There is increasing evidence that amorphous inorganic materials play a key role in biomineralisation in many organisms, however the inherent instability of synthetic analogues in the absence of the complex in vivo matrix limits their study and clinical exploitation. To address this, we report here an approach that enhances long-term stability to >1 year of biologically relevant amorphous metal phosphates, in the absence of any complex stabilisers, by utilising pyrophosphates (P 2 O 7 4À ); species themselves ubiquitous in vivo. Ambient temperature precipitation reactions were employed to synthesise amorphous Ca 2 P 2 O 7 .nH 2 O and Sr 2 P 2 O 7 .nH 2 O (3.8 < n < 4.2) and their stability and structure were investigated. Pair distribution functions (PDF) derived from synchrotron X-ray data indicated a lack of structural order beyond $8 A in both phases, with this local order found to resemble crystalline analogues. Further studies, including 1 H and 31 P solid state NMR, suggest the unusually high stability of these purely inorganic amorphous phases is partly due to disorder in the P-O-P bond angles within the P 2 O 7 units, which impede crystallization, and to water molecules, which are involved in H-bonds of various strengths within the structures and hamper the formation of an ordered network. In situ high temperature powder X-ray diffraction data indicated that the amorphous nature of both phases surprisingly persisted to $450 C. Further NMR and TGA studies found that above ambient temperature some water molecules reacted with P 2 O 7 anions, leading to the hydrolysis of some P-O-P linkages and the formation of HPO 4 2À anions within the amorphous matrix. The latter anions then recombined into P 2 O 7 ions at higher temperatures prior to crystallization. Together, these findings provide important new materials with unexplored potential for enzyme-assisted resorption and establish factors crucial to isolate further stable amorphous inorganic materials. IntroductionA growing awareness of the existence and potential of amorphous inorganic materials has led to a rapid increase in their study in recent years. Often previously neglected or undetected as a consequence of their lack of long-range structural order and relative instability, amorphous inorganic materials are now more readily evident, partly through the continuing development of techniques sensitive to local order, such as magic angle spinning (MAS) solid state NMR and atomic pair distribution functions. Of particular interest has been the discovery of their prevalence in a diverse range of biological systems, 1-8 where it appears organisms exploit their unique properties in a number of processes.Amorphous calcium carbonate (ACC) is one of the most widely studied amorphous materials in nature, 1,2 with numerous studies on marine creatures, 9 where ACC has been shown to be a transient precursor to hard external shell formation. In effect, the highly unstable and soluble ACC is used as a reactive store of calcium and carbonate ions whose transformation in...

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The compounds (Ti1‐xZrx) (HPO4)2·H2O (x = 0—1) are synthesized from aqueous solutions of TiCl4, ZrOCl2, and H3PO4 (autoclave, 150 °C, 7 d) and characterized by synchrotron powder XRD and pair distribution function analysis of high energy synchrotron X‐ray total scattering data.