| Abstract: | Contributions on Crystal Structures and Thermal Behaviour of Anhydrous Phosphates. XXIII. Preparation, Crystal Structure, and Thermal Behaviour of the Mercury(I) Phosphates α-(Hg2)3(PO4)2, β-(Hg2)3(PO4)2, and (Hg2)2P2O7 Light-yellow single crystals of (Hg2)2P2O7 have been obtained via chemical vapour transport in a temperature gradient (500 °C → 450 °C, 23 d) using Hg2Cl2 as transport agent. Characteristic feature of the crystal structure (P2/n, Z = 2, a = 9,186(1), b = 4,902(1), c = 9,484(1) Å, β = 98,82(2)°, 1228 independent of 5004 reflections, R(F) = 0,066 for 61 variables, 7 atoms in the asymmetric unit) are Hg22+-units with d(Hg1–Hg1) = 2,508 Å and d(Hg2–Hg2) = 2,519 Å. The dumbbells Hg22+ are coordinated by oxygen, thus forming polyhedra (Hg12)O4] and (Hg22)O6]. These polyhedra share some oxygen atoms. In addition they are linked by the diphosphate anion P2O74– (ecliptic conformation; ∠(P,O,P) = 129°) to built up the 3-dimensional structure. Under hydrothermal conditions (T = 400 °C) orange single crystals of the mercury(I) orthophosphates α-(Hg2)3(PO4)2 and β-(Hg2)3(PO4)2 have been obtained from (Hg2)2P2O7 and H3PO4 (c = 1%). The crystal structures of both modifications have been refined from X-ray single crystal data α-form (β-form): P21/c (P21/n), Z = 2 (2), a = 8,576(3) (7,869(3)), b = 4,956(1) (8,059(3)), c = 15,436(3) (9,217(4)) Å, β = 128,16(3) (108,76(4))°, 1218 (1602) independent reflections of 4339 (6358) reflections, R(F) = 0,039 (0,048) for 74 (74) variables, 8 (8) atoms in the asymmetric unit]. In the structure of α-(Hg2)3(PO4)2 three crystallographically independent mercury atoms, located in two independent dumbbells, are coordinated by three oxygen atoms each. Thus, (Hg2)O6] dimers with a strongly distorted tetrahedral coordination of all mercury atoms are formed. Such dimers are present besides (Hg2)O5]-polyhedra in the less dense crystal structure of β-(Hg2)3(PO4)2 (d(Hg–Hg) = 2,518 Å). The mercury(I) phosphates are thermally labile and disproportionate between 200 °C (β-(Hg2)3(PO4)2) and 480 °C (α-(Hg2)3(PO4)2) to elemental mercury and the corresponding mercury(II) phosphate. |
