Electrochemical behaviour of LiM y Mn2−y O4 (M = Cu, Cr; 0 ≤y ≤ 0.4)

Spinel lithium manganese oxide, LiMn₂O₄, is beset with problems of capacity fade upon repeated cycling. The loss in capacity upon cycling is attributable to Jahn-Teller distortion and manganese dissolution in the electrolyte in the charged state. One way to circumvent this capacity fade is to introduce other 3d bdtransition metal ions in the LiMn₂O₄ lattice. In this paper, we report on the effect of partial substitution of manganese in the LiMn₂O₄ phase with copper (II) and chromium (III) ions. It has been shown that the higher octahedral stabilization energy of trivalent chromium imparts greater structural stability to chromium-doped LiMn₂O₄ spinels. Both copper and chromium reduce the capacity of the spinel in the 4 V region. In terms of its good reversible capacity and ability to sustain cycling with minimal capacity fade, LiCr_0.1Mn_1.9O₄ may be considered as a potential cathode material for lithium rechargeable cells.     

Effect of praseodymium substitution on the growth and properties of Y1−x Pr x Ba2Cu3O7−δ (o <x < 0·4) single crystals

In the present paper, a modified self-flux technique has been successfully employed for the growth of pure and praseodymium substituted (partially) large single crystals of high temperature superconducting Y_1−x Pr _x Ba₂Cu₃O_7−δ (x = 0·0,0·2,0·4). Typical sizes of the platy and bulky crystals of pure YBCO(123) material are ≈ 2 × 2 × 0·1 mm³ and 4 × 1 × 1 mm³, respectively. In case of Pr-substitution, the typical sizes of platy and bulky crystals of Y_0·8Pr_0·2Ba₂Cu₃O_7−δ and Y_0·6Pr_0·4Ba₂Cu₃O_7−δ materials are ≈ 2 × 3 × 0·1 mm³ and 5 × 1 × 1 mm³ and ≈ 1 × 1·5 × 0·1 mm³ and 7 × 0·2 × 0·1 mm³, respectively. The morphology and growth habit of the as-grown single crystals and the critical transition temperature (T _c) of the oxygenated crystals were found to depend on the Pr-content. Paper presented at the poster session of MRSI AGM VI, Kharagpur, 1995     

(PEG) x NH4ClO4: a new polymeric fast proton conductor

A new polymer electrolyte (PEG) _x NH₄ClO₄(x = 5, 10, 15, 20) has been prepared that shows protonic conduction. The room temperature conductivities are of the order of 10⁻⁷S/cm, and increase with decrease in salt concentration. NMR line width studies indicate fairly low glass transition temperatures of the polymer salt complexes. Paper presented at the poster session of MRSI AGM VI. Kharagpur, 1995     

Preparation and properties of (CdS) x -(PbS)1 −x thin-film composites

A modified chemical deposition process is employed for the preparation of thin-film (CdS) _x -(PbS)_{1 −x} composites with 0·2 ≤x ≤ 0·8. Cadmium sulphate, lead acetate and thiourea are used as the basic source materials. The electrical conductivity is found to decrease with increasing composition parameterx up to 0·5 and increase for further increase inx. The composites are polycrystalline as is revealed from XRD and microscopic observations and show phases of both cubic and hexagonal CdS, cubic CdO and PbS, and tetragonal PbO and PbO₂. Additional peaks of free elemental Cd and S have also been observed. For all the phases no significant variation in lattice parameters withx has been observed. The optical absorption studies show the presence of four well-defined absorption edges at approximately 0·45 eV, 1·05eV, 1·80 eV and 2·35 eV, at the same energies for allx values. The absorption coefficient is of the order of 10⁴ cm⁻¹ and mode of band-to-band transition is of the direct type.     

Photoluminescence and Raman study of Cu2ZnSn(SexS1 − x)4 monograins for photovoltaic applications

The quaternary semiconductors Cu₂ZnSnSe₄ and Cu₂ZnSnS₄ have attracted a lot of attention as possible absorber materials for solar cells due to their direct bandgap and high absorption coefficient (> 10⁴ cm⁻¹). In this study we investigate the optical properties of Cu₂ZnSn(Se_xS_1 − x)₄ monograin powders that were synthesized from binary compounds in the liquid phase of potassium iodide (KI) flux materials in evacuated quartz ampoules. Radiative recombination processes in Cu₂ZnSn(Se_xS_1 − x)₄ monograins were studied by using low-temperature photoluminescence (PL) spectroscopy. A continuous shift from 1.3 eV to 0.95 eV of the PL emission peak position with increasing Se concentration was observed indicating the narrowing of the bandgap of the solid solutions. Recombination mechanisms responsible for the PL emission are discussed. Vibrational properties of Cu₂ZnSn(Se_xS_1 − x)₄ monograins were studied by using micro-Raman spectroscopy. The frequencies of the optical modes in the given materials were detected and the bimodal behaviour of the A₁ Raman modes of Cu₂ZnSnSe₄ and Cu₂ZnSnS₄ is established.     

Low-temperature synthesis and characterization of Ag2S1−x Te x (0≤x≤1)

A low-temperature route for the synthesis of Ag₂S, Ag₂Te and their solid solutions Ag₂S_{1 −x} Te _x (0 ≤x ≤ 1) is reported. Ag₂S is prepared by the direct addition of silver nitrate solution to thiourea, while Ag₂ Te is prepared by reacting silver nitrate solution with tellurium in nitric acid and subsequently reducing it with hydrazine hydrate. The solid solutions of Ag₂S and Ag₂Te are obtained by the addition of nitrate solutions of silver and tellurium to thiourea followed by its reduction with hydrazine hydrate. The method enables the synthesis of low-temperature crystalline phase of Ag₂S_{1 −x} Te _x solid solutions. The powder X-ray diffraction studies suggest that the solid solutions of compositionsx 〈 0·3 have a phase akin toα-Ag₂S and those with compositionsx 〉 0·6 are similar toα-Ag₂ Te. In the intermediate range of compositions (x=0·4 and 0·5), the solid solutions are found to be mixtures ofα-Ag₂S andα-Ag₂ Te phases which transform totally toα-Ag₂S phase on prolonged annealing at about 473 K.     

Superstructures in Ba0·6K0·4BiO3−y and BaPb1−x Bi x O3−y

The perovskite compounds Ba_0·6K_0·4BiO₃ and BaPb_1−x Bi _x O₃ (x=0·9, 0·5, 0·25) have been investigated by high resolution electron microscopy and selected area electron diffraction. Several superlattices produced byin situ electron beam reduction have been observed. Ordered atomic arrangements in the reduced materials are discussed.     

Composition dependence of magnetocaloric effect in Pr0.6Ca0.4Mn(1-x)Cr(x)O3 (x = 0.02-0.08)

We report the effect of varying Cr content on magnetic and magnetocaloric properties of Pr0.6Ca0.4Mn(1-x)Cr(x)O3 samples (x = 0, 0.02, 0.04, 0.06 and 0.08). While the parent compound (x = 0) is a charge ordered and antiferromagnetic insulator, Cr doped compounds are ferromagnetic metals with nearly same Curie temperature (T(c) approximately 140 K). We find unusual field-induced meta-magnetic transition above T(c) in x = 0.02 and 0.04 which is absent in x = 0.06 and 0.08. It is suggested that the paramagnetic phase in these compounds is inhomogeneous with coexistence of nano-size ferromagnetic clusters and short range charge ordered clusters. Field induced growth of ferromagnetic nano-clusters and destruction of short-range charge ordering leads to the observed metamagnetic transition, which results in large magnetic entropy change of -deltaS(M) = 5.043, 6, 5.509 and 4.375 J/kg K under deltaH = 5 T, for x = 0.02, 0.04, 0.06 and 0.08, respectively. In addition, large relative cooling power (RCP) found in these materials (327.384, 286.36, 272.22 and 279.936 J/kg) makes it interesting for practical applications. Our study suggests that creation of ferromagnetic nano-clusters in the paramagnetic phase by Mn-site doping in charge ordered compounds provides an alternative approach to achieve high AS(M) and RCP values.     

Luminescence properties of Ca4Y6(SiO4)6O:RE (RE = Eu, Tb, Dy, Sm and Tm) under vacuum ultraviolet excitation

The vacuum ultraviolet excited luminescent properties of Eu³⁺, Tb³⁺, Dy³⁺, Sm³⁺ and Tm³⁺ in the matrices of Ca₄Y₆(SiO₄)₆O were investigated. The bands at about 173 nm in the vacuum ultraviolet excited spectra were attributed to host lattice absorption of the matrix Ca₄Y₆(SiO₄)₆O. For Eu³⁺-doped samples, the O²⁻ → Eu³⁺ CTB was identified at 258 nm. Typical 4f-5d absorption bands in the region of 195-300 nm were observed in Tb³⁺-doped samples. For Dy³⁺-doped and Sm³⁺-doped samples, the broad excitation bands consisted of host absorptions, CTB and f-d transition. For Tm³⁺-doped samples, the O²⁻ → Tm³⁺ CTB was located at 191 nm. About the color purity and emission intensity, Ca₄Y₆(SiO₄)₆O:Tb³⁺ is an attractive candidate of green light PDP phosphor, and Ca₄Y₆(SiO₄)₆O:Dy³⁺ has potential application in the field of mercury-free lamps.     

Magnetic properties of the mixed spinel Co1+x Si x Fe2−2x O4

The structural and magnetic properties of the mixed spinel Co_1+x Si _x Fe_2?2x O₄ system for 0·1≤x≤0·6 have been studied by means of X-ray diffraction, magnetization, and Mössbauer spectroscopy measurements. X-ray intensity calculations indicate that Si⁴⁺ ions occupy only tetrahedral (A) sites replacing Fe³⁺ ions, and the added Co²⁺ ions substitute for (B) site Fe³⁺ ions. The Mössbauer spectra at 300 K have been fitted with two sextets in the ferrimagnetic state corresponding to Fe³⁺ at the A and B sites, forx≤0·3. The Mössbauer intensity data shows that Si possesses a preference for the A site of the spinel. The variation of the saturation magnetic moment per formula unit measured at 300 K with the Si content, is explained on the basis of Neel’s collinear spin ordering model forx≤0·3 which is supported by Mössbauer, and X-ray data. The Curie temperature decreases nearly linearly with increase of the Si content, forx=0·1–0·6.     

Characterization of MMoO4 (M = Ba, Sr and Ca) with different morphologies prepared using a cyclic microwave radiation

Scheelite molybdates (MMoO₄, M = Ba, Sr and Ca) were successfully prepared by the reactions of M(NO₃)₂·2H₂O and Na₂MoO₄·2H₂O in propylene glycol and NaOH using a microwave radiation. The phases were detected using XRD and SAED. TEM analysis revealed the presence of micro-sized bi-pyramids with a square base, nano-sized particles in clusters, and dispersed nano-sized particles for BaMoO₄, SrMoO₄ and CaMoO₄, respectively. Diffraction patterns of the bi-pyramids were simulated, and are in accord with the experimental results. Raman and FTIR spectra provide the evidence of scheelite structure with Mo-O stretching vibration in MoO₄²⁻ tetrahedrons at 742-901 cm^− 1.     

Ca4REO(BO3)3 crystals for green and blue microchip laser generation: from crystal growth to laser and nonlinear optical properties

We have taken advantage of congruent melting behavior of the nonlinear rare-earth oxoborate Ca₄REO(BO₃)₃ family to perfect a process of collective fabrication of self-frequency doubling microchip laser based on Nd:GdCOB (Ca₄Gd_1−xNd_xO(BO₃)₃) crystals. The process goes from Czochralski boule to 1 × 3 mm² chips perfectly oriented (better than 0.1°) to the phase matching direction (θ=90°, φ=46°) in the XY principal plane, with dielectric mirrors directly deposited on both faces of the chips. 20 mW of self-frequency doubling output power at 530 nm was performed under 800 mW of diode laser as incident pump power at 812 nm. In addition, new compositions from the solid solution Ca₄Gd_1−xY_xO(BO₃)₃ (Gd_1−xY_xCOB) (x=0.13, 0.16, 0.44) have been grown by the Czochralski pulling method, in order to achieve noncritical phase matching (NCPM) second harmonic generation of ⁴F_3/2 → ⁴I_9/2 Nd³⁺ doped laser hosts. Three types of laser wavelengths have been chosen: Nd:YAP (YAlO₃) at 930 nm, Nd:YAG (Y₃Al₅O₁₂) at 946 nm, and Nd:ASL (Nd_ySr_1−x La_x−yMg_x Al_12−xO₁₉) at 900 nm. Angular acceptance measurements of these three types of compositions present very large values, compared to pure GdCOB or YCOB oriented in critical phase matching configurations.     

Thermal and mechanical properties of novel substrate crystal Mg0.4Al2.4O4

Mg_0.4Al_2.4O₄ single crystal was grown by the Czochralski method. The measured specific heat values are 0.804-1.06 J g^− 1 K^− 1 in the temperature range from 298.15 to 573.15 K. The calculated thermal conductivity components are 11.37, 11.47 and 10.77 W m^− 1 K^− 1 along the [111], [004] and [22?0] direction at 298.15 K. The Vickers microhardness values are 1328-1414 kg mm^− 2. These experimental results show that Mg_0.4Al_2.4O₄ crystal is a promising substrate for GaN-based LEDs.     

Transport properties of MoSe x Te2−x (0 ≤x ≤ 2) single crystals

The layer type MoSe _x Te_2−x (0 ≤x ≤ 2) have been grown in single crystalline form by chemical vapour transport technique using bromine as the transporting agent. The electrical resistivity and Hall mobility perpendicular to thec-axis of the crystals were measured at room temperature. The variation of the Seeback coefficient with temperature was also investigated.     

Synthesis and characterization of novel nanostructured fishbone-like Cu(OH)2 and CuO from Cu4SO4(OH)6

The Cu₄SO₄(OH)₆ was synthesized by a simple hydrothermal reaction with a yield of ~ 90%. Using Cu₄SO₄(OH)₆ as the starting material, novel fishbone-like Cu(OH)₂ was produced by a direct reaction of Cu₄SO₄(OH)₆ with NaOH solution. The Cu(OH)₂ consists of many needle-like nanorods parallel to each other and perpendicular to the direction of backbone, forming fishbone-like structure. Using the fishbone-like Cu(OH)₂ as the sacrificial precursor, CuO with similar size and morphology was obtained through a simple heat treatment. X-ray diffraction, scanning electron microscopy, energy dispersive X-ray, X-ray photoelectron spectroscopy, BET nitrogen adsorption, and UV-Vis absorption spectroscopy were employed to characterize the as-prepared samples. The conversion of the Cu₄SO₄(OH)₆ to the fishbone-like Cu(OH)₂ was visualized by time-dependent SEM images. A mechanism was also proposed based on the observed results.     

Radioluminescence of SrAl2O4:Ln (Ln = Eu, Sm, Dy) phosphor ceramic

Phosphors for radiation detection require efficient energy transfer from the ionization track to the luminescent centers. In this work, the radioluminescence (RL) spectra of SrAl₂O₄ phosphor ceramics doped with individual trivalent rare earth element (REE) ions (Sm, Eu and Dy) are reported at the room temperature. Although there is some intrinsic UV/blue emission from the host lattice, the dominant signals are from the rare-earth sites, with signals characteristic of the REE²⁺ and REE³⁺ states. The shapes of the emission bands are different for each dopant. The sharp emission properties show that the SrAl₂O₄ is a suitable host for rare-earth ion doped phosphor material.     

Superconducting YBa2Cu3O7-δ thick films on Ba2RETaO6 (RE = Pr5 Nd, Eu, and Dy) substrates

Thick films of YBa₂Cu₃O_7-δ fabricated on polycrystalline Ba₂RETaO₆ (where RE = Pr, Nd. Eu, and Dy) substrates by dip-coating and partial melting techniques are textured and oaxis oriented, showing predominantly (00/) orientation. All the thick films show a superconducting zero resistance transition of 90 K. SEM studies clearly indicate platelike and needlelike grain growth over a wide area of the thick films. The values of the critical current density for these thick films are ∼10⁴ A/cm² at 77 K as determined by the nonresonant R.F. absorption method. Various processing conditions that affect the critical current density of thick films are also discussed.     

Controlled synthesis and photoluminescence properties of BaXO4 (X = W, Mo) hierarchical nanostructures via a facile solution route

Shape-controlled synthesis of BaWO₄ hierarchical nanostructures has been achieved in a mixed solvent of water and ethanol at room temperature. By simply adjusting the volume ratio of C₂H₅OH and H₂O (R ratio), the size and shape of BaWO₄ nanostructures, such as shuttle-like and ellipsoid-like, are successfully controlled. This simple method has been extended to synthesize BaMoO₄ hierarchical nanostructures. Both BaWO₄ and BaMoO₄ hierarchical nanostructures exhibited new green emission peaks at 558 and 560 nm, respectively.     

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.     

Ar assisted carbidization of TiO2 (1 1 0) supported Mo nanoparticles by decomposition of C2H4

Ar⁺ assisted carbidization of Mo nanoparticles supported on TiO₂ (1 1 0) is studied by scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS). In order to activate the diffusion of carbon into the bulk of Mo nanoparticles we applied Ar ions (1 keV) during the exposure of C₂H₄. XPS exhibited that the decomposition of C₂H₄ at 850 K accompanied by ion bombardment results in an almost complete carbidization of nanocrystalline Mo while this treatment performed without ion bombardment results only in the carbidization of the particle surface. The modification of the crystallinity of the Mo-carbide particles was deduced from STM measurements.