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Optically transparent single crystal fibers of calcium niobate (CaNb 2 O 6 ) were investigated by polarized micro-Raman spectroscopy. The fibers were obtained by the laser heated pedestal growth technique, in the form of rods with about 0.4 mm diameter and up to 20 mm long. The crystalline axes were determined by mid-infrared reflectivity and X-ray diffraction. The fibers grew within the Pbcn columbite structure, with their longitudinal axis parallel to the crystallographic a-axis. They were free of cracks or facets, and no secondary phases were detected by the spectroscopic or structural characterizations. Polarized Raman spectra were obtained in adequate parallel and crossed configurations for the orthorhombic D 2h point group. In short, a total of 48 Raman phonon modes, which are characteristic of the CaNb 2 O 6 columbite crystal, were discerned, in good agreement with group theoretical predictions. The selection rules for the columbite group were generally well obeyed, confirming the high purity, ordering, and quality of the material. The knowledge of the optical phonon characteristics of the CaNb 2 O 6 single crystals presented here is important to design compact optical devices, such as solid-state lasers operating in the near-infrared region.

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At room temperature, calcium tantalite (CaTa 2 O 6 ) is known to exhibit three stable polymorphs, depending on the crystal growth conditions. In this work, the laser-heated pedestal growth method was used to obtain CaTa 2 O 6 single crystal fibers in its quenched (ordered) cubic polymorph, attractive for optical applications. X-ray diffraction, Raman scattering and infrared spectroscopy showed that the fibers grew into the centrosymmetrical Pm3 space group. The growth axis was determined as [001]. The cylindrical fibers (20 mm long and 400 μm wide) showed good optical quality, without cracks or striations, but with small facets, parallel to crystal edges or to in-plane edge bicetrices. A comprehensive set of the optical phonons for this cubic structure was obtained, consisting of 13 (of 16 foreseen) triply degenerate polar phonons, and all the 21 predicted Raman modes. By using special scattering geometries, the Raman bands were attributed to their corresponding irreducible representations. In general, the phonons showed rather large damping constants, which do not compromise the designed optical applications in compact lasers but increase the losses in the microwave range, even though the dielectric response is quite adequate for microwave applications. Quenched defects and crystal polymorphism can be at the origin of the large phonon damping.

The experimental setup used for mid-and far-infrared measurements under microscope.The figure below (Fig. S1) shows a sketch of the detector selection inside the microscope.