EFFECT OF IRON SUBSTITUTION ON THE STRUCTURAL, ELECTRIC, AND MAGNETIC-PROPERTIES IN R-TYPE PBFEXV6-XO11, A FRUSTRATED SYSTEM

Single-phase material of the solid solution PbFexV6-xO11 was prepared by solid-state reaction over the range 1 less than or equal to x less than or equal to 1.75. Powder preparations of PbV6O11, however, were a lways accompanied by a small amount of impurity. The noncentrosymmetri c space group P63mc of PbV6O11, which has an R-block structure, remain s unchanged with iron substitution. The crystal structure of PbFe1.75V 4.25O11 was refined from single-crystal X-ray data to R = 0.049. The i ron atoms are located preferentially in specific crystallographic site s. The triangular-bipyramidal site is completely occupied by Fe3+ whil e the interstices of the central octahedral layer are solely occupied by the vanadium V(1) atoms. The other two crystallographic sites have mixed Fe/V occupancies. Magnetic measurements performed on nearly sing le-phase PbV6O11 and the iron substituted phases reveal a spin-glass-t ype behavior with freezing temperatures T-f of 50 and 65 K, respective ly. T-f is independent of the iron ratio. This spin-disordered system is attributed to strong frustration occuring in the a-b plane because of the existence of V(1) trimers providing alternatively long and shor t V-V distances along the a axis. Resistance and Seebeck coefficient m easurements indicate p-type conduction above T-f while the system beco mes an insulator below T-f. Fe3+ ions do not participate in the hoppin g process, and they block conduction parallel to the c axis because of their location in the bridging dimeric octahedral and bipyramidal sit es. (C) 1997 Academic Press.