Tuning of physical properties of Fe7(PO4)6 by sodium intercalation

Abstract The sodium intercalation of mixed – valence iron phosphate Fe 3 2+ Fe 4 3+ (PO 4 ) 6 results in drastic transformation of its physical properties. The parent compound Fe 7 (PO 4 ) 6 reaches magnetically ordered state through succession of phase transitions at T N1  = 45.5 K and T N2  = 16 K marked by sharp singularities in both specific heat C p and magnetic susceptibility χ. The introduction of sodium suppresses the formation of antiferromagnetic state down to T N  = 33 K in Na 0.65 Fe 7 (PO 4 ) 6 . The low temperature phase transition in this compound smears being substituted by broad anomalies in magnetization and specific heat related to the spin/charge disorder effects. The sharp well resolved electron spin resonance spectra in parent material transform into asymmetric broad line in sodium – intercalated substance. The dielectric permittivity e of Fe 7 (PO 4 ) 6 demonstrates a kink at T N2 , while no singularity marks T N1 . No features accompany magnetic phase transition in dielectric property of Na 0.65 Fe 7 (PO 4 ) 6 but e reaches the relaxation maxima at high temperatures which can be attributed to the freezing of sodium ions.