arXiv subjects

Elissaios Stavrou

Publications and source records attributed to Elissaios Stavrou.

At least 19 recordsLinked to original sources

arXiv2026

Tellurium sublattice instability driven amorphization in the chalcogenide AgSbTe2 under pressure

Pressure provides a powerful thermodynamic route to access hidden structural states in functional materials, yet the microscopic origin of pressure-induced amorphization remains elusive in many complex chalcogenides. Here we report a detailed high-pressure structural study of AgSbTe2,combining synchrotron X-ray diffraction with density functional theory and molecular dynamics calculations up to 60 GPa. We uncover a pressure-driven transformation from the ambient R-3m phase to a fully disordered cubic Im3m phase, through an extended intermediate amorphous state. Enthalpy calculations reveal a near-degeneracy between the R3m and Im3m structures over a broad pressure range, dictating amorphization. Contrary to previously speculated cation vacancies, the amorphization is governed by a pronounced displacement instability of the Te sublattice. Remarkably, the time dependent decompression pathway controls the final structural state, resulting in either amorphous (slow decompression) or fully crystalline (fast decompression) states, indicative of a strong counterintuitive kinetic effect.

cond-mat.mtrl-sci↗

arXiv2026

High-pressure structural and lattice-dynamics study of Yttria-Stabilized Zirconia

The structural evolution of two selected compositions of Yttria-Stabilized Zirconia (YSZ), with 3mol% (3YSZ) and 8mol% (8YSZ) of Y2O3, have been investigated under pressure using in-situ synchrotron X-ray diffraction (XRD) and Raman spectroscopy in a diamond anvil cell up to 40 GPa (at room temperature).The close crystallographic relation between the observed structures and the relatively large difference in the atomic numbers of Y/Zr and O, imposes the simultaneous study using both techniques, aiming to fully elucidate the structural evolution under pressure. The results, by combining both techniques, reveal that for both 3YSZ and 8YSZ, pressure promotes higher-symmetry structures. Under initial compression, the minority at ambient conditions monoclinic phase (m-phase) gradually transforms towards t-phase, a transition that is concluded for both 3YSZ/8YSZ at ~10 GPa. At higher pressures, the solely remaining t-phase of 3YSZ transforms to the t'', that in turns transforms to the c-phase above 28 GPa. Likewise, for 8YSZ the coexistence of t- and t''-phases continue up to 31 GPa, where both transforms towards c-phase, that remains stable up to the highest pressure of this study. Upon pressure release, all observed transitions are fully reversible with negligible hysteresis, with the exception of the practical disappearance of the monoclinic phase at ambient conditions. Our study underscores the significance of simultaneously performing and analyzing the results of both XRD and Raman spectroscopy studies in relevant crystallographic systems. Moreover, it provides a route towards a ``structural purification'' of YSZ through the elimination of the m-phase aiming to improve material properties.

cond-mat.mtrl-sci↗

arXiv2025

2D MoS$_2$/Au interfaces for enhanced opto-electronic response with sub-bandgap photons

Monolayer MoS$_2$ is a direct band gap semiconductor with potential applications in optoelectronics and photonics. MoS$_2$ also has a large optical nonlinearity. However, the atomic thickness of the monolayer limits the strength of the measured functional signals, such as the photocurrent or photoluminescence, in optoelectronic devices. Here, we show that photocurrent in monolayer MoS$_2$ can be induced by sub-band gap photons by depositing Au nanoparticles on it. In this system, the nonlinear light-matter interaction in Au nanoparticles enhanced by the localized surface plasmons results in the generation of supercontinuum, which is reabsorbed by MoS$_2$ due to efficient resonant energy transfer. Au nanoparticle assisted photocurrent is more than an order of magnitude larger than two-photon photocurrent in monolayer MoS$_2$. Optimization of the shape, size and composition of the nanoparticle has the potential to enhance the photocurrent significantly with the prospect of applications in the detection of NIR photons, and related technologies including optical telecommunication.

physics.optics↗

arXiv2025

High pressure lattice dynamics study of few layer-$α$-In$_2$Se$_3$

Few-layer $α$-In$_2$Se$_3$ has been studied under pressure using Raman spectroscopy in a diamond anvil cell up to 60 GPa (at room temperature). A combination of AFM and Raman was used to estimate the thickness of the specimens. While few-layer $α$-In$_2$Se$_3$ shows identical structural evolution with the one of the bulk powder-like form of $α$-In$_2$Se$_3$ ( $α$ $\rightarrow$ $β^{'}$ $\rightarrow$ IV ), an abrupt $β^{'}$ $\rightarrow$ IV phase transition (at 45 GPa) was observed, in contrast with the case of the bulk specimen where the two phases coexist over a wide pressure range. This is attributed to the difference in specimens morphology, $i.e.$ single crystal and powder in the case of few-layer and bulk $α$-In$_2$Se$_3$, respectively. This study documents the significance of specimens morphology on the observed pressure-induced phase transitions. The methodology developed in this study for performing high-pressure Raman measurements can be applied to other nanodimensional layered materials.

cond-mat.mtrl-sci↗

arXiv2025

High pressure synthesis and structural study of AuGa2 intermetallic compound

We report the synthesis of the AuGa2 intermetallic compound, using a direct reaction of the relevant elements at room temperature and at very low pressure. The pressure needed to synthesize the AuGa2 compound is below 1 GPa, that is at the lower limit of modern large volume presses, routinely used to synthesize other commercially available materials. This study presents a new method of synthesizing AuGa2, which is much more cost efficient and environmentally friendly than the previously used high-temperature synthesis techniques, and will open new possibilities of synthesizing other intermetallic compounds using high-pressure athermal techniques.

cond-mat.mtrl-sci↗

arXiv2025

On the ambient conditions crystal structure of AgSbTe2

We present a combined X-ray and neutron diffraction, Raman spectroscopy, and 121Sb NMR studies of AgSbTe2, supported by first-principles calculations aiming to elucidate its crystal structure. While diffraction methods cannot unambiguously resolve the structure, Raman and NMR data, together with electric field gradient calculations, strongly support the rhombohedral R-3m phase. Moreover, the agreement between experimental and calculated Raman spectra further corroborates this result, resolving the 60-year sold debate about the exact crystal structure of the AgSbTe2 compound.

cond-mat.mtrl-sci↗

arXiv2025

Comparative high-pressure structural and electrical transport properties study of thermoelectric (Bi1-xSbx)2Te3 compounds

Thermoelectric (Bi1-x Sbx)2Te3 (BST-x) compounds with x=0.2, 0.7 and 0.9 have been studied using synchrotron angle-dispersive powder x-ray diffraction in a diamond anvil cell up to 25 GPa (at room temperature). The results clearly indicate that all compounds of this study follow a similar structural evolution with the one of pure Bi2Te3 and Sb2Te3 under pressure. From the comparison between the critical pressures of the corresponding phase transitions, a clear trend of increasing critical pressure for the transition to the disordered solid-solution BCC phase was observed with the increase of Sb concentration. In the case of the BST-0.7, an extended stability of the solid-solution BCC phase up to, at least, 180 GPa was observed. Finally, electrical transport properties measurements under pressure for BST-0.7, document a reversible pressure-induced metallization above 12 GPa.

cond-mat.mtrl-sci↗

arXiv2025

High pressure structural and lattice dynamics study of α-In$_2$Se$_3$

Layered $α$-In$_2$Se$_3$has been studied using a concomitant in-situ synchrotron angle dispersive powder x-ray diffraction and Raman spectroscopy study in a diamond anvil cell up to 60+ GPa, at room temperature. Helium, that remains fairly hydrostatic up to the highest pressure in this study, was used as the pressure-transmitting medium. The results from both experimental methods reveal a pressure-induced structural phase transition from $α$-In$_2$Se$_3$ to a monoclinic $β$'-In2Se3 structure at $\approx$1 GPa, in agreement with previous studies. Based on our detailed measurements using both experimental techniques and F-f formalism, the $β$'-In$_2$Se$_3$ structure remains stable up to 45 GPa, without a clear indication of a phase transition towards the previously reported $β$-In2Se3 phase. Above this pressure, In$_2$Se$_3$ adopts a disordered solid-solution-like orthorhombic structure, phase IV. The results are discussed in comparison with the relevant previous studies of $α$-In$_2$Se$_3$ under pressure.

cond-mat.mtrl-sci↗

arXiv2021

Structural study of TATB under detonation-induced shock conditions

We explore the response of the insensitive high explosive (IHE) 1,3,5-Triamino-2,4,6- trinitrobenzene (TATB) under detonation-induced shock conditions using in-situ synchrotron X-ray diffraction in the 100 ns time scale using either a conventional or a colliding detonation drive. In all of the detonation experiments on various sizes and morphologies of TATB, we observe an extended stability of the TATB triclinic crystal structure. As the detonation front passes through the TATB, X-ray diffraction indicates a portion of the TATB exhibits a compression up to 30+ GPa, followed subsequently by a pressure release and continued decomposition over a few hundred nanoseconds. Likewise, for colliding detonation-driven shock compression of single crystals of TATB, a significant portion of the triclinic crystal structure appears to be stable up to 60+ GPa. Conversely, in similar detonations of a LLM-105 PBX, X-ray diffraction is simply indicative of decomposition without the apparent compression and slow decomposition seen in TATB. The results indicate the surprising resilience of TATB under these high-pressure, temperature and shock conditions, providing a baseline for understanding the insensitivity of TATB that is considered the industry standard for an insensitive high explosive. The results also provide intriguing information for the extended reaction zone in TATB, and the hot-spot mechanisms for initiating and propagating detonation in this uniquely insensitive explosive.

cond-mat.mtrl-sci↗

arXiv2021

Ethane and methane at high pressures: structure and stability

We have performed a combined experimental and theoretical study of ethane and methane at high pressures up to 120 GPa at 300 K using x-ray diffraction and Raman spectroscopy and the USPEX ab-initio evolutionary structural search algorithm, respectively. For ethane, we have determined the crystallization point, for room temperature, at 2.7 GPa and also the low pressure crystal structure (Phase A). This crystal structure is orientationally disordered (plastic phase) and deviates from the known crystal structures for ethane at low temperatures. Moreover, a pressure induced phase transition has been identified, for the first time, at 13.6 GPa to a monoclinic phase B, the structure of which is solved based on a good agreement of the experimental results and theoretical predictions. For methane, our XRD measurements are in agreement with the previously reported high-pressure structures and EOS. We have determined the equations of state of ethane and methane, which provides a solid basis for the discussion of their relative stability at high pressures.

cond-mat.mtrl-sci↗

arXiv2020

High-pressure structural study of a-Mn: solving a three decades-old mystery

Manganese, in the a-Mn structure, has been studied using synchrotron powder x-ray diffraction in a diamond anvil cell up to 220 GPa at room temperature combined with density functional calculations (DFT). The experiment reveals an extended pressure stability of the a-Mn phase up to the highest pressure of this study, in contrast with previous experimental and theoretical studies. On the other hand, calculations reveal that the previously predicted hcp-Mn phase becomes lower in enthalpy than the a-Mn phase above 160 GPa. The apparent discrepancy is explained due to a substantial electron transfer between Mn ions, which stabilizes the a-Mn phase through the formation of ionic bonding between monatomic ions under pressure.

cond-mat.mtrl-sci↗

arXiv2020

Detonation-induced transformation of graphite to hexagonal diamond

We explore the structural evolution of highly oriented pyrolytic graphite (HOPG) under detonation-induced shock conditions using in-situ synchrotron X-ray diffraction in the ns time scale. We observe the formation of hexagonal diamond (lonsdaleite) at pressures above 50 GPa, in qualitative agreement with recent gas gun experiments. First-principles density functional calculations reveal that under uniaxial compression the energy barrier for the transition towards hexagonal diamond is lower than cubic diamond. Finally, no indication of cubic diamond formation was observed up to >70 GPa.

cond-mat.mtrl-sci↗

arXiv2018

Observation of a metastable intermediate during solid-solid phase transformation in response to rapid compression

In order to probe the mechanism of solid-solid phase transformations, we have applied ultrafast shock wave compression (120 picosecond duration) and ultrashort (130 femtosecond) x-ray diffraction at the Linac Coherent Light Source (LCLS) to probe the compression-induced phase transition pathway in zirconium. Surprisingly, rather than transform from alpha-Zr to the more disordered hex-3 equilibrium omega-Zr phase, in its place we find the formation of a non-equilibrium body-centered cubic (bcc) metastable intermediate. Theoretically hypothesized for several decades, this bcc intermediate state has now been found to be dynamically stabilized under uniaxial loading at sub-nanosecond timescales. Molecular dynamics simulations of shock-wave propagation in zirconium predict this transformation via the dynamical intermediate state. In contrast with longer timescale experiments where the phase diagram alone is an adequate predictor of the crystalline structure of a material, our recent study highlights the importance of metastability and time-dependence in the kinetics of phase transformation at extreme conditions.

cond-mat.mtrl-sci↗

arXiv2018

Anharmonicity-induced isostructural phase transition of Zirconium under pressure

We have performed a detailed x-ray diffraction structural study of Zr under pressure and unambiguously identify the existence of a first-order isostructural bcc-to-bcc phase transition near 58 GPa. First-principles quantum molecular dynamics lattice dynamics calculations support the existence of this phase transition, in excellent agreement with experimental results, triggered by anharmonic effects. Our results highlight the potential ubiquity of anharmonically driven isostructural transitions within the periodic table under pressure and calls for follow-up experimental and theoretical studies.

cond-mat.mtrl-sci↗

arXiv2017

Synthesis of Xenon and Iron/Nickel intermetallic compounds at Earth's core thermodynamic conditions

Although Xe is known to form stable compounds with strong electronegative elements, evidence on the formation of stable compounds with electropositive elements, such as Fe and Ni, was missing until very recently. In addition to the significance of the emerging field of noble gas elements chemistry, the possible formation of Xe-Fe/Ni compounds has been proposed as a plausible explanation of the so-called "missing Xe paradox". Here we explore the possible formation of stable compounds in the Xe-Fe/Ni systems at thermodynamic conditions representative of Earth's core. Using in situ synchrotron X-ray diffraction and Raman spectroscopy in concert with first principles calculations we demonstrate the synthesis of stable Xe(Fe,Fe/Ni)$_3$ and XeNi$_3$ compounds. The results indicate the changing chemical properties of elements under extreme conditions where noble gas elements can form stable compounds with elements which are electropositive at ambient conditions but become slightly electronegative at high pressures.

physics.chem-ph↗

arXiv2017

A stable compound of helium and sodium at high pressure

Helium is generally understood to be chemically inert and this is due to its extremely stable closed-shell electronic configuration, zero electron affinity and an unsurpassed ionization potential. It is not known to form thermodynamically stable compounds, except a few inclusion compounds. Here, using the ab initio evolutionary algorithm USPEX and subsequent high-pressure synthesis in a diamond anvil cell, we report the discovery of a thermodynamically stable compound of helium and sodium, Na2He, which has a fluorite-type structure and is stable at pressures >113 GPa. We show that the presence of He atoms causes strong electron localization and makes this material insulating. This phase is an electride, with electron pairs localized in interstices, forming eight-centre two-electron bonds within empty Na8 cubes. We also predict the existence of Na2HeO with a similar structure at pressures above 15 GPa.

cond-mat.mtrl-sci↗

arXiv2016

High-Pressure Synthesis of a Pentazolate Salt

The pentazolates, the last all-nitrogen members of the azole series, have been notoriously elusive for the last hundred years despite enormous efforts to make these compounds in either gas or condensed phases. Here we report a successful synthesis of a solid state compound consisting of isolated pentazolate anions N5-, which is achieved by compressing and laser heating cesium azide (CsN3) mixed with N2 cryogenic liquid in a diamond anvil cell. The experiment was guided by theory, which predicted the transformation of the mixture at high pressures to a new compound, cesium pentazolate salt (CsN5). Electron transfer from Cs atoms to N5 rings enables both aromaticity in the pentazolates as well as ionic bonding in the CsN5 crystal. This work provides a critical insight into the role of extreme conditions in exploring unusual bonding routes that ultimately lead to the formation of novel high nitrogen content species.

cond-mat.mtrl-sci↗

arXiv2015

Formation of As-As Interlayer Bonding in the collapsed tetragonal phase of NaFe$_2$As$_2$ under pressure

NaFe$_2$As$_2$ is investigated experimentally using powder x-ray diffraction and Raman spectroscopy at pressures up to 23 GPa at room temperature and using ab-initio calculations. The results reveal a pressure-induced structural modification at 4 GPa from the starting tetragonal to a collapsed tetragonal phase. We determined the changes in interatomic distances under pressure that allowed us to connect the structural changes and superconductivity. The transition is related to the formation of interlayer As-As bonds at the expense of weakening of Fe-As bonds in agreement with recent theoretical predictions.

cond-mat.supr-con↗