Ultra-incompressible Material Research Articles

An unexpectedly stable Y2B5 compound with the fractional stoichiometry under ambient pressure

Boron-rich yttrium borides are an exceptional group of compounds not only with excellent mechanical properties, but also with particular superconducting and thermoelectric properties. Although the Y–B compounds with integral components have been extensively investigated experimentally and theoretically, the yttrium borides with the fractional stoichiometries are rarely observed. Herein, utilizing a combination of the CALYPSO method for crystal structure prediction and first-principles calculations, we made an investigation on a broad range of stoichiometries of yttrium borides. An extraordinary stable Y2B5 compound possessing the fractional stoichiometry with the monoclinic P121/c1 phase is firstly uncovered. Structurally, the P121/c1-Y2B5 crystalline consists of the distorted B6 octahedrons and seven-member B rings. Remarkably, the B–B covalent network following the increment of the boron content in six concerned yttrium borides undergoes an increasing dimension, quasi one-dimensional chain → two-dimensional B ring → a combination of two-dimensional B ring and three-dimensional B6 octahedron → three-dimensional B24 cage. According to a microscopic hardness model, P121/c1–Y2B5 is considered as an incompressible and hard material with the hardness of 18.83 GPa. More importantly, Fm-3 m-YB12 can be classified into an ultra-incompressible material with the appreciable Vickers hardness of 33.16 GPa. The present consequences can provide important insights for understanding the complex crystal structures of boron-rich yttrium borides and stimulate further experimental synthesis of novel multifunctional materials with the fractional compositions.

Nitride Spinel: An Ultraincompressible High-Pressure Form of BeP2 N4.

Owing to its outstanding elastic properties, the nitride spinel γ‐Si3N4 is of considered interest for materials scientists and chemists. DFT calculations suggest that Si3N4‐analog beryllium phosphorus nitride BeP2N4 adopts the spinel structure at elevated pressures as well and shows outstanding elastic properties. Herein, we investigate phenakite‐type BeP2N4 by single‐crystal synchrotron X‐ray diffraction and report the phase transition into the spinel‐type phase at 47 GPa and 1800 K in a laser‐heated diamond anvil cell. The structure of spinel‐type BeP2N4 was refined from pressure‐dependent in situ synchrotron powder X‐ray diffraction measurements down to ambient pressure, which proves spinel‐type BeP2N4 a quenchable and metastable phase at ambient conditions. Its isothermal bulk modulus was determined to 325(8) GPa from equation of state, which indicates that spinel‐type BeP2N4 is an ultraincompressible material.

Nitride Spinel: An Ultraincompressible High‐Pressure Form of BeP2N4

AbstractOwing to its outstanding elastic properties, the nitride spinel γ‐Si3N4 is of considered interest for materials scientists and chemists. DFT calculations suggest that Si3N4‐analog beryllium phosphorus nitride BeP2N4 adopts the spinel structure at elevated pressures as well and shows outstanding elastic properties. Herein, we investigate phenakite‐type BeP2N4 by single‐crystal synchrotron X‐ray diffraction and report the phase transition into the spinel‐type phase at 47 GPa and 1800 K in a laser‐heated diamond anvil cell. The structure of spinel‐type BeP2N4 was refined from pressure‐dependent in situ synchrotron powder X‐ray diffraction measurements down to ambient pressure, which proves spinel‐type BeP2N4 a quenchable and metastable phase at ambient conditions. Its isothermal bulk modulus was determined to 325(8) GPa from equation of state, which indicates that spinel‐type BeP2N4 is an ultraincompressible material.

Structural, elastic properties and electronic structure of Cr3B4-type borides: An ultra-incompressible material

The structural, elastic properties, hardness and electronic structure of TM3B4 (TM=Cr, Re, Ru and Os) borides are investigated by using first-principles approach. The calculated results show that Re3B4 is more stable than that of other Cr3B4-type borides. These borides all have high bulk modulus and high elastic constant C22 compared with the corresponding diborides. The calculated value of B/G ratio of Cr3B4 (1.404) is lower than 1.75, indicating that this boride exhibits brittle behavior. The calculate bulk modulus of Re3B4 and intrinsic hardness of Cr3B4 is 380GPa and 38.3GPa, respectively. The high bulk modulus and hardness derive from the strong B–B and Cr–B covalent bonds along the a–c plane because of the strong charge accumulation between B–B and Cr–B atoms.

Ultra-incompressible superconducting phase of OsC predicted by phonon calculations

The earlier proposed WC-type and NiAs-type structures of the synthesized potential superhard material OsC were found to be dynamically unstable at ambient pressure by ab initio phonon calculations. An energetically more favorable phase with the orthorhombic P m n 2 1 structure consisted of pyramids was then predicted by employing frozen-phonon technique. The P m n 2 1 structure is stable up to about 80 GPa and then transforms to the WC-type structure. Results show that OsC with the P m n 2 1 structure is an ultra-incompressible material originated from the three-dimensional network of the covalent Os C bond. Electronic and electron–phonon coupling calculations suggest that the P m n 2 1 OsC is metallic and possesses superconductivity.

Crystal structure and physical properties of OsN: First-principle calculations

Using ab initio evolutionary methodology for crystal structure prediction, we have found two orthorhombic structures of P m n 2 1 and C m c 2 1 for potential superhard OsN, energetically much superior to the previously proposed NaCl-type and WC-type structures. The P m n 2 1 structure which consisted of distorted OsN 4 tetrahedra is stable up to 62 GPa, above which C m c 2 1 becomes energetically more favorable. The C m c 2 1 structure contains the Os–Os and Os–N–N chains and possesses the unique diatomic N–N bond. OsN within two orthorhombic phases is found to be an ultra-incompressible material due to the high bulk modulus (∼350 GPa), which originates from the strong and directional covalent bonds in two structures. Analysis of the calculated formation energy suggested that the two structures could be synthesized at moderate pressures of ∼20 GPa.

Mechanical stability and thermodynamic properties of OsC from first-principles calculations

The elastic and thermodynamic characteristics of OsC crystal have been predicted through a method of density functional theory within the generalized gradient approximation (GGA). Compared with WC-type OsC, NaCl-type OsC is not only energy unfavorable but also mechanics unstable. The five independent elastic constants ( C ij ), bulk modulus ( B 0 ), the dependence of bulk modulus on temperature and pressure as well as the thermal expansion coefficient ( α V ) at various temperatures for WC-type OsC are discussed. According to our calculations, WC-type OsC should be an ultra-incompressible material with high bulk modulus about 381 GPa. In addition, the bulk modulus will increase with increasing pressure while decrease with increasing temperature. The researches on the thermal expansion coefficient indicate that there will be a knee point during the process of thermal expansion coefficient variation versus increasing temperature. Our results may provide useful information for theoretical and experimental investigation of OsC.

Structural stability and mechanical property of WN from first-principles calculations

The structure and mechanical stability of WN crystal are investigated using the first-principle calculations based on density functional theory (DFT). Eight possible crystal structures are studied and the results reveal that WC-type WN is metastable, which is different from the conclusions obtained by Schoenberg. Our calculated results show that NiAs-type WN is not only the most energy favorable but also meets the requirements of mechanical stability. The investigation on anisotropic incompressibility indicates that NiAs-type WN is an ultra-incompressible material, especially in c-axis direction, which is much more incompressible than that of diamond. Calculated density of states, charge density distribution as well as Mulliken population analysis show that the chemical bonding of WN is a mixture of covalent, ionic and metallic bonding.

Osmium Metal Studied under High Pressure and Nonhydrostatic Stress

Interest in osmium as an ultra-incompressible material and as an analog for the behavior of iron at high pressure has inspired recent studies of its mechanical properties. We have measured elastic and plastic deformation of Os metal at high pressures using in situ high pressure x-ray diffraction in the radial geometry. We show that Os has the highest yield strength observed for any pure metal, supporting up to 10 GPa at a pressure of 26 GPa. Furthermore, our data indicate changes in the nonhydrostatic apparent c/a ratio and clear lattice preferred orientation effects at pressures above 15 GPa.

Crystal structure and physical properties ofOsN2andPtN2in the marcasite phase

We have theoretically evaluated a recently synthesized ${\mathrm{OsN}}_{2}$ that was shown to have a large bulk modulus, and we propose a structure that is isostructural with marcasite with the space group Pnnm. The calculated lattice constants, bulk modulus, and x-ray diffraction spectrum of ${\mathrm{OsN}}_{2}$ are in good agreement with the experimental results. Calculations of bulk modulus and hardness indicate that the marcasite-type ${\mathrm{OsN}}_{2}$ is an ultraincompressible material instead of a superhard one. The possibility of the marcasite ${\mathrm{PtN}}_{2}$ has also been investigated, showing that the marcasite ${\mathrm{PtN}}_{2}$ could exist as a metastable phase at pressures up to $50\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$, being elastically stable with a slightly higher energy ($32\phantom{\rule{0.3em}{0ex}}\mathrm{meV}∕\mathrm{atom}$ at $0.1\phantom{\rule{0.3em}{0ex}}\mathrm{MPa}$) than that in the pyrite structure.

First-principles study of low compressibility osmium borides

Using first-principles total energy calculations we investigate the structural, elastic, and electronic properties of OsB2 and OsB, respectively. The calculated equilibrium structural parameters of OsB2 are in agreement with the available experimental results. The calculations indicate that OsB in tungsten carbide is more energetically stable under the ambient condition than the metastable cesium chloride phase of OsB. Results of bulk modulus show that they are potential low compressible materials. The hardness of OsB2 is estimated by employing a semiempirical theory. The results indicate that OsB2 is an ultraincompressible material, but not a superhard material. The method designing superhard materials is different from one creating ultraincompressible materials.