Mild hydrothermal synthesis, structure and characterization of the vanadyl phosphate hydrate Pb(VOPO4)2·3H2O: the formation of spin dimers in a three dimensional crystal structure

Abstract

Crystals of the vanadyl phosphate hydrate Pb(VOPO4)2·3H2O were synthesized by mild hydrothermal treatment and characterized by X-ray diffraction techniques, infrared spectroscopy, XPS, ICP, magnetic and heat capacity measurements. A new structure type (space group P21/c (14), a = 10.292(4) A, b = 7.552(3) A, c = 12.557(5) A, β = 90.007(4)°, Z = 4 and V = 976.0(7) A3) has been found. Pb(VOPO4)2·3H2O exhibits a 3D open-framework structure where the lead and water molecules were located orderly in the channel. Magnetic measurements indicated the antiferromagnetic properties and the presence of predominant short-range interactions in the investigated temperature range of 2–300 K, which was further confirmed by heat capacity measurements. The magnetic exchange pathways between the V4+ magnetic ions are via the bridging phosphates and can therefore be classified as a V–O⋯O–V super–superexchange. Magnetic analysis shows that Pb(VOPO4)2·3H2O is an interesting spin gapped antiferromagnet in a pure SSE system with a 3D open-framework crystal structure. Analysing the exchange couplings based on the extended Huckel tight binding calculations, it was possible to identify one dominant spin dimer interaction in Pb(VOPO4)2·3H2O, which effectively produces a lower magnetic dimensionality. The magnetic data fit also shows that Pb(VOPO4)2·3H2O can be taken to be a magnetic spin dimer system with inter-dimer coupling. It is a good example of effective magnetic dimensionality being reduced from the geometric characteristics of the crystal structure. In addition, a known compound β-PbVO2PO4 can be synthesized with a different ratio of reactants, yet keeping all the other conditions the same as for Pb(VOPO4)2·3H2O. Both compounds are formed as high quality crystals. For comparison, the infrared spectroscopy and XPS of β-PbVO2PO4 and Pb(VOPO4)2·3H2O are reported.