The mixed-valent mercury compounds Hg3(HAsO4)2 (= HgIIHgI 2(HAsO4)2) and Hg6As2O10 (= HgI 2HgII 2(AsO4)2·2HgIIO) were obtained from the reaction of (Hg3)3(AsO4)4 with arsenic acid (60 wt-%) carried out at 65 ºC and as a by-product under hydrothermal conditions, respectively. The crystal structure of Hg3(HAsO4)2 (Z = 2, C2/c, a = 13:010(3), b = 8:149(2), c = 9:496(3) Å, b = 126:864(4)º, 1427 structure factors, 64 parameters, R[F2 > 2 s(F2)] = 0:0272; wR (all F2) = 0.0490) contains a

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... ic O-Hg-Hg-O unit (d(Hg-Hg) = 2.5226(8) Å, d(Hg-O) = 2.141(5) Å), ∠(Hg-Hg-O) = 167:68(13)º) and an Hg2+ cation that is located on a twofold rotation axis showing an almost linear coordination to two O atoms (d(Hg-O) = 2.059(5) Å; ∠(O-Hg-O) = 177:3(2)º). The O-Hg-Hg-O and O-Hg-O units are linked by AsO3OH tetrahedra resulting in the formation of chains extending parallel to [102]. Strong hydrogen bonding (d(O···O) = 2.523(6) Å) between AsO3OH tetrahedra of neighbouring chains is observed. The crystal structure of Hg6As2O10 (Z = 2, P21=n, a = 6:94750(10), b = 8:9837(2), c = 8:9966(2) Å, b = 98:1630(10)º, 4842 structure factors, 83 parameters, R [F2 > 2 s(F2)] = 0:0269, wR (all F2) = 0.0577) contains an O-Hg-Hg-O unit (d(Hg-Hg) = 2.5337(3) Å, d(Hg-O) = 2.181(3) Å, ∠(Hg-Hg-O) = 168:05(10)º) and two O- Hg-O units (d(Hg-O) = 2.048(3), 2.072(3) Å, ∠(O-Hg-O) = 168:78(15)º; 2.149(4), 2.150(3) Å, 166:65(12)º) that are linked into a three-dimensional framework structure with As atoms of the AsO4 tetrahedra in the voids. An alternative structural description of Hg6As2O10 is presented under consideration of oxygen-centred [OHg4] tetrahedra.