-temperature tetragonal (I4/mmm) phase for La₂NiO₄ were calculated in order to study the transport properties of the as-obtained samples.