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    STUDIA PHYSICA - Issue no. SPECIAL%20ISSUE / 2001  
         
  Article:   X AND W ESR STUDIES OF TRANSITION METAL CLUSTERS ENCAPSULATED IN SANDWICH-TYPE POLYOXOMETALATES.

Authors:  O. COZAR, L. DAVID, VASILE CHIŞ.
 
       
         
  Abstract:  The series of sodium/potassium salts of the sandwich-type heteropolyanions [Mn+3(H2O)x(BiW9O33)2](183n) (Mn+ = (VO)II, x = 0 and Mn+ = CrIII, MnII, FeIII, CoII, NiII, CuII, x = 3) was investigated by X and W bands ESR spectroscopy. In the complex with vanadium(IV) the unpaired electrons are trapped on the parent ions (gII = 1.899, g = 1.974, AII = 184 G, A = 69 G) and the VIV ions are prevalently dipolar coupled. In the case of the other trinuclear clusters, the metallic coupling is based on the superexchange mechanism realized through the heteropolyoxometalate frame. The isotropic exchange constants were estimated for the CrIII (J = 4.116 cm1) and MnII (J = 2.074 cm1) clusters from the temperature dependance of the linewidths of the HF-EPR signals. The g tensor is isotropic for the complexes with CrIII, MnII and FeIII (g  2.0), axial for the CuII cluster (gII = 2.080, g = 2.237) and rhombic for the NiII complex (gx = 2.201, gy = 2.211, gz = 2.292). The zero field splitting tensor is rhombic for the clusters of MnII (D = 0.381 cm1, E = 0.054 cm1 for the spin state S = 7/2) and NiII (D = 1.66 cm1, E = 0.18 cm1 for the spin state S = 1). Two species with different degrees of the rhombical distortion have been identified in case of the FeIII (D1 = 1.5 cm1, E1 = 0.12 cm1 and D2 = 1.5 cm1, E2 = 0.5 cm1) and CoII (gx1 = 5.021, gy1 = 3.561, gz1 = 2.401 and gx2 = 5.776, gy2 = 3.813, gz2 = 2.850) samples.  
         
     
         
         
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