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Synthesis of metastable rare-earth-iron mixed oxide with the hexagonal crystal structure
Rare-earth-iron mixed oxides with the rare earth/iron ratio=1 have either orthorhombic (o-REFeO{sub 3}) or hexagonal (h-REFeO{sub 3}) structure. h-REFeO{sub 3} is a metastable phase and the synthesis of h-REFeO{sub 3} is usually difficult. In this work, the crystallization process of the precursors obtained by co-precipitation and Pechini methods was investigated in detail to synthesize h-REFeO{sub 3}. It was found that the crystallization from amorphous to hexagonal phase and the phase transition from hexagonal to orthorhombic phase occurred at a similar temperature range for rare earth elements with small ionic radii (Er-Lu, Y). For both co-precipitation and Pechini methods, single-phase h-REFeO{sub 3} was obtained by shortening the heating time during calcination process. The hexagonal-to-orthorhombic phase transition took place by a nucleation growth mechanism and vermicular morphology of the thus-formed orthorhombic phase was observed. The hexagonal YbFeO{sub 3} had higher catalytic activity for C{sub 3}H{sub 8} combustion than orthorhombic YbFeO{sub 3}. - Graphical abstract: Although the synthesis of metastable hexagonal REFeO{sub 3} by the conventional method is difficult, we found that this phase is obtained by shortening the heating time of the precursor prepared by co-precipitation method. Highlights: Black-Right-Pointing-Pointer Synthesis of metastable REFeO{sub 3} with hexagonal structure by the co-precipitation method. Black-Right-Pointing-Pointer Hexagonal REFeO{sub 3} is obtained for the rare earth elements with small ionic radii. Black-Right-Pointing-Pointer Hexagonal-to-orthorhombic transformation of REFeO{sub 3}. Black-Right-Pointing-Pointer Catalytic activity of hexagonal REFeO{sub 3} for C{sub 3}H{sub 8} combustion.
WE-G-204-03: Photon-Counting Hexagonal Pixel Array CdTe Detector: Optimal Resampling to Square Pixels
Purpose: Detectors with hexagonal pixels require resampling to square pixels for distortion-free display of acquired images. In this work, the presampling modulation transfer function (MTF) of a hexagonal pixel array photon-counting CdTe detector for region-of-interest fluoroscopy was measured and the optimal square pixel size for resampling was determined. Methods: A 0.65mm thick CdTe Schottky sensor capable of concurrently acquiring up to 3 energy-windowed images was operated in a single energy-window mode to include ≥10 KeV photons. The detector had hexagonal pixels with apothem of 30 microns resulting in pixel spacing of 60 and 51.96 microns along the two orthogonal directions. Images of a tungsten edge test device acquired under IEC RQA5 conditions were double Hough transformed to identify the edge and numerically differentiated. The presampling MTF was determined from the finely sampled line spread function that accounted for the hexagonal sampling. The optimal square pixel size was determined in two ways; the square pixel size for which the aperture function evaluated at the Nyquist frequencies along the two orthogonal directions matched that from the hexagonal pixel aperture functions, and the square pixel size for which the mean absolute difference between the square and hexagonal aperture functions was minimized over all frequencies up to the Nyquist limit. Results: Evaluation of the aperture functions over the entire frequency range resulted in square pixel size of 53 microns with less than 2% difference from the hexagonal pixel. Evaluation of the aperture functions at Nyquist frequencies alone resulted in 54 microns square pixels. For the photon-counting CdTe detector and after resampling to 53 microns square pixels using quadratic interpolation, the presampling MTF at Nyquist frequency of 9.434 cycles/mm along the two directions were 0.501 and 0.507. Conclusion: Hexagonal pixel array photon-counting CdTe detector after resampling to square pixels provides high-resolution imaging suitable for fluoroscopy.
Atomic Structure of the Initial Nucleation Layer in Hexagonal Perovskite BaRuO 3 Thin Films
Hexagonal perovskites are an attractive family of materials due to their various polymorphs and rich structure–property relationships. BaRuO 3 (BRO) is a prototypical hexagonal perovskite, in which the electromagnetic properties are significantly modified depending on its lattice structure. Whereas thin-film synthesis would allow the study of the diverse properties of hexagonal perovskites by epitaxially stabilizing various metastable polymorphs, the epitaxial growth of hexagonal perovskites, especially at the initial growth stage, is yet to be addressed. In this study, it is shown that an intriguing nucleation behavior takes place during the initial stabilization of a hexagonal 9R BRO thin film deposited on a (111) SrTiO 3 (STO) substrate. It is found that a nanoscale strained layer composed of different RuO 6 octahedral stacking is initially formed at the interface followed by a relaxed single crystal 9R BRO thin film. Within the interface layer, hexagonal BRO is nucleated on the (111) STO substrate by both face- and corner-sharing octahedra. More interestingly, it is found that boundaries between differently stacked nucleation layers (heterostructural boundaries) facilitate strain relaxation, in addition to the formation of conventional misfit dislocations evolved from homostructural boundaries. The observations reveal an important underlying mechanism to understand the thin-film epitaxy and strain accommodation in hexagonal perovskites.
Synthesis of hexagonal wurtzite Cu{sub 2}ZnSnS{sub 4} prisms by an ultrasound-assisted microwave solvothermal method
Wurtzite Cu{sub 2}ZnSnS{sub 4} (CZTS) hexagonal prisms were synthesized by a simple ultrasound-microwave solvothermal method. The product was characterized by XRD, FESEM, EDS, TEM, Raman and UV–vis spectrometer. The hexagonal prisms were 0.5–2 μm wide and 5–12 μm long. The PVP played an important role in the formation of the CZTS hexagonal prisms. In addition, the ultrasound-assisted microwave process was helpful for synthesis of wurtzite rather than kesterite phase CZTS. A nucleation–dissolution–recrystallization mechanism was also proposed to explain the growth of the CZTS hexagonal prisms. - Graphical abstract: Wurtzite Cu{sub 2}ZnSnS{sub 4} hexagonal prisms were synthesized by ultrasound-microwave solvothermal method. The ultrasound-assisted microwave process and PVP were useful to the growth of CZTS. A nucleation–dissolution–recrystallization growth mechanism was also proposed. - Highlights: • Wurtzite Cu{sub 2}ZnSnS{sub 4} was prepared by ultrasound-assisted microwave solvothermal method. • The wurtzite CZTS hexagonal prisms are demonstrated a band gap of 1.49 eV. • Synergistic effect of ultrasound and microwave is helpful to prepare Wurtzite CZTS. • PVP plays an important role in the formation of the CZTS hexagonal prisms. • Nucleation–dissolution–recrystallization growth mechanism of the CZTS was proposed.
Hydrothermal synthesis of hexagonal magnesium hydroxide nanoflakes
Graphical abstract: Hexagonal Mg(OH){sub 2} nanoflakes were synthesized via hydrothermal method in the presence of PEG-20,000. Results show that PEG-20,000 plays an important role in the formation of this kind of nanostructure. The SAED patterns taken from the different positions on a single hexagonal Mg(OH){sub 2} nanoflake yielded different crystalline structures. The structure of the nanoflakes are polycrystalline and the probable formation mechanism of Mg(OH){sub 2} nanoflakes is discussed. - Highlights: • Hexagonal Mg(OH){sub 2} nanoflakes were synthesized via hydrothermal method. • PEG-20,000 plays an important role in the formation of hexagonal nanostructure. • Mg(OH){sub 2} nanoflakes show different crystalline structures at different positions. • The probable formation mechanism of hexagonal Mg(OH){sub 2} nanoflakes was reported. - Abstract: Hexagonal magnesium hydroxide (Mg(OH){sub 2}) nanoflakes were successfully synthesized via hydrothermal method in the presence of the surfactant polyethylene glycol 20,000 (PEG-20,000). Results show that PEG-20,000 plays an important role in the formation of this kind of nanostructure. The composition, morphologies and structure of the Mg(OH){sub 2} nanoflakes were characterized by X-ray diffraction (XRD), field-emission scanning electron microscopy (FE-SEM), high-resolution transmission electron microscopy (HRTEM), and selected area electron diffraction (SAED). The SAED patterns taken from the different positions on a single hexagonal Mg(OH){sub 2} nanoflake show different crystalline structures. The structure of the nanoflakes are polycrystalline and the probable formation mechanism of Mg(OH){sub 2} nanoflakes is discussed. Brunauer–Emmett–Teller (BET) analysis were performed to investigate the porous structure and surface area of the as-obtained nanoflakes.
First 14-Layer Twinned Hexagonal Perovskite Ba 14 Mn 1.75 Ta 10.5 O 42 : Atomic-Scale Imaging of Cation Ordering
Formation of hexagonal perovskite with mixed cubic and hexagonal stacking of AO 3 layers becomes more and more difficult when the number of layers in the stacking repeating unit increases. So far, the highest number of layers reported for twinned hexagonal perovskite is 12, with alternative 5 consecutive cubic layers and one hexagonal layer in the (ccccch) 2 sequence. Here, we present the unexpected formation of a 14-layer twinned hexagonal perovskite with a stacking sequence (cccccch) 2 for the BaO 3 layers on the Ba 14 Mn 1.75 Ta 10.5 O 42 (Ba 8 MnTa 6 O 24 ) composition, the first example of twinned hexagonal perovskite with a periodicity exceeding 12-layers. The B-cation and vacancy distributions are characterized by multiple efficient and complementary techniques including neutron and synchrotron powder diffraction, scanning transmission electron microscopy-high angle annular dark field (STEM-HAADF) imaging, and electron energy loss spectroscopy (EELS) and X-ray energy dispersive spectroscopy (EDS) elemental mapping. Atomic-resolution STEM-HAADF imaging and EELS/EDS elemental mapping enables direct observation of high-spin d 5 Mn 2+ cation ordering in the d 0 Ta 5+ host, thus demonstrating the great potential of this technique for probing cation ordering and performing structure determination. Moreover, atomic mapping allows for the observation of local defect structure variants, which can be a powerful tool for future new material design. The large high-spin Mn 2+ cation and Ta-vacancy pair formation in face-sharing octahedral sites play key roles on both the stabilization of this 14-layer twinned hexagonal perovskite structure and the Mn 2+ ordering in the central corner-sharing octahedral (CSO) positions within the five-consecutive CSO layers. Here, compared with the 8-layer twinned Ba 8 ZnTa 6 O 24 material, the low quality factor in microwave frequency and enhanced ultraviolet and visible light absorption of Ba 14 Mn 1.75 Ta 10.5 O 42 as well as the photocatalytic activity on water splitting are discussed in terms of the presence of high-spin Mn 2+ cations in the structure.
Characterization of the secondary flow in hexagonal ducts
In this work we report the results of DNSs and LESs of the turbulent flow through hexagonal ducts at friction Reynolds numbers based on centerplane wall shear and duct half-height Re τ,c ≃ 180, 360, and 550. The evolution of the Fanning friction factor f with Re is in very good agreement with experimental measurements. A significant disagreement between the DNS and previous RANS simulations was found in the prediction of the in-plane velocity, and is explained through the inability of the RANS model to properly reproduce the secondary flow present in the hexagon. The kinetic energy of the secondary flow integrated over the cross-sectional area < K > yz decreases with Re in the hexagon, whereas it remains constant with Re in square ducts at comparable Reynolds numbers. Close connection between the values of Reynolds stress uw¯ on the horizontal wall close to the corner and the interaction of bursting events between the horizontal and inclined walls is found. This interaction leads to the formation of the secondary flow, and is less frequent in the hexagon as Re increases due to the 120° aperture of its vertex, whereas in the square duct the 90° corner leads to the same level of interaction with increasing Re . Analysis of turbulence statistics at the centerplane and the azimuthal variance of the mean flow and the fluctuations shows a close connection between hexagonal ducts and pipe flows, since the hexagon exhibits near-axisymmetric conditions up to a distance of around 0.15 D H measured from its center. Spanwise distributions of wall-shear stress show that in square ducts the 90° corner sets the location of a high-speed streak at a distance z + v ≃50 from it, whereas in hexagons the 120° aperture leads to a shorter distance of z + v ≃38. Finally, at these locations the root mean square of the wall-shear stresses exhibits an inflection point, which further shows the connections between the near-wall structures and the large-scale motions in the outer flow.
Cubic to hexagonal tuning in Fe 2 Mn(Si 1– x Ge x ) Heusler alloys
Here, the competition between the stability of the cubic and hexagonal full Heusler alloys and the implications concerning their magnetic properties were systematically studied through the detailed structural and magnetic characterization of the Fe 2 Mn(Si 1– x Ge x ) system. This system was specifically chosen as the parent compositions are cubic ( x = 0) and hexagonal ( x = 1). It is found that the formation of hexagonal phases occurs for the x ≥ 0.6 samples, whereas its phase fraction monotonically increases with x until the pure hexagonal Fe 2 MnGe is formed. The change in structure results in high sensitiveness of both the saturation of magnetization ($M_S$) and Curie temperature ($T_C$) with x values, related to a strong magnetocrystalline anisotropy of the hexagonal phase. Both cubic and hexagonal magnetic features were qualitatively reproduced by Density Functional Theory (DFT) calculations. This work provides an experimental and theoretical foundation for further design of Heusler systems with controlled structures and magnetic properties.
Structural, magnetic and electrical properties of the hexagonal ferrites MFeO{sub 3} (M=Y, Yb, In)
The hexagonal ferrites MFeO{sub 3} (M=Y, Yb, In) have been studied using a combination of neutron and X-ray powder diffraction, magnetic susceptibility, dielectric measurements and {sup 57}Fe Moessbauer spectroscopy. This study confirms the previously reported crystal structure of InFeO{sub 3} (YAlO{sub 3} structure type, space group P6{sub 3}/mmc), but YFeO{sub 3} and YbFeO{sub 3} both show a lowering of symmetry to at most P6{sub 3}cm (ferrielectric YMnO{sub 3} structure type). However, Moessbauer spectroscopy shows at least two distinct Fe sites for both YFeO{sub 3} and YbFeO{sub 3} and we suggest that the best model to rationalise this involves phase separation into more than one similar hexagonal YMnO{sub 3}-like phase. Rietveld analysis of the neutron diffraction data was carried out using two hexagonal phases as a simplest case scenario. In both YFeO{sub 3} and YbFeO{sub 3}, distinct dielectric anomalies are observed near 130 K and 150 K, respectively. These are tentatively correlated with weak anomalies in magnetic susceptibility and lattice parameters, for YFeO{sub 3} and YbFeO{sub 3}, respectively, which may suggest a weak magnetoelectric effect. Comparison of neutron and X-ray powder diffraction shows evidence of long-range magnetic order in both YFeO{sub 3} and YbFeO{sub 3} at low temperatures. Due to poor sample crystallinity, the compositional and structural effects underlying the phase separation and possible magnetoelectric phenomena cannot be ascertained. - Graphical abstract: Hexagonal MFeO{sub 3} (M=Y, Yb) exhibit phase separation into two YMnO{sub 3}-like phases. Variable temperature crystallographic, electrical and magnetic studies suggest weak correlations between electrical and magnetic responses and long-range magnetic order at low temperature. Highlights: Black-Right-Pointing-Pointer Multi-technique study of multiferroic hexagonal MFeO{sub 3}. Black-Right-Pointing-Pointer Phase separation into two similar hexagonal phases. Black-Right-Pointing-Pointer Weak coupling of electrical and magnetic responses. Black-Right-Pointing-Pointer Long-range magnetic order at low T.
Hexagonal phase WO{sub 3} nanorods: Hydrothermal preparation, formation mechanism and its photocatalytic O{sub 2} production under visible-light irradiation
WO{sub 3} with various morphologies and crystal phases were prepared via a hydrothermal process in the presence of different Na{sub 2}SO{sub 4} concentrations. Experimental results indicated that the preferred growth face ((200) crystal plane) of hexagonal WO{sub 3} could adsorb abundant Na{sup +}, which induced the WO{sub 3} growing along [001] crystal direction to form nanorods, and hexagonal phase WO{sub 3} nanorods with diameter of 30-150 nm and length of 0.5-5 {mu}m were obtained in the presence of 0.25 M Na{sub 2}SO{sub 4}, while excessive Na{sub 2}SO{sub 4} led to a mixed crystal consisting of cubic H{sub 2}W{sub 2}O{sub 7} and hexagonal WO{sub 3}. The photoactivities for O{sub 2} evolution for various products were investigated by using Fe(NO{sub 3}){sub 3} as a sacrificial reagent under visible-light irradiation, and the hexagonal phase WO{sub 3} nanorods show a better photoactivity than the other products with different morphologies and crystal phases due to its fast three-dimensional photogenerated carrier transfer along its special rod-shaped structure. - Graphical abstract: As a preferred growth plane, (200) plane of h-WO{sub 3} can adsorb abundant of Na{sup +}, which induces the WO{sub 3} growing along [001] direction to form nanorods with high length-diameter ratio. Highlights: Black-Right-Pointing-Pointer WO{sub 3} with various morphologies were hydrothermally prepared. Black-Right-Pointing-Pointer Hexagonal WO{sub 3} nanorods with diameter of 30-150 nm can be obtained from 0.25 M Na{sub 2}SO{sub 4} solution. Black-Right-Pointing-Pointer Hexagonal WO{sub 3} nanorods show a better photoactivity than the other products. Black-Right-Pointing-Pointer The good photoactivity can be due to its fast carrier transfer along its rod-shaped structure.
Phase stabilisation of hexagonal barium titanate doped with transition metals: A computational study
Interatomic potentials recently developed for the modelling of BaTiO{sub 3} have been used to explore the stabilisation of the hexagonal polymorph of BaTiO{sub 3} by doping with transition metals (namely Mn, Co, Fe and Ni) at the Ti-site. Classical simulations have been completed on both the cubic and hexagonal polymorphs to investigate the energetic consequences of transition metal doping on each polymorph. Ti-site charge compensation mechanisms have been used for the multi-valent transition metal ions and cluster binding energies have been considered. Simulations show a significant energetic gain when doping occurs at Ti sites in the face sharing dimers (Ti{sub 2} sites) of the hexagonal polymorph compared with the doping of the cubic polymorph. This energetic difference between the two polymorphs is true for all transition metals tested and all charge states and in the case of tri- and tetra-valent dopants negative solution energies are found for the hexagonal polymorph suggesting actual polymorph stabilisation occurs with the incorporation of these ions as observed experimentally. Oxidation during incorporation of Ni{sup 2+} and Fe{sup 3+} ions has also been considered. - Graphical abstract: The representation of the strongest binding energy clusters for tri-valent dopants—(a) Ti{sub 2}/O{sub 1} cluster and (b) Ti{sub 2}/O{sub 2} cluster. Highlights: ► Classical simulations show a significant energetic gain when doping occurs at Ti sites in the face sharing dimers (Ti2 sites) of the hexagonal polymorph compared with the doping of the cubic polymorph. ► This energetic difference between the two polymorphs is true for all transition metals tested and all charge states. ► In the case of tri- and tetra- valent dopants negative solution energies are found for the hexagonal polymorph suggesting actual polymorph stabilisation occurs with the incorporation of these ions.
Synthesis of hexagonal Zn{sub 3}(OH){sub 2}V{sub 2}O{sub 7}·2H{sub 2}O nanoplates by a hydrothermal approach: Magnetic and photocatalytic properties
Hexagonal Zn{sub 3}(OH){sub 2}V{sub 2}O{sub 7}·2H{sub 2}O nanoplates have been successfully synthesized via a facile and template-free hydrothermal method. The nanocrystals have a hexagonal shape with 650–750 nm in diameter and 120–140 nm in thickness. The possible mechanism of forming such hexagonal Zn{sub 3}(OH){sub 2}V{sub 2}O{sub 7}·2H{sub 2}O nanoplates may be due to its inherent anisotropic crystal structure. Magnetic hysteresis measurement indicates that the as-synthesized hexagonal Zn{sub 3}(OH){sub 2}V{sub 2}O{sub 7}·2H{sub 2}O nanoplates have weak ferromagnetic property at room temperature. Compared to the floriated-like nanostructured Zn{sub 3}V{sub 2}O{sub 7}(OH){sub 2}(H{sub 2}O){sub 2} synthesized by a hydrothermal route, the as-prepared hexagonal Zn{sub 3}(OH){sub 2}V{sub 2}O{sub 7}·2H{sub 2}O nanoplates exhibited a significant increase in the methylene blue (MB) photodegradation rate under UV irradiation. Highlights: • Hexagonal Zn{sub 3}(OH){sub 2}V{sub 2}O{sub 7}·2H{sub 2}O nanoplates was synthesized via a hydrothermal method. • Magnetic study indicates that the nanoplates are of weak ferromagnetic property at room temperature. • The nanoplates exhibit greatly enhanced activity in the UV-light photocatalytic degradation of methylene blue.
Photon-counting hexagonal pixel array CdTe detector: Spatial resolution characteristics for image-guided interventional applications
Purpose: High-resolution, photon-counting, energy-resolved detector with fast-framing capability can facilitate simultaneous acquisition of precontrast and postcontrast images for subtraction angiography without pixel registration artifacts and can facilitate high-resolution real-time imaging during image-guided interventions. Hence, this study was conducted to determine the spatial resolution characteristics of a hexagonal pixel array photon-counting cadmium telluride (CdTe) detector. Methods: A 650 μm thick CdTe Schottky photon-counting detector capable of concurrently acquiring up to two energy-windowed images was operated in a single energy-window mode to include photons of 10 keV or higher. The detector had hexagonal pixels with apothem of 30 μm resulting in pixel pitch of 60 and 51.96 μm along the two orthogonal directions. The detector was characterized at IEC-RQA5 spectral conditions. Linear response of the detector was determined over the air kerma rate relevant to image-guided interventional procedures ranging from 1.3 nGy/frame to 91.4 μGy/frame. Presampled modulation transfer was determined using a tungsten edge test device. The edge-spread function and the finely sampled line spread function accounted for hexagonal sampling, from which the presampled modulation transfer function (MTF) was determined. Since detectors with hexagonal pixels require resampling to square pixels for distortion-free display, the optimal square pixel size was determined by minimizing the root-mean-squared-error of the aperture functions for the square and hexagonal pixels up to the Nyquist limit. Results: At Nyquist frequencies of 8.33 and 9.62 cycles/mm along the apothem and orthogonal to the apothem directions, the modulation factors were 0.397 and 0.228, respectively. For the corresponding axis, the limiting resolution defined as 10% MTF occurred at 13.3 and 12 cycles/mm, respectively. Evaluation of the aperture functions yielded an optimal square pixel size of 54 μm. After resampling to 54 μm square pixels using trilinear interpolation, the presampled MTF at Nyquist frequency of 9.26 cycles/mm was 0.29 and 0.24 along the orthogonal directions and the limiting resolution (10% MTF) occurred at approximately 12 cycles/mm. Visual analysis of a bar pattern image showed the ability to resolve close to 12 line-pairs/mm and qualitative evaluation of a neurovascular nitinol-stent showed the ability to visualize its struts at clinically relevant conditions. Conclusions: Hexagonal pixel array photon-counting CdTe detector provides high spatial resolution in single-photon counting mode. After resampling to optimal square pixel size for distortion-free display, the spatial resolution is preserved. The dual-energy capabilities of the detector could allow for artifact-free subtraction angiography and basis material decomposition. The proposed high-resolution photon-counting detector with energy-resolving capability can be of importance for several image-guided interventional procedures as well as for pediatric applications.
Ab initio density functional theory investigation of the structural, electronic and optical properties of Ca{sub 3}Sb{sub 2} in hexagonal and cubic phases
A density functional theory study of structural, electronical and optical properties of Ca{sub 3}Sb{sub 2} compound in hexagonal and cubic phases is presented. In the exchange–correlation potential, generalized gradient approximation (PBE-GGA) has been used to calculate lattice parameters, bulk modulus, cohesive energy, dielectric function and energy loss spectra. The electronic band structure of this compound has been calculated using the above two approximations as well as another form of PBE-GGA, proposed by Engle and Vosko (EV-GGA). It is found that the hexagonal phase of Ca{sub 3}Sb{sub 2} has an indirect gap in the Γ→N direction; while in the cubic phase there is a direct-gap at the Γ point in the PBE-GGA and EV-GGA. Effects of applying pressure on the band structure of the system studied and optical properties of these systems were calculated. - Graphical abstract: A density functional theory study of structural, electronic and optical properties of Ca{sub 3}Sb{sub 2} compound in hexagonal and cubic phases is presented. Display Omitted - Highlights: • Physical properties of Ca{sub 3}Sb{sub 2} in hexagonal and cubic phases are investigated. • It is found that the hexagonal phase is an indirect gap semiconductor. • Ca{sub 3}Sb{sub 2} is a direct-gap semiconductor at the Γ point in the cubic phase. • By increasing pressure the semiconducting band gap and anti-symmetry gap are decreased.
Analysis of growth kinetics and impact of NH{sub 3} on the morphology evolution of hexagonal-prism shaped Y{sub 4}O(OH){sub 9}NO{sub 3}/Y{sub 2}O{sub 3} single crystals
Highlights: • Tailored the morphology of Y{sub 4}O(OH){sub 9}NO{sub 3}and Y{sub 2}O{sub 3} crystals using different electrolytes by hydrothermal method. • Role of NH{sub 3} in the formation of hexagonal prism like morphology. • Time dependent experiments to study the growth process and formation of hexagonal prisms. - Abstract: The effect of NH{sub 3} on the morphology evolution of single crystalline hexagonal-prism like Y{sub 4}O(OH){sub 9}NO{sub 3} and Y{sub 2}O{sub 3} has been systematically investigated, as a function of reaction time, by using a combination of powder X-ray diffraction (XRD) and scanning electron microscopy (SEM). Rational control over the morphology has been achieved by the choice of suitable electrolytes like NaNO{sub 3}, KNO{sub 3}, NH{sub 4}NO{sub 3}, KCl and NH{sub 4}Cl in the presence of NH{sub 4}OH. We report that the cross-sectional geometry of hydrothermally grown one-dimensional Y{sub 4}O(OH){sub 9}NO{sub 3} crystals can be tuned from spherical to hexagonal-prism like shape by introducing a particular reagent like NH{sub 4}NO{sub 3} in the presence of NH{sub 4}OH. An ‘oriented attachment’ mediated growth process takes place along with the ‘dissolution-recrystallization’ route which eventually resulted in the generation of the single crystals with hexagonal-prism like morphology.
Elastic moduli of hexagonal diamond and cubic diamond formed under shock compression
The relative stiffnesses and strengths of hexagonal diamond and cubic diamond constitute long standing unresolved issues, because relevant experimental results are lacking. Laser interferometry was used to experimentally determine the longitudinal sound speeds and moduli in hexagonal diamond and cubic diamond formed during the shock compression of graphite. The hexagonal diamond longitudinal moduli are significantly larger than the cubic diamond longitudinal moduli, and even exceed averaged cubic diamond single crystal values. Here, the measured hexagonal diamond longitudinal moduli, combined with high-pressure bulk moduli for cubic diamond single crystals, show that shock-formed hexagonal diamond shear moduli are larger than the shear moduli for cubic diamond single crystals.
Tuning the Néel Temperature of Hexagonal Ferrites by Structural Distortion
To tune the magnetic properties of hexagonal ferrites, a family of magnetoelectric multiferroic materials, by atomic-scale structural engineering, we studied the effect of structural distortion on the magnetic ordering temperature ( T N ) in these materials. Using the symmetry analysis, we show that unlike most antiferromagnetic rare-earth transition-metal perovskites, a larger structural distortion leads to a higher T N in hexagonal ferrites and manganites, because the K 3 structural distortion induces the three-dimensional magnetic ordering, which is forbidden in the undistorted structure by symmetry. We also revealed a near-linear relation between T N and the tolerance factor and a power-law relation between T N and the K 3 distortion amplitude. Following the analysis, a record-high T N (185 K) among hexagonal ferrites was predicted in hexagonal ScFeO 3 and experimentally verified in epitaxially stabilized films. Furthermore these results add to the paradigm of spin-lattice coupling in antiferromagnetic oxides and suggests further tunability of hexagonal ferrites if more lattice distortion can be achieved.