Crystal Structure and Phase Transformations of Sodium Diarsenate, Na₄As₂O₇

Abstract

Sodium diarsenate at room temperature is monoclinic with lattice parameters a = 5.969(1), b = 10.126(1), c = 12.272(3) A and P = 92.87(2)". There are four formula units per cell with space group C2/c. The structure was refined by full-matrix least-squares to a final R value of 0.027 utilizing 1157 reflections measured on a Syntex P i automatic diffractometer. The anion, consisting of two AsO, tetrahedra sharing a common oxygen atom, with a bridging angle of 123.5" at that oxygen atom, lies on a two-fold axis. The bridging As-0 bond lengths are 1.783 A and the average terminal As-0 bond length is 1.658 8, uncorrected for thermal motion. Both Na ions show fivefold coordination to oxygen atoms. One of these sodium ions has a long interaction (2.852 A) with the bridging oxygen atom of the anion. Na,As207 undergoes a series of phase transformations between room temperature and 220 "C, and a further major transformation near 680 "C.Le sodium diarsenate est monoclinique a la temperature ambiante avec comme parametres du reseau a = 5.969(1), b = 10.126(1), c = 12.272(3) A et P = 92.87(2)". La maille contient quatre unites et le groupe spatial est C2/c. La structure a ete raffinte par la matrice des moindres carres jusqu'a une valeur finale R de 0.027, apres utilisation de 1157 reflexions mesurtes sur un diffractometre Syntex Pi auto-

Crystal structure of CaK2As2O7 and CdK2P2O7

“…The average terminpl bond length is 1.5 13 A in Na4P207 and 1.510 A in CdK2P207. The diarsenate configuration also compares very well in the present compound and in Na4As207 (11). It is nearly staggered in both compounds with the bridging angle differing by about 3".…”

Crystal structure of sodium dihydrogen arsenate

“…With respect to sodium arsenates, numerous crystal structures have been determined so far, including arsenic in tetrahedral and/or in octahedral coordination by oxygen atoms. Arsenate structures with arsenic exclusively in tetrahedral coordination resemble those of the related phosphates and in some cases show isotypism with them (marked by an asterisk): Na 3.25 (AsO 4 )(OH) 0.25 (H 2 O) 12 * (Tillmanns & Baur, 1971), Na 4 (AsO 4 )OH (zur Loye et al, 2015), Na 2 (HAsO 4 )(H 2 O) 7 * (Baur & Khan, 1970;Ferraris et al, 1971), Na(H 2 AsO 4 )(H 2 O) , Na 3 (H 2 As 3 O 10 ) (Driss & Jouini, 1990), Na 4 As 2 O 7 (Leung & Calvo, 1973), Na(AsO 3 ) (Liebau, 1956) and Na 5 (AsO 5 ) (Haas & Jansen, 2001). Arsenate structures with arsenic in (complete or partial) octahedral coordination include Na(H 2 As 3 O 9 ) (Driss, Jouini, Durif et al, 1988), Na 3 (H 5 As 4 O 14 ) (Driss & Jouini, 1989), Na(HAs 2 O 6 ) (Dung & Tahar, 1978), Na 2 As 4 O 11 (Driss, Jouini & Omezzine, 1988) and Na 7 As 11 O 31 (Guesmi et al, 2006).…”

Structure of potassium arsenate K5As3O10

“…Condensed alkali arsenates generally contain chains or rings of corner-sharing AsO 4 tetrahedra. (LiAsO3) x (von Hilmer & Dornberger-Schiff, 1956) and (NaAsOa) x (Liebau, 1956) contain chains of infinite length, whereas Na4As207 (Leung & Calvo, 1973) contains pyroarsenate groups, built up from two AsO 4 tetrahedra.…”