The Raman scattering behavior of anatase nanocrystals with average diameters of 4, 8, 20, and 34 nm has been compared with bulk crystal data in order to establish size-dependent changes to the phonon spectrum at ambient conditions. Further, the high-pressure behavior of the anatase nanocrystals was examined at room-temperature using in situ Raman scattering data obtained in diamond-anvil cells to a maximum pressure of 41 GPa. The size-dependent changes to the Raman spectrum are best explained in terms of three-dimensional confinement of phonons in finite-sized nanocrystalline anatase. The difference in slopes obtained for the pressure shifts of Raman modes between nanocrystalline and single crystal anatase is in conformity with the observed size-dependent bulk modulus values. The metastability of anatase as a function of pressure is demonstrated to be size dependent, with smaller crystallites preserving the structure to higher pressures. Three size regimes have been recognized for the pressure-induced phase transition of anatase at room temperature: an anatase-amorphous transition regime at the smallest crystallite sizes, an anatase-baddeleyite transition regime at intermediate crystallite sizes, and an anatase–$\alpha \text{−}\mathrm{Pb}{\mathrm{O}}_2$ transition regime comprising large nanocrystals to macroscopic single crystals. This size-dependent phase selectivity of anatase at high pressures explains the recent contradictory experimental data. A semiquantitative phase diagram for anatase metastability as a function of size and pressure at room temperature is proposed.

We present a new design of a universal diamond anvil cell, suitable for different kinds of experimental studies under high pressures. Main features of the cell are an ultimate 90-degrees symmetrical axial opening and high stability, making the presented cell design suitable for a whole range of techniques from optical absorption to single-crystal X-ray diffraction studies, also in combination with external resistive or double-side laser heating. Three examples of the cell applications are provided: a Brillouin scattering of neon, single-crystal X-ray diffraction of α-Cr₂O₃, and resistivity measurements on the (Mg_0.60Fe_0.40)(Si_0.63Al_0.37)O₃ silicate perovskite.

A material as hard as diamond or cubic boron nitride has yet to be identified^1,2,3,4,5,6, but here we report the discovery of a cotunnite-structured titanium oxide which represents the hardest oxide known. This is a new polymorph of titanium dioxide, where titanium is nine-coordinated to oxygen in the cotunnite (PbCl₂) structure. The phase is synthesized at pressures above 60 gigapascals (GPa) and temperatures above 1,000 K and is one of the least compressible and hardest polycrystalline materials to be described.

Since invention of the diamond anvil cell technique in the late 1950s for studying materials at extreme conditions, the maximum static pressure generated so far at room temperature was reported to be about 400 GPa. Here we show that use of micro-semi-balls made of nanodiamond as second-stage anvils in conventional diamond anvil cells drastically extends the achievable pressure range in static compression experiments to above 600 GPa. Micro-anvils (10–50 μm in diameter) of superhard nanodiamond (with a grain size below ∼ 50 nm) were synthesized in a large volume press using a newly developed technique. In our pilot experiments on rhenium and gold we have studied the equation of state of rhenium at pressures up to 640 GPa and demonstrated the feasibility and crucial necessity of the in situ ultra high-pressure measurements for accurate determination of material properties at extreme conditions.

Lonsdaleite, also called hexagonal diamond, has been widely used as a marker of asteroidal impacts. It is thought to play a central role during the graphite-to-diamond transformation, and calculations suggest that it possesses mechanical properties superior to diamond. However, despite extensive efforts, lonsdaleite has never been produced or described as a separate, pure material. Here we show that defects in cubic diamond provide an explanation for the characteristic d-spacings and reflections reported for lonsdaleite. Ultrahigh-resolution electron microscope images demonstrate that samples displaying features attributed to lonsdaleite consist of cubic diamond dominated by extensive {113} twins and {111} stacking faults. These defects give rise to nanometre-scale structural complexity. Our findings question the existence of lonsdaleite and point to the need for re-evaluating the interpretations of many lonsdaleite-related fundamental and applied studies.