Effect of Iron Substitution on the Structural, Electric, and Magnetic Properties inR-Type PbFexV6−xO11, a Frustrated System

Single-phase material of the solid solution PbFe_xV_6−xO₁₁was prepared by solid-state reaction over the range 1≤x≤1.75. Powder preparations of PbV₆O₁₁, however, were always accompanied by a small amount of impurity. The noncentrosymmetric space groupP6₃mcof PbV₆O₁₁, which has anR-block structure, remains unchanged with iron substitution. The crystal structure of PbFe_1.75V_4.25O₁₁was refined from single-crystal X-ray data toR=0.049. The iron atoms are located preferentially in specific crystallographic sites. The triangular–bipyramidal site is completely occupied by Fe³⁺while 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 single-phase PbV₆O₁₁and the iron substituted phases reveal a spin-glass-type behavior with freezing temperaturesT_fof 50 and 65 K, respectively.T_fis independent of the iron ratio. This spin-disordered system is attributed to strong frustration occuring in thea–bplane because of the existence of V(1) trimers providing alternatively long and short V–V distances along theaaxis. Resistance and Seebeck coefficient measurements indicatep-type conduction aboveT_fwhile the system becomes an insulator belowT_f. Fe³⁺ions do not participate in the hopping process, and they block conduction parallel to thecaxis because of their location in the bridging dimeric octahedral and bipyramidal sites.