One-dimensional antiferromagnetic behavior in AVOPO₄ (A=NH ₄, Na) prepared from hydrothermal conditions

Two vanadyl phosphates have recently been prepared in superheated aqueous solutions. These materials crystallize as infinite chains of corner sharing VO₆ octahedra. The magnetic susceptibility of NaVOPO₄ and NH₄VOPO₄ has been recorded over the 2-300 K temperature region. At higher temperatures, both compounds exhibit Curie-Weiss paramagnetism, where C=0.416 emu K/mole, θ=-31 K; C=0.417 emu K/mole, θ=-4.7 K, for NaVOPO₄ and NH₄VOPO₄, respectively. At lower temperatures, there is a maximum in the temperature-dependent magnetic susceptibility of each compound. The data for the two compounds have been successfully analyzed following the Bonner-Fisher numerical calculations that describe the magnetic behavior of antiferromagnetically coupled S=1/2 one-dimensional chains. The results of the Bonner-Fisher analysis of the magnetic data are g=2.00, J/k=-18.6 for NaVOPO₄; and g=2.1, J/k=-3.26 K, for NH₄VOPO₄. The theoretical analysis of the NH₄VOPO₄ analog also required the addition of a molecular field correction (zJ'/k=-2.1 K) to the Bonner-Fisher calculation in order to adequately fit the data. The trans crosslinking via phosphate bridges of the magnetic chains in NH ₄VOPO₄ gives rise to the onset of long-range antiferromagnetic ordering at T_AF=3.71 K.