NOVELTY - An ion/proton conducting membrane comprises a substrate; and a crystalline ion-conducting thin film having a thickness (t), where the thin film has single apatite crystals each having its c-axis oriented normal to the substrate in the form of a gas tight film, where each of the single crystals has a crystal domain that spans the film thickness (t).    USE - For making an ion-/proton conducting membrane used in hydrogen fuel cell (claimed), electrochemical membrane reactors, and in chemical sensors.    ADVANTAGE - The membrane has a proton conductivity of greater than or equal to 1E-6 Siemens per centimeter over a temperature of 300-900 degrees C. The membrane has selectively oriented crystal c-axes that facilitate (and are intended to optimize) ion/proton transport. Compared to current ceramic membranes, the membrane is relatively thin and has single crystal domains spanning the membrane thickness. The membrane is also sufficiently dense to provide a gas-tight barrier. By optimizing proton transport, a fuel cell incorporating such a membrane operates at lower temperatures than is currently possible with standard ceramic membranes.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are included for the following:    (1) a hydrogen fuel cell comprising an anode, a cathode, and an ion/proton-conducting membrane disposed intermediate the anode and the cathode, where the ion/proton-conducting membrane further comprises a palladium hydrogen substrate, and a hydroxyapatite (HAP) thin film having a thickness (t); and    (2) a method for making an ion-/proton conducting membrane, involving creating a seeded surface by depositing a sufficiently dense HAP seed layer on a hydrogen membrane cathode electrode; hydrothermally synthesizing a c-axis-oriented, crystalline HAP film on the seeded surface in a secondary, single crystallization normal to the seeded surface; and hydrothermally densifying the HAP film in an a-axis orientation perpendicular to the c-axis orientation in a tertiary crystallization to form a gas tight thin film.