arXiv subjects

Alain Polian

Publications and source records attributed to Alain Polian.

15 recordsLinked to original sources

arXiv2025

Lattice dynamics of hexagonal ZnMgS

Inelastic neutron scattering measurements on the hexagonal Zn67Mg33S semiconductor alloy reveal a bimodal pattern of the optical modes across the Brillouin zone, confirmed by first-principles simulations. Such modes are sensitive to the local fluctuations in the composition inherent to random Zn/Mg alloying, distinguishing homo from hetero environments of a given bond (1-bond/2-mode), as is formalized for cubic alloys by the percolation model. The latter model thus emerges as a generic framework for systematizing the optical modes of semiconductor alloys in various crystal structures.

cond-mat.mtrl-sci↗

arXiv2023

Multiscale insight into the Cd1-xZnxTe vibrational-mechanical properties -- High-pressure experiments and ab initio calculations

The Cd1-xZnxTe semiconductor alloy is a regular system regarding its macroscopic mechanic properties in that its experimental bulk modulus exhibits a linear x-dependence, in line with ab initio predictions. Complexity arises at the bond scale, referring to the intricate Cd1-xZnxTe percolation-type Raman pattern [T. Alhaddad et al., Journal of Applied Physics 133, 065701 (2023)]. This offers an appealing benchmark to test various phonon coupling processes at diverse length scales in a compact multi-oscillator assembly, presently tuned by pressure. At x around 0, an inter-bond long-range/macro electric coupling between the matrix and impurity polar phonons is detuned under pressure. Inversely, at x around 1, an intra-bond short-range/nano mechanic coupling is enforced between the two Zn Te apolar sub-phonons stemming from same and alien percolation-type environments. The pressure-induced macro/nano polar/apolar coupling/decoupling processes are compared within a model of two coupled electric/mechanic harmonic oscillators in terms of a compromise between proximity to resonance and strength of coupling, impacting the degree of mode mixing, with ab initio (apolar case) and analytical (polar case) Raman calculations in support. Notably, the free mechanic coupling at x around 1 opposes the achievement of a phonon exceptional point, manifesting the inhibition of mechanic coupling, earlier evidenced with similar bonds for x smaller than 0.5. Hence, the pressure dependence of a given bond vibration in a disordered alloy basically differs depending on whether the bond is matrix-like, i.e., self-connected in bulk (free coupling), or dispersed, i.e., self-connected in a chain (inhibited coupling). This features pressure-tunable percolation-based on-off phonon switches in complex media.

cond-mat.mtrl-sci↗

arXiv2020

Iodine molecule modifications with high pressure

Metallization and dissociation are key transformations in diatomic molecules at high densities particularly significant for modeling giant planets. Using X-ray absorption spectroscopy and atomistic modeling, we demonstrate that in halogens, the formation of a \textit{connected} molecular structure takes place at pressures well below metallization. Here we show that the iodine diatomic molecule first elongates of $\sim$0.007 Å~up to a critical pressure of $P_c$ $\backsim$7~GPa developing bonds between molecules. Then its length continuously decreases with pressure up to 15-20~GPa. Universal trends in halogens are shown and allow to predict for chlorine a pressure of 42$\pm$8~GPa for molecular bond-length reversal. Our findings tackle the molecule invariability paradigm in diatomic molecular phases at high pressures and may be generalized to other abundant diatomic molecules in the universe, including hydrogen.

physics.chem-ph↗

arXiv2020

Mechanism of pressure induced amorphization of SnI4: a combined X-ray diffraction -- X-ray absorption spectroscopy study

We have studied the amorphization process of SnI4 up to 26.8GPa with unprecedented experimental details by combining Sn and I K edge X-ray absorption spectroscopy and powder X-ray diffraction. Standards and reverse Monte Carlo extended X-ray absorption fine structure (EXAFS) refinements confirm that the SnI4 tetrahedron is a fundamental structural unit that is preserved through the crystalline phase-I to crystalline phase-II transition about 7 to 10GPa and then in the amorphous phase that appears above 20GPa. Up to now unexploited Iodine EXAFS reveals to be extremely informative and confirms the formation of iodine iodine short bonds close to 2.85Å in the amorphous phase at 26.8 GPa. A coordination number increase of Sn in the crystalline phase-II appears to be excluded, while the deformation of the tetrahedral units proceeds through a flattening that keeps the average I-Sn-I angle close to 109.5°. Moreover, we put in evidence the impact of pressure on the Sn near edge structure under competing geometrical and electronic effects.

cond-mat.mtrl-sci↗

arXiv2019

Epsilon-iron as a spin-smectic state

Using x-ray emission spectroscopy, we find appreciable local magnetic moments until 30-40 GPa in the high-pressure phase of iron, however no magnetic order is detected with neutron powder diffraction down to 1.8 K contrary to previous predictions. Our first-principles calculations reveal a "spin-smectic" state lower in energy than previous results. This state forms antiferromagnetic bilayers separated by null spin bilayers, which allows a complete relaxation of the inherent frustration of antiferromagnetism on a hexagonal close-packed lattice. The magnetic bilayers are likely orientationally disordered, owing to the soft interlayer excitations and the near-degeneracy with other smectic phases. This possible lack of long-range correlation agrees with the null results from neutron powder diffraction. An orientationally-disordered, spin-smectic state resolves previously perceived contradictions in high pressure iron and could be integral to explaining its puzzling superconductivity.

cond-mat.mtrl-sci↗

arXiv2015

Phonon study of rhombohedral BS under high pressure

Raman spectra of rhombohedral boron monosulfide (r-BS) were measured under pressures up to 34 GPa at room temperature. No pressure-induced structural phase transition was observed, while strong pressure shift of Raman bands towards higher wavenumbers has been revealed. IR spectroscopy as a complementary technique has been used in order to completely describe the phonon modes of r-BS. All experimentally observed bands have been compared with theoretically calculated ones and modes assignment has been performed. r-BS enriched by 10B isotope was synthesized, and the effect of boron isotopic substitution on Raman spectra was observed and analyzed.

cond-mat.mtrl-sci↗

arXiv2013

Vitreous silica distends in helium gas: acoustic vs. static compressibilities

Sound velocities of vitreous silica are measured under He compression in the pressure range 0-6 GPa by Brillouin light scattering. It is found that the well-known anomalous maximum in the pressure dependence of the compressibility is suppressed by He incorporation into the silica network. This shows that the elastic anomaly relates to the collapse of the largest interstitial voids in the structure. The huge difference between the static and the acoustic compressibilities indicates that the amount of incorporated helium still increases at 6 GPa.

cond-mat.mtrl-sci↗

arXiv2011

Structural and optical studies of FeSb2 under high pressure

Nanostructured orthorhombic FeSb2 and an amorphous phase were formed by mechanical alloying starting from a mixture of high purity elemental Fe and Sb powders. The effects of high pressures on structural and optical properties were studied using X-ray diffraction (XRD) and Raman spectroscopy (RS). XRD patterns showed the presence of the orthorhombic FeSb2 phase up to the maximum pressure applied (28.2 GPa). The XRD patterns showed also an increase in the amount of the amorphous phase with increasing pressure up to 23.3 GPa. At 14.3 GPa, together with the former phases, a new phase was observed and indexed to a tetragonal FeSb2 phase, but its volume fraction is small at least up to 23.3 GPa. For the orthorhombic FeSb2 phase, the pressure dependence of the volume fitted to a Birch-Murnaghan equation of state gave a bulk modulus = 74.2 +- 3.0 GPa and its pressure derivative = 7.5 +- 0.6. RS measurements were performed from atmospheric pressure up to 45.2 GPa. For the orthorhombic FeSb2 phase, the Raman active mode was observed up to the maximum pressure applied, while the mode disappeared at 16.6 GPa. For pressures higher than 21 GPa, the Raman active mode of a tetragonal FeSb2 phase was observed, confirming ab initio calculations reported in the literature.

cond-mat.mtrl-sci↗

arXiv2011

New pressure-induced monoclinic β-Sb2Te3 phase with sevenfold symmetry

A nanometric Sb2Te3 rhombohedral phase was produced from Sb and Te by mechanical alloying for 3 hours and its structural stability was studied by synchrotron X-ray diffraction (XRD) and Raman spectroscopy (RS) measurements as a function of pressure. A phase transformation from the ambient pressure rhombohedral phase into a β-Sb2Te3 monoclinic structure between 9.8 and 13.2 GPa is observed by XRD. This phase transformation is confirmed by the Raman spectroscopy measurements. The pressure dependence of the volume fited to a Birch-Murnaghan equation of state gives a bulk modulus B0 = 40.6 +- 1.5 GPa and B'0 = 5.1 +- 0.6. The bulk modulus of the nano-Sb2Te3 seems to be slightly smaller than that for its bulk counterpart (44.8 GPa).

cond-mat.mtrl-sci↗

arXiv2007

High-pressure study of X-ray diffuse scattering in ferroelectric perovskites

We present a high-pressure x-ray diffuse scattering study of the ABO$_3$ ferroelectric perovskites BaTiO_3 and KNbO_3. The well-known diffuse lines are observed in all the phases studied. In KNbO_3, we show that the lines are present up to 21.8 GPa, with constant width and a slightly decreasing intensity. At variance, the intensity of the diffuse lines observed in the cubic phase of BaTiO_3 linearly decreases to zero at $\sim 11$ GPa. These results are discussed with respect to x-ray absorption measurements, which leads to the conclusion that the diffuse lines are only observed when the B atom is off the center of the oxygen tetrahedron. The role of such disorder on the ferroelectric instability of perovskites is discussed.

cond-mat.mtrl-sci↗

arXiv2004

High Pressure X-Ray Diffraction Study of UMn2Ge2

Uranium manganese germanide, UMn2Ge2, crystallizes in body-centered tetragonal ThCr2Si2 structure with space group I4/mmm, a = 3.993A and c = 10.809A under ambient conditions. Energy dispersive X-ray diffraction was used to study the compression behaviour of UMn2Ge2 in a diamond anvil cell. The sample was studied up to static pressure of 26 GPa and a reversible structural phase transition was observed at a pressure of ~ 16.1 GPa. Unit cell parameters were determined up to 12.4 GPa and the calculated cell volumes were found to be well reproduced by a Murnaghan equation of state with K0 = 73.5 GPa and K' = 11.4. The structure of the high pressure phase above 16.0 GPa is quite complicated with very broad lines and could not be unambiguously determined with the available instrument resolution.

cond-mat.mtrl-sci↗

arXiv2004

XMCD under pressure at the Fe K edge on the energy dispersive beamline of the ESRF

The present paper demonstrates the feasibility of X-ray Absorption Spectroscopy (XAS) and X-ray Magnetic Circular Dichroism (XMCD) at high pressure at the Fe-K edge on the ID24 energy dispersive beamline of the ESRF. In 3d transition metals, performing experiments at the hard X-ray K-edge rather than at the magnetically interesting soft X-ray L-edges represents, the only way to access the high pressure regime obtainable with Diamond Anvil Cells. The simultaneous availability of a local structure (XAS) and of a magnetic (XMCD) probe on the sample in identical thermodynamical conditions is essential to study correlations between local structural and magnetic properties. We briefly summarize the state of the art theoretical understanding of K-edge XMCD data, then illustrate the setup of beamline ID24 for high pressure XMCD experiments and underline the conditions required to perform measurements at the K-edges of 3d transition metals. Finally, we present two examples of recent high pressure results at the Fe-K edge in pure Fe and Fe3O4 powder.

physics.ins-det↗

arXiv1998

On the High-Pressure Phase Transition in GaPO4

X-ray diffraction (XRD) experiments have been carried out on quartz-like GaPO4 at high pressure and room temperature. A transition to a high pressure disordered crystalline form occurs at 13.5 GPa. Slight heating using a YAG infrared laser was applied at 17 GPa in order to crystallize the phase in its stability field. The structure of this phase is orthorhombic with space group Cmcm. The cell parameters at the pressure of transition are a=7.306 A, b=5.887 A and c=5.124 A.

cond-mat↗

arXiv1997

XAS Study of the High Pressure Behaviour of Quartzlike Compounds

EXAFS spectroscopy experiments have been carried out on quartz-like GaAsO4 and AlAsO4 at high pressure and room temperature. It has been shown that these materials exhibit two structural phase transitions; the first transition to a high pressure crystalline form occurs at 9 GPa and is reversible upon decompression, whereas the second transition occurs at higher pressures and is irreversible. In GaAsO4, EXAFS measurements agree with the predicted transition from four- to six-fold coordination of oxygen atoms around the cations, but the two local coordination transformations are not dissociated; in fact, both As and Ga atoms exhibit a coordination change at the onset of the first phase transition, the rate of transformation being significantly higher for Ga atoms. In both cases, the average bond length increases very rapidly with pressure thus yielding the first compression stage after the transition. In the second stage, the average bond lengths increase slowly, ultimately reaching six-fold coordination above 28 GPa and 24 GPa for As and Ga respectively. The behaviour of the As K-edge EXAFS is the same for both compounds, and enables us to link the behaviour of Ga and Al atoms. The local transformations are well described and a direct link with phosphate berlinites seems timely.

cond-mat.mtrl-sci↗

arXiv1997

A combined XAS and XRD Study of the High-Pressure Behaviour of GaAsO4 Berlinite

Combined X-ray absorption spectroscopy (XAS) and X-ray diffraction (XRD) experiments have been carried out on GaAsO4 (berlinite structure) at high pressure and room temperature. XAS measurements indicate four-fold to six-fold coordination changes for both cations. The two local coordination transformations occur at different rates but appear to be coupled. A reversible transition to a high pressure crystalline form occurs around 8 GPa. At a pressure of about 12 GPa, the system mainly consists of octahedral gallium atoms and a mixture of arsenic in four-fold and six-fold coordinations. A second transition to a highly disordered material with both cations in six-fold coordination occurs at higher pressures and is irreversible.

cond-mat.mtrl-sci↗