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The alluaudite-type of structure is of huge research interest as an open matrix ensuring fast alkali-metal ion mobility, a property that could contribute to the development of novel electrode materials for rechargeable alkali-metal ion batteries. In this contribution, we provide new data on the formation of well-crystallized sodium manganese sulfates Na 2+d Mn 2Àd/2 (SO 4 ) 3 with an alluaudite-type of structure by simple dehydratation of the corresponding dihydrate Na 2 Mn(SO 4 ) 3 $2H 2 O with a kröhnkite-type of structure. The structure of Na 2+d Mn 2Àd/2 (SO 4 ) 3 is determined on the basis of Rietveld refinement of powder XRD patterns, infrared (IR) and Raman spectroscopy and electron paramagnetic resonance at X-and Q-band frequencies (EPR). From a structural point of view, the release of two H 2 O molecule from the kröhnkite phase takes place by a transformation of the infinite [Mn(SO 4 ) 2 (H 2 O) 2 ] chains into Mn 2 O 10 dimers bounded by distorted Na(1)O-polyhedra. The anhydrous sulfates are able to participate in the electrochemical reaction delivering a reversible capacity of 135 mA h g À1 , when they are used as cathode materials in lithium ion cells. The stability of the alluaudite phase Na 2+d Mn 2Àd/2 (SO 4 ) 3 in the lithium electrolyte solution and the mechanism of the electrochemical reaction are discussed on the basis of ex situ EPR, IR and Raman spectroscopy. This is a first report on electrochemical activity of manganese-based sulfate with an alluaudite-type of structure.

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Elpidite from the Lovozero alkaline complex, Kola Peninsula, Russia, and Ag-exchanged forms of elpidite from two different localities (Lovozero and Khan Bogdo, Mongolia) were studied by means of single-crystal X-ray diffraction, electron microprobe analysis, thermogravimetry and IR spectroscopy. All studied samples retain the heteropolyhedral framework consisting of double Si6O15 chains (ribbons) and isolated ZrO6 octahedra. Zeolitic cavities in the initial elpidite from Lovozero (space group Pbm2, a = 14.6127(7), b = 7.3383(4), c = 7.1148(3) Å, V = 762.94(6) Å3) are occupied by Na+ cations and H2O molecules. Both Ag-exchanged forms are characterized by evident distortions of the heteropolyhedral framework and a strongly disordered arrangement of extra-framework cations which results in the appearance of the 14-14-14 Å unit cell (a = 14.1755(7), b = 14.6306(9), c = 14.2896(7) Å, V = 2963.6(3) Å3 for the Ag-exchanged form of elpidite from Lovozero and a = 14.1411(5), b = 14.5948(4), c = 14.3035(5) Å, V = 2952.04(17) Å3 for the Ag-exchanged form of elpidite from Khan Bogdo) and space group Cmce. Elpidite from both localities demonstrates a high exchange capacity to Ag. Exchanged Ag+ cations preferably occupy the sites that are close to the Na sites in the initial elpidite. The paper also contains a review of crystal chemical data on elpidite and its laboratory-modified forms.

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