Hai-tao Yu

First-principles calculations of the stability and electronic properties of the PbTiO3 (110) polar surface

The structural and electronic properties of five terminations of cubic lead titanate (PbTiO3) (110) polar surface were investigated by first‐principles total‐energy calculations using a periodic slab model. On the PbTiO termination, an anomalous filling of conduction band was observed, whereas on the O2 termination, two surface oxygen atoms formed a peroxo group, demonstrating that the electronic structures of the two stoichiometric terminations undergo significant changes with respect to bulk materials. However, for the three nonstoichiometric TiO‐, Pb‐, and O‐terminated surfaces, their electronic structures are very similar to bulk. Charge redistribution results for the five terminations confirmed that electronic structure and surface composition changes are responsible for their polarity compensation. However, which mechanism actually dominates the stabilization process depends upon energetic considerations. A thermodynamic stability diagram suggested that the two stoichiometric terminations are unstable; however, the three nonstoichiometric terminations can be stabilized in some given regions. Furthermore, this study indicates that the very different stabilities and surface states filling behaviors of the PbTiO3 (110) polar surface with respect to SrTiO3 and BaTiO3 ones seem to originate from the partially covalent characteristics of PbO pairs.

Lattice dynamics investigation of different transition behaviors of cubic BaTiO3 and SrTiO3 by first-principles calculations

The lattice dynamics of cubic BaTiO3 and SrTiO3 at different crystal lattice volumes were investigated using first-principles density functional theory calculation. The computational results indicate that the Ti–O and A–O (A = Sr or Ba) interactions are responsible for the unstable ferroelectric Γ15 and antiferrodistortive R25 phonons, respectively. With decreasing volume, the Γ15 phonon behaviors of cubic SrTiO3 and BaTiO3 show a similar trend and the Ti–O repulsions are significantly enhanced, leading to the disappearance of ferroelectric instability. However, for BaTiO3 the large ionic radius of Ba with respect to Sr results in a repulsive Ba–O interaction and thereby the disappearance of the R25 instability, while the long-range characteristic of Sr–O pair are crucial for the antiferrodistortive instability of cubic SrTiO3. By analyzing the real-space interatomic force constants, it can be confirmed that the different antiferrodistortive behaviors of the two compounds are dictated by different short-range repulsions of A–O pairs. Moreover, owing to the smaller ionic radius of Sr relative to Ba, the lattice constant of SrTiO3 can be significantly reduced, leading to the absence of ferroelectric instability for SrTiO3. The substitution and different crystal volumes make the phase transitions of the two compounds entirely different.

Electronic structures and ferroelectric instabilities of cubic AV O3 (A = Sr, Ba, and Pb) perovskites by first-principles calculations

The electronic properties and ferroelectric transition behaviors of three AVO(3) (A = Sr, Ba, and Pb) compounds are investigated by first-principles density functional theory (FP-DFT) in combination with soft-mode theory. The band structures and projection density of states (PDOS) confirm that the bonding properties of SrVO(3) and BaVO(3) are rather similar to each other, but different to that of PbVO(3). The bonding differences determine the ferroelectric transition behaviors of these compounds. For SrVO(3) and BaVO(3), no ferroelectric instability is observed and they possess cubic structures. The covalent interactions of Pb-O pairs are very important for the ferroelectric instability of PbVO(3). In comparison to PbTiO(3), we found that the V-O interactions further enhance the ferroelectric instability of PbVO(3), and therefore PbVO(3) shows a much larger tetragonal distortion than PbTiO(3).

Hollow and hierarchical Na2Li2Ti6O14 microspheres with high electrochemical performance as anode material for lithium-ion battery

Relying on a solvent thermal method, spherical Na2Li2Ti6O14 was synthesized. All samples prepared by this method are hollow and hierarchical structures with the size of about 2–3 μm, which are assembled by many primary nanoparticles (~300 nm). Particle morphology analysis shows that with the increase of temperature, the porosity increases and the hollow structure becomes more obvious. Na2Li2Ti6O14 obtained at 800°C exhibits the best electrochemical performance among all samples. Charge-discharge results show that Na2Li2Ti6O14 prepared at 800°C can delivers a reversible capacity of 220.1, 181.7, 161.6, 144.2, 118.1 and 97.2 mA h g−1 at 50, 140, 280, 560, 1400, 2800 mA g−1. However, Na2Li2Ti6O14-bulk only delivers a reversible capacity of 187, 125.3, 108.3, 88.7, 69.2 and 54.8 mA h g−1 at the same current densities. The high electrochemical performances of the as-prepared materials can be attributed to the distinctive hollow and hierarchical spheres, which could effectively reduce the diffusion distance of Li ions, increase the contact area between electrodes and electrolyte, and buffer the volume changes during Li ion intercalation/deintercalation processes.摘要本文采用溶剂热法合成了球形Na2Li2Ti6O14材料. 所有溶剂热法制备得到的材料均具有中空的分级结构, 并且均由粒径约为300 nm的 初级粒子通过组装形成, 微球的直径大约为2−3 μm. 粒子的形貌分析表明, 随着合成温度的增加, 孔隙率逐渐增加且中空结构更加明显. 在 所有材料中, 800°C合成的Na2Li2Ti6O14具有最好的电化学性能. 充放电测试表明, 在电流密度为50、140、280、560、1400、2800 mA g−1时, 800°C合成的Na2Li2Ti6O14样品的可逆容量分别为220.1、181.7、161.6、144.2、118.1、97.2 mA h g−1. 但是在相同电流密度条件下, 块状的 Na2Li2Ti6O14的可逆容量分别为187、125.3、108.3、88.7、69.2、54.8 mA h g−1. 中空分级结构微球可以有效地减小锂离子的扩散距离、增 加电极与电解液的接触面积、以及缓冲锂离子嵌脱过程中的体积变化, 从而使其具有较高的电化学性能.

A first-principles investigation into the ferroelectric and antiferrodistortive instabilities of cubic SrTiO3

The lattice dynamics and potential energy curves of cubic SrTiO3 as a function of lattice volume were investigated by density functional theory (DFT) calculations. The calculated results indicate that the lowest optical phonon in the Brillouin-zone center is real and an unstable R25 mode, corresponding to the antiferrodistortive transition, is found at the equilibrium volume. The results are consistent with previous experimental predictions. Moreover, increasing volume can improve the ferroelectric instability and restrain the antiferrodistortive one. A competition between the ferroelectric and antiferrodistortive instabilities does exist. The smaller lattice constant of cubic SrTiO3 than of BaTiO3 is crucial both for the appearance of the antiferrodistortive instability and the disappearance of the ferroelectric instability, which makes the transition behaviors of the two compounds entirely different.

Theoretical prediction regarding structural and thermodynamical characteristics of stable CH3PO2 isomers and unimolecular decomposition mechanisms of species CH3P(O)2, CH3OPO, and CH2P(O)OH

The detailed isomerization and dissociation reaction potential energy profile of the CH3PO2 system was established at the UCCSD(T)/6‐311++G(3df,2p)//UB3LYP/6‐311++G(d,p) level of theory. Seventy minimum isomers were located and connected by 93 optimized interconversion transition states. Furthermore, 32 isomers with high kinetic stability were predicted to be possible candidates for further experimental detection. The bonding nature of the suggested stable isomers was analyzed while their molecular properties including heats of formation, adiabatic ionization potentials, and adiabatic electronic affinities were calculated at the G2, G2(MP2), G3, and CBS‐Q levels. Based on the isomerization and dissociation potential energy surface, possible unimolecular decomposition mechanisms and pathways of the low‐lying molecules CH3P(O)2, CH3OPO, and CH2P(O)OH were discussed. Furthermore, the transition state theory rate constants of the primary unimolecular dissociation channels were also calculated.

Combined DFT, QCISD(T), and G2 mechanism investigation for the reactions of carbon monophosphide CP with unsaturated hydrocarbons allene CH2CCH2 and methylacetylene CH3CCH

The possible reaction product distribution and mechanism of carbon monophosphide CP with unsaturated hydrocarbons allene CH2CCH2 and methylacetylene CH3CCH are investigated at the B3LYP/6‐311+G(d,p), QCISD(T)/6‐311++G(2df,2p), and G2 levels of theory. Corresponding reactants, products, intermediates, and interconversion and dissociation transition states are located on the reaction potential energy profiles. Computation results show that in the reaction of CP with CH2CCH2 the dominant reaction product should be species CH2CCHCP. Also, we can suggest species HCCCH2CP as a secondary reaction product despite of only minor contribution to reaction products. In the reaction of CP with CH3CCH, the primary and secondary products are suggested to be two important molecules HCCCP and CH3CCCP, respectively. The predicted mechanisms for the two reactions are not in parallel with the reactions of CN with allene CH2CCH2 and methylacetylene CH3CCH given in previous studies. The present calculations provide some useful information for future possible experimental isolation and observation for some interesting unsaturated carbon–phosphorus‐bearing species.

First-principles study on the electronic and bonding properties of PbTiO3 (110) polar terminations

The electronic structures and bonding properties of the (110) polar terminations of cubic PbTiO3 were examined by the first-principles calculations at the generalized gradient approximation level. Two stoichiometric (PbTiO and O2) and three nonstoichiometric(TiO, Pb, and O) terminations were considered in this study. With the aid of the calculated electron density differences, atomic charges, band structures, and densities of states, the charge redistributions and electronic properties were evaluated in detail. Furthermore, based on the calculated results of the cleavage energies, relaxation energies, and surface energies of the investigated terminations, the charge compensation by the modification of the surface stoichiometry and the fillings of surface states were thermodynamically evaluated.

Theoretical investigation on the protonation reactions and products of the stable [N,C,C,S] isomers

A theoretical study on the protonation system of [N,C,C,S], [H,N,C,C,S]+, was performed at the B3LYP/6‐311++G(d,p) and CCSD(T)/6‐311++G(2df,2p) (single point) levels of theory. On the doublet [H,N,C,C,S]+ surface, 24 species were located as energy minima and 10 of them were considered as kinetically stable species. The species HNCCS+ with 2A′ state and a shallow W‐shaped skeleton was predicted to be the global minimum and kinetically the most stable species, being in good agreement with previous experimental findings. Furthermore, the protonation reactions of the stable [N,C,C,S] isomers were investigated in detail. The calculation results indicated that the [N,C,C,S] isomers may be significantly stabilized upon protonation. Finally, the possible covalent structures of the [H,N,C,C,S]+ isomers with considerable stability were briefly discussed.