Effect of irradiation on CsNO3, RbNO3., and NaNO2 crystal structure

Studies of the structure changes in heated (20–200°C) and γ –irradiated (10⁹- 5×10⁹ R, E = 1.3 Mev, Tirrad. = 40°C) crystals CsNO₃, RbNO₃ and NaNO₂ have been carried out by means of X-ray diffraction methods. The investigation of temperature effect on the crystal structure has shown that the phase transformation in CsNO₃ and NaNO₂ are of continuous type while that in RbNO₃ is discontinuous. It has been found that crystal structures of CsNO₃ and NaNO₂ change under irradiation in the same way, as they change under heating, No changes of RbNO₃ crystal structure caused by irradiation have been found. Experimental results agree with an assumption that radiation-induced structure changes of high-temperature type can be observed only in the compounds, the phase transition of which is of continuous type.     

Anomalous structural states of rare-earth oxides in solid-phase synthesis during continuous heating

The kinetics of solid-phase synthesis of complex rare-earth (RE) oxides from an amorphous precursor state during continuous heating is studied by X-ray diffraction. Using the synthesis of RE borates (ReBO₃) as an example, it is shown that a continuous heating of such precursors stimulates the formation of new phases which are unknown in an equilibrium state of one rare-earth element but known for other rare-earth borates. It is established that the regime of continuous heating during solid-phase synthesis of garnets (Re ₃ M ₅O₁₂) exhibiting the only crystal phase state for all rare-earth atoms results in an accelerated growth of crystallites and that the higher the heating rate the higher the growth rate.     

Structural characterization,thermal, vibrational properties and molecular motions in perovskite-type diaminopropanetetrachlorocadmate NH3(CH2)3NH3CdCl4 crystal

The work presents a detailed analysis of the sequencing of the structural phase transitions in NH₃(CH₂)₃NH₃CdCl₄ crystal by differential scanning calorimetry (DSC), X-ray, infrared, far infrared and Raman spectroscopy. DSC studies have shown that in analyzed crystal occurring one reversible continuous phase transition at 375/374 K (on heating/cooling). Observed in Nujol and Fluorolube mulls in the wide temperature range between 296 K and 413 K spectral changes through the structural phase transition can be attributed to an onset of motion of cations. An assignment of some bands due to internal modes has been also proposed.     

Nucleation behaviour and anomalous eutectic formation in highly undercooled Fe2O3-La2O3 eutectic melts

Employing an aero-acoustic levitator, the Fe₂O₃-16.5 mol% La₂O₃ eutectic alloy was levitated, melted, undercooled and then solidified under a containerless condition when a continuous laser beam heating system was incorporated. By revealing the surface and cross-sectional microstructures, copious nucleation is confirmed to take place in the undercooled melts solidified by either an external seeding or spontaneous crystallization. The nucleation behaviour of eutectic alloys is summarized, indicating that copious nucleation may be an intrinsic attribute of a eutectic system with the exact eutectic composition in unconstrained solidification. Considering the complexity level of crystal structures of eutectic oxide phases in the Fe₂O₃-La₂O₃ alloy, the linear kinetic constants are approximately estimated and the growth kinetics are discussed. The sluggish growth kinetics of the perovskite-type phase, that is LaFeO₃ with a higher complexity level in a unit cell leads to a decoupled growth within a single eutectic colony when the melt is undercooled to exceed the critical undercooling ΔT = 125 K. It is the decoupled growth that results in the formation of anomalous eutectics. The present concept based on the kinetic constant of different phases can also be applicable to account for the growth behaviour of other oxide eutectics when considering the stable and metastable eutectic reactions in different solidification conditions. The nucleation behaviour and growth modes in some oxide eutectic systems have been predicted in the free solidification from an undercooled state.     

In‐situ X‐ray diffraction of mechanically milled β‐Al3Mg2 powders

The structure evolution during heating of mechanically milled single‐phase β‐Al₃Mg₂ has been investigated by in‐situ X‐ray diffraction. The nanoscale supersaturated Al(Mg) solid solution formed during milling transforms back to the original β‐Al₃Mg₂ phase through a sequence of phase transformations. At low temperatures, an increasing amount of Mg is rejected from the solid solution with increasing temperature. At intermediate temperatures, the β′‐phase, a hexagonal phase with approximate composition Al₃Mg₂, forms. Finally, at higher temperatures the original β‐Al₃Mg₂ phase is restored, indicating that the formation of the supersaturated solid solution during milling can be reversed by appropriate heat treatment. The phase transformations during heating are gradual and the temperature ranges of stability of the different structure configurations are quite large, all exceeding 50 K. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Specific heat and critical behavior of Tl0.3MoO3 near the Peierls phase transition

The specific heat of the quasi-one-dimensional charge-density-wave (CDW) compound Tl_0.3MoO₃ has been measured using an adiabatic continuous heating method from 100 to 220 K. A specific heat jump associated with the Peierls phase transition occurs at 172.3 K. A good scaling relation between the excess specific heat and the susceptibility is found between 140 K and 190 K. Further analysis indicates that the width of the critical region of Tl_0.3MoO₃ is about 10 K and the specific-heat critical behavior can be well described by the three-dimensional XY model.     

Dielectric properties of solid solutions PbMg1/3Nb2/3O3-SrTiO3

Ceramic samples of the solid solutions PbMg_1/3Nb_2/3O₃-SrTiO₃ are synthesized. The dielectric properties are studied and a phase diagram is constructed. The results obtained are discussed in terms of ideas concerning relaxor ferroelectrics for solid solutions on the PMN side and from the standpoint of potential ferroelectrics on the SrTiO₃ side. The introduction of SrTiO₃ into PMN gradually degrades relaxor properties, and the introduction of PMN into SrTiO₃ does not result in initiation of the ferroelectric phase transition, possibly because of the appearance of random fields in the lattice. The possibility of practical applications of the synthesized system is also discussed. Fiz. Tverd. Tela (St. Petersburg) 41, 1091–1095 (June 1999)     

Low-temperature phase transition in Ba2TiGe2O8

Previous work on Ba₂TiGe₂O₈ crystals has shown an unusual low-temperature (～ 223 K on cooling, ～ 273 K on heating) phase transition. Precession x-ray photographs on Ba₂TiGe₂O₈ single crystals show an incommensurate modulation along b*, and, for the first time, also along a*. Single crystal intensity data confirm the average structure in space group Cmm2. There is positional disorder in the pyrogermanate groups, and this is the probable cause for the modulated structure. The low-temperature phase transition is proposed to be a lock-in transition, with the modulation along a* locking in at a value of 1/3. Several properties, as well as other unusual features of the low-temperature phase transition, are discussed in light of the proposed lock-in transition. Domain studies show that the ferroelastic domains are unstable in the low-temperature phase.     

Al-Ti-V-M四元系合金TiAl3-VAl3-MAl3(M=Ni,Fe)赝三元系相图的室温截面

本文用X射线衍射法测定了Al-Ti-V-M四元系合金TiAl₃-VAl₃-MAI₃(M=Ni,Fe)两个赝三元系相图的室温截面。(1)TiAl₃-VAl₃NiAl₃赝三元系相图室温截面由一个单相点ε(NiAl₃)、一个单相区δ(TiAl₃和VAl₃形成的连续固溶体)和一个两相区δ+s所组成。(2)TiAl关键词：     

Mechanism and modelling of aluminium nanoparticle oxidation coupled with crystallisation of amorphous Al2O3 shell

The oxidation of aluminium nanoparticles coupled with crystallisation of amorphous alumina shell is investigated through the thermogravimetric analyser and differential scanning calorimetry (TGA-DSC) and the transmission electron microscope (TEM). The thermogravimetric (TG) curves show stepwise shapes with temperature increase and could be divided into four stages. The reaction at the second stage is complex, including the simultaneous crystallisation of amorphous alumina (am-Al₂O₃) and Al oxidation. The crystallisation of am-Al₂O₃ promotes the reaction through generating fast diffusion channels, like micro-cracks and grain boundaries in the oxide shell to accelerate the ionic diffusion. An enhancement factor (f_react), which follows a power-law formula with the crystallisation rate, is introduced to quantify the impact of crystallisation on reaction. With heating rate increase, the second stage of TG curves shifts to the high temperature regime and the total weight gain at the second stage decreases slowly. A crystallisation-reaction model is constructed to fit and predict the weight gain after derivation of diffusivities and crystallisation kinetics. Modelling indicates that with heating rate rise, the mass increment at the second stage of TG curves decreases owing to the reduced reaction time, although the reaction is accelerated. The shift of TG curve to higher temperature is due to the polymorphic phase transition. Actually the derived kinetics of the crystallisation of amorphous alumina indicates that the polymorphic phase transformation mechanism works mainly below the heating rate of 3 K s^–1. At higher heating rate, the melting of Al takes place firstly and the crystallisation of am-Al₂O₃ follows to enhance the ionic diffusion. Therefore, when the heating rate is fast during ignition or combustion, the Al nanoparticles undergo both the melting of Al and the polymorphic phase transition of am-Al₂O₃ to accelerate the reaction.     

Magnetic field diffusion during a metal-insulator phase transition

An analysis is made of the diffusion of a static (slowly varying) magnetic field in a conductor in which a first-order phase transition to the insulating state takes place under the action of the Joule heating. An investigation is made of the case of subsonic propagation of the phase boundary. A (V_1−X Cr_X)₂O₃ solid solution is analyzed as a model substance. The application of this effect in pulsed high-current circuit breakers is discussed. Zh. Tekh. Fiz. 68, 43–48 (December 1998)     

Devitrification behaviour of Cu-Zr-Ti-Pd bulk glassy alloys

The glass formation, mechanical properties and devitrification of the Cu-Zr-Ti-Pd glassy alloys are studied. The formation of a nanoscale icosahedral phase in Cu-based alloys is observed. The nanoscale particles with icosahedral symmetry are primarily formed in the Cu-Zr-Ti-Pd glassy alloys in the initial stage of the devitrification process. As the icosahedral phase is metastable it transforms to the equilibrium oC68 (Cu, Pd)₁₀(Zr, Ti)₇ single phase or its mixture with the oP44 Cu₈Zr₃ phase, depending upon the alloy composition, through an intermediate crystalline compound. The structure changes on heating were studied by X-ray diffraction, transmission electron microscopy, differential scanning calorimetry and isothermal calorimetry.     

In situ synchrotron high‐energy X‐ray diffraction analysis on phase transformations in Ti–Al alloys processed by equal‐channel angular pressing

Mixtures of 47‐Al and 53‐Ti powders (atomic %) have been consolidated using back pressure equal‐channel angular pressing starting with both raw and ball‐milled powders. In situ synchrotron high‐energy X‐ray diffraction studies are presented with continuous Rietveld analysis obtained upon a heating ramp from 300 K to 1075 K performed after the consolidation process. Initial phase distributions contain all intermetallic compounds of this system except Al, with distribution maxima in the outer regions of the concentrations (α‐Ti, TiAl₃). Upon annealing, the phase evolution and lattice parameter changes owing to chemical segregation, which is in favour for the more equilibrated phases such as γ‐TiAl, α₂‐Ti₃Al and TiAl₂, were followed unprecedentedly in detail. An initial δ‐TiH₂ content with a phase transition at about 625 K upon heating created an intermediate β‐Ti phase which played an important role in the reaction chain and gradually transformed into the final products.     

Effect of nitrogen on some properties of vapor-deposited Nb3Sn

Measurements of critical currentsI _c, transition temperaturesT _c, as well as transmission electron microscopy, scanning electron microscopy and X-ray analysis have established a correlation between critical currentI _c, the tape heating temperature during deposition, and the presence of a fine dispersion of precipitates. As the tape heating current (temperature) increased from 3 to 5 Amps,I _c increased from 20 to 430 Amps/3 mm tape width at 50 kG. A fine precipitate of the Nb₂N phase was observed for highest tape heating temperatures. It is concluded that the precipitate particles may act as flux-pinning centers in the Nb₃Sn layer.     

Nonlinear properties of Pb5Ge3O11:Gd3+ in an electric field: Paramagnetic resonance

This paper discusses the field and temperature dependences of the shift in position of EPR signals in an external electric field, which is linearly related to the polarization in the paraelectric phase of Pb₅Ge₃O₁₁:Gd³⁺. Fiz. Tverd. Tela (St. Petersburg) 39, 1643–1644 (September 1997)     

A neutron diffraction study of the high pressure structural phase transition in (CD3ND3)2MnCl4

Abstract

High-pressure neutron diffraction experiments have been performed at room temperature on a powdered sample of the perovskite type-layer compound (CD₃ND₃)₂MnCl₄. A phase transition from the orthorhombic room-temperature phase (ORT) to a new high-pressure phase (HP) is demonstrated at 20.5 ± 0.2 kbars. A monoclinic unit cell with lattice parameters a = 6.824 (5) Å; b = 7.409 (8) Å c = 17.126 (12) Å and β = 82.94(9)° has been inferred for the HP phase, consistent with a two-dimensional perovskite-type structure. The HP phase appears to be much more compact than ORT; it is characterized, in particular, by an important compression (?10%) of the inter-layer distance. Space groups P2/c or P2₁/c consistent with the experimental data have been deduced for the HP phase, after group theoretical considerations based on shear transformation and order-disorder mechanisms.     

Photoinduced phase transition in Ag3AsS3 crystals

It is shown that the illumination of samples leads to the appearance of anomalies in the temperature dependences of the propagation velocity of longitudinal ultrasonic waves along the principal crystallographic directions of proustite (Ag₃AsS₃) at T∼150 K. The features discovered are associated with a photoinduced phase transition caused by restructuring of the cation sublattice of proustite. Fiz. Tverd. Tela (St. Petersburg) 41, 702–704 (April 1999)     

Infrared Studies on Structural Phase Transition in [N(CH3)4]3Bi2Br9

Abstract

FT-IR (4000 - 400 cm^?1) spectroscopy was used for the study of the phase transition at 183 K in [N(CH₃]₄]₃Bi₂Br₉. The changes in the spectra confirmed the presence of the phase transition of first order type at 183 K. The most spectacular changes in the vicinity of the phase transition are found for the stretching CH3 and skeletal NC4 modes. The mechanism of the phase transition of the order-disorder type is believed to be connected with the reorientation motions of the tetramethylammonium cation.     

Thermal behavior of paramagnetic Gd3+ centers in CsSrCl3

The temperature dependence of the order parameters and the axial initial splitting parameter of Gd³⁺ impurity complexes is measured in the first low-symmetry phase of CsSrCl₃, and the relationship between these parameters is found. The parameters of the thermodynamic potential, which takes into account the interaction of the order parameter with strains, are estimated. The nature of the temperature dependence of the axial initial splitting parameter in the cubic phase is explained. Fiz. Tverd. Tela (St. Petersburg) 41, 2065–2069 (November 1999)     

81Br NQR Study of [NH3(CH2) n NH3]CdBr4 (n = 4 and 5) and [NH3(CH2) n NH3]ZnBr4 (n = 5 and 6)

⁸¹Br NQR frequencies and differential scanning calorimetry (DSC) were measured as a function of temperature. [NH₃(CH₂)₄ NH₃]CdBr₄ (1) and [NH₃(CH₂)₅NH₃]CdBr₄ (2) showed a doublet and quartet ⁸¹Br NQR spectrum, respectively. [NH₃(CH₂)₅NH₃]ZnBr₄ (3) and [NH₃(CH₂)₆NH₃]ZnBr₄ (4) exhibited a four-line ⁸¹Br NQR spectrum. From the NQR results, it is inferred that (1) and (2) consist of infinite two-dimensional sheets of corner-sharing CdBr₆ octahedra, whereas (3) and (4) have isolated [ZnBr₄]²⁻ tetrahedra. All of the crystals except (1) showed at least one structural phase transition above 380 K.