REFINEMENT OF THE CRYSTAL STRUCTURE OF β-Mg₂P₂O₇

Abstract

'The crystal s t r~~c t~~r e of Mg?PrG.i reported by Lukaszewicz has been reiined by least squares analysis. Data of the lzkl type with 0 < k ,< 6 have been ~~s e d to obtain improved atomic parameters. The central oxygen atom of the P?O?-' is found to have enhanced thermal motion ill the plane perpendicular to the 1' -P vector. Thus the P-0-P bold angle appears to be linear in this phase because of a thermal averaging of the position of the central oxygen atom. This conclusion is in agreement with that obtained by Lazarev from inlrared studies.

Crystal structure of Cd2P2O7

“…The cation coordination numbers are the same as inaMg2P207 (16) but there the anion has essentially a staggered config~~ration while in Cd2P207 it is nearest to an eclipsed configuration. In all the A2B207 compounds, where the B 2 0 7 anion is formed by corner sharing of a pair of tetrahedra, the eclipsed anion configuration appears to be stabilized by having one of the cations sharing oxygen atoms across the anion.…”

Crystal chemistry of Mg2P2O7.nH2O, n = 0, 2 and 6: magnesium-oxygen coordination and pyrophosphate ligation and conformation

“…In the dihydrate (Oka & Kawahara, 1982) and in the a-anhydride (Calvo, 1967), octahedra share edges to form infinite zigzag chains, which are crosslinked through the tetrahedra via shared vertices. In the fl-anhydride (Calvo, 1965), rings of six octahedra are formed by edge sharing, and each ring is linked to two neighboring rings, also by edge sharing. Only pyrophosphate oxygens are shared in the Mg--O--Mg bridges between octahedra; each water molecule in the hydrate structures ligates only one magnesium ion.…”

“…Octahedral distortion reaches an extreme in the fl-anhydride (Calvo, 1965), in which the Mg--O distances range from 2.06 to 2.14A in the one octahedron, but from 1.99 to 2.12 to 3.35 A in the other 'octahedron'. The latter is perhaps better described as a distorted square pyramid, which shares a pair of edges with two neighborin~ octahedra, and shares one vertex and has a 3.35 A nonbonded Mg---O contact with a third octahedron.…”

“…The dihydrate (Oka & Kawahara, 1982) crystallized under hydrothermal conditions at -700 K from a mixture of precipitated MgHPO4.3H30 (newberyite) and H3PO4. Both anhydrous phases (Calvo, 1967(Calvo, , 1965Lukaszewicz, 1967Lukaszewicz, , 1961 crystallized from melts obtained after thermal decomposition of precipitated MgNH4PO4.6H20 (struvite). The lowtemperature a-phase and the high-temperature fl-phase (isostructural with thortveitite, Sc2Si207) * To whom correspondence should be addressed.…”

Crystal structure of β-Cu2P2O7

“…This can be seen by the appearance A number of pyrophosphates, Mn2P207 (I), Zn2P207 (2), and Mg2P207 (3), are known to have the thortveitite structure (4) as their high temperature modifications. These compounds show linear P-0-P groups in the P2074-anion with cations having irregular octahedral coordination.…”