(NH4)image: A new mixed valence vanadium(III,IV) fluoro-arsenate with ferromagnetic interactions and electronic conductivity

A new mixed valence vanadium(III,IV) fluoro-arsenate compound, with formula (NH4)image and KTP structure-type, has been synthesized by mild hydrothermal techniques. The crystal structure has been solved from single crystal X-ray diffraction data in the Pna21 orthorhombic space group. The unit-cell parameters are a=13.196(2) Å, b=6.628(1) Å and c=10.7379(7) Å with Z=8. The final R factors were R1=0.0438 and wR2=0.0943 [all data]. The crystal structure consists of a three-dimensional framework formed by (VIII,IVO4F2) octahedra and (AsO4)3− tetrahedra arsenate oxoanions. The vanadium(III,IV) cations, from the (VIII,IVO4F2) octahedra, are linked through the fluorine atoms giving rise to zigzag chains. The ammonium cations are located in the cavities of the structure compensating the anionic charge of the image inorganic skeleton. The thermal stability limit of the phase is 345 °C, around to this temperature the ammonium cation and fluoride anion are lost. The IR spectrum shows the characteristic bands of the (NH4)+ and (AsO4)3− ions. Magnetic measurements indicate the existence of weak ferromagnetic interactions. Electronic conductivity, via a hopping mechanism, occurs with an activation energy of 0.66 eV.