Synthesis and in-depth structure determination of a novel metastable high-pressure CrTe3 phase.

This study reports the synthesis and crystal structure determination of a novel CrTe3 phase using various experimental and theoretical methods. The average stoichiometry and local phase separation of this quenched high-pressure phase were characterized by ex situ synchrotron powder X-ray diffraction and total scattering. Several structural models were obtained using simulated annealing, but all suffered from an imperfect Rietveld refinement, especially at higher diffraction angles. Finally, a novel stoichiometrically correct crystal structure model was proposed on the basis of electron diffraction data and refined against powder diffraction data using the Rietveld method. Scanning electron microscopy-energy-dispersive X-ray spectrometry (EDX) measurements verified the targeted 1:3 (Cr:Te) average stoichiometry for the starting compound and for the quenched high-pressure phase within experimental errors. Scanning transmission electron microscopy (STEM)-EDX was used to examine minute variations of the Cr-to-Te ratio at the nanoscale. Precession electron diffraction (PED) experiments were applied for the nanoscale structure analysis of the quenched high-pressure phase. The proposed monoclinic model from PED experiments provided an improved fit to the X-ray patterns, especially after introducing atomic anisotropic displacement parameters and partial occupancy of Cr atoms. Atomic resolution STEM and simulations were conducted to identify variations in the Cr-atom site-occupancy factor. No significant variations were observed experimentally for several zone axes. The magnetic properties of the novel CrTe3 phase were investigated through temperature- and field-dependent magnetization measurements. In order to understand these properties, auxiliary theoretical investigations have been performed by first-principles electronic structure calculations and Monte Carlo simulations. The obtained results allow the observed magnetization behavior to be interpreted as the consequence of competition between the applied magnetic field and the Cr-Cr exchange interactions, leading to a decrease of the magnetization towards T = 0 K typical for antiferromagnetic systems, as well as a field-induced enhanced magnetization around the critical temperature due to the high magnetic susceptibility in this region.

High pressure synthesis and the valence state of vanadium ions for the novel transition metal pernitride, CuAl2-type VN2.

A novel transition metal pernitride, CuAl2-type VN2, has been synthesized at a pressure above 73.3 GPa. The bulk modulus has been determined to be K0 = 347(12) GPa. By hard X-ray absorption spectrum measurements of VN2, the valence state of transition metal ions in pernitrides has been for the first time experimentally reported.

Pressure-Tunable Crystal Structure and Magnetic Transition Temperature of the Nowotny Chimney-Ladder CrGeγ Phase.

A Nowotny chimney-ladder (NCL) chromium germanide (CrGeγ) with varying compositions has been synthesized under high pressure. Crystal structure parameters of the NCL CrGeγ have been calculated by Le Bail refinement based on the superspace group. The refined γ of CrGeγ increases with the synthesis pressure, indicating an increasing Ge content. The NCL CrGeγ phases are ferromagnetic at T = 2 K regardless of their composition, and the magnetic transition temperature (TC) increases when the γ becomes higher. It is noteworthy that CrGe1.763 and CrGe1.774 synthesized at P = 10 and 14 GPa have magnetic transition temperatures of T = 295 and 333 K above room temperature, respectively. Surprisingly, the magnetic transition temperature has changed by ΔTC = 270 K, although the γ values of the raw material and the sample synthesized at P = 14 GPa differ by only Δγ = 0.05, corresponding to an atomic concentration of 0.62 atom %. The synthesis pressure acts as an essential parameter in tuning the composition of the NCL phase. Accordingly, the high-pressure synthesis may significantly control several physical characteristics of NCL phases by utilizing compositional and structural modulation.

Crystal and Electronic Structure of U7Te12-Type Tungsten Nitride Synthesized under High Pressure.

Tungsten nitride continues to drive fundamental interests because of its potential as a functional compound, which combines features such as high hardness together with thermal, chemical, and wear resistance. Here, we report a novel tungsten nitride phase synthesized from MoC-type WN0.6 and molecular nitrogen after laser irradiation at P = 70 GPa in a diamond anvil cell. This novel phase is quenchable at ambient pressure and determined to be U7Te12-type tungsten nitride and crystallizes in the hexagonal space group (P6) having lattice parameters of a = 8.2398(3) Å, c = 2.94948(14) Å, and V = 173.423(13) Å3. Tungsten atoms are coordinated to eight and nine nitrogen atoms, higher than previously reported tungsten nitrides. The bulk modulus is determined to be K0 = 312 (5) GPa (K0' = 4.0 fixed), and DFT calculations predict that U7Te12-type W7N12 has a metallic nature.

Phase relations and thermoelasticity of magnesium silicide at high pressure and temperature.

Within the exploration of sustainable and functional materials, narrow bandgap magnesium silicide semiconductors have gained growing interest. Intriguingly, squeezing silicides to extreme pressures and exposing them to non-ambient temperatures proves fruitful to study the structural behavior, tune the electronic structure, or discover novel phases. Herein, structural changes and thermoelastic characteristics of magnesium silicides were probed with synchrotron x-ray diffraction techniques using the laser-heated diamond anvil cell and large volume press at high pressure and temperature and temperature-dependent synchrotron powder diffraction. Probing the ambient phase of Mg2Si (anti-CaF2-type Mg2Si, space group: Fm3¯m) at static pressures of giga-Pascals possibly unveiled the transformation to metastable orthorhombic anti-PbCl2-type Mg2Si (Pnma). Interestingly, heating under pressures introduced the decomposition of Mg2Si to hexagonal Mg9Si5 (P63) and minor Mg. Using equations of state (EoS), which relate pressure to volume, the bulk moduli of anti-CaF2-type Mg2Si, anti-PbCl2-type Mg2Si, and Mg9Si5 were determined to be B0 = 47(2) GPa, B0 ≈ 72(5) GPa, and B0 = 58(3) GPa, respectively. Employing a high-temperature EoS to the P-V-T data of anti-CaF2-type Mg2Si provided its thermoelastic parameters: BT0 = 46(3) GPa, B'T0 = 6.1(8), and (∂BT0/∂T)P = -0.013(4) GPa K-1. At atmospheric pressure, anti-CaF2-type Mg2Si kept stable at T = 133-723 K, whereas Mg9Si5 transformed to anti-CaF2-type Mg2Si and Si above T ≥ 530 K. This temperature stability may indicate the potential of Mg9Si5 as a mid-temperature thermoelectric material, as suggested from previous first-principles calculations. Within this realm, thermal models were applied, yielding thermal expansion coefficients of both silicides together with estimations of their Grüneisen parameter and Debye temperature.

Crystal and Electronic Structures of MoSi2-Type CrGe2 Synthesized under High Pressure.

Chromium germanides, namely, Nowotny chimney-ladder-phase CrGe1.77 and MoSi2-type CrGe2, were synthesized above 15 GPa or more via laser heating using a diamond anvil cell (LHDAC). MoSi2-type CrGe2, which is the most Ge-rich compound in the Cr-Ge system, crystallizes in the tetragonal structure with a space group of I4/mmm (no. 139) and lattice parameters of a = 3.24919(6) Å and c = 8.0523(3) Å and is isostructural with MoSi2. MoSi2-type CrGe2 has a deep pseudogap caused by the splitting of 3d orbitals with Cr, as evidenced by ab initio calculation. In this article, we have succeeded in synthesizing a binary compound between transition-metal and metalloid elements for the first time at high pressures above 10 GPa using the LHDAC. This pathway opens the possibility to explore more compounds in this system and may provide new insights into the fundamental interaction between these two elements.

Thermal Expansion of Incompressible U2S3-Type Nb2N3 Synthesized in a Diamond Anvil Cell.

A novel niobium nitride, U2S3-type Nb2N3, has been successfully synthesized by nitridation of δ-NbN above approximately 30 GPa in a laser-heated diamond anvil cell. Nb2N3 crystallizes in the same orthorhombic structure (space group: Pnma) as η-Ta2N3. Nb2N3 consists of regular-shaped polyhedra, and the bulk modulus has been determined to K0 = 300(2) GPa. The low-temperature X-ray diffraction measurements have been successfully conducted for the tiny novel Nb2N3 between 297.7(5) and 106.3(3) K under ambient pressure. Nb2N3 shows no structural phase transition down to 106.3(3) K, and investigation of the linear thermal expansion coefficients yields αa = 3.36(9) × 10-6 K-1, αb = 5.39(10) × 10-6 K-1, αc = 6.77(15) × 10-6 K-1, respectively. Our study reveals that the incompressible novel nitride shows low thermal expansion behavior, which offers new insights for the development of functional nitrides and their crystal chemistry.