Rostislav Hrubiak

New developments in laser-heated diamond anvil cell with in situ synchrotron x-ray diffraction at High Pressure Collaborative Access Team

An overview of the in situ laser heating system at the High Pressure Collaborative Access Team, with emphasis on newly developed capabilities, is presented. Since its establishment at the beamline 16-ID-B a decade ago, laser-heated diamond anvil cell coupled with in situ synchrotron x-ray diffraction has been widely used for studying the structural properties of materials under simultaneous high pressure and high temperature conditions. Recent developments in both continuous-wave and modulated heating techniques have been focusing on resolving technical issues of the most challenging research areas. The new capabilities have demonstrated clear benefits and provide new opportunities in research areas including high-pressure melting, pressure-temperature-volume equations of state, chemical reaction, and time resolved studies.

The laser micro-machining system for diamond anvil cell experiments and general precision machining applications at the High Pressure Collaborative Access Team

We have designed and constructed a new system for micro-machining parts and sample assemblies used for diamond anvil cells and general user operations at the High Pressure Collaborative Access Team, sector 16 of the Advanced Photon Source. The new micro-machining system uses a pulsed laser of 400 ps pulse duration, ablating various materials without thermal melting, thus leaving a clean edge. With optics designed for a tight focus, the system can machine holes any size larger than 3 μm in diameter. Unlike a standard electrical discharge machining drill, the new laser system allows micro-machining of non-conductive materials such as: amorphous boron and silicon carbide gaskets, diamond, oxides, and other materials including organic materials such as polyimide films (i.e., Kapton). An important feature of the new system is the use of gas-tight or gas-flow environmental chambers which allow the laser micro-machining to be done in a controlled (e.g., inert gas) atmosphere to prevent oxidation and other chemical reactions in air sensitive materials. The gas-tight workpiece enclosure is also useful for machining materials with known health risks (e.g., beryllium). Specialized control software with a graphical interface enables micro-machining of custom 2D and 3D shapes. The laser-machining system was designed in a Class 1 laser enclosure, i.e., it includes laser safety interlocks and computer controls and allows for routine operation. Though initially designed mainly for machining of the diamond anvil cell gaskets, the laser-machining system has since found many other micro-machining applications, several of which are presented here.

Origins of ultralow velocity zones through slab-derived metallic melt

Significance Nearly three decades ago, seismologists discovered peculiarly dense and slow patches just above Earth’s core−mantle boundary (CMB), known as the ultralow velocity zones (ULVZs). The origin of the ULVZs has remained enigmatic because silicate partial melt models face challenges in accounting for the nonubiquitous occurrence of ULVZs and explaining the observed density excess, whereas iron-rich solid models may have difficulty reproducing the sound velocity observations. Here we propose hypotheses involving slab-derived metallic melt as a critical component to explain the density and velocity features of the ULVZs. These hypotheses differ fundamentally from existing models and may provide insights into the influence of the deep carbon cycle on Earth’s dynamics. Understanding the ultralow velocity zones (ULVZs) places constraints on the chemical composition and thermal structure of deep Earth and provides critical information on the dynamics of large-scale mantle convection, but their origin has remained enigmatic for decades. Recent studies suggest that metallic iron and carbon are produced in subducted slabs when they sink beyond a depth of 250 km. Here we show that the eutectic melting curve of the iron−carbon system crosses the current geotherm near Earth’s core−mantle boundary, suggesting that dense metallic melt may form in the lowermost mantle. If concentrated into isolated patches, such melt could produce the seismically observed density and velocity features of ULVZs. Depending on the wetting behavior of the metallic melt, the resultant ULVZs may be short-lived domains that are replenished or regenerated through subduction, or long-lasting regions containing both metallic and silicate melts. Slab-derived metallic melt may produce another type of ULVZ that escapes core sequestration by reacting with the mantle to form iron-rich postbridgmanite or ferropericlase. The hypotheses connect peculiar features near Earths core−mantle boundary to subduction of the oceanic lithosphere through the deep carbon cycle.

High P-T phase transitions and P-V-T equation of state of hafnium

We measured the volume of hafnium at several pressures up to 67 GPa and at temperatures between 300 to 780 K using a resistively heated diamond anvil cell with synchrotron x-ray diffraction at the Advanced Photon Source. The measured data allows us to determine the P-V-T equation of state of hafnium. The previously described [Xia et al., Phys. Rev. B 42, 6736–6738 (1990)] phase transition from hcp (α) to simple hexagonal (ω) phase at 38 GPa at room temperature was not observed even up to 51 GPa. The ω phase was only observed at elevated temperatures. Our measurements have also improved the experimental constraint on the high P-T phase boundary between the ω phase and high pressure bcc (β) phase of hafnium. Isothermal room temperature bulk modulus and its pressure derivative for the α-phase of hafnium were measured to be B0 = 112.9 ± 0.5 GPa and B0′ = 3.29 ± 0.05, respectively. P-V-T data for the α-phase of hafnium was used to obtain a fit to a thermodynamic P-V-T equation of state based on model by Brosh ...

Microstructures define melting of molybdenum at high pressures

High-pressure melting anchors the phase diagram of a material, revealing the effect of pressure on the breakdown of the ordering of atoms in the solid. An important case is molybdenum, which has long been speculated to undergo an exceptionally steep increase in melting temperature when compressed. On the other hand, previous experiments showed nearly constant melting temperature as a function of pressure, in large discrepancy with theoretical expectations. Here we report a high-slope melting curve in molybdenum by synchrotron X-ray diffraction analysis of crystalline microstructures, generated by heating and subsequently rapidly quenching samples in a laser-heated diamond anvil cell. Distinct microstructural changes, observed at pressures up to 130 gigapascals, appear exclusively after melting, thus offering a reliable melting criterion. In addition, our study reveals a previously unsuspected transition in molybdenum at high pressure and high temperature, which yields highly textured body-centred cubic nanograins above a transition temperature.

Pressure-induced structural transition in chalcopyrite ZnSiP2

The pressure-dependent phase behavior of semiconducting chalcopyrite ZnSiP2 was studied up to 30 GPa using in situ X-ray diffraction and Raman spectroscopy in a diamond-anvil cell. A structural phase transition to the rock salt type structure was observed between 27 and 30 GPa, which is accompanied by soft phonon mode behavior and simultaneous loss of Raman signal and optical transmission through the sample. The high-pressure rock salt type phase possesses cationic disorder as evident from broad features in the X-ray diffraction patterns. The behavior of the low-frequency Raman modes during compression establishes a two-stage, order-disorder phase transition mechanism. The phase transition is partially reversible, and the parent chalcopyrite structure coexists with an amorphous phase upon slow decompression to ambient conditions.

Laser-assisted processing of Ni-Al-Co-Ti under high pressure

ABSTRACT Laser-assisted processing and in-situ characterization of a Ni0.7-Al0.1235-Co0.15-Ti0.0265 alloy were carried out under a range of simultaneous hydrostatic high pressures of ∼30 GPa and high temperature conditions ∼2000°C using a laser-assisted heating in diamond anvil cell with synchrotron X-ray micro-diffraction. The characterization of the microstructure and X-ray diffraction analysis at ambient conditions confirmed the formation of the cuboids of ordered γ′ phase in the disordered γ matrix. The isothermal bulk modulus (B0) and its first-order derivative (B0’) of the alloy were determined to be B0 = 123 ± 9 GPa and B0’ = 5.7 ± 2.8. The in-situ characterization of the alloy at high temperatures under high pressures revealed that the γ′ phase transforms into the tetragonaly distorted D022-type structure. This transformation is similar to the transformation that occurs in the ordered Ni3Al, responsible for the improved strength at high temperatures. High pressure was found to increase the onset temperature of the structural distortion. The pressure–temperature phase diagram of the Ni0.7-Al0.1235-Co0.15-Ti0.0265 up to ∼30 GPa and ∼2000°C was determined and is reported here.

A CO2 laser heating system for in situ high pressure-temperature experiments at HPCAT

We present a CO2 laser heating setup for synchrotron x-ray diffraction inside a diamond anvil cell, situated at HPCAT (Sector 16, Advanced Photon Source, Argonne National Lab, Illinois, USA), which is modular and portable between the HPCAT experiment hutches. The system allows direct laser heating of wide bandgap insulating materials to thousands of degrees at static high pressures up to the Mbar regime. Alignment of the focused CO2 laser spot is performed using a mid-infrared microscope, which addressed past difficulties with aligning the invisible radiation. The implementation of the mid-infrared microscope alongside a mirror pinhole spatial filter system allows precise alignment of the heating laser spot and optical pyrometry measurement location to the x-ray probe. A comparatively large heating spot (∼50 μm) relative to the x-ray beam (<10 μm) reduces the risk of temperature gradients across the probed area. Each component of the heating system and its diagnostics have been designed with portability in mind and compatibility with the various experimental hutches at the HPCAT beamlines. We present measurements on ZrO2 at 5.5 GPa which demonstrate the improved room-temperature diffraction data quality afforded by annealing with the CO2 laser. We also present in situ measurements at 5.5 GPa up to 2800 K in which we do not observe the postulated fluorite ZrO2 structure, in agreement with recent findings.

High-pressure, high-temperature equations of state using nanofabricated controlled-geometry Ni/SiO2/Ni double hot-plate samples

We have fabricated novel controlled-geometry samples for the laser-heated diamond-anvil cell (LHDAC) in which a transparent oxide layer (SiO2) is sandwiched between two laser-absorbing layers (Ni) in a single, cohesive sample. The samples were mass manufactured (>104 samples) using a combination of physical vapor deposition, photolithography, and wet and plasma etching. The double hot-plate arrangement of the samples, coupled with the chemical and spatial homogeneity of the laser-absorbing layers, addresses problems of spatial temperature heterogeneities encountered in previous studies where simple mechanical mixtures of transparent and opaque materials were used. Here we report thermal equations of state (EOS) for nickel to 100 GPa and 3000 K and stishovite to 50 GPa and 2400 K obtained using the LHDAC and in situ synchrotron X-ray microdiffraction. We discuss the inner core composition and the stagnation of subducted slabs in the mantle based on our refined thermal EOS.

Surprising Stability of Cubane under Extreme Pressure

The chemical stability of solid cubane under high-pressure was examined with in situ Raman spectroscopy and synchrotron powder X-ray diffraction (PXRD) in a diamond anvil cell (DAC) up to 60 GPa. The Raman modes associated with solid cubane were assigned by comparing experimental data with calculations based on density functional perturbation theory, and low-frequency lattice modes are reported for the first time. The equation of state of solid cubane derived from the PXRD measurements taken during compression gives a bulk modulus of 14.5(2) GPa. In contrast with previous work and chemical intuition, PXRD and Raman data indicate that solid cubane exhibits anomalously large stability under extreme pressure, despite its immensely strained 90° C-C-C bond angles.