Ferroelectricity of Ca9Fe(PO4)7 and Ca9Mn(PO4)7 ceramics with polar whitlockite-type crystal structure

Ca9Fe(PO4)7 is a member of the double phosphate family having polar whitlockite-typecrystal structure. The phase transition from the room temperature polar R3c to the hightemperature non-polar R c phase has been called a ferroelectric phase transition usingcomplementary experiments such as temperature dependent second harmonic generation anddielectric constant measurements however no ferroelectric hysteresis measurement has beenreported. In order to be able to call these polar materials ferroelectric, measurement of asaturated ferroelectric hysteresis loop is necessary to demonstrate that the electricalpolarization of these materials is switchable. In order to realize this goal, we havesynthesized Ca9Fe(PO4)7 as well as structurally identical Ca9Mn(PO4)7 using solid statesynthesis. Crystal structure of the ceramics were confirmed using Rietveld refinement of thex-ray diffraction (XRD) patterns. Differential scanning calorimetry (DSC) measurementsrevealed phase transition temperatures of 848 and 860 K for Ca9Fe(PO4)7 and Ca9Mn(PO4)7,respectively. Our ferroelectric hysteresis measurements and current electric field loops (I-E)derived from the hysteresis loops showed that the loops cannot be saturated and the directionof the electrical polarization of both materials cannot be switched up to the largest appliedelectric field of 100 kV/cm. Possible origins of this behaviour are discussed.

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