Raman study of Rb_1-X(NH₄)_XH₂P_1-YAs_YO₄ mixed crystal: Order parameters in competing and frustrated interactions

Abstract

Mixed crystals of Rb_1-x (NH₄) _x H₂P_1-y As _y O₄ (RADPA-xy) between ferroelectric RbH₂PO₄/RbH₂AsO₄ and antiferroelectric NH₄H₂PO₄/NH₄H₂AsO₄ substances for (x = 0.35, y = 1) and (x = 0.55, y = 0.65) are studied by Raman scattering in the temperature range from room temperature to T = 15 K. Low temperature Raman spectra of both RADPA (x = 0.55, y = 0.65) and RADA (x = 0.35, y = 1) crystals show a gradual change with the spectral bands maintaining one to one correspondence with the room temperature paraelectric spectra characteristic of the glass transition rather than new additional bands or band splittings of the structural phase transition associated with the ferroelectric or antiferroelectric ordering. Temperature dependence of the Raman spectral change seems to show that condensation of the B₂-symmetry soft-mode band, the intensity saturation of the E-mode NH₄-librational band and frequency changes of v ₂ mode PO₄/AsO₄ internal vibrations are all correlated to each other with each one related to a different aspect of the glass freezing dynamics. Lattice external bands of RADPA-xy crystal are observed to be relatively strong while molecular internal bands are observed to be very weak even at low temperature: the former may be another confirmation of the coherent ionic lattice of tetragonal symmetry in the glass phase and the latter may be speculated as due to the site symmetry frustration in RADPA-xy crystal.