INSULATOR-METAL AND STRUCTURAL PHASE PHENOMENA IN KXBA1-XCOS2 (X-LESS-THAN-0.07)

KxBa1-xCoS2 was synthesized by a new technique using a closed vitreous carbon crucible. Single crystals prepared from materials of nominal c omposition x = 0.2 were found to have only modest substitution levels (x approximate to 0.07) by microprobe and X-ray analysis. The substitu ted compounds crystallize in P4/nmm, a = 4.557(1), c = 8.957(2), R-1 = 2.03%, and wR(2) = 4.20%, as determined by single-crystal X-ray diffr action. Measurement of electrical resistivity by the van der Pauw meth od reveals a phase transition with hysteresis at about 170 K for cooli ng and 210 K upon warming, with semiconducting behavior above (rho app roximate to 10(-2) Omega cm) and metallic-like behavior (rho approxima te to 10(-3) Omega cm) below the transition. Magnetic susceptibility d ata show a second-order transition around 270 K, along with a first-or der transition around 150-200 K that displays significant hysteresis. The transition at 270 K is probably due to the onset of antiferromagne tic order, whereas the transition at lower temperatures is associated with structural modifications. Low-temperature X-ray diffraction, from 300 to 100 K, of single crystals indicates tetragonal --> monoclinic --> triclinic symmetry shifts upon cooling through this temperature ra nge, with corresponding superlattice formation.