High-pressure studies on lithium fast-ion conductors
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G. Parthasarathy and E. S. R. Gopal Department of Physics, Indian Institute of Science, Bangalore, India 560 012 (Received 18 August 1986; accepted 1 April 1987) The variation of resistivity of the lithium fast-ion conductor Li 3 +y Ge^V, _j,O4 {y = 0.25, 0.6, 0.72) has been studied with hydrostatic pressure up to 70 kbar and compared with that of Li16_2xZnx (GeO 4 ) 4 (x = 1, 2). Both types showed pronounced resistivity maxima between 20-30 kbar and marked decrease thereafter. Measurements as a function of temperature between 120-300 K permitted the determination of activation energies and prefactors that also showed corresponding maxima. The activation volumes (AF) of the first type of compound varied between 4.34 to — 4.90 cmVmol at 300 K and decreased monotonically with increasing temperature. For the second type AFwas much smaller, varied with pressure between 0.58 and — 0.24 cmVmol, and went through a maximum with increasing temperature. High-pressure studies were also conducted on aged samples, and the results are discussed in conjunction with results of impedance measurements and nuclear magnetic resonance (NMR) studies. The principal effect of pressure appears to be variations of the sum of interatomic potentials and hence barrier height, which also causes significant changes in entropy.
I. INTRODUCTION The study of ionic conductivity in solids under hydrostatic pressure has become of interest due to the possibility of obtaining information about conduction mechanisms. The subject has been recently reviewed by Samara,' who has pointed out that while the behavior of conventional ionic conductors such as alkali halides is reasonably well understood, in the case of fast-ion conductors even qualitative understanding is lacking. Thus, while it may be expected that the application of pressure would suppress ionic conductivity as in the case of 0alumina,2 pronounced enhancement of conductivity by a factor as much as 250 has been found3 recently in lithium fast-ion conductors at pressures near 70 kbar. In this paper we report on an investigation of the variation of resistivity with pressure up to 70 kbar and temperature between 120-300 K for the lithium fast-ion conductor Li 3+J ,Ge J ,V 1 _ J) O 4 (y = 0.25, 0.6, 0.72). These data are compared with previous results3 on Wxb_2x7j\x (GeO 4 ) 4 (;c = 1, 2), where the composition with x = 2 has been named Lisicon. The variations of activation energy and prefactor with pressure have been determined and go through maxima similar to the variation in resistivity. The activation volume (A F), which is one of the key parameters in interpreting pressure dependence of conductivity, decreased monotonically with temperature for the vanadates, while for Lisicon its value was much smaller and it was found to go through a maximum. Many of the earlier studies of fast-ion conductors 366
J. Mater. Res. 2 (3), May/Jun 1987
http://journals.cambridge.org
were limited to pressures less than 10 kbar. Radzilowski and Kummer, 4 for example, had examined the conductivity of /?-aluminas w
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