Ordinary materials can transform into novel phases at extraordinary high pressure and temperature. The recently developed method of ultrashort laser-induced confined microexplosions initiates a non-equilibrium disordered plasma state. Ultra-high quenching rates overcome kinetic barriers to the formation of new metastable phases, which are preserved in the surrounding pristine crystal for subsequent exploitation. Here we demonstrate that confined microexplosions in silicon produce several metastable end phases. Comparison with an ab initio random structure search reveals six energetically competitive potential phases, four tetragonal and two monoclinic structures. We show the presence of bt8 and st12, which have been predicted theoretically previously, but have not been observed in nature or in laboratory experiments. In addition, the presence of the as yet unidentified silicon phase, Si-VIII and two of our other predicted tetragonal phases are highly likely within laser-affected zones. These findings may pave the way for new materials with novel and exotic properties.

Thermally induced phase transformation of Si-III/Si-XII zones formed by nanoindentation has been studied during low temperature ͑200Ͻ T Ͻ 300°C͒ thermal annealing by Raman microspectroscopy and transmission electron microscopy. Two sizes of spherical indenter tips have been used to create substantially different volumes of phase transformed zones in both crystalline ͑c-Si͒ and amorphous silicon ͑a-Si͒ to study the zone size and starting matrix effects. The overall transformation is from Si-III/XII to poly-or nanocrystalline Si-I through intermediate phases of Si-XIII and Si-IV. Attempts have been made to determine the exact transformation pathways. Two scenarios are possible: either Si-XII first transforms to Si-III before transforming to Si-I through the intermediate phases or that Si-XII goes through the intermediate phases while Si-III transforms directly to Si-I. Finally, the phase transformations are slower in the larger indents and the starting matrix ͑crystalline or amorphous͒ has a substantial effect on the transformation kinetics of the small indents compared to the larger ones. We attribute this increased stability to both matrix effects ͑nucleation͒ and a difference in overall residual stress in indents made in a-Si compared to c-Si.

We investigate the structure and mechanical properties of pressure-induced ͑PI͒ amorphous silicon ͑a-Si͒ and compare this to the more extensively characterized case of a-Si created by ion implantation. To study the effect of thermal history we also examine the structure of both PI and ion-implanted a-Si after a lowtemperature "relaxation" anneal ͑450°C͒. Indentation testing suggests that structural changes are induced by thermal annealing. As-prepared forms of a-Si deform via plastic flow, while relaxed forms of a-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 lowtemperature anneal. Fluctuation electron microscopy demonstrates that the as-prepared PI a-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-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.

Articles you may be interested inThe roles of hydrogen in the diamond/amorphous carbon phase transitions of oxygen ion implanted ultrananocrystalline diamond films at different annealing temperatures AIP Advances 2, 042109 (2012); 10.1063/1.4759087 n-type conductivity and phase transition in ultrananocrystalline diamond films by oxygen ion implantation and annealing J. Appl. Phys. 109, 053524 (2011); 10.1063/1.3556741 Effect of oxygen concentration on nanoindentation-induced phase transformations in ion-implanted amorphous silicon J. Appl. Phys. 105, 083520 (2009); 10.1063/1.3097752 Phase transformations induced in relaxed amorphous silicon by indentation at room temperatureThe deformation behavior of ion-implanted ͑unrelaxed͒ and annealed ion-implanted ͑relaxed͒ amorphous silicon ͑a-Si͒ under spherical indentation at room temperature has been investigated. It has been found that the mode of deformation depends critically on both the preparation of the amorphous film and the scale of the mechanical deformation. Ex situ measurements, such as Raman microspectroscopy and cross-sectional transmission electron microscopy, as well as in situ electrical measurements reveal the occurrence of phase transformations in all relaxed a-Si films. The preferred deformation mode of unrelaxed a-Si is plastic flow, only under certain high load conditions can this state of a-Si be forced to transform. In situ electrical measurements have revealed more detail of the transformation process during both loading and unloading. We have used ELASTICA simulations to obtain estimates of the depth of the metallic phase as a function of load, and good agreement is found with the experiment. On unloading, a clear change in electrical conductivity is observed to correlate with a "pop-out" event on load versus penetration curves.

The nucleation of metastable germanium polymorphs on decompression is studied using in situ synchrotron xray diffraction. We show that the transition pathway is critically dependent on the hydrostaticity. Quasihydrostatic conditions result in the nucleation of the rhombohedral r8 phase, followed by the cubic bc8 and hexagonal diamond phases. In contrast, the presence of shear yields the tetragonal st12 phase. Thus, targeted nucleation of a metastable polymorph is now possible. This observation has implications for the technological exploitation of Ge, but also for other tetrahedral systems.

The deformation behavior of self-ion-implanted amorphous-Si (a-Si) has been studied using spherical nanoindentation in both relaxed (annealed) and unrelaxed (as-implanted) a-Si. Interestingly, phase transformations were clearly observed in the relaxed state, with the load–unload curves from these samples displaying characteristic discontinuities and cross-sectional transmission electron microscopy images indicating the presence of high-pressure crystalline phases Si-III and Si-XII following pressure release. Thus, an amorphous to crystalline phase transformation has been induced by indentation at room temperature. In contrast, no evidence of a phase transformation was observed in unrelaxed a-Si, which appeared to deform via plastic flow of the amorphous phase. Furthermore, in situ electrical measurements clearly indicate the presence of a metallic Si phase during loading of relaxed a-Si but no such behavior was observed for unrelaxed a-Si

The Group 14 element silicon possesses a complex free-energy landscape with many (local) minima, allowing for the formation of a variety of unusual structures, some of which may be stabilized at ambient conditions. Such exotic silicon allotropes represent a significant opportunity to address the ever-increasing demand for novel materials with tailored functionality since these exotic forms are expected to exhibit superlative properties including optimized band gaps for solar power conversion. The application of pressure is a well-recognized and uniquely powerful method to access exotic states of silicon since it promotes large changes to atomic bonding. Conventional high-pressure syntheses, however, lack the capability to access many of these local minima and only four forms of exotic silicon allotropes have been recovered over the last 50 years. However, more recently, significant advances in high pressure methodologies and the use of novel precursor materials have yielded at least three more recoverable exotic Si structures. This review aims to give an overview of these innovative methods of high-pressure application and precursor selection and the recent discoveries of new Si allotropes. The background context of the conventional pressure methods and multitude of predicted new phases are also provided. This review also offers a perspective for possible access to many further exotic functional allotropes not only of silicon but also of other materials, in a technologically feasible manner.

The suite of neutron powder diffractometers at Oak Ridge National Laboratory (ORNL) utilizes the distinct characteristics of the Spallation Neutron Source and High Flux Isotope Reactor to enable the measurements of powder samples over an unparalleled regime at a single laboratory. Full refinements over large Q ranges, total scattering methods, fast measurements under changing conditions, and a wide array of sample environments are available. This article provides a brief overview of each powder instrument at ORNL and details the complementarity across the suite. Future directions for the powder suite, including upgrades and new instruments, are also discussed.