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

The sodium intercalation of mixed - valence iron phosphate Fe32+Fe43+(PO4)6 results in drastic transformation of its physical properties. The parent compound Fe7(PO4)6 reaches magnetically ordered state through succession of phase transitions at TN1 = 45.5 K and TN2 = 16 K marked by sharp singularities in both specific heat Cp and magnetic susceptibility χ. The introduction of sodium suppresses the formation of antiferromagnetic state down to TN = 33 K in Na0.65Fe7(PO4)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 ε of Fe7(PO4)6 demonstrates a kink at TN2, while no singularity marks TN1. No features accompany magnetic phase transition in dielectric property of Na0.65Fe7(PO4)6 but ε reaches the relaxation maxima at high temperatures which can be attributed to the freezing of sodium ions.