The significance of the Jahn-Teller effect for the phase transitions of K2NbCl6 and Rb2NbCl6

X-ray powder measurements were used to investigate the phase transitions and crystal structures of K2NbCl6 and Rb2NbCl6. In both cases, the metrical changes of the unit cell are accompanied by a compressive distortion of the NbCl62- groups from O-h to D-4h symmetry. This static manifestation of the Jahn-Teller effect is ascribed to electrostatic interactions with the cations. Thus, Rb2NbCl6 transforms from Fm $$(3) over barm (Z = 4) to I4/mmm (Z = 2) due to a cation-supported compression in the c direction, whereas K2NbCl6 expands along c when converting from Fm $$(3) over barm to P4/mnc (Z = 2). On further cooling a second transition leads to Cccm (Z = 4) where b as the shortest axis is approximately parallel to the direction of the octahedral contraction. The oxidation state of Nb4+ (d(1)) is checked via bond-length bond-strength correlations and magnetic measurements.