Structural properties of pure and Fe-doped Yb films prepared by vapor condensation

Highlights

• Pure and Fe-doped Yb films have been prepared by vapor condensation.

• Coexistence of fcc- and hcp-type structures was observed.

• No oxide phases have been detected.

• Fe-clustering does not affect the fcc/hcp ratio, but favors a crystalline texture.

• A schematic model is proposed to describe microscopically the microstructure.

Abstract

Ytterbium and iron-doped ytterbium films were prepared by vapor quenching on Kapton substrates at room temperature. Structural characterization was performed by X-ray diffraction and transmission electron microscopy. The aim was to study the microstructure of pure and iron-doped films and thereby to understand the effects induced by iron incorporation. A coexistence of face centered cubic and hexagonal close packed-like structures was observed, the cubic-type structure being the dominant contribution. There is an apparent thickness dependence of the cubic/hexagonal relative ratios in the case of pure ytterbium. Iron-clusters induce a crystalline texture effect, but do not influence the cubic/hexagonal volume fraction. A schematic model is proposed for the microstructure of un-doped and iron-doped films including the cubic- and hexagonal-like structures, as well as the iron distribution in the ytterbium matrix.

Introduction

It is well known that bulk rare-earth (RE) metals present very rich phase diagrams, revealing structural changes either with temperature or pressure. However, there exists the problem of fast oxidation of these materials that may reduce their suitability for technological applications. Even though these materials have been considered to be applied in several devices [1], [2], [3], [4], [5], [6], [7], [8], [9], [10] and in the form of thin films or nanoparticles, they also exhibit unusual physical properties due to the enhancement of the interface/bulk ratio. Consequently, RE-metal films and their compounds are important from the viewpoint of both basic research and their applications in several functional devices [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16].

For vapor-deposited lanthanide films, the deposition conditions (temperature, time, residual gas during the deposition, etc.) have a strong influence on their morphology and physical properties [15], [16]. It has been reported that ytterbium (Yb) films have slightly different behavior related to its lower tendency for oxidation [5], [13], an effect that could enhance its suitability for technological application [6], [7], [8], [9], [10]. Specifically, results reported for Yb-films suggest that it grows, on glass substrate, with nanoparticle-like shape when the deposition is performed in both He and N₂ atmospheres [15], [16]. These authors have also shown that: (i) pure Yb-like grains are not found in amorphous state when vapor-deposited, but they reveal a coexistence of a fcc crystalline structure with a small amount of hcp phase and (ii), when Yb-films are exposed to air, their top surfaces will become oxidized. The latter result is different from that previously reported by our group for deposition of Yb films under ultra-high vacuum (no measurable oxidation was detected for Yb-films exposed to air after a few months) [12], [13].

In fact, during the last 20 years, we prepared by vapor co-deposition, various film systems with dilute Fe atoms in several metallic matrices that have positive mixing enthalpy with Fe, including some RE-matrices [11], [12], [13]. By doping with magnetic impurities, we expected to induce not only magnetic and electronic changes, but also structural phase transformations. The aims of our previous works were first to investigate the occupation of Fe in substitutional and interstitial sites in distinct crystalline matrices and later on, the formation of Fe nano-clusters upon annealing of the films at temperatures higher than deposition temperature (T_S) or when they are directly deposited at T_S ≥ 300 K. Studies were mainly performed using in-situ ⁵⁷Fe Mössbauer spectroscopy, i.e. without removing the freshly prepared films from the preparation chamber. Under these conditions, film thickness on the order of micrometers had to be used in order to obtain Mössbauer signals of reasonable strengths. Our results suggest that the sizes of Fe nano-clusters depend on the Fe concentration as well as on annealing temperature and time [11], [12], [13]. Therefore, a study that brings information about crystalline structure of the RE-based films as well as the location of Fe nano-clusters within the crystalline structure will help in a better understanding of the physical properties of RE-based films. In particular, for films of pure ytterbium and/or containing Fe there are open questions to be investigated related to: (i) the film crystalline structures (fractions of fcc and hcp Yb phases for micrometer thick films) and (ii) the function of Fe impurity and its localization when atomic diffusion takes place and nano-clusters are formed in Yb matrix [11], [12], [13], [14], [15], [16].

Pure bulk Yb metal stabilizes with a fcc structure and has a divalent character (4f¹⁴) at room temperature (RT). At low temperatures (< 260 K) an hcp phase has been found [14]. Theoretical calculations predicted that the fcc structure is the most stable phase for Yb at RT, when bulk phases are considered [14]. The energy difference per atom between fcc- and hcp-type structures for Yb is about 40 meV [14]. In addition, previous studies have shown that vapor-quenched Fe-doped [12], [13] Yb films contain different types of Fe aggregates. In these works [12], [13], it has been proposed that the Fe-nanoclusters are related to a coexistence of fcc and hcp Yb when the film deposition occurs on a substrate at RT (T_S ~ 300 K). For Fe-doped Yb films, prepared at low substrate temperatures (T_S ~ 20 K), a fcc-like structure was proposed, but there is yet no experimental proof for this assumption [11]. Motivated by the above questions, in this work, the presence of the two phases in vapor quenched films of pure and Fe-doped Yb was systematically studied by X-ray diffraction (XRD) and transmission electron microscopy. A model for the microstructure of our Yb films is suggested. This will help to understand the formation of Fe clusters as well as their location in the Yb lattice, consequently contributing to the literature about physical properties of Yb films. In a first step, pure Yb films, with different thicknesses, were systematically studied to understand the stabilization of the different crystalline structures. Furthermore, Fe-doped films were prepared and the Fe-distribution was characterized. From the dependence of the observed fractions of the crystalline phases and their microstructure of the pure Yb films with film thickness together with those observed for the Fe-doped Yb films we are able to describe the Fe-incorporation in the different crystalline phases.