Crystal structure, conductivity and reversible water uptake of new layered potassium antimonates KxL(1+x)/3Sb(2−x)/3O2 (L=Ni2+, Mg2+, Co2+)

Novel mixed potassium antimonates K0.59Mg0.53Sb0.47O2, K0.5Ni0.5Sb0.5O2, K0.5Co0.5Sb0.5O2 (rhombohedral P3-type structure), K0.56Ni0.52Sb0.48O2 and K0.86Co0.62Sb0.38O2 (hexagonal P2 type) have layered structures based on brucite-like (L,Sb)O6/3 sheets of edge-shared octahedra and interlayer K+ cations in trigonal prismatic coordination. The preference to form P2 and P3 structures rather than closely related O3 type is dictated by the large radius of K+ and the value of unit cell parameter a, restricted by average size of the cations randomly distributed in the octahedral sites within (L,Sb)O6/3 layer. The new phases reversibly absorb atmospheric moisture leading to the formation of hydrates with ca. 11% larger interlayer distances. The impedance spectroscopy of P2-type K0.56Ni0.52Sb0.48O2 and P3-type K0.59Mg0.53Sb0.47O2 ceramics shows relatively high ionic conductivity, presumably due to potassium cationic transport, with activation energies of 35±2 and 33±1 kJ/mol, respectively. At 573 K, the conductivity values are 0.016 S/cm for K0.56Ni0.52Sb0.48O2 and 0.021 S/cm for K0.59Mg0.53Sb0.47O2. Interaction with water vapor leads to increasing total conductivity.