Unusual Metal−Insulator Transitions in the LaTi₁_-<i>_x</i>V<i>_x</i>O₃ Perovskite Phases

Abstract

A series of LaTi 1 - x V x O 3 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., La 0.97 Ti 0.8 V 0.2 O 3 ). Like the LaMO 3 end members where M = Ti, V, the LaTi 1 - x V x O 3 phases are barely orthorhombic (almost tetragonal) and adopt the perovskite-type GdFeO 3 structure (space group Pnma ). Most of the LaTi 1 - x V x O 3 phases are antiferromagnetic (AF) insulators including LaTiO 3 ( T N = 148 K) and LaVO 3 ( T N = 140 K). For the LaTi 1 - x V x O 3 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 (ρ 298 ≈ 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 La 0.97 Ti 0.8 V 0.2 O 3 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 GdFeO 3 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 LaMO 3 end members as expected. Electron diffraction experiments on the La 0.97 Ti 0.8 V 0.2 O 3 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.