Modeling of the Magnetic Susceptibilities of the Ambient- and   High-Pressure Phases of (VO)_{2}P_{2}O_{7}
  The magnetic susceptibilities chi versus temperature T of powders and single crystals of the ambient-pressure (AP) and high-pressure (HP) phases of (VO)_{2}P_{2}O_{7} are analyzed using an accurate theoretical prediction of chi(T, J1, J2) for the spin-1/2 antiferromagnetic alternating-exchange (J1, J2) Heisenberg chain. The results are consistent with recent models with two distinct types of alternating-exchange chains in the AP phase and a single type in the HP phase. The spin gap for each type of chain is derived from the respective set of two fitted alternating exchange constants and the one-magnon dispersion relation for each of the two types of chains in the AP phase is predicted. The influences of interchain coupling on the derived intrachain exchange constants, spin gaps, and dispersion relations are estimated using a mean-field approximation for the interchain coupling. The accuracies of the spin gaps obtained using fits to the low-T chi(T) data by theoretical low-T approximations are determined. The results of these studies are compared with previously reported estimates of the exchange couplings and spin gaps in the AP and HP phases and with the magnon dispersion relations in the AP phase measured previously using inelastic neutron scattering. 
D.C. Johnston, T. Saito, M. Azuma, M. Takano, T. Yamauchi, Y. Ueda