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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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DOE OSTI2018

Controlling the structure and ferroic properties of strained epitaxial NiTiO3 thin films on sapphire by post-deposition annealing

Ferroelectrically induced weak ferromagnetism had been predicted earlier in compounds MTiO3 (M = Fe, Mn, Ni) with the LiNbO3-type structure. In order to stabilize this metastable structure by oxide heteroepitaxy, we attempted to grow epitaxial NiTiO3 films on the nonpolar, non-magnetic substrate Al2O3 by pulsed laser deposition. Given the structural imperfections of the as-deposited films arising from the large lattice mismatch, which resulted in weak ferroic ordering, we investigated the effect of post-synthesis annealing on the films’ properties. Our structural data from x-ray diffraction and electron microscopy suggest that the crystalline quality of the NiTiO3 films was greatly improved by annealing the films at 600-1000 oC for 8 hours, with new phase formation along the interface at 1000 oC. Our physical property characterization indicates increased ferromagnetism and polarization after 600 oC, but a change in the magnetic state to likely ferrimagnetism, and a loss of polarization after the 1000 oC treatment. Our results suggest that the ferroic domain properties of the NiTiO3 films can be favorably altered by post-synthesis heat treatment at the proper temperature to manipulate interfacial strain and enhance the multiferroic properties. Improvement in the ferroic properties can be explained by a combination of crystallite growth, interfacial strain relief, and thermodynamic phase stability arguments. At the same time, exceeding temperatures may drive the system to a thermodynamically more favored crystalline order, and eventually to phase segregation and decomposition.

Varga, Tamas↗
DOE OSTI2018

Hydrogen Bonding Controls the Structural Evolution in Perovskite-Related Hybrid Platinum(IV) Iodides

In this paper, we describe the solid-state structural evolution in four hybrid hexaiodoplatinate(IV) compounds, demonstrating the increasingly important role that extended hydrogen bonding plays in directing the structure across the series. The compounds are A 2 PtI 6 , where A is one of the following amines: ammonium, NH 4 + ; methylammonium, CH 3 NH 3 + ; formamidinium, CH(NH 2 ) 2 + ; guanidinium, C(NH 2 ) 3 + . These are closely related in structure and properties to the hybrid halide perovskites of lead(II) that have recently established their prowess in optoelectronics. The first three of these compounds crystallize in the vacancy-ordered double perovskite A 2 Pt$\square$I 6 ($\square$ indicates a vacant site) structure in the K 2 PtCl 6 archetype, despite the relatively large perovskite tolerance factors involved. The last compound, (GUA) 2 PtI 6 , crystallizes in a vacancy-ordered variant of the hexagonal CsNiCl 3 structure: the K 2 MnF 6 structure. A combination of solid-state 195 Pt and 1 H NMR spectroscopy and detailed density functional theory calculations helps to reveal structural trends and establish the hydrogen-bonding tendencies. Finally, the calculations and measured optical properties support the surprising observation in these iodosalt compounds that, for smaller A cations, the conduction bands are considerably disperse, despite lacking extended I–Pt–I connectivity.

36 MATERIALS SCIENCE↗