The present invention relates to a process for preparing a material having the formula:+4\u22124:where M is a cation selected from the group consisting of monovalent cations, divalent cations, and trivalent cations; H is hydrogen, P2O7 is the pyrophosphate anion, and y is a number between 2. The movable design objects are unplaced. This layer has a layer thickness of at least 15 \u03bcm and has a specific heat capacity per volume that is at least a factor of 1.