The isomorphous pair of hydrated salts Na2HAsO4.7H20 and Na2HPO4.7H20 crystallizes in space group P21/n, Z=4 with a=9.40 (9.26), b= 11"03 (11"01), c= 10"67 (10"44)/~ and fl=95.5 (95.6) ° (the values for the phosphate compound are in parentheses). The crystal structures have been determined and refined with the use of three-dimensional counter diffractometer data to R=0-048 (0.081) for 1162 (1252) Fnk~. The m~st prominent feature of the structure is a column of face and edge sharing coordination octahedra which has the composition [NaE(OH2)7] 2+ and is hydrogen bonded to the tetrahedral groups AsO3OH (or PO3OH). The salts should therefore be formulated as [Na2 (OH2)7] [XO3OH]. A plausible hydrogen bonding scheme which accounts for all hydrogen atoms has been established from steric considerations. The average Na-O(w) distances differ in the two compounds significantly: in the arsenate salt Na-O(w) it is 2.437/~, in the phosphate it is 2.418/~. The shapes of the orthoarsenate and orthophosphate groups in these two structures as well as in other phosphate and arsenate compounds display a systematic deviation from the ideal tetrahedral angles: the 'tetrahedral' angles are the larger the shorter the bonds are, which form their sides, and vice versa, while the O-O distances (the edges of the tetrahedra) stay relatively constant.
(NH4)2HPO4 crystallizes in the space group P21/c with cell constants a = 11"043 (6), b = 6.700 (3), c = 8.031 (4)/~, fl = 113"42 (3) ° and Z= 4, and is isomorphous with (NHa)2HAsO4. The structure is refined using three-dimensional data collected on a four-circle automatic diffractometer. The final R value is 0.015 for 649 observed reflections. The positions of the hydrogen atoms indicate that only four among five N. • • O contact distances (less than 3.2/~ from each NH4 + ion) represent the actual N-H... O bonds. The length of the O-H..-O bond is 2.615 (1) A and differs significantly from 2.669 (13)/~ found in the isomorphous arsenate. It is suggested that this is a consequence of the size difference between the ps+ and As 5+ ions.
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