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) Å, b = 7.552(3) Å, c = 12.557(5) Å, β = 90.007(4)°, Z = 4 and V = 976.0(7) ų) 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 Hückel 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.