Guo-Hua Zhong

Phase transformations and vibrational properties of coronene under pressure

Both the vibrational and structural properties of coronene have been investigated upon compression up to 30.5 GPa at room temperature by a combination of Raman scattering and synchrotron x-ray diffraction measurements. The spectroscopic and crystallographic results demonstrate that two pressure-induced structural phase transitions take place at 1.5 GPa and 12.2 GPa where the high-pressure phases are identified as monoclinic and orthorhombic crystal structures with space groups of P2/m and Pmmm, respectively. A kink in the slope of the cell parameters as a function of pressure is associated with the disappearance of several internal Raman modes, which suggests the existence of structural distortions or reorganizations at approximately 6.0 GPa. Above 17.1 GPa, almost no evidence of crystallinity can be observed, indicating a possible transformation of coronene into an amorphous phase.

Constraint on the potassium content for the superconductivity of potassium-intercalated phenanthrene

Raman-scattering measurements were performed on K(x)phenanthrene (0 ⩽ x ⩽ 6.0) at room temperature. Three phases (x = 3.0, 3.5, and 4.0) are identified based on the obtained Raman spectra. Only the K3phenanthrene phase is found to exhibit the superconducting transition at 5 K. The C-C stretching modes are observed to broaden and become disordered in K(x)phenanthrene with x = 2.0, 2.5, 6.0, indicating some molecular disorder in the metal intercalation process. This disorder is expected to influence the nonmetallic nature of these materials. The absence of metallic character in these nonsuperconducting phases is found from the calculated electronic structures based on the local density approximation.

Structural and vibrational properties of phenanthrene under pressure

The structural and vibrational properties of phenanthrene are measured at high pressures up to 30.2 GPa by Raman spectroscopy and synchrotron X-ray diffraction techniques. Two phase transitions are observed in the Raman spectra at pressures of 2.3 GPa and 5.4 GPa which correspond to significant changes of intermolecular and intramolecular vibrational modes. Above 10.2 GPa, all the Raman peaks are lost within the fluorescence background; however, upon further compression above 20.0 GPa, three broad peaks are observed at 1600, 2993, and 3181 cm(-1), indicating that phenanthrene has transformed into amorphous phase. Using X-ray diffraction, the structures of corresponding phases observed from Raman spectra are indexed with space groups of P2(1) for phase I (0-2.2 GPa), P2/m for phase II (2.2-5.6 GPa), P2/m+Pmmm for phase III (5.6-11.4 GPa) which has a coexistence of structures, and above 11.4 GPa the structure is indexed with space group of Pmmm. Although phenanthrene has transformed to a hydrogenated amorphous carbon structure above 20.0 GPa, these amorphous clusters still show characteristic crystalline behavior based on our X-ray diffraction patterns. Our results suggest that the long-range periodicity and the local disorder state coexist in phenanthrene at high pressures.

Electronic and magnetic structures of 4f in Ga1-xGdxN

Electronic structures and magnetic properties of Gd-doped GaN have been investigated within the framework of density functional theory. First, the density of states and band structure of GdN (terminal compound of Ga1−xGdxN) are presented. Second, the wurtzite type GaN:Gd magnetic semiconductors are studied by changing the Gd content. Magnetic stability, net spin exchange splitting and correlation of 4f electrons are analyzed, comparing them to the GaN:TM cases. Finally, the magnetic moment in Gd-doped GaN is calculated. Changing the Gd concentration hardly influences the magnetic moment of the system. Analyzing the x = 0.03 125 case, we find that the polarized magnetic moment of N by Gd atoms is very small, only about 0.01 μB and the polarization of N away from Gd can be ignored. Both long range spin polarization of the GaN matrix by the Gd atoms and the obvious magnetic moment change with Gd content are not found in our work. This study provides a further understanding of electronic and magnetic structures for Ga1−xGdxN.

Ionic transport properties in doped δ-Bi2O3

We have performed the first principles calculations on doped δ-Bi2O3 to investigate ionic conductivity. The crystal structure and the ionic conductivity are discussed in total energy and density of states (DOS) from the calculations. The stabilized δ-phase Bi2O3 doped rare-earth metal was explained from DOS data. By doping Ca, Sr, La, Gd or Sm, the ion conductivity monotonically decreases, while doping with impurity Y, Tb, Dy, Er or Tm, the ion conductivity firstly increases and then decrease. Our results support the effective oxygen vacancies mechanism.

Antiferromagnetism in Potassium-Doped Polycyclic Aromatic Hydrocarbons

First-principles density functional calculations are performed to investigate the magnetic characteristics in K-doped polycyclic aromatic hydrocarbons (PAHs) including phenanthrene, picene, 1,2:5,6-dibenzanthracene, 7-phenacene, and 1,2;8,9-dibenzopentacene. With the help of lowest energy crystal structures, the calculated total energies indicate that all five K-doped PAHs are stabilized in an antiferromagnetic ground state, with antiparallel spins between two molecular layers. The magnetic moment in these K-doped PAHs is increased with the increase of benzene rings number, while it is not sensitive to the arrangement of benzene rings. The enhancement of the magnetic moment is caused by a stronger spin splitting near the Fermi level and an increase of magnetic C atoms induced by K atoms with the increase of molecular size. Our results also indicate that the magnetism strongly depends on the crystal structure.

First-principles investigations on the magnetic property in tripotassium doped picene

First-principles calculations are performed to investigate the magnetic characteristics in the tripotassium doped picene, especially for the effects induced by the volume variations. When changing volume, both crystal lattice constants and atomic positions are optimized. For the system with the experimental crystal volume, the doped picene shows a weak antiferromagnetic instability. When the volume expands from this experimental crystal volume, the antiferromagnetic spin ordering becomes clear. The electronic structures show that the magnetism comes from the spin unbalance on the π orbitals of the C atoms. On the contrary, both ferromagnetic and antiferromagnetic spin orderings are strongly suppressed as the volume is reduced. Our results indicate that the magnetism is sensitive to the variation of volume or pressure in the tripotassium doped picene. No metal-insulator transition is observed for several considered volumes.

Pressure-induced superconductivity in H2-containing hydride PbH4(H2)2

High pressure structure, stability, metallization, and superconductivity of PbH4(H2)2, a H2-containing compound combining one of the heaviest elements with the lightest element, are investigated by the first-principles calculations. The metallic character is found over the whole studied pressure range, although PbH4(H2)2 is metastable and easily decompose at low pressure. The decomposition pressure point of 133 GPa is predicted above which PbH4(H2)2 is stable both thermodynamically and dynamically with the C2/m symmetry. Interestedly, all hydrogen atoms pairwise couple into H2 quasi-molecules and remain this style up to 400 GPa in the C2/m structure. At high-pressure, PbH4(H2)2 tends to form the Pb-H2 alloy. The superconductivity of Tc firstly rising and then falling is observed in the C2/m PbH4(H2)2. The maximum of Tc is about 107 K at 230 GPa. The softening of intermediate-frequency phonon induced by more inserted H2 molecules is the main origin of the high Tc. The results obtained represent a significant step toward the understanding of the high pressure behavior of metallic hydrogen and hydrogen-rich materials, which is helpful for obtaining the higher Tc.

Vibrational and structural properties of tetramethyltin under pressure

The vibrational and structural properties of a hydrogen-rich group IVa hydride, Sn(CH(3))(4), have been investigated by combining Raman spectroscopy and synchrotron x-ray diffraction measurements at room temperature and at pressures up to 49.9 GPa. Both techniques allow the obtaining of complementary information on the high-pressure behaviors and yield consistent phase transitions at 0.9 GPa for the liquid to solid and 2.8, 10.4, 20.4, and 32.6 GPa for the solid to solid. The foregoing solid phases are identified to have the orthorhombic, tetragonal, monoclinic crystal structures with space groups of Pmmm for phase I, P4/mmm for phase II, P2/m for phase III, respectively. The phases IV and V coexist with phase III, resulting in complex analysis on the possible structures. These transitions suggest the variation in the inter- and intra-molecular bonding of this compound.

Aluminum-Doped Cesium Lead Bromide Perovskite Nanocrystals with Stable Blue Photoluminescence Used for Display Backlight

Abstract Bright and stable blue emitters with narrow full‐width at half‐maxima are particularly desirable for applications in television displays and related technologies. Here, this study shows that doping aluminum (Al3+) ion into CsPbBr3 nanocrystals (NCs) using AlBr3 can afford lead‐halide perovskites NCs with stable blue photoluminescence. First, theoretical and experimental analyses reveal that the extended band gap and quantum confinement effect of elongated shape give rise to the desirable blueshifted emission. Second, the aluminum ion incorporation path is rationalized qualitatively by invoking fundamental considerations about binding relations in AlBr3 and its dimer. Finally, the absence of anion‐exchange effect is corroborated when green CsPbBr3 and blue Al:CsPbBr3 NCs are mixed. Combinations of the above two NCs with red‐emitting CdSe@ZnS NCs result in UV‐pumped white light‐emitting diodes (LED) with an National Television System Committee (NTSC) value of 116% and ITU‐R Recommendation B.T. 2020 (Rec. 2020) of 87%. The color coordinates of the white LED are optimized at (0.32, 0.34) in CIE 1931. The results suggest that low‐cost, earth‐abundant, solution‐processable Al‐doped perovskite NCs can be promising candidate materials for blue down‐conversion layer in backlit displays.