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.     

Congruent Composition and Li-Rich Phase Boundary of LiNbO3

The congruent composition of LiNbO₃ was determined precisely by correlating crystal growth compositions with the Curie temperatures of samples quenched from a melt before and after crystal growth and of various sections of crystals grown from that melt. The initial melt composition was determined by control of Li₂CO₃ and Nb₂O₅ contents. The melt compositions were varied from 47 to 49 mol% Li₂O. The variation of T_e with melt composition was found to follow T_e = 9095.2 − 369.05C + 4.228C², where C is mol% Li₂O. High-temperature DTA was used to determine T_e with a precision of ±2°C. Above 1000°C the Li-rich phase boundary was found to be a function of temperature by determining the weight gain after complete lithiation of congruently grown crystals. This boundary curves slightly toward lower Li₂O content as temperature increases. The congruent composition of LiNbO₃ contains 48.45 mol% Li₂O and has a measured Curie temperature of 1138°± 2°C.     

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.     

Formation of Small-Sized CdSxSe1-x Crystals in Sol-Gel-Derived SiO2 Glasses

Silica glasses doped with small-sized CdS_x-Se_1-x crystals were prepared by the sol-gel method. Gels synthesized by the hydrolysis of Si(OC₂H₅)₄, in the presence of CdSeO₄ with NH₄SCN dissolved in HNO₃ or NH₄OH, were heated in H₂-N₂ atmosphere. The pH value of solutions for CdSeO⁴ and NH⁴SCN primarily determined the fraction of anions in CdS^xSe^1-x crystals. The anion content in crystals was dependent on the pH value of the solvent and/or heat-treatment temperature, and the sulfur fraction changed from 0.1 to 0.6. The optical absorption spectra were red-shifted as the selenium content and the crystal size increased, and the emission spectra showed a sharp band near the absorption edge position and a broad tail extending into the long wavelength. The optical band gap energies increased reciprocally proportional to the square of the crystal size.     

Detection of a Lithium-Rich Skin on Lithium Niobate Single Crystals

Optical and DTA measurements of a single crystal of LiNbO₃ pulled from the congruent melt indicate a radial composition gradient leading to a Li-rich surface. From T_c measurements it is concluded that the surface is ≥0.15 mol% Li₂O richer than the bulk composition. The skin thickness is ≤1 mm and is generally removed prior to device preparation; however, some possible sources of concern are discussed for subsequent high-temperature processing.     

Patterning of Non-Linear Optical Crystals in Glass by Laser-Induced Crystallization

This paper reports recent progress in the patterning of non-linear optical crystals on the glass surface by laser irradiation. Two techniques for the writing of crystal lines have been developed, i.e., rare-earth (samarium) atom heat processing and transition metal atom heat processing, in which a continuous wave Nd:YAG laser (wavelength: λ=1064 nm) is irradiated to the glasses containing rare-earth (RE: Sm³⁺, Dy³⁺) ions or transition metal (TM: Ni²⁺, Fe²⁺, V⁴⁺) ions. The writing of crystal lines such as β-BaB₂O₄, Sm _x Bi_{1− x} BO₃, and Ba₂TiGe₂O₈ showing second harmonic generations has been successful. It is clarified from the azimuthal dependence of second harmonic intensity and polarized micro-Raman scattering spectra that crystal lines consist of highly oriented crystals along the crystal line growth direction. It is also possible to write two-dimensional crystal bending or curved lines by just changing the laser scanning direction. The mechanism of the laser-induced crystallization has been proposed.     

Concurrent Measurement of Volume, Grain-Boundary, and Surface Diffusion Coefficients in the System NiO-Al2O3

Surface, grain-boundary, and volume inter diffusion coefficients for the NiO-Al₂O₃ system were measured concurrently by using a diffusion couple consisting of an A1₂O₃ bicrystal and an NiO single crystal. The A1₂O₃ bicrystals having various tilt angles were fabricated by firing 2 single crystals to be joined in an H₂ atmosphere at 1800°C for 30 h. Diffusion profiles over the surface, along the grain boundary, and in the bulk of the bicrystal were determined with an electron probe microanalyzer. Mathematical analysis of the diffusion profiles gives D _s = 7.41×10^-2 exp (-35,200/ RT ), D _gb = 2.14×10^-1 exp (-63,100/ RT ) (tilt angle =30°), and D _v = 1.26×10⁴ exp (-104,000/ RT ). The grain-boundary diffusion coefficient increases with the mismatch at the boundary.     

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.     

Optical Spectra of 3d Transition Metal Ions in MgO 3·52O3 Spinel

Large gem-quality boules of MgO·3.5A1₂, O₃ spinel, both pure and incorporating controlled amounts of Ti³, V₃₊, Cr³⁺, Mn₂, Fe₂₊, Co²⁺, or Ni₂₊ were synthesized by a modified verneuil technique. Each crystal was characterized by measuring the lattice parameter, chemical composition, and density. Using thin polished slices as specimens, the absorption spectrum of each crystal was measured from 400 to 50, 000 cm⁻¹, and band centers and order-of-magnitude oscillator strengths were calculated from the spectrum for each specimen. For these well characterized crystals, the absorption spectra were interpreted satisfactorily in terms of crystal field theory when the complete individual spectrum as well as the spectrum of the host matrix was considered. On this basis, the oxidation state and coordination of the 3d ion could be determined. Covalent bonding effects were of definite significance in these spinels and caused an inexact correspondence between observed band centers and those predicted by the Orgel diagrams. The ratio Dq (tetrahedral)/ Dg (octahedral) was 0.67 in the 1:3.5 spinel and verified the theoretical relation expressing this ratio in terms of a constant, 4/9, times the fifth power of the ratio of the site radii. Coordination preferences of the incorporated transition metal ions are also discussed.     

Conversion of Polycrystalline Al2O3 into Single-Crystal Sapphire by Abnormal Grain Growth

In a given batch more than 30%–40% of polycrystalline, MgO-doped Al₂O₃ tubes were converted into single crystals of sapphire by abnormal grain growth (AGG) in the solid state at 1880°C. Most crystals grew 4–10-cm in length in tubes with wall thicknesses of 1/2 and 3/4 mm and outer diameters of 5 and 7 mm, respectively, and had their c -axes oriented ∼ 90° and 45° to the tube axis. Initiation of AGG was associated with low values of bulk MgO concentration near 50 ppm. The unconverted tubes did not develop centimeter-size crystals but instead exhibited millimeter-size grains. The different grain structures in converted and unconverted tubes may be related to nonuniform concentration of MgO in the extruded tubes. The growth front of the migrating crystal boundary was typically nonuniformly shaped, and the interface between the single crystal and the polycrystalline matrix was composed of many "curved" boundary segments indicative of classical AGG in a single-phase material. The average velocities of many migrating crystal boundaries were quite high and reached ∼1.5 cm/h. The average grain boundary mobility at 1880°C was calculated as 2 × 10⁻¹⁰ m³/(N·s), representing the highest value reported so far in Al₂O₃ and within a factor of 2.5 of the calculated intrinsic mobility. Under similar experimental conditions sapphire crystals did not grow when a codopant of CaO, La₂O₃, or ZrO₂ was added in concentrations of several hundred ppm.     

Structural and Electrical Properties of Er2O3-Doped Na1/2Bi1/2TiO3 Lead-Free Piezoceramics

Sodium bismuth titanates Na_1/2Bi_1/2TiO₃ (NBT) doped with 0–3 wt% Er₂O₃ were prepared by the conventional solid-state reaction method. The X-ray diffraction results revealed that the sintered Er-doped NBT ceramics exhibited a pure perovskite structure with Er³⁺ concentrations ranging from 0 to 1 wt%. At a low Er₂O₃ concentration, the Er-doped NBT ceramics showed enhanced electrical properties with dielectric constant ɛ₃₃^T/ɛ₀=636, a low dielectric dissipation factor (tan δ=3.3%), a low coercive field ( E _c=4.56 kV/mm), and a high piezoelectric constant ( d ₃₃=75 pC/N). The relationship between the composition and properties of Er-doped NBT ceramics has been discussed.     

Structural Characterization and Luminescent Properties of a Red Phosphor Series: Y2−xEux(MoO4)3 (x=0.4–2.0)

A series of rare earth molybdates, Y_{2− x} Eu _x (MoO₄)₃ for x =0.4, 0.8, 1.2, 1.6 and 2.0 were prepared by solid-state method and their crystal structures, photo luminescent characteristics were investigated. The powders are mainly studied for their red light emission efficiency under near UV excitation. The crystal structures of the powders were found to depend on annealing temperature and the yttrium concentration. Mixtures of monoclinic ( C 2 /c ) and orthorhombic ( Pba 2, Pbna ) structures were formed in varying proportions depending on the value of x and annealing temperatures (700°–800°C). The luminescence behavior depended on the resultant composition of the crystal phase and the Eu³⁺ concentration. The excitation spectra showed the characteristic and broad O→Mo charge transfer (CT) band of the MoO₄ tetrahedra and the sharp intra-configurational 4 f –4 f transitions of Eu³⁺ in the host lattice. The integrated emission ratio (⁵D₀→⁷F₂/⁵D₀→⁷F₁) of Eu³⁺ depends on the annealing temperature and reveals that the local site symmetry of Eu³⁺ ions decreases with increasing concentration of Eu³⁺. The emission spectra obtained by exciting at 396 nm, gave highest red emission intensity for Y_0.4Eu_1.6(MoO₄)₃ annealed at 700°C/6 h among this series of samples.     

XRD and Raman Studies on the Ordering/Disordering of Ba(Mg1/3Ta2/3)O3

The samples of complex perovskite Ba(Mg_1/3Ta_2/3)O₃ (BMT) were synthesized at various sintering temperatures and the X-ray diffraction (XRD) patterns and Raman spectra of the samples were collected. Both the structure refinements using the XRD data and the Raman spectra show that the samples sintered above 1500°C with high pellet relative density (>95%) take almost-perfect B-site-ordered structure; while the samples sintered below 1400°C with low pellet relative density (<70%) take the more disordered structure at B-sites. The vibrational modes of 1:2 ordered BMT     were obtained and illustrated by using first-principle calculations. With the assistance of the calculation results, Raman peaks of BMT were assigned as A _1g(1) (107 cm⁻¹), A _1g(2) (212 cm⁻¹), A _1g(3) (433 cm⁻¹), A _1g(4) (798 cm⁻¹), and E _g(1) (104 cm⁻¹), E _g(2) (160 cm⁻¹), E _g(3) (264 cm⁻¹), E _g(4) (386 cm⁻¹), E _g(5) (576 cm⁻¹). The highly ordered BMT sample at B-site shows longer phonon lifetime and weaker coupling among phonons than the disordered BMT, which probably are the intrinsic reasons for the highly ordered BMT sample to have the low dielectric loss.     

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.     

Habit Changes of Sapphire Grown from PbO-PbF2, and MoO3-PbF2 Fluxes

Single crystals of Al₂O₃ (sapphire) were grown from PbO-PbF₂ and MoO₃-PbF₂ fluxes; they varied from flat plates (PbF₂-rich melts) to equidimensional crystals (PbO- or MoO₃-rich melts). The primary growth planes are basal {0001}, first-order rhombohedral {1011}, and second-order rhombohedral {0112}. The habit change is interpreted on the basis of F^- contamination and Pb²⁺ surface adsorption. Possible ion species in the melts and their relative importance on crystal growth from these systems are discussed.     

Infrared Spectra of Rapidly Quenched Glasses in the Systems Li2O-RO-Nb2O5 (R=Ba, Ca, Mg)

Twin-roller quenching produced wide ranges of glass formation in the systems Li₂O-RO-Nb₂O₅ (R=Ba,Ca,Mg). The glass-forming ability is improved with an increase in the ionic radius of R²⁺ ions. The crystallization temperature is increased as Li₂O is replaced by RO or the ionic radius of R²⁺ ions is increased. Infrared spectra revealed that the glass LiNbO₃ (=50Li₂O-0Nb₂O₅) was composed of six-coordinated NbO₆ octahedra, which were joined together by corner-sharing only. In the Li₂O-RO-Nb₂O₅ glasses there exist edge-shared as well as corner-shared NbO₆ octahedra. The edge-shared NbO₆ octahedra in the glasses are increased with an increase in the content of RO or Nb₂O₅, and also with an increase in the ionic radius of R²⁺ ions.     

Large Pyroelectric Effect in Relaxor-Based Ferroelectric Pb(Mg1/3Nb2/3)O3–PbTiO3 Single Crystals

The pyroelectric properties of (1− x )Pb(Mg_1/3Nb_2/3)O_{3− x} PbTiO₃ (PMN− x PT) single crystals with various compositions and orientations have been investigated using a dynamic method. Excellent pyroelectric performances can be achieved in 〈111〉-oriented rhombohedral PMN− x PT (0.24≤ x ≤0.30) crystals, where the measurement direction corresponds to the polar axis of the crystal. At room temperature, the pyroelectric coefficient and the detectivity figure of merit ( F _d ) for the 〈111〉-oriented PMN–0.28PT single crystal are 8.55 × 10⁻⁴ C·(m²·K)⁻¹ and 9.89 × 10⁻⁵ Pa^−1/2 (100 Hz), respectively, superior to those of the widely used pyroelectric materials. They are also weak temperature dependent and nearly independent of frequency. These outstanding pyroelectric performances make the single crystals a promising candidate for uncooled infrared detectors and thermal imagers.     

Phase Separation in 12 mol% Ceria-Doped Zirconia Induced by Heat Treatment in H2 and Ar

The phase separation in 12 mol% CeO₂─ZrO₂ ceramic heattreated in a mixture of H₂ and Ar was investigated by X-ray diffractometry (XRD), X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy, and Raman scattering. After heat treatment at temperatures above 1200°C, the tetragonal solid-solution phase separated into Zr₂Ce₂O₇ and the monoclinic phase. Raman scattering measurements also provided supplementary evidence for the phase separation. XPS showed that the valence change from Ce⁴⁺ to Ce³⁺ predominantly occurred, whereas the reduction from Zr⁴⁺ to Zr³⁺ took place above 1200°C. It is concluded, that in the highly reduced sample, where the valence changes from Ce⁴⁺ (Zr⁴⁺) to Ce³⁺ (Zr³⁺), the phase separation is noticeably promoted. Below 1000°C the phase separation was suppressed because of no appreciable valence change to trigger the phase separation, and the single tetragonal phase was retained.     

Correlation of Structure and Electrical Properties of 55FeO-45P2O5 Glass

The ac and dc characteristics of 55 mol% FeO-45 mol% P₂O₅ glass were measured as functions of time of heat treatment at 600° and of the ratio Fe³⁺/Fe_total. The dc resistivity behavior at constant temperature (for bulk specimens) was correlated with the appearance and growth of crystals in a glassy matrix, as indicated by data obtained with a Guinier-De Wolff X-ray camera. Dispersions appeared in plots of tan δ_ne vs frequency; a hypothesis explaining the appearance of these peaks is presented.     

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