6 research outputs found

    Nanoindentation, High-Temperature Behavior, and Crystallographic/Spectroscopic Characterization of the High-Refractive-Index Materials TiTa<sub>2</sub>O<sub>7</sub> and TiNb<sub>2</sub>O<sub>7</sub>

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    Colorless single crystals, as well as polycrystalline samples of TiTa<sub>2</sub>O<sub>7</sub> and TiNb<sub>2</sub>O<sub>7</sub>, were grown directly from the melt and prepared by solid-state reactions, respectively, at various temperatures between 1598 K and 1983 K. The chemical composition of the crystals was confirmed by wavelength-dispersive X-ray spectroscopy, and the crystal structures were determined using single-crystal X-ray diffraction. Structural investigations of the isostructural compounds resulted in the following basic crystallographic data: monoclinic symmetry, space group <i>I</i>2<i>/m</i> (No. 12), <i>a</i> = 17.6624(12) Å, <i>b</i> = 3.8012(3) Å, <i>c</i> = 11.8290(9) Å, β = 95.135(7)°, <i>V</i> = 790.99(10) Å<sup>3</sup> for TiTa<sub>2</sub>O<sub>7</sub> and <i>a</i> = 17.6719(13) Å, <i>b</i> = 3.8006(2) Å, <i>c</i> = 11.8924(9) Å, β = 95.295(7)°, <i>V</i> = 795.33(10) Å<sup>3</sup>, respectively, for TiNb<sub>2</sub>O<sub>7</sub>, <i>Z</i> = 6. Rietveld refinement analyses of the powder X-ray diffraction patterns and Raman spectroscopy were carried out to complement the structural investigations. In addition, <i>in situ</i> high-temperature powder X-ray diffraction experiments over the temperature range of 323–1323 K enabled the study of the thermal expansion tensors of TiTa<sub>2</sub>O<sub>7</sub> and TiNb<sub>2</sub>O<sub>7</sub>. To determine the hardness (<i>H</i>), and elastic moduli (<i>E</i>) of the chemical compounds, nanoindentation experiments have been performed with a Berkovich diamond indenter tip. Analyses of the load–displacement curves resulted in a hardness of <i>H</i> = 9.0 ± 0.5 GPa and a reduced elastic modulus of <i>E</i><sub>r</sub> = 170 ± 7 GPa for TiTa<sub>2</sub>O<sub>7</sub>. TiNb<sub>2</sub>O<sub>7</sub> showed a slightly lower hardness of <i>H</i> = 8.7 ± 0.3 GPa and a reduced elastic modulus of <i>E</i><sub>r</sub> = 159 ± 4 GPa. Spectroscopic ellipsometry of the polished specimens was employed for the determination of the optical constants <i>n</i> and <i>k</i>. TiNb<sub>2</sub>O<sub>7</sub> as well as TiTa<sub>2</sub>O<sub>7</sub> exhibit a very high average refractive index of <i>n</i><sub>D</sub> = 2.37 and <i>n</i><sub>D</sub> = 2.29, respectively, at λ = 589 nm, similar to that of diamond (<i>n</i><sub>D</sub> = 2.42)

    Mechanical Properties, Quantum Mechanical Calculations, and Crystallographic/Spectroscopic Characterization of GaNbO<sub>4</sub>, Ga(Ta,Nb)O<sub>4</sub>, and GaTaO<sub>4</sub>

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    Single crystals as well as polycrystalline samples of GaNbO<sub>4</sub>, Ga­(Ta,Nb)­O<sub>4</sub>, and GaTaO<sub>4</sub> were grown from the melt and by solid-state reactions, respectively, at various temperatures between 1698 and 1983 K. The chemical composition of the crystals was confirmed by wavelength-dispersive electron microprobe analysis, and the crystal structures were determined by single-crystal X-ray diffraction. In addition, a high-P–T synthesis of GaNbO<sub>4</sub> was performed at a pressure of 2 GPa and a temperature of 1273 K. Raman spectroscopy of all compounds as well as Rietveld refinement analysis of the powder X-ray diffraction pattern of GaNbO<sub>4</sub> were carried out to complement the structural investigations. Density functional theory (DFT) calculations enabled the assignment of the Raman bands to specific vibrational modes within the structure of GaNbO<sub>4</sub>. To determine the hardness (<i>H</i>) and elastic moduli (<i>E</i>) of the compounds, nanoindentation experiments have been performed with a Berkovich diamond indenter tip. Analyses of the load–displacement curves resulted in a high hardness of <i>H</i> = 11.9 ± 0.6 GPa and a reduced elastic modulus of <i>E</i><sub>r</sub> = 202 ± 9 GPa for GaTaO<sub>4</sub>. GaNbO<sub>4</sub> showed a lower hardness of <i>H</i> = 9.6 ± 0.5 GPa and a reduced elastic modulus of <i>E</i><sub>r</sub> = 168 ± 5 GPa. Spectroscopic ellipsometry of the polished GaTa<sub>0.5</sub>Nb<sub>0.5</sub>O<sub>4</sub> ceramic sample was employed for the determination of the optical constants <i>n</i> and <i>k</i>. GaTa<sub>0.5</sub>Nb<sub>0.5</sub>O<sub>4</sub> exhibits a high average refractive index of <i>n</i><sub>D</sub> = 2.20, at λ = 589 nm. Furthermore, <i>in situ</i> high-temperature powder X-ray diffraction experiments enabled the study of the thermal expansion tensors of GaTaO<sub>4</sub> and GaNbO<sub>4</sub>, as well as the ability to relate them with structural features

    Superstructure of Mullite-type KAl<sub>9</sub>O<sub>14</sub>

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    Large whiskers of a new KAl<sub>9</sub>O<sub>14</sub> polymorph with mullite-type structure were synthesized. The chemical composition of the crystals was confirmed by energy-dispersive X-ray spectroscopy, and the structure was determined using single-crystal X-ray diffraction. Nanosized twin domains and one-dimensional diffuse scattering were observed utilizing transmission electron microscopy. The compound crystallizes in space group <i>P</i>2<sub>1</sub>/<i>n</i> (<i>a</i> = 8.1880(8), <i>b</i> = 7.6760(7), <i>c</i> = 8.7944(9) Å, β = 110.570(8)°, <i>V</i> = 517.50(9) Å<sup>3</sup>, <i>Z</i> = 2). Crystals of KAl<sub>9</sub>O<sub>14</sub> exhibit a mullite-type structure with linear edge-sharing AlO<sub>6</sub> octahedral chains connected with groups of two AlO<sub>4</sub> tetrahedra and one AlO<sub>5</sub> trigonal bipyramid. Additionally, disproportionation of KAl<sub>9</sub>O<sub>14</sub> into K β-alumina and corundum was observed using in situ high-temperature optical microscopy and Raman spectroscopy

    Atomic-Scale <i>in Situ</i> Observations of Crystallization and Restructuring Processes in Two-Dimensional MoS<sub>2</sub> Films

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    We employ atomically resolved and element-specific scanning transmission electron microscopy (STEM) to visualize <i>in situ</i> and at the atomic scale the crystallization and restructuring processes of two-dimensional (2D) molybdenum disulfide (MoS<sub>2</sub>) films. To this end, we deposit a model heterostructure of thin amorphous MoS<sub>2</sub> films onto freestanding graphene membranes used as high-resolution STEM supports. Notably, during STEM imaging the energy input from the scanning electron beam leads to beam-induced crystallization and restructuring of the amorphous MoS<sub>2</sub> into crystalline MoS<sub>2</sub> domains, thereby emulating widely used elevated temperature MoS<sub>2</sub> synthesis and processing conditions. We thereby directly observe nucleation, growth, crystallization, and restructuring events in the evolving MoS<sub>2</sub> films <i>in situ</i> and at the atomic scale. Our observations suggest that during MoS<sub>2</sub> processing, various MoS<sub>2</sub> polymorphs co-evolve in parallel and that these can dynamically transform into each other. We further highlight transitions from in-plane to out-of-plane crystallization of MoS<sub>2</sub> layers, give indication of Mo and S diffusion species, and suggest that, in our system and depending on conditions, MoS<sub>2</sub> crystallization can be influenced by a weak MoS<sub>2</sub>/graphene support epitaxy. Our atomic-scale <i>in situ</i> approach thereby visualizes multiple fundamental processes that underlie the varied MoS<sub>2</sub> morphologies observed in previous <i>ex situ</i> growth and processing work. Our work introduces a general approach to <i>in situ</i> visualize at the atomic scale the growth and restructuring mechanisms of 2D transition-metal dichalcogenides and other 2D materials

    Atomic-Scale <i>in Situ</i> Observations of Crystallization and Restructuring Processes in Two-Dimensional MoS<sub>2</sub> Films

    No full text
    We employ atomically resolved and element-specific scanning transmission electron microscopy (STEM) to visualize <i>in situ</i> and at the atomic scale the crystallization and restructuring processes of two-dimensional (2D) molybdenum disulfide (MoS<sub>2</sub>) films. To this end, we deposit a model heterostructure of thin amorphous MoS<sub>2</sub> films onto freestanding graphene membranes used as high-resolution STEM supports. Notably, during STEM imaging the energy input from the scanning electron beam leads to beam-induced crystallization and restructuring of the amorphous MoS<sub>2</sub> into crystalline MoS<sub>2</sub> domains, thereby emulating widely used elevated temperature MoS<sub>2</sub> synthesis and processing conditions. We thereby directly observe nucleation, growth, crystallization, and restructuring events in the evolving MoS<sub>2</sub> films <i>in situ</i> and at the atomic scale. Our observations suggest that during MoS<sub>2</sub> processing, various MoS<sub>2</sub> polymorphs co-evolve in parallel and that these can dynamically transform into each other. We further highlight transitions from in-plane to out-of-plane crystallization of MoS<sub>2</sub> layers, give indication of Mo and S diffusion species, and suggest that, in our system and depending on conditions, MoS<sub>2</sub> crystallization can be influenced by a weak MoS<sub>2</sub>/graphene support epitaxy. Our atomic-scale <i>in situ</i> approach thereby visualizes multiple fundamental processes that underlie the varied MoS<sub>2</sub> morphologies observed in previous <i>ex situ</i> growth and processing work. Our work introduces a general approach to <i>in situ</i> visualize at the atomic scale the growth and restructuring mechanisms of 2D transition-metal dichalcogenides and other 2D materials

    Atomic-Scale <i>in Situ</i> Observations of Crystallization and Restructuring Processes in Two-Dimensional MoS<sub>2</sub> Films

    No full text
    We employ atomically resolved and element-specific scanning transmission electron microscopy (STEM) to visualize <i>in situ</i> and at the atomic scale the crystallization and restructuring processes of two-dimensional (2D) molybdenum disulfide (MoS<sub>2</sub>) films. To this end, we deposit a model heterostructure of thin amorphous MoS<sub>2</sub> films onto freestanding graphene membranes used as high-resolution STEM supports. Notably, during STEM imaging the energy input from the scanning electron beam leads to beam-induced crystallization and restructuring of the amorphous MoS<sub>2</sub> into crystalline MoS<sub>2</sub> domains, thereby emulating widely used elevated temperature MoS<sub>2</sub> synthesis and processing conditions. We thereby directly observe nucleation, growth, crystallization, and restructuring events in the evolving MoS<sub>2</sub> films <i>in situ</i> and at the atomic scale. Our observations suggest that during MoS<sub>2</sub> processing, various MoS<sub>2</sub> polymorphs co-evolve in parallel and that these can dynamically transform into each other. We further highlight transitions from in-plane to out-of-plane crystallization of MoS<sub>2</sub> layers, give indication of Mo and S diffusion species, and suggest that, in our system and depending on conditions, MoS<sub>2</sub> crystallization can be influenced by a weak MoS<sub>2</sub>/graphene support epitaxy. Our atomic-scale <i>in situ</i> approach thereby visualizes multiple fundamental processes that underlie the varied MoS<sub>2</sub> morphologies observed in previous <i>ex situ</i> growth and processing work. Our work introduces a general approach to <i>in situ</i> visualize at the atomic scale the growth and restructuring mechanisms of 2D transition-metal dichalcogenides and other 2D materials
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