This paper presents a new mixed valence heptavanadate (NH₄)₂V₇O₁₆, obtained from a two-stage treatment in a single container of ammonium metavanadate with hexadecylamine in an acetic medium. The hydrolysis of the precursor under normal conditions leads to an intermediate, layered hybrid nanocomposite, V₂O₅/alkylamine, which after an in situ hydrothermal treatment is transformed almost quantitatively into an amine-free microcrystalline phase constituted by perfect square microparticles of (NH₄)₂V₇O₁₆. The analysis of composition, structure and morphology of the product points to a mixed valence vanadium oxide with a high content of V_(IV) (approximately 73%). The microsquares have a tetragonal structure similar to that of BaV₇O₁₆, as well as to those proposed for the VO_x/amine hybrid nanocomposite series: nanotubes, nano urchins and the compound (en)V₇O₁₆. The results suggest that all these phases belong to the V₇O₁₆ family, but that they differ in the amine content, the degree of reduction and the curvature of the network. The feasibility of obtaining flat nanostructures based on V₇O₁₆ without templates, beyond demonstrating the robustness of the structural unit V₇O₁₆ in networks with different degrees of reduction and curvatures, clarifies the role of alkylamines in this type of anti-entropic supramolecular process. First, the amine provides a stable platform that allows for an orderly reduction of the network under hydrothermal conditions and, when the medium favours the stability of the V-amine bond, the alkylamine contributes to the driving force that leads to the curving of the V-O network.