The synthesis, crystal structure and vibrational spectra of α-Sr(PO3)2 containing an unusual catena-polyphosphate helix

The phosphate α-Sr(PO3)2 was synthesised as a single-phase crystalline powder starting from strontium carbonate and phosphorous acid at a temperature of 1150 K. α-Sr(PO3)2 crystallises in a novel structure type in which helical PO3−catena-polyphosphate ions interpenetrate a substructure of Sr2+ closely related to cubic diamond (Sr4(PO3)8, P21, Z=2, a=735.37(8), b=1868.2(2), c=735.81(8)pm, β=90.021(9)°). The catena-polyphosphate chains form helices running along [010]. Their chain-periodicity is 16 and the length of the repeating unit complies with b=1868pm, the chain stretching factor amounts to fs=0.48 and the pitch to radius ratio is ξ=2.49. The vibrational spectra of α-Sr(PO3)2 are also reported.