High-pressure magnetic and structural properties of TiOX (X=Cl,Br)

We report high-pressure diffraction and magnetization measurements to demonstrate that the partial collapse of electronic gap at high-pressure insulator to metal transition reported in TiOCl (C. Kuntscher et al. Phys. Rev. B 74 184402 (2006).) corresponds to a Ti³⁺–Ti³⁺ dimerization at room temperature within the space group P2₁/m. The shortest Ti–Ti distance is comparable to that of the Ti metal, but a Peierls-like distortion prevents a metallic behaviour.     

Phonon softening in (C2H5)4NFeX4 (X=Cl,Br)

Temperature-dependent¹⁶⁹Tm Mössbauer measurements are reported for the ternary intermetallic compounds TmT₂Ge₂ (T=Fe, Cu). Based on comparison with results for their TmT₂Si₂ counterparts, it is verified that the T constituent has the more significant influence on the crystal field interaction at the rare-earth site.     

Lattice and molecular dynamics in (C2H5)4NFeX4(X=Cl,Br)

We have measured the thermal dependence of thef factor and the line shift of the title compounds, in order to determine the (−2) and (+2) moments of the Fe frequency spectrum. The latter is approximated by a superposition of an acoustic-wave band and an intramolecular optical mode. Our results show that both frequency bands are well separated, owing to a relatively weak interaction between tetrahedral units. Optical frequencies thus determined are consistent with Raman data. Debye temperatures were obtained for the low-frequency vibrations of both crystals.     

Incommensurate magnetic ordering in Cu2Te2O5X2 (X = Cl,Br) studied by neutron diffraction

We present the results of the first neutron powder and single crystal diffraction studies of the coupled spin tetrahedra systems Cu2Te2O5X2 (X = Cl,Br). Incommensurate antiferromagnetic order with the propagation vectors kCl approximately [0.150,0.422,1/2], kBr approximately [0.158,0.354,1/2] sets in below TN = 18 K for X = Cl and 11 K for X = Br. No simple collinear antiferromagnetic or ferromagnetic spin arrangements within Cu2+ tetrahedra fit these observations. Fitting the diffraction data to more complex but physically reasonable models with multiple helices leads to a moment of 0.67(1)microB/Cu2+ at 1.5 K for the Cl compound. The reason for such a complex ground state may be geometrical frustration of the spins due to the intratetrahedral and intertetrahedral couplings having similar strengths. In neither compound has any evidence for a structural transition accompanying the magnetic ordering been found.     

NQR study of phase transitions in CH$\mathsf{_3}$HgX (X = Cl,Br, I)

The temperature dependence of halogen nuclear quadrupole resonance (NQR) in the series CH₃HgX (X = Cl, Br, I) is measured with special emphasis on the structural phase transitions at T _c = 162 K, 310 K, and 400 K, respectively. In the temperature dependences of NQR frequencies similarities are observed and discussed in relation with the structure and thermal vibrations on both sides of the phase transition. On the basis of known data a mechanism for the three analogous phase transitions is proposed. The chlorine spin-lattice relaxation behaviour in CH₃HgCl can be explained by a competition of fast thermal fluctuations of MMX molecules across the high temperature (h.t.) mirror plane and of infrequent transitions to the other equivalent fluctuation mode across the orthogonal h.t. mirror plane. Proton high temperature relaxation is probably dominated by the same slow motion, but at lower temperatures some other mechanism involving magnetic coupling prevails.Received: 23 June 2003, Published online: 15 October 2003PACS: 64.70.Kb Solid-solid transitions - 61.66.Fn Inorganic compounds - 76.60.Gv Quadrupole resonanceSupported in part by the Ministry of Education, Science and Sport, Republic of Slovenia     

Proton-conducting biopolymer electrolytes based on pectin doped with NH4X (X=Cl,Br)

Research has been undertaken to develop polymer electrolytes based on biodegradable natural polymers such as cellulose acetate, starch, gelatin, and chitosan, which are being used as polymer hosts for obtaining new polymer electrolytes for their applications in various electrochemical devices such as batteries, sensors, and electrochromic windows. Pectin is a naturally available material which is extracted from the skin of citrus fruits. Pectins, also known as pectic polysaccharides, are rich in galacturonic acid. The present study focuses on the proton-conducting polymer electrolytes based on the biopolymer pectin doped with ammonium chloride (NH₄Cl) and ammonium bromide (NH₄Br) prepared by solution casting technique. The prepared membranes are characterized using XRD, FTIR, and AC impedance techniques to study their complexation behavior, amorphous nature, and electrical properties. The conductivity of pure pectin membrane has been found to be 9.41 × 10⁻⁷ S cm⁻¹. The polymer systems with 30 mol% NH₄Cl-doped pectin and 40 mol% NH₄Br-doped pectin have been found to have maximum ionic conductivity of 4.52 × 10⁻⁴ and 1.07 × 10⁻³ S cm⁻¹, respectively. The conductivity value has increased by three orders of magnitude compared to pure pectin membrane. The dielectric behavior of both the systems has been explained using dielectric permittivity and electric modulus spectra.

     

Structural and magnetic phase transitions in Pr2NiO4

Neutron powder diffraction and magnetization measurements have been performed in stoichiometric Pr₂NiO₄₊(0), which at room temperature (RT) is orthorhombic (Bmab). Ni²⁺ becomes three dimensionally (3D) antiferromagnetically ordered atT _N 325K, with a propagation vectork=[100], and spins oriented along thea axis (parallel to the propagation vector). The magnetic structure belongs to the _7g (––+)-representation (g _x mode, Shubnikov groupB _pmab') ofBmab, and a magnetic moment of 1.5 _B is measured at 155K. This compound undergoes two different structural phase transitions. First, going down in temperature, Pr₂NiO₄ transforms by a first order phase transition from orthorhombic (Bmab) to tetragonal (P4₂/ncm) atT _c1115K. At high temperature, we can predict a transformation from orthorhombic (Bmab) to tetragonal (I4/mmm) in a continuous way. The extrapolated temperature for this second structural transition isT _c21500±100K. The low temperature structural transformation allows a change in the magnetic structure which forT<T _c1 is better described in the orthorhombic symmetry. Just belowT _c1 the magnetic structure is described by the: _3g (–+)-representation ofPccn(g_xc_yf_z mode, Shubnikov groupPccn), this magnetic phase begins to disappear going down in temperature and at the same time a new magnetic phase grows. This new magnetic structure is described by the _1g ofPccn (c _xg_ya_z mode, Shubnikov groupPccn). Both magnetic structures coexist in a certain temperature range. At 1.5 K thec _xg_ya_z mode represents the total of the magnetic ordering. To reproduce the observed magnetic intensities we are forced to consider that Pr³⁺ is polarized below 40 K, with a magnetic structure which is coupled to the Ni sublattices (i.e. _1g and _3g ). The magnetic moment at low temperature for Pr is about 1.28 _B .     

Molecular Rydberg transitions: The lowest-energy Rydberg transitions of s-type in CH3X and CD3X,X = Cl,Br, and I

The absorption spectra associated with transitions to the lowest-energy s-type Rydberg states of CH₃X and CD₃X, X = Cl, Br, and I, have been measured and analyzed. The spectra of the bromides and iodides consist, individually, of four electronic origins of s-excitation type. The vibrational frequency of a given normal mode is more or less identical in all four excited states of any one molecule; and the excited state/ground state ratios of the frequencies of any given normal vibrational mode are essentially identical for all four molecules (i.e., for 16 states, four for each of two bromides and two iodides). The spectra of the chlorides are amenable to a number of different vibronic analyses, none of them unique; these analyses are discussed.     

Luminescence of MeFX : Eu2+ (Me = Sr,Ba; X = Cl,Br)

Compounds of the type MeFX : Eu²⁺ with Me = Sr, Ba and X = C1, Br are efficient phosphors. Emission consists of both a 5d-4f band as well as 4f-4f lines, their relative intensities being strongly dependent on host lattice and temperature.     

Photostimulated luminescence in BaX 2 :Eu 2+ (X=Br,Cl) X-ray storage phosphors

The photostimulated luminescence (PSL) effect in BaX 2 :Eu 2+ (X=Br, Cl) is comparable to that observed in BaFBr:Eu 2+ which is used in commercial X-ray storage phosphor screens. After X-irradiation the PSL stimulation spectra of BaX 2 :Eu 2+ (X=Br, Cl) single crystals are identical to the F centre absorption spectra, i.e. the F centres are the PSL-active electron trap centres. The nature of the hole centres is still unknown. The PSL response time of about 0.70 v s is within experimental error of 0.02 v s identical to the Eu 2+ radiative lifetime, whereas in BaCl 2 :Eu 2+ the PSL response time is 0.60 v s, and thus longer than the Eu 2+ radiative lifetime of 0.47 v s.     

晶体NiX2(X=Cl,Br,I)的光谱和电子顺磁共振谱研究

在强场耦合图像中,采用双自旋-轨道耦合(SO)参量模型建立了过渡族3d2(3d8)离子的三角对称下全组态光谱能级和电子顺磁共振(EPR)公式.与经典的晶体场理论(仅考虑中心金属离子的自旋-轨道耦合作用)相比较,该公式还包括了配体离子的自旋-轨道耦合作用的贡献,这一模型在应用于计算共价性较强的晶体光谱和电子顺磁共振谱可得到合理的结果.作为验证,用完全对角化方法研究了品体NiX2(X=Cl,Br,I)的光谱和电子顺磁共振谱,结果表明,理论与实验很好地符合.建立的全组态谱能级和电子顺磁共振公式为更精确地计算光谱和电子顺磁共振谱提供了一条可行方法.     

An EPR investigation of the phase transitions and cooperative ordering phenomena of pentaamminecopper(II) polyhedra in Cu(NH3)5X2 [X=BF4, CIO4; Br]

The results of powder and single-crystal EPR studies on Cu(NH₃)₅X₂ compounds [X=BF₄, CIO₄; Br] give evidence that phase transitions from a cubic α-phase (g _i ?2.12) with an anti-K₂PtCl₆ type structure to low-temperature β-phases with reduced symmetry occur, which are induced by ordering processes of the vacancies □ of elongated [Cu(NH₃)₅□]²⁺ pseudo-octahedra. The type of order is crucially controlled, by the nature of the anion. In the case of the BF ₄ ^? and ClO ₄ ^? salts the large anions are structure-determining leading to a slightly disturbed antiferrodistortive order of the elongated Cu^IIN₅ square pyramids atT _c?155 K and <130 K, respectively. The resulting pseudo-tetragonal β-structure of the BF ₄ ^? compound is characterized by two rhombic crystalg tensors (g ₂ ^B∥g ₁ ^A=2.173;g ₁ ^B∥g ₂ ^A=2.124;g ₃ ^B∥g ₃ ^A=2.071). The dibromide salt undergoes a first-order phase transition atT _c=285 K from cubic to tetragonal with a rather lowc/a ratio of about 0.9. The Cu^IIN₅ square pyramids in the low-temperature β-structure are arranged in this case according to a “mixed ferrodistortive/antiferrodistortive” order pattern. Interestingly enough the angular dependence of theg tensor components indicate further structural changes with decreasing temperature, which are of rather local character, however, not correlated with a second phase transition. Atomic displacements lead to reduced Cu^II?Cu^II distances within certain antiferrodistortive pairs, as evidenced by EPR spectroscopy. The molecularg-values at higher temperatures (g _∥=2.243;g _⊥=2.056) transform to those of pairs with canting angles of about 80° (g ₁=2.168;g ₂=2.138;g ₃=2.046).     

Charge transfer transitions in the excitation spectra of PrX3:Ce (X=Cl, Br) scintillators

The Cl⁻→Pr³⁺ charge transfer transition is identified to occur in the excitation spectrum of PrCl₃:Ce³⁺ at 211 nm (47,393 cm⁻¹). A model based on the dissociation of the charge transfer state is proposed to explain the presence of the ligand-to-Pr³⁺ charge transfer transition band in the excitation spectrum of PrX₃:Ce³⁺ (X=Cl, Br) when the Ce³⁺ emission is monitored.     

Ultraviolet photoelectron spectra of gas phase and condensed tin and lead dihalides (MX2 ; M = Sn,Pb; X = Cl,Br, I)

The photoelectron spectra and electronic structures of the methylene dihalides, CH₂X₂ (X = F, Cl, Br and I), have been calculated by the overtapping-spheres SCF-Xα-MS method. The results are in good agreement with experimental data. Calculated assignments of the spectra are also presented and interpreted by assuming interaction between lone-pair and bonding orbitals.     

Polarized Raman scattering from oriented single crystals of A2BX4 halides (A = Cs,B = Zn,X = Cl,Br, I)

Raman spectra of isostructural tetrahalozincates viz. Cs₂ZnI₄, Cs₂ZnBr₄ and Cs₂ZnCl₄ single crystals have been measured in various scattering orientations using polarized laser excitation. The lattice dynamics of the crystals is probed down to 70 K in order to cover the successive phase sequences. Except Cs₂ZnI₄, none of the other salts shows the characteristic commensurate-incommensurate features of lattice instabilities. The unit cell dynamics is understood in terms of 84 phonon branches originating from various internal modes of tetrahedral ZnX²⁻₄ group and external lattice modes. The enormous splittings, responsible for a complete loss of degeneracy in the internal modes for all the three salts, suggest a considerable distortion particularly at low temperatures in the tetrahedral geometry of the ZnX²⁻₄ group.     

KX（X=Cl,Br,I）晶体中Eu^2＋离子的4f^65d态能级计算

按着4f~6(~7F_J)和5dΓ_1耦合模型,提出了计算KX(X=Cl,Br,I)晶体中Eu~(2+)离子4f~65d能级的方法,导出了能级的参数表达式,并进行了数值计算,计算结果和实验符合较好.     

Halogen bond between hypervalent halogens YF3/YF5 (Y=Cl,Br, I) and H2X (X= O,S, Se)

ABSTRACT

The complexes of H₂X (X?=?O, S, Se) with hypervalent halogens YF₃ and YF₅ (Y?=?Cl, Br, I) have been studied. The σ-hole on the Y atom participates in a halogen bond with the lone pair on the chalcogen atom. In addition, some secondary interactions coexist with the halogen bond in most complexes. The interaction energy correlates with the nature of both X and Y atoms. In most cases, the complex is more stable for the heavier Y atom and the lighter X atom. Of course, there are some exceptions in H₂X···YF₃. YF₃ forms a more stable complex with H₂X than does YF₅. These complexes are dominated by electrostatic interaction and the halogen bond involving H₂S and H₂Se exhibits some covalent character.

Halogen bond plays an important role in chemical reactions and multivalent halogens can regulate chemical reactions by participating in a halogen bond. Thus we compare the effect of the chalcogen electron donor on the strength and nature of halogen bonding involving multivalent halogens.     

Electronic states and transitions of the TeX (X = C1, Br,I) radicals

Ab initio multireference configuration interaction calculations including spin-orbit coupling have been carried out for the first time for valence electronic states of the TeX (X = Cl, Br, I) radicals and compared with the results for the isovalent TeF and IO systems obtained earlier at a similar level of theoretical treatment. The calculated spectroscopic constants are in good agreement with experimental data in the rare cases when the latter are available. It is shown that the X² II(σ²π⁴π?³) ground state bonding becomes consistently weaker down the TeX group (calc. D_e, = 25480cm^?1 for TeF, 12 100cm^?1 for Tel) due to the more covalent character of bonding in the heavier radicals. The first excited state, A ⁴Σ^? (π?→ σ?), is calculated to be bound in all systems. It is split into Ω 1/2 and 3/2 components, with regular ordering in the Franck-Condon region, opposite to that of the ground X²II state. For larger internuclear distances, the A₁ ⁴Σ^? _1/2 state undergoes an avoided crossing with X₂ ²II_1/2, which causes a shoulder in the X₂ potential curve and also leads to a crossing between the A₁, and A₂ curves and large distinctions in their vibrational frequencies. The π? → σ? B²Σ^?, C²δ, and 1 ²Σ⁺ states are calculated to lie next in energy. They are all bound in the lightest of the TeX radicals, TeF, but successively lose their bonding character down the group. In contrast to oxygen monohalides, the 2²II(σ²π³ π?⁴) state has a repulsive potential curve. Electric dipole transition moments and radiative lifetimes have also been calculated for the low lying bound states in all systems. Most of them are found to be quite weak. The A^1,2 → X^1,2 spectra are dominated by parallel contributions, with the A₂ → X₁ being the strongest one. The T values for this transition are quite similar and lie in the 17–30 μs range. Radiative lifetime values for the B → X^1,2 transitions demonstrate very irregular behaviour for various, TeX radicals, due to strong admixture of A⁴Σ^? character to the X^1,2 states near the B²Σ^? potential minimum. The A^{1,2 4}Σ^? _1/2,3/2 and B²Σ^? _1/2 states of TeX (X = Cl, Br, I) still await their experimental observation.     

Spectroscopy and dissociation of sulfuryl halides SO2X2 (X=F,Cl)

The spectroscopy and dissociation of the sulfuryl halides SO₂F₂ and SO₂Cl₂ have been studied in detail using ab initio methods. The possibility of various dissociation channels has been explored taking into account that the fragmented atoms and molecules can stay in their ground state only. An interesting pattern was observed in their dissociation energy spectra for the dissociation channels. The singlet potential energy surfaces for the exit channels of these molecules have been analysed. The release of halogen molecules after dissociation is discussed from an industrial point of view. Finally, the enthalpy of formation of these molecules was computed using the ab initio results. Our results agree very well with the experimental values available.     

Raman and IR spectra of new organic metals (ET)8[Hg4X12(C6H5Y)2] (X, Y=Cl, Br)

The Raman and IR reflectivity spectra of the new organic metals (ET)₈[Hg₄X₁₂(C₆H₅Y)₂] (X, Y=Cl, Br) based on the molecule bis(ethylenedithio)tetrathiafulvalene (BEDT-TTF or ET in abbreviated form) have been investigated. These metals differ from the previously studied compounds, particularly, in the type of ET molecular packing in the quasi-two-dimensional conducting layers. A high reflectivity and a plasma minimum observed in the IR reflectivity spectrum indicate the presence of quasi-free charge carriers (holes), as in other conducting ET salts. However, the A _g vibrational modes in the Raman spectrum are not activated in the IR spectrum, as is the case of conducting ET salts with other packing types. The Raman lines are assigned to the normal vibrations in the ET molecule, and their ionization shifts are determined. It is demonstrated that the frequencies of the most intense line ν₃(A _g) show a linear dependence on the cation charge, which is characteristic of different ET salts. No correlations are revealed between the ν₃(A _g) frequencies and the packing type. The strong background with a broad maximum at a Raman shift of about 3000 cm^?1 is observed in the Raman spectra upon excitation with the 2.54-and 2.41-eV lines of an Ar⁺ laser. The assumption is made that such a background can be associated with the scattering by one-particle and collective electronic excitations.