Abstract
The anomalous optical dispersion and the existence of an isotropic point in the organic-inorganic double-halide (${\mathrm{C}}_{2}$${\mathrm{H}}_{5}$${\mathrm{NH}}_{3}$${)}_{2}$${\mathrm{CuCl}}_{4}$ crystal (in short EACuC) around its thermochromic low-temperature phase transition were investigated by means of birefringence, thermal expansion, and calorimetric measurements as well as by direct conoscopic and orthoscopic observations under polarized light. The birefringence dispersion data are analyzed using a simple two-oscillator model based on the optical band structure of EACuC. The model adjustable parameters are related with the oscillator strength and average positions, respectively. The thermal behavior of the involved parameters is compatible with the existence of the isotropic point below the thermochromic phase transition. These results, together with the optical extinction measurements and the domain pattern observations, suggest a complicated structural phase-transition sequence which is also discussed.
Published Version
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