Jahn-Teller effect in the [CuEn<sub>3</sub>]CrO<sub>4</sub> structure
A change in the coordination of the copper atom in the crystal structure of [CuEn<sub>3</sub>]CrO<sub>4</sub> (En is ethylenediamine) in studied in the range 150-300 K. According to the single crystal X-ray diffraction (XRD) data at 150 K, the single crystal has a complicated twining character based on the triclinic cell ( a = 9.027(2) Å, b = 13.335(3) Å, c = 13.339(3) Å, a = 71.77(3)°, b = 70.53(3)°, g = 70.42(3)°) composed of two crystallographically independent [CuEn<sub>3</sub>]<sup>2+</sup> complex cations. The coordination of copper atoms is a distorted square bipyramid; four short Cu-N distances lie in the range 2.049-2.082 Å; two long ones are 2.415 Å and 2.470 Å. According to the differential scanning calorimetry (DSC) data, near 218 K there is a phase transition. The single crystal XRD experiment performed at 250 K ( a = 15.6992(19) Å, c = 9.7573(13) Å, V = 2082.6(6) Å<sup>3</sup>, space group P <sub></sub> c 1 No. 165), Z = 6) shows that chromate anions are disordered over three positions about the с axis, and Cu-N distances are 2.120-2.177 Å. According to the DSC data, on further heating the structure undergoes yet another two alterations (260 K and 270 K) due to the disordering of oxygen atoms of chromate anions and the subsequent equalization of Cu-N distances. At 300 K in the structure ( a = 9.0778(6) Å, c = 9.7715(5) Å, V = 697.4 Å<sup>3</sup>, space group P <sub></sub> 1 c No. 163), Z = 2) all Cu-N distances are 2.155 Å, and chromate anions are disordered over six positions about the с axis. A comparative crystal chemical analysis of the packing of the studied structures is carried out.