Diamond-anvil-cell and shock-wave technologies now permit the study of matter under multimegabar pressure (that is, of several hundred GPa). The properties of matter in this pressure regime differ drastically from those known at 1 atm (about 105 Pa). Just how different chemistry is at high pressure and what role chemical intuition for bonding and structure can have in understanding matter at high pressure will be explored in this account. We will discuss in detail an overlapping hierarchy of responses to increased density: a) squeezing out van der Waals space (for molecular crystals); b) increasing coordination; c) decreasing the length of covalent bonds and the size of anions; and d) in an extreme regime, moving electrons off atoms and generating new modes of correlation. Examples of the startling chemistry and physics that emerge under such extreme conditions will alternate in this account with qualitative chemical ideas about the bonding involved.

The chemical imagination at work in very tight places.

Jianwei Li,† Shouzhi Ma,‡ Xinjuan Liu, Zhaofeng Zhou,† and Chang Q Sun*,†,‡ †Institute for Quantum Engineering and Micro-Nano Energy Technology, Key Laboratory of Low-Dimensional Materials and Application Technologies, and Faculty of Materials and Optoelectronics and Physics, Xiangtan University, Hunan 411105, China ‡School of Electrical, and Electronic Engineering, Nanyang Technological University, Singapore 639798 Engineering Research Center for Nanophotonics & Advanced Instrument, Ministry of Education, Department of Physics, East China Normal University, Shanghai, 200062 China

https://www3.ntu.edu.sg/home/ecqsun/rtf/CR-ZnO.pdf

ZnO meso-mechano-thermo physical chemistry.

The stability of rocksalt (B1) phase of ZnO under high pressure up to 2Mbars was studied using angular dispersive x-ray diffraction at room temperature and compared to the structurally analogous CdO where both theoretical and experimental results are available. First-principles total energy calculations predict the B1-to-B2 (CsCl-type) phase transition in ZnO and CdO to occur at 261 and 83GPa, respectively. The bulk moduli, K0, of the B1 structure of ZnO were measured and the possible high pressure CsCl-type (B2) structure was looked for. The potential application of ZnO as an internal pressure marker in x-ray diffraction study at the megabar range is proposed.

Stability of rocksalt phase of zinc oxide under strong compression: Synchrotron x-ray diffraction experiments and first-principles calculation studies

Series of metastable Me1-xZnxO solid solutions (Me = Ni2+, Co2+, Fe2+, Mn2+) with the rocksalt (rs) crystal structure have been synthesized from the binary oxides by quenching from 7.7 GPa and 1450-1650 K. Phase composition of the samples, as well as structural properties and stoichiometry of synthesized solid solutions have been studied by X-ray powder diffraction, both conventional and with synchrotron radiation. The widest (0.3 \leq x \leq 0.8) composition range of the existence of individual rock-salt solid solution has been established for the NiO-ZnO system. The bulk rs-Co1-xZnxO, rs-Fe1-xZnxO and rs-Mn1-xZnxO solid solutions may be quenched down to ambient conditions only with twice lower ZnO content, i.e. x \leq 0.5, 0.5 and 0.4, respectively; while formation of rock-salt solid solutions in the CdO-ZnO system has not been observed in the whole concentration range.

Synthesis of rock-salt MeO-ZnO solid solutions (Me = Ni2+, Co2+, Fe2+, Mn2+) at high pressure and high temperature

A series of metastable Me1−x Zn x O solid solutions (Me = Ni2+, Co2+, Fe2+, Mn2+) with the rock-salt (rs) crystal structure have been synthesized from binary oxides by quenching at 7.7 GPa and 1450–1650 K. Phase composition of the samples, as well as structural properties and stoichiometry of synthesized solid solutions, have been studied by X-ray powder diffraction, both conventional and with synchrotron radiation. The widest (0.3 ≤x≤0.8) composition range of the existence of individual rs solid solution has been established for the NiO–ZnO system. The bulk rs-Co1−x Zn x O, rs-Fe1−x Zn x O and rs-Mn1−x Zn x O solid solutions may be quenched down to ambient conditions with only twice as low ZnO content, i.e. x≤0.5, 0.5 and 0.4, respectively; while formation of rs solid solutions in the CdO–ZnO system has not been observed in the entire concentration range.

Synthesis of rock-salt MeO–ZnO solid solutions (Me=Ni 2+, Co2+, Fe2+, Mn2+) at high pressure and high temperature

We have carried out high-pressure X-ray structural studies of BeO and ZnO up to 200 GPa to find the phase transition of BeO from the wurzite to the NaCl structure and that of ZnO from the NaCl to the CsCl structure, which are expected theoretically. The phase transition of BeO occurred at 137 GPa, which is in good agreement with the theoretical value of 139 GPa. The wurzite phase persisted up to 175 GPa. The bulk modulus of BeO at zero pressure is determined to be 223 GPa by the Birch's equation. As previously reported, the ZnO sample transformed from the wurzite structure to the NaCl structure at 9.1 GPa. Extending the pressure range up to 202 GPa no phase transition to the CsCl structure occurred. The bulk modulus of the NaCI structure was determined to be 194 GPa.

High-pressure X-ray structural study of BeO and ZnO to 200 GPa

High-pressure X-ray diffraction studies of Co x Zn 1-x O (x=0, 0.05, 0.10) were carried out with a synchrotron radiation source. The crystal parameters for three samples under pressure were refined by the Rietveld method. The bulk moduli for Co x Zn 1-x O (x = 0, 0.05, 0.10) were determined by Birch's equation of state to be 144.9 ± 0.6, 143.4 ± 4.2, and 138.5 ± 1.9 GPa, and also the transition pressure from the wurtzite structure to the rocksalt structure were determined to be 8.2, 7.7, and 6.1 GPa, respectively. On increasing the Co-doping into the ZnO, both the bulk moduli and transition pressures decrease, but the rates are quite different. The bulk modulus decreases by 4.8% with 10% Co-doping but the transition pressure decreases by 25%.

N. Niiya

Papers

High-pressure X-ray structural study of BeO and ZnO to 200 GPa

Pressure-induced phase transition in Co-doped ZnO