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Henning Henke

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

Keywords: Jahn-Teller effect

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 NbCl₆^2– 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, Rb₂NbCl₆ trans forms from Fm-3m (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-3m 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⁺ (d¹) is checked via bond-length bond-strength correlations and magnetic measurements.

Zeitschrift für Kristallographie, Oldenbourg Wissenschaftsverlag

Print ISSN: 0044-2968
Volume: 222, 09/2007
Pages: 477 - 486

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