Vapor transport equilibration was used to prepare lithium niobate crystals of a variety of controlled off-congruent compositions. Crystals were characterized through measurement of the ferroelectric Curie temperature Tc and measurement of the temperature for noncritical phase matching TPM of second-harmonic generation from both 1.064- and 1.32-μm laser sources. Across the majority of the single-phase region, both Tc and TPM were observed to vary nearly linearly with Li/Nb ratio. The variation in TPM with Li/Nb ratio was observed to change direction within the single-phase region at some small but finite compositional interval away from the Li-rich phase boundary. Crystals equilibrated to the Li-rich phase boundary had excellent optical homogeneity. Preparation of Li-poor crystals was hampered by extremely slow lithium diffusivity and problems with second-phase precipitation.

Articles you may be interested inPredicting temperature dependence of the refractive index and nonlinear optical coefficients in lithium niobate An investigation was performed on the diffusivity of lithium in lithium niobate at 1100 "C in the crystallographic z direction over the composition range from 48.38 to 49.85 mol % L&O. A vapor transport technique was applied to produce a lithium niobate crystal with a widely varying diffusion-limited lithium concentration profile. The profile was measured through spatially resolved measurement of the phase-match temperature for frequency doubling of a Nd:YAG laser. A Boltzmann-Matano analysis was applied to the profile to estimate diffusivity as a function of composition. The validity of the Boltzmann-Matano results was examined through numerical simulation, taking into account finite interfacial mass transport. A dramatic increase of lithium diffusivity with Li/Nb ratio was observed, ranging from roughly 3 X 10d9 cm27s at the 48.38 mol % L&O congruent composition to roughly 50X lop9 cm2/s at 49.85 mol % Li,O.

No abstract

Stoichiometry issues in lithium tantalate were studied through Czochralski growth of single crystals and vapor transport equilibration (VTE) of crystalline samples to specified Li/Ta compositions. Crystals were characterized through measurement of the ferroelectric Curie temperature Tc. The VTE experiments indicated a linear variation of Tc with Li/Ta ratio across the investigated range of 47.5–49.5 mol % Li2O. By control of initial melt stoichiometry, crystals representing melt fractions ≳90% were grown having compositional uniformity within ±0.015 mol % Li2O. Growth of the compositionally uniform crystals was determined to take place through incongruent solidification, with lithium rejection from the growth interface compensating for lithium volatilization from the melt surface. With the growth process applied, a melt composition of 48.39 mol % Li2O yielded grown crystals of a uniform 47.85 mol % Li2O solid composition. The lithium tantalate congruent composition was determined to lie at a value less than or equal to 47.70 mol % Li2O.

Lithium niobate single-crystal wafers were chemically reduced using nitrogen/hydrogen atmospheres at a variety of temperatures between 350 and 750 °C. Reduced wafers were measured to have increased optical absorptivity and increased direct current (dc) electrical conductivity as a function of reduction-processing temperature. Absorptivity increases were measured larger than 1000×; conductivity could be increased by more than a factor of 105. Standard 78 MHz surface acoustic wave (SAW) filters were successfully fabricated using reduced wafers, and shown to have increased resistance to spurious pyroelectric charging effects during thermal cycling. The observed increases in optical absorptivity and dc electrical conductivity can also be expected to provide benefits for SAW device fabrication processing.