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A full-matrix least-squares refinement using Jost's 1961 data reduced R(hkl) for 760 reflexions to 0.076.The refined bond lengths of the (PO3)~ chain, P-O (bridge) 1.610 and P-O (terminal) 1.479/~, agree with results for (RbPO3)z.The crystal structure of sodium Kurrol salt, type A, a polyphosphate of composition (NaPO3)z, was determined by Jost (1961). The structure was then refined by means of difference syntheses using only hOl and hkO data, although sign determination was carried out by direct methods for the layers hll to h51 also. The present three-dimensional refinement by least squares uses all the data except the separate hkO reflexions, which are probably more affected by absorption than the others. No absorption corrections were applied.Of a possible 1000 reflexions in the layers hOl to h51 (ca. 60% of the Cu sphere), 760 were observed. The starting point of the new refinement, which was carried out (in Glasgow) with the full-matrix leastsquares program of Smith and Cruickshank, was the set of atomic coordinates and isotropic vibrational parameters from the earlier refinement. Atomic form factors for neutral atoms were taken from International Tables for X-ray Crystallography (1962). The weighting scheme used was of the form Vw= ([1 --exp (-PI( sin20)/22)]where the constants took the final values P1=500, 1°2=0.01, P3=0.001 and P4=0.00001. Initially, atomic coordinates, isotropic vibrational parameters and layer scale factors were refined. After three cycles, the vibrational parameters were allowed to vary anisotropically, with the layer scale factors, but not the overall scale factor, held constant at their converged values. Complete convergence was attained in six more cycles; final R(hkl) =0.076 for the 760 observed reflexions. A final difference synthesis showed no peaks over 1 e./~ -3. Librational corrections were considered, but are of rather dubious value in an infinite chain structure; however, calculations based on the P207 group and individual PO4 tetrahedra were carried out; the results, which gave corrections of less than a half an e.s.d., were not applied. The final atomic coordinates, vibrational parameters and their e.s.d, are given in Table 1.

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