Revisiting Purported Solvothermal “Black Phosphorus”: Beyond Elemental Allotrope to Organic Ammonium Polyphosphides

Black phosphorus, featuring a distinctive layered structure and exceptional properties, has driven the exploration of scalable fabrication strategies, among which solvothermal synthesis is pursued for its mild conditions. However, the true identity of the resulting products remains obscured by persistent misinterpretations. In this work, we systematically revisit the purported solvothermal "black phosphorus", particularly that synthesized in ethylenediamine. In contrast to authentic black phosphorus, this material exhibits intrinsic discrepancies that challenge its conventional assignment as an elemental allotrope, thereby clarifying the origins of such confusion. Employing continuous rotation electron diffraction, we have determined the crystallographic parameters of its average structure, revealing a quasi-one-dimensional architecture with significant disorder. Multimodal characterizations redefine this material as a salt-like assembly of ethylenediammonium polyphosphides. Further structural rationale is guided by the Baudler rules of phosphorus catenation and generalized to a broader series of organic ammonium polyphosphide hybrids that feature an invariant backbone. While acknowledging the reported functional potential of these materials, this study prompts a fundamental reappraisal of the structure-property relationship, establishing an interpretive framework for the rational design of phosphorus-based materials.