Unusual Metal−Insulator Transitions in the LaTi1-xVxO3 Perovskite Phases

Abstract

A series of LaTi1-xVxO3 compounds (0 ≤ x ≤ 1) have been prepared by the dc arc-melting method and characterized by conventional powder X-ray diffraction, thermogravimetric analysis, four-probe resistivity, and magnetic susceptibility studies. Selected members of this series have been characterized by synchrotron X-ray diffraction, neutron diffraction, selected area electron diffraction. Samples in the 0.2 ≤ x ≤ 0.3 region are ∼3−4% La deficient (e.g., La0.97Ti0.8V0.2O3). Like the LaMO3 end members where M = Ti, V, the LaTi1-xVxO3 phases are barely orthorhombic (almost tetragonal) and adopt the perovskite-type GdFeO3 structure (space group Pnma). Most of the LaTi1-xVxO3 phases are antiferromagnetic (AF) insulators including LaTiO3 (TN = 148 K) and LaVO3 (TN = 140 K). For the LaTi1-xVxO3 phases in the 0 < x < 0.10 region, the AF ordering temperatures and resistivities are reduced with increasing vanadium concentration. The 0.10 ≤ x ≤ 0.25 region comprises poorly metallic (ρ298 ≈ 10-2 Ω cm), paramagnetic phases that represent rare examples of B-site-substituted metallic perovskites. Magnetic susceptibility studies in this region show Curie−Weiss behavior below 200 K with large temperature independent susceptibilities of ca. 10-3 emu/mol. Variable-temperature synchrotron X-ray diffraction experiments on the La0.97Ti0.8V0.2O3 phase showed a marked orthorhombic distortion below 150 K but a nearly tetragonal cell at 298 K. Rietveld refinements of neutron diffraction data for this phase at 50 K showed a prototypical GdFeO3 structure with disordered Ti and V (refined occupancies of 82(1)% and 18(1)%, respectively) distributed over the octahedral site and a slight La deficiency (refined occupancy = 97(1)%). The structural parameters (bond distances, angles, cell constants) are intermediate to those of the LaMO3 end members as expected. Electron diffraction experiments on the La0.97Ti0.8V0.2O3 phase revealed diffraction patterns that were also consistent with the prototypical Pnma cell with no evidence of microdomain ordering or supercell formation. In the 0.25 < x ≤ 1.0 region, the samples again show AF ordering and insulating behavior. The magnetic susceptibilities of the samples in this region show unusual peaking that is compositionally dependent. The field-cooled magnetic susceptibility of the x = 0.90 and 1.0 samples show large diamagnetic signals of unexplained origin. The zero-field cooled susceptibilities of these samples showed paramagnetic behavior. The formation of metallic phases from B-site substitution is discussed in terms of the Hubbard model.