Pb_3Fe~(3+)_2(PO_4) _4(H_2O), a new octahedral-tetrahedral framework structure with double-strand chains

摘要

A new lead iron(III) hydrated phosphate, Pb_3Fe _2(P0_4)_4(H_2O), has been synthesised hydrothermally in Teflon-lined stainless steel autoclaves at 220 0C for 7 days, with an initial pH of 1.5. It is the first example of a synthetic hydrous Pb-Fe~(3+) phosphate. Crystals are small, colourless to white prisms, uniaxial (+) and non-pleochroic. The calculated refractive index for white light is n = 1.95. Single-crystal structure determination (R(F) = 0.0457) shows Pb_3Fe~2(PO_4)_4(H_2O) to be tetragonal, space group P4_1{2_1{2 (no. 92), with a = 9.0440(10), c = 16.766(3) ?, V = 1371.4(3) ?~3 and Z = 4. Pb_3Fe_2(PO_4MH_2O) has a structure type which is based on a tetrahedral-octahedral framework of FeO_6 octahedra sharing corners with PO_4 tetrahedra, with Pb atoms and H_2O molecules occupying voids in the framework. Pb_3Fe _2(PO_4)_4(H_2O) is homeotypic with synthetic Pb_3Cr_2(PO_4)4; both compounds share the same heteropolyhedral topology and the same space group, but only Pb _3Fe_2(PO_4)_4(H20) contains an additional position occupied by a water molecule. The octahedral-tetrahedral chain of Pb_3Fe_2(PO_4)_4(H _2O) is topologically similar to double-strand chains found in the structures of hannayite, galliskiite, kapundaite, PbIn(AsO_4)(AsO _3OH) and Na_(2.88)Fe(PO_4)_2. Infrared spectroscopy is used to describe the vibrational properties of Pb _3Fe_2(PO_4)_4(H_2O).