Phase transitions and temperature changes of the optical absorption edge in (NH2(C2H5)2)2CoCl4 layered crystal

This work was devoted to X-ray diffraction study and investigations oftemperature changes of the optical absorption edge of (NH2(C2H5)(2))(2)CoCl4 crystals in the region of possible phase transitions. The X-ray powder diffraction data revealed the monoclinic phase at room temperature - space group P2/n. The cobalt atom was found to be square-plane coordinated by four chlorine atoms resulting [CoCl4](2-) anion, which is surrounded by two DEA(+) cations. It was shown that the low-energy tail of the absorption edge in these materials possesses an exponential shape. In the temperature range above 255K it follows the empirical Urbach's rule. The obtained experimental data confirmed the existence of the ferroelastic phase in (NH2(C2H5)(2))(2)CoCl4 in the temperature range between 255 and 326K. The anomalous behaviour of the investigated parameters observed at the temperatures below 255K would be related to earlier unknown phase transitions.