We used X-ray/neutron diffraction to determine the low temperature (LT) structure of IrTe2. A structural modulation was observed with a wavevector of k =(1/5, 0, 1/5) below Ts≈285 K, accompanied by a structural transition from a trigonal to a triclinic lattice. We also performed the first principles calculations for high temperature (HT) and LT structures, which elucidate the nature of the phase transition and the LT structure. A local bonding instability associated with the Te 5p states is likely the origin of the structural phase transition in IrTe2.The competition between charge density wave (CDW) state and superconductivity is one of mostly interesting phenomena in transition metal dichalcogenides and has been widely studied due to possible relation to high-Tc superconductivity. [1-5] Classically, CDW transitions are second order transitions driven by Fermi surface nesting in a metal, i.e. a Kohn anomaly leading to a soft mode instability of the high tempearature structure. A key feature of this type of transition is a coupling of the electronic structure at the Fermi energy to the structural distortion leading to strong signatures of the phase transition in transport and also in some systems an interplay between the CDW and superconductivity.Recently IrTe 2 , a new member of the TX 2 family incorporating a 5d transition metal, presents the superconductivity when its first-order structural transition is suppressed through doping. [6][7][8][9][10][11] Its HT structure has a trigonal symmetry with edge-sharing IrTe 6 octahedra forming layers stacked along the c-axis with the Ir ions forming an equilateral triangular lattice ( Fig. 1(a)). The LT structure was proposed to be monoclinic based on powder X-ray diffraction.[12] Accompanied with the structural transition, the resistivity shows a hump-shaped maximum and the magnetic susceptibility drops, which is similar to that of the CDW state in other TX 2 systems.However, recent measurements for IrTe 2 imply that the physics is more complicated than a simple CDW. [6-8, 10, 13] In particular, while optical and transport measurements do imply a strong reconstruction of the electronic structure at E F through the transition, other measurements show that the transition is first order, which is not the generic behavior of a standard CDW. There are many possible origins for a first order transition. One is that the mechanism is still CDW type related to Fermi surface nesting, but that the transition becomes first order due to coupling with strain. Another is that it is driven by local ordering, such as orbital ordering on the transition metal. Finally, a transition can be driven by chemical bonding effects.Up to now, all the reported studies used a proposed LT structure model from powder X-ray diffraction [12]. Given that electron diffraction revealed the existence of superlattice peaks [6], which principally also can be from the structure, the LT structure is probably more complicated than the proposed model. The correct LT structure of IrTe 2 is essential to explore t...
A new type of superhalogen-(super)alkali compound, BF 4 -M (M ¼ Li, FLi 2 , OLi 3 , NLi 4 ), is theoretically characterized at the MP2/6-311þG(3df) level. The interaction between superhalogen BF 4 and different shaped (super)alkali M is found to be strong and ionic in nature. Bond energies of these BF 4 -M species are in the range of 200.0-226.7 kcal/mol at the CCSD(T)/6-311þG(3df) level, which are much larger than the traditional ionic bond energy of 130.1 kcal/mol of FLi. In addition, different from the alkali halides, the BF 4 -M compounds prefer to dissociate into ions rather than neutral fragments. The energetic properties of BF 4 -M are found to be closely related to the size of the M subunit. The different effects of superalkali and superhalogen subunits on the nonlinear optical (NLO) properties of such superatom compounds are also revealed.
The consistency of the quantum adiabatic theorem has been doubted recently. It is shown in the present paper that the difference between the adiabatic solution and the exact solution to the Schrödinger equation with a slowly changing driving Hamiltonian is small; while the difference between their time derivatives is not small. This explains why substituting the adiabatic solution back into the Schrödinger equation leads to "inconsistency" of the adiabatic theorem. Physics is determined completely by the state vector, and not by its time derivative. Therefore the quantum adiabatic theorem is physically correct.
Focusing on the interesting new concept of all-metal electride, centrosymmetric molecules e–+M2+(Ni@Pb12)2–M2++e– (M = Be, Mg, and Ca) with two anionic excess electrons located at the opposite ends of the molecule are obtained theoretically. These novel molecular all-metal electrides can act as infrared (IR) nonlinear optical (NLO) switches. Whereas the external electric field (F) hardly changes the molecular structure of the all-metal electrides, it seriously deforms their excess electron orbitals and average static first hyperpolarizabilities (β0 e(F)). For e–+Ca2+(Ni@Pb12)2–Ca2++e–, a small external electric field F = 8 × 10–4 au (0.04 V/Å) drives a long-range excess electron transfer from one end of the molecule through the middle all-metal anion cage (Ni@Pb12)2– to the other end. This long-range electron transfer is shown by a prominent change of excess electron orbital from double lobes to single lobe, which forms an excess electron lone pair and electronic structure Ca2+(Ni@Pb12)2–Ca2++2e–. Therefore, the small external electric field induces a dramatic β0 e(F) contrast from 0 (off form) to 2.2 × 106 au (on form) in all-metal electride molecule Ca(Ni@Pb12)Ca. Obviously, such switching is high sensitive. Interestingly, in the switching process, such long-range excess electron transfer does not alter the valence and chemical bond nature. Then, this switching mechanism is a distinct nonbonding evolution named electronic structure isomerization, which means that such switching has the advantages of being fast and reversible. Besides, these all-metal electride molecules also have a rare IR transparent characteristic (1.5–10 μm) in NLO electride molecules, and hence are commendable molecular IR NLO switches. Therefore, this work opens a new research field of electric field manipulated IR NLO switches of molecular all-metal electrides.
TM -doped IrTe2 (TM =Mn, Fe, Co, Ni) compounds were synthesized by solid state reaction. Single crystal x-ray diffraction experiments indicate that part of the doped TM ions (TM =Fe, Co, and Ni) substitute for Ir, and the rest intercalate into the octahedral interstitial sites located in between IrTe2 layers. Due to the lattice mismatch between MnTe2 and IrTe2, Mn has limited solubility in IrTe2 lattice. The trigonal structure is stable in the whole temperature range 1.80 K≤ T ≤ 300 K for all doped compositions. No long range magnetic order or superconductivity was observed in any doped compositions above 1.80 K. A spin glass behavior below 10 K was observed in Fe-doped IrTe2 from the temperature dependence of magnetization, electrical resistivity, and specific heat. The low temperature specific heat data suggest the electron density of states is enhanced in Feand Co-doped compositions but reduced in Ni-doped IrTe2. With the 3d transition metal doping the trigonal a-lattice parameters increases but the c-lattice parameter decreases. Detailed analysis of the single crystal x-ray diffraction data shows that interlayer Te-Te distance increases despite a reduced c-lattice. The importance of the Te-Te, Te-Ir, and Ir-Ir bonding is discussed.
Using a new two-step synthesis method -ultrasound treatment and low temperature annealing, we explore superconductivity in potassium-doped triphenylbismuth, which is composed of one bismuth atom and three phenyl rings. The combination of dc and ac magnetic measurements reveals that one hundred percent of synthesized samples exhibit superconductivity at 3.5 K and/or 7.2 K at ambient pressure. The magnetization hysteresis loops provide a strong evidence of type-II superconductor, with the upper critical magnetic field up to 1.0 Tesla. Both calculated electronic structure and measured Raman spectra indicate that superconductivity is realized by transferring electron from potassium to carbon atom. Our study opens an encouraging window for the search of organic superconductors in organometallic molecules.PACS number(s):74.70. Kn, 78.30.Jw
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