Raman and Infrared Phonon Features in a Designed Cubic Polymorph of CaTa₂O₆

Abstract

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.

Optical phonon characteristics of an orthorhombic-transformed polymorph of CaTa2O6single crystal fibre

“…The optically-active vibrational modes of CaTa 2 O 6 crystal fibres crystallized within the ordered Pm3 cubic structure were previously obtained by Raman and infrared spectroscopies [9,11]. A total of 21 Raman-active modes at the Brillouin-zone centre (5A g ⊕ 5E g ⊕ 11F g ), along with their appropriate symmetries were discerned by special scattering configurations.…”

“…A total of 21 Raman-active modes at the Brillouin-zone centre (5A g ⊕ 5E g ⊕ 11F g ), along with their appropriate symmetries were discerned by special scattering configurations. In addition, 13 of the 16 predicted triply degenerate (F u ) infrared modes were obtained by micro-infrared reflectivity [9]. After long-time annealing, the Raman spectra became completely different, owing to the polymorphic transformation to an orthorhombic phase.…”

“…Structural characterization of the as-grown fibres by single crystal x-ray diffraction (Gemini-Oxford diffractometer, Mo-Kα source, 45 kV, 40 mA) showed that they grew into a single Pm3 ordered cubic phase, with a lattice parameter a = 7.745 Å and Z = 4. The fibre's longitudinal axis was identified as nearly parallel to one of the cubic edges, and the small facets were observed to be mainly normal to the cubic [110] directions [9]. Thermal treatments to induce polymorphic transformation were done in air, at 1200°C, for 24 h. The heating/cooling rates used were ±5°C min −1 .…”

“…In a previous work, we have investigated the optical vibrational properties of a CaTa 2 O 6 single crystal fibre grown by the Laser-Heated Pedestal Growth method (LHPG) [9]. The samples obtained by this method always crystallize within an ordered cubic structure belonging to the Pm3 space group [10].…”

“…Therefore, the investigation of the complete phonon spectra of CaTa 2 O 6 in its different polymorphic phases contributes to the understanding of its dielectric behavior and optimization of the technical applications within the appropriate crystal phase. Concerning the ordered Pm3 cubic phase, we have already reported a thorough Raman and infrared investigation of its optical vibrational modes, with a comprehensive set of phonon modes and their characteristics [9]. Very recently, we were able to obtain single crystal fibres of CaTa 2 O 6 with the orthorhombic Pnma structure, by long-time annealing (24 h) at 1200°C [11].…”