A New Series of Heptamolybdonickelate(IV) and Manganate(IV). Investigation of the Unusual Magnetic and Electronic Spectral Behavior of the Nickel(IV) Compound

Two new heteropolymolybdates of nickel(IV) and manganese(IV) of composition K2H8NiMo7O28·6H2O and K2H8MnMo7O28·4H2O with a hitherto unknown 1 : 7 (Ni/Mn : Mo) stoichiometry are reported. X-Ray powder patterns show that they are isomorphous. The electronic spectrum of the nickel(IV) compound exhibits four absorption bands at 16.4, 17.5, 18.3, and 24×103 cm−1, and the room temperature magnetic moment of the compound is abnormally high, 1.78 μB. The rationale for these observations is that there are Ni3+ impurity which occupy interstitial positions in the crystal lattice and as a consequence of dynamic Jahn-Teller distortion the high-spin 4A2g(4T1g) state of the Ni3+ ion lies close to the low-spin ground state 2A1g(2Eg) to get thermally populated to an appreciable extent leading to significant triplet state contribution.