An arsenate with the α-CrPO₄structure type, NaCa_1–xNi_3–2xAl_2x(AsO₄)₃(x= 0.23): crystal structure, charge-distribution and bond-valence-sum analyses

Abstract

Since the discovery of electrochemically active LiFePO, materials with tunnel and layered structures built up of transition metals and polyanions have become the subject of much research. A new quaternary arsenate, sodium calcium trinickel aluminium triarsenate, NaCaNiAl(AsO) (x = 0.23), was synthesized using the flux method in air at 1023 K and its crystal structure was determined from single-crystal X-ray diffraction (XRD) data. This material was also characterized by qualitative energy-dispersive X-ray spectroscopy (EDS) analysis and IR spectroscopy. The crystal structure belongs to the α-CrPO type with the space group Imma. The structure is described as a three-dimensional framework built up of corner-edge-sharing NiO, (Ni,Al)O and AsO polyhedra, with channels running along the [100] and [010] directions, in which the sodium and calcium cations are located. The proposed structural model has been validated by bond-valence-sum (BVS) and charge-distribution (CHARDI) tools. The sodium ionic conduction pathways in the anionic framework were investigated by means of the bond-valence site energy (BVSE) model, which predicted that the studied material will probably be a very poor Na ion conductor (bond-valence activation energy ∼7 eV).

Structural and Spectroscopic Studies of NaCuCr2(PO4)3: A Noncentrosymmetric Phosphate Belonging to the α-CrPO4-Type Compounds

“…They are identified with the general chemical formula □ 3 M 3 (XO 4 ) 3 , where □ is a vacancy that can be filled by monovalent and/or divalent cations, introducing an oxidation state decrease of the transition metal M and XO 4 referring to tetrahedral entity, respectively. They all crystallize with a centrosymmetric Imma space group. − Interestingly, some of them with the following general formula A 2 M 2+ 2 M 3+ (PO 4 ) 3 , where A is an alkali and M are metals able to show an oxidation equal to 2 and/or 3, were recently described as low-temperature polymorphs of the alluaudite-type structure. , …”