B. L. Choudhary

Molecular precursors for the preparation of homogenous zirconia-silica materials by hydrolytic sol-gel process in organic media. Crystal structures of [Zr{OSi(O(t)Bu)3}4(H2O)2]·2H2O and [Ti(O(t)Bu){OSi(O(t)Bu)3}3].

[Zr(OPr(i))(4)·Pr(i)OH] reacts with [HOSi(O(t)Bu)(3)] in anhydrous benzene in 1:1 and 1:2 molar ratios to afford alkoxy zirconosiloxane precursors of the types [Zr(OPr(i))(3){OSi(O(t)Bu)(3)}] (A) and [Zr(OPr(i))(2){OSi(O(t)Bu)(3)}(2)] (B), respectively. Further reactions of A or B with glycols in 1:1 molar ratio afforded six chemically modified precursors of the types [Zr(OPr(i))(OGO){OSi(O(t)Bu)(3)}] (1A-3A) and [Zr(OGO){OSi(O(t)Bu)(3)}(2)] (1B-3B), respectively [where G = (-CH(2)-)(2) (1A, 1B); (-CH(2)-)(3) (2A, 2B) and (-CH(2)CH(2)CH(CH(3)-)} (3A, 3B)]. The precursors A and B are viscous liquids, which solidify on ageing whereas the other products are all solids, soluble in common organic solvents. These were characterized by elemental analyses, molecular weight measurements, FAB mass, FTIR, (1)H, (13)C and (29)Si-NMR studies. Cryoscopic molecular weight measurements of all the products, as well as the FAB mass studies of 3A and 3B, indicate their monomeric nature. However, FAB mass spectrum of the solidified B suggests that it exists in dimeric form. Single crystal structure analysis of [Zr{OSi(O(t)Bu)(3)}(4)(H(2)O)(2)]·2H(2)O (3b) (R(fac) = 11.9%) as well as that of corresponding better quality crystals of [Ti(O(t)Bu){OSi(O(t)Bu)(3)}(3)] (4) (R(fac) = 5.97%) indicate the presence of a M-O-Si bond. TG analyses of 3A, B, and 3B indicate the formation of zirconia-silica materials of the type ZrO(2)·SiO(2) from 3A and ZrO(2)·2SiO(2) from B or 3B at low decomposition temperatures (≤200 °C). The desired homogenous nano-sized zirconia-silica materials [ZrO(2)·nSiO(2)] have been obtained easily from the precursors A and B as well as from the glycol modified precursors 3A and 3B by hydrolytic sol-gel process in organic media without using any acid or base catalyst, and these were characterized by powder XRD patterns, SEM images, EDX analyses and IR spectroscopy.

Sol–gel synthesis of highly pure α-Al2O3 nano-rods from a new class of precursors of salicylaldehyde-modified aluminum(III) isopropoxide. Crystal and molecular structure of [Al(OC6H4CHO)3]

Reactions of aluminum isopropoxide with salicylaldehyde in 1 : 1, 1 : 2 and 1 : 3 molar ratios in anhydrous benzene yield complexes of the type [(OPri)3−nAl(OC6H4CHO)n] {where n = 1(1); n = 2 (2); n = 3 (3)}. All the products are yellow solids and are soluble in common organic solvents. They were characterized by elemental analysis, ESI-mass spectrometry, FT-IR and 1H, 13C and 27Al NMR studies. The ESI-mass spectral studies indicate dimeric nature for (1) and (2) and monomeric nature for compound (3). Crystal and molecular structures of [Al(OC6H4CHO)3] (3) suggest that salicylaldehyde ligands bind to the metal in a side-on dihapto η2-(O,O) manner, leading to the formation of a hexa-coordinated environment around the aluminum atom. Powder XRD, SEM image, and EDX analysis appear to indicate formation of nano-sized rods for precursor (3). Sol–gel hydrolysis of all the precursors Al(OPri)3, (1), (2) and (3) followed by sintering at 1100 °C yielded α-Al2O3 (a), (b), (c) and (d), respectively. The powder X-ray diffraction patterns and the SEM images of all the oxides exhibit nano-sized microcrystalline morphology for (a) and mixed platelike nano-rod morphology for (b), (c) and (d). The EDX and TEM studies of (d) also corroborate the formation of α-Al2O3. The IR spectral studies of all the oxides indicate the formation of pure α-alumina, a versatile ceramic oxide known for exhibiting a wide variety of applications in engineering and biomedical areas.

Preparation and Magnetic Studies of Mn Substituted Analogues of BiFeO3

Manganese substituted samples (for iron) in the multiferroic BiFeO3 have been prepared using solid state ceramic route. It has been possible to prepare a sample with the highest 40% Mn content. Reitveld analysis of the X‐ray diffraction patterns shows that the substituted analogues crystallize in rhombohedral symmetry in R‐3c space group. Effort to prepare 50 atomic% Mn substituted sample did not succeed. Magnetization measurements have been made in the temperature range 20K–300K and in fields upto 8 kOe. Magnetic nature of the 40 atomic% Mn substituted sample is alike that of 30 atomic% Mn substituted one. Mn induces weak ferromagnetism with the average magnetic moment increasing with its concentration.

Changes in optical behaviour of iron pyritohedron upon microwave treatment

We have utilized the volumetric heating of materials by microwave energy absorption for investigating the changes in the optical behavior of a well characterized natural crystal of iron pyritohedron (FeS2). For microwave treatment virgin central core pieces of the FeS2 crystal were ground to fine powder and then heated in a microwave oven for half an hour. Powder XRD measurements confirmed that the microwave treatment on FeS2 does not affect the face centered cubic structure of FeS2. The UV-Visible optical spectrum of the microwave treated FeS2 display a narrow optical absorption peak at ∼315 nm, on the other hand in the UV-Vis spectrum of pure FeS2 a broad absorption band with a maximum centered ∼310-330 nm was observed. The band gap energies for pure and microwave treated FeS2 are estimated to be 1.09 eV and 1.35 eV respectively. This study clearly indicates that microwave treatment results in a blue shift in the absorption edge and enhancement in the band gap energy.

Structural, optical and magnetic behaviour of nanocrystalline Volborthite

Nanocrystalline sample of Volborthite (Copper Pyrovanadate: Cu3V2 (OH)2O7.2H2O) has been synthesized using wet chemical route and characterized by XRD, SEM, FTIR, UV-Vis-NIR spectroscopic and magnetization measurements. Room temperature X-ray diffraction analysis confirms the single phase monoclinic structure and nanocrystalline nature of Volborthite. The UV-Visible optical absorption spectrum displays two broad absorption peaks in the range of 200-350 nm and 400-1000 nm. The direct band gap is found to be Eg= ∼2.74 eV. Bulk Volborthite was reported to be a natural frustrated antiferromagnet, however our nanocrystalline Volborthite display week ferromagnetic hysteresis loop with very small coercivity and retentivity at room temperature.