Leilei Zhang

Micro-stress dominant displacive reconstructive transition in lithium aluminate

It is supposed that diffusive reconstructive transitions usually take place under hydrostatic pressure or low stresses, and displacive reconstructive phase transitions easily occur at nonhydrostatic pressure. Here, by in-situ high pressure synchrotron X-ray diffraction and single-crystal Raman scattering studies on lithium aluminate at room temperature, we show that the reconstructive transition mechanism is dependent on the internal microscopic stresses rather than the macroscopic stresses. In this case, even hydrostatic pressure can favor the displacive transition if the compressibility of crystal is anisotropic. During hydrostatic compression, γ-LiAlO2 transforms to δ-LiAlO2 at about 4u2009GPa, which is much lower than that in previous nonhydrostatic experiments (above 9u2009GPa). In the region where both phases coexist, there are enormous microscopic stresses stemming from the lattice mismatch, suggesting that this transition is displacive. Furthermore, the atomic picture is drawn with the help of the shear R...

Strengthening effects of interstitial nitrogen on rhenium

Introducing interstitial nitrogen atoms into the rhenium lattices is accepted for enhancing the mechanical and thermal properties of the metal. Here, we present an experimental study on the strengthening effects of interstitial nitrogen in rhenium on bulk modulus B0 and elastic shear constant C44 by comparing the difference in the compression and vibration behaviors between Re3N and Re in the in situ high pressure angle dispersive X-ray diffraction and high-pressure Raman scattering. The incorporation of interstitial N atoms introduces the covalent bonding and the special Z-type structure along the c-axis, which caused the bulk modulus B0 and elastic shear constant C44 increase by 14% and 15%, and the incompressibility of the c-axis is obviously enhanced. In addition, this work well explains the strengthening effects of interstitial N atoms on mechanical and vibrational properties.Introducing interstitial nitrogen atoms into the rhenium lattices is accepted for enhancing the mechanical and thermal properties of the metal. Here, we present an experimental study on the strengthening effects of interstitial nitrogen in rhenium on bulk modulus B0 and elastic shear constant C44 by comparing the difference in the compression and vibration behaviors between Re3N and Re in the in situ high pressure angle dispersive X-ray diffraction and high-pressure Raman scattering. The incorporation of interstitial N atoms introduces the covalent bonding and the special Z-type structure along the c-axis, which caused the bulk modulus B0 and elastic shear constant C44 increase by 14% and 15%, and the incompressibility of the c-axis is obviously enhanced. In addition, this work well explains the strengthening effects of interstitial N atoms on mechanical and vibrational properties.

Reciprocating Compression of ZnO Probed by X-ray Diffraction: The Size Effect on Structural Properties under High Pressure

Zinc oxide, ZnO, an important technologically relevant binary compound, was investigated by reciprocating compress the sample in a diamond anvil cell using in situ high-pressure synchrotron X-ray diffraction at room temperature. The starting sample (∼200 nm) was compressed to 20 GPa and then decompressed to ambient condition. The quenched sample, with average grain size ∼10 nm, was recompressed to 20 GPa and then released to ambient condition. The structural stability and compressibility of the initial bulk ZnO and quenched nano ZnO were compared. Results reveal that the grain size and the fractional cell distortion have little effect on the structural stability of ZnO. The bulk modulus of the B4 (hexagonal wurtzites structure) and B1 (cubic rock salt structure) phases for bulk ZnO under hydrostatic compression were estimated as 164(3) and 201(2) GPa, respectively. Importantly, the effect of pressure in atomic positions, bond distances, and bond angles was obtained. On the basis of this information, the B4-to-B1 phase transformation was demonstrated to follow the hexagonal path rather than the tetragonal path. For the first time, the detail of the intermediate hexagonal ZnO, revealing the B4-to-B1 transition mechanism, was detected by experimental method. These findings enrich our knowledge on the diversity of the size influences on the high-pressure behaviors of materials and offer new insights into the mechanism of the B4-to-B1 phase transition that is commonly observed in many other wurzite semiconductor compounds.

Direct observation of melted Mott state evidenced from Raman scattering in 1T-TaS2 single crystal*

The evolution of electron correlation and charge density wave (CDW) in 1T-TaS2 single crystal has been investigated by temperature-dependent Raman scattering, which undergoes two obvious peaks of A 1g modes about 70.8 cm−1 and 78.7 cm−1 at 80 K, respectively. The former peak at 70.8 cm−1 is accordant with the lower Hubbard band, resulting in the electron-correlation-driven Mott transition. Strikingly, the latter peak at 78.7 cm−1 shifts toward low energy with increasing the temperature, demonstrating the occurrence of nearly commensurate CDW phase (melted Mott phase). In this case, phonon transmission could be strongly coupled to commensurate CDW lattice via Coulomb interaction, which likely induces appearance of hexagonal domains suspended in an interdomain phase, composing the melted Mott phase characterized by a shallow electron pocket. Combining electronic structure, atomic structure, transport properties with Raman scattering, these findings provide a novel dimension in understanding the relationship between electronic correlation, charge order, and phonon dynamics.

Neutron powder diffraction and high-pressure synchrotron x-ray diffraction study of tantalum nitrides*

Tantalum nitride (TaN) compact with a Vickers hardness of 26 GPa is prepared by a high-pressure and hightemperature (HPHT) method. The crystal structure and atom occupations of WC-type TaN have been investigated by neutron powder diffraction, and the compressibility of WC-type TaN has been investigated by using in-situ high-pressure synchrotron x-ray diffraction. The third-order Birch–Murnaghan equation of state fitted to the x-ray diffraction pressure–volume (P – V) sets of data, collected up to 41 GPa, yields ambient pressure isothermal bulk moduli of B 0 = 369(2) GPa with pressure derivatives of for the WC-type TaN. The bulk modulus of WC-type TaN is not in good agreement with the previous result (B 0 = 351 GPa), which is close to the recent theoretical calculation result (B 0 = 378 GPa). An analysis of the experiment results shows that crystal structure of WC-type TaN can be viewed as alternate stacking of Ta and N layers along the c direction, and the covalent Ta–N bonds between Ta and N layers along the c axis in the crystal structure play an important role in the incompressibility and hardness of WC-type TaN.