A new flexible ultramicroporous solid, La(H₅DTMP)·7H₂O (1), has been crystallized at room temperature using the tetraphosphonic acid H₈DTMP, hexamethylenediamine-N,N,N′,N′-tetrakis(methylenephosphonic acid). Its crystal structure, solved by synchrotron powder X-ray diffraction, is characterised by a 3D pillared open-framework containing 1D channels filled with water. Upon dehydration, a new related crystalline phase, La(H₅DTMP) (2) is formed. Partial rehydration of 2 led to La(H₅DTMP)·2H₂O (3). These new phases contain highly corrugated layers showing different degrees of conformational flexibility of the long organic chain. The combination of the structural study and the gas adsorption characterization (N₂ and CO₂) suggests an ultramicroporous flexible framework. NO isotherms are indicative of a strong irreversible adsorption of NO within the pores. Impedance data indicates that 1 is a proton-conductor with a conductivity of 8 × 10⁻³ S cm⁻¹ at 297 K and 98% of relative humidity, and an activation energy of 0.25 eV.

We review the progress in metal phosphate structural chemistry focused on proton conductivity properties and applications. Attention is paid to structure–property relationships, which ultimately determine the potential use of metal phosphates and derivatives in devices relying on proton conduction. The origin of their conducting properties, including both intrinsic and extrinsic conductivity, is rationalized in terms of distinctive structural features and the presence of specific proton carriers or the factors involved in the formation of extended hydrogen-bond networks. To make the exposition of this large class of proton conductor materials more comprehensive, we group/combine metal phosphates by their metal oxidation state, starting with metal (IV) phosphates and pyrophosphates, considering historical rationales and taking into account the accumulated body of knowledge of these compounds. We highlight the main characteristics of super protonic CsH₂PO₄, its applicability, as well as the affordance of its composite derivatives. We finish by discussing relevant structure–conducting property correlations for divalent and trivalent metal phosphates. Overall, emphasis is placed on materials exhibiting outstanding properties for applications as electrolyte components or single electrolytes in Polymer Electrolyte Membrane Fuel Cells and Intermediate Temperature Fuel Cells.