Browsing by Author "N.P. Lalla"

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Coherent orientation relationship among various quasicrystalline and crystalline phases in rapidly solidified Al78Mn20Ge2 alloy
(1992) N.P. Lalla; R.S. Tiwari; O.N. Srivastava
Rapidly solidified Al78Mn20Ge2 alloy exhibits a curious structural feature; this corresponds to the simultaneous occurrence of almost all the quasicrystalline phases, e.g., primitive and face-centered icosahedral phase, rational approximant, and a new metastable cubic phase. The cubic crystalline phase shows yet another type of recently observed twin relation which effectively produces an icosahedral symmetry, a feature that has so far been specific to alloy systems Al-Mn-Si and Al-Mn-Fe-Si only. The diffraction evidences obtained in the present investigation reveal that the orientation of the structural subunit, which is a MacKay icosahedron, remains invariant across the various interphase boundaries. These results elucidate clearly the structural interrelationship among the various phases.
Electrical and XPS studies of 100 MeV Si 7+ ion irradiated Pd/n-GaAs devices
(Elsevier, 2004) O.P. Sinha; T. Shripathi; N.P. Lalla; P.C. Srivastava
The effect of Swift Heavy Ion (100MeV Si 7+ ) irradiation on electronic-transport of Pd/n-GaAs devices has been studied by I-V and C-V techniques. The chemical compositions of the interface have been studied by XPS/EDAX techniques. It is observed that the irradiated devices show a reduction in current and capacitance by few orders of magnitude. The C-V characteristics show a change in conductivity type from n- to p-type after the irradiation. On hydrogenation, the irradiated devices show a capacitance peak in C-V characteristics, which has been ascribed to As vacancies. The XPS studies of these devices, for various etching durations, show that the ratio of As:Ga has reduced after the irradiation, which indicates the formation of irradiation-induced As vacancies. This reduction in As:Ga ratio is also confirmed by EDAX measurement. The observed conductivity type change from n- to p-type (on the irradiation) seems to be due to the change of substitutional sites of dopant silicon atoms from Ga to As sites due to the irradiation-induced As vacancies. © 2004 Elsevier B.V. All rights reserved.
Electrical conductivity and structural behaviour of Al65Cu20Ru15 quasicrystalline alloy
(1995) N.P. Lalla; R.S. Tiwari; O.N. Srivastava; B. Schnell; G. Thummes
The low temperature conductivity σ (T) and the magnetoresistance (up to 8 T) of a phason disorder free pure Al65Cu20Ru15 quasicrystalline alloy, prepared by melt casting, has been investigated in the temperature range from 1.3 K to 200 K. In the temperature range 1.3 to 20 K the conductivity exhibits a {Mathematical expression}-variation, whereas for the range 20 to 200 K a linear T-dependence has been observed. The analysis of magnetoresistance data reveals the presence of weak localization and electron-electron interaction effects. The positive nature of the magnetoresistance indicates the presence of spin orbit scattering. The σ∞ {Mathematical expression} behaviour prevailing over such a wide temperature range has been observed for the first time for Al65Cu20Ru15. A quantitative analysis of the {Mathematical expression}-variation in terms of electron-electron interaction yields a very low density of states at the Fermi level. © 1995 Springer-Verlag.
Electrical resistivity characteristics of the decagonal quasicrystalline alloy Al78Mn22
(1989) N.P. Lalla; R.S. Tiwari; O.N. Srivastava
The variation of electrical resistivity rho with temperature T for the decagonal phase of the Al72Mn22 has been measured in the temperature range 300-680 K. The rho -T variation exhibits two main characteristics. One relates to the negative slope in sharp contrast to the corresponding crystalline phase. Evidence and arguments have been advanced to show that the effective transport mechanism in the temperature region studied is thermally activated hopping. The other observed characteristic relates to the possible fine structure in the rho -T curve, which may be expected to reflect the expected fine structure in the density of states of the quasicrystalline phase.
Highly efficient field emission properties of radially aligned carbon nanotubes
(Royal Society of Chemistry, 2018) Prashant Tripathi; Bipin Kumar Gupta; Ashish Bhatnagar; C.R.P. Patel; Prashant K. Banker; Dattatray J. Late; Mahendra A. More; N.P. Lalla; D.M. Phase; R.J. Choudhary; M.A. Shaz; P.M. Ajayan; O.N. Srivastava
Here, we report extraordinary field emission properties from one pot synthesized aligned carbon nanotubes endowed with related Fe nanoparticles (NPs). The CNT configuration is in the form of a carbon hollow cylinder (CHC) with CNTs radially aligned towards the CHC axis. The structure generates electron field emission properties such as an ultralow turn on field (0.35 V μm-1 at 10 μA cm-2), a low threshold field (0.41 V μm-1 at 100 μA cm-2) and a high field emission current density (7.71 mA cm-2 at 0.78 V μm-1). It also exhibits multi-fold improvement in the field enhancement factor (1.34 × 104) with highly stable current emission at 100 μA measured for 14 h. No post synthesis treatment is required for enhanced field emission characteristics. The growth related Fe NPs assist in lowering the work function and hence enhancing the field emission properties. The possibility of assembling nano-structured field emitters into macroscale architectures suggests new prospects for next generation three dimensional electron sources. © 2018 The Royal Society of Chemistry.
Investigation on the synthesis, characterization and electronic behaviour of Al65Cu20+xRu15-x (x = 2, 1, 0 and -1) quasi-crystalline alloys
(1995) N.P. Lalla; R.S. Tiwari; O.N. Srivastava
The sigma -T variation in the quasi-crystalline alloys Al 65Cu20+xRu15-x (x = 2, 1, 0 and -1) has been investigated in the temperature range 15 K
On the curious electrical resistivity behaviour of Al86Mn14 quasicrystalline phase
(1987) N.P. Lalla; A.K. Singh; R.S. Tiwari; O.N. Srivastava
The variation of electrical resistivity (ρ{variant}) with temperature (T) for the Al86Mn14 quasicrystalline phase has been measured in the temperature range 300-750 K. The ρ{variant}-T variation for this quasicrystalline phase has been found to exhibit a fine structure (small, ∼ 1 to 5% increase and decrease and no variation in resistivity regions). This fine structure in ρ{variant}-T variation for the quasicrystalline phase which bears sharp contrast to the corresponding behaviour for the crystalline phase has been observed for the first time. It is thought to reflect broadly the recently suggested form of the density of states for the quasicrystalline phase. © 1987.
On the new structural phases in Al65Cu20Cr15 quasicrystalline alloy
(1995) Varsha Khare; N.P. Lalla; R.S. Tiwari; O.N. Srivastava
The quasicrystalline (qc) alloy Al65Cu20Cr15, unlike its Ru- and Fe-bearing counterparts like Al65Cu20Ru15 and Al65Cu20Fe15, is a metastable phase. This qc alloy has been shown to possess several structural variants and curious structural characteristics. We have investigated the qc alloy Al65Cu20Cr15 with special reference to the possible occurrence of new structural variants. TEM exploration of the as-quenched qc alloy has indeed revealed the existence of several new phases. These are (i) body-centered cubic (bcc) (a = 12.60 Å, disordered) and simple cubic (s.c.) (a = 12.60 Å, ordered), which are the 1/1 approximants of the primitive icosahedral phase (i phase); (ii) a twice order-induced modulated cubic phase (bcc, a = 25.20 Å) which has been shown to correspond to 1/1 approximant of the ordered i phase [i.e., face-centered icosahedral (FCI)]; and (iii) real crystalline bcc (a = 8.90 Å) and face-centered cubic (fcc) (a = 17.98 Å) phases possessing a specific orientation relationship with the icosahedral matrix phase. Tentative structural models showing the interrelationships between the bcc/fcc phases have been outlined. © 1995, Materials Research Society. All rights reserved.
Pd and Pt nucleated nano Ni-particles: Evidence for heterogeneous nucleation and growth by transmission electron microscopy
(2000) N.P. Lalla; M.S. Hegde; L. Dupont; K. Tekaia-Elhsissen; J.-M. Tarascon; O.N. Srivastava
Transmission electron microscopy (TEM) study of mono-dispersed Ni-particles, prepared by the reduction of Ni(OH)2 in ethylene glycol with Pd/Pt as nucleating agent, has been carried out. Microstructural analysis employing bright-field and dark-field electron microscopy reveals polymicrocrystalline nature of each Ni-particle. TEM studies of Ni particles show presence of nearly 20 Å thick NiO layer over each 300 Å Ni-particles. Presence of 40 to 60 Å Pd/Pt nucleus has been confirmed in high magnification bright-field micrographs. The observed radial growth morphology supports heterogeneous nucleation of Pd/Pt over which Ni grows.
Some new structural and electronic characteristics of quasicrystals
(Indian Acad of Sciences, 1999) O.N. Srivastava; R.S. Tiwari; N.P. Lalla; R.K. Mandal; M.A. Shaz
The quasicrystals being based on quasiperiodic order other than crystal like periodic translational order and embodying self similarity, present unique condensed matter phases. This article describes and discusses some new developments in regard to structural and electronic aspects of quasicrystalline materials. In regard to the structural aspects, two comparatively newer features are described: one relating to the observation of variable strain approximants (VSA) first found in Ti68Fe26NiSi5, qc alloys; and one relating to the structure of decagonal phases.
Structural correlation of magneto-electric coupling in polycrystalline TbMnO3 at low temperature
(Elsevier Ltd, 2019) Harshit Agarwal; Poonam Yadav; N.P. Lalla; J.A. Alonso; O.N. Srivastava; M.A. Shaz
The present study is focused on the structural correlation of magneto-electric coupling that exists in polycrystalline TbMnO3. The X-ray diffraction patterns are collected at various conditions, which are (1) at 300 K without a magnetic field, (2) at 2 K without magnetic field and (3) at 2 K with 7 Tesla magnetic fields. The structural transition in TbMnO3 has been observed at 2 K and some lattice modulation after applying the magnetic field at 2 K. The Rietveld refinement of TbMnO3 confirms the orthorhombic phase with centrosymmetric space group Pnma at 300 K. We have observed that the inversion symmetry breaks at 2 K and the polycrystalline TbMnO3 has been refined using a non-centrosymmetric orthorhombic space group which can be either Pn21a or P21ma. This structural transition confirms the presence of a ferroelectric phase at 2 K. After the application of 7 Tesla magnetic field, the signature of an incommensurate phase has been observed in polycrystalline TbMnO3 at 2 K. The DC magnetization behaviour with temperature M(T) and field M(H) reveals the antiferromagnetic behaviour of polycrystalline TbMnO3 below 42 K. We have also measured magneto-dielectric property of polycrystalline TbMnO3 at the low temperature, which confirms the strong magneto-electric coupling in polycrystalline TbMnO3 below the transition temperature. © 2019 Elsevier B.V.
Structural studies of Ni2+x Mn1-x Ga by powder x-ray diffraction and total energy calculations
(2007) S. Banik; R. Ranjan; A. Chakrabarti; S. Bhardwaj; N.P. Lalla; A.M. Awasthi; V. Sathe; D.M. Phase; P.K. Mukhopadhyay; D. Pandey; S.R. Barman
The crystal structure of Ni2+x Mn1-x Ga has been studied as a function of composition (x) by powder x-ray diffraction. For Ni2.24 Mn0.75 Ga, where one-fourth of the Mn atoms are replaced by Ni, the experimentally determined lattice constants are in good agreement with theoretical equilibrium lattice constants calculated by minimization of total energy using full potential linearized augmented plane-wave method. For 0.15≤x≤0.35, a nonmodulated tetragonal martensitic phase is obtained at room temperature, whose lattice constant c increases and a decreases linearly with increasing x following Vegard's law. A 7M modulated monoclinic phase is obtained for x=0.2 due to annealing. The small width of the hysteresis of the martensitic transition shows its thermoelastic nature that is the characteristic of a shape memory alloy. Phase coexistence is observed for 0.1≤x≤0.15 at room temperature, confirming the first-order nature of the martensitic transition. © 2007 The American Physical Society.
Structural, microstructural and electronic characteristics of quasicrystalline and carbon containing fullerene (C60) and graphitic tubule, solids
(Elsevier Ltd, 1997) N.P. Lalla; O.N. Srivastava
The quasicrystalline and recently discovered carbon bearing exotic solids-the fullerenes (e.g. C60) and their tubule relatives Nano-tubules have a common linkage in the sense that they have built in icosahedral symmetry either at the level of lattice - as in QC materials, or motif e.g. in fullerene or tubule (caps). It is, therefore, of interest to explore the structural features and properties (e.g. electronic behaviour) of the solids on a comparative basis. The basic theme of this paper centres around this aspect. We have discussed and described the structural/microstructural characteristics and properties (with emphasis on electronic behaviour) of the QC, C60 and Nano-tubule. Attempts have then been made to bring out their comparisons for these type of icosahedron bearing solids. The case of comparison of structural behaviours and grass root electronic characteristic e.g. Brillouin zones and density of states of the QC and C60 solid - the special carbon bearing material - have been taken first. Thereafter, the case of tubule-solids which are related to C60 have been taken up. Then inter-relationship between tubule and C60 and the exotic electronic characteristics of the tubules have been discussed and described. Finally the similarities and dissimilarities of the three types of solids in regard to their structural and electronic behaviours have been outlined.
Transmission electron microscopic investigations of rapidly solidified Al-Mn-Ni quasicrystalline alloys
(1991) N.P. Lalla; R.S. Tiwari; O.N. Srivastava
The formation of quasicrystalline phases in rapidly solidified Al-Mn-Ni alloys (Al60Mn11Ni4and Al80Mn13Ni7) has been investigated by X-ray diffraction and TEM techniques. X-ray diffraction studies indicate that the icosahedral phase preferentially nucleates on the side of the ribbon that is in contact with the copper wheel during melt spinning. As indicated by the width (f.w.h.m.) of the XRD peaks, the substitution of Ni in Al-Mn results in the formation of more ordered quasicrystalline phases. Microstructural and electron diffraction (SAD and CBED) investigations show the simultaneous occurrence of icosahedral and decagonal phases. The present investigation reveals the epitaxial-like growth mode of the decagonal phase on the icosahedral phase. For example, it has been found that a pseudo-fivefold axis of the decagonal phase grows parallel to fivefold axis of the icosahedral phase. Other orientational relationships have also been studied. The occurrence of five-times modulation along a tenfold axis in the decagonal phase is also reported. © 1991 Taylor & Francis Ltd.
Transmission electron microscopic studies of rapidly solidified Al-Cu-Gd alloys
(Elsevier BV, 2001) N.P. Lalla; O.N. Srivastava
We report the results of the synthesis and structural explorations of the rapidly solidified Al-Cu-Gd alloys. Interesting structural phases have been observed for Al33.3Cu33.3Gd33.3, Al25Cu25Gd50, Al30Cu30Gd40, Al65Cu20Gd15 and Al42Cu50Gd8. For the Gd-rich compositions Al33.3Cu33.3Gd33.3, Al30Cu30Gd40 and Al25Cu25Gd50 the observed phases have been found to be dominantly amorphous and glassy. The metallic-glass nature of these phases has been confirmed through the occurrence of glass-transition observed in differential thermal analysis measurements. On the other hand, the Al65Cu20Gd15 and Al42Cu50Gd8 alloys exhibit some ordered crystalline phases. The Al65Cu20Gd15 alloy exhibits a tetragonal phase (a = 6.2±0.1 Å and c = 10.1±0.1 Å) and a monoclinic phase (a = 6.499±0.002 Å, b = 5.028±0.002 Å, c = 4.372±0.002 Å and β = 108.43±0.01°). After preliminary analysis of electron diffraction patterns it appears that these phases may correspond to the vacancy ordered phases based on CsCl-type structure. In the case of Al42Cu50Gd8 alloy, we have observed a cubic phase (a = 8.742± 0.005Å) which is a τ3 modulated phase based on the CsCl-type structure. © 2001 Elsevier Science B.V. All rights reserved.