Configuration of the dimolybdate in salt inclusion type of compounds, Cs₂Mo₂O₇ ? CsX (X=Cl, Br, and I)

Cs2Mo2O7 center dot CsI is obtained as a member of the series Cs2Mo2O7 center dot CsX with X=Cl, Br, and I from the synthesis of Cs2MoO4 with MoO3 and CsX. At room temperature, the compounds crystallize in the space group P6(3)/mmc with the dimolybdate, [Mo2O7](2-), in a linear and eclipsed conformation. Low temperature Raman spectroscopy and low temperature X-ray diffraction were used to investigate the effect of the hetero-honeycomb lattice on the bridging angle of the dimolybdate. For X=Cl, a phase transition occurs below 150 K (P6(3)/mmc -> Cmcm). The phase transition relates to a lock-in that involves bending of the (Mo-O-Mo) bridge. For the Pearson softer iodide compound, no phase transition is observed down to 4 K; thus, the linear configuration remains stable. Specific heat data and their evaluation in terms of phonon contributions are given.