Non-adiabatic small polaron hopping transport above metal-like to insulator transition in the vacant 3d -orbital Tb 2 Ti
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Non-adiabatic small polaron hopping transport above metal-like to insulator transition in the vacant 3dorbital Tb2Ti2O7 pyrochlore oxide B. Santhosh Kumar1, Y. Naveen Kumar1, V. Kamalarasan1, and C. Venkateswaran1,*
1
Department of Nuclear Physics, University of Madras, Guindy Campus, Chennai 600 025, India
Received: 29 June 2020
ABSTRACT
Accepted: 21 October 2020
We report the validity of Mott variable range hopping (VRH) conduction mechanism and small polaron hopping in Tb2Ti2O7 in the temperature range 603–803 K. The temperature-dependent resistivity data are in good agreement with Mott 3d VRH. The other parameters estimated such as hopping range, hopping energy and density of states near Fermi level are in good agreement with those of many semiconductor oxides. The Holstein’s condition for nonadiabatic conduction mechanism is also satisfied. The ac conductivity is governed by Jonscher’s power law as r0a:c ðt; T Þ ¼ rd:c ðTÞ þ aðTÞtn and the exponent n values in the range of 1.02–1.23 suggest that conduction takes place due to the small polaron hopping mechanism. The possible polaron formation and their mechanism are discussed systematically.
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1 Introduction In the last five decades, a great deal of attention has been drawn by the study of geometrically frustrated magnetic systems from both theoretical and experimental standpoint of view due to their fascinating structural and physical properties [1–8]. The term geometrical frustration refers to the ordered spin of ions in the lattice which can interact magnetically, but their individual interactions cannot reach the equilibrium (least energy). A kagome´ lattice/staircase, corner-shared triangular lattice (2D) with the antiferromagnetic nearest neighbour is one of the best examples for the geometrically frustrated system. The
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https://doi.org/10.1007/s10854-020-04732-6
3D example of such corner-shared tetrahedral lattice is found in the pyrochlore lattice [1,9]. Pyrochlores are the compound with the general formula of A2B2O7, where A is a trivalent rare-earth ion and B is a tetravalent transition metal ion. Depending on the radius of A and B cation (rA and rB) the compound formation will vary. If rA/rB \ 1.48, the compound crystallises into fluorite, if rA/rB is between 1.48 and 1.78, pyrochlore lattice is obtained and if this ratio is greater than 1.78 then a perovskite is obtained [10–11]. The oxides of A2B2O7 (A = Tb, Y, Dy, Ho, Gd, Er, Pb and B = Ti, Mo, Tl, Mn, Sn, Ir, Ru, Os) crystallise into the cubic (face-centred) structure with space group Fd-3m. The A and B cations have
J Mater Sci: Mater Electron
eightfold and sixfold coordination with oxygen anions. The six oxygen anions are equidistant with B cation, whereas the A cation is surrounded by eight oxygen with six oxygen placed equidistant and the other two slightly at a shorter distance due to spatial arrangement [1]. Due to their structural arrangement of ions and ele
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