Crystal Structure of Ammonium Catena ‐polyphosphate IV [NH 4 PO 3 ] x

Phase‐pure ammonium catena ‐polyphosphate IV [NH 4 PO 3 ] x was synthesized by heating NH 4 H 2 PO 4 in a tube furnace under an ammonia gas flow. The product contained single crystals of [NH 4 PO 3 ] x IV appropriate for an X‐ray structure determination enabling structure refinement of this compound. The pseudo‐merohedrally twinned crystals of [NH 4 PO 3 ] x crystallize in the monoclinic crystal system ( P 2 1 / c , no. 14, a = 2270.3(5), b = 458.14(9), c = 1445.1(3) pm, β = 108.56(3)°, Z = 4, 2264 data, R 1 = 0.076). In the unit cell the catena ‐polyphosphate chain anions run parallel [010] with a chain‐periodicity P = 2 and a stretching factor f s = 0.94. The chain anions are interconnected through extensive hydrogen bonding towards the ammonium ions. Due to ‘chemical twinning’ a novel catena ‐polyphosphate structure type is realized in [NH 4 PO 3 ] x IV. The vibrational spectra of [NH 4 PO 3 ] x IV are reported as well.