  The solid solution of antimonide-oxides Ba1-xKxTi2Sb2O (0 < x < 1) has been
synthesized by solid-state reactions and characterized by X-ray powder
diffraction (CeCr2Si2C-type structure; P4/mmm, Z = 1). The crystal structure
consists of Ti2Sb2O-layers that are stacked with layers of barium atoms along
the c-axis. BaTi2Sb2O is a known superconductor with a critical temperature
(Tc) of 1.2 K. Substitution of barium through potassium raises Tc up to 6.1 K
at 12 % potassium, while no superconductivity emerges with concentrations
higher than 20 %. Anomalies in electrical transport and magnetic susceptibility
indicate charge density wave (CDW) instabilities. The CDW transition
temperatures (Ta) decrease from 50 K in the parent compound to 28 K at 10 %
potassium substitution. No CDW transition was detected at higher
concentrations, and no evidence for a reduction of the lattice symmetry below
Ta was found. The lattice parameters vary linearly while the unit cell volume
increases with higher potassium concentrations. The phase diagrams Tc(x) and
Ta(x) of Ba1-xKxTi2Sb2O are remarkably similar to the known series
Ba1-xNaxTi2Sb2O (0 < x < 0.33) in spite of the reverse volume effect. From this
we conclude that the charge and not the volume determines the phase diagrams of
these superconducting antimony oxides.
