Browsing by Author "D. Pandey"

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A faulted matrix model for the spiral growth of polytype structures
(1975) D. Pandey; P. Krishna
A faulted matrix model for the spiral growth of polytype structures is proposed and takes into account the presence of stacking faults in the basic structure. The most probable structures theoretically deduced from such a model are shown to be in excellent agreement with those actually observed in PbI2. © 1975.
Application of 1d kinetic Ising model for studying the evolution of diffuse scattering during HCP (2H) to FCC (3C) martensitic transition
(1996) S.P. Shrestha; D. Pandey
The evolution of diffuse-scattering patterns and dynamical scaling behaviour of pair correlation functions for crystals undergoing HCP (2H) to FCC (3C) martensitic transition are studied using 1d Ising model with non-conserved dynamics by Monte Carlo technique.
Characterization of ferroelectric and superconducting ceramics prepared from precursor carbonates
(Springer India, 1989) D. Pandey; V.S. Tiwari; A.K. Singh; Sangeeta Chaudhry
The advantages of a new semi-wet method for the synthesis of (Ba, Ca)TiO3 and YBa2Cu3O7 - y powders from precursor carbonates are described. The precursor carbonates are prepared using chemical coprecipitation technique under controlled pH conditions to ensure uniform distribution of Ba+2/Ca+2 and Y+3/Ba+2 ions. It is shown that the powders synthesized by this route possess better chemical homogeneity as compared to those obtained by the conventional ceramic technique involving calcination of a mixture of BaCO3, CaCO3, TiO2 and Y2O3, BaCO3, CuO powders. The behaviour of the ferroelectric (Ba, Ca)TiO3 and the superconducting YBa2Cu3O7 - y ceramics prepared from powders obtained by our method are found to be markedly different from those known for the conventionally prepared ceramics. © 1989 Indian Academy of Sciences.
Diffuse ferroelectric transition and relaxational dipolar freezing in (Ba,Sr)TiO3
(1995) V.S. Tiwari; N. Singh; D. Pandey
The effect of Sr2+ substitution on the structure and dielectric behaviour of BaTiO3 is reported. It is shown that the smearing of the variation of dielectric constant with temperature in such samples is not due to compositional inhomogeneities. Observation of relaxational dipolar freezing below the hump temperature indicates the similarity of diffuse ferroelectric transition and orientational glass behaviour.
Effect of particle size on the ferroelectric behaviour of tetragonal and rhombohedral Pb(ZrxTi1-x)O3 ceramics and powders
(1995) S.K. Mishra; D. Pandey
The effect of particle size on the structure and ferroelectric behaviour of PZT powders and ceramics with bulk tetragonal and rhombohedral compositions is reported. To our knowledge, this is the first report of particle size effects for the rhombohedral PZT composition. It is pointed out that, below a certain critical size, the formation of ferroelectric domains is not energetically favourable. As a result, the ferroelectric transition may be suppressed due to depolarization and/or elastic energy contributions.
Fluctuations of Mediterranean Outflow Water circulation in the Gulf of Cadiz during MIS 5 to 7: Evidence from benthic foraminiferal assemblage and stable isotope records
(Elsevier, 2015) A.D. Singh; A.K. Rai; M. Tiwari; P.D. Naidu; K. Verma; M. Chaturvedi; A. Niyogi; D. Pandey
We studied variations in benthic foraminiferal assemblages and δ13C for the last 225kyr at IODP site U1387 which is currently bathed by upper core of the Mediterranean Outflow Water (MOW). The MOW paleocirculation and sea-floor environment (oxygen level, trophic condition, bottom current strength) have been inferred from faunal composition; species diversity, abundances of selected index species/groups, microhabitat preferences combined with δ13C record of the epifaunal Cibicidoides pachyderma. The faunal and isotope records indicate relatively better ventilation at sea-floor and low trophic condition during MIS 1, 5 and 7 possibly due to increased influence of upper MOW in the Gulf of Cadiz. Our multi-proxy record reflects significant and rapid changes during cold (stadial) and warm (interstadial) phases within the interglacials MIS 5 and 7 and at Termination II. The faunal and isotope records reveal strong MOW flow and better ventilated, oligotrophic bottom-water conditions during stadials MIS 5b, 5d, 7b and 7d. The study further demonstrates weakened MOW intensity associated with poor ventilation and increased trophic level at sea-floor during interstadials MIS 5a, 5e, 7a and 7c. MOW flow was relatively sluggish at Termination II, followed by its strengthening at the end of MIS 5e. The chronology of these events suggests that periods of weakened MOW correlate with sapropel layers of the Mediterranean Sea, implying strong coupling between glacial-interglacial climate and MOW circulation in the Gulf of Cadiz. © 2015 Elsevier B.V.
Magnetoresistance behavior of ferromagnetic shape memory alloy Ni1.75 Mn1.25 Ga
(2008) S. Banik; R. Rawat; P.K. Mukhopadhyay; B.L. Ahuja; Aparna Chakrabarti; P.L. Paulose; Sanjay Singh; Akhilesh Kumar Singh; D. Pandey; S.R. Barman
A negative-positive-negative switching behavior of magnetoresistance (MR) with temperature is observed in a ferromagnetic shape memory alloy Ni1.75 Mn1.25 Ga. In the austenitic phase between 300 and 120 K, MR is negative due to s-d scattering. Curiously, below 120 K MR is positive, while at still lower temperatures in the martensitic phase, MR is negative again. The positive MR cannot be explained by Lorentz contribution and is related to a magnetic transition. Evidence for this is obtained from ab initio density-functional theory, a decrease in magnetization and resistivity upturn at 120 K. Theory shows that a ferrimagnetic state with antiferromagnetic alignment between the local magnetic moments of the Mn atoms is the energetically favored ground state. In the martensitic phase, there are two competing factors that govern the MR behavior: a dominant negative trend up to the saturation field due to the decrease in electron scattering at twin and domain boundaries and a weaker positive trend due to the ferrimagnetic nature of the magnetic state. MR exhibits a hysteresis between heating and cooling that is related to the first-order nature of the martensitic phase transition. © 2008 The American Physical Society.
Measurement of intensity of directionally diffuse streaks on a four-circle diffractometer: divergence correction factors for bisecting setting
(Wiley-Blackwell, 1987) D. Pandey; L. Prasad; S. Lele; J.P. Gauthier
Because of the inherent beam divergence, point intensities measured using a four-circle diffractometer at equispaced points along the diffuse streak correspond to variable lengths of the streak. Correction factors required to convert the measured intensities into those corresponding to a fixed length at each equispaced point along the diffuse streak are derived for two different crystal mountings under bisecting geometry. A procedure for experimentally verifying the mathematical approach employed in these calculations is also described. © 1987, Wiley-Blackwell. All rights reserved.
Nomenclature of magnetic, incommensurate, composition-changed morphotropic, polytype, transient-structural and quasicrystalline phases undergoing phase transitions. II. Report of an IUCr Working Group on Phase Transition Nomenclature
(International Union of Crystallography, 2001) J.-C. Tolédano; R.S. Berry; P.J. Brown; A.M. Glazer; R. Metselaar; D. Pandey; J.M. Perez-Mato; R.S. Roth; S.C. Abrahams
A general nomenclature applicable to the phases that form in any sequence of transitions in the solid state has been recommended by an IUCr Working Group [Acta Cryst. (1998). A54, 1028-1033]. The six-field notation of the first Report, hereafter I, was applied to the case of structural phase transitions, i.e. to transformations resulting from temperature and/or pressure changes between two crystalline (strictly periodic) phases involving modifications to the atomic arrangement. Extensive examples that illustrate the recommendations were provided. This second Report considers, within the framework of a similar six-field notation, the more complex nomenclature of transitions involving magnetic phases, incommensurate phases and transitions that occur as a function of composition change. Extension of the nomenclature to the case of phases with less clearly established relevance to standard schemes of transition in equilibrium systems, namely polytype phases, radiation-induced and other transient phases, quasicrystalline phases and their transitions is recommended more tentatively. A uniform notation for the translational periodicity, propagation vector or wavevector for magnetic and/or incommensurate substances is specified. The notation adopted for incommensurate phases, relying partly on the existence of an average structure, is also consistent with that for commensurate phases in a sequence. The sixth field of the nomenclature is used to emphasize the special features of polytypes and transient phases. As in I, illustrative examples are provided for each category of phase sequence. © 2001 International Union of Crystallography Printed in Great Britain - all rights reserved.
On the characterization of basal plane stacking faults in N9R and N18R martensites
(1988) V.K. Kabra; D. Pandey; S. Lele
Theory of diffraction from 9R martensites containing sequence and deformation type faults is developed. It is shown that a distinction can be made between the sequence and deformation faults by analysing diffuse streaks on the diffraction patterns obtained from such crystals. Further, a distinction amongst the three sequence faults themselves on the one hand and the two deformation faults on the other hand is also shown to be possible. Such distinctions are not possible for the N9R or N18R structures using the g · R criterion of the conventional transmission electron microscopy. From a comparison of the theoretically predicted diffraction effects with those experimentally observed by earlier workers, it is concluded that the faults present in the N9R martensites are of predominantly sequence type. The theory developed for N9R is also applicable to N18R structure for the reciprocal lattice rows common with the fundamental structure. © 1988.
On the spiral growth of polytype structures in SiC from a faulted matrix I. Polytypes based on the 6H structure
(1975) D. Pandey; P. Krishna
The different intrinsic and extrinsic fault configurations that can occur in the close-packed 6H structure, ABCACB, are considered and their stacking fault energies calculated to determine the relative probability of their occurrence. Two-fault configurations are found to be most probable. Polytype structures that can result by spiral growth around single screw dislocations of different Burgers vector created in a 6H matrix containing the most probable fault configurations have been deduced. It is shown that polytype structures can result from screw dislocations of integral as well as non-integral Burgers vectors and a much larger number of structure series are actually possible than those deduced earlier from a perfect 6H matrix. A screw dislocation step of a given number of layers can give rise to several different polytype structures depending on the position of the fault in the exposed ledge. Of these the most probable structures have been determined by calculating their stacking fault energies. The most probable series of structures predicted theoretically from such a faulted matrix model for the growth of polytypes are compared with the structures observed so far. There is an excellent agreement between the theoretically predicted and experimentally observed structures. The model explains the observed limitations on the Zhdanov number of the SiC polytypes and can be used in structure determination for predicting the most probable structures for a new polytype. © 1975.
On the spiral growth of polytype structures in SiC from a faulted matrix. III. Polytypes based on the 15R and 4H structures
(1976) D. Pandey; P. Krishna
The faulted-matrix model of polytypism is applied to the deduction of the most probable series of structures that can result from faulted 15R and 4H matrices in SiC. The different possible intrinsic and extrinsic fault configurations which can occur in the 15R and 4H SiC structures are considered and their relative probability of occurrence determined by calculating the stacking fault energy. It is shown that the most probable fault configuration in 15R depends on whether the 15R structure is stable or metastable in the temperature range in which it is formed. The most probable fault configuration so obtained is considered to lie at different distances from the surface of the basic matrix at the time of the origin of a screw dislocation ledge. Polytype structures that can result from the spiral growth of such faulted ledges of screw dislocations with different Burgers vectors have been deduced. The most probable series of structures are predicted from a calculation of stacking fault energy. The theoretically predicted structures are compared with those experimentally observed. © 1976.
On the study of the f.c.c.-h.c.p. martensitic transformation using a diffraction approach-I. F.c.c. → h.c.p. transformation
(1986) D. Pandey; S. Lele
A new approach based on diffraction evidence for the involvement of shear at the atomic level is proposed to elucidate the mechanism of the f.c.c. to h.c.p. martensitic transformation. This approach is based on the analysis of the intensity distribution along streaked reciprocal lattice rows observed on diffraction patterns recorded from partially transformed f.c.c. crystals. The relevant theory of diffraction from crystals undergoing f.c.c.-h.c.p. transformation by non-random insertion of deformation faults is developed. This approach, as distinct from the conventional microstructural approach, can be especially useful in situations where the transformation strains are self accommodating in nature, such that there is no observable surface relief effect. © 1986.
On the study of the f.c.c.-h.c.p. martensitic transformation using a diffraction approach-II. H.c.p. → f.c.c. transformation
(1986) D. Pandey; S. Lele
Theory of diffraction from h.c.p. crystals undergoing martensitic transformation to the f.c.c. structure through a non-random insertion of deformation faults is developed. From a comparison of the theoretically predicted diffraction effects with those experimentally observed on single crystals of ZnS, it is shown that the thermally induced transformations in this material are growth controlled while the deformation induced transformation is nucleation controlled. © 1986.
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(1976) D. Pandey; P. Krishna
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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.
Structure determination of one-dimensionally disordered polytypes.
(1986) D. Pandey; V.K. Kabra; S. Lele
Crystals of polytypic materials often contain a high concentration of stacking faults giving rise to characteristic diffuse streaks along certain reciprocal lattice rows. Structure determination of such one-dimensionally disordered crystals involves the following steps: 1) geometrical specification of stacking faults which requires postulation of physical processes like accidents during crystal growth, plastic deformation, structural transformations responsible for their formation; 2) a model for random or non-random distribution of faults to be developed, depending on the physical process of formation, to calculate the continuous distribution of diffracted intensity along the streaked reciprocal lattice rows; 3) the correctness of the proposed model has to be verified by matching the theoretically calculated intensity distributions with those experimentally observed for single crystals. Instructions for the structure determination of one-dimensionally disordered polytypes are discussed with suitable examples. -J.M.H.
Studies on anticonvulsant agents. Achievements and prospects
(2011) S.H. Pandey; S.H. Shukla; D. Pandey; R.S. Srivastava
The data published over the past 15 years on the search for newer anticonvulsant drugs are generalized. Pyrrolidinedione, quinazolinone, xanthone, hydrazine and thiadiazole derivatives manifesting anticonvulsant activity in model in vivo tests in rodents are considered. The bibliography includes 69 references. © 2010 Russian Academy of Sciences and Turpion Ltd.
Suspension of the tongue to the digastric tendon following resection of the anterior mandibular arch for oral cancer prevents postoperative tongue fall and avoids the need for tracheostomy
(2012) D. Pandey
Background: Resection of the anterior arch of the mandible leads to tongue fall and postoperative stridor because of the detachment of tongue musculature from the mandible. In this article, a simple method of tongue suspension is described which would prevent such complications and the need for tracheostomy. Settings and Design: This study was carried out on patients with oral cancer requiring resection of the anterior arch of the mandible as a part of the surgical resection at a tertiary centre. Materials and Methods: This study was performed on 32 patients with oral cancer requiring resection of the anterior arch of the mandible as a part of the surgical resection. Following an appropriate resection of the oral cancer (including the anterior mandibular arch) and neck dissection, a silk suture is used to loop the tongue musculature on either side to the intermediate tendon of the digastric muscle. Result: This technique of tongue suspension was used in 32 patients who required resection of the anterior mandibular arch for oral cancer. Prophylactic tracheostomy was not performed. One patient developed stridor at extubation and required temporary tracheostomy. All other patients could be successfully extubated within 12 h of surgery and none experienced postoperative stridor or sleep apnea. One other patient required temporary tracheostomy for pulmonary toilet as he developed aspiration pneumonitis related to nasogastric feed. Conclusion: This simple method of tongue suspension to the digastric tendon prevents postoperative tongue fall and obviates the need for tracheostomy in most instances.
The influence of a powder processing technique on chemical homogeneity and the diffuse phase transition behaviour of Ba0 9Ca0 1TiO3 ceramics
(1989) V.S. Tiwari; D. Pandey; P. Groves
Ba0.9Ca0.1TiO3 ceramics have been fabricated using powders obtained by calcining mixtures of (i) BaCO3, CaCO3 and TiO2 and (ii) Ba0.9Ca0.1CO3 and TiO2. The precursor Ba0.9Ca0.1CO3 was prepared by a wet technique to ensure homogeneous mixing of Ba and Ca ions at the atomic level. DTA analysis of solid state thermochemical reactions in the two types of mixtures shows that the temperature of reaction completion is 140 degrees C lower for the mixture containing the precursor carbonate solid solution. XRD results indicate better chemical homogeneity for powders obtained by the new route. A powder diffraction pattern for Ba0.9Ca0.1TiO3 has been theoretically computed assuming partial infstitution of Ba2+ by Ca2+. The theoretically predicted intensities are in good agreement with those experimentally observed for powders prepared by the precursor route only. Better chemical homogeneity with respect to the distribution of Ca2+ ions leads to more pronounced diffuse phase transition in Ba0.9Ca0.1TiO3 ceramics. © 1989 IOP Publishing Ltd.