Formation of magnetic clusters in the strongly frustrated hollandite vanadium oxide V7.22O8(OH)8Cl0.77(H3O)2.34

Magnetic properties of the first hollandite-type vanadium oxide containing anions in the 2x2 channels, V7.22O8(OH)(8)(Cl)(0.77)(H3O)(2.34), are studied via static (dc) and dynamic (ac) magnetic susceptibilities and heat capacity. At 25 K a rapid increase of magnetic susceptibility is observed; the field-cooled and zero-field-cooled susceptibilities diverge below this temperature and magnetization shows a pronounced hysteresis. The temperature dependence of dc susceptibility follows the Curie-Weiss law above 350 K; the effective magnetic moment per vanadium ion is consistent with the 3+ oxidation state. The negative Curie-Weiss temperature Theta approximate to-336 K indicates strong antiferromagnetic exchange; combined with low transition temperature, this reveals significant spin frustration. The real chi(') and imaginary chi(') components of the ac susceptibility show two frequency-dependent maxima at 17.5 and 7 K that shift toward lower temperatures with decreasing frequency. The temperature dependences of the spin relaxation time tau in various magnetic fields H were obtained by the Cole-Cole method and analyzed using the scaling law. A very slow relaxation with characteristic times of the order of 10(-3) s was found, consistent with fluctuation of cluster magnetic moments. Temperature dependence of the specific heat does not reveal long-range magnetic order down to 2 K. Magnetic specific heat shows a broad maximum centered at 25 K, which is consistent with formation and blocking of cluster magnetic moments. Magnetic exchange pathways in the hollandite structure are analyzed, and a mechanism of magnetic cluster formation is proposed.