Structure Of Niobate Research Articles

Single‐crystal LiNbO3 surfaces processed in low‐temperature hydrogen plasma: XPS, REELS and AFM study

AbstractSingle crystals of lithium niobate, which is a widely used material in opto‐electronics, holography, surface acoustic wave devices, etc., have been processed in a low‐temperature plasma of radiofrequency discharge in hydrogen. The aim was to examine a new kind of surface modification of LiNbO3. The surface modification has been studied by XPS, reflection electron energy‐loss spectroscopy (REELS) and atomic force microscopy (AFM). The XPS study via the Nb 3d spectral lineshape revealed the plasma effect: a decomposition of the niobate structure and the appearance of niobium oxides. The subsequent REELS study performed at 4 keV electron energy showed a transfer of characteristic electron‐loss energies of relevant transitions. Oxygen and lithium escape from the surface and hydrogen is incorporated from the plasma, producing niobium hydride. The deterioration of the niobate unit cells is accompanied by an enhancement of conduction electrons. The subsequent thermal reoxidation in medical oxygen did not restore the original niobate structure. The surface morphology after the reoxidation is characterized by elongated 100 nm grains situated normally to the surface. The main constituents of this new top layers are niobium oxides Nb2O5 and NbO. The present results form part of a complex study of the plasma modification of lithium niobate. The plasma processing induces lattice defects that are similar to those of ion implantation or metal ion doping: the electrical and optical properties of the surface layer drastically differ from those of the bulk. Copyright © 2002 John Wiley & Sons, Ltd.

Structure Refinement of (Sr,Ba)Nb2O6 Ceramic Powder from Neutron and X-Rays Diffraction Data

The structure of polycrystalline strontium barium niobate at room temperature was refined by the Rietveld method. Sintered ceramic samples were used to collect powder neutron and X-ray diffraction data. The ratio Sr/Ba ~ 64/36 was found from the initial batch composition Sr0.61Ba0.39Nb2O6, corroborating with the quantitative X-ray dispersive spectroscopy (EDS) measurements. The structure is tetragonal with cell parameters a, b = 12.4504(3) Å and c = 3.9325(1) Å and space group P4bm. It was not necessary to introduce any positional disorder for the oxygen atoms. Cation Nb+5 displacements not parallel to the c direction are presented, which can influence the behavior of the ferroelectric properties.

Structure, Morphology, and Properties of Strontium Barium Niobate Thin Films Grown by Pulsed Laser Deposition

Structure, Morphology, and Properties of Strontium Barium Niobate Thin Films Grown by Pulsed Laser Deposition

Piezoelectric Properties in LiTaO 3 -CaTiO 3 Solid Solution Ceramics

As a part of sequential work developing new lead-free piezoelectric ceramics, solid solution ceramic system in (1 m x)LiTaO 3 -xCaTiO 3 , 0 o x o 0.20, has been investigated. Samples sintered at 1250°C for 5 h in air have resulted in dense and white ceramics with single phase lithium niobate structure. The Curie temperature observed from dielectric properties was lowered with increasing amount of CaTiO 3 , accompanying with a feature of diffused phase transition. The ceramics, 16mm in diameter, subjected to poling treatment at a high temperature near the phase transition showed clear vibration resonance in radial and thickness modes. In using such ceramics, an arch-shaped interdigital transducers was useful for generating or detecting Lamb waves.

Doping effect in layer structured SrBi2Nb2O9 ferroelectrics

This article reports a systematic study of doping effects on the crystal structure, microstructure, dielectric, and electrical properties of layer-structured strontium bismuth niobate, SrBi2Nb2O9 (SBN), ferroelectrics. Substitution in both the A site (Sr2+ by Ca2+ and Ba2+) and B site (Nb5+ by V5+) up to 30 at % were studied. It was found that crystal lattice constant, dielectric, and electrical properties of SBN ferroelectrics varied appreciably with the type and amount of dopants. The relationships among the ionic radii, structural constraint imposed by [Bi2O2]2+ interlayers, and properties were discussed.

Crystal growth and structure of pure and rare-earth doped barium sodium niobate (BNN)

Through the exploitation of the corresponding ternary phase diagram, the barium sodium niobate (BNN) crystal growth by several techniques: Czochralski, Laser Heated Pedestal Growth and Micro-Pulling Down is surveyed. It is pointed out that the fiber crystal growth contributed to obtain important advances on the crystal structure, doping by rare-earth ions and growing behavior leading to what can be considered as an attractive new material. The further growth of high optical quality bulk crystals could now lead to the commercial development of this material with outstanding non-linear optical properties.

Domain walls characterization of the opposite domain lithium niobate structures

Opposite domain lithium niobate structures have been grown using erbium as a dopant. The “head-to-head” and “tail-to-tail” domain walls show a variation in the Li/Nb ratio, while the Er concentration is constant along the periodic domain structure. The influence of the etching time in the domain pattern was studied. After a vapor transport equilibration process the domain structure is destroyed, and the Li/Nb ratio is constant along the whole sample due to the Li in-diffusion process.

Surface analysis of LiNbO3 single crystals modified by radiofrequency hydrogen plasma

Lithium niobate (LiNbO 3 ) is a widely used ferroelectric material with high electro-optic, non-linear optic, birefringent and photorefractive activities. Modification of the surface of LiNbO 3 single crystals is essential for its application in optoelectronics. We accomplished the surface modification of single-domain LiNbO 3 wafers (z-cut) in the hydrogen plasma of radiofrequency (r.f.) discharge (13.56 MHz) under pressures of 0.5-3 mbar. Modification of the wafer surface is demonstrated mainly by oxygen and lithium depletion. The overall depth of the modified surface region is ∼0.5 μm. There is a thin near-surface layer (<10 nm) where the niobate structure is destroyed and only niobium oxide remains there. Towards the bulk the niobate structure is preserved, although with many point defects. From a depth of 1 μm the genuine niobate composition remains. A double inversion of the spontaneous polarization in the layer was found after reoxidation of the plasma-processed sample. The polarity inversion is adjoined to the internal fields induced by point defect gradients in the plasma-modified surface. The following methods are used: neutron depth profiling for lithium depth distribution; RBS for heavier element depth profiles; x-ray diffraction for evaluation of crystallinity changes; and XPS for composition and chemical bonding of elements in the actual surface.

Continuous wave simultaneous multi-self-frequency conversion in Nd3+-doped aperiodically poled bulk lithium niobate

We report simultaneous participation of the fundamental laser wave in self-frequency doubling and self-sum-frequency mixing with pump radiation in Nd3+-doped bulk aperiodically poled lithium niobate structures. A green continuous wave visible laser output power of 1.5 mW generated by self-frequency doubling and 0.5 mW of blue generated by self-sum-frequency mixing have been obtained simultaneously from 30 mW of laser output power available in the fundamental. Stable and efficient infrared laser action at room temperature is obtained in both F3/24→I11/24 and F3/24→I13/24 laser channels of Nd3+ ion without oxide codoping, although some residual photorefractive damage for the visible outputs is observed.

Sheet resistance of LiNbO3 wafers processed in radio-frequency plasma of hydrogen

Radio-frequency discharge of 13.56MHz in hydrogen was used for processing single domain crystalline LiNbO3 wafers under an electrodeless capacitive coupling. At a pressure of 0.5 torr and RF input power of 250 W a surface layer of approx. 0.5 μm on the LiNbO3 wafers was created in which the niobate structure was strongly injured. The Li concentration dropped almost to zero at the very surface and only niobium oxides remained there. After the surface modification the wafers lost their insulating properties and became electrically conducting. The sheet resistance was measured and revealed a semiconducting character. We discuss possible mechanisms of the charge carriers creation in the material during the plasma processing. The nuclear method NDP (Neutron Depth Profiling) has been used for the lithium depth profiling and the four-point probe technique for the sheet resistanace measurements.

Electronic structure of layered perovskite-relatedSr1−yLayNbO3.5−x

The electronic properties and the crystal structure of layered perovskite-related niobates ${\mathrm{Sr}}_{1\ensuremath{-}y}{\mathrm{La}}_{y}{\mathrm{NbO}}_{3.5\ensuremath{-}x}$ can be tuned by gradually changing the oxygen stoichiometry. We have studied the electronic structure of this series in detail by angle-integrated and angle-resolved (ARPES) photoemission and x-ray absorption (NEXAFS) on ${\mathrm{SrNbO}}_{3.45}$ and ${\mathrm{La}}_{0.1}{\mathrm{Sr}}_{0.9}{\mathrm{NbO}}_{3.39}$ single crystals. ARPES shows a one-dimensional dispersion and a purely one-dimensional Fermi surface with a nesting vector ${2k}_{F}=0.56{\mathrm{\AA{}}}^{\ensuremath{-}1}.$ A strongly anisotropic electronic character appears in polarization-dependent O $1s$ NEXAFS as well. Local-density approximation (LDA) band-structure calculations support this dispersion behavior of the lowest-lying conduction band and find additional Fermi surface sheets with a more two-dimensional character.

Synthesis, structure and ferroelectric property of a new niobate: Sr 6CrNb 9O 30

A new niobate Sr 6CrNb 9O 30 was found in the ternary system SrOCr 2O 3Nb 2O 5. The transparent crystal which has a green color and a need-like shape can be grown by the flux method. The crystal structure was determined by x-ray diffraction analysis and dielectric measurement, and it belongs to the orthorhombic tungsten bronze structure at room temperature with space group Cmm2, and unit cell parameters a=1.7505(4) nm, b=1.7510(1) nm, c=7.768(4)nm, Z=4. Dielectric constant measurement show that Sr 6CrNb 9O 30 has two phase transitions, paraelectric to ferroelectric at 110°C and ferroelectric to ferroelastic at 205°C.

Calculation of local electric fields in displacive-type ferroelectrics: LiNbO3

The method is suggested for calculating the intensity and potential of a local electric field in displacive-type ferroelectrics which is based on the introduction of a transitional zone between the region of the discrete localization of ions and the homogeneously polarized medium. The method is exemplified on the calculation of the contribution of Li+ ions to the local electric field in the lithium niobate structure. It is shown that the method provides fast convergence of the results in the calculation of the field intensity and high relative accuracy of the calculated electric-field potential inside the crystal. The contribution of the induced dipoles to the local electric field is calculated, and it is shown that the stability of the LiNbO3 structure is provided by considerable anisotropy in the polarizability of Nb-O bonds. The method is applicable to any ionic crystal.

Effect of mechanical constraint on the dielectric and piezoelectric behavior of epitaxial Pb(Mg1/3Nb2/3)O3(90%)–PbTiO3(10%) relaxor thin films

The effect of heteroepitaxy-induced constraint on the structure and piezoelectric properties of the relaxor ferroelectric lead magnesium niobate–lead titanate (PMN–PT) were investigated. Relaxor PMN–PT epitaxial thin films with oxide electrodes were grown by pulsed-laser deposition on (100) LaAlO3 substrates. We observe a systematic decrease in the phase transition temperature (temperature at which a maximum in dielectric response occurs), from around 250 to around 60 °C as the relaxor thickness is increased from 100 to 400 nm. This is accompanied by an increase in the relative dielectric constant (εr), measured at room temperature and 10 kHz, from 300 to 2000. The piezoelectric coefficient d33 measured using a scanned probe microscope, increase by almost an order of magnitude with increasing film thickness.

Garnet-perovskite transformation under conditions of the Earth's lower mantle: an analytical transmission electron microscopy study

Natural pyrope garnets with three different chemical compositions have been transformed at 30–60 GPa in the laser-heated diamond-anvil cell (DAC). Recovered samples were examined by analytical transmission electron microscopy (ATEM). For all three pyropes, the dominant post-garnet phase was Mg-rich orthorhombic silicate perovskite. The Al content of Mg-perovskite increased significantly with increasing pressure and temperature, and its chemical composition became close to that of the starting material. Mg-perovskite with Al 2O 3 content less than 25 mol% was quenched as a single phase of orthorhombic perovskite at ambient conditions, whereas Mg-perovskite with Al 2O 3 content of 25–28 mol% transformed into the alternating lamellae of orthorhombic perovskite and lithium niobate phases. With further increasing the Al 2O 3 content, Mg-perovskite converted into a single phase of the lithium niobate structure with polysynthetic twinning on {101̄2̄} at ambient conditions. The high Al content may also induce the stabilization of Fe 3+ in the perovskite structure accompanied by metallic iron. Two Al-rich phases, corundum and a new Al-rich phase (the NAL phase) were recognized with Mg-perovskite mostly at lower pressures. The NAL phase is close to M 3Al 4Si 1.5O 12 (M=Mg, Fe, Ca, Na, K), i.e., the middle of A 2O 3 and AB 2O 4 type compounds, and is accompanied by stishovite. The electron diffraction patterns are consistent with the space group P6 3/m or P6 3 with a=8.85 (2) Å, c=2.78 (2) Å, V=188 (1) Å 3. The existence of large cations such as Ca, K, and Na seems to stabilize the NAL phase relative to the corundum phase, but excess Ca and Na contents are likely to induce the formations of Ca-rich cubic perovskite and the calcium ferrite phase, respectively.

On the cooling effect in the formation of periodic poled lithium niobate crystals grown by Cz technique

The effect of the cooling rate after the off-centered Cz growth process in the formation of the periodic poled lithium niobate structures has been studied. It has been observed that for a slow cooling process the periodic structures appear only at the edges of the crystals, while for a quenching to RT process the periodic structures are also formed at the center of the crystals. This fact has been related to the shape of the solid–liquid interface and the competence of the built-in internal fields created by the thermal and compositional gradient. Moreover, it has been observed that the Nb and dopant concentration along the periodic domain structure is different in both flat and convex interface.

The MgTiO3-FeTiO3join at high pressure and temperature

The phase relations at high pressure and high temperature for the FeTiO 3 -MgTiO 3 join were de­termined using several different experimental methods. Through a series of multi-anvil experiments, a phase boundary with a negative slope was observed between MgTiO 3 I (ilmenite structure) and a high pressure phase with the MgTiO 3 II (lithium niobate structure) after quenching. The enthalpy of transformation of MgTiO 3 I to MgTiO 3 II was determined through transposed-temperature-drop calo­rimetry to be 28.78 ± 1.45 kJ/mol. The enthalpy of transformation from ilmenite to lithium niobate structure was also determined for three intermediate compositions on the FeTiO 3 -MgTiO 3 join, Fe 0.2 Mg 0.8 TiO 3 , Fe 0.5 Mg 0.5 TiO 3 and Fe 0.8 Mg 0.2 TiO 3 , and confirmed for FeTiO 3 , and was found to be a linear function of composition. These experiments represent one of the first successful calorimetric measurements on small samples (1 to 3 mg) synthesized at high pressures (15 to 21 GPa). X-ray analysis during compression of Fe 0.5 Mg 0.5 TiO 3 II in a diamond cell confirmed a room temperature transition at 28 GPa to Fe 0.5 Mg 0.5 TiO 3 III (a GdFeO 3 -type perovskite structure), similar to the transi­tions previously observed in FeTiO 3 and MnTiO 3 . The Fe 0.5 Mg 0.5 TiO 3 sample was heated to 802 ℃ at 21 GPa, and it was observed that the stable high temperature, high pressure phase is perovskite, Fe 0.5 Mg 0.5 TiO 3 III. The above data combined confirm the stability of a continuous perovskite solid solution at high pressure and temperature for the FeTiO 3 -MgTiO 3 join.

On the effect of Li diffusion in Er-doped bulk periodic poled lithium niobate crystals

The impurity distribution of either Nb and Er ions, during a Li diffusion process, has been studied in the as-grown Er-doped periodic poled lithium niobate structures. It has been observed that there is a periodic variation of the Nb concentration along the periodic domain structure for the as-grown crystals. The Nb variation in the as-grown crystal is modified when the samples are submitted to a vapor transport equilibration process, which is related with Li diffusion. The Er concentration is constant along the domain structure.

Dependences of the refractive indices on the proton concentration in H:LiNbO3 waveguides

Proton-exchanged optical waveguides in lithium niobate crystals exhibit a rich variety of structures and phases. It is established that seven HxLi1−xNbO3 crystalline phases with a lithium niobate structure may form under various conditions of proton exchange and post-exchange annealing. A method is proposed to determine the proton concentration in the various phases identified. Relationships are established between the structural parameters, the proton concentration, and the ordinary and extraordinary refractive indices of various HxLi1−xNbO3 crystalline phases. The results can explain various optical phenomena observed in proton-exchanged waveguides and permit prediction of the characteristics of light-guide structures.

Clinopyroxene-perovskite phase transition of FeGeO 3 under high pressure and room temperature

In order to confirm the possible existence of FeGeO3 perovskite, we have performed in situ X-ray diffraction measurements of FeGeO3 clinopyroxene at pressures up to 40 GPa at room temperature. The transition of FeGeO3 clinopyroxene into orthorhombic perovskite is observed at about 33GPa. The cell parameters of FeGeO3 perovskite are a=4.93(2) A, b=5.06(6) A, c=6.66(3) A and V=166(3) A3 at 40 GPa. On release of pressure, the perovskite phase transformed into lithium niobate structure. The previously reported decomposition process of clino-pyroxene into Fe2GeO4 (spinel)+GeO2 (rutile) or FeO (wustite) +GeO2 (rutile) was not observed. This shows that the transition of pyroxene to perovskite is kinetically accessible compared to the decomposition processes under low-temperature pressurization.