Ab initio band-structure calculations were performed for bulk, single slab, and thin films of TiX 2 ͑XϭS, Se͒ using the localized spherical wave method. According to these calculations, bulk TiS 2 and TiSe 2 are semimetallic. The calculations show that TiS 2 thin films are semiconductors, but thin films of TiSe 2 are semimetallic. The indirect gap for single slab TiS 2 is about 1.0 eV, and the gap becomes smaller with increasing number of layers. When the number of layers increases to 11, the TiS 2 thin films are semimetallic. All but the surface layers are found to be electrically neutral. The density of states as a function of the energy for the surface layer is different from that of the bulk. The Madelung energy of the Ti atoms on the surface is about 0.35 eV lower than that for the Ti atoms in the bulk. The calculations are compared with photoemission spectra, reported in the literature. ͓S0163-1829͑97͒05532-X͔

Prussian blue analogs with an open framework are ideal cathodes for Na‐ion batteries. A superior high‐rate and highly stable monoclinic nickel hexacyanoferrate (NiHCF‐3) is synthesized via a facile one‐step crystallization‐controlled co‐precipitation method. It gives a high specific capacity of 85.7 mAh g−1, nearly to its theoretical value. It also exhibits an excellent rate capability with a high capacity retention ratio of 78% at 50 C and a stable cycling performance over 1200 cycles. Through the ex situ X‐ray diffraction and pair distribution function measurements, it is found that the monoclinic structure with distorted framework is greatly related to the high Na content. The electronic structure studies by density functional theory (DFT) calculation demonstrate that NiHCF‐3 deformation promotes the framework conductivity and improves the electrochemical activity of Fe, which results in an ultrahigh‐rate performance of monoclinic phase. Furthermore, the high‐quality monoclinic (NiHCF‐3) exhibits excellent compatibility with both hard carbon and NaTi2(PO4)3 anodes in full cells, which shows great prospects for the application in the large‐scale energy storage systems.

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Atomistic simulations with atomic potentials including anion polarizibility have been performed for the low-index surfaces of spinel MgAl 2 O 4 with various terminations. The calculations show that for the most stable surface the surface energy is 2.27 J/m 2 for the {100}, about 2.85 J/m 2 for the {110}, and 3.07 J/m 2 for the {111} orientation. The ratio between the experimental values to the calculated relaxed surface energies is about 1.5. Strong surface relaxation was found for the {110} and {111} orientation but only moderate surface relaxation for the {100} surface.S URFACE energy is an important parameter of ceramic materials.It controls many chemical and physical processes and properties, such as sintering, mass loss, adherence, crack nucleation and growth, catalysis, sensing, etc. A precise knowledge of this quantity would be very useful in predicting and optimizing processing and properties of ceramic materials. 1 For example, in the fracture process of brittle materials surface energies play an important role according to the Griffith criterion. 2 The fracture of MgAl 2 O 4 has been a subject of several investigations. 3-8 Stoichiometric spinel MgAl 2 O 4 is a brittle single crystal, which exhibits plastic flow only at elevated temperature. 3 Stewart and Bradt 4 studied the fracture of single-crystal, stoichiometric MgAl 2 O 4 for the low-index planes and found that the fracture toughness at room temperature is 1.18, 1.54, and 1.90 MN⅐m Ϫ3/2 for {100}, {110}, and {111}, respectively. They also estimated the surface energy to be 3.57 J/m 2 for {100}, 4.07 J/m 2 for {110}, and 4.85 J/m 2 for {111}. 5,6 Early theoretical calculations by Mishra and Thomas 9 indicated that surfaces parallel to the {111} plane are of the lowest energy for spinel MgAl 2 O 4 , in contrast to the experimental data. 4 -6 Recently Davies et al. performed atomistic simulations for the low-index surfaces of spinel MgAl 2 O 4 , but their calculations were limited to certain terminations with half anion/cation occupation. 10