Meng Ju

Ab Initio Search for Global Minimum Structures of Pure and Boron Doped Silver Clusters.

The global minimum structures of pure and boron doped silver clusters up to 16 atoms are determined through ab initio calculations and unbiased structure searching methods. The structural and electronic properties of neutral, anionic, and cationic Ag(n)B (n ≤ 15) and Ag(n)B2 (n ≤ 14) clusters are much distinct from those of the corresponding pure silver. Considering that Ag and B possess one and three valence electrons, respectively, both the single and the double boron-atom doped silver clusters with even number of valence electrons are more stable than those with odd number of electrons, a feature also observed in the pure silver clusters. We demonstrate that the species with a valence count of 8 and 14 appear to be magic numbers with enhanced stability irrespective of component or the charged state. A new putative global minimum structure of Ag13(-) cluster, with high symmetry of C(2v), is unexpectedly observed as the ground state, which is lower in energy than the previous suggested bilayer structure.

Insights into the geometries, electronic and magnetic properties of neutral and charged palladium clusters.

We performed an unbiased structure search for low-lying energetic minima of neutral and charged palladium PdnQ (n = 2–20, Q = 0, + 1 and –1) clusters using CALYPSO method in combination with density functional theory (DFT) calculations. The main candidates for the lowest energy neutral, cationic and anionic clusters are identified, and several new candidate structures for the cationic and anionic ground states are obtained. It is found that the ground state structures of small palladium clusters are more sensitive to the charge states. For the medium size Pdn0/+/– (n = 16–20) clusters, a fcc-like growth behavior is found. The structural transition from bilayer-like structures to cage-like structures is likely to occur at n = 14 for the neutral and cationic clusters. In contrast, for the anionic counterparts, the structural transition occurs at Pd13–. The photoelectron spectra (PES) of palladium clusters are simulated based on the time-dependent density functional theory (TD-DFT) method and compared with the experimental data. The good agreement between the experimental PES and simulated spectra provides us unequivocal structural information to fully solve the global minimum structures, allowing for new molecular insights into the chemical interactions in the Pd cages.

Systematic theoretical investigation of geometries, stabilities and magnetic properties of iron oxide clusters (FeO)nμ (n = 1–8, μ = 0, ±1): insights and perspectives

The structural properties of neutral and charged (FeO)nμ (n = 1–8, μ = 0, ±1) clusters have been studied using an unbiased CALYPSO structure searching method. As a first step, an unbiased search relying on several structurally different initial clusters has been undertaken. Subsequently, geometry optimization by means of density-functional theory with the Perdew and Wang (PW91) exchange–correlation functional is carried out to determine the relative stability of various candidates for low-lying neutral, anionic and cationic iron oxide clusters obtained from the unconstrained search. It is shown that the mostly equilibrium geometries of iron oxide clusters represent near planar structures for n ≤ 3. No significant structural differences are observed between the neutral and charged iron oxide clusters beyond sizes with n = 6. The relative stabilities of (FeO)nμ clusters for the ground-state structures are analyzed on the basis of binding energies and HOMO–LUMO gaps. Our theoretical results confirm that the binding energies of neutral and anionic (FeO)n0/− tend to increase with cluster size. Cationic (FeO)n+ exhibits a slight downward trend. It is worth noticing that (FeO)5 and (FeO)4−/+ are the most stable geometries for (FeO)nμ (n = 1–8, μ = 0, ±1) clusters. Lastly, an evident local oscillation of magnetic behavior is present in the most stable (FeO)nμ (n = 1–8, μ = 0, ±1) clusters, and the origin of this magnetic phenomenon is analyzed in detail.

Determination of the microstructure, energy levels and magnetic dipole transition mechanism for Tm3+ doped yttrium aluminum borate

Yttrium aluminum borate (YAB) doped with rare-earth ions are promising materials for infrared lasers and self-frequency summing laser systems. The stable crystal forms of YAB doped with rare-earth ions are of great scientific interest. Here, we systematically study the structural evolution of Tm3+ doped YAB, by using an unbiased CALYPSO structure search method in conjunction with first principles calculations. We are able to identify a unique semiconducting phase with P321 space group that reveal Tm3+ ions occupy Y ion sites of octahedral symmetry. The electronic band structure shows an extremely narrow conduction band above the Fermi level of Tm3+ in YAl3(BO3)4, indicating that the impurity Tm3+ ions lead to an insulator to semiconductor transition. The atomic energy structure of the 4f12 configuration of Tm3+ in the YAB has been calculated by a crystal-field theory method, which includes the major electrostatic and spin–orbit interactions as well as various minor contributions, for 4fN tripositive lanthanide ions. Electric dipole induced transitions are calculated to be much stronger than magnetic dipole induced ones in most situations. However, detailed magnetic dipole calculations for individual crystal field levels indicate a large number of strong absorption lines and spontaneous emissions, including many in the visible spectrum and at longer wavelengths that would be an attractive medium for investigating the magnetic portion in the light-matter interactions.

Exploration of stable stoichiometries, physical properties and hardness in the Rh-Si system: a first-principles study†

To understand the structural stability, physical properties, and hardness of the Rh–Si system, we have performed systematic first-principles crystal structure searches for various stoichiometries of rhodium silicides, utilizing the particle swarm optimization method. A new stable stoichiometry, Rh4Si5 with space group P21/m, has been found at atmospheric pressure, complementing the three well-known rhodium silicides of Rh2Si (Pnma), Rh5Si3 (Pbam), and RhSi (Pnma). Our calculations of the structural and mechanical properties of the known phases are in excellent agreement with the available experimental data and similar theoretical calculations. The elastic, electronic, and hardness properties of the Rh–Si stoichiometries are discussed. Our results suggest that the new rhodium silicide Rh4Si5 (P21/m) is a potentially hard material with the hardness of 20.1 GPa.

Theoretical investigation of the electronic structure and luminescence properties for NdxY1−xAl3(BO3)4 nonlinear laser crystal

Neodymium ion (Nd3+)-doped yttrium aluminum borate (YAB) nonlinear laser materials show strong prospects for highly efficient laser oscillations in kinds of multi-frequency conversion systems. Although excellent optical and spectroscopic properties for Nd-doped YAB have been demonstrated, detailed information of its microstructure as well as the incorporation of the laser ion Nd3+ is still lacking. Herein, the structural evolution of NdxY1−xAl3(BO3)4 systems are systematically investigated using the CALYPSO structure search method in conjunction with first-principles calculations. Our study demonstrates a stable configuration with C2 space group for a Nd-doped YAB crystal, which suggests that the impurity Nd3+ ions can accurately substitute for Y3+ sites. With the increase in Nd concentration, two traditional structures of NdAl3(BO3)4, γ-NAB and β-NAB, are identified and compared with previous experimental measurements. For the local [NdO6]9− unit, we introduced the correlation crystal field Hamiltonian to analyze the energy levels and have obtained a new set of crystal field parameters which leads to an improved fit with a RMS deviation of 13.32 cm−1 between the 135 theoretical and observed Stark levels. Our results could largely account for the well-known anomalous splitting of the 2H11/2 multiplets. Additionally, the transition intensities from the excited states to ground 4I9/2, including electric dipole and magnetic dipole contributions, are calculated. It is found that the characterization of two emission lines 4F5/2 → 4I9/2 and 2H(2)9/2 → 4I9/2, occurring at approximately 800 nm, is totally different. These findings provide a deep understanding of rare-earth doped laser materials and suggest a new way to explore the luminescence properties of such materials.

Structure and luminescence properties of a Nd3+ doped Bi4Ge3O12 scintillation crystal: new insights from a comprehensive study

The rare-earth Nd3+ doped bismuth ortho-germanate Bi4Ge3O12 (BGO), serving as an excellent kind of fast scintillator, has been widely used in many scientific and technical areas. However, there are two unsolved problems which hinder its practical applications, namely: (i) there is no detailed information about the microstructure and location sites of Nd3+ ions in the BGO lattice; (ii) it is difficult to determine the electric and magnetic dipole transition mechanisms of Nd3+ doped BGO. Here, the microstructure and site location of Nd3+ ions in BGO have been systematically investigated by means of an unbiased CALYPSO structure search method coupled with first principles calculations. As a result, we have for the first time identified a unique semiconducting phase of the R3 space group where impurity Nd3+ ions occupy exactly the host Bi3+ ion sites with trigonal symmetry. Based on our developed WEPMD method, new sets of free-ion, crystal field and orthogonal correlation crystal field parameters are obtained, yielding a much better agreement between the calculated and observed values for both optical energy levels and Zeeman splitting g-factors of the ground state. Starting from these new set parameters, the majority of electric dipole and magnetic dipole transition lines, including a large number of absorption and emission lines, in the region of visible and near-infrared spectra of Nd3+ ions in BGO are predicted and discussed. It is shown that the main emission channel of Nd3+ doped BGO occurs at the inter-Stark laser transitions of 4F3/2 → 4I11/2 corresponding to an emission wavelength of approximately 1064 nm.

Insights into the Microstructure and Transition Mechanism for Nd3+-Doped Bi4Si3O12: A Promising Near-Infrared Laser Material

Due to its unusual optical properties, neodymium ion (Nd3+)-doped bismuth silicate (Bi4Si3O12, BSO) is widely used for its excellent medium laser amplification in physics, chemistry, biomedicine, and other research fields. Although the spectral transitions and luminescent mechanisms of Nd3+-doped BSO have been investigated experimentally, theoretical research is severely limited due to the lack of detailed information about the microstructure and the doping site of Nd3+-doped BSO, as well as the electric and magnetic dipole transition mechanisms. Herein, we systematically study the microstructure and doping site of Nd3+-doped BSO using an unbiased CALYPSO structure search method in conjunction with first-principles calculations. The result indicates that the Nd3+ ion impurity occupies the host Bi3+ ion site with trigonal symmetry, forming a unique semiconducting phase. Based on our newly developed WEPMD method, the electric dipole and magnetic dipole transition lines, including a large number of absorption and emission lines, in the region of visible and near-infrared spectra of Nd3+-doped BSO are calculated. It is found that the 4G5/2 → 4I9/2, 2H9/2 → 4I9/2, and 4F3/2 → 4I11/2 channels are promising laser actions of Nd3+-doped BSO. These findings indicate that Nd3+-doped BSO crystals can serve as a promising multifunctional material for optical laser devices.