Radiation-induced Phase Transition Research Articles

Effect of radiation-induced vacancy saturation on the first-order phase transformation in nanoparticles: insights from a model.

By employing a model of nanomaterials with polymorphic phase transitions and using a thermodynamic approach to describe the effects of vacancy saturation, irradiation dose, powder dispersion, and surface energies, we demonstrate the possibility of radiation-induced phase transitions and the zones of radiation stability for nanoparticles. We utilize nanoparticles exhibiting transitions from the body-centered cubic α phase to the face-centered cubic β phase, and the reverse transition from β phase to α phase, as a model system for first-order phase transformations. We incorporate nucleation through the appearance and growth of the nucleus of a new phase, resulting in the formation of a two-phase α+β system, and we highlight the importance of accounting for nucleation. Our model study reveals that very small α-phase particles are unstable (while very small β-phase particles are stable) because of surface effects. There is an intermediate zone of sizes and parameters where radiation-induced defects become important so that the α-phase particle is unstable without irradiation but becomes stable under irradiation. For large sizes and low temperatures, the α→β transformation cannot occur regardless of irradiation because of bulk driving forces; initially, α-phase particles are stable, whereas the β-phase particles are unstable. In some cases, nucleation requires a large additional energy change, resulting in a low probability of phase change fluctuations. This behavior is confirmed by calculations for iron particles under irradiation. Substances characterized by high vacancy migration energy, small diffusion coefficients of defects, and low temperatures of first-order phase transitions can serve as suitable candidates for radiation-induced phase transitions in nanosystems. Ceramic nanomaterials, which possess high vacancy migration energy, will have their behavior significantly influenced by radiation doses. In contrast, most metals exhibit small vacancy migration energy and demonstrate better resistance to irradiation, making them recommended candidates for nuclear materials.

Phase-field simulation of radiation-induced phase transition in binary alloys

In this study, we simulate the radiation-induced phase transition in the binary alloy employing the modified Cahn–Hilliard (CH) equation that accounts for the process of radiation-enhanced diffusion, ballistic mixing, and compositional fluctuations. The influence of displacement rate on the dynamics of the average radius, number density, nucleation rate, and volume fraction of the second phase is discussed. Also, the mechanism of precipitate vanishing under irradiation condition is revealed.

Crystal structure of thin CaSi2 films grown by radiation-induced epitaxy

The electron-beam irradiation (electron energy 20 keV, current density 50 μA/m2) effects on the CaF2 films epitaxially grown on a Si(111) substrate have been studied with Raman spectroscopy (RS). It was found that the radiation-induced phase transition of CaF2 into CaSi2 films takes place at electron-beam irradiation both as during the epitaxial CaF2 film growth and as after completed growth of the film. The temperature dependence studies of the observed radiation-induced phase transition show needs to use substrate heating above 300 °C. The crystal structure of CaSi2 films was found to depend on the thickness of the grown CaF2 film. For thin films (less than 20 nm) crystal structure of CaSi2 films belongs to the trigonal rhombohedral modification 3R (space group R3̅m with a three-layer translational period of silicon substructures in the unit cell 3R). The transition of CaSi2 crystal structure to the trigonal rhombohedral modification a six-layer period material 6R was found for thicker films (>20 nm).

Ion-irradiation resistance of the orthorhombic Ln2TiO5 (Ln = La, Pr, Nd, Sm, Eu, Gd, Tb and Dy) series

The response of Ln2TiO5 (where Ln is a lanthanide) compounds exposed to high-energy ions was used to test their suitability for nuclear-based applications, under two different but complementary conditions. Eight samples with nominal stoichiometry Ln2TiO5 (Ln = La, Pr, Nd, Sm, Eu, Gd, Tb and Dy), of orthorhombic (Pnma) structure were irradiated, at various temperatures, with 1 MeV Kr2+ ions in-situ within a transmission electron microscope. In each case, the fluence was increased until a phase transition from crystalline to amorphous was observed, termed critical dose Dc.At certain elevated temperatures, the crystallinity was maintained irrespective of fluence. The critical temperature for maintaining crystallinity, Tc, varied non-uniformly across the series. The Tc was consistently high for La, Pr, Nd and Sm2TiO5 before sequential improvement from Eu to Dy2TiO5 with Tc's dropping from 974 K to 712 K.In addition, bulk Dy2TiO5 was irradiated with 12 MeV Au+ ions at 300 K, 723 K and 823 K and monitored via grazing-incidence X-ray diffraction (GIXRD). At 300 K, only amorphisation is observed, with no transition to other structures, whilst at higher temperatures, specimens retained their original structure. The improved radiation tolerance of compounds containing smaller lanthanides has previously been attributed to their ability to form radiation-induced phase transitions. No such transitions were observed here.

Radiation-induced phase transition in a film of niobium-tin solid solution

A 5-μm-thick film of a niobium-tin solid solution (18.3 at % Sn) grown by ion-plasma sputtering with subsequent codeposition of ultrafine metal particles is irradiated by a fast proton flux with a fluence of 1019 p +/cm2. X-ray diffraction analysis carried out using CuK α (λ = 0.154178 nm), CoK α (λ = 0.179021 nm), and MoK α (λ = 0.071069 nm) radiations shows the presence of a radiation-induced stannide niobium (Nb3Sn) phase in the region of proton energy dissipation (at a depth of 2.5 μm from the surface of the solid solution film). It is found that at the end of the particle range, nonlocal interaction between the protons and the concentrated matrix solid solution takes place. When interacting with the supersaturated solid solution, a bombarding particle covers a tangible area of the solution, so that an intermetallic phase greater than critical in size nucleates. The feasibility is demonstrated of using a fast particle flux to produce an intermetallic (superconducting) phase inside a solid solution layer with a composition close to the stoichiometric composition of the intermetallic.

Fabrication of submicrocrystalline structure in solid solutions by radiation-induced phase transition

The method of field ion microscopy was used to study the submicrocrystalline structure of the surface of PdCuAg solid solutions obtained after irradiation with Ar+ ions. It was shown that there are radiationinduced structural phase transitions in the subsurface volume of solid solutions irradiated by gas ions with an energy of 20 keV and a fluence of 1018 ion/cm2. The regimes were determined of radiation treatment of structural alloys Pd50Cu30Ag20 in which a microduplex-type structure with the phases’ particle sizes being up to 100 nm with the necessary set of properties arises in the subsurface volume.

Experimental set-up and procedures for the investigation of XUV free electron laser interactions with solids

In this article, we describe the experimental station and procedures for investigating the interaction of short-wavelength free-electron lasers (FELs) pulses with solids. With the advent of these sources, a unique combination of radiation properties (including wavelength range from tens of nanometers down to sub-Angstroms, femtosecond pulse duration, and high pulse energy reaching milli-Joules level) creates new research possibilities for the systematic studies of radiation-induced structural changes in solids. However, the properties of the intense FEL radiation generate, apart from the new experimental opportunities, extreme demands on the experimental set-up (mostly in terms of radiation hardness of detectors and their saturation levels). Thus, radiation-induced phase transitions in solids, beyond the fundamental scientific interest, are of importance for the design of FEL beamlines and instruments which interact with the direct beam. In this report, we focus on the instrumentation and experimental techniques used in the recent studies performed at the FLASH facility in Hamburg.

Phase transition kinetics in Langmuir and spin-coated polydiacetylene films

Thin films of 10,12-pentacosadiynoic acid were prepared using Langmuir and spin-coating techniques and polymerized using a controlled dosage of UV radiation. The radiation-induced phase transitions: from the colorless monomer, via the metastable blue phase, to the red polydiacetylene phase, and finally to degradation of the material, were monitored by optical absorbance spectroscopy. Deconvolution analysis of the absorbance curves allowed us to monitor quantitatively the dynamical changes in the chromatic properties of the films as a function of applied UV radiation dose. Several reaction kinetics models were applied in order to describe the phase transitions in the films. The results present the phase evolution in PDA and compare the kinetics for Langmuir films vs. spin-coated films. Polymerization directly at the air-water interface was found to be two-to-three orders of magnitude faster compared to solid-supported films of the same material. Moreover, we show that the data of the solid supported films is considerably better fitted when a reversible intermediate phase between the blue and the red phases is considered. Furthermore, a shift of the Raman active triple bond supports the presence of the intermediate phase.

Structural modifications of Gd2Zr2-xTixO7 pyrochlore induced by swift heavy ions: Disordering and amorphization

The isometric, pyrochlore structure type, A2B2O7, exhibits a wide variety of properties that find application in a large number of different technologies, from electrolytes in solid oxide fuel cells to actinide-bearing compositions that can be used as nuclear waste forms or inert matrix nuclear fuels. Swift xenon ions (1.43 GeV) have been used to systematically modify different compositions in the Gd2Zr2-xTixO7 binary at the nanoscale by radiation-induced phase transitions that include the crystalline-to-amorphous and order-disorder structural transformations. Synchrotron x-ray diffraction, Raman spectroscopy, and transmission electron microscopy provide a complete and consistent description of structural changes induced by the swift heavy ions and demonstrate that the response of pyrochlore depends strongly on chemical composition. The high and dense electronic energy deposition primarily results in amorphization of Ti-rich pyrochlore; whereas the formation of the fully disordered, defect-fluorite structure is the dominant process for Zr-rich pyrochlore.

Effect of neutron irradiation on the optical properties and lattice dynamics of α-SiO2

The effect of reactor irradiation on the IR reflectivity spectrum of α-SiO2 single crystals is studied in a broad range of fast-neutron fluences. Data are presented on the irradiation-induced changes in the spectral characteristics of antisymmetric and degenerate vibrational modes of bridge bonds and in dynamic parameters. Critical lattice dynamics is revealed near the antisymmetric stretching and degenerate modes. The ionic polarizability and dynamic constants of the crystals are shown to be nonlinear functions of neutron fluence. The results are interpreted in terms of radiation damage, a radiation-induced phase transition, and breaking and stretching of some of the silicon-oxygen bonds. A mechanism of radiation-induced structural changes is discussed.

Radiation-Induced Phase Transition of Quartz

The effect of high-energy, high-flux neutron irradiation on the phonon spectrum and structure of α-quartz is studied using IR reflectivity, Raman scattering, and x-ray diffraction measurements. The results demonstrate that neutron irradiation to a fluence of about 7 × 1019 n/cm2 transfers low-quartz to a state similar in structure to high-quartz. The dynamics and mechanism of the radiation-induced phase transition are discussed in relation to the associated changes in silicon–oxygen bond distances and angles.

Spectroscopic Study of a Radiation-Induced Phase Transition in Quartz Crystals

An infrared and Raman spectroscopic study of a radiation-induced phase transition has been performed for quartz crystals irradiated with a set of fast neutron fluxes. Under the influence of radiation, the spectral characteristics of some bands change consistently due to structural rearrangement of the crystal, as do the parameters of the stretching and deformation vibrations of the bridging bonds. Two processes are assumed to take place in the course of irradiation: cleavage and deformation of some Si–O–Si bonds, changing the initial structure. A parallel has been drawn with X-ray spectroscopy data in respect of the individual parameters and with the scattering spectrum of heated quartz crystals; a correlation between the corresponding dependences has been found. At a certain stage of irradiation, a state similar to the high-temperature modification of quartz is formed.

Modification of graphite surface layers by nitrogen ion irradiation

To study the modified surface layer of polycrystalline graphite MPG-8 (produced by NIIGraphite, Moscow, Russia) under high-fluence irradiation by 30 keV N 2 + ions, electron paramagnetic resonance (EPR) spectra of surface layer and the dependence of the ion–electron emission yield γ on the target temperature have been measured. The dependence of γ( T) manifests a step-like behaviour at the temperature T a, which is typical of the radiation-induced phase transition. The EPR analysis shows that below T a, which corresponds to annealing of radiation damage created by ion bombardment, paramagnetic defects are typical of carbon as well as defects connected with bonding of the carbon atom with three 14N nuclei. Above T a, the defects are typical of graphite-like structures. The results for MPG-8 are compared with those for MPG-LT and POCO-AXF-5Q graphites published earlier.

Study of near surface layer of graphite produced by nitrogen ion bombardment at high doses

To study the modified surface layers of graphites and deposited films of sputtered material, the dependences of sputtering yield Y , and ion-electron emission coefficient γ on ion incidence angle and target temperature under high dose 30 keV N+ 2 ion irradiation have been measured. In the angular range θ=0-80° Y and γ increase approximately as inverse cosθ, Y of POCO-AXF-5Q are 1.5 times larger than of MPG-LT. The dependences of γ (T) manifests a step-like behaviour typical for the radiation induced phase transitions. EPR analysis shows that at near room temperatures the point electron defects are typical of carbon and the defects due to carbon atoms interacting with 14 N nuclei. At elevated temperatures (≥ 300°C) there are the defects typical of graphite-like structures. The films deposited on glass collectors shows for cold targets only the defects typical of carbon, for the heated graphites - also the defects associated with C-14N nuclei interaction.

Transformations in the electronic subsystem of metal solid solutions near a radiation-induced phase transition

Nonmonotonic changes in the mechanical properties of various alloys are demonstrated under the action of radiation which stimulates phase transitions. A comparison with the results of measurements of the thermoelectric power confirmed the assumption that the nature of the interparticle bonds changes.

Magnetic field and microwave radiation induced phase transitions in anisotropic Jahn-Teller elastic KEr(MoO4)2

ESR and magnetic studies in KEr(MoO4)2 were carried out in the external magnetic fields up to 8T at 1.9–5 K. Phase transition of Jahn-Teller type was induced by external magnetic field due to crossover of the electron levels of Er3+. A series of nonequilibrium phase transitions at a different power of the microwave radiation was discovered.

Diffuse Phase Transition in Ferroelectric Polymers and Its Irradiation Effect

The ferroelectric phase transition was investigated in the VDF/TrFE copolymer films before and after irradiations by means of dielectric, X-ray diffraction, IR and gel content measurements. The diffuse phase transition was explained in terms of the Ginzburg-Landau theory, in which the domain boundary effect was taken into account. On irradiation, the peak temperature on the ε'-Tcurve shifted to lower temperatures, but did not decrease below 40°C even irradiated with more than 200 Mrad. On the other hand, the result of X-ray diffraction measurement exhibited a radiation-induced phase transition at room temperature after irradiation with 120 Mrad. This discrepancy may be attributed to the phase coexistence over a wide temperature range.

Spatial nonuniformity of cross-linking in crystalline alkanes of different chain length

Regarding their irradiation behaviour, crystalline n-alkanes can be devided into three groups according to their chain length: (i) very long alkanes (polyethylene, degraded polyethylene of approximately 100 carbon atoms, alkane C 94), (ii) moderately long alkanes (C 40, C 28), and (iii) shorter alkanes (C 23). Within the crystal lattice of very long alkanes (group i) there are no large-scale heterogeneities in the distribution of cross-links; extensive distortion of the lattice is thus gradually introduced at high irradiation doses. In contrast, in crystals of moderately long alkanes (group ii) cross-links form almost entirely in the fully amorphous domains which appear in early stages of irradiation. In order to explain this non-uniformity a novel migration mechanism of cross-link precursors over micron distances is invoked. Going on to still shorter alkanes (iii) the evidence for large-scale heterogeneity of cross-link formation is again absent, at least in the case of high dose-rate irradiation (>0.3 Mrad s ). This is due to the tendency of shorter alkane crystals towards a radiation-induced phase transition. In the resulting plastic crystal modification the newly postulated migration mechanism appears to be ineffective. Some possible factors influencing the operativity of this mechanism in hydrocarbon radiolysis are suggested.