Investigation of specific features of the lattice dynamics and the ferroelectric transition in perovskite crystals

The specific features of diffuse X-ray scattering in BaTiO₃, KNbO₃, and PbTiP₃ perovskite crystals have been investigated. The former two perovskite compounds in cubic, tetragonal, and orthorhombic phases exhibit anomalous sheets due to diffuse X-ray scattering, whereas no similar sheets are observed in the case of diffuse X-ray scattering in PbTiO₃. For these compounds, the phonon spectra are calculated in the quasi-harmonic approximation within the polarizable-shell model, and the mechanism of stabilization of the soft mode above the temperature of the phase transition to the ferroelectric state is considered. It is demonstrated that, in the cubic phase of BaTiO₃ and KNbO₃ crystals, there exist quasi-one-dimensional “soft” modes of vibrations of ions in M-O-M-O- chains, where M = Ti or Nb. In PbTiO₃, this feature of the soft mode has not been revealed. The pair correlation functions of simultaneous atomic displacements in BaTiO₃, KNbO₃, and PbTiO₃ are determined and used to calculate the intensity of diffuse X-ray scattering. The results obtained are in good agreement with experimental data. This is a strong argument in support of the hypothesis that the specific features of diffuse scattering are associated with the existence of quasi-one-dimensional correlations of atomic displacements in the soft optical mode and that the ferroelectric transition in perovskites is a displacive ferroelectric phase transition. The possible influence of the specific features revealed in the phonon spectra of the perovskite crystals on the processes of nuclear magnetic resonance and X-ray absorption (extended X-ray absorption fine structure spectra) is briefly discussed.     

Local atomic structure of niobates and titanates from X-ray absorption spectroscopic data

The local atomic structure of PbTiO₃, BaTiO₃, and KNbO₃ perovskite-type crystals and K _x Na_{1 ? x} NbO₃ solid solutions in different phases is investigated using the angular dependence of the pre-edge structure of the Ti and Nb K X-ray absorption spectra and the EXAFS data. In noncubic phases, a considerable deviation of the local structure from the structure determined from diffraction data is observed only for the tetragonal phase of the BaTiO₃ crystal. It is revealed that, in the cubic phase of niobates, the niobium atoms are characterized by significant displacements from the centrosymmetric positions along the threefold axes, so that they are close in the magnitude and the direction to the displacements in the low-temperatures rhombohedral phases.     

Electron paramagnetic resonance of Fe3+ in orthorhombic KNbO3

EPR spectra of KNbO₃:Fe single crystals are obtained in the - 10–110°C temperature range. The angular dependence of resonant lines is well reproduced by spin Hamiltonian parameters relevant to a Fe³⁺ impurity ion substituted to Nb ion in KNbO₃ crystal. The temperature behaviour of resonant lines is explained by a quadratic dependence of axial parameter D vs polarization P_s of the form D = βP²_s in the orthorhombic phase.     

Theory of the successive phase transitions in KNbO3

The microscopic mechanism of the successive cubic-tetragonal-orthorhombic-rhombohedral phase transitions in KNbO₃ is discussed quantitatively from the microscopic free energy based upon the mean field approximation where the Nb ions are displaced to create spontaneous deformations. From the calculation of the microscopic free energy, it is shown that the order of the phase transitions and the experimental values of the transition entropy in KNbO₃ are well explained by this model.     

Desorption of carbon dioxide from small potassium niobate particles induced by the particles’ ferroelectric transition

The aim of this work is to understand surface properties of ferroelectric crystals related to gas adsorption. Various ferroelectric crystals involved in these studies readily adsorb carbon dioxide, thus our studies were centered on adsorption studies of this molecule. It has been claimed that a dipole moment is induced on carbon dioxide molecules that are near an oxide surface. Our experiments explored the possibility of a dipole-dipole interaction between the gas molecule and the ferroelectric oxide surface in order to explain its adsorption. We characterized the samples with scanning electron microscopy, X-ray diffraction and Raman spectroscopy. We determined the ferroelectric nature of the particles and studied the temperature-dependent phase transitions in small particles of KNbO₃ using Raman spectroscopy. We were able to correlate desorption of CO₂ from one surface state of KNbO₃ with the occurrence of the orthorhombic to tetragonal transition in KNbO₃ in particles of 1 μm size. This CO₂ surface site was not observed in KTaO₃, which does not show ferroelectricity at room temperature.     

KNb0.77Al0.23O2.77的高压相变研究

 利用水热法合成了立方相KNb_0.77Al_0.23O_2.77。在水热晶化过程中得到了包括正交相钙钛矿和一新相在内的中间相产物。结构相变行为采用XRD、DSC、高压Raman和XPS进行了表征。结果表明，低于1 000 ℃的热处理不能导致KNb_0.77Al_0.23O_2.77发生结构相交。而在4.49 PGa高压下可能发生了到一新相的结构转变。高温高压处理可以导致另一新相的出现。最后对该立方相的相变机理进行了研究。     

Spontaneous local distortions and structural phase transitions

Xray Absorption Fine Structure (XAFS) measurements of the local atomic structure of perovskite crystals undergoing various structural phase transitions are summarized and discussed. The results show that the local structure of crystals undergoing ferroelectric antiferroelectric and antiferrodistortive transitions is distorted in a disordered fashion far above the transition to the high symmetry phase. The size of the distortions is a large fraction of the distortion at temperatures far below T _c. Based on these results we propose a model of ferroelectricity which accounts quantitatively for the temperature dependence of the dielectric function the soft mode frequency, the imaginary part of the dielectric constant and the central peak in PbTiO₃ and KNbO₃.     

Transmission electron microscopy investigation of domains and boundaries in bulk La2/3Ca1/3MnO3

The microstructure of domains and boundaries in a bulk orthorhombic La_2/3Ca_1/3MnO₃ material is analysed by applying symmetry group theory. Group theory predicts that the 120° and 90° orientation domains occur in bulk La_{1? x} Ca _x MnO₃ (LCMO) when the cubic LCMO transforms into the orthorhombic LCMO by means of a structural phase transition. The orientation domains are observed and characterized by means of electron diffraction and high-resolution electron microscopy. Antiphase boundaries due to the loss of translation operations have displacement vectors of 1/2[010]o, 1/2[101]o and 1/2[111]0, according to the group theory. Diffraction contrast technique reveals that the displacive vector of most antiphase boundaries is 1/2[111]o, which is also confirmed by means of high-resolution electron microscopy.     

Theoretical study on the fine structure of PbFe0.5Nb0.5O3

The full potential linearized augmented plane wave method within the generalized gradient approximation was used to determine the tetragonal fine structure for PbFe_0.5Nb_0.5O₃, i.e., the equilibrium configuration of B-site compound cations (Fe and Nb). It was found that the displacement of atom Fe along [001] direction is 0.022 nm and that of Nb is 0.0020 nm. The ferroelectric instability of complex compound PbFe_0.5Nb_0.5O₃ is mainly coming from the displacement of atom Fe along [001] direction.     

Investigation of the phase transitions of ferroelectric KIO3 and KNbO3 crystals by optical diffraction

Optical diffraction is reviewed as a technique for investigation of the phase transitions in crystals with a multidomain structure. It has been used to study the phase transitions in KIO₃ and KNbO₃ single crystals. Strong optical diffraction bands resulted from electric domains in KNbO₃ crystals and their change with temperature were observed when a laser beam passed through the crystals. The diffraction patterns observed changed abruptly at 427°C, 223°C, and -50°C respectively, at which KNbO₃ crystals undergo structural phase transitions. It is considered that the change of the diffraction patterns with temperature is due to change of the electric domains in the crystals.     

Formation of ferroelectric phases in KNbO<Subscript>3</Subscript> and other niobates with perovskite structure

The Curie-Weiss temperatures T _CW of NaNbO₃ and AgNbO₃ are determined for the first time by extrapolating the temperature dependence of the reciprocal permittivity from the cubic phase. It is established that the values of T _CW of ANbO₃ perovskites (A = Na, Ag, K) are practically equal, i.e., that the cubic phases of all the compounds are potentially unstable to an almost equal extent with respect to polarization. From an analysis of the EXAFS spectra of the NaNbO₃ and KNbO₃ cubic phases, it follows that the potential-energy surfaces of the Nb cation in both compounds are similar and exhibit eight minima shifted by 0.16 and 0.19 Å, respectively, from the oxygen-octahedron center along the [111]-type directions. Based on these facts, it is inferred that the ANbO₃ instability with respect to the occurrence of polarization is due to the tendency of the Nb cations to be ordered over the eight minima and that the phase transitions induced by this instability are of the order-disorder type. A statistical model capable of describing the sequence of ferroelectric phase transitions occurring in KNbO₃ and in (Ba, Sr)TiO₃ is proposed and studied.     

Fluctuation-induced broadening of Mn2+ EPR lines in the incommensurate phase of Rb2ZnCl4 crystals

The EPR spectra of Mn²⁺ ions in Rb₂ZnCl₄ crystals is investigated in the vicinity of the transition from the paraelectric phase to an incommensurate modulated phase. When these crystals are cooled below the transition temperature T _i =304 K, a splitting of the resonance lines is observed in the singular spectrum. A one-harmonic model is used to discuss the contributions that fluctuations in the amplitude and phase of the incommensurate displacement wave make to the local width of the singular spectra. It is shown that anomalies in the local width of the low-temperature singular peaks observed in the vicinity of T _i are caused by amplitude fluctuations. Fiz. Tverd. Tela (St. Petersburg) 41, 1668–1674 (September 1999)     

Integrated electro-optic platform using ferroelectric crystals with the point symmetry of orthorhombic mm2

We theoretically and numerically investigate the modulation efficiencies of integrated electro-optic (EO) modulators based on ferroelectric crystals with the point symmetry of orthorhombic mm2. An analytical expression describing the index-ellipsoid under the EO effect is derived, where all 5 non-zero linear EO coefficients of mm2-crystals are considered. As practical candidates for integrated EO platform, three mm2-crystals (KNbO₃, KTiOPO₄, and Ba₂NaNb₅O₁₅) that are well-established and commercially available are chosen for numerical simulations. The refractive-index (RI) changes caused by an external electric field applied to each crystal with an arbitrary direction are analyzed in detail for the given optical polarization states of input light wave. For each crystal, the modulation efficiency is evaluated in terms of the half-wave voltage required for a π-phase shift of input wave, which is compared with the established counterpart based on LiNbO₃. The results indicate that higher modulation efficiencies are achievable in the two schemes using KNbO₃ and Ba₂NaNb₅O₁₅ than those for other cases, meaning that either lower power-consuming or larger beam-window integrated EO devices are feasible.     

New crystallographic dielectric phase boundary in K0.5Na0.5NbO3‐based lead‐free ceramics

0.979K_0.5Na_0.5Nb_1‐xSb_x O₃‐0.021Bi_0.5Na_0.5TiO₃ (KNNSx ‐BNT) lead‐free piezoelectric ceramics were fabricated by conventional solid state reaction technique, and their phase transition and electrical properties were studied. With the increase of x, the rhombohedral‐orthorhombic phase transition temperature of the ceramics increases. Finally, both the rhombohedral‐orthorhombic and orthorhombic‐tetragonal phase transitions of the ceramics were modified to be around room tempera‐ ture when about 6% Sb were substituted for the Nb site, resulting in the formation of a new phase boundary separating rhombohedral and tetragonal phases. The formation of the new phase boundary results in excellent properties for the ceramics, that is, the KNNS0.05‐BNT ceramic shows an enhancement in piezoelectric properties: d₃₃ = 380 pC/N and k_P = 0.438. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

OH stretching vibrations in monodomain KNbO3

Infrared absorption bands due to OH stretching vibrations have been investigated in monodomain KNbO₃ at different temperatures. The results obtained in the orthorhombic phase for polarized light suggest hydrogen positions predominantly along the O-O directions of the oxygen octahedron parallel to the spontaneous polarization.     

Temperature dependence of acoustic modes in KNbO3 near the orthorhombic-tetragonal phase transition

The wave vector and temperature dependences of transverse acoustic modes propagating along the [001] and polarized along the [100] orthorhombic axis in KNbO₃ have been studied by means of coherent inelastic neutron scattering methods. The results are consistent with previous elastic constant and neutron scattering measurements. The mechanism of the orthorhombic-tetragonal phase transition is discussed in the light of all these data.     

静水压下锆锡钛酸铅铁电陶瓷相变和介电性能研究

研究了铌掺杂锆锡钛酸铅铁电陶瓷Pb_0.99Nb_0.02[(Zr_0.90Sn_0.10)_0.96Ti_0.04]_0.98O₃(PZST 90/10-4-2Nb)在静水压(0-300 MPa)下的电荷释放量和介电性能. 对压力诱导的低温铁电三方(F_R(LT))→反铁电正交(A_O)相变进行了研究. PZST 90/10-4-2Nb铁电陶瓷分为未极化、极化和压力去极化三种. 极化PZST 90/10-4-2Nb陶瓷F_R(LT)→A_O相变过程中,电荷释放量为29.3 μC/cm²,相变压力为140 MPa. 介电性能表明:极化PZST 90/10-4-2Nb陶瓷相变压力为136 MPa,而未极化陶瓷相变压力为104 MPa,压力去极化陶瓷未表现出明显的相变特征. 关键词： 静水压 铁电陶瓷 相变 介电     

Effect of oxygen segregation on the surface structure of single-crystalline niobium films on sapphire

Single-crystalline Nb films are grown on (1120) oriented sapphire substrates by electron-beam evaporation in ultra-high vacuum. The films are studied in-situ by RHEED and Auger analysis. At a substrate temperature T _S=750° C the RHEED pattern shows a smooth growth of bcc-Nb in the [110] direction. In addition to the fundamental streaks, we observe superlattice streaks of fractional order in several azimuthal directions. The reciprocal lattice of the surface is determined. The basic vectors of the superlattice in real space are given by b ₁=2a ₁, b ₂=–a ₁+3a ₂ where a ₁ and a ₂ are the basic vectors of the Nb (110) surface. Auger analysis shows that the surface of these films is contaminated with oxygen. Therefore, the superstructure is attributed to a modified surface structure due to segregated oxygen, possibly having diffused from the sapphire to the film surface. The superstructure dissappears during further evaporation of Nb at T _S<450° C with a concomitant decrease of the oxygen signal. Nb films on sapphire with a clean, oxygen-free surface can only be prepared at lower temperatures in an island-growth mode.     

Ordered orthorhombic phases of titanium monoxide

Symmetry analysis is carried out for the ordered phases of cubic monoxide TiO_y with relative oxygen contents y<1 and y>1. It is established that a partially ordered orthorhombic phase (space group Immm)—a derivative of the orthorhombic M₃X₂□ superstructure (at y<1.0) or the inverse superstructure M₂■X₃ (at y>1)—may arise in TiO_y. The distribution of Ti and O atoms, oxygen vacancies □, and titanium vacancies ■ in unit cells of the orthorhombic ordered phases is determined. The phases are formed through the order-disorder transition channel along two rays of a non-Lifshitz star {k ₄}, and the ordering proceeds as a first-order phase transition. The distribution functions of Ti atoms over the sites of metallic and O atoms over the sites of nonmetallic sublattices are calculated for the orthorhombic superstructures of cubic titanium monoxide TiO_y.     

Structural state of β-solid solution in rapidly quenched alloys of Al50Cu50− x Fex (x = 17, 13, 6) and ordered η1-AlCu phase formation

Conventional transmission and scanning electron microscopy were employed to investigate the structural state and phase transformations of the β(CsCl)-solid solution in rapidly quenched alloys of Al₅₀Cu_{50? x} Fe _x with respect to the Cu/Fe ratio. We studied the alloys from the central (x?=?17 and 13) and border areas (x?=?6) of the β-solid solution homogeneity region. The structural state of the β-solid solution was characterized by premartensitic structural instability of the bcc lattice for x?=?17 and 13, and the presence of combined short-range order accompanied by the intense diffuse scattering for x?=?6. This short-range order can be described by the ordering of atoms and vacancies in the planes (111)β and ω-like atomic displacements (longitudinally polarized waves of displacement in the direction of the [111]β). The structural state of the β-solid solution with the combined short-range order was regarded as a pretransition state for the revealed transformation with homogeneous precipitation of the nanodispersed phase. The precipitation phase was attributed to an orthorhombic Al(Cu, Fe) η₁-phase, belonging to the family of ordered β-based phases with orientation relationships of [100]_η1 ||[110]_β, [010]_η1 ||[110]_β, [001]_η1 ||[001]_β. We concluded that the atomic structure of the η1-phase is characterized by ordering, accompanied by ω-like atomic displacements of the adjacent layers in 3d metals (Cu, Fe) and aluminum.