Bromine metallization studied by X-ray absorption spectroscopy

The experimental data carried out by M?ssbauer and magnetic resonances investigations of the structural phase transitions in K₂ZnCl₄ crystals are discussed by a simple electrostatic model, calculating, the lattice contributions to the local electric potential V(r), electric field intensity E(r) and electric field gradient tensor, (r) and taking into account both the fractional electric point charges and rigid lattice approximations. The validity of the model is proved by a good fit of the computing results and experimental data of quadrupole splitting parameters at K sites obtained by ³⁹K-NMR methods in high temperature incommensurate phase ( Pnam symmetry). The experimental results obtained by M?ssbauer and EPR methods in commensurate phase (Pna2₁ symmetry) of iron and copper doped K₂ZnCl₄ crystals are explained by relaxing the rigid lattice approximation. The insertion of probe ions appear to be done on not-exactly-Zn²⁺ site. Received 3 February 1999 and Received in final form 4 May 1999     

The crystal structure of Rb2CdCl4 in para- and ferroelastic phases

Abstract

The crystal structure of Rb₂CdCl₄ has been studied by X-ray diffraction at 295 and 160 K in the initial phase D ¹⁷ _4h as well as at 105 K in the ferroelastic phase. It was found that the phase transition D ¹⁷ _4h ? D ¹⁰ _2h takes place instead of D ¹⁷ _4h ? D ¹⁸ _2h as proposed earlier. The first of the transitions corresponds to unequal and the second to equal Φ-tilts of CdCl₆-octahedra around the a and b axes of the tetragonal unit cell.     

Group-theoretical analysis of possible structural phase transitions in the high-temperature superconductors

A group-theoretical analysis is performed for the complete condensation of order parameters at structural phase transitions (SPT's) in the high-temperature superconductors belonging to the D ¹⁷ _4h -14/mmm space group in the high-symmetry phase. As a rule, such transformations are due to a successive softening of phonons with wave vectors k ₁ = 1/2 b ₃ and k ₂ = 1/2 (b ₁ - b ₃) belonging to the K13(X) star of the Brillouin zone of a tetragonal body-centered Bravais cell. SPT's in system La_2-x Ba _x CuO₄ are considered in detail.     

Modeling of a structural phase transition in La2−x SrxCuO4

The structural, vibrational, and elastic properties of La₂CuO₄ are calculated using a model for calculating the energy of the crystal based on interionic potentials with the multiparticle Jahn-Teller contribution included explicitly. The microscopic reasons for the structural instability of the La₂CuO₄ lattice relative to rotations of the oxygen octahedra are investigated. A structural phase transition from the orthorhombic phase (space group D _2h ¹⁸ ) into the tetragonal phase (space group D _4h ¹⁷ ) under hydrostatic compression of an La_2−x Sr_xCuO₄ crystal is modeled. The (P,x) structural phase diagram for La_2−x Sr_xCuO₄ is constructed. Fiz. Tverd. Tela (St. Petersburg) 41, 1096–1102 (June 1999)     

Structural stability,phase transition and dielectric properties of Pb(Mg1/2W1/2)1−xMxO3 (M = Zr,Sn, Ti) ceramics

ABSTRACT

Pb(Mg_1/2W_1/2)_1?xM_xO₃ (M?=?Zr,Sn,Ti) ceramics have been prepared by the conventional ceramic process. Their crystallographic, phase transition and dielectric properties have been investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM) and impedance analyzer in this paper. The solubility of the tetra-valent cation in Pb(Mg_1/2W_1/2)_1?xM_xO₃ decreases in the sequence of Ti^4+?>?Zr^4+?>?Sn⁴⁺. The doping led to the phase transition from orthorhombic antiferroelectric to cubic paraelectric phase, the microscopic nature of which could be attributed to the contraction of Pb-O₁₂ dodecahedron. A definition of the ordering parameter of the cubic phase was deduced to quantitatively evaluate the B-site ordering degree which decreased with the increase in doping concentration. Anti-site disordering of Mg²⁺ and W⁶⁺ occurred in the high-level doping compositions, which led to the relaxor behavior observed in Ti⁴⁺-doped series with x?=?0.01 composition. Both of the maximum dielectric permittivity and loss in the paraelectric phase increased with the increasing doping concentration.     

Evolution of the structure of Rb2ZnCl4 over the temperature range 4.2–310 K

X-ray diffraction is used to study the temperature dependence of the lattice parameters and the sequence of structural realignments in crystalline Rb₂ZnCl₄ over temperatures of 4.2–310 K. The appearance of and changes in the system of satellite reflexes indicative of structural ordering are studied. Below 74 K, on going into the monoclinic phase (space group A11a), anomalies are observed in the behavior of the lattice parameters, and superstructural reflexes develop with wave vectors q=a ^*/3+b ^*/2+c ^*/2 corresponding to an increase by a large factor in initial parameters a, b, and c of the Pnma-phase. Fiz. Tverd. Tela (St. Petersburg) 41, 1084–1090 (June 1999)     

Pressure and temperature induced phase transitions in LiInSe2

Abstract

At 4.1 GPa LiInSe₂ transforms from the β-NaFeO₂ - type structure to the NaCl-type structure LiInSe₂-hpI (cubic; Fm3m; a=546.4(3)pm, Z=2, D _x =5.75g/cm³; 4.1GPa) which remains metastable at normal conditions. Heating to 210°C at 1.8 GPa causes ordering of the cations and a phase transition from LiInSe₂-hpI to the α-NaFeO₂ - type structure LiInSe₂-hpII (rhombohedral; R3m; a=393.4pm, c=1919.7pm, Z=3, D _x =5.53g/cm³; 1.8GPa). Heating to 210°C at 0.27 GPa results in a phase transformation from LiInSe₂-hpII to the chalcopyrite-type phase LiInSe₂-hpIII (tetragonal; 142d; a=580.7(8)pm, c=1181.0(31)pm, Z=4, D _x =4.66g/cm³; 0.27 GPa).     

Nonphonon mechanism of superconductivity in compounds with quasi-two-dimensional complexes RuO2

The characteristics of nonphonon pairing of hybridized p and d electrons in planar complexes RuO₂ are studied in the presence of a strong short-range Hubbard repulsion. The phase diagram of superconductivity as a function of the degree of underfilling of the 2p ⁶ and 4d ⁶ shells in the RuO₂ complexes is calculated in a generalized Hubbard model. Fiz. Tverd. Tela (St. Petersburg) 40, 980–983 (June 1998)     

Atomic 6p-Tl0 centers in ferroelectric Rb2ZnCl4 crystals

Abstract

Several Tl⁰ (6s²6p ¹)-type paramagnetic centers, produced by low temperature X-ray irradiation, were observed and studied by electron spin resonance (ESR) in the orthorhombic ferroelectric phase of thallium doped Rb₂ZnCl₄ crystals. The centers were formed by electron trapping at Tl⁺ ions localized substitutionally at Rb⁺ sites. The number and properties of the observed centers account for the tripling of the unit cell in the ferroelectric phase.     

Energy Shift of the 4f 136s 1 Excited States of Yb2+ from Gas Phase to the CsCaBr3 Solid

ABSTRACT

Ab initio calculations on Yb²⁺-doped CsCaBr₃ show a large increase (20000 cm^?1) of the 4f ¹⁴ → 4f ¹³6s ¹ transitions from gas phase to CsCaBr₃, due to large embedding effects originated beyond the first bromide coordination shell. The 4f ¹³6s ¹ states of (YbBr₆)^4- in vacuo undergo a sudden energy and electron density change at short Br-Yb distances resulting in the formation of an Yb-trapped exciton, which evolves to full ionization, making Yb(II) unstable in hexabromide coordination in gas phase. Embedding in CsCaBr₃ localizes the 6s electron inside the Br₆ cage, which increases the 6s-Br₆ repulsion and increases the energy of the 4f ¹³6s ¹ states.     

Fluctuation-induced broadening of Mn2+ EPR lines in the incommensurate phase of Rb2ZnCl4 crystals

The EPR spectra of Mn²⁺ ions in Rb₂ZnCl₄ crystals is investigated in the vicinity of the transition from the paraelectric phase to an incommensurate modulated phase. When these crystals are cooled below the transition temperature T _i =304 K, a splitting of the resonance lines is observed in the singular spectrum. A one-harmonic model is used to discuss the contributions that fluctuations in the amplitude and phase of the incommensurate displacement wave make to the local width of the singular spectra. It is shown that anomalies in the local width of the low-temperature singular peaks observed in the vicinity of T _i are caused by amplitude fluctuations. Fiz. Tverd. Tela (St. Petersburg) 41, 1668–1674 (September 1999)     

The effect of ligands on the first hyperpolarizabilities of rich d electron molecular system: iridium clusters,a TDDFT study

A systematically varied series of tetrahedral iridium clusters have been studied using a TDDFT method focusing on their electronic and nonlinear optical properties. The clusters W₂Ir₂(CO)₁₀(η⁵-C₅H₄Me)₂ (1), WIr₃(μ-dppe)(CO)₉(η⁵-C₅H₄Me (2), W₂Ir₂(μ-L)(CO)₈(η⁵-C₅H₄Me)₂(L = dppe 3, dppf 4), Ir₄(μ-L)(CO)₁₀(L = dppm 5, dppe 6, Ph₂P(CH₂)₃PPh₂ 7, Ph₂P(CH₂)₄PPh₂ 8, (Ph₂P)₂CHMe 9), Ir₄(CO)₁₀(phen)(phen = 1,10-phenanthroline) (10) exhibit the first static hyperpolarizabilities of medium magnitude (β _tot ～ 10 × 10^?30esu). The origination of β is discussed in terms of the electronic structure calculation and the expanded orbital decomposition scheme. The result suggests the origination of β for all the clusters are mainly d–d electron transitions intra metal skeleton, and d–π* electron transitions from metals to carbonyls. For cluster 5, which contains the ferrocenyl group, the main origination of β involves charge transfer from d orbitals of ferrocene to d orbirals of Ir and W.     

The structural transformation of monoclinic [(R)-C5H14N2][Cu(SO4)2(H2O)4]·2H2O into orthorhombic [(R)-C5H14N2]2[Cu(H2O)6](SO4)3: crystal structures and thermal behavior

Single crystals of [(R)-C₅H₁₄N₂][Cu(SO₄)₂(H₂O)₄]·2H₂O (1) were grown through the slow evaporation of a solution containing H₂SO₄, (R)-C₅H₁₂N₂ and CuSO₄·5H₂O. These crystals spontaneously transform to [(R)-C₅H₁₄N₂]₂[Cu(H₂O)₆](SO₄)₃ (2) over the course of four days at room temperature. The same single crystal on the same mounting was used for the determination of the structure of (1) and the unit cell determination of (2). A second single crystal of the transformed batch has served for the structural determination of (2). Compound 1 crystallizes in the noncentrosymmetric space group P2₁ (No. 4) and consists of trimeric [Cu(SO₄)₂(H₂O)₄]^2? anions, [(R)-C₅H₁₄N₂]²⁺ cations and occluded water molecules. Compound 2 crystallizes in P2₁2₁2 (No. 18) and contains [Cu(H₂O)₆]²⁺ cations, [SO₄]^2? anions and occluded water molecules. The thermal decompositions of compounds 1 and 2 were studied by thermogravimetric analyses and temperature-dependent X-ray diffraction.     

A luminescence study of B-type Eu2O3 under pressure

Abstract

Luminescence spectra from Eu³ + ion in B-type (monoclinic) ₂O₃ powder have been recorded at room temperature as a function of pressure using a diamond anvil cell. Changes in the spectral pattern of the Eu³ + ion emission at about 4 GPa indicated that a phase transition to the A-type (hexagonal) structure had taken place. Upon release of the applied pressure, the B-type structure was regained with hysteresis. The spectral shifts with pressure have been used to study the effect of pressure on the spin-orbit interaction of the 4f electrons in the Ed + ion. The relationship between the relative changes in the spin-orbit coupling constant, ζ_4f, and the volume accompanying the phase transition is also discussed.     

Crystal structure,characterisation and vibrational study of a mixed compound Cs0.4Rb0.6H2Po4

Abstract

The crystal structure of Cs_1-x Rb _x H₂PO₄, x = 0.6 (CRDP) which crystallises in space group P2₁/m and is isostructural with the monoclinic phase of CsH₂PO₄ (CDP), has been refined at room temperature using single-crystal X-ray diffractometer data. The cell parameters are a = 4.8183(9)å, b = 6.2671(6) å, c = 7.7620(10)å, β= 108.260(10)°, V = 222.58(5)å³, Z = 2, D_x = 3.009g cm^?3. F(000)=187, T = 298(2)K (room-temperature phase), R = 0.0355 and wR = 0.0949 for 654 observed reflections. CRDP contains two crystallography inequivalent hydrogen bonds in the unit cell. The shorter bond (Ko – o = 2.453(7) A) links the phosphate groups into chains running along the b-axis and the longer bond (Ko – o = 2.488(6) A) which is always ordered, crosslinks the chains to form (001) layers. The phase transitions in the mixed Cs_0.4Rb_0.6H₂PO₄ (CRDP) were characterised by differential scanning calorimetry which shows two anomalies at about 293 and 525 K. The Raman and infrared spectra at room temperature were investigated in the frequency ranges 10–3500 and 200–4000 cm^?1 respectively. An assignment of all the bands is given. The bands are in agreement with the monoclinic room-temperature phase implying high dynamical disorder of the acidic proton O-H s–O hydrogen bond.     

Far infrared and phase transitions in ferroelectric materials

Abstract

A number of far infrared absorption bands are found in molecular and hydrogen-bonded crystals when there are several formula units in the primitive cell. They are very sensitive to the crystal structure and constitute a convenient probe for looking at small changes in the structure, especially at low temperatures where there is a lack of X-Ray data. As well known examples, the case of LTT (Lithium Thallium Tartrate) where the absorption band observed at 21 cm^?1 at room temperature softens down to 8 cm^?1 at the Curie temperature (T_c = 12 K), and LiNH₄SO₄ which has a phase transition at 26 K, will be considered.

It is less known that some ferroelectric crystals can be studied as very thick single crystal plates (thickness t up to 10 mm) at 4 K, and they show in some cases a far IR transmission that is much higher than expected from the study of thin plates (i.e. t = 20 μm). In fact the transmission does not decrease very much when thickness is higher than some specific value t ₀/2. The crystal is not homogeneous at 4 K. A model with a far IR absorbing surface layer (thickness t ₀/2) and a transparent bulk is a good first approximation.

At some temperature T ₀ located between liquid helium and liquid nitrogen temperatures, the center of the bulk undergoes a phase transition from the absorbing phase into the transparent one. When temperature is still lowered, the transparent phase is extended towards the surface with an incommensurate phase between the surface and the bulk.     

Superionic phase transition in (NH4)2SeO4·2NH4HSeO4 single crystal at 378 K

Abstract

DTA, structural and electric conductivity investigations were made for (NH₄)₄H₂(SeO₄)₃ single crystals. A high-temperature phase transition at 378 K to a superionic phase was found. The phase is characterized by a high electrical conductivity (～4.10^?3 Ω^?1 cm^?1) and a low activation energy (0.11 eV).     

X-ray diffraction analysis of the modulated structure of Bi2Sr2CaFe2O y

The modulated structure of the Bi₂Sr₂CaFe₂O _y compound isomorphous with the Bi₂Sr₂CaCu₂O _y phase was refined by the method of the full profile analysis on the basis of the powder X-ray diffraction data. The analysis was carried out in the superspace group ofN:Abmm: 1 – 11 using the parameters of the unit cell of the basic structurea = 5.466(3),b = 5.446(5) andc = 31.25(9) Å and the modulation vectorq = 0.220(5)b* +c*. The model of excess oxygen interstitials into the BiO plane was used by applying a saw-tooth-type function of the form:U = 2U ₀ [(X ₄ -X ₄ ⁰ /)] [1]. The results point to a strong displacement of the atoms from their positions in the basic structure. An essential improvement of theR factors was obtained by taking into account the modulation waves of the higher harmonics.     

Optical properties and radiation effects in layered crystals of (CnH2n+nNH3)2MCl4: n = 1, 2, 3; M = Cd,Cu, Mn

Abstract

A new absorption band has been found at 5.10 eV in (C _n H_{2n + 1}NH₃)₂CdCl₄: n = 1, 2, 3 in addition to the absorption bands of CdCl₂ whose electronic structure resembles the former crystals. The energy of the additional peak shifts with temperature by as much as 0.38 eV from 5.10eV at room temperature (RT) to 5.48 eV at liquid nitrogen temperature. This large peak shift is attributed to a structural phase transition between these two temperatures. A new type of electron center has been found in these crystals (M = Cd, Mn; n = 1, 2, 3) irradiated with X-rays at 15 K in addition to the Cl₂ ^?. This shows optical absorption bands (IR bands) in the infrared region of 10 ～ 20 kcm ^?1. The IR bands are assigned to an electron center where an electron is trapped at an ammonium site in the neighborhood of a Cl^? vacancy.     

Dielectric relaxation in ferroelectric (CH3)2NH2AL(so4)2 · 6H2O crystal

Abstract

This paper presents the results of the investigation of dielectric dispersion and ultrasonic velocity in the ferroelectric (CH₃)₂NH₂Al(SO₄)₂ · 6H₂O crystal. The crystal shows a critical slowing down process of polarization with an extremely long relaxation time of the dipole system (τ = 1.6 · 10^?7s at the phase transition point). The dielectric response over the frequency range up to 56 GHz in the paraelectric phase can be well described in terms of a monodispersive Debye-type formula. The activation energy of dipoles in the paraelectric phase is 0.11 eV = 8.5 kT_c . The results show that the proper ferroelectric phase transition is nearly critical and of the order-disorder type.