39K NMR study of the low temperature phase transitions in KLiSO4

The³⁹K NMR spectra, spin-spin (T ₂) and spin-lattice (T ₁) relaxation times of KLiSO₄ have been measured in the temperature range from 300 K to 90 K. The temperature dependence of the³⁹K (I=3/2) NMR spectra demonstrates the occurrence of a first order phase transition atT _c1=217 K which occurs without a change in the K⁺ site symmetry and another first order transition atT _c2=190 K which is connected with a lowering of the K⁺ site symmetry and the formation of three kinds of ferroelastic domains. From the angular dependence of the second order quadrupole shifts of the³⁹K NMR 1/2–1/2 transitions the electric-field gradient (EFG) tensors at the potassium sites were determined at 290 K, 204 K and 180 K. The symmetry of the ferroelastic phase is monoclinic and not orthorhombic as the K⁺ EFG tensors are tilted away from thec-axis belowT _c2. TheT ₁ data further show the freezing in of the slow reorientational motion 10^–8 s with decreasing temperature from 300 to 90 K.On leave from: J. Stefan Institute, E. Kardelj University of Ljubljana, Ljubljana, Yugoslavia     

Proton magnetic relaxation in ethylene oxide,tetrahydrofuran, and triethylene diamine as clathrate deuterates

Proton spin-lattice relaxation times (T ₁) have been measured for triethylene diamine, ethylene oxide, and tetrahydrofuran as clathrate deuterates. The results are interpreted in terms of anisotropic rotation of the guest molecules. Triethylene diamine is thought to be undergoing rotation about its C ₃ axis with a correlation time given by τ_c/s = 4·87 × 10^-14 exp (1680 K/T) at temperatures between 120 K and the decomposition point (308 K). Between 77 K and 120 K, T ₁ is dominated by conformational distortions of the guest molecule. Ethylene oxide and tetrahydrofuran rotate about at least two axes in the deuterate at rates sufficient to produce some motional narrowing. At high temperatures the relaxation is caused in both cases by rotation about an axis perpendicular to the C ₂ axis, and at lower temperatures by rotation about the C ₂ axis itself. The correlation times are for ethylene oxide τ_c/s = 6·76 × 10^-14 exp (450 K/T), T < 160 K; and for tetrahydrofuran τ_c/s = 4·79 × 10^-14 exp (470 K/T), T < 140 K.

The free induction decay shapes indicate that, in each case, low frequency motion is occurring about all axes throughout the temperature range studied (77 K to the decomposition temperature in each case). From the lack of an observable signal from the clathrate deuterates of hexamethylene tetramine and dioxan, it is deduced that there is no reorientational motion of these guests at frequencies greater than their rigid-lattice linewidths.     

14N NMR study and Landau theory of phase transitions in [N(CH3)4]2ZnI4

The¹⁴N NMR spectra and spin-lattice relaxation timeT ₁ of [N(CH₃)₄]₂ZnI₄ have been studied between room temperature and 200 K. Two phase transitions atT _c 1=255 K and atT _c 1=217 K are observed. The¹⁴N NMR lineshape andT ₁ data suggest that the intermediate phase is commensurate rather than incommensurate in spite of the presence of a Lifshitz invariant in the expansion of the free energy density in powers of the order parameter. We also discuss the phenomenological theory of structural phase transitions in [N(CH₃)₄]₂ZnI₄.     

39K quadrupole coupling and spin-lattice relaxation in the high temperature phases in KLiSO4

³⁹K quadrupole perturbed nuclear magnetic resonance spectra show that in KLiSO₄ atT _c =743 K a phase transition from a room temperature hexagonal to a high temperature orthorhombic phase takes place. The high temperature phase is definitely not incommensurately modulated. The huge shortening of the³⁹K spin-lattice relaxation time on approachingT _c from below demonstrates that KLiSO₄ becomes a superionic conductor above 743 K. The self-diffusion coefficient of the Li-ions is estimated asD=10^–6 cm²/s at 780 K.     

Electronic states of boron in superconducting MgB2 studied by11B NMR

Nuclear magnetic resonance (NMR) spectra and nuclear spin-lattice relaxation rateT ₁⁻¹ of¹¹B have been measured in superconducting polycrystalline MgB₂ with 7_c^ons = 39.5 K. It is shown that (T ₁T⁻¹ and the Knight shiftK _s are independent of temperature and nearly isotropic aboveT _c. Both of these quantities are decreased gradually in going to the superconducting state. According to NMR data the density of states near the Fermi level is flat at the scale of about 500 K. Some conclusions on the orbital content of the density of states at the Fermi level were drawn and compared with the results of the band structure calculations.     

Optical study of the antiferromagnetic-paramagnetic phase transition in chromium oxide Cr2O3

Abstract

Optical effects of the first and second order with respect to the order parameter (1 is antiferromagnetic vector) have been studied in Cr₂O₃ around its phase transition at T_N = 306 K from the antiferromagnetic to paramagnetic state. The magnetic linear birefringence is characterized by a rather large magnitude Δn _sp ? 10^?3 and by a large contribution of fluctuations of the order parameter to the birefringence. The study of the nonreciprocal optical rotation induced by an electric field has shown that the phase transition has a well defined first-order character. This result is also supported by the observation of a nonlinear (quadratic in the electric field) nonreciprocal rotation in a narrow temperature region ΔT = 0.15 K around T_N . The temperature variation of the order parameter l(T) below T_N is well described by a power low l where τ^β = (T_N - T)/T_N and β = 0.355. We also observed a very reproducible effect of the rotation of the optical indicatrix in opposite directions for two types of antiferromagnetic domains. The possible explanation of this effect could be related to the so-called gyrotropic birefringence, an effect related to k_il_i terms in the dielectric permeability.     

1H NMR study of molecular dynamics and phase transition in (NH4)2ZnBr4

The proton second moment (M ₂) and spin-lattice relaxation time (T ₁) have been measured in (NH₄)₂ZnBr₄ in the range 77–300 K. The room-temperature spectrum shows a structure which disappears around 243 K. The signal is strong and narrow even at 77 K. Proton T ₁ shows a maximum at 263 K, caused by spin rotation interaction and decreases with decreasing temperature till 235 K, where it shows a sudden increase. Below 235 K, again it decreases and shows a slope change around 216.5 K (reported T_c ). From 216.5 K, T ₁ decreases continuously without exhibiting any minimum down to 77 K. The narrow line at 77 K, and absence of a T ₁ minimum down to 77 K indicate the possibility of quantum mechanical tunnelling in this system. Motional parameters such as activation energy and pre-exponential factor have been evaluated for the reorientational motion of the NH⁺ ₄ ion.     

Cubic-tetragonal phase transition in SrTiO3 revisited: Landau theory and transition mechanism

Abstract

Existing experimental data for the antiferroelastic phase transition in strontium titanate are reviewed and analysed using a Landau free energy of the form ΔG = 1/2Aθs (cothθs/ Tc-colb.θ/T)Q² + 1/4BQ ⁴ + 1/6CQ ⁶, with A = 0·6472 J K^?1mol^?1, B = 29·12 Jmol^?1, C = 39·27 Jmol, T _c= 105·6 K, θ _S = 60·8 K. The temperature dependence of the critical exponent is found to be due to the delicate balance between the Q ⁴ and Q ⁶ terms in the free energy expansion, and the saturation of the order parameter at low temperatures.

The spontaneous strains observed in this phase transition are not consistent with simple rotation of the TiO₆ octahedra around [001], An alternative model is proposed, where these octahedra expand in order to preserve the volume of the twelve-fold co-ordinated Sr site and the spacing between SrO₃ pseudo-closepacked layers.     

Polar relaxation mode in PbZrO3 crystals

Abstract

In antiferroelectric lead zirconate crystal with one phase transition a nearly monodispersive dipolar relaxation has been found in the paraelectric phase and temperature range 20 K below T _c in the frequency region 20—3 ′ 10⁵ Hz. This relaxation has a dominating influence on the temperature dependence of dielectric susceptibility. Relaxation time obeys the Arrhenius law increasing up to 1.5 ′ 10^?2s (11 Hz) at T _c and then exhibiting a distinct jump.     

Helical magnetic structure in a quasi-one-dimensional LiCu<Subscript>2</Subscript>O<Subscript>2</Subscript> multiferroic crystal according to <Superscript>63,65</Superscript>Cu NMR studies

The NMR spectra of ⁶³Cu and ⁶⁵Cu natural copper isotopes in a LiCu₂O₂ multiferroic single crystal compound have been measured above and below the temperature of magnetic phase transition (T _c = 23 K) in zero magnetic field and in applied magnetic field H ₀ = 94 kOe parallel to the c axis of the crystal. In LiCu₂O₂ below T _c, a complicated helical magnetic structure with the magnetic moment of copper ions Cu²⁺ varying along the chain according to the harmonic law with the wave vector being incommensurate to the crystal lattice constants has been revealed. The experimental results have been successfully interpreted using the model based on the planar helical magnetic structure. It has been found that the plane of rotation for Cu²⁺ magnetic moments in LiCu₂O₂ does not coincide at H ₀ = 0 with the ab plane. The high magnetic field (H ₀ = 94 kOe) applied along the c axis of the single crystal does not affect the spatial orientation of the plane of rotation.     

The effect of a magnetic field on Jahn-Teller ordering in the monoclinic crystal RbDy(WO4)2

This paper presents the results of a study of the thermal properties of monoclinic single-crystal RbDy(WO₄)₂ at temperatures of 2–15 K and in magnetic fields up to 6 T. From the results of measurements of the heat capacity and thermograms, two structural phase transitions are detected, at T _c1=4.9 K and T _c2=9.0 K. The transformation from the high-temperature phase to the low-temperature phase occurs via an intermediate phase. The field dependences of the critical temperatures are found for various magnetic-field orientations. H-T phase diagrams are constructed for H‖a and H‖c. An anomalous increase (by almost an order of magnitude) of the relaxation time of the system, associated with structural instability of the crystal lattice, is detected in the region of the structural phase transitions. A symmetry analysis is carried out, and possible crystal structures of the low-temperature phase are indicated. Fiz. Tverd. Tela (St. Petersburg) 40, 2221–2225 (December 1998)     

Computational investigations of inhomogeneous elastic effects near phase transitions

Abstract

We present different approaches to computer simulations of the order-disorder phase transition in thiocyanate crystals. In these systems the order parameter is coupled to the strain by coupling of the type ?²ε. This coupling plays the dominant role in the behavior of the system below T _c. Molecular Dynamics and Monte Carlo techniques give the proper temperature dependence of the order parameter and the behavior of the fluctuations similar to that observed experimentally. The ab initio calculations allow us to render properly the structural properties of crystals like KSCN and additionally can become the basis for more realistic phenomenological models.     

Spin susceptibility of neutron-irradiation-disordered YBa2Cu3O6.9 in the normal and superconducting states

The spin-spin relaxation rate ⁶³ T ₂ ⁻¹ of ⁶³Cu nuclei in CuO₂ layers is measured in the normal and superconducting states of the compound YBa₂Cu₃O_6.9 (T _c ^onset =94 K) subjected to radiation-induced disordering by a fast-neutron flux Φ to T _c ^onset =68 K (Φ=7×10¹⁸ cm⁻²) and T _c ^onset <4 K (Φ=12×10¹⁸ cm⁻²). It is found that as the structural disorder increases, the contribution of the indirect spin-spin interaction ⁶³ T _2G ⁻¹ , which is related to the value of the spin susceptibility at the boundary of the Brillouin zone of the copper planes χ_s(q={π/a; π/a}), decreases slightly at the transition to the superconducting state for the initial sample and remains unchanged for the weakly disordered sample. This behavior of the short-wavelength contribution to the spin susceptibility attests to the stability of the x ²−y ² symmetry of the energy gap against structural disorder, in accordance with proposed theoretical models of Cooper pairing for high-T _c cuprates. Pis’ma Zh. éksp. Teor. Fiz. 67, No. 3, 172–177 (10 February 1998)     

<Emphasis Type="Italic">T</Emphasis>-odd asymmetries for evaporation γ-rays in nuclear fission

T-odd asymmetries in angular distributions of evaporation γ-rays emitted by thermalized fragments resulting from the fission of axially symmetric deformed nuclei induced by cold polarized neutrons are investigated within the quantum theory of fission. The asymmetries in question are due to the anisotropy of the angular distributions of evaporation γ-rays, caused by zero wriggling vibrations of the fissioning nucleus and associated with the orientation of large fission fragment spins in the direction perpendicular to the direction n ₀ of the symmetry axis of the fissioning nucleus at the time of its separation into fragments. The angle of rotation of the vector n ₀ with respect to the asymptotic direction of the light fission fragment emission is calculated with allowance for the interference of fission amplitudes of neutron resonances excited in the fissioning nucleus as it captures the incident neutron. It is shown that the angular and energy characteristics of the T-odd asymmetry calculated for evaporation γ-rays agree with the characteristics of the experimentally investigated T-odd asymmetry in angular distributions of all γ-rays emitted by a fissioning ²³⁶U nucleus.     

Ferroelectric phase transition in Cs3Bi2I9

Single crystals of Cs₃Bi₂I₉ have been grown. Polarized-optical, dielectric, and calorimetric studies have been performed, along with measurements of birefringence, and the coefficients of elasticity c ₄₄, and linear expansion a over a wide temperature range. It is found that an intrinsic (pseudo-intrinsic) second-order phase transition to the ferroelastic phase takes place at T ₀=220.0±0.6 K accompanied by a change of the crystal symmetry from 6/mmc to 2/m. The transition parameter depends linearly on the shear strain x ₅ and varies with temperature according to a power law with exponent β=0.5. Zh. éksp. Teor. Fiz. 39, 1850–1852 (October 1997)     

Effect of Metal Substitution in BSCCO Ceramic Superconductors

The fabrication, electrical, and optical properties of ceramic Bi_2-xM_xSr₂Ca₂Cu₃O_y (where M=Al or Ni and x=0.3) superconductors are described. Resistivity measurements reveal that when Al is partially substituted at the bismuth site in the BiSrCaCuO compound, T _c decreases to 77 K and the nickel-substituted compound shows a T _c of 70 K, compared to the undoped ceramic BiSrCaCuO samples with T _c of 85 K. Infrared reflectance measurements, which cover the 50-4000 cm^–1 range, find that the reflectance from the ceramic samples decreases in the metal-doped samples. The observed phonon modes in the infrared conductivity spectra (obtained by Kramers-Kronig analysis) decrease in strength and some features are completely smeared out. Furthermore some of the modes are observed to shift slightly in the frequency on doping, the change in the T _c and vibrational modes is attributed to destabilization of 2223 phase.     

Effect of pressure on the superconducting transition in Bi-Sr-Ca-Cu-O system

Abstract

The effect of pressure on T_c was studied on 110 K class Bi-Sr-Ca-Cu-O (1:1:1:2) compounds. T_c was varied with current in I^3/2 , and defined at the value extrapolated to zero current. dT_c /dP = + 0.18 K/kbar was obtained up to 10.9 kbars.     

Incommensurate phase in the layered hexagonal crystal Cs3Bi2I9

An incommensurate phase is observed in a Cs₃Bi₂I₉ crystal, which belongs to the family of layered A₃B₂X₉ structures. Data from ¹²⁷I NQR and x-ray powder diffraction patterns show that the second-order normal phase—incommensurate phase transition at T _i=220 K is accompanied by a P6₃/mmc↔P2₁/m symmetry change and takes place at the center of the Brillouin zone. The nature of the instability leading to the appearance of this little-studied form of incommensurability is discussed. Fiz. Tverd. Tela (St. Petersburg) 39, 1105–1108 (June 1997)     

μSR and NMR investigations on electronic and magnetic state aroundx=0.12 in La2−x Sr x CuO4 and La2−x Ba x ;CuO4

The muon spin rotation ( ⁺SR) and NMR measurements provides clear evidences of the antiferromagnetic order of Cu-moment below 35 K for La_2–x Ba _x CuO₄ and below 15 K for La_2–x Sr _x CuO₄ in the narrow range ofx where the high-T _c superconductivity (SC) is suppressed remarkably. The results suggest that the freezing of spin fluctuations of Cu-moment is relevant to the local suppression of SC under an change of the electronic state coupled with the lattice instability.     

LiKSO4低温相变的核磁共振研究

研究了LiKSO₄中⁷Li的核磁共振谱与温度及晶体取向的关系。证实了在T_c=195K存在一个结构相变,并由实验数据推导出在这一温度以下,晶体的对称性低于P31c,每单胞至少含有六个分子式。没有证据说明有一个无公约相存在。测得181K的四极耦合常数为38.9kHz,不对称因子η=0.46,电场梯度主轴方向z与原c轴成11.6°角。 关键词：