The effect of partial substitution of Ni by Mg on the structural, magnetic and spectroscopic properties of the double perovskite Sr2NiTeO6.

In this report, the structural, magnetic and spectroscopic properties of the freeze-drying synthesized Sr2Ni1-xMgxTeO6 (x = 0.0, 0.1, 0.2, 0.3 and 0.5) oxides are analyzed by means of X-ray powder diffraction (XRPD) and neutron powder diffraction (NPD), electron paramagnetic resonance, diffuse reflectance and magnetic susceptibility. The XRPD and NPD data analysis using the mode-crystallography approach have revealed that at room temperature (RT), all the compositions are monoclinically distorted with the space group I2/m. The high and low temperature analyses have shown that these materials suffer a series of three structural phase transitions. The EPR results have shown that the spectra of all the compositions are centred at g≈ 2.28, indicating a slightly distorted octahedral environment of Ni(2+), which is in agreement with the crystal structure analysis. The increase of the Mg(2+) content in Sr2Ni1-xMgxTeO6, provokes a decrease of the dipolar interaction effects and thus, the resonance becomes narrower. This resonance does not completely disappear which leads to the idea that the long-range magnetic order is not completely established when x≥ 0.3. The substitution of the Ni(2+) (S = 1) ions by Mg(2+) (S = 0) ions, also induces a weakening of the antiferromagnetic interactions, which is reflected in the diminishing of the absolute value of θ and the Néel temperature TN. The magnetic structure determination revealed the existence of an antiferromagnetic coupling for x- and z-spin components of the nickel atoms.

Synthesis, structural, magnetic and phase-transition studies of the ferromagnetic La2CoMnO6 double perovskite by symmetry-adapted modes.

A powdered La2CoMnO6 double perovskite was synthesized by the solid-state reaction method, and its crystal structure was investigated by (mode-crystallography) Rietveld analysis using X-ray and neutron powder diffraction data. La2CoMnO6 material is a monoclinic perovskite at room temperature, adopting the space group P21/n (a(-)a(-)b(+)), , c ≈ 2ap and Z = 2. The P21/n phase can be described effectively by three distortion modes (GM4(+), X3(+), X5(+)) of the Fm3[combining macron]m (a(0)a(0)a(0)) parent phase. The comparative study of the material and those in the Ln2CoMnO6 and Ln2NiMnO6 families has shown a general trend in nearly all the materials, has served to select a common direction in the sub-space spanned by X5(+), tri-linearly coupled to the order parameters of the cubic to monoclinic first order phase transition. This direction has been used to parametrize the refinements and to perform reliable refinements in the high-temperature intermediate distorted trigonal phase, R3[combining macron] (a(-)a(-)a(-)), for which only one effectively acting irrep has been deduced: GM5(+), physically a tilt of the oxygen sharing octahedra of Co and Mn. Its temperature evolution up to the prototype cubic phase has been fitted in the framework of the Landau Theory of Phase Transitions, showing a behavior typical of a tricritical point. The low-temperature neutron powder diffraction data have served to solve the magnetic structure: three indistinguishable ferromagnetic models with the space groups P21/n and P2/n' are proposed.

Structural phase transitions and magnetic and spectroscopic properties of the double perovskites Sr2Co1-xMgxTeO6 (x = 0.1, 0.2 and 0.5).

The structural and magnetic properties of a series of ordered double perovskites with the formula Sr(2)Co(1-x)Mg(x)TeO(6) (x = 0.1, 0.2 and 0.5) are investigated by X-ray diffraction, low temperature neutron diffraction, electron paramagnetic resonance and magnetic susceptibility. The progressive substitution of the paramagnetic Co(2+) high spin ion by the diamagnetic Mg(2+), of about the same size, induces changes in the room temperature crystal structure, from a distorted P2(1)/n phase for the undoped Sr(2)CoTeO(6) oxide to the I4/m of the end member (Sr(2)MgTeO(6)). These perovskites experience structural transitions on heating, the temperature at which the transitions occur being smaller as x increases. The novel approach of mode-crystallography is used for the analysis. All oxides show antiferromagnetic exchange interactions between Co(2+) ions but the long range antiferromagnetic order is not achieved for the phase with x = 0.5. The low temperature neutron diffraction data have been evaluated using a full symmetry analysis. Results are consistent with an unquenched orbital contribution of a high spin Co(2+) ion.

Alluaudite Na2Co2Fe(PO4)3 as an electroactive material for sodium ion batteries.

The electroactive orthophosphate Na2Co2Fe(PO4)3 was synthesized using a solid state reaction. Its crystal structure was solved using the combination of powder X-ray- and neutron-diffraction data. This material crystallizes according to the alluaudite structure (S.G. C2/c). The structure consists of edge sharing [MO6] octahedra (M = Fe, Co) resulting in chains parallel to [-101]. These chains are linked together via the [PO4] tetrahedra to form two distinct tunnels in which sodium cations are located. The electrochemical properties of Na2Co2Fe(PO4)3 were evaluated by galvanostatic charge-discharge cycling. During the first discharge to 0.03 V, Na2Co2Fe(PO4)3 delivers a specific capacity of 604 mA h g(-1). This capacity is equivalent to the reaction of more than seven sodium ions per formula unit. Hence, this is a strong indication of a conversion-type reaction with the formation of metallic Fe and Co. The subsequent charge and discharge involved the reaction of fewer Na ions as expected for a conversion reaction. When discharged to 0.9 V, the material intercalated only one Na(+)-ion leading to the formation of a new phase Na3Co2Fe(PO4)3. This phase could then be cycled reversibly with an average voltage of 3.6 V vs. Na(+)/Na and a capacity of 110 mA h g(-1). This result is in good agreement with the theoretical capacity expected from the extraction/insertion of two sodium atoms in Na3Co2Fe(PO4)3.

α-Na2Ni2Fe(PO4)3: a dual positive/negative electrode material for sodium ion batteries.

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.