In this paper the thermochromic effect in [NH₄]₂CuCl₄ (A-CuCl) crystal is discussed. It belongs to a group of A₂MX₄ crystals (where A- ammonium group or organic cation, M – metal ion, X – halogen ion) [1]. It is known, that for some of them in case when M = Cu²⁺ the thermochromic effect has been observed, which is demonstrated primarily by the change a color from green to yellow [2]. For A-CuCl crystal author observed the change of color from blue to green at 345K.

The A-CuCl crystal has been grown from water solution of 0.1 moles of CuCl₂ and 0.2 moles of NH₄Cl using slow evaporation method at the room temperature. The sample was split and polished to obtain the plate with 1mm thickness. The transmission spectra were measured using AVS-S2000 fiber optics spectrometer in wavelength range of 250 – 850nm and in temperature from room temperature to 350K. The results are presented in fig.1.

Fig.1. Transmittance spectra of A-CuCl crystal in different temperatures.

Spectral properties of A₂MX₄ (M = Cu, Co, Mn) crystals are correlated to the electronic transitions in metal ions [3]. In crystals with Cu²⁺ ion an electrostatic influence of ligands leads to splitting of ²D term. Additionally, due to Jahn-Teller phenomenon the CuCl₄ tetrahedra are strongly deformed.

Theoretical model of interaction of ligands with 3d orbitals based on crystal field theory [4] was elaborated. It allows to estimate the influence of molecule deformation on splitting of energy level. Simulations show that the value of the splitting can be clearly different from that for the case of perfect tetrahedron. Transmission spectra for the investigated crystal are limited by absorption bands connected with electronic transitions in Cu²⁺ ion and also probably by ligand-metal charge transfer. With increase of the temperature a shift the absorption edge is observed. Simulations show that it may be connected with reduction of deformation of the polyhedra. The appearance of transparency in the region above 650nm has been observed at 343K. It may be connected with a change of symmetry of the polyhedra and in consequence with a change of selection rules.

[1] Kałuża S., Suchańska M., Belka R., Leśniewski S.: The phase transitions in ferroelectric crystals of A₂MX₄ and AMX₃ type, Ferroelectrics, 2002, vol. 273, pp.143-148.

[2] Polovinko I., Kałuża S., Korczak Yu., Kapustianik V.: Thermochromic properties of the A₂CuCl₄ crystals, Molecular Physics Reports 18/19, 67, 1997

[3] Kałuża S., Suchańska M., Polovinko I.: Studies of absorption spectra of A₂BCl₄ (B=Cu, Co) crystals with organic cations, Ferroelectrics, vol. 239, 2000, pp. 189-196.

[4] Ballhausen C.: Introduction to Ligand Field Theory, McGraw-Hill, New York, 1962

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