Evidence for phase transitions and pseudospin phonon coupling in K0.9(NH4)0.1H2AsO4

This work reports a detailed temperature dependent Raman study on the mixed crystals of K0.9(NH4)0.1H2AsO4 (KADA) from 5K to 300K in the spectral range of 60-1200cm−1, covering tetragonal to orthorhombic structural phase transition accompanied by paraelectric to ferroelectric transition at Tc* ∼ 60K. Multiple phase transitions below transition temperature (Tc* ∼ 60K) are marked by the appearance of new modes, splitting of existing ones as well as anomalies in the self-energy parameters (i.e. mode frequencies and damping coefficient) of the phonon modes. Temperature independent behaviour of damping coefficient and abrupt jump in the mode frequency of some of the internal vibrations of AsO4 tetrahedra as well as external vibrations clearly signal long range ferroelectric ordering and proton ordering below Tc*. In addition, we observed that temperature dependence of many prominent phonon modes diverges significantly from their normal anharmonic behaviour below Tc* suggesting potential coupling between pseudospins and phonons.