ELECTRICAL-PROPERTIES OF THE SQUARE-ANTIPRISMATIC CHAIN COMPOUNDS M4BR4OS (M=Y, ER) AND M4TE4Z (M=NB, TA Z=SI, CR, FE, CO)

The resistivities of the rare-earth metal halides, R4Br4Os (R = Y, Er) and several of the ternary tellurides M4Te4Z (M = Nb, Z = Si, Fe, Co; M = Ta, Z = Si, Cr, Co) were measured using standard four-probe ac an d dc techniques at temperatures ranging from 4.2 to 350 K. The rare-ea rth halides R4Br4Os show semiconducting behavior: conductivity increas es exponentially with temperature. Activation energies (E(a)) for cond uction in Y4Br4Os and Er4Br4Os are 0.16 and 0.11 eV, respectively. The resistivity of Er4Br4OS is 2 orders of magnitude smaller than that of Y4Br4Os at 273 K. The behavior of these compounds is in accord with t he calculated band structure presented. The isoelectronic compounds M4 Te4Si (M = Ta, Nb) behave as electronic conductors at high temperature . Ta4Te4Si shows a resistivity maximum at 160 K and below 60 K exhibit s a broad metal-insulator transition. The absolute resistivity measure d for Ta4Te4Si is 2 orders of magnitude higher than the other tellurid es. The analogue Nb4Te4Si shows a similar anomalous resistivity maximu m near 280 K and at low temperature (approximately 60 K) exhibits a gr adual rise in resistivity that suggests a semiconducting ground state. Transition-metal-centered M4Te4Z compounds all behave as simple metal s in the temperature range 4.2-300 K; metal-insulator transitions are not seen at temperatures greater than 4.2 K. Nb4Te4Fe shows a small re sistivity anomaly near 25 K.