Fine structure and isotope shift of the 3s4d 3 D-3s3p 3 P, 3s5d 3 D-3s3p 3 P and 3p 2 3 P-3s3p 3 P Transitions of MgI

Summary We report the measurements of the³ D(3s4d)-³ P(3s3p)³ D(3s5d)-³ P(3s3p), and³ P(3p ²)-³ P(3s3p) transition frequency of MgI, the fine-structure separation and isotope shift between²⁴Mg and²⁶Mg. The measurements have been performed in a metastable atomic beam; a good agreement is found for data already existing in the literature. The accuracy of the measurements reported in this paper is mainly limited by the Doppler broadening of theI ₂ transitions used as a reference and by the precision in the knowledge of the related wavelengths.     

Elastic pion-deuteron scattering near the 3-3 resonance

The elastic scattering of positive pions by deuterium has been studied at seven energies between 82 MeV and 292 MeV laboratory kinetic energy in the angular range between 30° and 130° (lab). The results are compared to recent relativistic three-body calculations.     

Electrostriction at the LaAlO3/SrTiO3 interface

We present a direct comparison between experimental data and ab initio calculations for the electrostrictive effect in the polar LaAlO(3) layer grown on SrTiO(3) substrates. From the structural data, a complete screening of the LaAlO(3) dipole field is observed for film thicknesses between 6 and 20 uc. For thinner films, an expansion of the c axis of 2% matching the theoretical predictions for an electrostrictive effect is observed experimentally.     

Elastic pion-deuteron scattering near the 3-3 resonance

Elastic pion-deuteron scattering at medium energies is analyzed on the basis of the Brueckner model, particular attention being paid to the kinematic approximations that have to be made. In contrast to an earlier Glauber-type calculation, the energy dependence of the large angle cross section is reasonably well reproduced.     

E-T phase diagrams of a solid solution of the multicomponent PbZn1/3Nb2/3O3-PbMg1/3Nb2/3O3-PbNi1/3Nb2/3O3-PbTiO3 system near the morphotropic phase boundary

The specific features of the dielectric and pyroelectric responses of a solid solution in the barium-doped multicomponent yPbZn_1/3Nb_2/3O₃-mPbMg_1/3Nb_2/3O₃-nPbNi_1/3Nb_2/3O₃-xPbTiO₃ system of the composition y = 0.0982, m = 0.4541, n = 0.1477, and x = 0.3 near the morphotropic phase boundary have been investigated. It has been assumed that the maxima revealed in the dependences of the reversible permittivity on the electric field strength with both the forward and backward changes in the field (E ₌) are associated with the induced phase transition. Based on the experimental results, the E-T phase diagrams are constructed in the temperature range from ?100 to 150°C for different temperature-field regimes: (i) variation in the electric field E ₌ at a fixed temperature of the sample and (ii) variation in the temperature of the sample at a constant value of E ₌. It has been found that there is a singular point in the E-T phase diagram and that, in the vicinity of this point, the dielectric and pyroelectric responses of the studied ceramics exhibit specific features.     

Intrinsic Instability of the Hybrid Halide Perovskite Semiconductor CH_3NH_3PbI_3

The organic-inorganic hybrid perovskite CH_3NH_3PbI_3 has attracted significant interest for its high performance in converting solar light into electrical power with an efficiency exceeding 20%. Unfortunately, chemical stability is one major challenge in the development of CH_3NH_3PbI_3 solar cells. It was commonly assumed that moisture or oxygen in the environment causes the poor stability of hybrid halide perovskites, however, here we show from the first-principles calculations that the room-temperature tetragonal phase of CH_3NH_3PbI_3 is thermodynamically unstable with respect to the phase separation into CH_3NH_3I + PbI_2, i.e., the disproportionation is exothermic,independent of the humidity or oxygen in the atmosphere. When the structure is distorted to the low-temperature orthorhombic phase, the energetic cost of separation increases, but remains small. Contributions from vibrational and configurational entropy at room temperature have been considered, but the instability of CH_3NH_3PbI_3 is unchanged. When I is replaced by Br or Cl, Pb by Sn, or the organic cation CH_3NH_3 by inorganic Cs, the perovskites become more stable and do not phase-separate spontaneously. Our study highlights that the poor chemical stability is intrinsic to CH_3NH_3PbI_3 and suggests that element-substitution may solve the chemical stability problem in hybrid halide perovskite solar cells.     

Structural,spectroscopic, and thermophysical investigations of the oxyfluorides CsZnMoO3F3 and CsMnMoO3F3 with the pyrochlore structure

The structure, infrared and Raman spectra, heat capacity, and thermal expansion have been investigated. It has been shown that, down to liquid-helium temperatures, the cubic pyrochlore structure of the CsMnMoO₃F₃ and CsZnMoO₃F₃ oxyfluorides remains stable. The influence of cation substitution on individual features of the properties of the oxyfluorides has been analyzed.     

Ferroelastic transition and the relaxor state in SrTiO3-PbMg1/3Nb2/3O3

Dielectric and acoustic properties of the (1 ? x)SrTiO₃?xPbMg_1/3Nb_2/3O₃ solid solutions (0 ? x ? 1) have been studied at temperatures ranging from 4.2 to 350 K. It has been found that the improper ferroelastic transition exists up to a concentration x = 0.4 and that the phase transition temperature grows with x increasing from 0 to 0.4. As the concentration of the second component is further increased, the transition becomes suppressed by the relaxor phase forming at x _c ≈ 0.22. The results obtained are discussed in terms of the current concepts of relaxor ferroelectrics.     

Raman spectroscopic study of the phase transitions in the (NH4)3WO3F3 oxyfluoride

The Raman spectra of (NH₄)₃WO₃F₃ perovskite-like oxyfluorides are measured in the frequency range 70–3600 cm^?1. The anomalies observed in the spectral parameters upon phase transitions at a temperature of 200 K under atmospheric pressure and at room temperature under a pressure of 1.4 GPa are interpreted.     

Description of the symmetry groupSU 3/Z 3 of the octet model

A study is made of the relationship ofS U 3 and its adjoint groupS U3/Z3 to the subgroup ofR8 which leaves invariant not only the lengtha _i a _i of a real eight component vectora _i but also the cubic invariantd _{i j k} a _i a _j a _k ,d _{i j k} being the totally symmetric isotropicS U3 tensor introduced byGell-Mann. A formula for the rotationR ε R8 corresponding toU ε S U3, and a formula inverse to this, which provides a way of parametrizingU ε S U3, are derived.     

Enhancing the photo-luminescence stability of CH3NH3PbI3 film with ionic liquids

The methylammonium lead triiodide (CH₃NH₃PbI₃)-based perovskite shows a great alluring prospect in areas of solar cells, lasers, photodetectors, and light emitting diodes owing to their excellent optical and electrical advantages. However, it is very sensitive to the surrounding oxygen and moisture, which limits its development seriously. It is urgent to spare no effort to enhance its optical and electrical stability for further application. In this paper, we synthesize the MAPbI₃ perovskite film on the glass substrate with/without the ionic liquid (IL) of 1-Butyl-3-methylimidazolium tetrafluoroborate (BMIMBF₄) by a simple two-step sequential solution method. The additive of BMIMBF₄ can improve the quality of crystal structure. Moreover, the photo-luminescence (PL) intensity of MAPbI₃ film with BMIMBF₄ is much stronger than the pure MAPbI₃ film after a week in the air, which is almost ten-fold of the pure one. Meanwhile, under the illumination of 405-nm continuous wave (CW) laser, the fluorescent duration of the MAPbI₃ film with BMIMBF₄ is approximately 2.75 min, while the pure MAPbI₃ film is only about 6 s. In fact, ionic liquid of BMIMBF₄ in the perovskite film plays a role of passivation, which prevents the dissolution of MAPbI₃ into CH₃NH₃ and PbI₂ and thus enhances the stability of environment. In addition, the ionic liquid of BMIMBF₄ possesses high ionic conductivity, which accelerates the electron transport, so it is beneficial for the perovskite film in the areas of solar cells, photodetectors, and lasers. This interesting experiment provides a promising way to develop the perovskite's further application.     

Dynamics of the Yb3+ to Er3+ energy transfer in LiNbO3

The energy transfer dynamics between Yb³⁺ and Er³⁺ ions in lithium niobate is investigated after ytterbium-pulsed excitation at 920 nm. The sensitisation of the LiNbO₃:Er³⁺ system with Yb³⁺ ions does not modify the lifetime of the ⁴I_13/2 erbium level (1.5-μm emission), whereas it induces a marked, concentration-dependent change in the lifetime of the ²F_5/2 (Yb³⁺) and ⁴S_3/2 (Er³⁺) multiplets (1060-nm and 550-nm emissions, respectively). The results are analysed by using the rate-equation formalism and cross-relaxation model for the energy transfer. Received: 15 October 1998 / Revised version: 24 November 1998 / Published online: 24 February 1999     

Experimental observation of the Σ hypernuclei, ΣH and ΣC

The hypernuclei _Σ⁶H and _Σ¹⁶C were observed by the (K⁻, π⁺) reaction on targets of ⁶Li and liquid O, respectively, at 713 MeV/c incident K⁻ momentum. Structure is seen in _Σ⁶H which may be interpreted in terms of particle-hole excitations similar to the previously observed states in _Λ⁶Li. The inablitity to resolve individual Σ hypernuclear levels in _Σ¹⁶C, due in part to the excitation of non-coherent states as a result of the large momentum transfer of about 130 MeV/c, precludes the extraction of the Σ-nucleus spin-orbit potential strength. The Σ-nucleus well depth appears to be 7 to 10 MeV less than that for the Λ.