Mechanochemically assisted room temperature solid state metathesis reaction for the synthesis of MMoO4 (M = Ca, Sr and Ba)

This communication reports a novel mechanochemically assisted room temperature solid state metathesis reaction for the synthesis of submicron-size alkaline-earth molybdates crystallizing in a tetragonal Scheelite structure. The solids were characterized by powder XRD, FTIR, TGA, DTA, SEM, EDAX and TEM to ascertain their composition, phase homogeneity and morphology.     

Controlled synthesis and photoluminescence properties of hierarchical BaXO4 (X = Mo, W) nanostructures at room temperature

Controlled synthesis of hierarchical Barium molybdate (BaMoO₄) nanostructures with different morphologies, such as peanut-like, cube-like and flower-like, was successfully achieved in aqueous solution at room temperature. The obtained products were characterized by a scanning electron microscope (SEM) and an X-ray power diffractometer (XRD). The morphologies of the obtained products were found to be greatly dependent on reaction time, EDTA concentration and the [Ba²⁺]/[MoO₄²⁻] ratio. This controllable method could be readily extended to produce hierarchical Barium tungstate (BaWO₄) nanostructures with peanut-like, dumbbell-like, sphere-like and flower-like morphologies. The photoluminescence (PL) properties of the obtained BaMoO₄ and BaWO₄ nanostructures exhibited strong dependence on the morphologies and sizes, respectively.     

Hydrothermal synthesis and luminescence of MWO4:Tb3+ (M = Ca,Sr, Ba) microsphere phosphors

Spherical MWO₄:Tb³⁺ (M = Ca, Sr, Ba) particles were synthesized by a hydrothermal route at 180 °C for 10 h. The synthesized MWO₄:Tb³⁺ particles were characterized by X-ray powder diffraction (XRD), Fourier transform infrared (FT-IR) spectroscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM), and luminescence spectroscopy. The XRD and FT-IR results show that MWO₄:Tb³⁺ particles with a scheelite-type crystal structure were synthesized successfully. The SEM and TEM results show that uniform spherical particles in the range of hundreds of nanometers were obtained. The possible growth mechanism may be attributed to a typical Ostwald ripening process. The excitation spectra of MWO₄:Tb³⁺ phosphors show a strong absorption band of the WO₄ ^2? group and some weak absorption bands of Tb³⁺ ions. The emission spectra of MWO₄:Tb³⁺ phosphors show the characteristic emission bands of Tb³⁺ ions. CaWO₄:Tb³⁺ sample has the highest excitation and emission intensity.     

Soft-Mode Behavior in Ternary Silicides MAlSi (M=Ca, Sr, Ba)

We present a combined experimental and theoretical investigation of the lattice dynamics and electron–phonon coupling of the ternary silicides MAlSi (M = Ca, Sr, Ba). Temperature dependent inelastic neutron-scattering measurements of the generalized phonon density-of-states provide evidence for a soft-mode behavior for CaAlSi which is absent for the other two compounds. Density-functional-based calculations of phonon spectra and electron–phonon coupling support this interpretation and identify the soft mode as strong coupling out-of-plane Al vibrations, in agreement with previous theoretical work. Observed differences in superconducting properties, in particular in T _c , among the investigated compounds can be explained by the presence of the soft mode.     

Electrical resistivity of La0.7A0.3O3 (A = Ca, Sr and Ba) at low temperature

Abstracts are not published in this journal This revised version was published online in November 2006 with corrections to the Cover Date.     

A structural study on MFCl (M=Ca, Sr, Ba) and BaFX (X=Br, I)

Here we present the electronic band structure calculations of CaFCl, SrFCl, BaFCl, BaFBr and BaFI performed using tight binding linear muffin-tin orbital method. The calculated ground state properties namely the lattice parameters, bulk moduli and the pressure-volume relations were found to be in good agreement with the experimental values. The electronic band structure of these compounds were calculated at ambient as well as in the high pressure region. The band gap values of these insulating systems were calculated and compared with available experimental values. At high pressure these compounds exhibit the interesting phenomenon of band overlap metallization.     

Structure determination at room temperature and phase transition studies aboveT c in ABi4Ti4O15 (A = Ba, Sr or Pb)

The room temperature structure of three compounds belonging to the Aurivillius family (n = 4), ABi₄Ti₄O₁₅ (A = Ba, Sr or Pb) has been analysed. BaBi₄Ti₄O₁₅ crystallizes in a tetragonalI4/mmm space group whereas SrBi₄Ti₄O₁₅ and PbBi₄Ti₄O₁₅ crystallize in the orthorhombic space group A2₁am. The starting model for the Sr and Pb analogues was derived fromab initio methods and refined using the Rietveld method. The cations Ba and Sr are disordered over the Bi sites while the Pb cation is found exclusively in the [Bi₂O₂]²⁺ layers. The TiO₆ octahedra are tilted with the Ti-O bonds forming zigzag chains along the “c” axis. The displacement of Bi atoms along the “a” axis might be responsible for ferroelectricity in these compounds. The high temperature X-ray data aboveT _c indicate no structural transition for A = Ba and Pb while A = Sr transforms to the tetragonal structure.     

Antiferromagnetic interactions and Mössbauer study of LnMCuFeOδ Phases (Ln = Y,La; M = Ca,Sr, Ba)

A three-dimensional antiferromagnetic ordering 3D-AF atT _N ≈12K has been observed in these nonsuperconducting phases. In the compounds where a partial trivalent-divalent cationic substitution (Y by Ca or La by Sr) has been performed, this antiferromagnetic ordering is not observed above 4 K. Mössbauer spectroscopy studies indicate a unique iron environment and magnetic ordering; the spectra are consistent with the classic high spin Fe³⁺ although in an unusual fivefold coordination. Both the herein described transition and the formerly reported one at ～460 K are affected upon doping.     

Physicochemical properties of La(Ba,M)CuFeO5+δ (M = Sr, Ca, Mg) solid solutions

The crystal structure of LaBa_1?x M_xCuFeO_5+δ (M = Sr, Ca, Mg; 0 < x ≤ 0.5) solid solutions has been studied by x-ray diffraction and IR absorption spectroscopy, and their thermal stability, thermal expansion, and electrical conductivity have been determined in air between 290 and 1270 K. The structure of LaBa_1?x Sr_xCuFeO_5+δ has been shown to change from cubic to tetragonal at x ? 0.4?0.45, whereas the LaBa_1?x M_xCuFeO_5+δ (M = Ca, Mg) solid solutions have a cubic structure up to x = 0.5. The composition dependences of structural parameters and physicochemical properties for LaBa_1?x Sr_xCuFeO_5+δ show anomalies at x = 0.25, which suggests ordering of the La³⁺, Ba²⁺, and Sr²⁺ cations in the structure of La(Ba,Sr)CuFeO_5+δ, leading to the formation of a new chemical compound, LaBa_3/4Sr_1/4CuFeO_5+δ.     

T c Versus Lattice Constants in MBE-Grown M2CuO4 (M=La, Sr, Ba)

Cuprates with a K₂NiF₄ structure are prototypes of p-type high-T _c superconductors and may provide a good opportunity for understanding the relationship between structure and T _c. However, until now they have all been La-based compounds, which limits the structural parameter range. Recently, by means of molecular beam epitaxy (MBE) and subsequent ozone oxidation, we newly synthesized Sr₂CuO_4± (T _c ^onset75 K) and Ba₂CuO_4± (T _c ^onset90 K) in addition to La₂CuO_4± (T _c ^onset52 K). These new compounds enabled us to undertake an investigation over a wider range than before. The results show the tendency for a longer Cu-O_apex to give a higher T _c.     

Composition dependent intrinsic defect structures in ASnO_3(A=Ca,Sr,Ba)

Perovskite stannates are promising semiconductors that are widely used in optoelectronic devices.Here,the composition dependent intrinsic point defects of stannate perovskites ASnO_3(A=Ca,Sr,Ba) are studied by first-principles calculations.The prefe rences of de fects under stoichiometric and nonstoichiometric conditions are unsealed,meanwhile the charge states of each intrinsic defect varying with the change of electron Fermi energy are clarified.For stoichiometric BaSnO_3 and SrSnO_3,the Schottky defect complexes are predicted as the most stable defect structure,while the antisite defect complexes are the most stable one in CaSnO_3.In nonstoichiometric ASnO_3,excessive AO is beneficial to the formation of oxygen vacancies and A-Sn antisite defects in all ASnO_3;while the Ca interstitial is another major defect existing in CaSnO_3.In the case of SnO_2 excess,the predominant de fects are the Sn-A antisite defects and A vacancies.Since the functionality of these perovskite oxides is closely related to the types and concentrations of their point defects,the present results are expected to guide the future experiments to optimize the function of the perovskite oxides by tailoring the intrinsic defects through controlling the composition of AO and SnO_2.     

白光LED用红色荧光粉MMoO4:Eu3+（M=Ca,Sr,Ba）的制备与表征

采用高温固相法制备了红色荧光粉MMoO4:Eu3+(M=Ca,Sr,Ba),用XRD和荧光分光光度计对其物相及发光性能进行表征和研究。结果表明,在800℃时可得到MMoO4(M=Ca,Sr,Ba)物相结构。分别以395nm的近紫外光和465nm的可见光激发样品,MMoO4:Eu3+(M=Ca,Sr,Ba)荧光粉发红光,对应于Eu3+的4f-4f跃迁,其中以616nm发光最强。荧光粉在395nm和465nm的吸收分别与紫外光和蓝光LED芯片相匹配。     

Phase evolution and microstructural development in sol-gel derived MSnO3 (M = Ca, Sr and Ba)

Alkaline-earth stannates having the general chemical formula MSnO₃ (M = Ca, Sr and Ba) have been projected as potential electronic ceramics. In view of the information gaps in the reported research, a vigorous and systematic investigation on these exotic materials has been carried out. In this communication, the synthesis of CaSnO₃, SrSnO₃ and BaSnO₃ via sol-gel technique is reported. Infrared spectroscopy and X-ray analyses of various gel samples with different thermal history helped in identifying the reaction pathways and the stage where amorphous gel to crystalline phase transition occurred. Grains of submicron size with narrow size distribution and spherical morphology, were the most noticeable characteristics of sintered calcium metastannate derived by sol-gel method. In the case of barium analogue, a fascinating sugar cube structure (akin to that observed in solid-state reaction and the self-heat-sustained reaction derived samples) having improved density characteristics evolved at low sintering temperatures. This gradually transformed into a more familiar spherical granular motif with improved density characteristics as the sintering profiles were varied from 1200 °C/24 h to 1500 °C/2 h. This seems to be an inherent feature of this system, irrespective of the method of synthesis.     

钙钛矿型复合氧化物MZrO3（M=Ba,Sr）的水热合成

利用水热晶化法对ＢａＺｒＯ３和ＳｒＺｒＯ３的制备进行了研究，对不同水热条件下的产物进行了结构表征，并探讨了最佳合成条件，结果表明，１５０℃，晶化１ｄ为最佳水热晶化条件，介质的适宜碱度为ＫＯＨ／Ｚｒ≈４。在此条件下，可获得３－７μｍＢａＺｒＯ３和５－１５μｍＳｒＺｒＯ３的多晶粉末。     

Microwave dielectric properties of alkaline earth orthosilicates M2SiO4 (M=Ba, Sr, Ca)

Alkaline earth orthosilicates M₂SiO₄ (M=Ba, Sr, Ca) ceramics were prepared by solid state ceramic route and their microwave dielectric properties were investigated. M₂SiO₄ ceramics have ε_r in the range 8.5-13. At microwave frequencies, the Q_u × f obtained were 17,900 GHz, 19,100 GHz and 26,100 GHz for Ba₂SiO₄, Sr₂SiO₄ and Ca₂SiO₄ respectively. The τ_f of Ba₂SiO₄ was − 17 ppm/°C, whereas Sr₂SiO₄ and Ca₂SiO₄ exhibited high values of τ_f, − 205 ppm/°C and − 89 ppm/°C respectively. The coefficient of thermal expansion (α_l) of the orthosilicates was also studied.     

Structural, dielectric and electrical properties of ACu3Ti4O12 (A = Ca, Sr and Ba)

Polycrystalline compounds of a general formula ACu₃Ti₄O₁₂ (A = Ca, Sr, Ba) were synthesized by a high-temperature solid-state reaction technique. Crystal structure of the compounds at room temperature studied by an X-ray diffraction (XRD) technique was found to be cubic. Detailed studies of dielectric constant () and loss tangent (tan ) of the compounds as a function of frequency (100 Hz to 1 MHz) and temperature (125 K to 600 K) did not show any ferroelectric phase transition. The ac and dc conductivity studied over a wide range of temperature provided activation energy of the compounds. Variable range phonon-assisted quantum mechanical tunneling technique of correlated barrier hopping (CBH) model has been applied to interpret the temperature and frequency dependent ac conductivity. Using these theoretical models, various model parameters have also been calculated. The temperature variation of resistivity shows that the compounds have negative temperature coefficient of resistance (NTCR).     

Thermochemistry and thermal characteristics of Ba2MIIUO6 (MII = Mg, Ca, Sr, Ba)

A series of uranium compounds with the composition Ba₂M^IIUO₆ (M^II = Mg, Ca, Sr, Ba) were synthesized by a solid-phase procedure. The polymorphism for Ba₂SrUO₆ was studied by high-temperature X-ray diffraction, and the coefficients of thermal expansion were determined. The standard enthalpies of formation of crystalline Ba₂M^IIUO₆ at 298.15 K were determined by the reaction calorimetry.     

Li_(0.5)MAl_(0.5)SiO_4:Eu,Bi的合成和发光特性(M=Mg,Ca,Sr,Ba)

首次用高温固相反应合成了 Li_(0.5)MAl_(0.5)SiO_4:Eu,Bi(M=Mg,Ca,Sr,Ba)发光体,研究了基质中不同碱土金属离子对 Eu~(3+)和 Bi~(3+)的发光特性以及 Bi~(3+)敏化 Eu~(3+)发光性能和能量传递特点,得到了良好的基质组成和一些规律性结果。用395nm 紫外线激发,M=Mg 时的发光强度比 Y(V,P)O_4:Eu强约60%。Bi~(3+)发光的 Stokes 位移与 M(Ⅱ)的离子半径呈线性关系。     

Hydrothermal Synthesis of nanocrystalline powders of alkaline-earth hydroxyapatites, A10(PO4)6(OH)2 (A = Ca, Sr and Ba)

In the present work, we report a direct precipitation of nanocrystalline powders of alkaline-earth hydroxyapatites of the compositions, A₁₀(PO₄)₆(OH)₂ (A = Ca, Sr or Ba) from aqueous solutions containing Na₃PO₄ and MCl₂ (M = Ca, Sr or Ba) at 150°C for 2 days and autogeneous pressure under hydrothermal conditions. The products were characterized by X-ray powder diffraction, transmission electron microscopy and scanning electron microscopy. The paper also discusses a convenient and economical hydrothermal route for the extraction of nanocrystalline calcium hydroxyapatite, from fish bone waste.