Mercury Phosphates with the Triangular Hg4+3 Cluster: (Hg3)3(PO4)4 and (Hg3)2(HgO2)(PO4)2

The crystal structures of two calcium aluminogallates have been investigated by single-crystal X-ray diffraction methods. Ca2AlGaO5 is a quenched high-pressure phase prepared at 1250°C and 2.5 GPa. The compound belongs to the brownmillerite structure type (space group I2mb, a=5.2823(6) ?, b=14.613(2) ?, c=5.4739(8) ?, V=422.5(9) ?3, Z=4, Dcalc=4.04 g cm-3, wR2=0.072 for 586 reflections with I>2(I)). Main building units are (1) layers of perovskite-type corner-connected MeO6octahedra perpendicular to [010] and (2) zweier single chains of MeO4-tetrahedra running parallel to [100] (Me=Al, Ga). The stacking of the layers and sheets of single chains results in a three-dimensional network with Ca ions in the interstitial voids. Single crystals of CaAlGaO4 (space group P121/c1, a=7.9817(1) ?, b=8.7415(1) ?, c= 10.4430(2) ?, =94.84(2)°, V= 726.0(3) ? 3, Z=8, Dcalc=3.67 g cm-3, wR2=0.079 for 971 reflections with I>2(I)) were synthesized at ambient pressure and a temperature of 1500°C. The structure consists of layers of S6R of MeO4-tetrahedra perpendicular to [100]. The stacking of the layers parallel to the a-axis results in a framework with channels occupied by the Ca cations. The sequence of up (U) and down (D) pointing apices of adjacent tetrahedra within a single ring is UUDUDD. CaAlGaO4 is isotypic with the monoclinic modification of CaGa2O4.