Influence of B2O3 on microstructure and microwave dielectric properties of 0.4Nd(Mg0.4Zn0.1Sn0.5)O3–0.6Ca0.8Sr0.2TiO3 ceramic system

The effects of B₂O₃ on the microstructure and microwave dielectric properties of the 0.4Nd(Mg_0.4Zn_0.1Sn_0.5)O₃–0.6Ca_0.8Sr_0.2TiO₃ ceramic system were investigated with a view to their use in microwave devices. A B₂O₃-doped 0.4Nd(Mg_0.4Zn_0.1Sn_0.5)O₃–0.6Ca_0.8Sr_0.2TiO₃ ceramic system was prepared by the conventional solid-state method. The X-ray diffraction patterns of the B₂O₃-doped 0.4Nd(Mg_0.4Zn_0.1Sn_0.5)O₃–0.6Ca_0.8Sr_0.2TiO₃ ceramic system did not significantly vary with sintering temperature. A 0.5 wt% B₂O₃-doped 0.4Nd(Mg_0.4Zn_0.1Sn_0.5)O₃–0.6Ca_0.8Sr_0.2TiO₃ ceramic system that was sintered at 1350 °C for 4 h had a dielectric constant of 38.3, a quality factor Qf of 35,000 GHz, and a temperature coefficient of resonant frequency of ?4.8 ppm/°C.     

Vibration Spectra and Structural Characteristics of Ba[(Zn1-x Mg x )1/3Nb2/3]O3 Solid Solutions

Abstract

The Ba[(Zn_1-x Mg _x )_1/3Nb_2/3]O₃ (BZMN, x = 0.0, 0.2, 0.4, 0.6, 0.8, 1.0) solid solution ceramics were synthesized by the conventional solid-state reaction method. Vibration spectra (Raman spectroscopy and Fourier transform far-infrared reflection [FTIR] spectroscopy) and X-ray diffraction (XRD) were employed to evaluate the correlation between crystal structures and vibration modes of these solid solutions as a function of Zn²⁺ ions replaced by Mg²⁺ ions. Spectroscopic and structural data show sensitivity to the sample structural evolution with Mg²⁺ concentration, and a 1:2 ordered structure appears for Mg-rich samples where x ≥ 0.6. The Ba[(Zn_0.4Mg_0.6)_1/3Nb_2/3]O₃ has a 1:2 ordered monoclinic unit cell, which is distorted by the antiphase tilting of the oxygen octahedra. The phonon modes were assigned, and a correlation of phonon vibrations with the microstructure was found. The position and width of the phonon modes were determined and correlated to the ionic radii, mass, and tolerance factors for the different atoms substituted in the B′-site.     

Ab‐initio thermodynamic calculations of Mg2BIV (BIV = Si,Ge, Sn) solid solutions

The thermodynamic properties of ternary Mg₂B^IV (B^IV = Si, Ge, Sn) solid solutions were first calculated by the ab‐initio density functional method. The results showed that there exist composition regions with d²G /dx² < 0 in Mg₂Si_1–x Sn_x and Mg₂Ge_1–x Sn_x systems, implying the possibility of spinodal decomposition of the pseudobinary solid solutions. It is suggested that the spinodal decomposition would be a potential way to obtain Mg₂B^IV based bulk in‐situ nanocomposites with reduced grain sizes and enhanced phonon scattering, and hence an improved thermoelectric figure of merit. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

XRD, microstructural and EPR susceptibility characterization of combustion synthesized nanoscale Mg1−xNixO solid solutions

The lattice parameter a(x) of the stoichiometric Mg_1−xNi_xO (0?x?1) solid solutions prepared by urea-based combustion synthesis with fuel to oxidizer ratio (ψ=1) was determined by X-ray diffraction. It was found that the dependence of the lattice parameter a(x) on the composition deviated more from the linear Vegard's model (VM) when compared to Kuzmin-Mironova (KM) model. a(x) in the Mg_1−xNi_xO system differs nontrivially from the predictions of both VM and KM models. For x=0.4 (Mg_0.6Ni_0.4O), the maximum deviation was about 2 and 1.7 pm, respectively. The increase in the intensity of (1 1 1) peak in XRD with increase of nickel concentration confirms that the substitution induces changes at the unit cell level. Nelson-Riley function (NRF) and Williamson-Hall plots are used to calculate micro strain in the solid solution. This analysis indicates that the micro strain is maximum for the compositions 60-40 (Mg_0.6Ni_0.4O), 50-50 (Mg_0.5Ni_0.5O) and 40-60 (Mg_0.4Ni_0.6O). The crystallite size was estimated using Williamson-Hall plot. We conclude that almost similar sized crystallite is formed in all the compositions studied. Porosity determined using XRD increases with a raise in the nickel concentration. The SEM morphology provides corroborative evidence. EPR susceptibilities of solid solution Mg_1−xNi_xO are determined at room temperature. Variable temperature of EPR allowed to check the Curie-Weiss law for solid solution. The linearity of C_M(x) and Θ(x) with concentration of nickel has ruled out chemical clustering in the samples.     

Thermoelectric properties of p‐type Mg2Si0.6Ge0.4 fabricated by bulk mechanical alloying and hot pressing

Bulk mechanical alloying (BMA) followed by hot pressing (HP) was used to prepare Mg₂Si_0.6Ge_0.4 thermoelectric material with high densification. Starting from the elemental power mixture, the Mg₂Si_0.6Ge_0.4 solid solution was solid‐state synthesized via BMA. In fact, the peaks for the cubic‐structured Mg₂Si_0.6Ge_0.4 solid solution phase were detected after 300 cycles in BMA. The single phase of Mg₂Si_0.6Ge_0.4 was synthesized at 600 cycles in BMA. Mg₂Si_0.6Ge_0.4 showed p‐type semiconduction without doping. Effects of hot pressing conditions on thermoelectric properties were investigated. With increasing hot pressing temperature from 673 to 773 K and pressure from 500 MPa to 1 GPa, the electrical conductivity increased and the Seebeck coefficient decreased. The maximum figure of merit was obtained with the processing parameter of 600 cycles BMA and hot pressing at 773 K, 1 GPa for 1 h. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Ionic conductivity of Li4−2xMgxSiO4

Solid solutions Li_4−2xMg_xSiO₄ in the system Li₄SiO₄Mg₂SiO₄ were prepared at 1000°C. The single phase region extended up to x=0.5. The conductivity is much higher than that of the end member Li₄SiO₄, and passes through a maximum at Li_3.2Mg_0.4SiO₄ with values of 2.3 × 10⁻⁵ S cm⁻¹ at 200°C, rising to 1.5 × 10⁻² S cm⁻¹ at 400°C. The solid solutions are easy to prepare, are stable in air, and retain their conductivity over a long period of time.     

Kinetic properties of p-Mg2SixSn1 ? x solid solutions for x < 0.4

The results of a study of Mg₂Si _x Sn_{1 − x} solid solutions (x = 0.25, 0.3, 0.35, 0.4) are reported. The measurements performed cover the Seebeck coefficient, electrical conductivity and the Hall coefficient over broad ranges of temperatures (80–700 K) and carrier concentrations (10¹⁸ to 6 × 10²⁰ cm⁻³). These measurements were used to derive the band structure parameters (band gap, hole mobility, hole effective mass). The effective mass of holes was found to grow strongly with an increase in their concentration.     

Thermoelectric performance of quaternary Mg2(1+x)Si0.2Ge0.1Sn0.7 (0.06 ≤ x ≤ 0.12) solid solutions with band convergence

The study of Mg₂Si based thermoelectric materials has received widespread attention. In this research, quaternary Mg_2(1+x)(Si_0.2Ge_0.1Sn_0.7)_0.99Sb_0.01 (0.06≤x ≤ 0.12) solid solutions with an optimized Sb doping were prepared by B₂O₃ flux method combined with spark plasma sintering (SPS) technique. The Seebeck coefficient, electrical conductivity and thermal conductivity were measured as a function of Mg excess between 300 K and 780 K. The electron concentration, electrical conductivity and lattice thermal conductivity increase while the Seebeck coefficient decreases with increasing magnesium excess content. The electron effective mass enhancement for x ≥ 0.08 suggests the conduction band convergence of Mg₂Si_0.2Ge_0.1Sn_0.7. Mg_2.16(Si_0.2Ge_0.1Sn_0.7)_0.99Sb_0.01 with a maximum dimensionless figure of merit of 0.94 at 780 K stand out as one of the best materials for intermediate temperature applications, providing a good nontoxic alternative to PbTe.     

High temperature electrical properties and defect structure of Co1−xMgxO

The electrical conductivity and thermoelectric power of Co_1−xMg_xO solid solutions with x ⩽ 0.85 were investigated in the range 700–1100°C and for oxygen partial pressures in the range 1–10⁻¹⁶ atm. The experimental results show that an ideal point defect model of isolated cobalt vacancies cannot explain nonstoichiometry of Co_1−xMg_xO materials with x, at least, less than 0.3. Furthermore, the hole mobility in Co_1−xMg_xO with x < 0.6 was found to be nonactivated suggesting that holes in these materials are itinerant carriers. For x > 0.6, the mobility is thermally activated indicating a change to small polaron conduction. The experimental evidence for complex defect structure and bandlike conduction in CoO is in agreement with the conclusions of recent theoretical studies of 3-D transition metal monoxides.     

Observation of Bose condensation in (Pb<Subscript>0.4</Subscript>Sn<Subscript>0.6</Subscript>)<Subscript>0.86</Subscript>In<Subscript>0.14</Subscript>Te semiconductor solid solutions using Mössbauer spectroscopy

⁷³As(⁷³Ge) Mössbauer emission spectroscopy is used to ascertain that the transition of the (Pb_0.4Sn_0.6)_0.86In_0.14Te solid solution to a superconducting state is accompanied by a change in electronic density at the cation sites, and spatial inhomogeneities in the Bose condensate of Cooper pairs are revealed.     

Penetration depths of superconducting U6X (X = Fe,Co, Mn)

The penetration depths, λ(T), of the heavy-fermion uranium-based superconductors U₆X (X = Fe, Co, Mn) have been measured as a function of temperature at ∼ 35 MHz. It was found that the temperature dependence of λ(T) for these compounds agrees well with the BCS theory. However, the values of λ(0) are large. By comparing the Slater-Pauling curve of the X elements with λ(0)^-1 for the U₆X superconductors, we have found that λ(0)^-1 is proportional to the saturation spin moments of elemental X, suggesting that there is a correlation between the superconductivity of these U₆X superconductors and the magnetic nature of the X elements.     

Microstructures and microwave dielectric properties of La1-xBx(Mg0.5Sn0.5)O3 ceramics

The microwave dielectric properties of La_1-xB_x(Mg_0.5Sn_0.5)O₃ ceramics were examined with a view to their exploitation for mobile communication. The La_1-xB_x(Mg_0.5Sn_0.5)O₃ ceramics were prepared by the conventional solid-state method with various sintering temperatures. The X-ray diffraction patterns of the La_0.995B_0.005(Mg_0.5Sn_0.5)O₃ ceramics revealed no significant variation of phase with sintering temperatures. A maximum apparent density of 6.58 g/cm³, a dielectric constant (ε_r) of 19.8, a quality factor (Q × f) of 41,800 GHz, and a temperature coefficient of resonant frequency (τ_f) of −86 ppm/°C were obtained for La_0.995B_0.005(Mg_0.5Sn_0.5)O₃ ceramics that were sintered at 1500 °C for 4 h.     

Magnetic ordering in manganites substituted by chromium ions

NdMn_1?x Cr _x O₃ and Nd_0.6Ca_0.4Mn_1?x Cr _x O₃ solid solutions have been studied by neutron diffraction and magnetic measurements. NdMn_0.5Cr_0.5O₃ is found to have a magnetic structure consisting of an antiferromagnetic G-type component and a ferromagnetic component, which are caused by 3d ions. The magnetic moments of the neodymium ions are parallel to the ferromagnetic component. Nd_0.6Ca_0.4Mn_0.5Cr_0.5O₃ mainly has a G-type magnetic structure, and the magnetic moments of the neodymium ions are normal to the antiferromagnetism vector. Magnetic phase diagrams are plotted for both systems. They are interpreted on the assumption that the Mn³⁺-O-Cr³⁺ superexchange interactions are positive and the Mn⁴⁺-O-Cr³⁺ interactions are negative; the fact that manganese and chromium ions are not ordered in a crystal lattice is taken into account. Concentration magnetic phase transformations proceed through a two-phase state because of the internal chemical inhomogeneity of the solid solutions.     

Photoluminescence and persistent luminescence properties of non-doped and Ti4+-doped Mg2SnO4 phosphors

Mg₂SnO₄ exhibits green photoluminescence and persistent luminescence, which originate from oxygen vacancies. When Ti⁴⁺ ions were doped, an interesting Mg₂SnO₄:Ti⁴⁺ phosphor with bluish white photoluminescence under ultraviolet irradiation and with green persistent luminescence was first obtained. Our investigation reveals that two emission centres exist in Mg₂SnO₄:Ti⁴⁺. The centres responsible for the green emission are considered to be the F centres (oxygen vacancies) and the blue centres are the TiO₆ complex. Trap clusters in the band gap with different depths, such as [Sn_Mg^‥—O_i^″], [Sn_Mg^‥—V_O^·], [Sn_Mg^‥—V_O^×] and Mg_Sn^″, correspond to the components at 85 ℃, 146 ℃ and 213 ℃ of the thermoluminescence curve.     

The great potential of coupled substitutions in In2O3 for the generation of bixbyite-type transparent conducting oxides, In2−2xMxSnxO3

The study of coupled substitution of In³⁺ by Sn⁴⁺/M²⁺ species in In₂O₃ has allowed In_2−2xSn_xM_xO₃ solid solutions with bixbyite structure to be synthesized for M=Ni, Mg, Zn, Cu and Ca. The latter exhibit a rather broad homogeneity range and are characterized by an ordered cationic distribution. More importantly, these novel oxides are transparent conductors, and among them the Zn and Cu phases show a great potential, since one observes a semi-metallic behavior with conductivity up to 3×10² and 3×10³ (Ω cm)⁻¹, respectively, to be compared to 2×10³ (Ω cm)⁻¹ for reduced ITO. Moreover, in contrast to the latter no reducing conditions are required for reaching such performances.     

Synthesis and properties of Mn1−XFeXS single-crystals at room temperature

Results of the first synthesis of Mn_1−XFe_XS single-crystals and its structural, electrical and magnetic properties at room temperature are presented. The Mn²⁺→Fe²⁺ substitution in Mn_1−XFe_XS solid solutions is accompanied by a compression of the NaCl lattice and a small deformation of the octahedral environments, and the concentration transition from dielectric to semimetal. Single-line Mossbauer spectra indicate the paramagnetic state Mn_1−XFe_XS sulfides at room temperature.     

Effect of co-dopant on the formation and properties of anatase-type titania solid solutions doped with niobium

The effect of co-dopant M (M=gallium (Ga), aluminum (Al), and scandium (Sc)) on the formation, crystallite growth, optical band gap, photocatalytic activity, and phase stability of anatase-type titanium dioxide solid solutions (Ti_1−2XNb_XM_XO₂) containing the same amount of dopant niobium (Nb) that were directly formed as nanoparticles under mild hydrothermal conditions at 180 °C for 5 h was investigated. The composition range X of the anatase-type solid solutions (Ti_1−2XNb_XM_XO₂) depended on the co-dopant M, i.e., X=0.15-0.20 for M=Ga and Al, and X=0.33 for M=Sc. A remarkable increase in the lattice parameter c₀ was detected in the solid solutions co-doped with M=Sc. The increase in the amount of co-dopant M=Ga and Al enhanced the crystallite growth of the anatase-type solid solutions under the hydrothermal conditions. The photocatalytic activity of the solid solutions (Ti_0.80Nb_0.10M_0.10O₂) co-doped with M=Sc, Ga, and Al increased in that order. The co-dopant M=Ga promoted the anatase-to-rutile phase transformation of the solid solutions at lower temperature.     

Enhancement microwave dielectric properties of La(Mg0.5Sn0.5)O3 ceramics by substituting La3+ with Sm3+

The microwave dielectric properties of La_1?xSm_x(Mg_0.5Sn_0.5)O₃ ceramics were examined with a view to their exploitation for mobile communication. The La_1?xSm_x(Mg_0.5Sn_0.5)O₃ ceramics were prepared by the conventional solid-state method with various sintering temperatures. The X-ray diffraction patterns of the La_0.97Sm_0.03(Mg_0.5Sn_0.5)O₃ ceramics revealed no significant variation of phase with sintering temperatures. Apparent density of 6.59 g/cm³, dielectric constant (ε_r) of 19.9, quality factor (Q×f) of 70,200 GHz, and temperature coefficient of resonant frequency (τ_f) of ?77 ppm/°C were obtained for La_0.97Sm_0.03(Mg_0.5Sn_0.5)O₃ ceramics that were sintered at 1500 °C for 4 h. The dielectric constant, and τ_f of La_0.97Sm_0.03(Mg_0.5Sn_0.5)O₃ ceramics were almost independent with the sintering temperature as the sintering temperature varied from 1450 to 1600 °C.     

Observation of charged X−and X+excitons and metal-to-insulator transition in CdTe/CdMgZnTe modulation-doped quantum wells

The positively and negatively charged excitons,X⁺andX⁻, respectively, are identified by their magnetic circular dichroism in the absorption spectra of modulation-doped CdTe/Cd_0.69Mg_0.23Zn_0.08Te multiple quantum wells at small carrier densities (≈10¹⁰cm⁻²). In these quantum wells with a width of 8 nm the binding energies of the second electron of theX⁻exciton and of the second hole of theX⁺exciton are very similar, 2.9 meV and 2.6 meV, respectively. At larger hole densities a transition to the conducting state is observed. In a sample with intermediate hole density (low 10¹¹cm⁻²) the insulating phase is restored by application of a magnetic field.     

Synthesis,structural and electrical properties of Ti modified Bi2Sn2O7 pyrochlore

Polycrystalline ceramic samples of Bi₂Sn_2−xTi_xO₇ (x=0.00, 0.2, 0.4, 0.6 and 0.8) have been synthesized by standard high temperature solid state reaction method. The effect of homovalent cation (titanium) substitution on the Sn-site on the structural and electrical properties of the pure Bi₂Sn₂O₇ ceramic have been studied by X-ray diffraction followed by SEM, dielectric and dc conductivity studies. The structural analysis indicates that the increase of titanium contents do not lead to any secondary phase. The frequency and temperature dependent dielectric studies have been carried out. It is found that the Ti doping reduces the material particle size. The size of the particles are strongly influenced by the addition of titanium to the system. The substitution of Ti for Sn ions affected the degree of disorder and modified the dielectric properties leading to more resistive ceramic compounds. The activation energies of all the compounds were calculated using the relation σ=σ₀exp(−Ea/kT).