Optical studies of phase transitions in the (NH4)3VO2F4 crystal

(NH₄)₃VO₂F₄ crystals were grown, and polarization-optical studies and measurements of birefringence were conducted on crystal plates of various cuts over a wide temperature range. Phase transitions were detected at temperatures T _1↑ = 417 K, T _3↑ = 211 K, and T _4↑ = 205 K (on heating) and at T _1↓ = 413 K, T _3↓ = 210 K, and T _4↓ = 200 K (on cooling). The transitions are accompanied by anomalies of the birefringence and by twinning. The sequence of changes in the phase symmetry is assumed to be as follows: cubic Fm m ↔ orthorhombic Immm (I222₁) ↔ monoclinic 112/m) ↔ triclinic P . Near temperatures T ₂ ≈ 240–250 K, an additional anomaly of the birefringence is observed, with the crystal retaining the orthorhombic symmetry. Original Russian Text ? S.V. Mel’nikova, A G. Kocharova, 2009, published in Fizika Tverdogo Tela, 2009, Vol. 51, No. 3, pp. 562–564.     

Continuum bound states K L, D 1(2420), D s1(2536), and their partners K S, D 1(2400), D * sJ(2463)

The very recently observed D^*_sJ(2463) meson is described as a bound state in a unitarised meson model, owing its existence to the strong OZI-allowed coupling to the nearby S-wave threshold. By the same non-perturbative mechanism, the narrow axial-vector D_s1(2536) resonance shows up as a quasi-bound-state partner embedded in the continuum. With the same model and parameters, it is also shown that the preliminary broad 1 ⁺ D₁(2400) resonance and the established narrow 1 ⁺ D₁(2420) may be similar partners, as a result of the strong OZI-allowed coupling to the nearby S-wave threshold. The continuum bound states D₁(2420) and D_s1(2536) are found to be mixtures of 33% and 67% , whereas their partners D₁(2400) and D^*_sJ(2463) have more or less the opposite -state content, but additionally with some or admixture, respectively. The employed mechanism also reproduces the ratio of the K_L-K_S mass difference and the K_S width, by describing K_L as a bound state embedded in the continuum. The models results for states containing one b quark are also discussed.Received: 7 June 2003, Revised: 20 October 2003, Published online: 18 December 2003PACS: 12.40.Yx, 14.40.Aq, 14.40.Lb, 13.25.Es, 13.25.Ft, 13.75.Lb     

Simulation of elasto optical properties of K2SO4 crystals

The electronic optical spectra of the mechanically free and stressed crystals of potassium sulfate, K₂SO₄, in the orthorhombic phase Pnma have been calculated by the Cambridge Serial Total Energy Package (CASTEP) code. On the basis of these calculations, the components of stress elasto optical tensors based on the changes of refractive index n (π_im) and birefringence Δn () (i, k, m=1, 2, …, 6) have been obtained for the indices i, k ,m=1, 2, 3. Absolute magnitudes of the calculated tensor π_im are probably underestimated because the magnitudes of the calculated elastic stiffness tensor c_rm are found to be overestimated about two times. Features of the spectral dependences n(E) and k(E) of refractive and absorption indices of the mechanically free and stressed potassium sulfate crystals have been analyzed.     

Running mass m s at low scale from the heavy-light meson decay constants

It is shown that a 25(20)% difference between the decay constants ( ) and f _D (f _B ) occurs due to large differences in the pole masses of the s and d(u) quarks. The values η _D = /f _D ∼ 1.23(15), recently observed in the CLEO experiment, and η _B = /f _B ∼ 1.20, obtained in unquenched lattice QCD, can be reached only if, in the relativistic Hamiltonian the running mass, m _s at low scale is m _s (∼0.5 GeV) = 170–200 MeV. Our results follow from the analytical expression for the pseudoscalar decay constant f _P based on the path-integral representation of the meson Green’s function. The text was submitted by the authors in English.     

Temperature dependence of the indirect band gap, steepness parameter and related optical constants of [Kx(NH4)1−x]2ZnCl4 mixed crystals

Optical transmittance measurements near the absorption edge of [K_x(NH₄)_1−x]₂ZnCl₄ mixed crystals, where x=0.00, 0.232, 0.522, 0.644, 0.859 and 1.00, are reported over 276–350 K range. Analysis reveals that the type of transition is the indirect allowed one. The absorption edge shifted towards lower energy with increasing temperature. It is shown that [K_x(NH₄)_1−x]₂ZnCl₄ mixed crystals with x0.644 reveal a phase transition at 319 K, this phase disappeared at high concentrations of K⁺ ions. The steepness parameter is given, its value is used to estimate the temperature dependence of the indirect energy gap. In the region of the absorption edge, the absorption coefficient obeys Urbach's rule. Urbach parameters are investigated as a function of temperature.     

A nuclear magnetic resonance study of the phase transitions and electric quadrupole Raman processes of M5H3(SO4)4·H2O (M=Na, K, Rb, and Cs) single crystals

The spin–lattice relaxation times and spin–spin relaxation times for ¹H and M in M₅H₃(SO₄)₄·H₂O (M=Na, K, Rb, and Cs) single crystals grown using the slow-evaporation method were measured as functions of temperature. Two kinds of protons were identified in the M₅H₃(SO₄)₄·H₂O structure: acid protons and water protons. Our experimental results show that the acid and water protons in Cs₅H₃(SO₄)₄·H₂O are involved in phase transitions of this crystal, whereas neither type of proton is involved in the phase transitions of the other three crystal type (M₅H₃(SO₄)₄·H₂O; M=Na, K, and Rb). Moreover, the relaxation times for the M (=Na, K, and Rb) nuclei in these crystals were found to decrease with increasing temperature and can be described with (k=2). The T₁ results for M (=Na, K, and Rb) in M₅H₃(SO₄)₄·H₂O crystals can be explained in terms of a relaxation mechanism in which the lattice vibrations are coupled to the nuclear electric quadrupole moments.     

Brillouin scattering study of phase transitions in LiK0.80(NH4)0.20SO4 mixed crystals

Brillouin spectroscopy was used to study the phase transitions of LiK_0.80(NH₄)_0.20SO₄ mixed crystals in the temperature range 10-300 K. The relevant elastic stiffness coefficients were evaluated at room temperature. The quasi-longitudinal γ₁₆ and the quasi-transverse γ₁₇ mode frequencies were measured in the above temperature range. From their frequency vs. temperature curve, three different phase transitions were determined. Two of the four phases presented by the crystal were found to be ferroelastic. The observed phases are tentatively assigned through a comparison with the phase transitions undergone by LiKSO₄ and LiK_0.96(NH₄)_0.04SO₄ crystals. An anomalous behavior of the Brillouin linewidth near the 260 K phase transition was observed.     

Phase transition and analysis of AC conductivity data of the (Cs)0.26(Rb)0.74H(SO4)0.89(SeO4)0.11 compound

The X-ray diffraction, vibrational and impedance spectroscopy studies of (Cs)_0.26(Rb)_0.74H(SO₄)_0.89(SeO₄)_0.11 (CsRbHSSe) new solid solution are presented. The title compound undergo a superionic phase transition (SPT) at This transition was confirmed by an abrupt increase of conductivity. The bulk impedance parameters of CsRbHSSe, RbH(SO₄)_0.81(SeO₄)_0.19 (RbHSSe) and CsH(SO₄)_0.76(SeO₄)_0.24 (CsHSSe) were determined from an analysis of AC conductivity data measured in a wide temperature range. The charge carriers concentration in the samples investigated has been evaluated using the Almond-West formalism and shown to be independent of temperature.     

EPR studies of Gd3+-doped Ce2(SO4)3.8H2O and La2(SO4)3.9H2O single crystals

EPR spectra of Gd³⁺-doped Ce₂(SO₄)₃.8H₂O and La₂(SO₄)₃.9H₂O single crystals have been measured with an X-band spectrometer at room and low temperatures. The absolute signs of spin Hamiltonian parameters have been determined for the La₂(SO₄)₃.9H₂O host from intensities of lines at liquid helium temperature; for the Ce₂(SO₄).8H₂O host the lines broaden considerably below 60 K, not permitting the determination of absolute signs of spin Hamiltonian parameters. The data are analysed using a rigourous least-squares procedure, fitting simultaneously all lines obtained for several orientations of the external magnetic field. The zero-field splittings have been computed for both the hosts. The characteristics of EPR spectra of Gd³⁺ in these hosts are compared with those obtained in other rare-earth trisulphate octahydrate hosts.     

Action of monoatomic cations on the structure and spectra of Mx(1)M1-x(2)UO2(NO3)3 crystals

The action of the monovalent M⁺ cations on the luminescent properties of the mixed M _x ⁽¹⁾ M _1-x ⁽²⁾ UO₂(NO₃)₃ crystals, where M is Na, K, Rb, Cs, or NH₄ , has been investigated. It has been established that the spectral positions of the bands of vibronic transitions depend linearly on the ratio between the concentrations of the M⁽¹⁰⁾ and M⁽²⁾ cations. It is shown that the crystals considered are composed of l[RbUO₂(NO₃)₃]n[CsUO₂(NO₃)₃] clusters, where l/n = x/(1 - x). The spectral regularities revealed are determined by the partial contributions of the M⁽¹⁾ and M⁽²⁾ cations to their combined, polarizing action on the uranyl complex and are explained by the ligand nature of its highest occupied molecular orbital.Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 71, No. 6, pp. 827–830, November–December, 2004.This revised version was published online in April 2005 with a corrected cover date.     

In situ pressure-induced solid-state amorphization in Sm2(MoO4)3, Eu2(MoO4)3 and Gd2(MoO4)3 crystals: chemical decomposition scenario

The effect of pressure on the phase transformations in Sm₂(MoO₄)₃, Gd₂(MoO₄)₃ and Eu₂(MoO₄)₃ crystals has been studied in situ using synchrotron radiation. All three isostructural compounds undergo a structural phase transition at 2.2-2.8 GPa to a new phase, which is interpreted as a possible precursor of amorphization. Amorphization in these crystals occurs irreversibly over a wide pressure range, and its mechanism, interpreted as a chemical decomposition, is found to be weakly affected by the degree of hydrostaticity.     

Crystal structure and thermal behavior of two new organic monosulfates NH3(CH2)5NH3SO4 1.5H2O and NH3(CH2)9NH3SO4 H2O

The chemical preparation, the calorimetric studies and the crystal structure are given for two new organic sulfates NH₃(CH₂)₅NH₃SO₄ 1.5H₂O (DAP-S) and NH₃(CH₂)₉NH₃SO₄·H₂O (DAN-S). DAP-S is monoclinic P2₁/n with unit cell dimensions: a=11.9330(2) Å; b=10.9290(2) Å; c=17.5260(2) Å; β=101.873(1)°; V=2236.77(6) Å³; and Z=8. Its atomic arrangement is described as inorganic layers of units and water molecules separated by organic chains. DAN-S is monoclinic P2₁/c with unit cell parameters: a=5.768(2) Å; b=25.890(10) Å; c=11.177(5) Å; β=115.70(4)°; V=1504.0(11) Å³ and Z=4. Its structure exhibits infinite chains, parallel to the [100] direction where the organic cations are interconnected. In both structures a network of strong and weak hydrogen bonds connects the different components in the building of the crystal.     

H nuclear magnetic resonance study of the ferroelastic and superionic phase transitions of (NH4)4LiH3(SO4)4 single crystals

We investigated the temperature dependences of the line shape, spin-lattice relaxation time, T₁, and spin-spin relaxation time, T₂, of the ¹H nuclei in (NH₄)₄LiH₃(SO₄)₄ single crystals. On the basis of the data obtained, we were able to distinguish the “ammonium” and “hydrogen-bond” protons in the crystals. For both the ammonium and hydrogen-bond protons in (NH₄)₄LiH₃(SO₄)₄, the curves of T₁ and T₂ versus temperature changed significantly near the ferroelastic and superionic phase transitions at T_C (=232 K) and T_S (=405 K), respectively. In particular, near T_S, the ¹H signal due to the hydrogen-bond protons abruptly narrowed and the T₂ value for these protons abruptly increased, indicating that these protons play an important role in this superionic phase transition. The marked increase in the T₂ of the hydrogen-bond protons above T_S indicates that the breaking of O-H?O bonds and the formation of new H-bonds with HSO₄^- contribute significantly to the high-temperature conductivity of (NH₄)₄LiH₃(SO₄)₄ crystals.     

Au10(TBBT)10:Aun(TBBT)m纳米团簇的起点与终点

Gold(I) thiolate compounds (i.e. Au^I-SR) are important precursors for the synthesis of atomically precise Au_n(SR)_m nanoclusters. However, the nature of the Au^I-SR precursor remains elusive. Here, we report that the Au₁₀(TBBT)₁₀ complex is a universal precursor for the synthesis of Au_n(TBBT)_m nanoclusters (where TBBT=4-tertbutylbenzenethiol/thiolate). Interestingly, the Au₁₀(TBBT)₁₀ complex is also found to be re-generated through extended etching of the Au_n(SR)_m nanoclusters with excess of TBBT thiol and O₂. The formation of well-defined Au₁₀(TBBT)₁₀ complex, instead of polymeric Au^I-SR, is attributed to the bulkiness of the TBBT thiol. Through 1D and 2D NMR characterization, the structure of Au₁₀(TBBT)₁₀ is correlated with the previously reported X-ray structure, which contains two inter-penetrated Au₅(TBBT)₅ rings. The photophysical property of Au₁₀(TBBT)₁₀ complex is further probed by femtosecond transient absorption spectroscopy. The accessibility of the precise Au₁₀(TBBT)₁₀ precursor improves the efficiency of the synthesis of the Au_n(TBBT)_m nanoclusters and is expected to further facilitate excellent control and understanding of the reaction mechanisms of nanocluster synthesis.     

High pressure behaviour of KHSO4 studied by electrical impedance spectroscopy

Measurements of the electrical conductivity were performed in KHSO₄ at pressures between 0.5 and 2.5 GPa and in the temperature range 120-350 °C by the use of the impedance spectroscopy. The temperatures of the α-β phase transition (T_Tr) and of the melting (T_m), determined from the Arrhenius plots ln(σT) vs. 1/T, increase with pressure up to 1.5 GPa having dT/dP∼+45 K/GPa. Above the pressure 1.5 GPa, the pressure dependencies of T_Tr and T_m are negative dT/dP∼−45 K/GPa. At pressures above 0.5 GPa, the reversible decomposition of KHSO₄ into K₃H(SO₄)₂+H₂SO₄ (and probably into K₅H₃(SO₄)₄+H₂SO₄) affects the electrical conductivity of KHSO₄, with the typical values of the protonic electrical conductivity, c. 10⁻¹ S/cm at 2.5 GPa.     

Crystal structure and structural transition caused by charge-transfer phase transition for iron mixed-valence complex (n-C3H7)4N[FeFe(dto)3] (dto=C2O2S2)

(n-C₃H₇)₄N[Fe^IIFe^III(dto)₃] shows a new type of first order phase transition called charge-transfer phase transition around 120 K, where the charge transfer between Fe^II and Fe^III occurs reversibly. Recently, we have succeeded in obtaining single crystals of the title complex and determined the crystal structure at room temperature. Crystal data: space group P6₃, Z=2. Moreover, we have investigated the structural transition caused by the charge-transfer phase transition by means of powder X-ray diffraction measurement. When the temperature is decreased, the a-axis, which corresponds to the hexagonal ring size in two-dimensional honeycomb network structure of [Fe^IIFe^III(dto)₃]_∞, contracts by 0.1 Å at the charge-transfer transition temperature (T_CT), while the c-axis, perpendicular to the honeycomb network layer, elongates by 0.1 Å at T_CT. Consequently, when the temperature is decreased, the unit cell volume decreases without noticeable anomaly around T_CT, which is responsible for the quite small vibrational contribution to the entropy change, compared with usual spin crossover transition. Thus, the charge-transfer phase transition around 120 K for (n-C₃H₇)₄N[Fe^IIFe^III(dto)₃] is regarded as spin entropy driven phase transition.     

Primary crystallization field of YnBamCum + nOy oxides with YBa2Cu3O6 tetragonal structure

The phase composition of specimens in the primary crystallization field of the Y _n Ba _m Cu _{m + n} O _y oxide series with compositions of (Y:Ba:Cu) 235, 123, and 257 was investigated by means of X-ray powder diffraction, X-ray microprobe analysis, electron diffraction, and high resolution imaging in a transmission electron microscope. Y _n Ba _m Cu _{m + n} O _y oxides have the YaBa₂Cu₃O₆ tetragonal structure and can be represented in the general form as 123 oxides. It was found that the crystallization field of 123 oxides corresponds to the crystallization fields of the Ba _m Cu _{m + n} O _y oxide series with the BaCuO₂ cubic structure. These crystallization fields are located in the matrix phase diagram of the BaO-CuO _x system at P(O₂) = 21 kPa in the range of 50–90 mol % CuO and 910–1000°C. The 123 oxides are formed according to an intercalation mechanism with the participation of barium-copper oxide matrices with the composition (Ba: Cu) 3: 5, 2: 3, and 5: 7