Where is the unpaired transition metal in substoichiometric diboride line compounds?

Justinas Palisaitis,Martin Dahlqvist,Allen J Hall,Jimmy Thörnberg,Ingemar Persson,Nils Nedfors,Lars Hultman,J.E Greene,Ivan Petrov,Johanna Rosen,Per O.Å Persson

doi:10.1016/j.actamat.2020.116510

Abstract

The atomic structure and local composition of high quality epitaxial substoichiometric titanium diboride (TiB1.9) thin film, deposited by unbalanced magnetron sputtering, were studied using analytical high-resolution scanning transmission electron microscopy, density functional theory, and image simulations. The unpaired Ti is pinpointed to inclusion of Ti-based stacking faults within a few atomic layers, which terminates the {11¯00} prismatic planes of the crystal structure and attributed to the absence of B between Ti planes that locally relaxes the structure. This mechanism allows the line compound to accommodate off-stoichiometry and remain a line compound between defects. The planar defects are embedded in otherwise stoichiometric TiB2 and are delineated by insertion of dislocations. An accompanied decrease in Ti-Ti bond lengths along and across the faults is observed.

Highlights

In the search for novel coating materials that are capable of withstanding harsh environments and extreme conditions while maintaining stable phase structures, increased attention has been devoted to exploring transition metal boride compounds
Electron microscopy was performed on a substoichiometric TiB1.9 thin film
The gathered experimental, theoretical and image simulation data leads us to deduce that the unpaired Ti is accommodated by the structure through the formation of Ti planar defects in the TiB2 crystal structure that can be considered as inclusions of Ti-based stacking faults within a few atomic layers, which terminates the {11 ̄ 00} prismatic planes of the TiB2 crystal structure

Summary

Introduction

In the search for novel coating materials that are capable of withstanding harsh environments and extreme conditions while maintaining stable phase structures, increased attention has been devoted to exploring transition metal boride compounds. Titanium diboride (TiB2) is regarded as an outstanding ceramic material that possess an excellent thermal stability, high electrical conductivity, high melting temperature, chemical inertness, and resistance to mechanical wear [1,2,3,4]. These unique properties make thin films and coatings of TiB2 and its alloys (e.g., Ti1-xAlxB2) highly attractive for a range of applications in erosive, abrasive, corrosive and hightemperature environments [5,6,7].

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Discussion

Conclusion