Control of Laser Damage in LiNbO3

Low-frequency (510 MHz) paramagnetic resonance studies of all available undoped LiNbO₃ crystals revealed Fe contamination of the order of 10 to 100 ppm. By using care during crystal growth, the amount of this impurity can be substantially reduced, and crystals with reduced susceptibility to laser damage are produced.     

Characterization of LiNbO3 by Dilatometry and DTA

DTA can successfully measure T_c and hence the lithium oxide content for samples over most of the single-phase range of LiNbO₃. Above 7sim;1190°C, however, the enthalpy associated with the decomposition and melting of the near stoichiometric material masks the weak thermal change associated with T_c . Fortunately, dilatometry can be also used to detect T_c . The values of T_c determined by these thermoanalytical techniques are 1130° and 1198°C for the congruent and stoichiometric compositions, respectively. Careful dilatometric measurement of the thermal expansion in the temperature range from 350 to 1300 K indicates that, while the a axis expands continuously, the c axis passes through a maximum at 640°C for the congruent material and 700°C for the stoichiometric material. Thermal expansion for the stoichiometric compared to the congruent composition is slightly greater along the c axis and somewhat smaller along the a axis. These expansions compensate to make the volume expansion of the two materials virtually identical.     

Determination of Stoichiometry Variations in LiNbO3 and LiTaO3 by Raman Powder Spectroscopy

Raman spectroscopy of powders is shown to be a sensitive technique for detecting stoichiometry variations. The method makes use of the broadening of the Raman-active modes which occurs when the translational symmetry of the lattice is reduced on deviation from simple, fixed cation ratios in the structure. Application of the technique to the LiNbO₃( ss ) system shows that the room-temperature solid-solution range does not exceed 50±0.5 mol% Nb₂O₅ and that congruent LiNbO₃( ss ) at 51.4 mol% Nb₂O₅ is thermodynamically unstable below ≅800°C. Similarly, application of the Raman method to the LiTaO₃( ss ) system indicates that congruent LiTaO₃( ss ) at 51.0 mol% Ta₂O₅ is unstable at temperatures <1000°C.     

Effect of BiScO3 and LiNbO3 on the Piezoelectric Properties of (Na0.5K0.5)NbO3 Ceramics

Lead-free potassium sodium niobate-based piezoelectric ceramics (1− y )(Na_{0.5−0.5 x} K_{0.5−0.5 x} Li _x )NbO₃− y BiScO₃ (  y =0.01, x= 0–0.06) have been prepared by an ordinary sintering process. The XRD analysis showed that the structure changes from orthorhombic to tetragonal with the increase of x (at y =0.01, abbreviated as KNNBSL100 x ). At room temperature, the polymorphic phase transition from the orthorhombic to the tetragonal phase was identified at approximately 0.02≤ x ≤0.04. The piezoelectric and ferroelectric properties were significantly enhanced. The temperature dependences of the relative permittivity revealed that the Curie temperature was increased with the addition of LiNbO₃. These solid solution ceramics are promising as potential lead-free candidate materials.     

CrO2-Cr2O3 Phase Boundary Under High O2 Pressures

The phase boundary between CrO₂ and Cr₂O₃ was reinvestigated under high O₂ pressures by using a new type of gas compressor. The boundary curve can be represented as log Po₂= 7.16-(3579/ T ). Using the observed data, Δ G °, Δ H °, and Δ S ° for the reaction 2CrO₂⇋Cr₂O₃+½O₂ were calculated to be: Δ G °= -(1.55/100) T +7.60 kcal/mol, Δ H °= -8.19 kcal/mol, and Δ S °= (-15.8/ T )+0.0155 kcal/mol.     

Selective Nucleation and Band-Gap Widening of LiNbO3 Nanocrystals on an α-Al2O3 Substrate

Selective nucleation of LiNbO₃ nanocrystals on the steps was observed during the initial stage of LiNbO₃ film growth in an α-Al₂O₃ (0001) substrate with a multiatomic step structure. In addition, stress stored in the nanocrystals as a result of the different thermal expansion coefficients of the film and the substrate caused band-gap widening. As the size of the nanocrystals decreased, the band gap shifted to a higher energy.     

Morphotropic Phase Boundary in the Pb(Zn1/3Nb2/3O3)-BaTiO3-PbTiO3 System

The morphotropic phase boundary (MPB) in the relaxor ferroelectric system Pb(Zn_1/3Nb_2/3O₃)-BaTiO₃-PbTiO₃ (PZN-BT-PT) with 15 mol% BT was investigated through dielectric permittivity and high-temperature X-ray diffraction measurements. It was revealed that MPB is a broad composition region extending from 12 to 18 mol% PT, within which the temperatures of the permittivity maximum are close to the ending temperatures for the phase transformation from coexisting rhombohedral and tetragonal phases to cubic phase on heating. When the specimen is cooled, the starting temperatures for the rhombohedral-to-tetragonal phase transition increase with increasing PT content. The large thermal hysteresis observed by X-ray diffraction is caused by the phase transformation between rhombohedral and tetragonal phases. On cooling, the MPB curves toward the PT-rich side, so that ceramics within this composition range undergoe successive phase transitions from cubic to rhombohedral and from rhombohedral to tetragonal phase. The diffuseness of the paraelectric-to-ferroelectric phase transition is remarkably decreased by the addition of PT. The enhanced dielectric permittivity peak values for the MPB compositions are correlated with the reduced lattice distortion and phase coexistence.     

Compositional Studies of Lanthanum-Modified Morphotropic Phase Boundary Pb(Mg1/3Nb2/3)O3—PbTiO3 Ceramics

Investigations have been performed on the La-modified (1 - x )Pb(Mg_1/3Nb_2/3)O₃—( x )PbTiO₃ crystalline solution for compositions close to the morphotropic phase boundary (MPB) using energy-dispersive X-ray spectrometry (EDS). Studies were performed on specimens with La contents between 0 and 10 at.% for x = 0.35 which were fabricated by keeping the average Mg/Nb/Ti ratio in the bulk ceramic unchanged. In this series of samples, La3 was compensated for by introducing B-site vacancies. Quantitative analysis by EDS revealed a change in the B-site cation concentrations with La substitution. It was observed that the Nb concentration decreases and the Ti concentration increases with increasing La content, indicating that local charge compensation in the perovskite phase is taking place by an adjustment in B-site cation concentrations. Phase analysis demonstrated the presence of a second phase for La contents above 2 at.%. Scanning electron microscopy then revealed the presence of a secondary pyrochlore phase with an octahedral-like morphology. The composition of the pyrochlore phase was found to be rich in Nb and poor in Ti. The effects of changes in composition on dielectric properties, polarization, and electrically induced strain were then investigated.     

Orientation Control and Laser-Beam-Assisted Crystallization of Sol-Gel-Derived, Titanium-Doped LiNbO3 Thin Films

Two processes for the crystallization of titanium-doped LiNbO₃ (LNT) gel thin films were investigated in the present study. LNT gel thin films were prepared on platinized glass and platinized sapphire substrates, as the application of a platinum intermediate layer greatly affected the orientation of LNT films on these substrates. X-ray pole figures revealed the well-oriented growth for LiNbO₃ both on the platinum (111)/sapphire C substrate and bare sapphire C. Another approach to crystallizing LNT gel film was attempted by developing a new technique to fabricate a LiNbO₃ waveguide on the sapphire substrate using a Nd:YAG laser beam. The crystallization of LNT was achieved successfully with no cracking or peeling. The prepared crystalline LNT films also were characterized by Raman microspectroscopy. S. E. Trolier-McKinstry—contributing editor     

Piezoelectric Resonances, Linear Coefficients and Losses of Morphotropic Phase Boundary Pb(Mg1/3Nb2/3)O3–PbTiO3 Ceramics

A method based on the use of four piezoelectric resonances for three sample geometries that allows obtaining the full set of linear electric, mechanical, and electromechanical coefficients, and all related losses of a piezoelectric ceramic has been applied to Mn-doped 0.655Pb(Mg_1/3Nb_2/3)O₃–0.345PbTiO₃ at the morphotropic phase boundary (MPB PMN–PT). Length-poled MPB PMN–PT ceramic plates presented piezoelectric shear double resonances associated with a thickness gradient of tetragonal and rhombohedral (or monoclinic) phases that originated during poling. The versatility of the method still allowed addressing these double resonances and obtaining all the linear coefficients and losses of the well-poled material. These are given for MPB PMN–PT and compared with those of a Navy type II Pb(Zr,Ti)O₃ (PZT) ceramic. MPB PMN–PT presents piezoelectric coefficients as high as soft PZT but significantly lower losses, and so less overheating and hysteresis under high driving fields. Its thermal stability has been studied up to 100°C, and the temperature dependence of a number of linear coefficients and of the thickness and planar coupling factors and frequency constants of disks has been obtained. The latter thickness parameters hardly changed with temperature, while planar ones showed a relative variation of 10%.     

Effects of TiO2 Addition to LiNbO3 on the Cation Vacancy Content, Curie Temperature, and Lattice Constants

The defect model of Abrahams and Marsh is expanded to consider the addition of TiO₂ and other oxides to form solid solutions with both the stoichiometric and congruent compositions of LiNbO₃. It is shown that the Curie temperature (T_c) and the limits of solubility are determined by the cation vacancy content regardless of whether these are formed by Li deficiency or the addition or substitution of aliovalent ions. X-ray powder diffraction patterns indicate that the a-axis parameter decreases markedly with additions of TiO₂ while the c-axis parameter remains essentially unchanged. These results are compared with prior investigations, and the technological implications of this work to lightguide devices are described.     

Locally Er-Doped Near-Stoichiometric LiNbO3 Crystals Prepared by Standard Er Diffusion and Post-VTE Treatment

The feasibility of preparing locally Er-doped near-stoichiometric (NS) LiNbO₃ crystals for integrated optics applications is demonstrated by a two-step process with standard diffusion (1130°C/154 h) of Er metal followed by vapor transport equilibration (VTE) treatment under three different conditions of 1135°C/22 h, 1115°C/50 h, and 1125°C/60 h. Detailed studies on the crystalline phase, Li composition, diffused surface roughness, and emission characteristics of Er³⁺ ions indicate that there is an upper limit on the initial Er metal film thickness: ∼20 nm for an X -cut crystal and ∼30 nm for a Z -cut crystal. When the initial Er film thickness is below this limit, the post-VTE does not induce formation of ErNbO₄ precipitate and the diffused surface retains high quality with a root mean square roughness <3 nm. Depending on the VTE condition adopted, the VTE results in the increase of ([Li]+[Er])/[Nb] ratio in the diffused layer from congruent point (94.5%) to 97.4%–99.4%. Secondary ion mass spectrometry study shows that the post-VTE does not affect the Gauss nature of the Er profile, but leads to the increase of diffusion depth by as much as 1.6 μm. In comparison with the standard Er diffusion, the post-VTE results in the decrease of Er diffusivity by three to nine times. The higher the VTE temperature is, the lower the Er diffusivity is. In addition, the post-VTE also results in definite reduction of OH⁻ content in crystal, slight lengthening of lifetime and slight narrowing of linewidth of Er³⁺ emission at 1530 nm.     

A Phase Diagram for Epitaxial PbZr1−xTixO3 Thin Films at the Bulk Morphotropic Boundary Composition

A phase diagram of temperature versus strain was constructed for a (001)-oriented PbZr_{1− x} Ti _x O₃ epitaxial single crystal thin film near the bulk morphotropic boundary composition ( x =0.47) on an (001)-oriented cubic substrate. The phase-field approach is employed. It is shown that a mixture of distorted rhombohedral, orthorhombic, and tetragonal phases exists under small values of strain, i.e., close to the in-plane clamped boundary condition. This result contradicts thermodynamic calculations assuming a single-domain state that predicted a single distorted rhombohedral phase under similar strains. Furthermore, it is demonstrated that under a large tensile strain current phase-field simulations showed a tetragonal phase with a ₁ /a ₂ twin structures as the stable state whereas thermodynamic calculations predicted an orthorhombic phase.     

Dielectric and Piezoelectric Properties of the Morphotropic Phase Boundary Composition in the (0.8−x) Pb(Mg1/3Ta2/3)O3−0.2PbZrO3−xPbTiO3 Ternary System

Morphotropic phase boundary (MPB) compositions separating rhombohedral and tetragonal phases in the (1− x − y )Pb(Mg_1/3Ta_2/3)O₃– y PbZrO₃– x PbTiO₃ (PMT–PZ–PT100 x ) ternary solid solution system were characterized using X-ray diffraction and dielectric, piezoelectric properties. This work focused on compositions with a PZ content fixed at y =0.2, with an MPB composition found to be located at x =0.4. Piezoelectric coefficients and dielectric permittivity were found to be on the order of d ₃₃=580 pC/N and 4100, respectively. Acceptor modification using manganese was found to induce a "hardening" effect in 0.4PMT–0.2PZ–0.4PT, with decreased piezoelectric coefficients d ₃₃ and dielectric loss and increased mechanical quality factor Q . Piezoelectric coefficients d ₃₃, Q values, and dielectric loss were found to be 500 pC/N, 2000, and 0.4%, respectively, for 0.4PMT–0.2PZ–0.4PT with MnO₂ dopant levels around 0.5 wt%. The figure of merit (product of Q and d ₃₃) was found to be on the order of 1 × 10⁶, significantly higher when compared with other hard piezoelectric PZT materials. Specifically, the PMT–PZ–PT materials may be attractive candidates for high-power ultrasonic applications, particularly fine-scale components that require relating high permittivities.     

The Influence of Piezoelectric Grain Resonance on the Dielectric Spectra of LiNbO3 Ceramics

It has often been suggested that the higher frequency dispersion ( f ∼10⁷∼10⁸ Hz) evident in the dielectric response of high-permittivity ferroelectric ceramics might be associated with piezoelectrically driven resonant mechanical motion of individual grains. The hypothesis certainly appears reasonable, but so far no direct verification appears to have been attempted. Our discovery that the individual crystallite resonances are well preserved in poled LiNbO₃ bicrystals suggested that we explore the dispersion of LiNbO₃ ceramics. In both conventionally sintered and uniaxially hot-pressed ceramics, the dielectric dispersion around 10⁸ Hz has obvious resonant character. Using a modified ECAP computer program demonstrated that both the real and imaginary components of the impedance can be quantitatively described using the equivalent circuit models for the coupled individual grain resonances.