The absorption spectra of the [NH 2 (C 2 H 5 ) 2 ] 2 CuCl 4 crystals grown from a solution in ethyl alcohol and also of solid solutions of [NH 2 (C 2 H 5 ) 2 ] 2 Zn 0.5 Cu 0.5 Cl 4 in the visible spectral region have been investigated in the vicinity of thermochromic phase transitions. It is shown that the phenomenon of thermochromism in these crystalline compounds is primarily related to the replacement of the square-planar geometry of the coordination surrounding of the Cu 2+ ion by a tetrahedral one.Introduction. Thermochromic crystals have attracted the attention of researchers in connection with their possible use as sensitive materials of sensor technology and also in recording optical information. The compounds of the group A 2 MeCl 4 and AMeCl 3 (Me = Cu, Co) [1-5] with an alkylammonium cation, to which the crystals of diethylaminetetrachlorocuprate -[NH 2 (C 2 H 5 ) 2 ] 2 CuCl 4 (DEACC) -are related, constitute a new class of thermochromic materials. According to [1], these crystals at T t = 323 K undergo a thermochromic phase transition of the first kind accompanied by a sharp change in the color of a sample from green to yellow in the regime of heating. The hydrogen bonds that influence the degree of deformation of the metal-haloid polyhedra play an important role in the phenomena of thermochromism, which, in turn, is manifested in the change in the absorption spectra.At room temperature, the [NH 2 (C 2 H 5 ) 2 ] 2 CuCl 4 crystals grown from a solution in ethyl alcohol (from now on, DEACC-I) belongs to the space group P2 1 ⁄ n. The parameters of the elementary cell are: a = 7.362 A°, b = 15.025 A°, c = 45.193 A°, β = 89.94 o , and Z = 12. One asymmetric structural group includes three tetrahedrally and nine octahedrally coordinated ions [CuCl 4 ] 2− in an abnormally large cell. All DEA + cations participate in the formation of the two-dimensional network of strong hydrogen bonds N-H⋅⋅⋅Cl parallel to the plane (bc). The high-temperature phase is also monoclinic with the space group P2 1 ⁄ c (a = 25.055 A°, b = 10.531 A°, c = 15.455 A°, β = 100.63 o , and Z = 8) [1].The temperature of the thermochromic phase transition in these crystals is greatly determined by the conditions of their growing. For the crystals grown from an aqueous solution (from now on, DEACC-II) it was equal to T t = 311 K (in the regime of heating) [2,4].In view of the thermochromic character of the indicated phase transition, absorption spectroscopy has turned out to be an effective tool for studying its nature [4]. The studies of DEACC-II crystals have shown that a phase transition is associated with changes in the coordination surrounding of the copper ion. Here, just as in the case of DEACC-I, the high-temperature phase is characterized by a distorted tetrahedral position of the surrounding of the copper ion, whereas the low-temperature one is characterized by the co-existence of the anions of the plane quadratic and tetrahedral forms (each surrounding of the copper ion yields two absorption bands). A sharp change in the config...

The dielectric parameters and electric conductivity of (NH2(CH3)2)2CoCl4 crystals are investigated. Existence of the ferroelectric phase was observed below Tc = 258 K. The occurrence of phase transitions at 244, 314, and 435 K is confirmed. The investigated crystals also manifest anomalously high conductivity commensurate with that in semiconductors. Its value as well as region of existence strongly depend on the preceding history of the sample. It is shown that the anomalous conductivity is connected with translocation of protons which participate in the formation of hydrogen bonds.

On the basis of dilatometric, optical, and dielectric investigations of (NH3C2H5)2CuC14 crystals the existence of phase transitions at T1 = 364 and T2 = 356 K was confirmed. Anomalous behaviour of the thermal expansion coefficient and optical birefringence around T3 = 330 K was related to earlier unknown phase transition. Besides, it was shown that these crystals are characterised by considerable protonic conductivity, the nature of which is fairly well explained in the framework of Grotthus mechanism of proton transport.

Optical and spectral investigations confirmed the availability of phase transitions in [C 2 H 5 NH 3 ] 2 CuCl 4 at T 1 = 364 K, T h 2 = 356 K, T 3 = 232 K. It has been found that the empirical Urbach rule is fulfilled in all phases. It is shown, that in the high-temperature phases skeletal vibrations of the C±N±C group interact with phonons, whereas in the low-temperature phases internal vibrations of the metal±halogen complex occur. A ferroelastic domain structure was observed using the polarising microscope in the temperature region 232 to 364 K.

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