The never-ending story of transition metals oxide systems: from dreams to reality - Augusto Marcelli 1,2,3
1. INFN - Laboratori Nazionali di Frascati, Via E. Fermi 40, 00044 Frascati, Italy
2. RICMASS, Rome International Center for Materials Science Superstripes, Via dei Sabelli 119A, 00185 Rome, Italy
3. University of Science and Technology of China, Hefei, Anhui 230026, P.R. China
Transition metal oxides are based on complex lattice architectures and offer a wide spectrum of properties, which provide the foundation for a broad range of potential applications. Many of these properties originate from interactions among spin, lattice, and charge degrees of freedom. Among the many, the VO2 is one of the most challenging and studied system because it undergoes to a hysteretic metal-insulator transition (MIT) and a structural phase transition (SPT) from monoclinic to rutile phases,
upon heating or cooling through a temperature range near room temperature (~340 K) with a change in conductivity by several orders of magnitude. MITs and SPTs are of extreme interest because of their potential applications in optics, sensing, micromechanics, new computing architectures and high-density memories. However, still now, the driving mechanism of the MIT transition in VO2 is a matter of debate with supporters of the Mott–Hubbard mechanism against the electron–lattice coupling described as a Peierls transition. Moreover, even the latest experimental and theoretical investigations have still not answered simples questions such as the nature of these transitions, their relationship, the sequence of occurrence, the existence of intermediate states, etc..
As required by many different and extremely demanding applications, continuous efforts have been made in the last years also to characterize and improve performances of metallic TMs films and coatings. Morphology, structure and electronic properties of TMs films are very important in different technologies. However, TM’s films such as Mo coatings are multiphase metallic films with not negligible contributions of disordered oxide phases. Typical Mo coatings exhibit a resistivity less than one order of magnitude higher than the Mo bulk comparable with that of bi-layer MoOx films (~10-5 W cm) growth with different crystalline phases. A proper combination of sputtering and post-deposition annealing is a powerful procedure to grow coatings suitable for many applications. Although the typical conductivity values remain low, an enhancement of the conductivity of Mo coatings is achievable tuning the growth parameters and the post treatment thermal process. All these multiphase films are characterized by percolative conductivity phenomena that make them extremely interesting to fabricate in the future hard metallic transparent coatings. Still considering TMs systems, the La2 CuO(4+y) is the simplest cuprate superconductor with mobile oxygen interstitials that exhibits a bulk multi-scale structural phase separation. In this material ac susceptibility and spectroscopy data clearly shown that multiple phases coexist on the mesoscopic scale. Different theoretical frameworks point out how this phenomenon is related to the superconductivity phenomenon.  Being a fast and local probe of a selected atom, XANES spectroscopy is a powerful technique suited to investigate the distribution of different polymorphs down to the micrometer scale and below using small spots. TM systems offer a rich variety of nanoscale structural, electronic and magnetic phases sometime coexisting among them, so that the XANES technique is almost unique to characterize and to investigate the nature of these multi-scale highly correlated systems. The definition of highly correlated systems is not easy, but they can be considered as complex systems having a multi-scale structure and a dynamics, unavoidably entangled and certainly well beyond the definition of a disordered system. The interplay of nano- and micrometer-scale factors is typically at the origin of the properties and the macroscopic behaviour of the above mentioned TM systems so that the capability to probe morphology and phase
distribution, in these complex systems and at multiple length scales, is mandatory. I will show and discuss how XANES and, in particular, μ-XANES mapping may identify chemical changes and correlate them with 2D and 3D morphologies giving, in particular, a strong experimental support to the percolative superconductivity scenario in high temperature superconductors .
 N. Poccia, A. Ricci, G. Campi, M. Fratini, A. Puri, D. Di Gioacchino, A. Marcelli, M. Reynolds, M. Burghammer, N.L. Saini, G. Aeppli, A. Bianconi, Optimum inhomogeneity of local lattice distortions in La2 CuO(4+y), Proc. Nat. Acad. Sci. 109, 15685-15690 (2012)
 N. Poccia, M. Chorro, A. Ricci, Wei Xu, A. Marcelli, G. Campi, A. Bianconi, Percolative superconductivity in La2CuO4.06 by lattice granularity patterns with μ-XANES scanning, Appl. Phys. Lett. 104, 221903-1/ 221903-5 (2014)