Electron microscopy of phase transitions

The possibilities of electron microscopy for the study of phase transitions are briefly outlined and illustrated by some particular aspects of phase transitions in Pb₃V₂O₈, Ba₂NaNb₅O₁₅ and Au3+xZn. Lead orthovanadate is used as an example for the application of a procedure to distinguish between the static or dynamic nature of diffuse intensity in diffraction patterns occurring near the transition temperature. For Ba₂NaNb₅O₁₅ the behaviour of the discommensurations in the vicinity of the ferroelastic transition has been studied and the low-temperature transition around 105 K is proven to be due to a doubling of the baxis and not to a restoration of the fourfold symmetry. The decomposition mechanism of the metastable <2³3> type long period super-lattices in the binary Au_3+xZn alloys is described in terms of the withdrawal of nodes of four <3> domains.     

Generation of broadband squeezed states pumped by CW mode-locked pulses

Broadband high level squeezing was clearly observed from 100 kHz to 80 MHz using crystals Ba₂NaNb₅O₁₅ of 5 mm length, MgO:LiNbO₃ of 19 mm length and KNbO₃ of 5.8 mm length. Maximum noise reductions detected on a spectrum analyzer were –1.2 dB (–24%), –1.25 dB (–25%), and –1.8 dB (–34%) for the three crystals, respectively. The maximum squeezing is limited mostly by optical index damage of the parametric crystals. A detailed analysis of the beam parameters traced along the pump beam, squeezed vacuum, etc. is given. A detailed discussion on the evaluation of the initial squeezed level is given. A preliminary experiment with compressed laser pulses to avoid the optical damage is also described.     

BPM simulation of SNOM measurements of waveguide arrays induced by periodically poled BNN crystals

The simulation of light propagation using standard beam propagation techniques, in periodically inverted domains in Ba₂NaNb₅O₁₂ crystals is presented. The poled domains pattern gives rise to periodic modulation of the refractive index in the crystal, which can be observed with diffraction experiments and simulated by one-dimensional beam propagation methods. The micro-domains are characterized by Scanning Near Field Microscopy, using both transmission and collection modes. In particular, under the collection mode, micro- pictures have been obtained with unexpected high optical contrast between positive and negative domains of up to 80%. Using a beam simulation method, the results are explained considering that the ferroelectric domains operate as a periodic array of planar waveguides.     

Time evolution of polar regions in (Sr0.65Ba0.35)2NaNb5 O 15 single crystal

The linear, second-order and third-order dielectric susceptibilities χ₁, χ₂, χ₃ of a (Sr_0.65Ba_0.35)₂NaNb₅O₁₅ (SBNN) single crystal have been measured. Strong relaxation was found in all investigated dielectric susceptibilities near the phase transition. The time dependence of linear and third-order dielectric susceptibility can be explained as the effect of polar nano-clusters evolution. Near the phase transition, the net polarization of the crystal, monitored by second-order dielectric susceptibility, changes both its value and orientation. Measurements of the pyroelectric effect indicate that the non-zero net polarization in SBNN crystals appears spontaneously. In addition, the ferroelectric phase transition in SBNN showed many features of a first-order phase transition.     

Light-induced charge transport processes in photorefractive crystals II: Materials

3 , LiTaO₃, BaTiO₃, Ba_1-xSr_xTiO₃ (, BST), Ba_1-xCa_xTiO₃ (, BCT), KNbO₃, KTa_1-xNb_xO₃ (, KTN), Sr_1-xBa_xNb₂O₆ (, SBN) and Bi₁₂(Si,Ti,Ge)O₂₀ (BSO, BTO, BGO) are discussed. Utilizing the knowledge on the charge transport processes, consequences for applications are deduced; improved techniques for nondestructive readout of holograms with light of the recording wavelength are described. Received: 14. October 1996     

Self-activated Nd:Ba2NaNb5O12 optical super-lattices: Micro characterization and non-collinear laser light generation

Czochralsky grown Nd³⁺-ion doped barium sodium niobate (Ba₂NaNb₅O₁₂, “banana”) optical super-lattices have been characterized by a combination of chemical etching, optical microscopy, micro-Raman and micro-photoluminescence experiments. The presence of ferroelectric domains, induced during growth process, is accompanied by a periodic modulation of Nd³⁺-ion concentration as well as by a small lattice distortion. When Nd³⁺-ion laser action at ≈1.06 μm is achieved under 0.808 μm laser diode pumping, the singularities of refractive index caused by opposed ferroelectric domains lead to non-collinear intra-cavity propagation.     

Wave-coupling theory of linear electro-optic effect for ultrashort laser pulses

A wave-coupling theory of linear electro-optic (EO) effect for ultrashort laser pulses has been developed. As one of its applications, the linear EO effect of ultrashort Gaussian pulses propagating in LiNbO₃ crystal is demonstrated, with the intensity lower than the damage threshold. It is found that the EO effect is sensitive to both the azimuth angle and the polar angle of wave vector of light. And the larger the applied electric field, the shorter the crystal used for optimization of EO coupling is. The numerical results show that, with the duration of input pulse changing from 150 fs to 5 fs, the durations of output pulses are broadened due to the group-velocity dispersion, as well as the first- and second-order refractive dispersion. In addition, when input pulse duration is larger than 20 fs, in contrast with zero chirp, the output durations can be slightly compressed if a positive chirp input pulse is used. The validities of slowly varying amplitude approximation and continuous wave approximation are also discussed. The influence of self-phase modulation and cross-phase modulation on the EO effect is also discussed briefly.     

Breakdown of translational symmetry at the 300°C ferroelastic transition of barium sodium niobate

The temperature dependence of superlattice reflections has been investigated, in Ba₂NaNb₅O₁₅, by X-ray oscillation photographs taken along the c-axis. The reflections vanish above the 300°C ferroelastic transition. This result confirms that the transition's order parameter corresponds to the Z-point of the Brillouin zone of the high-symmetry phase.     

Growth of Ba2−x Sr x NaNb5O15 thin film on La0.05Sr0.95TiO3 substrate by pulsed laser deposition

Ba_2−x Sr _x NaNb₅O₁₅ thin films were prepared on La_0.05Sr_0.95TiO₃ substrates by pulsed laser deposition. The structural and ferroelectric properties of the thin films depended on substrate temperature (T _sub) and Sr concentration. When T _sub was fixed at 700 °C, the Ba_2−x Sr _x NaNb₅O₁₅ (x = 0, 0.6, 1.0, and 1.4) thin films exhibited a high c-axis orientation. The thin films consisted of well-developed grains and exhibited a smooth surface. The c-axis-oriented Ba_0.6Sr_1.4NaNb₅O₁₅ thin film with the lowest Curie temperature also exhibited a high c-axis orientation and a P-E hysteresis loop with a high ferroelectricity at T _sub 650 °C. Thus, its remanent polarization (P _r) and coercive field (E _c) were 2P _r 24.9 μC/cm² and 2E _c 107 kV/cm, respectively. These values indicate that Ba_2−x Sr _x NaNb₅O₁₅ has ferroelectricity in the thin film form.     

铌酸锶钡钠系统X射线衍射数据的测定

用衍射仪和Guinier聚焦照相机收集了铌酸锶钡钠系统一系列化合物的X射线粉末衍射图谱(CuK_α辐射)。给出了Ba₂NaNb₅O₁₅,SrBaNaNb₅O₁₅和Sr_2.05Na_0.90Nb₅O₁₅详细指标化了的粉末衍射数据,可代替原有的JCPDSX射线粉末衍射标准数据卡(卡片号为23-654,27-1408,27-1409和27-787)的数据。 关键词：     

Photorefractive waveguides in oxide crystals: fabrication, properties, and applications

3 , LiTaO₃, BaTiO₃, KNbO₃, Sr_xBa_1-xNb₂O₆ (0.25≤x≤0.75, SBN), and Bi₁₂(Si,Ti,Ge)O₂₀ (BSO, BTO, BGO) waveguides are discussed. Furthermore, the suitability of photorefractive waveguides for nonlinear optical components is demonstrated in some examples. Received: 21 April 1998/Revised version: 28 May 1998     

Luminescence properties of Ba2NaNb5O15 crystals activated with Sm, Eu, Tb or Dy ions

Single crystals of Ba₂NaNb₅O₁₅ (BNN) singly doped with Sm³⁺, Eu³⁺, Tb³⁺ or Dy³⁺ have been grown by means of the flux growth method. Their visible emission and excitation spectra and the decay profiles of the luminescence have been measured at room temperature. All spectral features are significantly inhomogeneously broadened in consequence of the structural disorder of the host and of the doping mechanisms. The analysis of the observed spectra allows formulating an hypothesis about the site occupancy of the active ions in the BNN lattice.     

Nucleation of domains under the influence of temperature in Ba5Ti2O7Cl4

It is found that the nucleation of domains can take place in Ba₅Ti₂O₇Cl₄ under the influence of temperature unlike in many other ferroelectrics. The nucleated domain can also be removed from the structure under the randomizing effect of temperature. These observations have been explained on the basis of a nucleation model of Ingle and coworkers suggested for KNbO₃. Similarity of physical situation in KNbO₃ and Ba₅Ti₂O₇Cl₄ regarding the existence of ordered point defects inducing nucleation of domains around them has been highlighted. This surprising similarity is suggested to be due to the physical processes occurring at the phase transition rather than due to any structural resemblance. The nature of nucleation occurring under the influence of temperature is shown to be identical with the nature of nucleation occurring under the influence of externally applied stress, and/or DC electric field. A new technique of identifying point defects responsible for nucleation of domains is reported.     

Electron energy-loss spectroscopy at incommensurately modulated crystalline and glassy Ba2TiGe2O8

Based on cluster molecular orbital calculations, high-energy resolution (ΔE?～ 0.4?eV) Ti–L_2,3 electron energy loss near-edge structures of single-crystalline and glassy Ba₂TiGe₂O₈ are interpreted. The finding that the Ti–L_2,3 near-edge structure of the Ba₂TiGe₂O₈ single crystal possesses less pronounced peaks than the glass under identical experimental conditions can be attributed to distinct distortions of the titanium environment caused by the very strong one-dimensional structural modulation hosted by the Ba₂TiGe₂O₈ crystal lattice. As lattice periodicity is absent in the glass, the titanium environment is more regular in the vitreous surroundings. Moreover, the modulation in crystalline Ba₂TiGe₂O₈ is responsible for the virtually indiscernible O–K near-edge structures of the glassy and crystalline samples.     

Low-temperature phase transition in Ba2TiGe2O8

Previous work on Ba₂TiGe₂O₈ crystals has shown an unusual low-temperature (～ 223 K on cooling, ～ 273 K on heating) phase transition. Precession x-ray photographs on Ba₂TiGe₂O₈ single crystals show an incommensurate modulation along b*, and, for the first time, also along a*. Single crystal intensity data confirm the average structure in space group Cmm2. There is positional disorder in the pyrogermanate groups, and this is the probable cause for the modulated structure. The low-temperature phase transition is proposed to be a lock-in transition, with the modulation along a* locking in at a value of 1/3. Several properties, as well as other unusual features of the low-temperature phase transition, are discussed in light of the proposed lock-in transition. Domain studies show that the ferroelastic domains are unstable in the low-temperature phase.     

Highly Efficient Frequency Doubling with a KNbO3 Semi-Monolithic Resonator

A semi-monolithic standing-wave resonator has been constructed and employed for frequency doubling of a Ti:sapphire laser. The resonator consists of a 5 mm long KNbO₃ crystal with dielectric coatings on both end faces and a spherical mirror with 20 mm radius of curvature. With 31.2 mW input power at 861.4 nm second harmonic power of 11.4 mW has been generated corresponding to the conversion efficiency of 36.5%.     

New phase transition with increasing symmetry on cooling in barium sodium niobate

A new and unsual transition is reported in Ba₂NaNb₅O₁₅: the crystal, which was previously known to undergo at 573 K a ferroelastic transition 4mm → mm2 (C_4v → C_2v), reverts back at 110 ± 5 K to a low temperature phase of 4mm (C_4v) symmetry. Thus this crystal presents a unique example of a ferroelastic (mm2) phase stable in a wide temperature range (470 K) and sandwiched between two phases possessing the same higher point symmetry.     

Spontaneous twisting of an incommensurable improper ferroelastic Ba2NaNb5O15

The spontaneous twisting of a y-orientation single-crystal sample of barium-sodium niobate Ba₂NaNb₅O₁₅ at a phase transition and into an incommensurable ferroelastic phase is investigated. Anomalies are found in the temperature dependence of the torsional deformation at temperatures 453, 533, and 573 K and anomalous hysteresis of the temperature dependences of the torsional deformation, measured on heating and cooling of the sample. The results obtained are discussed on the basis of an analysis of the temperature dependence of incommensurable 1q and 2q structures and ferroelastic twins. Fiz. Tverd. Tela (St. Petersburg) 41, 1848–1850 (October 1999)     

Diffused ferroelastic phase transition in barium sodium niobate

We report the temperature dependence of the birefringence Δn_ab across the ferroelastic phase transition of Ba₂ NaNb₅O₁₅. The recorded variations correspond to a transition of the diffused type, with an inflection point at 275°C. This behaviour is consistent with the local disorder induced by a well-known lack of stoechiometry of the available crystals. A large thermal hysteresis is also pointed out.     

Structures and luminescence properties of Yb3＋ in the double perovskites Ba2YB＇O6 （B＇=Ta5＋, Nb5＋） phosphors

The Yb³⁺ doped Ba₂YB'O₆ (B'=Ta⁵⁺, Nb⁵⁺) were prepared by high temperature solid-state reaction method, their structures were determined by x-ray diffraction and refined by Rietveld method. The diffuse reflection absorption, excitation and emission spectra of Yb³⁺:Ba₂YB'O₆ (B'=,Ta⁵⁺, Nb⁵⁺) were measured at room temperature. Under the excitation of ultraviolet light, these phosphors exhibit broad charge transfer band emissions of TaO₆ or NbO₆ centre with large Stokes shift. The Yb³⁺ doped into these hosts are situated at Y³⁺ sites of cubic symmetry (O_h). The experimental energy levels of Yb³⁺ in Ba₂YTaO₆ and Ba₂YNbO₆ were determined by photoluminescence and diffuse reflection absorption spectra. Their wavefunctions and theoretical energy levels were obtained by diagonalising the Hamiltonian matrix. The experimental energy levels were fitted by Levenberg--Marquardt iteration algorithm to determine crystal field parameters. Then, the magnetic-pole transition line strengths of Yb³⁺:Ba₂YB'O₆(B'=Ta⁵⁺, Nb⁵⁺) from (²F_5/2)Γ₈^- to the low-energy states were calculated.