Deviatoric stress: a nuisance or a gold mine?

Pressure-induced high-spin/low-spin disproportionated state in the Mott insulator FeBO3

The pressure-induced Mott insulator-to-metal transitions are often accompanied by a collapse of magnetic interactions associated with delocalization of 3d electrons and high-spin to low-spin (HS-LS) state transition. Here, we address a long-standing controversy regarding the high-pressure behavior of an archetypal Mott insulator FeBO₃ and show the insufficiency of a standard theoretical approach assuming a conventional HS-LS transition for the description of the electronic properties of the Mott insulators at high pressures. Using high-resolution x-ray diffraction measurements supplemented by Mössbauer spectroscopy up to pressures ~ 150 GPa, we document an unusual electronic state characterized by a “mixed” HS/LS state with a stable abundance ratio realized in the \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$R\overline{3 }c$$\end{document}R3¯c crystal structure with a single Fe site within a wide pressure range of ~ 50–106 GPa. Our results imply an unconventional cooperative (and probably dynamical) nature of the ordering of the HS/LS Fe sites randomly distributed over the lattice, resulting in frustration of magnetic moments.

A simplified rapid-quench multi-anvil technique

We report a new rapid-quench technique for the Kawai-type multi-anvil press: several important improvements were made to our previous design. As a result, we are able to routinely quench melts with low glass-forming ability and form glasses. Owing to the use of 3D-printed parts to supply the coolant, the new design is easier to assemble and demonstrates better temperature stability and cooling rate. It was also found that the cooling rate is both pressure- and temperature-dependent. The cooling rate increases with increasing pressure from 6700 °C/s at 1 GPa to 8200 °C/s at 5.5 GPa and decreases with increasing temperature at a rate of 550 °C s^-1/100 °C. Taking these dependencies into account, the new rapid-quench design produces more than 15% higher cooling rate compared to the previous design. Moreover, enhancing coolant circulation, which was achieved by using tapered inner anvils with holes, additionally increases the cooling rate by about 4%. As the structure of the rapid-quench assembly differs dramatically from other existing designs, pressure calibration and temperature distribution in the experimental cell and sample capsule were determined for the first time. It was found that the first 0.6 MN of press load is not used to generate pressure due to the hard tungsten components in the assembly. At the current state-of-the-art, it is possible to routinely reach a pressure of 9 GPa and a temperature of 2200 K with the temperature variation not exceeding 70 K within the sample capsule.

Crystal Structure and Non-Hydrostatic Stress-Induced Phase Transition of Urotropine Under High Pressure

High-pressure behavior of hexamethylenetetramine (urotropine) was studied in situ using angle-dispersive single-crystal synchrotron X-ray diffraction (XRD) and Fourier-transform infrared absorption (FTIR) spectroscopy. Experiments were conducted in various pressure-transmitting media to study the effect of deviatoric stress on phase transformations. Up to 4 GPa significant damping of molecular librations and atomic thermal motion was observed. A first-order phase transition to a tetragonal structure was observed with an onset at approximately 12.5 GPa and characterized by sluggish kinetics and considerable hysteresis upon decompression. However, it occurs only in non-hydrostatic conditions, induced by deviatoric or uniaxial stress in the sample. This behavior finds analogies in similar cubic crystals built of highly symmetric cage-like molecules and may be considered a common feature of such systems. DFT computations were performed to model urotropine equation of state and pressure dependence of vibrational modes. The first successful Hirshfeld atom refinements carried out for high-pressure diffraction data are reported. The refinements yielded more realistic C-H bond lengths than the independent atom model even though the high-pressure diffraction data are incomplete.

In situ spectroscopic study of the plastic deformation of amorphous silicon under non-hydrostatic conditions induced by indentation

Indentation-induced plastic deformation of amorphous silicon (a-Si) thin films was studied by in situ Raman imaging of the deformed contact region of an indented sample, employing a Raman spectroscopy-enhanced instrumented indentation technique. Quantitative analyses of the generated in situ Raman maps provide unique, new insight into the phase behavior of as-implanted a-Si. In particular, the occurrence and evolving spatial distribution of changes in the a-Si structure caused by processes, such as polyamorphization and crystallization, induced by indentation loading were measured. The experimental results are linked with previously published work on the plastic deformation of a-Si under hydrostatic compression and shear deformation to establish a sequence for the development of deformation of a-Si under indentation loading. The sequence involves three distinct deformation mechanisms of a-Si: (1) reversible deformation, (2) increase in coordination defects (onset of plastic deformation), and (3) phase transformation. Estimated conditions for the occurrence of these mechanisms are given with respect to relevant intrinsic and extrinsic parameters, such as indentation stress, volumetric strain, and bond angle distribution (a measure for the structural order of the amorphous network). The induced volumetric strains are accommodated solely by reversible deformation of the tetrahedral network when exposed to small indentation stresses. At greater indentation stresses, the increased volumetric strains in the tetrahedral network lead to the formation of predominately five-fold coordination defects, which seems to mark the onset of irreversible or plastic deformation of the a-Si thin film. Further increase in the indentation stress appears to initiate the formation of six-fold coordinated atomic arrangements. These six-fold coordinated arrangements may maintain their amorphous tetrahedral structure with a high density of coordination defects or nucleate as a new crystalline β-tin phase within the a-Si network.

In-situ high-pressure powder X-ray diffraction study of α-zirconium phosphate

The high-pressure structural chemistry of α-zirconium phosphate, α-Zr(HPO4)2·H2O, was studied using in-situ high-pressure diffraction and synchrotron radiation. The layered phosphate was studied under both hydrostatic and non-hydrostatic conditions and Rietveld refinement carried out on the resulting diffraction patterns. It was found that under hydrostatic conditions no uptake of additional water molecules from the pressure-transmitting medium occurred, contrary to what had previously been observed with some zeolite materials and a layered titanium phosphate. Under hydrostatic conditions the sample remained crystalline up to 10 GPa, but under non-hydrostatic conditions the sample amorphized between 7.3 and 9.5 GPa. The calculated bulk modulus, K0 = 15.2 GPa, showed the material to be very compressible with the weak linkages in the structure of the type Zr-O-P.

Effects of deviatoric stresses in the diamond-anvil pressure cell on single-crystal samples

The nonhydrostatic stress states that are developed in the pressure media within diamond-anvil pressure cells have been investigated by single-crystal X-ray diffraction. Measurements of unit-cell parameters of small single crystals under nonhydrostatic conditions are used to calculate the deviatoric strains and, through knowledge of the elastic tensors of the crystals, the stress state of the media. The results confirm that the stress state is effectively cylindrically symmetrical with the stress parallel to the load axis being greater than the radial stresses. The stress state in a given medium can be predicted and can be used to design a specific response of the lattice parameters of small single crystals to pressure beyond the hydrostatic pressure limit of the pressure medium.

Invited article: High-pressure techniques for condensed matter physics at low temperature

Condensed matter experiments at high pressure accentuate the need for accurate pressure scales over a broad range of temperatures, as well as placing a premium on a homogeneous pressure environment. However, challenges remain in diamond anvil cell technology, including both the quality of various pressure transmitting media and the accuracy of secondary pressure scales at low temperature. We directly calibrate the ruby fluorescence R1 line shift with pressure at T=4.5 K using high-resolution x-ray powder diffraction measurements of the silver lattice constant and its known equation of state up to P=16 GPa. Our results reveal a ruby pressure scale at low temperatures that differs by 6% from the best available ruby scale at room T. We also use ruby fluorescence to characterize the pressure inhomogeneity and anisotropy in two representative and commonly used pressure media, helium and methanol:ethanol 4:1, under the same preparation conditions for pressures up to 20 GPa at T=5 K. Contrary to the accepted wisdom, both media show equal levels of pressure inhomogeneity measured over the same area, with a consistent DeltaP/P per unit area of +/-1.8 %/(10(4) microm(2)) from 0 to 20 GPa. The helium medium shows an essentially constant deviatoric stress of 0.021+/-0.011 GPa up to 16 GPa, while the methanol:ethanol mixture shows a similar level of anisotropy up to 10 GPa, above which the anisotropy increases. The quality of both pressure media is further examined under the more stringent requirements of single crystal x-ray diffraction at cryogenic temperature. For such experiments we conclude that the ratio of sample-to-pressure chamber volume is a critical parameter in maintaining sample quality at high pressure, and may affect the choice of pressure medium.