35Cl NQR study of the low temperature transition in ferroelectric (CH3NH3)HgCl3

In addition to the paraelectric-ferroelectric phase transition at T_c = + 62°C there is another low T phase transition in the −152 to −165°C range associated with a sudden change in the NQR spectra.The three ³⁵Cl NQR lines observed in the P3₂ ferroelectric phase of (CH₃NH₃)HgCl₃ below T_c = 62°C suddenly disappear on cooling below −152°C. No lines could be observed between 152 and −165°C. Below this temperature five ³⁵Cl NQR lines appear and remain down to liquid nitrogen temperature.     

Metal atom-induced diffuse scattering from short-range order among deuterium atoms in single crystal α-phase V2D

X-ray scattering has been studied above the critical ordering temperature, T_c (133°C), in b.c.c. α-phase V₂D. At 140 and 200°C quite weak diffuse intensity was observed centered on the reciprocal lattice points of the ordered superlattice. This intensity is due to short-range order among deuterium atoms which induces a local or short-range modulation of the parent vanadium lattice and suggests the possibility of local octahedral D—D fluctuations building up well above T_c.     

Inelastic neutron scattering study in cubic NaNbO3

Inelastic neutron scattering measurements performed on NaNbO₃ close to the cubic to tetragonal phase transition show that the 2D fluctuations earlier revealed by X-ray diffuse scattering correspond to an overdamped type dynamics. The sharp decrease in the spectral width observed at T-T_c =10° is interpreted in terms of the increase in correlation lengths and the cross-over between 2D and 3D behaviour of the fluctuations.     

Anomalous thermoelastic behaviour of cubic potassium cyanide

The temperature dependence of all elastic constants of cubic potassium cyanide has been measured by ultrasonic methods in the range from -105.4 to 181°C. Until about 180°C all elastic wave velocities increase with higher temperatures. The behaviour of the shear constant c₄₄ is given by c₄₄ = a · logT/T₀ with a = 0.219 · 10¹¹dyn.cm^?2 and T₀ = 153.7°K with a good approximation. These unusual effects are assigned to the librational vibrations and orientational jumping processes of the CN-ions. The phase transition at -105.5°C is a consequence of the low c₄₄ value.     

Phase transitions in the atomic, electronic, and magnetic subsystems of LaSrMnO films versus the synthesis temperature

The evolution of the cluster structure in amorphous LaSrMnO films as synthesis temperature T _s increases from 20 to 300°C is considered. Two order-disorder phase transitions with different scale parameters are observed. One of them, the aggregation of disordered atoms into small (～20 Å) amorphous clusters at T _s = 100°C, shows up as a sharp increase in the intensity of diffuse X-ray scattering (diffuse halo 1) with a simultaneous suppression of incoherent (background) scattering. At T _s > 150°C, disordering dominates (I _incoh = I _max) until the next stage of ordering sets in at T _s = 250?300°C. At this stage, the crystalline phase forms from large (>100 Å) crystalline clusters. This amorphous-crystalline phase transition is characterized by the appearance of Debye lines and a reduction of the halo intensity. The structural phase transition to long-range order is accompanied by a decrease in the LaSrMnO resistivity from 10¹⁰ to 10 Ω cm and a change from the tunneling mechanism of conductivity involving metallic clusters (which is typical of granulated systems) to the hopping mechanism with a hop variable length following the Mott law ρ ～ exp(T ^?1/4). In the magnetic subsystem, the paramagnetic-ferromagnetic phase transition occurs.     

Cl35 NQR study of the structural phase transition in (CH3NH3)HgCl3

The Cl³⁵ nuclear quadrupole resonance spectra of (CH₃NH₃)HgCl₃ have been measured between -150°C and + 100°C. The spectra clearly show that a structural phase transition of first order takes place around T_c? 60°C. The transition may be related to a disordering of the CH₃NH₃ groups which are reorienting both above and below T_c. The positive temperature coefficient of the Cl NQR frequency, dv/dT may be also explained by the CH₃NH₃ motion.     

Thermal conductivity of the amorphous and nanocrystalline phases of the beech wood biocarbon nanocomposite

Natural composites (biocarbons) obtained by carbonization of beech wood at different carbonization temperatures T _carb in the range of 800–2400°C have been studied using X-ray diffraction. The composites consist of an amorphous matrix and nanocrystallites of graphite and graphene. The volume fractions of the amorphous and nanocrystalline phases as functions of T _carb have been determined. Temperature dependences of the phonon thermal conductivity κ(T) of the biocarbons with different temperatures T _carb (1000 and 2400°C) have been analyzed in the range of 5–300 K. It has been shown that the behavior of κ(T) of the biocarbon with T _carb = 1000°C is controlled by the amorphous phase in the range of 5–50 K and by the nanocrystalline phase in the range of 100–300 K. The character of κ(T) of the biocarbon with T _carb = 2400°C is determined by the heat transfer (scattering) in the nanocrystalline phase over the entire temperature range of 5–300 K.     

Die Temperaturabhängigkeit der magnetischen Jordan-Nachwirkung von Nickel

The magnetic after effect of pure polycristalline nickel and of a single crystal was measured between ?196 and +340 °C by a Förster second harmonic magnetometer. The viscosity constantS _v is shown to be proportional toB(T)·H_c(T) with an increasing functionB(T) for specimens of different magnetic hardness. The temperature dependence S_v(T) atH _c is rather complex in comparison to the simpleT- or √T-behaviour of former theoretical models. The latter is observed only forT??100 °C, whileS _v is nearly independent ofT between ?100 and +300 °C, and drops abruptly to zero forT?300 °C.     

Inelastic light scattering studies of diffuse phase transition in ferroelectric Sr1.9Ca0.1NaNb5O15 thin films

The dynamic behavior of highly oriented ferroelectric Sr_1.9Ca_0.1NaNb₅O₁₅ thin films has been investigated by Brillouin light scattering over a wide temperature range between 25 and 450 °C. The temperature variations of the full‐width at half‐maximum (FWHM) of the central peak (CP) are found to exhibit marked changes around the Curie temperature (T_c = 246 °C). The CP reveals a relaxational mode behavior, which is attributed to thermally activated fast relaxation of polar nanoregions (PNRs). The width of the CP decreases steadily upon cooling from ∼400 °C to T_c and shows little subsequent change within the ferroelectric state, indicating an increase of the number of PNRs and a slowing down of their dynamics. By using a modified superparaelectric model, the activation energy (H₀) of the relaxation processes and the Burns temperature (T_B) are determined to be 2939 and 385 °C, respectively. The evidence for this diffuse phase transition provided by Brillouin scattering is in agreement with results obtained from the temperature‐dependent dielectric permittivity, refractive index, and Raman spectra. Copyright © 2011 John Wiley & Sons, Ltd.     

Superconductivity of niobium nitride single crystals after implantation of carbon and nitrogen

Single crystals of δ-NbN_0.85 with a superconducting transition temperature T_c of 14.3 K were implanted with nitrogen and carbon ions at room temperature and subsequently annealed at high temperatures. Implantation was also performed at high substrate temperatures. After implantation at about 920°C maximum T_c-values of 16.5 and 17.8 K were obtained with N- and C-ions respectively. Disorder observed after room temperature implantation consisted of displaced Nb-atoms which could not be completely annealed in an isochromous annealing process up to 1000°C. For annealing temperatures above 1100°C nitrogen diffusion out of the implanted layers resulted in a reduction of T_c.     

Hyper-Raman scattering spectra in the low frequency range in cubic BaTiO3 and the mechanism of the phase transition

The hyper-Raman spectra in the cubic phase of BaTiO₃ are studied with special emphasis on the lowest-freauency phonon mode. The spectral structure is found to change as the temperature T is raised from T_c (130 °C) to 400 °C. It is shown that the overall experimental results can be well accounted for in terms of the critical dynamical theory based on pseudospin-phonon coupled system developed by Y. Yamada et al. As the consequence, it is found that the coupling between TO phonons and pseudospins presumably constituted by flipping motion of Ti ions causes the cubic-tetragonal phase transition, and plays an important role in understanding the dynamical aspects including the anomalous increase in dielectric constant as T approaches T_c from above.     

Dielectric properties of deuterated ammonium Rochelle salt

The dielectric constant of deuterated ammonium Rochelle salt along the three crystallographic axes as well as the pyroelectric charge density developed on the crystal surface perpendicularly to the b-axis have been measured with a high temperature resolution. The para- to ferroelectric transition at T_c = ?159°C shows a thermal hysteresis of ΔT = 0.15°C. This and the discontinuities in the dielectric constant along all three axes show that the phase transition is of the first-order. No phase intermediate between the para- and the ferroelectric phases could be detected.     

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.     

Amorphization of ice near the melting point

In addition to reflections of the hexagonal phase of ice I _h, the intense diffuse scattering of X-rays mainly due to the amorphization of ice is revealed on the X-ray diffraction patterns of water ice samples prepared at liquid nitrogen (studied by the authors earlier) and samples prepared at T = ?10°C (this work). The measurements are performed in the temperature range from ?25 to 0°C. The existence of reflections of the crystalline phase and intense diffuse scattering on the X-ray diffraction patterns makes it possible make a conclusion about the coexistence of crystalline and amorphous structures of ice. Splitting of the first maximum on the electron-density radial distribution function is detected on the basis of an X-ray diffraction pattern recorded at T = ?3°C. This splitting is explained by an increase in the interatomic distances between the nearest-neighbor atoms located at different levels. Similar splitting was also detected on a radial distribution function constructed using an X-ray diffraction pattern recorded at ?10°C.     

Thermodynamic characteristics of the thermal polymerization of perfluoropropylvinyl ether at high pressures

The limiting polymerization temperatures of PFPVE at pressures 5.28–10.56 kbar (T _c = 200°C at 7.04 kbar and T _c = 240°C at 10.56 kbar) are measured. The volume of reaction ΔV and enthalpy of polymerization ΔH are determined.     

Self-organization of the structure in the Cu60Fe40 composite during deformation-thermal treatment

The deformation-thermal stability of a clusterized amorphous-crystalline structure prepared from a Cu₆₀Fe₄₀ powder mixture at a logarithmic strain e = 4.6 and subjected to isochronous (40 min) annealings at T _a = 200–800°C has been investigated. Periodic changes (ΔT = 300°C) in the order and disorder with a maximum ordering at T _a = 300 and 600°C and a maximum disordering at T _a = 400 and 700°C have been observed. The periodicity of the dominant crystallographic order with a period ΔT = 400°C in the annealing temperature has been revealed for face-centered cubic copper phase planes separated by a singular point at T = 500°C characterized by the dominant body-centered cubic iron phase ordering. It has been shown that the sawtooth shape of the size distribution of strain clusters formed within the crystal structure of deformed samples slowly changes with increasing annealing temperature from exponential (T _a = 200–700°C) to linear (T _a = 800°C). This indicates a high density of internal local distortions in structural units.     

Determination of inelastic critical scattering at a first order phase transition in the CDW structure of 1TTaS2 using Mössbauer diffraction

We report Mössbauer diffraction measurements of the temperature dependence of the elastic and inelastic intensities at the (100) Bragg reflection in 1TTaS₂. These measurements use a newly developed microfoil conversion electron (MICE) spectrometry. They cover the temperature range from 19°C to 100°C, bracketing the first order 1T₁ to 1T₂ phase transition in the charge density wave structure at 79°C. The elastic Bragg peak shows a discontinuity at the phase transition as reported by Moret and Colella. The inelastic scattering shows a significant peak near the phase transition. This peak is interpreted as inelastic critical scattering at this first order phase transition.     

Smectic A order parameter measurements and critical scattering near the smectic A↔nematic phase transition in CBOOA

The temperature dependence of the order parameter for the smectic A phase of CBOOA is determined by measuring, with elastic coherent neutron scattering, the intensity of the Bragg reflection from the smectic layers. Within the experimental accuracy, the transition smectic A? nematic appears continuous in this system. However, a fit with (T_c-T)^2β yields β = 0.18 ± 0.01. Critical scattering is also observed in both phases. It can be described with an anisotropic Ornstein-Zernike law. The correlation length perpendicular to the smectic layers behaves like (T-T_c)^?v, with an exponent v = 0.47 ± 0.06. In contrast, the lateral extension of the layers seems to increase uncritically.     

Ultrasonic attenuation near the cholesteric → smectic A transition

Ultrasonic attenuation at 2 MHz has been measured through the cholesteric → smectic A transition in cholestryl nonanoate. Very near the transition, 0.1 < (T ? T_c) < 0.6°C, the excess attenuation varies as (T ? T_c)ⁿ, where n for four separate runs varies between 0.30 and 0.34. These values are very close to the exponent of 0.33 predicted using a scaling laws and observed for the divergence of the twist viscosity in the nematic phase as the smectic A transition is approached.     

Halleffekt und anisotropie der beweglichkeit der elektronen in ZnO

Hallconstant, conductivity and Hall mobility of ZnO crystals were measured as function of temperature (4 °K < T < 370 °K) and orientation. Value and anisotropy of mobility can be explained (50 °K < T < 370 °K) by polar optical scattering, deformation potential sc., piezoelectric sc. and sc. by ionized impurities. The anisotropy of mobility is caused only by piezoelectric sc. Maximum values of μ_H are reached for μ_H ⊥ c, with 2400 cm²/V sec at 40 °K and for μ_H ¦ c with 1350cm²/Vsec at 60 °. Below 50 °K Hallconstant, conductivity and Hall mobility are influenced by impurity band conduction processes. The crystals have impurity concentration in the 10¹⁶ cm^?3 range, but they show different donor activation energies depending on growth conditions: Type I: 38,4 meV (50 °K < T < 100 °K) and Type II: 20,3 meV (50 °K < T < 100 °K) and 6 meV (25 °K < T < 50 °K).