UNUSUAL METAL-INSULATOR TRANSITIONS IN THE LATI1-XVXO3 PEROVSKITE PHASES

A series of LaTi1-xVxO3 compounds (0 less than or equal to x less than or equal to 1) have been prepared by the de are-melting method and ch aracterized by conventional powder X-ray diffraction, thermogravimetri c analysis, four-probe resistivity, and magnetic susceptibility studie s. Selected members of this series have been characterized by synchrot ron X-ray diffraction, neutron diffraction, selected area electron dif fraction. Samples in the 0.2 less than or equal to x less than or equa l to 0.3 region are similar to 3-4% La deficient (e.g., La0.97Ti0.8V0. 2O3) Like the LaMO(3) end members where M = Ti, V, the LaTi1-xVxO3 pha ses are barely orthorhombic (almost tetragonal) and adopt the perovski te-type GdFeO3 structure (space group Pnma). Most of the LaTi1-xVxO3 p hases are antiferromagnetic (AF) insulators including LaTiO3 (T-N = 14 8 K) and LaVO3 (T-N = 140 K). For the LaTi1-xVxO3 phases in the O < x < 0.10 region, the AF ordering temperatures and resistivities are redu ced with increasing vanadium concentration. The 0.10 less than or equa l to x less than or equal to 0.25 region comprises poorly metallic (rh o(298) approximate to 10(-2) Omega cm), paramagnetic phases that repre sent rare examples of B-site-substituted metallic perovskites. Magneti c susceptibility studies in this region show Curie-Weiss behavior belo w 200 K with large temperature independent susceptibilities of ca. 10( -3) emu/mol. Variable-temperature synchrotron X-ray diffraction experi ments on the La0.97Ti0.8V0.2O3 phase showed a marked orthorhombic dist ortion below 150 K but a nearly tetragonal cell at 298 K. Rietveld ref inements of neutron diffraction data for this phase at 50 K showed a p rototypical GdFeO3 structure with disordered Ti and V (refined occupan cies of 82(1)% and 18(1)%, respectively) distributed over the octahedr al site and a slight La deficiency (refined occupancy = 97(1)%). The s tructural parameters (bond distances, angles, cell constants) are inte rmediate to those of the LaMO(3) end members as expected. Electron dif fraction experiments on the La0.97Ti0.8V0.2O3 phase revealed diffracti on patterns that were also consistent with the prototypical Pnma cell with no evidence of microdomain ordering or supercell formation. In th e 0.25 < x less than or equal to 1.0 region, the samples again show AF ordering and insulating behavior. The magnetic susceptibilities of th e samples in this region show unusual peaking that is compositionally dependent. The field-cooled magnetic susceptibility of the x = 0.90 an d 1.0 samples show large diamagnetic signals of unexplained origin. Th e zero-field cooled susceptibilities of these samples showed paramagne tic behavior. The formation of metallic phases from B-site substitutio n is discussed in terms of the Hubbard model.