### Abstract

A new orthophosphate α-Na<inf>2</inf>Ni<inf>2</inf>Fe(PO<inf>4</inf>)<inf>3</inf> was synthesized using a solid state reaction route, and its crystal structure was determined from powder X-ray diffraction data. The physical properties of α-Na<inf>2</inf>Ni<inf>2</inf>Fe(PO<inf>4</inf>)<inf>3</inf> were studied by magnetic and electrochemical measurements and by Mössbauer and Raman spectroscopy. α-Na<inf>2</inf>Ni<inf>2</inf>Fe(PO<inf>4</inf>)<inf>3</inf> crystallizes according to a stuffed α-CrPO<inf>4</inf>-type structure with the space group Imma and the cell parameters a = 10.42821(12), b = 13.19862(15), c = 6.47634(8) Å, and Z = 4. The structure consists of a 3D-framework of octahedra and tetrahedra sharing corners and/or edges with channels along [100] and [010], in which the sodium atoms are located. The <sup>57</sup>Fe Mössbauer spectrum indicates that the Fe<sup>3+</sup> cation is distributed over two crystallographic sites implying the presence of a Ni<sup>2+</sup>/Fe<sup>3+</sup> statistical disorder. Magnetic susceptibility follows the Curie-Weiss behavior above 100 K with θ = -114.3 K indicating the occurrence of predominant antiferromagnetic interactions. Electrochemical tests indicate that during the first discharge to 1 V vs. Na<sup>+</sup>/Na in a sodium cell, one Na<sup>+</sup> ion could be inserted into the α-Na<inf>2</inf>Ni<inf>2</inf>Fe(PO<inf>4</inf>)<inf>3</inf> structure. This has led to the formation of a new phase Na<inf>3</inf>Ni<inf>2</inf>Fe(PO<inf>4</inf>)<inf>3</inf> which was found to be promising as a positive electrode material for sodium batteries. When α-Na<inf>2</inf>Ni<inf>2</inf>Fe(PO<inf>4</inf>)<inf>3</inf> is further discharged to 0.03 V, it delivers a capacity of 960 mA h g<sup>-1</sup>. This corresponds to the intercalation of more than seven sodium atoms per formula unit which is an indication of a conversion-type behaviour with the formation of metallic Fe and Ni. When cycled in the voltage range 0.03-3 V vs. Na<sup>+</sup>/Na, at 20 °C, under the current rates of 50, 100, 200, and 400 mA g<sup>-1</sup>, reversible capacities of 238, 196, 153, and 115 mA h g<sup>-1</sup>, were obtained, respectively. This journal is

Original language | English |
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Pages (from-to) | 4526-4532 |

Number of pages | 7 |

Journal | Dalton Transactions |

Volume | 44 |

Issue number | 10 |

DOIs | |

Publication status | Published - 14 Mar 2015 |

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### ASJC Scopus subject areas

- Inorganic Chemistry

### Cite this

*Dalton Transactions*,

*44*(10), 4526-4532. https://doi.org/10.1039/c5dt00021a