Vibrational spectroscopic and dielectric properties investigations of phase transitions in KMgPO4 compound

The potassium orthophosphate KMgPO4 with a β-tridymite structure was synthesized via solid-state reaction. X-ray diffraction study confirms the formation of a single phase material which crystallizes at room temperature in monoclinic system. This compound has been investigated by vibrational spectroscopy in the temperature range573-723 K. Thermal analysis shows that this composition undergoes two phase transitions at T1=633Kand T2=693 K.The evolution of Raman line ν and half -width Δν versus temperature introduces huge changes which are associated with the phase transitions originating from the reorientation of the PO4 tetrahedron. Besides, an analysis of the dielectric constants ε′ and ε″versus temperature at several frequencies shows a distribution of relaxation times. This relaxation is probably due to the change in dynamical state of the K+ cation. The ac conductivity behavior can be understood in terms of the motions of K+ cations along the tunnels which are formed by six-membered rings of MgO4 and PO4 tetrahedron linked by common vertices. The activation energies values obtained from the thermal evolution of the conductivity are: Ea1=0.52 eV (T<633 K), Ea2=1.15 eV (633 K 693 K).