Polaron hopping properties of Eu1-xCaxB6 (0.2 <= x <= 0.995)

Abstract

Using temperature- and field-dependent resistivity rho(T, H) and Hall coefficient R-H(T, H) measurements, we investigate the anomalous transport properties of single crystals of Eu1-xCaxB6 (x = 0.2-0.995). The replacement of Eu with Ca invokes drastic changes in the p(T) and RH (T, H) although Eu and Ca are isoelectric. It is found that the Hall mobility, defined as vertical bar mu(H)vertical bar = R-H (H = 1T)/rho, can be expressed by a non-adiabatic small polaron hopping model with four-neighbor hopping sites for Eu1-xCaxB6 (x = 0.2) and with three-neighbor hopping sites for Eu1-xCaxB6 (X = 0-9). The Hall mobility of Eu1-xCaxB6 (x = 0.6) is expressed well by either the four- or three-sites hopping model. On the other hand, the DC conductivity of Eu1-xCaxB6 (x >= 0.95) is found to be described well by an adiabatic small polaron hopping model. Furthermore, the DC conductivity of Eu1-xCaxB6 (x >= 0.95) can also be described well by the adiabatic polaron hopping process. From the above observations, we surmise that the electronic transport properties of Eu1-xCaxB6 are dominated by the small polaron hopping mechanism. The hopping mechanism changes gradually from adiabatic for x >= 0.9 to three-sites non-adiabatic for 0.9 >= x >= 0.6 and to four-sites non-adiabatic for x <= 0.6. (c) 2006 Elsevier B.V. All rights reserved.