Reversible Phase Transitions and Dielectric Properties in [(CH3)4P]2[Cr2O7] and [Et4P]2[Cr3O10]

Abstract Tetramethylphosphonium dichromate ([(CH 3 ) 4 P] 2 [Cr 2 O 7 ], 1 ) undergoes a reversible phase transition at around 221 K. The results of the variable‐temperature single‐crystal X‐ray structural analyses and the rotational potential energy calculations suggest that the phase transition is probably related to the order–disorder transition of the [Cr 2 O 7 ] 2– anion. The change of the space group from Pa ‐3 at room temperature to Pbca at low temperature is reflected in the peak‐like dielectric response. Tetraethylphosphonium trichromate ([Et 4 P] 2 [Cr 3 O 10 ], 2 ) crystallizes in the monoclinic space group P 2 1 / c at room temperature. Upon cooling and heating, the step‐like dielectric anomalies observed at around 270 K indicate a reversible phase transition. Moreover, the rotational potential energy calculations certify the potential motion of the [Cr 3 O 10 ] 2– anion, being related to the phase transition. These findings open a new approach to the design of complexes that show phase transitions coupled with unusual dielectric properties.