Browsing by Author "S. Tewari"

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Age of crustal melting in Higher Himalayan Crystalline Sequence (Darjeeling, Eastern Himalaya): Constraints from SHRIMP U–Pb geochronology
(John Wiley and Sons Ltd, 2018) S. Tewari; D. Prakash
The paper presents the petrological and geochronological studies of migmatite from an important section of Higher Himalayan Crystalline Sequence of Eastern Himalaya. Migmatites collected from the area characteristically contain biotite-garnet-sillimanite-plagioclase-K-feldspar-quartz as an important mineral assemblage, which has experienced extensive partial melting through dehydration melting reaction involving biotite. In this study, P–T evolution of these migmatites has been constrained through the use of multiequilibrium thermobarometry program winTWQ, conventional thermobarmetry, and pseudosection modelling in the MnNCKFMASHTO model system using Perple_X software. The unification of these three calculations demonstrates that the migmatite experienced peak pressure and temperature at 7.2 ± 0.5 kbar and 775 ± 20 °C, respectively. SHRIMP U–Pb chronological results yield the timing of crustal melting (21 Ma) in the migmatite. Copyright © 2017 John Wiley & Sons, Ltd.
Consensus Statement from India on the Renal Benefits of ARNi, SGLT-2i, and Bisoprolol in Chronic Kidney Disease
(Journal of Association of Physicians of India, 2024) H.K. Chopra; Dinesh Khullar; Tiny Nair; G.S. Wander; C.K. Ponde; Saumitra Ray; Navin C. Nanda; Ravi R. Kasliwal; D.S. Rana; Ashok Kirpalani; J.P.S. Sawhney; Praveen Chandra; Yatin Mehta; Viveka Kumar; S. Tewari; A.K. Pancholia; Vijay Kher; Sandeep Bansal; Sanjay Mittal; Praful Kerkar; P.K. Sahoo; Ramesh Hotchandani; Sunil Prakash; Nagendra Chauhan; Vishal Rastogi; A. Jabir; S. Shanmugasundaram; Mangesh Tiwaskar; Ajay Sinha; Vittul Gupta; S.S. Mishra; S.N. Routray; A.K. Omar; Onkar C. Swami; Aparna Jaswal; Shamsad Alam; Rajeev Passey; Rajeeve Rajput; Justin Paul; Aditya Kapoor; D. Prabhakar; Subhash Chandra; Poonam Malhotra; Vivudh Pratap Singh; Manish Bansal; Priyank Shah; Sanjay Jain; Mohan Bhargava; I.B. Vijayalakshmi; Kiron Varghaese; Dharmender Jain; Anupam Goel; Namrata Gaur; Rohit Tandon; Asha Moorthy; Sheeba George; V.K. Katyal; R.R. Mantri; Rahul Mehrotra; Dilip Bhalla; Vinod Mittal; Sarita Rao; Manish Jagia; Harmeet Singh; Surabhi Awasthi; Ameet Sattur; Rekha Mishra; Anand Pandey; Rajeev Chawla; Shalini Jaggi; Blessy Sehgal; Alok Sehgal; Naresh Goel; Ripen Gupta; Samir Kubba; Abhinav Chhabra; Saurabh Bagga; N.R. Shastry
Chronic kidney disease (CKD) is a major contributor to morbidity and mortality in India. CKD often coexists with heart failure (HF), diabetes, and hypertension. All these comorbidities are risk factors for renal impairment. HF and CKD are pathophysiologically intertwined, and the deterioration of one can worsen the prognosis of the other. There is a need for safe renal pharmacological therapies that target both CKD and HF and are also useful in hypertension and diabetes. Neurohormonal activation achieved through the activation of the sympathetic nervous system (SNS), the renin–angiotensin–aldosterone system (RAAS), and the natriuretic peptide system (NPS) is fundamental in the pathogenesis and progression of CKD and HF. Angiotensin receptor neprilysin inhibitor (ARNi), sodium-glucose cotransporter 2 inhibitors (SGLT-2i), and selective β1-blocker (B1B) bisoprolol suppress this neurohormonal activation. They also have many other cardiorenal benefits across a wide range of CKD patients with or without concomitant HF, diabetes, or hypertension. This consensus statement from India explores the place of ARNi, SGLT-2i, and bisoprolol in the management of CKD patients with or without HF and other comorbidities. ©The Author(s). 2024.
Current Place of SGLT2i in the Management of Heart Failure: An Expert Opinion from India
(Journal of Association of Physicians of India, 2024) H.K. Chopra; Tiny Nair; G.S. Wander; C.K. Ponde; Saumitra Ray; Dinesh Khullar; Navin C. Nanda; Jagat Narula; Ravi R. Kasliwal; D.S. Rana; Ashok Kirpalani; J.P.S. Sawhney; Praveen Chandra; Yatin Mehta; Viveka Kumar; S. Tewari; A.K. Pancholia; Vijay Kher; Sandeep Bansal; Sanjay Mittal; Praful Kerkar; P.K. Sahoo; Ramesh Hotchandani; Sunil Prakash; Nagendra Chauhan; Vishal Rastogi; A. Jabir; S. Shanmugasundaram; Mangesh Tiwaskar; Ajay Sinha; Vittul Gupta; S.S. Mishra; S.N. Routray; A.K. Omar; Onkar C. Swami; Aparna Jaswal; Shamsad Alam; Rajeev Passey; Rajeeve Rajput; Justin Paul; Aditya Kapoor; D. Prabhakar; Subhash Chandra; Poonam Malhotra; Vivudh Pratap Singh; Manish Bansal; Priyank Shah; Sanjay Jain; Mohan Bhargava; I.B. Vijayalakshmi; Kiron Varghaese; Dharmender Jain; Anupam Goel; Kiran Mehmood; Namrata Gaur; Rohit Tandon; Asha Moorthy; Sheeba George; V.K. Katyal; R.R. Mantri; Rahul Mehrotra; Dilip Bhalla; Vinod Mittal; Sarita Rao; Manish Jagia; Harmeet Singh; Surabhi Awasthi; Ameet Sattur; Rekha Mishra; Anand Pandey; Rajeev Chawla; Shalini Jaggi; Blessy Sehgal; Alok Sehgal; Naresh Goel; Ripen Gupta; Samir Kubba; Abhinav Chhabra; Saurabh Bagga; N.R. Shastry
Heart failure (HF) is a global health concern that is prevalent in India as well. HF is reported at a younger age in Indian patients with comorbidity of type 2 diabetes (T2DM) in approximately 50% of patients. Sodium-glucose cotransporter-2 inhibitors (SGLT2i), originally approved for T2DM, are new guideline-recommended and approved treatment strategies for HF. Extensive evidence highlights that SGLT2i exhibits profound cardiovascular (CV) benefits beyond glycemic control. SGLT2i, in conjunction with other guideline-directed medical therapies (GMDT), has additive effects in improving heart function and reducing adverse HF outcomes. The benefits of SGLT2i are across a spectrum of patients, with and without diabetes, suggesting their potential place in broader HF populations irrespective of ejection fraction (EF). This consensus builds on the updated evidence of the efficacy and safety of SGLT2i in HF and recommends its place in therapy with a focus on Indian patients with HF. ©The Author(s). 2024.
Field and textural relationship in pelitic schists and gneisses from the area around Mangpu, Darjeeling district, West Bengal
(2013) D. Prakash; S. Tewari
The metasedimentary rocks of the area around Mangpu constitute a portion of the hinge zone of the northern limb of the major synform of Lower Darjeeling Himalaya. The rocks display evidences of multiple deformation and at least three major phases of deformation have been recognized. The time relations between the phases of deformation (D1, D2, D3) and metamorphic crystallization reveal a single major prograde metamorphic event that initiated with the D1 deformation and finally outlasted it. The earlier phase of this metamorphism is essentially regional syn-tectonic lowgrade (greenschist facies) which may be designated (M1, early). This was followed by regional static metamorphism (M1, late) in the post-tectonic phase between D1 and D2 deformations (upper green schist and amphibolite facies). This M1 metamorphism is superposed by later retrogressive metamorphism (M2) during the D2 and D3 deformations (lower greenschist facies). Within the study area four isograds have been delineated by the first appearance of index minerals in the pelitic schists and gneiss which display Barrovian type of metamorphism. © 2013 Geological Society of India.
Geochronology and oxygen fugacity of the pelitic granulite from the Diwani hills, NE Gujarat (NW India)
(Cambridge University Press, 2023) Manish Kumar; D. Prakash; C.K. Singh; M.K. Yadav; S. Tewari; Pradip K. Singh; B. Mahanta
The Diwani hills are located SE of Balaram-Abu Road in the Banaskantha district of north Gujarat. The crystalline rocks of the Diwani hill area are a diverse assemblage of Precambrian metamorphic and igneous rocks. These rocks are petrologically more complex and date back to the Aravallis or earlier. The mineralogical assemblages such as grt-sp-opx-qz of these rocks indicate their origin in anhydrous or dry conditions, implying metamorphism under pyroxene granulite facies. These granulitic rocks were subjected to Delhi orogenic deformation and were later intruded by the Erinpura granite. Textural and microstructural relationships, mineral chemistry, P-T-X pseudosection modelling and the oxidation state of pelitic granulites from the Diwani hill area of north Gujarat are all part of the current approach. The winTWQ program and pseudosection modelling in the NCKFMASHTO model system utilizing Perple_X software were used to restrict the P-T evolution of these pelitic granulites. The unification of these estimates shows that the pelitic granulites reached their pressure and temperature maxima at 8.6 kbar and 770 °C, respectively. The oxygen fugacity (log fO2) versus temperature computations at 6.2 kbar revealed log fO2-T values of -13.0 and 765 °C, respectively. The electron microprobe dating of monazite grains separated from the granulites of the Diwani hills yields ages ranging from 769 Ma to 855 Ma. The electron microprobe dating presented here from the Diwani hills provides evidence for a Neoproterozoic (Tonian) metamorphic event in the Aravalli-Delhi Mobile Belt. © The Author(s), 2022. Published by Cambridge University Press.
Geochronology and phase equilibria modelling of ultra-high temperature sapphirine + quartz-bearing granulite at Usilampatti, Madurai Block, Southern India
(John Wiley and Sons Ltd, 2018) D. Prakash; R. Yadav; S. Tewari; H.E. Frimmel; N. Koglin; H.K. Sachan; M.K. Yadav
Sapphirine-bearing granulite from Usilampatti in the Madurai block of southern India preserves a variety of mineral textures and reactions that help in reconstructing a three-stage metamorphic evolution. Corroded biotite, sillimanite and quartz inclusions within garnet represent relics from the prograde history. Peak metamorphic conditions were attained with the development of sapphirine + quartz in textural equilibrium (Stage 1). This was followed by nearly isothermal decompression, leading to the formation of sapphirine + cordierite at Stage 2. Subsequent retrograde hydration (Stage 3) is only locally evident. Using the Perple_X software and the model system NCKFMASH, the peak P-T conditions were estimated from core compositions, and the retrograde evolution was deduced from rim or symplectite compositions of different minerals as computed by isopleths of XMg garnet, XCa garnet, XMg orthopyroxene, XMg sapphirine and XMg biotite. The P-T conditions for Stage 1 thus obtained, and supported by thermodynamic modelling using the winTWQ programme, is approximately 9 kbar and 940°C. Stage 2 conditions were constrained as 6.7 kbar and 900°C. Dating of zircon and monazite in the sapphirine-bearing granulite and associated gneisses by the U-Pb method using LA-ICP-MS indicates metamorphic overprint of zircon (lower intercept ages of discordant data arrays) at 546 ± 8 and 547 ± 11 Ma and metamorphic growth of monazite between 542 ± 3 and 551 ± 2 Ma. Upper intercept ages for zircon point to zircon growth at approximately 2514 ± 66 Ma. Although it remains unclear whether the metamorphic age data refer to Stage 1 or Stage 2 or, most likely, a continuum between both, they clearly document a late Ediacaran age for ultra-high temperature (UHT) metamorphism in the area, which, based on the obtained P-T path, was most likely the result of crustal thickening followed by uplift and erosion. Thus, it is concluded that the sapphirine-bearing granulites formed in response to Pan-African orogeny that led to the collision of the western and eastern Madurai domains, whereas initial zircon growth probably took place during late Neoarchaean arc magmatism that formed much of the western domain. Copyright © 2017 John Wiley & Sons, Ltd. Copyright © 2017 John Wiley & Sons, Ltd.
New Occurrence of Sapphirine-spinel-bearing Granulite from NW of Chilka Lake, Eastern Ghats Belt, Odisha
(Geological Society of India, 2019) D. Prakash; B. Vishal; A.S. Naik; R. Yadav; S.K. Rai; S. Tewari; M.K. Yadav; S. Tiwari; S. Dash; C. Pattnaik
A new locality of sapphirine-spinel-bearing granulites from the Kaithapalli area which lies NW of Chilka Lake, Odisha is reported. The area tectonically forms a northern part of Eastern Ghats belt. It occurs as small enclaves within the khondalite and pelitic granulite. The mineral assemblage includes spinel-sapphirine-garnet-cordierite-orthopyroxene-sillimanite-biotite-k-feldspar-plagioclase-quartz. Development of reaction textures and symplectites are common in the sapphirine-spinel granulites which have been used to describe reaction history. The relative XMg values among various minerals are as follows: cordierite > biotite > sapphirine > orthopyroxene > garnet > spinel. The P-T evolution of these sapphirine-spinel granulites constrained through the pseudosection modelling in the NCKFMASH model system using Perple_X software indicate conditions of ultra-high temperature (UHT) metamorphism. The P-T estimates computed by isopleths define a retrograde trajectory with decompression of c.2.5 kbar from P-Tmax of c.10.5 kbar at c.950 o_C. The sequence of reactions as deduced from the symplectite assemblages, together with pseudosection modelling, from the Kaithapalli area, offer greater opportunities for providing a better picture of petrological evolution of northern part of the Eastern Ghats Belt (EGB). © 2019, Geological Society of India.
Petrology and isotopic evolution of granulites from central Madurai Block (southern India): reference to Ediacaran crustal evolution
(Taylor and Francis Inc., 2018) S. Tewari; D. Prakash; M.K. Yadav; R. Yadav
The Madurai Block, constituting part of the southern granulite terrain in southern India, has contributed significantly towards understanding the UHT (ultrahigh-temperature) granulites that serve as a window into the mid-lower continental crust. The dominant rock types are charnockites, sapphirine-bearing granulites, garnet cordierite gneisses, and quartzites. Significant textural relations reveal multiphase reactions responsible for the formation of diverse mineral parageneses during prolonged metamorphic history of the area. Prograde reaction is evident from the textural relationship where biotite/sillimanite relics are seen as inclusion in garnet/orthopyroxene, suggesting dehydration reactions. The symplectitic assemblages that formed during isothermal decompression involve a series of cordierite-forming reactions, followed by retrogression and cooling. Variety of mineral assemblages present in the rocks of this area offer a wide spectrum of P–T sensors that provide details on the physical conditions of metamorphism. For the rigorous interpretation of the P–T path in the Perumalmalai area, quantitative phase diagrams (P–T pseudosections) have been constructed and contoured for the compositional as well as modal isopleths of involved mineral phases. The rocks of Perumalmalai area document a clockwise decompression P–T trajectory, consistent with crustal thickening followed by extensional collapse. SHRIMP U–Pb ages from zircon associated with sapphirine-bearing granulite facies rocks of Perumalmalai area suggest a widespread Ediacaran tectonothermal event. The occurrence of Ediacaran UHT metamorphism followed by isothermal decompression in the Madurai Block is consistent with the timing and physical conditions associated with the formation of East African Orogen during the amalgamation of Gondwana. © 2017, © 2017 Informa UK Limited, trading as Taylor & Francis Group.
Petrology, pseudosection modelling and U-Pb geochronology of silica-deficient Mg-Al granulites from the Jagtiyal section of Karimnagar granulite terrane, northeastern Dharwar Craton, India
(Elsevier B.V., 2017) D. Prakash; P. Chandra Singh; S. Tewari; M. Joshi; H.E. Frimmel; T. Hokada; T. Rakotonandrasana
The Jagtiyal section exposes a part of the Precambrian granulite – facies terrane in the northeastern Dharwar Craton, specifically charnockite, orthogneiss, mafic granulite, garnet-cordierite gneiss and silica-deficient Mg-Al granulite. The silica-deficient Mg-Al granulites exhibit well-preserved metamorphic reaction microstructures and were used to elucidate the metamorphic history of the study area. Textural evidence exists for the prograde break-down of biotite and corundum and formation of a peak metamorphic assemblage of spinel, sapphirine, orthopyroxene, cordierite and K-feldspar. KFMASHTO system based P-T pseudosections as well as the application of multi-equilibrium calculation for mineral core compositions, gave P-T conditions of 7.5–8 kbar and 800–840 °C. The thermal peak was followed by near isothermal decompression that led to a variety of coronal textures including, successive rims on corundum, consisting of spinel + sapphirine + cordierite ± orthopyroxene, and symplectitic intergrowth of sapphirine-cordierite, orthopyroxene-cordierite, spinel-cordierite, and biotite-cordierite. Modelled P-T conditions for this stage are 4.3–4.5 kbar and 750–800 °C. Few samples show textural evidence for further retrogression leading to the break-down of sapphirine and cordierite to form chlorite, corundum, spinel and andalusite. The proposed clockwise P-T path implies that granulites from the Jagtiyal section likely resulted from thickened continental crust undergoing decompression. New U-Pb zircon geochronologic data provide evidence for a Neoarchaean (2604 ± 25 Ma) age that likely corresponds to the timing of accretion of the terrane that was subjected to high temperature metamorphism at that time. Moreover, there is evidence of a Neoproterozoic thermal overprint in the area around 638 Ma. © 2017 Elsevier B.V.
Prograde Barrovian metamorphism along Darjeeling–Tista transect, Eastern Himalaya, India: constraints from textural relationship, phase equilibria and geothermobarometry
(John Wiley and Sons Ltd, 2018) D. Prakash; S. Tewari; P. Chandra Singh
The Darjeeling–Tista transect, located in Lesser Himalaya, is a part of the North-Eastern Himalaya. Along the transect, polyphase deformation and prograde Barrovian metamorphism have been delineated. The time relations between the phases of deformations (D1, D2 and D3) and metamorphic crystallization reveal a single major prograde metamorphic event that initiated with the D1 deformation and finally outlasted it. The earlier phase of this metamorphism is essentially regional syntectonic low grade, which may be designated (M1a). This was followed by regional static metamorphism (M1b) in the post-tectonic phase between the D1 and D2 deformations. This M1 metamorphism is superposed by later retrogressive metamorphism (M2) during the D2 and D3 deformations. The different parageneses of pelitic rocks containing chlorite, muscovite, biotite, garnet, staurolite, kyanite, sillimanite, K-feldspar and plagioclase show various textures that resulted from the continuous and discontinuous reactions in the different zones. The metamorphic zones and isograds delineated on the basis of specific metamorphic reactions, namely reaction isograd or isoreaction grad, and by critical mineral assemblages are as follows: (a) chlorite–biotite zone, (b) garnet–chlorite zone, (c) staurolite–biotite–chlorite zone, (d) kyanite–biotite–staurolite zone and (e) sillimanite–biotite–staurolite zone. Chemographic relations and inferred reactions for different zones are portrayed in the AKF and AFM projections. A sequence of metamorphic reactions at different isograds has been deduced through textural relations. The prograde P-T evolution of the study area has been constrained through the use of internally consistent winTWQ programme and Perple_X software in the MnNCKFMASHTO model system. The study has the potential to instigate future researches focusing on Himalayan metamorphic evolutionary trends using modern approaches. Copyright © 2017 John Wiley & Sons, Ltd. Copyright © 2017 John Wiley & Sons, Ltd.
Prograde polyphase regional metamorphism of pelitic rocks, NW of Jamshedpur, eastern India: Constraints from textural relationship, pseudosection modelling and geothermobarometry
(Cambridge University Press, 2020) D. Prakash; D.K. Patel; M.K. Yadav; B. Vishal; S. Tewari; R. Yadav; S.K. Rai; C.K. Singh
The study area belongs to the Singhbhum metamorphic belt of Jharkhand, situated in the eastern part of India. The spatial distribution of the index minerals in the pelitic schists of the area shows Barrovian type of metamorphism. Three isograds, viz. garnet, staurolite and sillimanite, have been delineated and the textural study of the schists has revealed a time relation between crystallization and deformation. Series of folds with shifting values of plunges in the supracrustal rocks having axial-planar schistosity to the folds have been widely cited. Development of these folds could be attributed to the second phase of deformation. In total, two phases of deformation, D1 and D2, in association with two phases of metamorphism, M1 and M2, have been lined up in the study area. Chemographic plots of reactant and product assemblages corresponding to various metamorphic reactions suggest that the pattern of metamorphic zones mapped in space is in coherence with the temporal-sequential change during prograde metamorphism. The prograde P-T evolution of the study area has been obtained using conventional geothermobarometry, internally consistent winTWQ program and Perple_X software in the MnNCKFMASHTO model system. Our observations suggest that the progressive metamorphism in the area is not related to granitic intrusion or migmatization but that it was possibly the ascending plume that resulted in the M1 phase of metamorphism followed by D1 deformation. The second and prime metamorphic phase, M2, with its possible heat source generated by crustal overloading, was preceded by D1 and it lasted until late-to post-D2 deformation. © 2019 Cambridge University Press.
Role of Bisoprolol in Heart Failure Management: A Consensus Statement from India
(Journal of Association of Physicians of India, 2023) H.K. Chopra; Tiny Nair; G.S. Wander; C.K. Ponde; Saumitra Ray; Dinesh Khullar; Navin C. Nanda; Ravi R. Kasliwal; D.S. Rana; Ashok Kirpalani; J.P.S. Sawhney; Praveen Chandra; Yatin Mehta; Viveka Kumar; S. Tewari; A.K. Pancholia; Vijay Kher; Sandeep Bansal; Sanjay Mittal; Praful Kerkar; P.K. Sahoo; Ramesh Hotchandani; Sunil Prakash; Nagendra Chauhan; Vishal Rastogi; A. Jabir; S. Shanmugasundaram; Mangesh Tiwaskar; Ajay Sinha; Vittul Gupta; S.S. Mishra; S.N. Routray; A.K. Omar; Onkar C. Swami; Aparna Jaswal; Shamsad Alam; Rajeev Passey; Rajeeve Rajput; Justin Paul; Aditya Kapoor; D. Prabhakar; Subhash Chandra; Poonam Malhotra; Vivudh Pratap Singh; Manish Bansal; Priyank Shah; Sanjay Jain; Mohan Bhargava; I.B. Vijayalakshmi; Kiron Varghaese; Dharmender Jain; Anupam Goel; Namrata Gaur; Rohit Tandon; Asha Moorthy; Sheeba George; V.K. Katyal; R.R. Mantri; Rahul Mehrotra; Dilip Bhalla; Vinod Mittal; Sarita Rao; Manish Jagia; Harmeet Singh; Surabhi Awasthi; Ameet Sattur; Rekha Mishra; Anand Pandey; Rajeev Chawla; Shalini Jaggi; Blessy Sehgal; Alok Sehgal; Naresh Goel; Ripen Gupta; Samir Kubba; Abhinav Chhabra; Saurabh Bagga; N.R. Shastry
In India, heart failure (HF) is an important health concern affecting younger age groups than the western population. A limited number of Indian patients receive guideline-directed medical therapy (GDMT). Selective β-1 blockers (BB) are one of the GDMTs in HF and play an important role by decreasing the sympathetic overdrive. The BB reduces heart rate (HR) reverse the adverse cardiac (both ventricular and atrial), vascular, and renovascular remodeling seen in HF. Bisoprolol, a β-1 blocker, has several advantages and can be used across a wide spectrum of HF presentations and in patients with HF and comorbid conditions such as coronary artery disease (CAD), atrial fibrillation (AF), post-myocardial infarction (MI), uncontrolled diabetes, uncontrolled hypertension, and renal impairment. Despite its advantages, bisoprolol is not optimally utilized for managing HF in India. This consensus builds on updated evidence on the efficacy and safety of bisoprolol in HF and recommends its place in therapy with a focus on Indian patients with HF. © The Author(s).
Studies on ac response of zinc oxide pellets
(2008) P.P. Sahay; S. Tewari; R.K. Nath; S. Jha; M. Shamsuddin
The ac responses of the ZnO pellets have been studied by ac measurements (impedance, capacitance and phase angle) over the temperature range 300-435 K. The ac conductivity of the ZnO pellets is observed to be proportional to ω s , where ω is the angular frequency and the exponent s is a temperature- and frequency-dependent parameter. Based on the existing theories of ac conduction, it has been concluded that for low frequency region (20 Hz-2 kHz), the dominant conduction mechanism in the ZnO pellets is multihopping at all temperatures, whereas for high frequency region (500 kHz-2 MHz), the small polaron tunneling model is the dominant mechanism in the pellets. Activation energies for ac conduction processes are estimated to be in the range of 0.028-0.277 eV which are found to vary with the frequency of the ac signal. These results are found to be consistent with the hopping model. The ac capacitance and the dielectric loss tangent are found to be dependent on both frequency and temperature. Such dependences have been explained taking into account the equivalent circuit model comprising a frequency-independent capacitive element in parallel with a temperature-dependent resistive element, both in series with a low value resistance. Impedance spectroscopy studies show single semicircular arcs in the complex impedance spectra at all temperatures in the range 300-435 K, with their centres lying below the real axis at a particular angle of depression indicating a multirelaxation behaviour in the pellets. © 2008 Springer Science+Business Media, LLC.