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In the present work, the oxidation and spin state of Fe and the local structure around Fe in the supercapacitor birnessite with different contents of the Fe dopant were investigated using Mössbauer spectroscopy. It was found that Fe ions were exclusively present as high spin Fe 3+ in octahedral coordination with about 70% iron occupying the Mn 3+ positions and about 30% iron occupying the Mn 4+ positions in the [MnO 6 ] octahedra for all Fe-doped birnessite samples. Based on these new findings, the trend of typical cell parameters, selected bond lengths of the Fe-doped birnessites and their corresponding quadrupole splittings in the Mössbauer spectra were well explained by considering both the weakened Jahn–Teller effect during the replacement of Mn 3+ by Fe 3+ and the expansion of octahedra during the replacement of Mn 4+ by Fe 3+ . The present work offers some new insights into the understanding of the mechanism of the heterogeneous atomic doping on the crystal structure of birnessite, with importance for both mineralogy and material science.

The compound NaFeGe 2 O 6 was grown synthetically as polycrystalline powder and as large single crystals suitable for X-ray and neutron-diffraction experiments to clarify the low temperature evolution of secondary structural parameters and to determine the low temperature magnetic spins structure. NaFeGe 2 O 6 is isotypic to the clinopyroxene-type compound aegirine and adopts the typical H T - C 2/ c clinopyroxene structure down to 2.5 K. The Na-bearing M2 polyhedra were identified to show the largest volume expansion between 2.5 K and room temperature, while the GeO 4 tetrahedra behave as stiff units. Magnetic susceptibility measurements show a broad maximum around 33 K, which marks the onset of low-dimensional magnetic ordering. Below 12 K NaFeGe 2 O 6 transforms to an incommensurately modulated magnetic spin state, with k  = [0.323, 1.0, 0.080] and a helical order of spins within the M1-chains of FeO 6 octahedra. This is determined by neutron-diffraction experiments on a single crystal. Comparison of NaFeGe 2 O 6 with NaFeSi 2 O 6 is given and it is shown that the magnetic ordering in the latter compound, aegirine, also is complex and is best described by two different spin states, a commensurate one with C 2′/ c ′ symmetry and an incommensurate one, best being described by a spin density wave, oriented within the (1 0 1) plane.

CoGeO 3 was synthesized at 1,273 and 1,448 K using ceramic sintering techniques in the monoclinic and orthorhombic modification, respectively. The two compounds were analysed by magnetic susceptibility measurements and neutron diffraction in order to study magnetic ordering and spin structures at low temperature. The monoclinic form of CoGeO 3 has C 2/ c symmetry and orders magnetically below 36 K with a small negative paramagnetic Curie temperature θ P  = −4.6 (2) K. The magnetic structure can be described with k  = (1, 0, 0) in the magnetic space group C 2′/ c ′ having a ferromagnetic spin arrangement within the chains of M1 sites, but a dominating antiferromagnetic coupling between the chains. At the M1 sites the magnetic spins are aligned within the a–c plane forming an angle of 120° with the + a -axis and they are not parallel to the spins at M2. Here spins are also ferromagnetically coupled within, but antiferromagnetically coupled between the M1/M2 site bands. The orthorhombic phase of CoGeO 3 displays Pbca symmetry and transforms to an antiferromagnetically ordered state [ θ P  = −18.6(2) K] below 33 K. The magnetic spin structure can be described with k  = (0, 0, 0) in space group Pbca ′ and it is similar to the one of the C 2/ c phase except that it is non-collinear in nature, i.e. there are components of the magnetic moment along all three crystallographic axes. Small magneto-elastic coupling is observed in the orthorhombic phase.

A synthetic clinopyroxene with composition LiFe 3+ Ge 2 O 6 , monoclinic s.g. P 2 1 / c , a  = 9.8792(7), b  = 8.8095(5), c  = 5.3754(3) Å, β  = 108.844(6)°, V  = 442.75(16) Å 3 , has been studied by in situ low- and high-temperature single-crystal X-ray diffraction. The variation of lattice parameters and the intensity of the b -type reflections ( h  +  k  = 2 n  + 1, only present in the P -symmetry) with increasing temperature showed a displacive phase transition from space group P 2 1 / c to C 2/ c at a transition temperature T tr  = 789 K, first order in character, with a sudden volume increase of 1.6% and a decrease of β by 1° at the transition. This spontaneous dilatation is reversible, shows a limited hysteresis of ±10°C, and corresponds to the vanishing of the b -type reflections, thus indicating a symmetry increase to space group C 2/ c . Below T tr an expansion is observed for all the cell parameters, while the β angle remained almost constant; at T  >  T tr the thermal volume expansion is due to dilatation of the structure in the plane, mostly along [0 1 0], and pure shear in the (0 1 0) plane due to the decrease of β . From comparison with silicate analogues, the germanate clinopyroxenes are more expansible, while the P 2 1 / c expands more than the C 2/ c phase. The evolution of Q 2 (calculated as the normalized intensity of b -type reflections) with T in the framework of the Landau theory has been done using a standard expression for a first order phase transition. We observe a jump of Q 0 2  = 0.538(2) at T tr , with T c of 481(7) K, b / a  = −2,290 K, and c / a  = 3,192 K, and thus far from being tri-critical point. A closely related composition (LiFe 3+ Si 2 O 6 ) shows an equivalent phase transition at 228 K, which is very close to the tri-critical point and 561 K cooler. This result indicates that a change in the composition of tetrahedral sites can have dramatic effects on the P 2 1 / c  ↔  C 2/ c displacive phase transition in clinopyroxenes. The major changes observed in the evolution of the crystal structure with T are observed in the M2 polyhedron, with a volume decrease by ca. 13.3%, compared to ca. 1.3% observed in the M1 polyhedron. The tetrahedra behave as rigid units with neither a significant change of volume at T  >  T tr (<1‰), nor a change of tilting of the basal plane. No change in coordination is observed at T  >  T tr in the M2 polyhedron, which remains sixfold coordinated although a strong deformation of this polyhedron is observed. This deformation is related to a strong change by 51.4° at T tr of the kinking angle (O3–O3–O3 angle) of the B-chain of tetrahedra, which switches from O-rotated to S-rotated [from 143.3(5)° to 194.7(6)°]. The A-chain is S-rotated at T  <  T tr [206.8(5)° at 703 K] and extends by 12° at the transition.

What can we learn from nature?The study of the physical, chemical and structural properties of well-known minerals in the geo- and biosphere creates new opportunities for innovative applications in technology, environment or medicine.

In the field of the renewables, a large effort has been devoted in the last years to obtain conventional and new materials for solar energy conversion by using methods which couple a good efficiency and scalability with energetic and environmental concerns. This research has included the so-called kesterites, materials considered interesting for the thin-film solar cell technology, consisting of relatively abundant and harmless elements: Cu3-x-yFexZnySn(S,Se)4. In this study, we undertook the synthesis of members of the kuramite-stannite (Cu3SnS4-Cu2FeSnS4) join by means of a two-step solvothermal approach, able to provide nanocrystalline products in an easy, low-temperature, and fast way. The sample with the highest Fe concentration was characterised by means of a multi-analytical approach, aimed to assess not only its final structural, chemical and micromorphological features, but also the redox speciation of the two transition metal cations, i.e. Cu and Fe, in relation to the overall charge balance. Namely, Electron Paramagnetic Resonance (EPR), Mössbauer and X-ray Absorption Spectroscopy (XAS) and SQUID magnetometry were involved. The main results point out an excellent control of the structural features, and an intermediate Fe content in the sample, leading to the following formula unit: Cu2.2Fe0.48Sn1.2S4. The overall findings of the multi-analytical characterization imply a complex redox balance, where inferring the site occupancy is not trivial; the charge balance, in fact, can only be achieved taking into account the presence of both Fe(III) and vacancies. Moreover, Fe is distributed over two different crystallographic sites.

The electronic and magnetic structure of the chain silicate orthoferrosilite Fe 2 2+ Si 2 O 6 has been investigated by electronic structure calculations in the local spin density approximation. All calculations are based on experimentally determined geometrical data at room temperature. The calculated spin-allowed d – d excitation energies and hyperfine parameters are in quantitative agreement with the respective experimental data from optical absorption and Mössbauer spectroscopy. Inside one ribbon that is parallel to the crystallographic c axis and contains two non-equivalent M1 and M2 sites, all iron spins are ferromagnetically coupled with coupling constants of about +16 cm −1 . Between these ribbons within the ( b ,  c )-plane a weak ferromagnetic coupling of about +2 cm −1 is obtained. Neighboured ( b ,  c )-planes are coupled antiferromagnetically via chains of Si B -tetrahedra but ferromagnetically via chains of Si A -tetrahedra. Such a theoretically determined \"double-plane antiferromagnetic\" spin structure is at variance with an experimentally derived magnetic structure. This discrepancy is attributed to differences between the geometry at room temperature and at temperatures below the Néel temperature currently not available.

A natural iron bearing omphacite with composition Ca 0.49 Na 0.525  Mg 0.424 Al 0.43 Fe 0.076 2+ Fe 0.056 3+ Ti 0.0024 Mn 0.005 Si 1.989 O 6 from Syros, Greece, has been investigated by Mössbauer spectroscopy. The interpretation of the spectra is based on electronic structure calculations in the local spin density approximation. The calculations emphasize that large clusters, extending beyond the second coordination sphere of iron, are necessary for a reliable description. As suggested by the electronic structure calculations, different environments around the Fe 2+ octahedra give rise to slightly different hyperfine parameters, especially affecting the quadrupole splitting. Hence, the measured spectrum has been evaluated based on quadrupole splitting distribution. The calculated values including the temperature dependence are in almost quantitative agreement with the experimentally derived values.

An analytical expression for the transfer integral H^sub AB^ between the localized magnetic orbitals in superexchange-coupled dimers as a function of the type of atoms and geometry of the molecule has been derived by explicitly including orbital interactions. It is shown that H^sub AB^ plays the key role for understanding magneto-structural correlations. The reliability and capability of this approach is confirmed by comparison with numerical electronic structure calculations in the local spin-density approximation on singly and doubly bridged Cu(II)-dimers with fluorine ligands. All results can be calculated and understood within the analytical formalism representing, therefore, a powerful tool for understanding the magneto-structural correlations and for constructing magnetic orbitals analytically. [PUBLICATION ABSTRACT]