As-deposited and unencapsulated GeSe1-xTex (x = 0, 0.25) 3-mu m-thick amorphous films on Si(001) were obtained via the co-evaporation technique to study the effect of selenium (Se) substitution for tellurium (Te) on the GeSe parent structure in the function of the heating temperature. In situ, grazing-incidence X-ray scattering (XRS), and fluorescence X-ray Absorption Near Edge Structure (XANES) data were collected in isochronal annealing conditions under nitrogen flow. The results show that the onset temperature of crystallization Tc and the crystallized phase symmetry are susceptible to the Te 25 at. % doping. Furthermore, Ge and Se K-edge XANES analyses reveal significant alterations in the local atomic environments surrounding Ge and Se atoms during the transition from the amorphous to the crystalline state. These modifications are accompanied by an observable increase in local structural disorder upon substitution with Te atoms.

The structure of evaporated amorphous GexSbxTe100-2x (x = 6, 9, 13) alloys was investigated by neutron diffraction, X-ray diffraction, and extended X-ray absorption spectroscopy at the Ge, Sb, and Te K-edges. Large-scale structural models were generated by fitting the experimental datasets (five for each composition) simultaneously in the framework of the reverse Monte Carlo simulation technique. It was found that the alloys are chemically ordered (Ge and Sb have predominantly Te neighbors) and within the experimental uncertainty, each component satisfies the 8 - N rule. A comparison with the pair correlation functions of melt-quenched Ge20Te80 revealed that the first minimum of gTeTe(r) is shallower in the ternary alloys than in Ge20Te80. On the other hand, the separation of the first and second coordination environments of Ge atoms is stronger in the Ge-Sb-Te alloys investigated.

As-deposited and unencapsulated GeTe1-xSex (x = 0, 0.5) 3-μm-thick amorphous films on Si(001) were obtained via the thermal co-evaporation technique. The crystallization and the structure of these phase change materials were examined using in situ temperature-dependent X-ray diffraction (XRD) and grazing-incidence fluorescence X-ray Absorption Near Edge Structure (XANES) in isochronal annealing conditions under nitrogen flow. The results show that the onset temperature of crystallization is highly sensitive to the substitution of the Te atoms by the Se ones and that the rhombohedral structure, with the space group R3m, is the one that crystallized for both samples without Ge or Te rejection. Moreover, the local order around the Ge and Se atoms, probed by Ge and Se K-edge XANES analyses, presents clear modifications from the amorphous state to the crystalline one expecting a Ge four-fold and a Se two-fold coordination in the amorphous state, with an increase of the local disorder with the Se substitution (x = 0.5).

Crystallization in unencapsulated GeTe(0.2)6Se(0.74) amorphous 3 mu m-thick films produced by thermal co-evaporation technique was studied by in situ temperature-dependent synchrotron powder X-ray diffraction from room temperature up to 418 degrees C and back to 30 degrees C. The experimental results obtained with a continuous heating rate of 2 degrees C/min under a protective nitrogen atmosphere showed that the crystallization process took place in several steps i) total crystallization of the amorphous phase to an intermediate crystallized phase, (ii) total transformation of the intermediate phase to a second crystallized phase. The intermediate phase corresponds to a solid solution with a metastable alpha-GeTe-like rhombohedral polar structure (R3m) presenting a uniaxial negative thermal ex-pansion along the c-axis. The onset of crystallization to the polar structure is around 270 degrees C. The high-temperature phase has a stable hexagonal structure with a P6(3)mc space group. When cooling, until back to room temperature, the hexagonal structure is conserved. The effect of a changing heating rate from 2 degrees to 0.1 degrees C/min was also investigated. In this context, the crystallization scheme is modified. (c) 2023 Elsevier B.V. All rights reserved.

The properties of xNa2S-(100-x)GeS2 glasses, which represent promising systems for all-solid-state batteries, are thoroughly investigated from a variety of experimental and theoretical techniques. The ionic conduction is measured as a function of composition. It reveals a powerlike behavior with a threshold composition found at low Na content. In contrast, temperature evolution suggests a typical Arrhenius behavior indicative of Na motions achieved by jumps between neighboring sites. Three particular compositions (0%, 33%, and 66% Na2S) are characterized by a combination of x-ray diffraction and density functional based molecular dynamics. Different structural properties are measured and calculated, such as structure factors, pair distribution functions, angular distributions, coordination numbers, and neighbor distributions. The comparison with experiments reveals a rather good agreement in real and reciprocal space. The short-range order is found to consist of a base network made of GeS4/2 tetrahedra (with Ge and S coordination numbers being of about 4 and 2) that are progressively depolymerized upon Na addition that also leads to a breakdown of the ring structure. Na coordination numbers are loosely defined, especially at high Na content. Typical features of alkali-modified silicates are also found, such as the presence of channel-like dynamics, a bond-length distribution that is different between Ge and bridging or nonbridging sulfur, a distribution Qn of Ge tetrahedra having n bonding sulfur, and a decoupling at low temperatures between network species (Ge, S) and Na dynamics. However, unlike such archetypal glasses, sodium thiogermanates contain homopolar Ge-Ge bonds that are specific to Ge chalcogenides and which lead to isolated (Ge2S6)6e anions at high Na content.

The structural, vibrational and electronic properties of several compositions of amorphous Ge-Se-Te are studied from a combination of x-ray diffraction and density functional-based molecular dynamics. Different structural properties are considered such as structure factors, pair distribution functions, angular distributions, coordination numbers, and neighbor distributions. We compare results with experimental findings and a satisfying agreement is found for the structure functions in real and reciprocal spaces. The short range order is found to be more complex than in related binaries that result in mixed geometries (similar or equal to 65%-75% tetrahedral, and remaining defect octahedral) for a dominant fourfold Ge (80%). The chalcogen atoms are dominantly twofold, the former having furthermore an important fraction of threefold coordinated atoms (30%-40%). The obtained model structures indicate that Ge-Ge, Ge-Se, and Ge-Te bonds dominate with small fractions of Te-Te bonds remaining from the base system GeTe. The investigation of electronic properties indicates that the addition of Se atoms will lead to Te-related bands that are much more localized so that Ge-Te-Se can be regarded as having an increased covalent character with respect to GeTe.

Phase Change Materials as those of the Ge-Sb-Te ternary system are of great interest for technological applications. Properties of these compounds are strongly related to presence of vacancies and structural investigations remain challenging. In this paper we evidence that 125Te NMR in natural abundance and using commercial systems at intermediate field (14.1 ​T) together with NMR parameters prediction can contribute to improve understanding of electronic structure of such systems. GeTe is a typical phase change material, whose structure contains germanium vacancies, even in its stoichiometric form, giving it metallic properties. Here, we use nominal Ge50Te50 and Ge48Te52 crystalline samples as an example to optimize the WURST-CPMG technique, a powerful technique to record wide NMR spectra which has not yet been used on 125Te. The goal was to minimize the time devoted to experiments as well as maximize the signal-to-noise ratio in order to detect small intensity signals directly linked to vacancies. Virtual Crystal Approximation (VCA) calculations performed with WIEN2K helped to interpret the NMR spectra. For Te-based crystalline conducting samples the best experimental results were obtained using 3.2 ​mm thin wall rotors with diluted samples 40 ​vol% GeTe-60 ​vol% SiO2. In addition to the WURST-CPMG technique, high resolution spectra using MAS as implemented in the pj-MAT technique allowed us to identify the distributions of chemical shift parameters in the high intensity contribution of the 1D spectra. The NMR spectra recorded on the samples showed that an addition of Tellurium in the stoichiometric Ge50Te50 sample leads to an important broadening of the spectrum together with a shift of the lines. According to VCA calculations it could be attributed to a distribution of concentrations of germanium vacancies in the sample and it would appear that Knight Shift but also Chemical Shift could contribute in similar proportion to the NMR line position when metavalent bonding is invoked.

Using thermal co-evaporation techniques, we show that various glassy compositions can be obtained along the GexSbxTe100-2x join in the ternary Ge-Sb-Te system which is known to display dramatic crystallization tendencies. Earlier attempts to produce bulk glasses have been limited to Sb-poor compositions close to the eutectic GeTe6. Results indicate a weak variation of T-g with composition x and also a thermal stability that is weak for most systems, and especially for compositions close to the domain where Ge2Sb2Te5 can be formed. Our results are put in perspective with other network-forming chalcogenides and the T-g variation suggests the preferential formation of Te-Sb-Te, at variance with isochemical compounds such as Ge-Sb-Se. Data are discussed within the framework of topological based approaches which not only indicate that optimal glass formation is achieved for compositions satisfying the Maxwell stability criterion but also predict a flexible to rigid transition at x = 8.5%. Most of our glasses could be formed around this composition.

Macroscopic and local studies of vibrational and mechanical properties of bulk Ag-x(Ge0.25Se0.75)(100-x) glasses were conducted using Raman (mapping) spectroscopy, Vickers microhardness H-v and Contact Resonance Atomic Force Microscopy (CR-AFM). For the glass containing 10 at% in Ag, Raman mapping gave evidence of a phase separation through continuous interpenetrating phases (Ag-rich and Ag-poor phases) in the spinodal decomposition process. Combined mechanical characterizations (H-v and CRAFM) indicated that the microhardness and rigidity modulus decrease with the silver content in the glass. At nanoscale level, CR-AFM measurements highlighted a modulation of the rigidity with Ag content. The structural origin of these changes was confirmed using Raman mapping evidencing modifications in the tetrahedral network between two phases. The results could suggest a different Ge/Se ratio in Ag-poor and Ag-rich phases. To further investigate the liquid-liquid phase separation morphology, small angle neutron scattering was performed at different temperatures in the liquid state. Whereas liquids with 15 and 25 at% in Ag showed a nucleation/growth morphology for all studied liquid temperatures, a spinodal decomposition with fractal dimension D from 2.6 to 3.4 was found for 5 at% in Ag when the temperature increases from 600 degrees C to 700 degrees C. (C) 2018 Elsevier B.V. All rights reserved.

Structural and calorimetric investigation of Ge(x)Te(100-x) films over wide range of concentration 10 < x < 50 led to evidence two structural singularities at x ∼ 22 at. % and x ∼ 33-35 at. %. Analysis of bond distribution, bond variability, and glass thermal stability led to conclude to the origin of the first singularity being the flexible/rigid transition proposed in the framework of rigidity model and the origin of the second one being the disappearance of the undercooled region resulting in amorphous materials with statistical distributions of bonds. While the first singularity signs the onset of the Ge-Ge homopolar bonds, the second is related to compositions where enhanced Ge-Ge correlations at intermediate lengthscales (7.7 Å) are observed. These two threshold compositions correspond to recently reported resistance drift threshold compositions, an important support for models pointing the breaking of homopolar Ge-Ge bonds as the main phenomenon behind the ageing of phase change materials.

One of the technological challenges of direct observation of extra-solar planets by nulling interferometry is the development of a modal filter operating from 6 to 20 mu m. In the present paper a candidate technology for the fabrication of such modal filters is presented: Integrated Optics. A solution based on alltelluride buried channel waveguides is considered. In the proposed waveguides, vertical guiding of light is achieved by a 15 mu m-thick Te83Ge17 core film deposited onto a lower-index Te75Ge15Ga10 substrate, and covered by a 15 mu m-thick Te76Ge24 superstrate. Horizontal guiding of light is obtained by modifying the geometry of the core layer by ion beam etching.As this stage, all-telluride buried channel waveguide prototypes demonstrate light guiding and transmission from 2 to 20 mu m. The validity of the technology and the good quality of the fabrication process, in particular the input and output facets surface finish are thus confirmed. These results consolidate the potential of Te-based integrated optics components for nulling interferometry. (C) 2015 Elsevier B.V. All rights reserved.

The feasibility of telluride based single-mode waveguides being able to guide light in the infrared region [1–16μm] is demonstrated. The rib waveguides comprise a silicon substrate, two Te–Ge–Se films deposited by thermal co-evaporation. The first one plays the role of a cladding layer while the second is the core layer. In order to control the light confinement, the second layer is further etched by reactive ion etching in CHF3/O2/Ar atmosphere. The obtained structures behaved as channel waveguides with a single-mode transmission demonstrated at 1.55μm. Transmission optical losses were evaluated by the cut-back method on the very first prototypes and were comprised between 0.6 and 1.8dBcm−1.

We report a thorough experimental study on the microstructure, thermal behavior and thermoelectric properties of the amorphous composition Cu15As30Te55 and the glass-ceramics related-compounds synthesized by using the Spark Plasma Sintering (SPS) technique. Varying the conditions of the SPS process enables the synthesis of composite glassy-crystalline samples with different crystal/glass ratios. Such treatments result in complex microstructures composed of large glassy domains where nanocrystals of the metastable beta-As2Te3 phase are embedded. These domains are separated by regions of the dendritic crystalline phase surrounded by a Cu-rich glassy matrix. The presence of beta-As2Te3, confirmed by both powder X-ray diffraction and scanning electron microscopy, suggests that pressure and/or internal stresses play an important role in stabilizing this phase. This conclusion is further supported by neutron thermodiffraction experiments revealing a sharp crossover from the beta-As2Te3 to the stable alpha-As2Te3 phase at temperatures below that of the SPS treatment. Transport properties measurements show that the presence of a crystalline fraction significantly lowers the electrical resistivity by four orders of magnitude. However, the probable intrinsic n-type behavior of beta-As2Te3 has a detrimental influence on the thermopower values. Even though the partial crystallization of the glassy matrix leads to an increase in the thermal conductivity, the measured values remain on the order of 1 W m(-1) K-1 at 300 K. Besides an overall increase in the dimensionless figure of merit ZT, our results demonstrate that the partial crystallization of an amorphous matrix is an efficient tool to tune the electrical resistivity over several orders of magnitude while maintaining low thermal conductivity values.

Amorphous GexTe100−x alloys were obtained over a broad composition range (12 ≤ x ≤ 44.6) by thermal co-evaporation. Their structure was investigated by x-ray diffraction and extended x-ray absorption fine structure measurements. Experimental datasets were fitted simultaneously by the reverse Monte Carlo simulation technique. It is concluded that Te is mostly twofold coordinated and the majority of Ge atoms have four neighbours. The number of Ge–Ge and Te–Te bonds evolves monotonically with composition. Ge–Ge bonding can be observed already at x = 24 while Te–Te bonds can be found even in Ge44.6Te55.4. The models obtained by simulation show that the structure of compositions with x > 24 should be considered as a random covalent network but there is chemical ordering for x ≤ 24, exactly in the composition range where glasses can be obtained from the melt by fast quenching. The composition dependences of some physical properties also point to the connection between chemical short range order and the stability of the amorphous phase: while the glass transition temperature and microhardness increase monotonically with the composition, the thermal stability of the amorphous films goes through a maximum around x = 20–24.

Nanostructured platinum-carbon thin films were prepared by magnetron co-sputtering method for designing efficient catalytic thin films, like fuel cells electrodes. The in-depth morphology of composite films was studied using surface sensitive X-ray techniques (grazing incidence small-angle scattering and reflectivity), consolidated by electron microscopy investigations. This study elucidates the growth mode of co-sputtered platinum-carbon thin film: 2-nm-sized platinum clusters are growing in surrounding simultaneously growing carbon columns (20-nm diameter range). In particular, the platinum cluster growth and distribution in the plane of the substrate surface are driven by surface diffusion and coalescence phenomena. Finally, this anisotropic distribution of platinum clusters correlated to the textured morphology of carbon matrix leads to a catalytic thin film morphology very suitable for electrochemical processes in fuel cell electrodes.