Optical Floating Zone Technique Research Articles

Growth and characterization of Dy1-xYxMnO3 single crystals by optical floating zone technique: A combined X-ray diffraction and DC magnetization study

Large size single crystals of DyMnO3, YMnO3 and their solid solutions, Dy1-xYxMnO3, have been grown by the optical floating zone technique. It is shown that pristine DyMnO3 crystals grown in air atmosphere correspond to the orthorhombic phase in the Pnma space group symmetry while growth in the argon atmosphere leads to the hexagonal phase in the P63cm space group symmetry. The crystals grown in air atmosphere for the composition range 0.10 ≤ x ≤ 0.50 show coexistence of orthorhombic and hexagonal phases with increasing hexagonal phase fraction until at x ~ 0.60, the orthorhombic phase completely transforms to the hexagonal phase even in the air atmosphere. On the other hand, all the crystals grown in argon atmosphere correspond to the hexagonal phase only. Our Rietveld refinements using X-ray powder diffraction data on powders obtained after crushing the crystals further confirm the hexagonal structure in the P63cm space group symmetry for all crystals grown in argon atmosphere. The unit cell volume and the lattice parameters obtained by Rietveld refinement are shown to decrease with increasing Y3+ substitution at the Dy3+ site in DyMnO3. DC magnetisation measurements on h-Dy1-xYxMnO3 single crystals reveal that Y3+ substitution marginally increases the antiferromagnetic ordering temperature (TN) from 67 K for h-DyMnO3 to 72 K for h-YMnO3. However, the first spin reorientation (SR) transition temperature (TSR1) of Mn3+ decreases with increasing Y3+ substitutions as per TSR1 ~ k(x-xc)0.3, where k is a proportionality constant and xc is the critical composition for which this transition temperature goes to 0 K. The remaining two magnetic transition temperatures related to Dy3+ reordering (TDy3+)/spin-glass transition (TSG) and second spin reorientation transition (TSR2) show very weak dependence on Y3+ substitution before going to 0 K around x = xc ~ 0.90 composition. A magnetic phase diagram of h-Dy1-xYxMnO3 showing stability field regions in respect of different magnetic phases is also presented.

Structural and optical properties of GdAlO3:Tb3+ crystal obtained by optical floating zone method

Gadolinium aluminate single crystal (GdAlO3: Tb3+, GAP:Tb) was obtained by optical floating zone technique (OFZ). The crystal was grown at a fast pulling rate (7.0 mm/h) in atmosphere ambient. The structural characterization proves the crystal having a high degree of crystallization and perovskite nature by XRD, SAED and IR. SEM/EDS results show neither element segregation nor defects in the crystal. Residual stress in this crystal, without annealing treatment, is close to YAG crystal made by Bridgman method. Absorption and photoluminescence spectra suggest a strong absorption in 200–400 nm (related to Tb3+-O2- and Gd3+ 8S7/2→6G/D/I/PJ) and a bright green emission centered at 548 nm (related to Tb3+ 5D4→7F5). The energy spectrum also show GdAlO3:Tb3+ crystal could be a good scintillator.

Photoluminescence, scintillation and thermoluminescence properties of Ce-doped SrLaAl3O7 crystals synthesized by the optical floating zone method

Photoluminescence, scintillation and thermoluminescence properties of undoped and Ce-doped SrLaAl 3 O 7 crystals grown using the optical Floating Zone technique were examined. An intense broad emission peak at 400 nm are observed from photoluminescence excitation-emission maps and scintillation spectra of the Ce-doped SrLaAl 3 O 7 crystals, with the origin is ascribed to the 5d-4f transitions of Ce 3+ . The primary decay of the Ce-doped and undoped SrLaAl 3 O 7 crystals are attributable to the Ce 3+ ions and intrinsic luminescence, respectively. The 3.0% Ce-doped SrLaAl 3 O 7 crystal shows the highest light yield of 760 ph/5.5 MeV-α among the investigated samples based on pulse height analysis under α-rays excitation from 241 Am. • Ce-doped SrLaAl 3 O 7 crystals were successfully synthesized and the samples show luminescence due to Ce 3+ ions at 400 nm. • The highest light yield among the present samples is 760 ph/5.5 MeV-α when excited by α-rays from 241 Am. • The scintillation properties of Ce-doped SrLaAl 3 O 7 crystals were clarified for the first time.

Microstructure of directionally solidified Al2O3/EAG eutectic ceramics prepared with high-temperature gradient

Directionally solidified Al2O3/Er3Al5O12 (EAG) eutectic ceramics are characterized by high melting point, improved high-temperature flexural strength, and high fracture toughness. Therefore, they have recently attracted considerable attention as candidates for high-temperature (>1500 °C) structural materials. In this work, directionally solidified Al2O3/EAG eutectic ceramics were prepared by the optical floating zone technique. 2D and 3D microstructures were observed by scanning electron microscopy and X-ray computerized tomography. The effect of thermal gradient and the solidification rate on Al2O3/EAG eutectic microstructure was investigated. The microstructure was completely irregular, exhibiting 3D interconnected microstructure. The size of eutectic microstructure decreased with increasing growth rate. Stable coupled-growth can be obtained when the solidification rate is less than 60 mm/h. Fracture toughness of Al2O3/EAG eutectic ceramics prepared with a withdrawal rate of 40 mm/h, measured by indentation tests, was KIC = 5.7 ± 0.3 MPa m1/2 and the hardness was HV = 14.5 ± 0.4 GPa. Interfaces between Al2O3 and the EAG phases play important role in improving fracture toughness. The characterization of microstructure and mechanical properties of Al2O3/EAG eutectic ceramics may provide guidance for microstructure design and reveal the deformation mechanisms of Al2O3/EAG eutectic ceramics.

Discovery and Single Crystal Growth of High Entropy Pyrochlores.

A family of high entropy oxides with the formula Mg2Ta3Ln3O14 (Ln = La, Pr, Nd, Sm, Eu, Gd) has been discovered and synthesized. Single crystals, 5 mm OD × 2.5 cm, for Ln = Nd have been grown using the laser optical floating zone technique. Crystal orientations are confirmed by Laue diffraction, and structure solutions were obtained via single crystal X-ray diffraction. The structure is found to be a partially disordered pyrochlore, space group Fd-3m, fractional chemical formula (Mg0.25Nd0.75)2(Mg0.25Ta0.75)2O7. Magnetization measurements indicate ordinary paramagnetic behavior in all compounds down to T = 2 K, except in the Eu variant which possesses Van Vleck paramagnetism. Specific heat measurements for Ln = Nd shows no phase transitions between T = 300 and 2 K. We demonstrate the ability to prepare magnetically disordered materials by substitution of Mg with Ni, Mn, and Co, demonstrating the flexibility of this family in accommodating defects. The stabilization of these compounds could be due to the entropy gain associated with defects, showcasing a "materials by design" approach by using disorder to stabilize novel magnetic and optical materials. Our work also demonstrates the feasibility of preparing high entropy oxides in single crystalline form.

Investigation of optical and spectroscopic properties of Nd co-doped Yb:YVO4 single crystals grown by OFZ method

Nd (0.8 at.%) co-doped Yb (0.8 and 3.0 at.%):YVO4 single crystals were grown by optical floating zone technique for the first time to the best of our knowledge. The absorption spectra revealed strong absorption for π-polarization (E||c) than the σ-polarization (E⊥c) for both the Yb concentrations due to anisotropy of the crystal. The estimated effective Judd-Ofelt parameters are 11.8675 × 10−20, 5.1577 × 10−20 and 7.3755 × 10−20 cm−2. Spectroscopic quality factor of Nd co-doped Yb:YVO4 are 0.70. which is lower than that of Nd (1.0 at.%) doped YVO4 (0.97), which indicates the energy transfer from Nd3+ ion to Yb3+ ion inside the host matrix. The estimated emission cross-section of 4F3/2 → 4I11/2 Nd transition with an excitation at 808 nm for both the concentrations of Yb is ∼4.0 × 10−19 cm2 which is almost same as estimated by Bhaumik et al. (2014) [21] for Nd:YVO4. Hence emission cross-section of 4F3/2 → 4I11/2 Nd transition is not effected by Yb co-doping. Also the PL intensity of Yb at 1010 nm enhances with Yb co-doping for same Nd (0.8 at.%) this is because the Nd3+ ion effectively excites the Yb3+ ion in upper state. Therefore Nd3+ ion acts as excellent sensitizer for Yb:YVO4 laser gain medium and make efficient pumping in the gain medium.

Studies on structural and magnetic properties of NdFeO3 single crystals grown by optical floating zone technique

NdFeO3 single crystals have been grown by optical floating-zone technique using Nd2O3 and Fe2O3 as the source by solid-state reaction method. The crystalline structure of NdFeO3 belongs to orthorhombic symmetry and the presence of a distorted perovskite structure was confirmed by powder X-ray diffraction (XRD) analysis. From the XRD results, it is visualized that the lattice expansion has been observed in the unit cell parameters which may appear due to interstitial defects in the NdFeO3 matrix. The deficiency of oxygen atoms and the presence of excess Fe ions in NdFeO3 have been confirmed with EDX spectra. The spin-reorientation region (T = 120 K) exhibits lower coercive field and the hysteresis loops depict enhanced magnetization (Ms and Mr) values of NdFeO3 single crystal. Presence of compensation temperature point at Tcomp = 7.5 K denotes the anti-parallel alignment of rare-earth ions with Fe magnetic moments. At high temperatures, Fe3+ magnetic moments show strong antiferromagnetic interactions ensuing in Neel temperature (TN) at 690 K.

Influence of Al content on the optical band-gap enhancement and lattice structure of (Ga1-xAlx)2O3 single crystal

Abstract Single crystals of undoped, 4.8, 9.3 and 14.1 at.% Al substituted β-Ga2O3 have been grown by optical floating zone technique. It has been observed that it is not possible to grow good quality single crystal with higher Al content unlike the thin film grown by pulsed laser deposition [Appl. Phys. Lett. 105 (2014) 162107]. It has been found that the incorporation of Al in β-Ga2O3 has resulted a shrinkage in the lattice parameters and volume. This leads to a systematic increase in the band gap of the material and broadens the transmission window in the deep UV range up to 5.08 eV for 14.1 at.% of Al substitution in the lattice.

Structural, optical and mechanical properties of Y2Ti2O7 single crystal

We report on the growth of Y2Ti2O7 single crystals by optical floating zone technique. X-ray diffraction and Raman spectroscopy studies confirm the structural quality of the crystal. The UV–Vis optical studies reveal that the grown crystals have a high optical transparency with an optical band gap of 3.44 eV. The hardness of Y2Ti2O7 single crystal is measured for the first time using nanoindentation. The measured hardness, indentation and bulk modulus are found to be 16.4 ± 0.4, 321.1 ± 6.9 and 243.3 ± 5.2 GPa respectively, which are higher than its polycrystalline counterpart and its constituent metal oxides, Y2O3 and TiO2.

Structural and optical properties of beta irradiated YAlO3 single crystals

We report on the growth, structural and optical properties of YAlO3 single crystals grown by optical floating zone technique. Powder X-ray diffraction and Raman spectroscopic studies confirm the phase purity of the crystals. Raman analysis reveals that the intensity and line-width of Raman bands increase significantly with beta irradiation indicating the formation of structural defects in YAlO3 lattice. The optical properties are studied through UV–visible absorption, and photoluminescence emission and excitation spectroscopies under pre- and post- beta irradiation. The optical studies indicate the presence of Sm and Cr impurities by exhibiting characteristic emission lines in the orange red region. Further, a systematic study on the thermoluminescence (TL) characteristics of the crystal is also carried out at different doses of beta irradiation. The crystals exhibit a prominent TL glow peak at 239 °C for less than 5 Gy doses while a weak second glow peak evolves at higher doses. Also, the crystals show a nearly linear dose response in the studied range from 0.1 to 10 Gy. The glow curve analysis reveals that the TL emission obeys the first order kinetics model. Based on the optical studies, the plausible mechanism for the TL glow curve is discussed in terms of the intrinsic defects and impurities that are present in the crystal.

Magnetic phase diagram and magnetoelastic coupling of NiTiO3

We report high-resolution dilatometry on high-quality single crystals of NiTiO3 grown by means of the optical floating-zone technique. The anisotropic magnetic phase diagram is constructed from thermal expansion and magnetostriction studies up to B=15T and magnetization studies in static (15T) and pulsed (60T) magnetic fields. Our data allow to quantitatively study magneto-elastic coupling and to determine uniaxial pressure dependencies. While the entropy changes are found to be of magnetic nature, Gr\"{u}neisen analysis implies only one relevant energy scale in the whole low-temperature regime. Thereby, our data suggest that the observed structural changes due to magneto-elastic coupling and previously reported magnetodielectric coupling[1] are driven by the same $magnetic$ degrees of freedom that lead to long-range magnetic order in NiTiO3, which in turn, establishes a linear magnetodielectric coupling in this compound.

Rietveld refinement of Sm0.55Sr0.45Mn0.4Fe0.6O3

Single crystals of Sm0.55Sr0.45Mn0.4Fe0.6O3were grown by an optical floating zone technique in an oxygen atmosphere. The powder X-ray diffraction pattern for the grown crystal revealed single-crystalline nature. The lattice parameters and atomic structure were refined and indexed using a General Structure Analysis System (GSAS) to an orthorhombic structure with lattice parametersa= 5.4415 (32) Å,b= 7.6994 (27) Å,c= 5.43868 (28) Å, and space group Pnma.

Torque magnetometry study of the spin reorientation transition and temperature-dependent magnetocrystalline anisotropy in NdCo5

We present the results of torque magnetometry and magnetic susceptibility measurements to study in detail the spin reorientation transition (SRT) and magnetic anisotropy in the permanent magnet NdCo5. We further show simulations of the measurements using first-principles calculations based on density-functional theory and the disordered local moment picture of magnetism at finite temperatures. The good agreement between theory and experimental data leads to a detailed description of the physics underpinning the SRT. In particular we are able to resolve the magnetization of, and to reveal a canting between, the Nd and Co sublattices. The torque measurements carried out in the ac and ab planes near the easy direction allow us to estimate the anisotropy constants, K1, K2 and K4 and their temperature dependences. Torque curves, τ(γ) recorded by varying the direction of a constant magnetic field in the crystallographic ac plane show a reversal in the polarity as the temperature is changed across the SRT (240 < T < 285 K). Within this domain, τ(γ) exhibits unusual features different to those observed above and below the transition. The single crystals of NdCo5 were grown using the optical floating zone technique.

Optical floating-zone growth of single crystals of Li-ion battery material LiCoO2

Large, high quality single crystals of Li-ion battery cathode material LiCoO2 have been grown using the optical floating zone technique employing a four-mirror furnace. Several growth experiments reveal that the calcination temperature, quantity of excess Li in the starting feed rod, the atmosphere in the growth chamber, and the growth speed are crucial parameters that govern the stability of the molten zone and in turn the phase purity, stoichiometry and crystallinity of the grown crystal. It is observed that high-quality, phase pure single crystals can only be grown in an Ar-atmosphere of 9 bar and using slow growth speeds of 1–2 mm/h, using feed rods containing 15% excess Li content. The structural refinement of the powder x-ray diffraction data along with the Laue diffraction patterns confirms a rhombohedral crystal structure (space group R3¯m) and the presence of a single grain along the length (~2.5 cm) of the grown crystal. Atomic emission spectrometry (ICP-AES) ascertained the uniformity in stoichiometry along the length of the grown crystals. The x-ray rocking curve of the crystal has a resolution limited full-width at half-maximum (FWHM) of 0.22°, confirming the high single crystalline quality of the sample.

Single crystal growth of GdB6 by the optical floating-zone technique

A high-quality and large-sized GdB6 single crystal was successfully prepared by using the optical floating zone method to accurately control the temperature and composition of the molten zone.

Growth of CuFeO2 single crystals by the optical floating-zone technique

CuFeO$_2$ single crystals up to 50 mm in length and up to 10 mm in diameter were grown by the optical floating-zone method. Stoichiometric polycrystalline rods with a diameter of 6-12 mm were used as feed materials to produce crystals of sufficient size to be used as substrates for the growth of thin films of delafossites. For stable growth along the $c$-axis, low growth rates of 0.4 mm/h are necessary. Due to the incongruent melting behavior of CuFeO$_2$, a stable melt zone requires adjustment of the lamp power during growth. The melting of CuFeO$_2$ is not simply incongruent because the thermodynamic equilibrium includes more than two solid phases and the melt; the gas phase is also involved. The crystals were characterized by X-ray diffraction and X-ray fluorescence measurements.

Preparation, characterization and properties of La0.6Ce0.3Pr0.1B6-ZrB2 directionally solidified eutectic grown via the optical floating zone technique

La0.6Ce0.3Pr0.1B6-ZrB2 directionally solidified eutectic (DSE) with the [100] orientation of the La0.6Ce0.3Pr0.1B6 matrix is firstly fabricated via the optical floating zone technique using LaB6, CeB6, PrB6 and ZrB2 powders as raw materials. The microstructure, mechanical and thermionic emission properties of the as-grown La0.6Ce0.3Pr0.1B6-ZrB2 DSE were systematically investigated. The ZrB2 fibers, whose average diameter is 0.8–1.2 μm, are embedded in the [100] La0.6Ce0.3Pr0.1B6 matrix and uniformly distributed. The Vickers hardness does not vary apparently between the transverse and longitudinal sections of La0.6Ce0.3Pr0.1B6-ZrB2, while the fracture toughness is of highly anisotropy. The fracture toughness of 5.71 MPa m1/2 on the longitudinal section, flexural strength of 826.50 MPa and thermionic emission current density of 84.96 A/cm2, which are all higher than that of LaB6 or LaB6-ZrB2, indicate that the La0.6Ce0.3Pr0.1B6-ZrB2 DSE grown here will have great application prospects as a candidate for high-performance thermionic emitters.

Effect of reduction on the optical properties of Sr0.61Ba0.39Nb2O6 single crystals grown by optical floating zone technique

The effect of thermal reduction during growth of strontium barium niobate Sr0.61Ba0.39Nb2O6 (SBN61) crystal on its optical and electrical properties has been investigated. Single crystals of SBN61 were grown by the Optical Floating Zone technique in air ambience. Growth challenges such as formation of core and crystal cracking were eradicated by optimizing the synthesis and growth process parameters. The evidence of reduction was observed in the grown crystals that resulted in a characteristic gray colour. Further, the crystal became colourless on annealing in oxygen ambiance at ∼1200 °C for 20 h. The as-grown crystal is therefore identified as reduced SBN crystals due to formation of oxygen vacancies and the accompanied colour centers and charge compensated defects e.g. Nb4+ polaron in the as-grown crystals. X-ray powder diffraction analysis confirmed the formation of the desired phase and no structural change in the reduced crystal. Also, the absorption spectra revealed a broad absorption band centred around 500 nm in the as-grown reduced crystals confirming the thermal reduction of the crystal. Reduced crystals depicted a slight decrease in the optical band gap energy due to defect induced band tailing. An increase in the pyroelectric coefficient (p3) was observed for reduced SBN61 crystal in comparison to the crystal annealed in oxygen. The impedance measurement revealed higher electrical conductivity with smaller activation energy for the reduced crystal in comparison to the annealed SBN61 crystal. It shows that the thermal reduction induced defects influence the electrical conduction processes, causing the materials to be more conducting, which is desirable for a relaxor ferroelectric materials to be used as a thermoelectric generator application.

Colossal dielectric behaviour of Sr2TiMnO6-δ single crystals

Single crystals of Sr2TiMnO6-δ were grown by optical floating zone technique. The single phase nature of the grown crystal was confirmed by powder X-ray diffraction studies. The permittivity of the grown crystal was about 104 at room temperature. The presence of Mn3+ and Mn4+ state resulted in the splitting of T2g mode which is evident from Raman spectrum. The mixed valence state of Mn for charge carrier hopping was determined by X-ray photoelectron spectroscopy.

Density of Feed Rod Dependence of the PrB6 Single Crystal Grown by the Optical Floating Zone Technique

AbstractRare‐earth hexaboride single crystals praseodymium hexaboride (PrB6) are successfully grown by the optical floating zone technique using the feed rods with different densities sintered by spark plasma sintering (SPS). Single crystal X‐ray diffraction and transmission electron microscopy analysis have found that the as‐grown PrB6 adopts a single crystalline structure which has a primitive cubic structure, and has grown along the &lt;100&gt; direction. Particular attentions are given to the effect of density of feed rod on the crystal quality and thermionic emission performance of PrB6 single crystals. The result of X‐ray rocking curve indicates that the quality of the as‐grown single crystals get better with increase in the density of feed rod, and an ultralow full‐width at half‐maximum of 0.066°, confirming the good crystal integrality, is resulted from the single crystal using the feed rod with relative density of 97.17%. Similarly, the result of thermionic emission shows that the PrB6 single crystal grown here using a high‐density feed rod has a large current density. The density of feed rod plays an important role in PrB6 single crystal growth by the optical floating zone technique.