Influence of defects on the phase transition in squaric acid (H2C4O4)

We have investigated the influence of defects induced by chromium and deuterium doping on the phase transition in solid squaric acid (H₂C₄O₄) by high resolution ¹³C NMR. Deuterium doping alone is observed to increase the phase transition temperature T_c linearly with the ²H concentration, whereas chromium defects, destroy the local order and lead to clusters of the high temperature phase. Correspondingly the critical temperature decreases with increasing chromium doping and the critical temperature region is smeared out.     

Vibronic model for antiferroelectric phase transition in squaric acid, H2C4O4

The antiferroelectric phase transition in H₂C₄O₄ crystals is analyzed using the neutron diffraction and high resolution ¹³C, ¹⁷O NMR data, Hartree-Fock, Moller-Plesset and DFT calculations. Is shown that AFE phase transition is of mixed order-disorder and of displacive type represented within of the unified vibronic interaction model.     

The phase transition of two-dimensional squaric acid H2C4O4 studied by neutron scattering

At T_c = 370 K squaric acid undergoes a phase transition, which was studied in detail by neutron scattering. The transition is essentially two-dimensional with β = 0.137 ± 0.010. Diffuse scattering above T_e is found to be anisotropic. From “diffuse crystallography” it is concluded that proton disorder is not the dominant source of diffuse scattering. Individual molecules retain their low-temperature shape in the high-temperature phase.     

Critical slowing down of fluctuations in p-terphenyl and p-quaterphenyl observed by proton spin-lattice relaxation

The temperature evolution of the proton spin-lattice relaxation time T₁ in p-terphenyl and in p-quaterphenyl around their order-disorder phase transition has been measured. In both cases pretransitional collective fluctuations destroy the high temperature Arrhenius behaviour of the relaxation rate corresponding to a single reorientational jump motion. The spin-lattice relaxation times present then a drastic decrease until the transition temperature (T₀ = 193 K in p-terphenyl, T₀ = 238 K in p-quaterphenyl). This decrease is associated to the critical slowing down of fluctuations. In the low temperature phase the ordering phenomena lead to a sharp drop of the spin-lattice relaxation rate.     

A microscopic model of squaric acid (H2C4O4) based on three-dimensional coupling of one-dimensional Ising chains

The phase transition in squaric acid is discussed on the basis of three-dimensionally coupled Ising chains, where the protons in the hydrogen bonds serve as the Ising spins. The Ising model is solved in an approximation treating the chains exactly and the interactions between them in mean-field approximation.     

Carbon-13 Isotopic Species of H2C3, H2C4, and H2C5: High-Resolution Rotational Spectra

The microwave rotational spectra of the carbon-13 isotopic species of H₂C₃, H₂C₄, and H₂C₅ have been observed in a pulsed supersonic molecular beam by Fourier transform microwave spectroscopy. At high resolution all of the rotational lines exhibit hyperfine structure produced by the magnetic interaction between the nuclear spin of ¹³C and the overall rotation of the molecule. The component of the nuclear spin-rotation tensor along the a-inertial axis is large for most isotopic species, especially at the carbene carbon; at this position C_aa is two to three times larger than at other substituted positions along the chain. In contrast to both H₂C₃ and H₂C₃, in H₂C₄C_aa exhibits a pronounced alternation along the carbon chain backbone. Following detection of the five carbon-13 isotopic species and D₂C₅, an experimental structure (r₀) has been determined to high accuracy for H₂C₅.     

Proton spin-lattice relaxation study of the phase transition in TlH2AsO4

Measurements of the temperature and frequency dependence of the proton laboratory and dipolar frame spin-lattice relaxation rates in powdered TlH₂AsO₄ demonstrate that the ⁷⁵As quadrupole resonance frequency changes from v_Q ≈ 38 MHz at T ?T_c to v_Q 〈 10 MHz at T 〉 T_c. The structural phase transition at T_c = 251 K is thus connected with a disordering of the protons in the O–H … O bonds surrounding the AsO₄ groups.     

13C chemical shift tensor investigation of a second order phase transion: squaric acid

The variation of the ¹³C chemical shift tensor was observed as a function of temperature when going through the antiferroelectric second order phase transition in a single crystal of squaric acid. From the temperature dependent line shift a non-classical critical exponent β = 0.14 was determined, which reflects the two dimensional character of this order-disorder transition.     

1H chemical shift tensor in squaric acid: C4O4H2

The proton chemical shift tensor in squaric acid, which undergoes an antiferrodistortive phase transition at T_c = 97°C, has been determined by means of a multiple-pulse NMR experiment. No change of the shift tensor is observed, when going through the phase transition, indicating a purely order—disorder type transition. The principal values of the shift tensor are: — 26.5, —20.2 and 1.0 ppm with respect ti TMS.     

23Na nuclear magnetic resonance study of the phase transition in ammonium rochelle salt,NaNH4C4H4O6 • 4H2O

The satellite NMR spectrum of ²³Na in ammonium Rochelle salt (NaNH₄C₄H₄O₆ ? 4H₂O) is investigated near the transition temperature. Each line in the paraelectric phase splits discontinuously into four lines at the transition temperature; this fact is compatible with the first-order phase transition and reveals the existence of the superstructure in the ferroelectric phase.     

Unexpected interference patterns observed in the 12C(14N, 13N)13C reaction at energies close to the Coulomb barrier

The reaction ¹²C(¹⁴N, ¹³N)¹³C has been measured at 28, 32, 34 and 36 MeV beam energies. The energy dependence of the measured interference pattern cannot be reproduced by DWBA calculations which take account of the interference between proton and neutron transfer. A rather strong asymmetry of the angular distributions about θ_c.m. = 90° has been observed.     

Proton spin-lattice relaxation time of triglycine sulfate (NH2CH2COOH)3 · H2SO4 near the transition point

The temperature dependence of the proton spin-lattice relaxation time T₁ of triglycine sulfate (NH₂CH₂COOH)₃ · H₂SO₄ is investigated near the transition point under several conditions. Any anomalous behavior of T₁ cannot be observed in contrast with an earlier report by Brosowski et al.     

Direct observation of ferroelasticity in squaric acid C4H2O4

Ferroelasticity of squaric acid C₄H₂O₄ has been directly confirmed by observations of elastic hysteresis loop under the polarizing microscope. Aizu and Sapriel theories predict the orientation of twin wall, which is not necessarily a low index plane. The wall orientation of this crystal is just the case, and it is not of low index. No temperature variation of the domain orientation was observed up to the transition temperature.     

Study of the reactions 12C(d,d)12C and 12C(d,p)13C across the resonance at Ed = 2.71 MeV

The second order processes in particle transfer reactions are tested by measuring the resonance structure of the 3⁺ state of ¹⁴N at 12.61 MeV for some outgoing channels of the ¹²C + d reaction.     

No pre-critical enhancement observed in 13C(π+, γ)13N(g.s.) at q ≅ 2mπ

The cross section and angular distribution for the reaction ¹³C(π⁺, γ)¹³N(g.s.) have been measured from 37 to 85° in the laboratory, at a pion energy of 115.5 MeV. The observed cross section ranges from 320 to 660 $\text{nb}\text{sr}$. These results do not show the large magnitude and wide-angle peaking expected if pre-critical effects due to nascent pion condensation were present. In addition, the observed cross section is less than one-half of the predictions of available theoretical calculations which do not include the pre-critical effect. Data on the reaction ¹H(π^?, γ)n at T_π = 116.6 MeV were also obtained for calibration purposes. These data agree with expectations based on knowledge of the inverse reaction and previous measurements.     

Critical slowing of local fluctuations near the paraelectric-incommensurate phase transition in Rb2ZnCl4

A study is reported of the anomalous broadening of Mn²⁺ EPR lines on the high-temperature side of the paraelectric-incommensurate phase transition in Rb₂ZnCl₄ crystals. It is shown that the resonant-line broadening is inhomogeneous and due to the contribution of low-frequency fluctuations corresponding to the central peak in the elementary-excitation spectrum. The data obtained have permitted us to obtain the critical correlation-length exponent ν=0.64±0.02 corresponding to the 3d XY model of Heisenberg. Fiz. Tverd. Tela (St. Petersburg) 41, 122–125 (January 1999)     

Statistical fluctuations and compound elastic scattering for 12C(12C, 12C)12C and 16O(16O, 16O)16O

The difference between the observed fluctuations in the elastic excitation functions for carbon-carbon and oxygen-oxygen scattering, just above the Coulomb barrier, is interpreted in terms of the compound nucleus model.     

Exact finite-range DWBA analyses of (12C, 11B) and (12C, 13C) reaction from 208Pb

A fast way to carry out the exact-finite range DWBA calculation is discussed, and is applied to the analyses of (¹²C, ¹¹B) and (¹²C, ¹³C) reaction from ²⁰⁸Pb.     

Rotating frame spin-lattice relaxation in the nematic phase of MBBA-d13

Measurements of the deutron and proton spin-lattice relaxation times in the rotating frame in the nematic phase of a partially deuterated MBBA-d₁₃ sample are reported. The rotating frame relaxation rate cannot be explained by the theoretical model of Freed.     

Evidence for inelastic processes in the 24Mg(13C, 12C)25Mg reaction at elab = 30 MeV

The ²⁴Mg(¹³C, ¹²C)²⁵Mg reaction has been studied at 30 MeV using a magnetic spectrometer. Differential cross sections for transitions to several final states in ²⁵Mg have been measured and analysed using an exact finite range DWBA code. The DWBA predictions have fitted the bell-shaped distributions satisfactorily, yielding spectroscopic factors which are in reasonable agreement with those obtained using (d, p) reactions. The exceptions are the $\text{3}\text{2}$⁺ state at 0.97 MeV which displays a marked departure from the bell-shaped angular distribution obtained for the other $\text{3}\text{2}$⁺ state at 2.80 MeV, and the $\text{7}\text{2}$⁺ state at 1.61 MeV whose angular distribution has an unusual shape, displaying a deep minimum located at the grazing angle. A semiquantitative model has been used to suggest that the angular distribution for the 0.97 MeV state is evidence for the coupling of inelastic processes in this transition. In the case of the 1.61 MeV state it is suggested that the angular distribution shows the influence of indirect Coulomb excitation on the transfer cross sections.