Refinement of the structure of sodium Kurrol salt (NaPO3)x, type A

Abstract

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.

Structural and physico-chemical analysis of calcium/strontium substituted, near-invert phosphate based glasses for biomedical applications

“…The measured Na-O coordination number of 3.77 is smaller than the value of 5.0 for crystalline -NaPO 3 [90], but the crystal has a longer average bond length, 2.43 Å, compared to the average for the glass, 2.36 Å, and so the crystal should be expected to have a larger coordination number. In a neutron diffraction study of sodium metaphosphate glass, Pickup et al [37] found a Na-O coordination number of 4.0 at a single distance of 2.33 Å, whilst in a combined neutron and X-ray diffraction study Hoppe et al [91] found an asymmetric distribution of Na-O distances with a total coordination number of 4.20.…”

Structural study of Al2O3-Na2O-CaO-P2O5 bioactive glasses as a function of aluminium content

“…The starting parameters used in the fitting process have been chosen on the basis of previous studies 1,12 and the chemical database of inorganic crystals. [26][27][28][29][30][31][32][33][34] The relative concentrations of the various aluminium environments were determined both from the MAS NMR data and corresponding neutron diffraction data. From the 27 Al MAS NMR spectra for each sample acquired at external B 0 field strengths of 9.4 and 14.1 T (see Figure 5), it is readily observed that aluminium speciation is described by 4-, 5-, and 6-fold coordinated oxo environments in each of these glass systems.…”

Effects Of Mno-Doping On The Structure Of Sodium Metaphosphate Glasses

“…This effect can be attributed to a decrease of the average P-O-P bond angle. The average P-O-P bond angle decreases from 130 • in crystalline NaPO 3 (Kurrol salt type A) [35] to 127.5…”

“…2 shows T (r) for the pure NaPO 3 glass with and without the Kaplow, Averbach, and Strong correction [26]. The average nearest-neighbour P-O, O-O and P-P distances in crystalline NaPO 3 (Kurrol salt type A) [35] are 1.54(1)Å, 2.57(7)Å and 2.91(4)Å, respectively. Therefore, the peaks at about 1.5Å, 2.5Å and 2.9Å in T (r) of the MN0 glass can be attributed to the P-O, O-O and P-P average nearest-neighbour distances in the glass structure.…”