Crystal structure and phase transition of n-alkyl ammonium dihydrogen phosphate CnH2n+1NH3H2PO4 (n=2∼6) crystals

Structure analyses in the ferroelastic crystals of C2H5NH3H2PO4 (C(2)ADP), C3H7NH3H2PO4 (C(3)ADP) and C4H9NH3H2PO4 (C(4)ADP) were performed by X-ray diffraction at room temperature. Space group of these three crystals was P2(1)/n (Z=8). The rule of even and odd effects about the number of carbon atoms in n-alkyl ammonium chain was found as the difference of packing of the organic chains between the layers of PO4 tetrahedron. In C(3)ADP and C(4)ADP crystals, all H-atoms were refined as the ordered state with the isotropic thermal parameters. This result from the structure analyses, that no disordered atom was observed at room temperature, corresponds with the result that no anomaly was observed by preliminary DSC measurements in C(n)ADP (n = 3 similar to 6) at low temperature range to -180 degrees C.