Frustrated magnetism and the effects of titanium substitution in the mixed-valence oxide Ba V10-x Tix O15

The low temperature magnetic phase transitions in Ba V10 O15 at ∼40 K and ∼25 K are investigated in detail. The topology of the V sublattice, which consists of linked V10 clusters formed by four tetrahedra sharing edges and corners, leads to geometric magnetic frustration and a depression of the ordering temperature characterized by the frustration index, f= θ Tc ∼30. Specific heat data show the coexistence of both long range and short range magnetic order at low temperature. The position of a λ peak, identified with Tc for long range antiferromagnetic order, depends on the V2+ V3+ ratio and varies from 43.0 K to 41.6 K for samples with the highest V2+ content but is suppressed to zero for highly oxidized samples. A broad maximum in the specific heat occurs at ∼25 K and is present in all samples regardless of the level of oxidation. For samples with a well defined Tc, estimates of the entropy removal over the range 5 K to 55 K are only 11% of that expected for 8 V3+ (S=1) and 2 V2+ (S=3 2) ions per formula unit, suggesting the presence of considerable short-range order. Neutron diffraction data support the presence of long range antiferromagnetic (AF) order with Tc ∼40 K. The magnetic structure below Tc is complex: there are five crystallographically independent vanadium ions in the unit cell, leading to 40 magnetic ions in the crystal unit cell and 80 spins per magnetic cell. Furthermore, the magnetic structure is of the multi- k type involving the two unrelated wave vectors k= (0 0 0) and k= (1 2 0 0). Zero field muon spin relaxation data show spin freezing below 30 K. ac susceptibility results are anomalous. Two peaks, at ∼0 K and ∼25 K are seen in χ′ but the maxima are frequency independent as is the prominent maximum in χ″ near 25 K, behavior atypical of spin glasses. Ti-doped samples, Ba V10-x Tix O15, show remarkable results as only 5% Ti doping is sufficient to destroy the AF long range order and induce conventional spin glass behavior in the dc and ac susceptibility, with freezing temperatures ranging from 15 K-5 K for doping levels from 5% to 50%. The frustration indices reach enormous values to f=130 for x=3. The spin glass state is quenched for 70% Ti doping and only paramagnetic behavior is seen.