Manifestation of ferroelastoelectric phase transition in temperature changes of the optical absorption edge in (NH₄)₂CuCl₄·2H₂O crystal

ABSTRACT

This work investigates temperature changes of the optical absorption edge of (NH₄)₂CuCl₄·2H₂O crystals in the region of the ferroelastoelectric (FEE) phase transition. The low-energy tail of the absorption edge in these materials shows an exponential shape. The temperature range above 200.5 K and below 100 K follows the empirical Urbach’s rule. The obtained experimental data confirmed the existence of the FEE phase transition at 200.5 K. Nonfulfillment of Urbach’s rule just below the phase transition temperature is explained by the scattering of light on the FEE domains.

KEYWORDS:

• Phase transition
• optical absorption edge
• empirical Urbach’s rule
• electron–phonon interaction
• ferroelastoelectric

PACS Numbers:

• 78.40, 64.70, 77.84, 78.30
• S12

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

This work was supported by the Ministry of Education and Science of Ukraine. This work was partly supported by the Programe Dnipro Cooperation Scientifique Franco-Ukrainian (PHC) № 46789RH.