Raman study of squaric acid

Raman spectra of squaric acid have been measured at various temperatures below and above the transition temperature. Six low frequency bands below 300 cm^?1 are assigned to translational and librational modes of the squaric molecule on the basis of the recent crystal determination by X-ray diffraction. A discussion is given of the striking temperature dependence of these external modes in the ordered phase in connection with the phase transition.     

Vibrational study of squaric acid

Infrared and Raman spectra of polycrystalline squaric acid and its deuterated derivative have been investigated at various temperatures in the 3600–30 cm^-1 range. Two infrared active lattice modes have been identified at 230 and 125 cm^?1 in the spectra at room temperature phase. A new assignment of some intramolecular vibrations involving OH and CO group is proposed. The behaviour of intramolecular and lattice bands as a function of temperature and phase transition is discussed.     

Temperature dependence of the order parameter of the two dimensional antiferroelectric squaric acid as determined from birefringence measurements

The temperature dependence of the partial birefringence Δn is measured on single crystals of squaric acid. In contrast to previous results a phase transition of the first order is detected. The temperature dependence of Δn close to T_c supports the validity of a classical Landau expansion which also has been suggested recently in order to account for the behaviour of the specific heat. The results are discussed in relation to those of previous works.     

Second harmonic generation in squaric acid

From the temperature-dependent optical second harmonic generation in squaric acid we conclude that no local symmetry breaking occurs in the high temperature phase of this material.     

The structure and spectra of squaric acid

Crystallographic data suggest that the squaric acid molecule has point symmetry 4/m in the high temperature phase. Symmetry as high as this is not however confirmed by Raman spectroscopy. The authors suggest that this discrepancy can be resolved by supposing that x-ray diffraction provides structural data that relates to the $\text{average}$ of the possible $\text{orientations}$ of the molecule in a disordered structure, while vibrational spectroscopy is mainly influenced by the symmetry of the molecules themselves.     

Specific heat of the two dimensional antiferroelectric squaric acid

The heat capacity of a single crystal of the two dimensional antiferroelectric squaric acid was measured particularly near the phase transition. In a wide temperature range belowT _c a strong temperature dependence of the anomaleous specific heat is found which can be described by a critical exponent=0.5. The behaviour close toT _c is determined by a latent heat thus indicating the phase transition to be of the first order. Near the phase transition the thermal behaviour can be described by a Landau type theory. The smallness of the transition entropy detected may be ascribed possibly to proton tunneling.     

A structural phase transition in squaric acid

Squaric acid (3,4-dihydroxy-3-cyclobutene-1,2-dione) is found to undergo a second order antiferrodistortive tetragonal to monoclinic phase transition at 97.7°C. The transition temperature for the fully deuterated compound is 243°C. The crystals are optically biaxial negative at room temperature, and the partial birefringence decreases with temperature as (n_z ? n_y)α(T_C ? T)^β, where β_H = 0.34 ± 0.02 and β_D = 0.37 ± 0.04.     

用电子散斑干涉法测量材料热膨胀系数

 利用双光束电子散斑干涉法（ESPI）对试件受热变形进行了实时观测，针对一次实验过程中得到的图片较多（300～500幅）的特点，在图像处理时摒弃了以往的手动识别等位移线的办法，用MATLAB语言编写了批处理程序，能够在采集的大量散斑图片中自动快速准确地标定等位移线，得到相应的位移和应变，并结合实时测量的温度值，获得了45钢和LY12铝合金在不同温升率下的热膨胀系数及其随温度的变化。实验结果表明，在涉及的温升率范围内，温升率的改变对材料热膨胀系数的影响不明显，材料的热膨胀系数随温度的升高略有上升。     

Dielectric relaxation near the antiferroelectric phase transition of squaric acid

The dielectric response of single crystals of squaric acid is studied around the antiferroelectric phase transition atT _c 100°C. The dielectric data between 64 and 400 GHz can be explained by a simple Debye-relaxation with a critical slowing-down of the relaxation frequency to about 55 Ghz on approachingT _c . The results in the paraelectric phase are discussed on the basis of weakly coupled one-dimensional Ising chains. Within the scope of this model we determine the intra and interchain coupling constants, the molecular dipolar moments and the noninteracting proton intrabond-jump time.     

High-resolution electroreflectance measurements of GaAs

High-resolution surface-barrier electroreflectance measurements are reported for GaAs, taken at 4.2 K in the Schottky barrier configuration. Interference effects between the bulk and space-charge regions are seen at the n = 1 exciton line for both E₀ and E₀ + Δ₀ transitions. We calculate the value μ_T = (0.055 ± 0.008) m_e for the transverse mass of the E₁ + Δ₁ transition from Franz-Keldysh oscillations observed at high fields. The E'₀ structure is resolved into separate critical points of Γ and Δ symmetry. Threshold and broadening energies are obtained at 4.2 K for the E₀,E₀, E₀ + Δ₀, E₁, E₁ + Δ₁, E'₀ (Γ and and E₂ (Σ and X) critical points.     

High-resolution resonance measurements of the physisorption interaction

Narrow selective adsorption resonances, with line widths given by the natural lifetime of the intermediate resonance state, have been observed directly in the angular distributions of Ne nozzle beams diffracted from a 20 K Cu(110) surface. Using beams of 20Ne and 22Ne isotopes we show that this high-resolution mode of resonance detection permits, even in a case of a small isotope effect, unique assignment of bound level sequences compatible with a single gas-surface potential curve.     

Polymorphic phase transition and thermal stability in squaric acid (H2C4O4)

Phase transformations in squaric acid (H₂C₄O₄) have been investigated by thermogravimetry and differential scanning calorimetry with different heating rates β. The mass loss in TG apparently begins at onset temperatures T_di=245±5 °C (β=5 °C min^?1), 262±5 °C (β=10 °C min^?1), and 275±5 °C (β=20 °C min^?1). A polymorphic phase transition was recognized as a weak endothermic peak in DSC around 101 °C (T_c⁺). Further heating with β=10 °C min^?1 in DSC revealed deviation of the baseline around 310 °C (T_i), and a large unusual exothermic peak around 355 °C (T_p), which are interpreted as an onset and a peak temperature of thermal decomposition, respectively. The activation energy of the thermal decomposition was obtained by employing relevant models. Thermal decomposition was recognized as a carbonization process, resulting in amorphous carbon.     

Interferometric measurements of the anomalous thermal expansion in single crystal dysprosium and 60% Gadolinium-Yttrium

An optical interferometer technique employed in the measurement of the c-axis thermal expansion in single crystal Dy confirms previous strain gauge observations that the expansion and Néel temperature are dependent on the internal strain in the crystal. The technique reveals that similar effects can be observed in single crystal 60% Gd-y although this material is less susceptible than dysprosium to internal strain produced by thermal cycling. The consequences of the deviations in c-axis expansion are discussed.     

Low temperature thermal expansion of NaNO2 along the ferroelectric b-axis

Measurements of the low temperature thermal expansion of ferroelectric NaNO₂ along the ferroelectric b-axis have been performed with a three terminal capacitance dilatometer, both in single domain and multidomain state.A contribution, due to domain walls, is present when the sample is in multidomain state. An Einstein contribution with ?_E ～ 120 K is present in both domain-states of the sample.     

High-resolution measurements of low-energy conversion electrons

High-resolution measurements of low-energy conversion electrons have been performed in odd and odd–odd nuclei with masses around A=182. The experimental setup, a magnetic spectrograph coupled to a specific tape transport system, is described. Three experiments have been performed and the main results are presented. This revised version was published online in July 2006 with corrections to the Cover Date.     

Molecular dynamics study of a two-dimensional model of squaric acid

The vibrational properties of the two-dimensional model of squaric acid are studied. Intermolecular and intramolecular interactions are taken to be either harmonic or of Lippincot-Schroeder type. Special attention is paid to the influence of the anharmonicity on the infrared active vibrations of the protons in the hydrogen bonds. The potential for the proton is chosen in such a way that in the room temperature phase the protons are found in an asymmetric position between two neighbouring oxygens. In the high temperature phase a symmetric distribution of protons is accessible indicating that a structural phase transition can occur.     

13C-NMR study of the phase transition in the layered antiferroelectric squaric acid

The temperature dependence of the¹³C-NMR spectra of undoped crystals of the layered antiferroelectric squaric acid (C₄H₂O₄; H₂SQ was measured at a resonance frequency of about 75 MHz by means of the¹H–¹³C double resonance technique. The relation between the order parameter and the chemical shift tensors at the¹³C nuclei is discussed in detail. The line splitting in the ordered phase belowT _c100°C is shown to be proportional to the order parameter. The phase transition is of first order and can be well described by a Landau expansion of the free energy. The results are compared to previous determinations of the temperature dependence of the order parameter. The spin lattice relaxation time of the¹³C system is estimated to be 5000–7500 s at 5°C and the given magnetic field.     

On the thermal expansion of nanoparticles

Magnetite nanoparticles with an average particle size of 7.5 nm were studied by Mössbauer spectroscopy in the 100–300 K temperature range. Using experimental data on the temperature variation of isomer shifts in the partial spectra of ⁵⁷Fe nuclei in the A and B crystal sublattices, the linear thermal expansion coefficient of the ultradisperse magnetite particles in the temperature range studied was estimated within the framework of the molecular orbital formalism: α=(1.2±0.2)×10^?4K^?1.