Electrically controlled transformation of memristive titanates into mesoporous titanium oxides via incongruent sublimation.

Perovskites such as SrTiO3, BaTiO3, and CaTiO3 have become key materials for future energy-efficient memristive data storage and logic applications due to their ability to switch their resistance reversibly upon application of an external voltage. This resistance switching effect is based on the evolution of nanoscale conducting filaments with different stoichiometry and structure than the original oxide. In order to design and optimize memristive devices, a fundamental understanding of the interaction between electrochemical stress, stoichiometry changes and phase transformations is needed. Here, we follow the approach of investigating these effects in a macroscopic model system. We show that by applying a DC voltage under reducing conditions on a perovskite slab it is possible to induce stoichiometry polarization allowing for a controlled decomposition related to incongruent sublimation of the alkaline earth metal starting in the surface region. This way, self-formed mesoporous layers can be generated which are fully depleted by Sr (or Ba, Ca) but consist of titanium oxides including TiO and Ti3O with tens of micrometre thickness. This illustrates that phase transformations can be induced easily by electrochemical driving forces.

Neutron-diffraction study of structural transition and magnetic order in orthorhombic and rhombohedral La(7/8)Sr(1/8)Mn(1-γ)O(3+δ).

We report on a systematic neutron powder diffraction (NPD) study of polycrystalline La(7/8)Sr(1/8)Mn(1-γ)O(3+δ) compounds. We investigated the structural and magnetic phases and transitions in the temperature range between 10 and 900 K for two different samples: an Ar-annealed sample with an orthorhombic Pbnm (Z = 4; O) structure at room temperature and an air-sintered sample with a rhombohedral [Formula: see text] (Z = 2;R) structure at room temperature. At higher temperatures, above 400 K, both samples exhibit a rhombohedral structure. For the Ar-annealed sample, a Jahn-Teller (JT) transition occurs in the orthorhombic phase at about 298 K with very large variations in the Mn-O2' and Mn-O2 bond lengths and Mn-O1-Mn bond angle on cooling from 298 to 180 K. For this sample the ferromagnetic moment at 10 K in the magnetic space group Pb'n'm amounts to 3.22(5) µ(B)/Mn. In contrast, the air-sintered sample undergoes on cooling a structural transition from rhombohedral (R) to orthorhombic (O) with a mixed phase of nearly equal R and O repartition at 120 K. Ferromagnetic order develops in this sample at about 240 K with a low-temperature moment of 3.35(4) µ(B)/Mn at 10 K. The coherent JT distortion in its orthorhombic phase occurs below some 170 K. In addition, we have determined the coherent JT distortion parameter Δ, the tolerance factor t and the one-electron bandwidth W of the e(g)-band.