New LGT crystal for ultra-stable resonators

Among the crystals belonging to the langasite family, the Langatate (La3Ga5.5Ta0.5O14 also called LGT) seems to be the best candidate to substitute quartz crystal for acoustic waves applications and frequency output devices. But, its capability to substitute quartz depends on the crystalline quality of the production of the harvested “boule”. Indeed, the crystal quality (its homogeneity and its atomic structure as well as the amount of chemical impurities) can be very different from one supplier to another. So, to compare and analyze the strategic interest of this crystal, we start a sufficiently researched program, supported by the French DGA, to enable us to address the parameters of the growth process, implemented on “Cristal Innov” platform. The Langatate is an incongruent material grown from a ternary melt of La2O3 — Ga2O3 and Ta2O5 by the Czochralski technique along <00.1> or <2−1.0> in iridium crucible and slightly oxidizing atmosphere. Furthermore, a post-growth annealing was performed in the pulling machine immediately after growth and in other oven under vacuum or in oxidizing/reducing atmosphere. Post-growth annealing is necessary to remove the thermal stress induced by the growth that are revealed by shadowscopy and it will determine the color of the boule, which varies from colorless to orange or pale-green color. We attach to analyze the stoechiometry (i.e. the amount of the majors La, Ga, Ta) by Electron Probe Micro-Analysis (EPMA) and the impurities levels by femtosecond laser ablation Inductively Coupled Plasma Mass Spectrometry (fs LA-ICP-MS) of different LGT crystals from different suppliers. The presence of point defects and inhomogeneity of the crystal composition significantly affect the optical properties and can be also revealed by optical transmission spectra in UV-VIS and IR ranges.