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X-ray powder measurements were used to investigate the phase transitions and crystal structures of K 2 NbCl 6  and Rb 2 NbCl 6 . In both cases, the metrical changes of the unit cell are accompanied by a compressive distortion of the NbCl 6  2−  groups from  O  h  to  D  4h  symmetry. This static manifestation of the Jahn-Teller effect is ascribed to electro static interactions with the cations. Thus, Rb 2 NbCl 6  trans forms from  Fm -3 m  ( Z  = 4) to  I 4/ mmm  ( Z  = 2) due to a cation-supported compression in the  c  direction, whereas K 2 NbCl 6  expands along  c  when converting from  Fm -3 m  to  P 4/ 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 Nb 4+  ( d  1 ) is checked via bond-length bond-strength correlations and magnetic measurements.

The significance of the Jahn-Teller effect for the phase transitions of K 2 NbCl 6 and Rb 2 NbCl 6