Effect Of Mg Substitution Research Articles

Ferromagnetic insulating and reentrant spin glass behavior in Mg doped La0.75Sr0.25MnO3 manganites

Abstract In this study, the effect of Mg substitution on structural, magnetic and electrical properties of La0.75Sr0.25Mn1−xMgxO3 and La0.75Sr0.25−xMgxMnO3 (nominal compositions) samples are investigated by XRD, Ac susceptibility and electrical resistivity measurements. It is found that Mg does not replace La in the perovskite lattice. Also the results show that by increasing Mg doping levels, the paramagnetic–ferromagnetic and metal–insulator transition temperatures decrease. The reason for decreasing transition temperatures with increasing Mg concentration is, that the long-range FM order has been destroyed by the Mg, which is randomly occupying Mn site. This leads to the suppression of double-exchange interaction in the Mn3+–O–Mn4+ networks. Also the reentrant spin glass (RSG) state accompanied by FM transition, exists in high doped samples. The RSG state could be understood on the basis of double exchange ferromagnetic interaction in Mn3+–O–Mn4+ and super-exchange antiferromagnetic interaction in the Mn4+–O–Mn4+ networks.

Enhanced giant dielectric response in Mg-substituted CaCu 3Ti 4O 12 ceramics

Effects of Mg-substitution on the giant dielectric response in CaCu 3Ti 4O 12 ceramics were investigated and discussed. A significantly enhanced giant dielectric response was obtained in Ca(Cu 1− x Mg x ) 3Ti 4O 12 ceramics, and the dielectric constant step increased with increasing Mg-substitution content. The results of X-ray photoelectron analysis confirmed the obvious increase of Ti 3+/Ti 4+ ratio, and the enhanced giant dielectric response should originate from the corresponding modification of the mixed-valent structure. The present results have great scientific significance in deepening the understanding on the origin of giant dielectric response and modulating the giant dielectric constant step in CaCu 3Ti 4O 12.

Effect of Mg substitution on photoluminescence of MgxCa1−xAl2O4: Eu2+, Nd3+

Abstract Rare earth ion-doped calcium aluminate (CaAl 2 O 4 ) is an efficient blue phosphor. The compositions in the series Mg x Ca 1− x Al 2 O 4 : Eu 2+ , Nd 3+ ( x =0.05–0.25) codoped with 1 mol% Eu and 3 mol% Nd were prepared by the solid state synthesis method. Crystalline phase, morphology and structural details were investigated by powder X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) techniques. Effect of Mg substitution on structure and photoluminescence characteristics was investigated. Photoluminescence characteristics show the intense emission for MgCaAl 2 O 4 : Eu 2+ , Nd 3+ in the blue region ( λ max =440 nm) with long persistence. The blue emission corresponds to transitions from 4f 6 5d 1 to 4f 7 of Eu 2+ ion.

The effect of Mg substitution for Ti on transport and thermoelectric properties of TiS2

Magnesium substituted compounds MgxTi1−xS2 (0⩽x⩽0.15) were prepared by solid-state reaction, and their transport and thermoelectric properties were investigated from 5to310K. The results indicate that at low temperatures (T<∼175K), a transition from metallike to semiconductorlike behavior occurred after the substitution of Mg for Ti, which implies that intrinsically TiS2 is a semiconductor and this transition can be ascribed to de-degeneration through the reduction in electron concentration due to Mg2+ substitution for Ti4+. Furthermore, it was found that dc conductivity σ for MgxTi1−xS2 (x>0) obeyed Mott’s two-dimensional (2D) variable range hopping law lnσ∝T−1∕3 at T<∼50K, indicating that TiS2 possess 2D transport characteristics. The appearance of Mott’s 2D law could originate from a potential disorder introduced by Mg substitution for Ti in S–Ti–S slabs. Meanwhile, the significant enhancement of absolute thermopower of MgxTi1−xS2 (x>0) in the whole temperature range investigated could also be attributed to the reduction of electronic concentration after doping. The thermoelectric figure of merit ZT of heavily substituted compounds (x=0.10 and 0.15) was smaller than that of TiS2, owing to the large increase of both their electrical resistivity and (lattice) thermal conductivity presumably caused by the reduced electron concentration and increased acoustic velocity, respectively. Nevertheless, ZT of the lightly substituted compound Mg0.04Ti0.96S2 enhanced substantially due to the remarkable increase in its thermopower, and specifically it is ∼1.6 times as great as that of TiS2 at 300K, indicating that doping (substitution) is an effective approach to enhance thermoelectric performance of TiS2.

Effects of Mg-substitution in Li(Ni,Co,Al)O 2 positive electrode materials on the crystal structure and battery performance

Mg-substituted Li(Ni,Co,Al)O 2 have been prepared by a co-precipitation method and examined by XRD, neutron diffraction, X-ray absorption, magnetization and electrochemical measurements. Neutron diffraction measurement indicated that the Mg ions were mostly substituted for Ni ions located in transition metal layers in LiNi 0.75Co 0.15Al 0.05Mg 0.05O 2. Although a tiny amount of Ni ions in lithium layers in LiNi 0.8Co 0.15Al 0.05O 2 was detected by magnetization measurements, such Ni ions decreased by Mg-substitution. The Mg-substitution for Ni in LiNi 0.75Co 0.15Al 0.05Mg 0.05O 2 was also confirmed by X-ray absorption measurements. The 500 mAh-type cylindrical lithium-ion cells of LiNi 0.8− x Co 0.15Al 0.05Mg x O 2 ( x = 0, 0.05) with graphite negative-electrode were prepared and their battery performances were evaluated by cycling test at 2 C charge/discharge rate at 60 °C. A lithium-ion cell of LiNi 0.75Co 0.15Al 0.05Mg 0.05O 2 exhibited better capacity retention and smaller battery resistance than that of LiNi 0.8Co 0.15Al 0.05O 2 after 500 cycles. Electrochemical impedance spectroscopy revealed that the Mg-substitution suppressed the increase in charge-transfer resistance upon cycling. The effects of Mg-substitution in LiNi 0.8Co 0.15Al 0.05O 2 were discussed in terms of crystal structure and battery performance.

Synchrotron x-ray study of polycrystalline wurtzite Zn1−xMgxO (⩽x⩽0.15): Evolution of crystal structure and polarization

The effect of Mg substitution on the crystal structure of wurtzite ZnO is presented based on synchrotron x-ray diffraction studies of polycrystalline Zn1−xMgxO (⩽x⩽0.15). Increase in Mg concentration results in pronounced c-axis compression of the hexagonal lattice, and in diminution of the off-center cation displacement within each tetrahedral ZnO4 unit. Going from ZnO to Zn0.85Mg0.15O, significant changes in the ionic polarization are observed (−5.6to−4.8μC∕cm2), despite only subtle increments in the cell volume (∼0.03%) and the ab-area dimension (∼0.1%).

Life cycle inventory study on magnesium alloy substitution in vehicles

Abstract Magnesium (Mg) alloys are suitable materials for weight reduction in vehicles because of their low density of 1.7 g/cm 3 and high specific strength. The effect of Mg substitution for conventional steel parts in a vehicle on total energy consumption and CO 2 emissions was evaluated through life cycle inventory calculation. The Mg substitution reduces the total energy consumption by weight reduction, although the production energy of a Mg-substituted vehicle is higher than those of conventional and Al-substituted vehicles. The Mg substitution can save more life cycle energy consumption than the Al substitution. Recycling of Mg parts is indispensable for efficient CO 2 reduction, because the CO 2 emissions during new ingot production of Mg are much higher than those of conventional steel and Al. Strengthening of the Mg parts also can reduce the total energy consumption and CO 2 emissions. If the main body and hood are made of Mg alloy and the ratio of recycled ingot is sufficiently high, the life cycle energy consumption and CO 2 emissions will be markedly reduced.

Effect of Mg substitution on electromagnetic properties of (Ni 0.25Cu 0.20Zn 0.55)Fe 2O 4 ferrite prepared by auto combustion method

The ferrite compositions of (Ni 0.25− x Mg x Cu 0.2Zn 0.55)Fe 2O 4 with x = 0.0 , 0.07, 0.13, 0.18, and 0.25 were synthesized through nitrate-citrate auto-combustion method. The as-burnt powders showed the presence of crystalline cubic spinel ferrite with about 19–22 nm crystallite sizes. The resultant powders were calcined at 700 °C/2 h and pressed ferrites were sintered at 950 °C/4 h. The initial permeability, magnetic loss and AC resistivity were measured in the frequency range 10 Hz–10 MHz. The permeability and AC resistivity were found to increase and the magnetic loss decreased with Mg substitution for Ni, up to x = 0.18 . The very high permeability in the composition x = 0.18 , was due to better densification, lower magnetostriction constant and inner stresses, etc. The AC resistivity of the composition was also highest. The composition would be better than NiCuZn-based material for more miniaturization of multi layer chip inductor.

Lattice dynamics and dielectric response of Mg-doped SrTiO3 ceramics in a wide frequency range

Atomic substitutions in the SrTiO3 (ST) perovskite lattice are foreseen to modify the lattice vibration modes, which are apparently sensitive to the site at which the substitution occurs. In order to understand the effect of Mg substitution in the ST lattice, the dielectric properties of Sr1−xMgxTiO3 and SrTi1−yMgyO3−δ ceramics were investigated in radio frequency (rf), microwave, terahertz (THz), and infrared (IR) ranges. Micro-Raman spectroscopy and rf tunability measurements were also conducted on these samples. The micro-Raman spectra and the high-frequency dielectric properties of Sr1−xMgxTiO3, including rf tunability, do not differ considerably from the properties of undoped SrTiO3, confirming only slight (if any) incorporation of Mg into the Sr site of ST perovskite lattice. At the same time, Ti-site Mg doping results in significant stiffening of the soft lattice mode observed in IR reflectivity spectra. Dielectric permittivity and dielectric losses decrease in both rf and THz ranges accompanied by the decrease of the tunability, in accordance with the stiffened ferroelectric soft mode. The appearance in the Raman spectra of the forbidden polar modes TO4 at 545cm−1 and TO2 at 174cm−1 at room temperature and their strengthening on cooling, demonstrate the local loss of the inversion center and the breaking of the cubic symmetry by the substitution of Ti4+ by Mg2+ ions. These results confirm more favorable occupation of Mg at the Ti site of the ST lattice and show the possibility of using Ti-site Mg-doped ST as an active material for low-loss microwave waveguides and resonators.

Effects of Al doping on the normal and superconducting properties ofMgB2:A specific heat study

The effects of Mg substitution by Al on the specific heat of ${\mathrm{MgB}}_{2}$ have been studied in the normal and in the superconducting state. The Sommerfeld coefficient, evaluated by measurements in magnetic field up to 7 T, progressively decreases from $\ensuremath{\gamma}=3.0\ifmmode\pm\else\textpm\fi{}0.2{\mathrm{m}\mathrm{J}/\mathrm{m}\mathrm{o}\mathrm{l}\mathrm{}\mathrm{K}}^{2}$ for undoped ${\mathrm{MgB}}_{2}$ down to $\ensuremath{\gamma}=1.9\ifmmode\pm\else\textpm\fi{}0.2{\mathrm{m}\mathrm{J}/\mathrm{m}\mathrm{o}\mathrm{l}\mathrm{}\mathrm{K}}^{2}$ for ${\mathrm{Mg}}_{0.6}{\mathrm{Al}}_{0.4}{\mathrm{B}}_{2}$ as a result of the change of the density of states and of the electron-phonon coupling constants. The superconducting contribution to the specific heat ${c}_{\mathrm{sc}}$ clearly shows an excess at low temperature compared with conventional single-gap BCS behavior also for doped samples with x up to 0.3. The two-band model has been used to fit the temperature dependence of ${c}_{\mathrm{sc}}$ and the amplitude of the two gaps has been evaluated for different Al concentrations. The changes of the energy gaps are in a rather poor agreement with those predicted by taking into account changes in the electronic and phononic structure only. We suggest that disorder also plays an important role when Al substitutes Mg.

Percolative conductivity in theLa0.67Sr0.33Mn1−xMgxO3system

The effects of Mg substitution on a Mn site have been studied in the series perovskite manganites ${\mathrm{La}}_{0.67}{\mathrm{Sr}}_{0.33}{\mathrm{Mn}}_{1\ensuremath{-}x}{\mathrm{Mg}}_{x}{\mathrm{O}}_{3}$ $(0.0<~x<~0.20).$ With increasing Mg content, the decrease of the magnetization, the widening of the paramagnetic-ferromagnetic transition, and the appearance of the cluster-spin-glass state have been observed. With decreasing temperature, all samples display a paramagnetic-ferromagnetic transition associated with the insulator-metal transition in the low doping region, but without the insulator-metal transition in the heavily doping zone. The phase separation in these doped samples results in the percolative conductivity.

Electronic structure of Li(Co0.7−xAl0.3)MgxO2 studied by electron energy-loss spectroscopy

Electronic structure of Li(Co0.7−xAl0.3)MgxO2 (x=0.01, 0.03, and 0.05) is studied via electron energy-loss spectroscopy. Oxygen K edge and Co L2,3 edge were used to investigate the effect of Mg substitution on both the valence state of Co ion and the density of unoccupied states of O ion. A change in the ratio of the white lines of Co ion, L3/L2, is used to identify the change in the valence state in Co ion, which reveals that Mg substitution gives rise to an increase of the average valence of Co ion. This is a mixed valence state of Co3+ and Co4+ in Mg substitution materials. Relative lowering of the integrated intensity of prepeak in oxygen K edge with the Mg concentration connects with a lower covalency of the Co–O bond, which indicates a significant electron feeding back from Co 3d to O 2p. This makes the oxygen ions closer to the feature of closed shell, which helps electron hopping between Co3+ and Co4+ based on the double-exchange process.

Magnesium-zinc ferrite with copper substitutions

The effects of Mg substitution by Cu on the properties of MgZn ferrites sintered at low temperature arc investigated. The densification of MgCuZn ferrites is dependent upon Cu content in the composition Mg 0.5-x Cu x Zn 0.5 Fe 2 O 4 + 0.5 MgO. From the sintering experiments carried out between 800 and 1 100°C it was established the optimum amount of CuO (x - 0.3) which is sufficient to promote sintering at low temperature (1050°C) and to yield a dense material (4.5 g/cm 3 ). It was evidended a break in the resistivity fo N > 0.3 when it decreases by four orders of magnitude. The strong dependence of the material parameters on the Cu ions content is consistent with the microstructural changes induced by copper ions which favour the lattice diffusion process.

Compositional Zoning in Pyroxenes from Lunar Rock 12021, Oceanus Procellarum

Electron probe study of the pyroxene phenocrysts in rock 12021 of the Apollo 12 sample reveals an extraordinary variety of zoning phenomena. These phenocrysts have cores of relatively homogeneous pigeonite which are mantled by Ca-rich pyroxenes; the mantles are zoned outward from about 20 mol per cent FeSiO3 at the core-mantle boundaries to 70 mol per cent FeSiO3 at the margins of the phenocrysts. The character of the normal zoning differs in each growth sector. Four different trends characterizing growth on (110), (100), (010), and parallel to c are described. Pronounced but erratic oscillatory effects are impressed on the normal zoning in all sectors. Normal zoning in lunar pyroxenes is generally more extensive than in terrestrial pyroxenes, probably because the lunar lavas were an order of magnitude more fluid than terrestrial basalts. Turbulent flow in lunar lavas may have caused some of the oscillatory effects. The cyclic nature of augite and pigeonite crystallization in these pyroxenes is probably kinetic in origin; conceivably it could reflect the presence of an augite-pigeonite peritectic. It is likely that further knowledge of the crystal structure of pigeonite at high temperatures will be required before a structural model can be formulated that will satisfactorily explain the details of the sector zoning. Coarse-grained pyroxferroite and subcalcic ferroaugite have crystallized in apparent ‘equilibrium’ in the final stages of formation of the groundmass in rock 12021. Nevertheless, the crystallization of the pyroxferroite was probably metastable relative to the assemblage ferroaugite+ fayalite+ tridymite. Data on the effect of Mg substitution on the stability of pyroxferroite will be needed to establish this point. We suggest that the pigeonite cores of the phenocrysts in rock 12021 crystallized under quiescent conditions, probably in a magma chamber below the lunar surface, and that the strongly zoned augite mantles crystallized during emplacement of the lava on the surface.