Pressure Induced Research Articles

Size Effects of Hardness and Strain Rate Sensitivity in Amorphous Silicon Measured by Nanoindentation

In this work, dynamic mechanical properties of amorphous silicon and scale effects were investigated by the means of nanoindentation. An amorphous silicon sample was prepared by plasma-enhanced chemical vapor deposition (PECVD). Next, two sets of the samples were investigated: as-deposited and annealed in 500 °C for 1 hour. A three-sided pyramidal diamond Berkovich’s indenter was used for the nanoindentation tests. In order to determine the strain rate sensitivity (SRS), indentations with different loading rates were performed: 0.1, 1, 10, 100 mN/min. Size effects were studied by application of maximum indentation loads in the range from 1 up to 5 mN (penetrating up to approximately one-third of the amorphous layer). The value of hardness was determined by the Oliver–Pharr method. An increase of hardness with decrease of the indentation depth was observed for both samples. Furthermore, the significant dependence of hardness on the strain rate has been reported. Finally, for the annealed samples at low strain rates a characteristic “elbow” during unloading was observed on the force-indentation depth curves. It could be attributed to the transformation of (β-Sn)-Si to the PI (pressure-induced) a-Si end phase.

Pressure-induced structural, electronic, and magnetic phase transitions inFeCl2studied by x-ray diffraction and resistivity measurements

High-pressure (HP) synchrotron x-ray diffraction (XRD) studies were carried out in ${\text{FeCl}}_{2}$ $({T}_{N}\ensuremath{\approx}24\text{ }\text{K})$ together with resistivity $(R)$ studies at various temperatures and pressures to 65 GPa using diamond-anvil cells. This work follows a previous HP $^{57}\text{F}\text{e}$ M\"ossbauer study in which two pressure-induced (PI) electronic transitions were found interpreted as: (i) quenching of the orbital-term contribution to the hyperfine field concurring with a tilting of the magnetic moment by $55\ifmmode^\circ\else\textdegree\fi{}$, and (ii) collapse of the magnetism concurring with a sharp decrease in the isomer shift. The $R(P,T)$ studies affirm that the cause of the collapse of the magnetism is a PI $p\text{\ensuremath{-}}d$ correlation breakdown, leading to an insulator-metal transition at $\ensuremath{\sim}45\text{ }\text{GPa}$ and is not due to a spin crossover $(S=2\ensuremath{\rightarrow}S=0)$. The structure response to the pressure evolution of the two electronic phase transitions starting at low pressures (LP), through an intermediate phase (IP) 30--57 GPa, and culminating in a high-pressure phase, $P>32\text{ }\text{GPa}$, can clearly be quantified. The IP-HP phases coexist through the 32--57 GPa range in which the HP abundance increases monotonically at the expense of the IP phase. At the LP-IP interface no volume change is detected, yet the $c$ axis increases and the $a$ axis shrinks by 0.21 and $0.13\text{ }\text{\AA{}}$, respectively. The fit of the equation of state of the combined LP-IP phases yields a bulk modulus ${K}_{0}=35.3(1.8)\text{ }\text{GPa}$. The intralayer Cl-Cl distances increase but no change is observed in Fe-Cl bond length nor are there substantial changes in the interlayer spacing. The pressure-induced electronic IP-HP transition leads to a first-order structural phase transition characterized by a decrease in Fe-Cl bond length and an abrupt drop in $V(P)$ by $\ensuremath{\sim}3.5%$ accompanying the correlation breakdown. In this transition no symmetry change is detected and the XRD data could be satisfactorily fitted with the ${\text{CdI}}_{2}$ structure. The bulk modulus of the HP phase is practically the same as that of the LP-IP phases suggesting negligible changes in the phonon density of state.

Structural characterization of pressure-induced amorphous silicon

We investigate the structure and mechanical properties of pressure-induced (PI) amorphous silicon $(a\text{-Si})$ and compare this to the more extensively characterized case of $a\text{-Si}$ created by ion implantation. To study the effect of thermal history we also examine the structure of both PI and ion-implanted $a\text{-Si}$ after a low-temperature ``relaxation'' anneal $(450\text{ }\ifmmode^\circ\else\textdegree\fi{}\text{C})$. Indentation testing suggests that structural changes are induced by thermal annealing. As-prepared forms of $a\text{-Si}$ deform via plastic flow, while relaxed forms of $a\text{-Si}$ transform to high-pressure crystalline phases. These structural changes are confirmed by more explicit measurements. Raman microspectroscopy shows that the short-range order as expressed by the average bond-angle distortion of the as-prepared amorphous phases is the same and reduced by the same amount following the low-temperature anneal. Fluctuation electron microscopy demonstrates that the as-prepared PI $a\text{-Si}$ displays a much lower variance of the diffracted intensity, a feature directly correlated with the medium-range order, than the as-prepared ion-implanted $a\text{-Si}$. However, relaxation brings this variance of the two networks to the same intermediate level. The mechanical tests and structural probes indicate that annealing the amorphous silicon network can bring it to a common state with the same structure and properties regardless of the initial state. This final state might be the closest attainable to the continuous random network model.

Crystallographic transitions related to magnetic and electronic phenomena in TM compounds under high pressure

Structural aspects of magnetic/electronic transitions in strongly correlated systems in a regime of very high static density are the main issues of this article. To achieve this objective, we have carried out series of X-ray diffraction (XRD) measurements up to 120 GPa using diamond anvil cells (DACs) to probe structural features specifically related to pressure-induced (PI) magnetic/electronic phenomena in transition metal (TM) compounds. The types of phenomena are the Mott transition (MT), high-spin (HS) to low-spin (LS) transition, valence transformations, the quenching of the orbital term, and Verwey transition. In all these cases, the electronic transition may induce or be a consequence of structural alterations. These studies provide essential information concerning: (i) structural alterations attributed to the valence transformation in some TM compounds; (ii) mechanisms and precursors responsible for the PI MT; features of the structural transformation specifically attributed to the MT for different types of the electronic transitions (Mott–Hubbard and Charge-Transfer); (iii) the mechanism of the PI degradation of the magnetic state due to HS–LS transition and their influence on the structural properties of material; (iv) mechanism of Verwey transition in magnetite; (v) structure of new phases at high pressure. As example cases we present recent results in some Fe-oxides, halides, and sulfides.

Crystallographic transitions related to magnetic and electronic phenomena under high pressure in iron oxides and compounds

The main objective of this paper is the study of structural aspects of strongly correlated systems in conjunctions with magnetic/electronic properties in a regime of matter under very high density. Synchrotron X-ray diffraction (XRD) measurements were carried out to 130 GPa to probe structural features specifically related to pressure-induced (PI) magnetic/electronic transitions in selected transition metal (TM) compounds, emphasizing those with geophysical interest. The types of electronic transitions discussed are the Mott transition (MT), and high-spin (HS) to low-spin (LS) crossover. In these cases the electronic transition may induce or be a consequence of structural alterations. For instance a first-order PI spin-crossover will be accompanied by an abrupt reduction of the TM ionic-radius, hence by a discontinuous volume decrease. On the other hand a “sluggish” or second-order spin-crossover may result in non-linear crystal elastic constants which will affect the equation of state. In case of a MT, a fundamental electronic process in which a strongly correlated magnetic-insulating system becomes non-magnetic and metallic, one expects major modifications of the lattice parameters or even structural symmetry changes. As example-cases we present results in Fe2O3, Fe3O4, FeI2, FeCr2S4, and the RFeO3 orthoferrites (R=rare earth).