Deepening the shear structure FeNb11O29: influence of polymorphism and doping on structural, spectroscopic and magnetic properties

FeNb11O29 is an intriguing and promising material that has been emerging in the last few years. It is isostructural with Nb12O29, one of the rare compounds in which Nb displays a local magnetic moment and shows both antiferromagnetic ordering and metallic conductivity at low temperatures. Both the two polymorphic monoclinic and orthorhombic forms have a mono-dimensional magnetic arrangement, but the different disposition of the structural building blocks leads to a strong frustration phenomenon of the magnetic order in the high-temperature orthorhombic form. Whereas Nb12O29 has been widely studied, barely few magnetic data can be found on its analogous FeNb11O29, for which a role of the Fe3+ localized d electrons in affecting the original magnetic behaviour can be foreseen. In this paper, we report how we synthesized undoped and, for the first time, Mn- and V-doped FeNb11O29. Both the monoclinic and orthorhombic polymorphs, stable in different temperature ranges, were then thoroughly structurally characterized. With the help of micro-Raman spectroscopy, we investigated the differences introduced into the vibrational levels by doping, while EPR data allowed us to obtain information on the transition metal ions and to point out the related peculiar structural features. Static magnetization measurements evidenced the paramagnetic character of the compounds and the high-spin configuration of Fe3+ ions.