Among them, Na 3 V 2 (PO 4 ) 3 (NVP) has been identified so far as the most interesting one as it possesses satisfactory energy density and high power for extended cycle life. Its crystal structure can be described as a 3D framework of VO 6 octahedra and PO 4 tetrahedra connected to each other by common corners forming so-called "lantern units" along the c direction of the commonly used hexagonal cell. Sodium cations were described as randomly disordered over two sodium sites (Na(1), 6b and Na(2), 18e) [18] until Chotard et al. discovered that below 280 K, the so called α-NVP form crystallized in a monoclinic superstructure due to a fully ordered distribution of Na +[19] similar to the ordering previously reported in α-Na 3 Ti 2 (PO 4 ) 3 . [51] The synthesis of Na 3 V 2 (PO 4 ) 3 was first reported by Delmas. [20] Gopalakrishnan [21] later on reported on the possible extraction of three Na + toward the novel sodium-free V IV V V (PO 4 ) 3 composition. Afterward, the electrochemical extraction of Na + from Na 3 V 2 (PO 4 ) 3 to NaV 2 (PO 4 ) 3 (with a theoretical capacity of 117.6 mAh g −1 at 3.4 V vs Na/Na) was extensively investigated. [22][23][24][25][26] A large number of special treatments (e.g., carbon coating, particle shape controlling) was also proposed to improve battery performances. [27][28][29][30][31] It is important to note that only 2Na formula unit −1 have been completely removed from the structure during charging up to now. A possible activation of the V 4+/5+ redox couple at higher voltages may also contribute to the increasing of the energy density of NVP-based materials, as demonstrated in a series of works by using a metal substitution of a part of V 3+ in the structure of NVP. In recent years, several elements have been chosen for the partial substitution of V into the crystal structure of this promising material (such as Ni, [32][33][34] Al, [35,36] Fe 3+ , [34,37] Zr 4+ , [38] Mn 3+ , [39] Mn 2+ , [34,40] Cr 3+ , [41][42][43] Ti 4+ , [44][45][46] Mo 6+ , [47] and Mg 2+[48] ).In this work, Mn 2+ was used as a substituting ion to enhance the capacity of the Na 3 V 2 (PO 4 ) 3 cathode material. Inspired by the recent work of Zhou et al., [34] nearly single-phase Na 4 MnV(PO 4 ) 3 (98.5 wt%) powders were synthesized and studied structurally and electrochemically in details. In operando X-ray diffraction (XRD) studies during electrochemical operation show for the first time that Na 4 MnV(PO 4 ) 3 can deliver 156 mAh g −1 toward the new composition NaMnV(PO 4 ) 3 . Results and DiscussionThe crystal structure of Na 4 MnV(PO 4 ) 3 has been fully determined using high-resolution synchrotron powder XRD (SXRD) dataThe mixed Mn 2+ /V 3+ Na-super-ionic-conductor (NASICON) cathode material Na 4 MnV(PO 4 ) 3 is prepared by solid-state reaction at 800 °C under argon. When used as a positive electrode in Na batteries, this material can exchange three electrons for two transition metals, that is, yielding a high gravimetric capacity of 156 mAh g −1 on charge when the upper cutoff voltage is set to 4.3 V...

The new mineral saranchinaite, ideally Na2Cu(SO4)2, was found in sublimates of the Saranchinaitovaya fumarole, Naboko Scoria Cone, Tolbachik volcano, Kamchatka, Russia. Its discovery and study has enabled the characterization of the thermal decomposition of kröhnkite and provided an insight into the high-temperature behaviour of other kröhnkite-type materials. Saranchinaite is monoclinic, P21, a = 9.0109(5), b = 15.6355(8), c = 10.1507(5) Å, β = 107.079(2)°, V = 1367.06(12) Å3, Z = 8 and R1 = 0.03. Saranchinaite is a unique mineral in that two of its four independent Cu sites display a very unusual Cu2+ coordination environment with two weak Cu–O bonds of ~2.9–3.0 Å, resulting in [4+1+2] CuO7 polyhedra. Each of the Cu-centred polyhedra shares common corners with SO4 tetrahedra resulting in a [Cu4(SO4)8]8– framework with a complex channel system occupied by Na atoms. Saranchinaite is sensitive to moisture and transforms into kröhnkite within one week when exposed to open air at 87% relative humidity and 25°C. High-temperature X-ray diffraction studies were performed for both kröhnkite (from La Vendida mine, Antofagasta Region, Chile) and saranchinaite. During thermal expansion kröhnkite retains its strongly anisotropic character up to its full dehydration and the formation of saranchinaite at ~200°C, which then transforms back into kröhnkite after exposure to open air. The thermal expansion of saranchinaite is more complex than that of kröhnkite. Saranchinaite is stable up to 475°C with subsequent decomposition into tenorite CuO, thénardite Na2SO4 and unidentified phases.