Chromium(II) diphosphate, Cr(2)P(2)O(7), has an incommensurately modulated structure at ambient conditions with a = 7.05, b = 8.41, c = 4.63 A, beta = 108.71 degrees and q = (-0.361, 0, 0.471). It undergoes a phase transition towards a commensurate structure with a commensurate q vector, q = (-1/3, 0, 1/2), at T(c) = 285 K. The incommensurate structure has been solved by the charge-flipping method, which yielded both the basic positions of the atoms and the shapes of their modulation functions. The structure model for the commensurate structure was derived directly from the incommensurate structure. The structure analysis shows that the modulation leads to a change of the coordination of the Cr(2+) ions from distorted octahedra in the average structure towards a sixfold coordination in the form of a more regular octahedron and a fivefold coordination in the form of a square pyramid. The fivefold and sixfold coordination polyhedra alternate along the lattice direction a with the pattern 5-6-5 5-6-5 in the commensurate structure. In the incommensurate structure this pattern is occasionally disturbed by a 5-6-5-5 motif. Both structures can be described in superspace using the same superspace group and a similar modulated structure model. The same superspace model can also be used for the low-temperature phases of other metal diphosphates with the thortveitite stucture type at high temperature. Their low-temperature structures can be obtained from the superspace model by varying the q vector and the origin in the internal dimension t(0).

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A new orthophosphate α-Na2Ni2Fe(PO4)3 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-Na2Ni2Fe(PO4)3 were studied by magnetic and electrochemical measurements and by Mössbauer and Raman spectroscopy. α-Na2Ni2Fe(PO4)3 crystallizes according to a stuffed α-CrPO4-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 (57)Fe Mössbauer spectrum indicates that the Fe(3+) cation is distributed over two crystallographic sites implying the presence of a Ni(2+)/Fe(3+) 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(+)/Na in a sodium cell, one Na(+) ion could be inserted into the α-Na2Ni2Fe(PO4)3 structure. This has led to the formation of a new phase Na3Ni2Fe(PO4)3 which was found to be promising as a positive electrode material for sodium batteries. When α-Na2Ni2Fe(PO4)3 is further discharged to 0.03 V, it delivers a capacity of 960 mA h g(-1). 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(+)/Na, at 20 °C, under the current rates of 50, 100, 200, and 400 mA g(-1), reversible capacities of 238, 196, 153, and 115 mA h g(-1), were obtained, respectively.

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