| Raman spectroscopic measurements have been carried out and compared for three ternary vanadate compounds: Mg2CrV3O11-x, Zn2CrV3O11-x, and Ni2CrV3O11-x. Each compound gave a distinctive Raman spectrum dominated by the vanadium-Óoxo and chromium-Óoxo stretching bands in the 800 - 1000 cm-1 region. Photoluminescence spectra have revealed possible cooperative emission bands of Ni2+ and Cr3+ ion pairs. Electron paramagnetic resonance (EPR) and magnetic susceptibility measurements of the Ni2CrV3O11-x compound have been carried out and analyzed. The EPR spectra of the Cr3+-Ni2+ and Ni2+-Ni2+ pairs have been observed and confirmed by static molar magnetic susceptibility measurements. In order to describe the EPR spectra of interacting pairs, the spin Hamiltonian has been used in its usual form -2JABSASB, but assuming a strong exchange limit. The temperature dependence of the magnetic susceptibility of Ni2+-Cr3+-Ni2+ trimers has been analyzed using standard Van Vleck formula. This analysis yielded an exchange constant, J1/kB, between Ni2+-Cr3 of - 32(5)K, in good agreement with JCr-Ni/kB = - 32(2)K calculated from the EPR results. Considerably positive exchange constant for Ni2+-Ni2+ species has been found, J2/kB = 90(7)K. Assuming that local g-factors are isotropic, we have calculated local Zeeman factors gCr = 1.977 and gNi = 2.157, which are in good agreement with the values observed in the other Cr3+-Ni2+ complexes. |
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