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Pivovarova, A. P.; Strakhov, V. I.; Smirnov, Yu. M.

doi:10.1023/a:1011360418213

Cited by 6 publications

(20 citation statements)

References 2 publications

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“…3) indicated a total of 5% weight-loss from 600 C to 715 C, which we attributed to the CO 2 loss from CaCO 3 compound precursor. The DTA analysis presented a chemical reaction band between the precursors, beginning at 718 C and ending at 1230 C. The DTA curve showed two endothermic peaks: one at 1561 C and the other at 1693 C. These peaks were associated with the different structural modifications existing in the CT compound, as reported by Pivovarova et al [22].…”

Section: Resultssupporting

confidence: 64%

“…In the specific case of CaTa 2 O 6 , very few reports have been published [20][21][22], and none explores its single crystal form. This compound has a polymorphic character and, according to Jahnberg [20,21], it exists in three different structural modifications: (i) a perovskite-type structure with cubic symmetry and a random distribution of the calcium atoms, if prepared at low temperatures; (ii) an orthorhombic modification above approximately 700 C; and (iii) a structure of deformed perovskite-type in samples rapidly cooled from the melt.…”

Section: Introductionmentioning

confidence: 99%

“…This compound has a polymorphic character and, according to Jahnberg [20,21], it exists in three different structural modifications: (i) a perovskite-type structure with cubic symmetry and a random distribution of the calcium atoms, if prepared at low temperatures; (ii) an orthorhombic modification above approximately 700 C; and (iii) a structure of deformed perovskite-type in samples rapidly cooled from the melt. Pivovarova et al [22], identified two hightemperature modifications: tetragonal (a 1 ) phase (a ¼ 3:882 ( A, c ¼ 7:828 ( A), stable from 1450 C to 1650 C, and cubic (a 2 ) phase (a ¼ 7:758 ( A), stable from 1650 C to the melting point.…”

Section: Introductionmentioning

confidence: 99%

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Laser heated pedestal growth and optical characterization of CaTa2O6 single crystal fiber

Ferrari

Camargo

Nunes

et al. 2004

Journal of Crystal Growth

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Section: Resultssupporting

confidence: 64%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

See 1 more Smart Citation

Laser heated pedestal growth and optical characterization of CaTa2O6 single crystal fiber

Ferrari

Camargo

Nunes

et al. 2004

Journal of Crystal Growth

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“…At room temperature, calcium tantalite can present three stable polymorphs, depending on the growth conditions: (i) a Ca-deficient simple cubic perovskite phase (half-occupied) with random occupation of vacancies on the A site, whose formula would be Ca 0.5 0 0.5 TaO 3 , obtained by low-temperature crystallization (below 650°C); (ii) a quenched cubic phase with ordered Ca ions obtained from rapid crystallization from the melt; (iii) an orthorhombic aeschynite phase, obtained by slow crystallization or by annealing one of the cubic phases above 700°C. 6À8 Two high temperature polymorphs were also reported (though the structures were not resolved yet) by Pivovarova et al, 9 a tetragonal one between 1450 and 1650°C, and a cubic phase from 1650°C up to the melting point at about 1950°C. All the mentioned phase transformations show kinetic behavior; that is, the phase transitions are not thermodynamically governed between the equilibrium states.…”

Section: ' Introductionmentioning

confidence: 95%

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

Teixeira

Moreira

Lobo

et al. 2011

Crystal Growth & Design

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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.

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“…13,14 Besides the above-mentioned room-temperature structures, two high-temperature phases were also observed in calcium tantalite by Pivovarova et al: a tetragonal phase, between 1450 and 1650°C, and a cubic one, from 1650°C up to the congruent melting point (around 1950°C). 4 The crystal structures of these phases have not been resolved yet, probably because the phase transformations show kinetic behavior, …”

mentioning

confidence: 99%

Polymorphic-Induced Transformations in CaTa₂O₆ Single-Crystal Fibers Obtained by Laser-Heated Pedestal Growth

Almeida

Matinaga

Andreeta

et al. 2013

Crystal Growth & Design

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Calcium tantalite (CaTa 2 O 6 ) single crystal fibers were obtained by the laser-heated pedestal growth method (LHPG). At room temperature, this material can present three polymorphic modifications. The rapid crystallization inherent to the LHPG method produced samples within the Pm3̅ space group, with some chemical disorder. In order to check for polymorphic-induced transformations, the CaTa 2 O 6 fibers have been submitted to different thermal treatments and investigated by micro-Raman spectroscopy. For short annealing times (15 min) at 1200°C, the cubic modification was maintained, though with an improved crystalline quality, as evidenced by the enhanced inelastic scattered intensity (by ca. 250%) and narrowing of Raman bands. The polarized Raman spectra respected very well the predicted symmetries and the selection rules for this cubic modification. On the other hand, long annealing times (24 h) at 1200°C led to a complete (irreversible) polymorphic transformation. The Raman bands became still more intense (ca. 15 times larger than for the as-grown fibers), narrower, and several new modes appeared. Also, the spectra became unpolarized, demonstrating a polycrystalline nature of the transformed crystals. The observed Raman modes could be fully assigned to an orthorhombic modification of CaTa 2 O 6 belonging to the Pnma space group. ■ INTRODUCTIONIn spite of its simple chemical formula, three different stable polymorphs have been claimed for the room-temperature structure of calcium tantalite (CaTa 2 O 6 ), as a function of the preparation conditions. 1−4 Low-temperature crystallization (below 650°C) for sufficient length of times would give rise to a cubic modification with half-occupation of calcium on its twelve-coordinated site, with a random distribution of vacancies. The chemical formula of such a phase would rather be Ca 0.5 □ 0.5 TaO 3 . Structural results on single crystals suggests that this highly disordered phase would belong to the ideal cubic perovskite Pm3̅ m space group, with a = 3.88 Å and Z = 1 (one motif per unit cell). 1,2 To the best of our knowledge, there are no other reports on the physical properties of this phase, even though these references have been often cited by other authors. 3,4 On the other hand, slow crystallization above ca. 700°Cgives rise to an orthorhombic phase of the aeschynite group. This is the case of CaTa 2 O 6 ceramic materials sintered at about 1550°C and slowly cooled down (1−2°C/min) to room temperature, which shows the orthorhombic aeschynite structure (Pnma space group, with a = 11.086 Å, b = 7.505 Å, and c = 5.378 Å, with Z = 4). The crystal structure, 2 microwave dielectric response, 5,6 and unpolarized Raman and infrared spectra 7 of the orthorhombic CaTa 2 O 6 ceramics have been previously reported. It is worth mentioning that orthorhombic CaTa 2 O 6 ceramics are considered as possible candidates for applications in microwave (MW) devices, for presenting reasonable dielectric response in the MW range. 5,6 The third crystal polymorph of CaTa 2 O 6 can be obta...

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Cited by 6 publications

References 2 publications

Laser heated pedestal growth and optical characterization of CaTa2O6 single crystal fiber

Laser heated pedestal growth and optical characterization of CaTa2O6 single crystal fiber

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

Polymorphic-Induced Transformations in CaTa₂O₆ Single-Crystal Fibers Obtained by Laser-Heated Pedestal Growth

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Untitled

Cited by 6 publications

References 2 publications

Laser heated pedestal growth and optical characterization of CaTa2O6 single crystal fiber

Laser heated pedestal growth and optical characterization of CaTa2O6 single crystal fiber

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

Polymorphic-Induced Transformations in CaTa2O6 Single-Crystal Fibers Obtained by Laser-Heated Pedestal Growth

Contact Info

Product

Resources

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

Polymorphic-Induced Transformations in CaTa₂O₆ Single-Crystal Fibers Obtained by Laser-Heated Pedestal Growth