A Raman scattering study of high-temperature phase transitions in potassium and rubidium dihydrogen phosphate

Abstract

The temperature dependence of the internal and external Raman-active modes of potassium and rubidium dihydrogen phosphate has been measured above room temperature near the high-temperature phase transitions. Broadening of the 187 cm-1B(z) lattice mode indicates the onset of order-disorder fluctuations in the crystal which may be a precursor to the high-temperature phase transition. A decrease in the intensity of one component of the E(x,y) coupled proton mode indicates a loss of the cooperative nature of the proton vibrations in the high-temperature phase. The intensity of the 361 cm-1E(v2) internal vibration of the PO4 decreases as the high-temperature phase is approached. Comparison of the effects in rubidium dihydrogen phosphate in which the transition occurs at lower temperature allows separation of the effects of decomposition in the Raman spectrum from those of the phase transition in potassium dihydrogen phosphate. A possible mechanism for the high-temperature phase transition is discussed.