We have studied the magnetic structure of the high symmetry vanadyl pyrophosphate ((VO)2P2O7, VOPO), focusing on the spin exchange couplings, using density functional theory (B3LYP) with the full three-dimensional periodicity. VOPO involves four distinct spin couplings: two larger couplings exist along the chain direction (a-axis), which we predict to be antiferromagnetic, JOPO = -156.8 K and JO = -68.6 K, and two weaker couplings appear along the c (between two layers) and b directions (between two chains in the same layer), which we calculate to be ferromagnetic, Jlayer = 19.2 K and Jchain = 2.8 K. Based on the local density of states and the response of spin couplings to varying the cell parameter a, we found that JOPO originates from a super-exchange interaction through the bridging -O-P-O- unit. In contrast, JO results from a direct overlap of 3dx2 - y2 orbitals on two vanadium atoms in the same V2O8 motif, making it very sensitive to structural fluctuations. Based on the variations in V-O bond length as a function of strain along a, we found that the V-O bonds of V-(OPO)2-V are covalent and rigid, whereas the bonds of V-(O) 2-V are fragile and dative. These distinctions suggest that compression along the a-axis would have a dramatic impact on JO, changing the magnetic structure and spin gap of VOPO. This result also suggests that assuming JO to be a constant over the range of 2-300 K whilst fitting couplings to the experimental magnetic susceptibility is an invalid method. Regarding its role as a catalyst, the bonding pattern suggests that O2 can penetrate beyond the top layers of the VOPO surface, converting multiple V atoms from the +4 to +5 oxidation state, which seems crucial to explain the deep oxidation of n-butane to maleic anhydride.
All Science Journal Classification (ASJC) codes
- Physics and Astronomy(all)
- Physical and Theoretical Chemistry