Single Crystals Of Mg Research Articles

White Laue and powder diffraction studies to reveal mechanisms of HCP-to-BCC phase transformation in single crystals of Mg under high pressure

Mechanisms of hexagonal close-packed (HCP) to body-centered cubic (BCC) phase transformation in Mg single crystals are observed using a combination of polychromatic beam Laue diffraction and monochromatic beam powder diffraction techniques under quasi-hydrostatic pressures of up to 58 ± 2 GPa at ambient temperature. Although experiments were performed with both He and Ne pressure media, crystals inevitably undergo plastic deformation upon loading to 40–44 GPa. The plasticity is accommodated by dislocation glide causing local misorientations of up to 1°–2°. The selected crystals are tracked by mapping Laue diffraction spots up to the onset of the HCP to BCC transformation, which is determined to be at a pressure of 56.6 ± 2 GPa. Intensity of the Laue reflections from HCP crystals rapidly decrease but no reflections from crystalline BCC phase are observed with a further increase of pressure. Nevertheless, the powder diffraction shows the formation of 110 BCC peak at 56.6 GPa. The peak intensity increases at 59.7 GPa. Upon the full transformation, a powder-like BCC aggregate is formed revealing the destructive nature of the HCP to BCC transformation in single crystals of Mg.

MODELING OF MONOCRYSTALLINE MAGNESIUM MICROBEAM BENDING

This paper presents a numerical simulation of a micro-scale experiment on a magnesium alloy. Micro cantilever beams were fabricated using Focused Ion Beam technology in a single crystal of Mg. The cantilever beams have dimensions in the order of a few micrometers and a pentagonal cross section. Nanoindenter was used for cantilever beam bending and load-displacement curve was received. Cantilevers with two different crystallographic orientations were chosen for the experiment. Three dimensional numerical FE model with elastoplastic behavior respecting crystal anisotropy was used to fit experimental load displacement curves. Strengths and deformation energy were evaluated from the models for each cantilever.

Growth, structure, crystalline perfection and characterization of Mg(II)-incorporated tris(thiourea)Zn(II) sulfate crystals: Enhanced second harmonic generation (SHG) efficiency

Single crystals of Mg(II)-incorporated tris(thiourea)Zn(II) sulfate (MZTS) have been grown from aqueous solution at room temperature by slow evaporation solution growth technique. The incorporation of Mg(II)- into the crystalline lattice was well confirmed by energy dispersive X-ray spectroscopy (EDS) and by single crystal X-ray diffraction technique. The reduction in the intensities observed in powder X-ray diffraction patterns of doped specimen and slight shifts in vibrational frequencies in FT-IR indicate the lattice stress as a result of doping. Thermal studies reveal the purity of the material and no decomposition is observed up to the melting point. High transmittance is observed in the visible region and the band gap energy is estimated by Kubelka–Munk algorithm. Surface morphology of doped material was observed by scanning electron microscopy (SEM). Decreased crystalline perfection by doping observed by high-resolution X-ray diffraction (HRXRD) analysis is justified by the crystal stress. Even a small quantity incorporation of Mg(II)- enhances the SHG efficiency significantly. The as-grown crystal is further characterized by microhardness and dielectric studies.

Magnetism of the Single Crystal Mg-V-O Spinel

In MgV2O4 with normal spinel structure (AB2O4), there is magnetic frustration of V3+ (3d2+) ions located on B-sites, if an antiferromagnetic interaction among B-site spins is essential for its magnetism. The excess Mg substituted materials such as Mg1+xV2−xO4, may construct isolated magnetic triangular lattices, because some of B-sites forming magnetic tetrahedral lattice are substituted by non-magnetic Mg ions, while the normal spinel MgV2O4 contains tetrahedral magnetic ion networks. Here, we have succeeded in growing single crystals of Mg–V–O system spinel oxide by conventional floating-zone method. The valence numbers and abundance ratio of V ions in the single crystal were estimated by wavelength-dispersive X-ray fluorescence analysis and X-ray photoelectron spectroscopy analysis. Although the composition ratio was Mg:V = 1:2, a small amount of Mg and V ion partly replace each other between A- and B- sites in this crystal. The composition formula is described as (Mg1−xVx)(V2−xMgx)O4. The single crystal shows magnetic transition at around 12 K and large negative antiferromagnetic Weiss temperature about −840 K, and the frustration parameter f = 70 estimated by those temperatures. This result suggests the existence of strong magnetic frustration in our material.

Structure of hexaaquamagnesium (II) bis(4-p-aminobenzenesulfamido-2,6-dimethoxypyrimidinate) pentahydrate

The X-ray analysis of a single crystal of Mg(C12H13N4O4S)2·11H2O, where (C12H13N4O4S)− is the anion of 4-p-aminobenzenesulfamido-2,6-dimethoxypyrimidine (sulfadimethoxine) is carried out. Unit cell parameters are: a = 19.753(4) A, b = 34.031(7) A, c = 13.859(3) A; β = 125.37(3)°, C2/c, Z = 8, R(F) = 0.042. The structure is built of [Mg(OH2)6]2+, (C12H13N4O4S)−, and water molecules and corresponds to the formula [Mg(OH2)6](C12H13N4O4S)2·5H2O. The IR bands of νasymSO2 and νsymSO2 are bathochromically shifted as a result of their participation in hydrogen bonding, and not because of any direct coordination of sulfadimethoxinate anion to the complexing atom through oxygen.

Single-crystal growth of the complex metallic alloy phase Mg32(Al,Zn)49

Abstract Single crystals of Mg 32 (Al,Zn) 49 , a complex metallic alloy phase with 162 atoms per unit cell and a lattice parameter of 14.16 A, were grown by means of the Bridgman and the Czochralski technique. We are able to produce single crystals of high structural quality with volumes of up to 3 cm 3 and a composition of about Mg 36 Al 27 Zn 37 .

Two-mode behavior for the E2g broad band in Mg(B1−xCx)2

A systematic Raman study has been carried out on single crystals of Mg(B1−xCx)2 with carbon concentrations in the range of x=0.0–0.15. We have found remarkable differentiations of the Raman spectra with doping. Even with small amounts of carbon substitution (x=0.027) the E2g-symmetry broad band at ∼600cm−1 (for pure MgB2) diminishes and new modes appear of the approximate A1g/B1g (∼750cm−1) and E2g (∼850cm−1) symmetries, though the transition temperature is only slightly reduced (<2K). The broad band does not evolve continuously into the mode of same symmetry at ∼850cm−1, as assumed by other investigators. The practical disappearance of the original ∼600cm−1 mode at x≃0.08 coincides with the fast reduction of Tc. The doping dependence of the phonons indicates a two-mode behavior, which could be driven by the 2D–3D electronic topological transition and associated with a possible nanoscale phase separation.

Synthesis, Crystal Growth and Structure of Mg Containing β‐Rhombohedral Boron: MgB17.4.

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Synthesis, crystal growth and structure of Mg containing β-rhombohedral boron: MgB 17.4

For the first time, single crystals of Mg containing β-rhombohedral boron MgB 17.4 were synthesised from the elements in a Mg/Cu melt at 1600 °C. The crystal structure determined by the refinement of single crystal data (space group R-3 m, a = 10.991 ( 2 ) Å , c = 24.161 ( 4 ) Å , 890 reflections, 123 variables, R 1 ( F ) = 0.049 , w R 2 ( I ) = 0.122 ) improves and modifies the former structure model derived from earlier investigations on powder samples. Mg is located on four different positions with partial occupation. While the occupation of the sites D (53.3%), E (91%) and F (7.2%) is already known from other boron-rich borides related to β-rhombohedral boron, the occupation of the fourth position (18 h, 6.7%) is observed for the first time. Two boron positions show partial occupation. The summation reveals the composition MgB 17.4 and Mg 5.85B 101.9, respectively, confirmed by WDX measurements. The single crystals of MgB 17.4 show the highest Mg content ever found. Preliminary measurements indicate no superconductivity.

In-situ nanoscale observations of the Mg(OH)2 dehydroxylation and rehydroxylation mechanisms

Environmental transmission electron microscopy has been used to probe the mechanisms that govern Mg(OH)2 dehydroxylation and rehydroxylation processes at the near-atomic level. Dehydroxylation and rehydroxylation rates for these in-situ observations were controlled by regulating the water vapour pressure over the sample. Generally, the dehydroxylation proceeded via the nucleation and growth of an oxide lamella, resulting in the formation of oxide and/or oxyhydroxide regions within the reaction matrix. Competition between rapid-nucleation–slow-growth and slow-nucleation–rapid-growth mechanisms can dramatically impact the nanostructure formed during dehydroxylation. Steps, both parallel and perpendicular to the {0001} planes, were observed to form during dehydroxylation. The nanocrystalline MgO formed was highly reactive and readily rehydroxylated with increasing water vapour pressure. Rehydroxylation proceeded via the nucleation and growth of Mg(OH)2 crystals in the heavily dehydroxylated matrix. The partial edge dislocations formed (both parallel and perpendicular to {0001}brucite) as the result of Mg(OH)2 nanocrystal intergrowth and anneal out with time, resulting in the formation of relatively large single crystals of Mg(OH)2. Such high mobility of Mg-containing species during rehydroxylation can be directly associated with the high chemical reactivity observed during rehydroxylation, which can facilitate key reaction processes, such as CO2 mineral sequestration.

Symmetry change of majorite solid solution in the system Mg3Al2Si3O12-MgSiO3

Six single crystals of Mg 3 (Mg x Si x Al (sub 2-2x) )Si 3 O 12 with x = 0.05, 0.13, 0.24, 0.38, 0.52, and 0.64 (the majorite solid-solution) were synthesized at 20 GPa and 2000 degrees C with a 6-8 type uniaxial split-sphere apparatus. Single-crystal X-ray diffraction studies revealed discontinuities in compositional dependence of the molar volume, equivalent isotropic temperature factors (Be q ), and mean bond lengths between x = 0.24 and 0.38. Single crystals in the compositional range 0 I4 1 /acd --> I4 1 /a, with increasing MgSiO 3 component. The symmetry changes from Ia3d to I4 1 /acd cannot be explained by the cation ordering on the octahedral site. Strong electrostatic interaction between the dodecahedral (Mg (super 2+) ) and tetrahedral (Si (super 4+) ) cations was observed from atomic thermal motion and electron density distribution. Because one of the site symmetries of the two nonequivalent tetrahedral sites in I4 1 /acd structure loses the center of symmetry with the symmetry reduction from Ia3d to I4 1 /acd, the symmetry reduction may be caused by the electronic polarization of the cations due to the neighboring cation-cation interaction.

Growth and characterization of Mg xCd 1 − xSe single crystals

Single crystals of Mg x Cd 1 − x Se over the range 0 ⩽ x ⩽ 0.46 were grown by the modified chemical transport reaction technique. The structural and optical properties of the Mg x Cd 1 − x Se single crystals were investigated at room temperature from X-ray powder diffraction and optical absorption measurements. It has been found that the Mg x Cd 1 − x Se has a wurtzite structure over the investigated composition range. The lattice constants of a and c decrease linearly with increasing a composition x. The band gap increases linearly with increasing a composition x, which is represented by a linear relationship E g( x) = 1.742 + 1.813 x.

EPR of Mn 2+-ion-doped single crystals of Mg[C 4H 3O 4] 2·6H 2O

Electron paramagnetic resonance of Mn 2+-ion-doped single crystals of magnesium bis(hydrogen maleate) hexahydrate, Mg[C 4H 3O 4] 2·6H 2O, has been studied at room and liquid nitrogen temperatures using X-band frequencies. Two differently oriented but otherwise identical complexes of Mn 2+ ions are found in these crystals. From the observed EPR spectra, spin-Hamiltonian parameters were calculated. It was found that the Mn 2+ ions substitute for Mg 2+ ions. The observed negative shift in g ∥-value of Mn 2+ at room temperature may be due to the polarization effects of the host Mg 2+ ions which produce a local static magnetic field at the Mn 2+sites. The observation of resolved Mn 2+ spectra at room temperature and its broadening on lowering the temperature in Mg(C 4H 3O 4) 2·6H 2O:Mn 2+ single crystals has been explained. The percentage of covalency in the MnO bond has been estimated to be 8%.

EPR of Mn2+-ion-doped single crystals of Mg[C4H3O4]2·6H2O

EPR of Mn2+-ion-doped single crystals of Mg[C4H3O4]2·6H2O

Ionic conductivity of crystalline and glassy Mg 4.5Na 7(P 2O 7) 4

In our work single crystals of Mg 4.5Na 7(P 2O 7) 4 were prepared, pulverized, pressed into pellets and sintered in order to measure the electrical conductivity of polycrystalline specimens. The conductivity was also measured on glassy specimens obtained by the melting of previously prepared crystals. The electrical conductivities at 25°C with values of the order of 10 −16 Ω −1 cm −1 for polycrystalline samples and a value of the order of 10 −14 Ω −1 cm −1 for glass, show that the glassy phase of Mg 4.5Na 7(P 2 because of its greater molar volume and loosely packed structure, is a better matrix for ionic motion.

A redetermination of the crystal structure of trimagnesium platinum, Mg 3Pt

Single crystals of Mg 3Pt have been obtained from the elements by high pressure-high temperature reaction in a modified Belt-type apparatus. The compound crystallizes hexagonally, space group P6 3 cm, with a=7.9103(8) A ̊ , c=8.3223(16) A ̊ , c/ a=1.0521 and Z=6. The structure refinement ( R=0.027, R w =0.022 for 264 absorption-corrected reflections) shows unequivocally that Mg 3Pt is isostructural with Cu 3P and that the structure models proposed by Ferro and Rambaldi (Na 3As type, space group P6 3/ mmc) and Mansmann (anti-LaF 3 type, space group P 3c1 ) are wrong.

Proton ENDOR of VO(H2O)52+in Mg(NH4)2(SO4)2)26H2O

ENDOR measurements at 25 K have been used to determine the hyperfine coupling tensors for all ten protons in the VO(H2O)5 2+ ion in single crystals of Mg(NH4)2(SO4)26H2O. The traceless components of all the tensors are close to axial and their use in a point dipole treatment enables a very plausible geometrical model of the complex ion to be constructed. Six of the protons in the equatorial water molecules have substantial positive isotropic couplings and it is suggested that these reflect the direct admixture of hydrogen 1s components into the singly occupied orbital.

ESR of vanadyl ions in Mg(H2O)6H2EDTA single crystals

An ESR study of single crystals of Mg(H2O)6H2EDTA doped with very small concentrations of vanadyl ions showed the spectra and spin-Hamiltonian parameters to be totally different from those obtained in an earlier study with higher concentrations of VO2+. The results obtained in the present study are comparable with the data of VO(H2O)52+ in other hydrated salts like alums, etc. In low concentrations, VO2+ takes up two preferred orientations along Mg–Ow1 and Mg–Ow2 in the host octahedron. The hyperfine splitting is slightly less than that in other hydrated salts, most probably due to the strong hydrogen bonding involving the metal–water octahedra in Mg(H2O)6H2EDTA.

Radiation damage in MgAl2O4

Exposure of single crystals of Mg${\mathrm{Al}}_{2}$${\mathrm{O}}_{4}$ to fast neutrons and to Van de Graaff electrons with energies in excess of 0.35 MeV introduces an optical-absorption band at 5.3 eV with a 1-eV half-width. This band can be partially bleached at temperatures as low as 40 K and a shoulder at 4.75 eV develops concurrently. This bleaching treatment also partially destroys a previously reported $V$-type absorption centered at 3.2 eV. Subsequent exposure to ionizing radiation destroys the 4.75-eV band and restores both the 5.3- and 3.2-eV bands to their original intensities. Since this behavior is analogous to the interconversion of $F$ to ${F}^{+}$ centers in ${\mathrm{Al}}_{2}$${\mathrm{O}}_{3}$, it is concluded that the 5.3-eV band is the principal optical transition of the $F$ center (two electrons trapped at an oxide-ion vacancy) and the 4.75-eV band is attributed to absorption by the ${F}^{+}$ center (one electron trapped at an oxide-ion vacancy). In electron-irradiated crystals the 5.3-eV absorption begins to anneal near 110\ifmmode^\circ\else\textdegree\fi{}C and is about 90% destroyed upon isochronal annealing (10-min pulses) up to 355\ifmmode^\circ\else\textdegree\fi{}C. Neutron-irradiated crystals behave similarly. Measurement of the threshold energy for damage by electrons at 77 K yields a displacement energy for the creation of ${\mathrm{O}}^{2\ensuremath{-}}$ interstitial-vacancy pairs of 59 eV. The defect yield drops off substantially with increasing temperature, and at room temperature the apparent ${\mathrm{O}}^{2\ensuremath{-}}$ displacement energy is 130 eV. Possible reasons for this strong temperature effect are discussed.

Macroscopic shape effect due to quadropole orientation in single crystals of Mg with heavy RE impurities

The magnetostriction of dilute Mg Tb, Mg Dy, Mg Ho and Mg Tm alloys was measured at 4.2 K and found to be comparable, per RE ion, to that of concentrated RE systems. The contribution of these impurities to the low-temperature thermal expansion of Mg was also measured between 1.5 and 15 K. Quadrupole orientation accounts only for the results on Mg Tb; in Mg Ho we find very strong indications for an effect of higher-order multipoles.