SOLID STATE PHYSICS Proceedings of the 57th DAE Solid State Physics Symposium 2012
Indian Institute of Technology, Bombay, Mumbai, India
3 – 7 December 2012
Solid State Physics (India) Vol. 57 (2012)
EDITORS A. K. Chauhan Chitra Murli S. C. Gadkari
AIP CONFERENCE PROCEEDINGS
1512
High-pressure Electrical Resistivity Studies on FeSe2 and FeTe2 G.Parthasarathy1 , D.K. Sharma2 , Y.K. Sharma2 and Usha Chandra2 1
CSIR-National Geophysical Research Institute, Hyderabad- 500007, India (
[email protected]) 2 Department of Physics, University of Rajasthan , Jaipur- 302004, India
Abstract. We report here for the first time the pressure dependence of the electrical resistivity of ferroselite (FeSe2) and frohbergite FeTe2 up to 8 GPa. The synthetic ferroselite shows a pressure induced marcasite-to- NiAs type structural phase transition at 6.8 GPa and frohbergite shows the transition at 5.7 GPa. The transition was observed with a discontinuous resistivity decrease by 0.9 times. We also present here the XRD results on the FeSe2 and FeTe2. The relevance of the phase transition to Martian mineral chemistry is discussed. Keywords: High-pressure ; marcasite; mineral physics, phase transitions, chalcogenides. PACS: 64.70kg;64.70K; 91.60Hg;91.60Gf;72.80.
resistive furnace. Initially the temperature was slowly raised from 300K to 1273K in the time interval of about 30 hours to ensure solid state diffusion of Se and Te. The temperature was fixed at 1273K for 30 hours which ensured the complete melting and then the ampoules were quenched into ice water. The furnace temperature was controlled by a temperature controller (! within ±10K). The black ingot , thus, obtained was crushed into fine powder and put again into evacuated quartz ampoules and were annealed at 873K for two weeks and quenched into water. The annealed ingots were crushed into fine powder using agate mortar and pestal in methanol and were dried at 350K. X-ray powder diffraction patterns were recorded at 300K for these specimens using a Phillips 1840 model diffractometer (operated at 30 kV and 30 mA) by varying the 2" in the step of 0.02o from 10o to 70o. XRD data were analyzed using a standard program. High-pressure electrical resistivity measurements were carried out in an opposed anvil cell with tungsten carbide anvils, with steatite as pressure transmitting medium. The calibration and methodology of the high-pressure four probe electrical resistivity measurements were discussed elsewhere [3-5].
INTRODUCTION The mineral ferroselite (FeSe2) is known to form in the vicinity of oxidizing sulfide and uranium deposits. Under reducing conditions, elemental Se either is incorporated within pyrite or forms the ferroselite (FeSe2).The minerals ferroselite and frohbergite FeTe2 are found as accessory minerals in uranium deposits [1]. Studies on the highpressure phase stability of Iron chalcogenides at mantle pressures are very relevant to understand the role of chalcogens in mineral chemistry of the Martian Core and subsurface geological processes [2]. In this work we report for the first time the high-pressure electrical resistivity measurements on the synthetic ferroselite FeSe2 and frohbergite FeTe2. Both ferroselite and frohbergite have the marcasite type structures with orthorhombic unit cell with Pnnm space group. To the best of our knowledge there are no previous report on the highpressure electrical resistivity studies on ferroselite FeSe2 and frohbergite FeTe2. .
EXPERIMENTAL METHODS High purity ( 99.999%) elements were weighed in stoichiometric proportion corresponding to FeSe2 and FeTe2 compositions of the alloys and were sealed under vacuum ( 10 -2 Torr) in quartz ampoule. The ampoules were kept in Kanthal wire wound
Results and Discussions Powder X-ray diffraction patterns show all the
Solid State Physics (India) Vol. 57 (2012) AIP Conf. Proc. 1512, 40-41 (2013); doi: 10.1063/1.4790900 © 2013 American Institute of Physics 978-0-7354-113-3/$30.00
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Earlier high-pressure studies on pyrite FeS2 predicted the pressure-induced metallization at about 14-30 GPa[12]. Mechanical alloying by ball milling induced a phase transition of FeSe2 from marcasite-to-NiAs type, for the milling time of 52 hours [13]. However, further EXAFS studies do not reproduce the phase transition in the FeSe2-FeSe system up to 19 GPa, but the pressure induced disorder-order transition as a function of milling time [14]. The Pauling electronegativity of oxygen is 3.44, where as that of S is 2.58, Se is 2.55, and Te is 2.1. Hence the phase transition pressure should decrease from Selenide to telluride, as observed in the present study (Fig.1). This work has implications to the core of the Mars, as it has been inferred that the core of the Mars is very rich in Iron-Nickel-chalcogenides. Further work on high-pressure Mössbauer spectroscopy and XRD to understand the nature of the phase transition in iron dichalcogenides are under progress.
major reflections corresponding to the crystal structure with orthorhombic marcasite phase , with unit cell parameters were determined to be a= 0.479(8)nm; b=0.578(5)nm; c=0.358(5) nm for FeSe2 and a= 0.526(5)nm; b=0.626(6)nm; c= 0.386(8)nm for FeTe2 . The conductivity activation energy for electrical conduction for FeSe2 is found to vary from 0.50 to 0.45 eV in the temperature range 300 to 400 K for FeSe2 and 0.35 eV for FeTe2 in the temperature range 300 to 400K. Figure-1 shows the pressure dependence of the normalized electrical resistivity of FeSe2 and FeTe2 up to 8 GPa at room temperature. It is well known that pure Te shows the pressure induced insulator-metal transition at 4.0 GPa [6,7]. The high-pressure phase was determined to be monoclinic and pure iron shows the phase transition from bcc to hcp phase at 13GPa with the sharp electrical resistivity increase at the transition pressure [8]. In case of Te it is both structural and electronic phase transition at 4GPa [7]. Our earlier study on CrSb2 demonstrated a pressure induced marcasite to pyrite transition at 6.8GPa. Since FeSe2 and FeTe2 have the marcasite structure at room temperature, the observed phase transition is interpreted as pressure induced structural phase transition from Marcasite [9] to thermodynamically stable phase NiAs type phase for dichalcogenides. In order to establish the insulator to metal transition [10], data on temperature dependence of the electrical resistivity at different pressures are needed and are under progress.
ACKNOWLEDGMENTS The authors thank CSIR, ISRO, PLANEX for funding the research. We are thankful to Professor Mrinal Sen , Director, CSIR-NGRI for his very kind encouragements and support. GP thanks Professor Charles Prewitt , Geophysical Laboratory , CIW, USA for suggesting the marcasite problem.
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FIGURE 1. Pressure dependence of the normalized electrical resistivity . R0 is the value of the room temperature resistivity. Kumar et al. have studied high-pressure behaviour of FeSe sample and found pressure induced enhancement in superconductivity due to a pressure-induced distortion of the low-temperature Cmma phase at around 1.6GPa and the appearance of a high-pressure Pbnm phase at 9 GPa [11]. However, there are no previous reports on the highpressure behaviour of FeSe2.
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