Isomorphous crystals: K2HPO4 and K5Na(HPO4)3

The crystal of dipotassium hydrogenphosphate, K2HPO4, are almost isomorphous with crystals of pentapotassium sodium tris(hydrogenphosphate), K5Na(HPO4)3. In both, the three crystallographically independent HPO42-anions are held together by O11-H11..O33 and O31-H31..O23 hydrogen bonds, forming chains. These chains are joined by symmetry-related hydrogen bonds, O24-H24..O21i and O21-H21..O24ii in K2HPO4 and K5Na(HPO4)3, respectively, forming infinite helical chains running parallel to the c axis. The K+ cations have different numbers of neighbours (from six to eleven) and the Na+ cations are six-coordinated. The P-O(H) distances range from 1.596(5) to 1.625(3) A and the P-O(terminal) distances range from 1.502(3) to 1.536(3) A. (orig.)