Octahedral tilting in the polar hexagonal tungsten bronzes RbNbW2O9 and KNbW2O9.

The ambient-temperature structures (orthorhombic, space group Cmc21) of the polar hexagonal tungsten bronzes RbNbW2O9 and KNbW2O9 have been determined by high-resolution powder neutron diffraction. Displacement of the A-site cation along the polar c axis with concomitant octahedral tilting occurs to optimize the A cation bonding environment, hence reducing the coordination from 18 to 16. This effect is more evident in KNbW2O9 due to decreased A cation size. The octahedral tilting in both compositions results in a doubling of the c axis that has not previously been reported, highlighting the importance of neutron diffraction as a complementary technique for structural determination of such systems.