Variety of LaSrMnO structures induced by growth conditions and laser irradiation

Crystallographic phase transitions in perovskite-like LaSrMnO metallic oxides are studied. The transitions are induced when internal stresses generated during film synthesis (at temperatures between 450 and 730°C) vary (decrease or increase) upon subsequent irradiation by a KrF laser emitting in the UV range. As the synthesis temperature T _s grows, the rhombohedral-to-orthorhombic phase transition occurs at 650–670°C. The resistivity is shown to be either temperature-independent, ρ(T)=const, at T<T _crit, or varies and reaches a maximum, ρ(T)=ρ_max, at the Curie temperature T _c. Optical transmission spectra taken at photon energies ℏω=0.5–2.5 eV exhibit both a high (0.8–0.9) and low (0.1–0.3) transmission coefficient t, depending on the synthesis temperature. As follows from X-ray diffraction data, the laser irradiation causes a phase transition only in LaSrMnO films grown at T _s<650°C. Phases of different size scales appear: the long-range-order orthorhombic matrix and mesoscopic-range-order rhombohedral clusters are observed in the films grown at T _s=450–550°C and the rhombohedral matrix with orthorhombic clusters, in the films grown at T _s=550–650°C.     

Changes in the electronic,optical, and magnetic properties of LaSrMnO films upon transition from the rhombohedral phase to the orthorhombic phase

The properties of LaSrMnO films are investigated in the temperature range of the transition from the rhombohedral phase to the orthorhombic phase (600–650° C). It is shown that, with a variation in the growth temperature T_s, the change in the film properties is governed by the interaction of Mn-O metallic (ferromagnetic) clusters in the dielectric (antiferromagnetic) matrix. At T_s≤600°C, the low density of e _g states and the dielectric gap (E _g =0.3–0.5 eV) are responsible for the following features: (i) the optical transparency in the range ?ω=0.5–2 eV, (ii) the difference between the FC and ZFC magnetizations M(T), (iii) the high resistance, and (iv) the appearance of the portions R(T) ≈ const in the dependence R(T) due to the transformation of clusters into a system of tunnel-coupled quantum dots. At T_s≥650°C, the local increase in the atomic and electronic densities leads to a decrease in the optical transmission and the resistance by three to nine orders of magnitude, the appearance of a maximum and a minimum in the dependences R(T) of the LaSrMnO films, and an increase in the magnetization M(10 K) by one order of magnitude. The inference is drawn that magnetic ordering of the system of tunnel-coupled clusters encourages an increase in the cluster size and in the content of the metallic (ferromagnetic) phase.     

Phase transitions in the CN<Subscript>x</Subscript>-TiN system attendant on the variation of the bias voltage during ion-stimulated growth

The structure of amorphous CN_x-TiN films grown by ion-stimulated deposition at a bias voltage U = 200–500 V is studied by X-ray diffraction. As the bias voltage increases in the range U = 300–360 V, the CN_x-TiN films are shown to undergo a phase transition in the amorphous phase having different order scales (20–50 Å): this transition is related to an increase in the content of the fraction of medium-cell (4 Å) carbon clusters as compared to the fractions of clusters with large (8 Å) and small (2 Å) cells. Under these conditions, 80–150 Å crystalline clusters undergo the phase transition from the Ti₂C(N) carbide into graphite (C _g) and diamond (C_d); the last two phases are represented by 100-Å clusters.     

Phase transitions in the CN<Subscript>x</Subscript>-Co system induced by changes in the film growth temperature

The structures of amorphous CN_x-Co films grown at different temperatures (T _s = 200–365°C) are studied by X-ray diffraction. As the growth temperature increases above T _s = 200°C, a concentration phase transition is found to occur in the amorphous state; this transition is related to a change in the major portion of carbon or cobalt in the structure of the cluster films. At T _s = 365°C, a disorder-order phase transition, which is accompanied by the transition from the amorphous to crystalline state, occurs in the films.     

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.     

Structural and magnetic properties of thin epitaxial Fe films on (1 1 0) GaAs prepared by metalorganic chemical vapor deposition

Iron films have been grown on (1 1 0) GaAs substrates by atmospheric pressure metalorganic chemical vapor deposition at substrate temperatures (T_s) between 135°C and 400°C. X-ray diffraction (XRD) analysis showed that the Fe films grown at T_s between 200°C and 330°C were single crystals. Amorphous films were observed at T_s below 200°C and it was not possible to deposit films at T_s above 330°C. The full-width at half-maximum of the rocking curves showed that crystalline qualities were improved at T_s above 270°C. Single crystalline Fe films grown at different substrate temperature showed different structural behaviors in XRD measurements. Iron films grown at T_s between 200°C and 300°C showed bulk α-Fe like behavior regardless of film thickness (100–6400 Å). Meanwhile, Fe films grown at 330°C (144 and 300 Å) showed a biaxially compressed strain between substrate and epilayer, resulting in an expanded inter-planar spacing along the growth direction. Magnetization measurements showed that Fe films (>200 Å) grown at 280°C and 330°C were ferromagnetic with the in-plane easy axis along the [1 1 0] direction. For the thinner Fe films (⩽200 Å) regardless of growth temperature, square loops along the [1 0 0] easy axis were very weak and broad.     

Structure-mediated transition in the behavior of elastic and inelastic properties of beach tree bio-carbon

Microstructural characteristics and amplitude dependences of the Young modulus E and of internal friction (logarithmic decrement δ) of bio-carbon matrices prepared from beech tree wood at different carbonization temperatures T _carb ranging from 600 to 1600°C have been studied. The dependences E(T _carb) and δ(T _carb) thus obtained revealed two linear regions of increase of the Young modulus and of decrease of the decrement with increasing carbonization temperature, namely, ΔE ～ AΔT _carb and Δδ ～ BΔT _carb, with A ≈ 13.4 MPa/K and B ≈ ?2.2 × 10^?6 K^?1 for T _carb < 1000°C and A ≈ 2.5 MPa/K and B ≈ ?3.0 × 10^?7 K^?1 for T _carb > 1000°C. The transition observed in the behavior of E(T _carb) and δ(T _carb) at T _carb = 900–1000°C can be assigned to a change of sample microstructure, more specifically, a change in the ratio of the fractions of the amorphous matrix and of the nanocrystalline phase. For T _carb < 1000°C, the elastic properties are governed primarily by the amorphous matrix, whereas for T _carb > 1000°C the nanocrystalline phase plays the dominant part. The structurally induced transition in the behavior of the elastic and microplastic characteristics at a temperature close to 1000°C correlates with the variation of the physical properties, such as electrical conductivity, thermal conductivity, and thermopower, reported in the literature.     

Influence of a strontium-enriched intergranular cluster network on the electrical conductivity of In2O3-SrO ceramics

A metastable hexagonal R-phase is revealed in polycrystalline In₂O₃-SrO samples, which has the form of a network made up of mesoscopic clusters (60–180 Å in size). The clusters arise from strontium-enriched regions near grain boundaries in the main cubic structure of indium oxide. It is shown that annealing in oxygen at T _a ? 300°C saturates dangling bonds between the R-phase and the matrix and makes the system metastable. This state shows up in the presence of (i) solitary diffuse maxima from the R-phase imposed on Debye lines from the main phase in the X-ray diffraction pattern and (ii) the electron cyclotron resonance (ECR) line with g = 1.875. In addition, the sample in this state acquires a high resistivity (ρ ～ 10⁶ Ω cm). Relaxation at T ? 300°C after annealing at T _a > 300°C disrupts bonds between the strontium-enriched clusters of the R-phase and the indium oxide matrix. This causes spatial separation of the clusters, disruption of their coherent bonds with the matrix structure, and escape of excess oxygen from the sample along grain boundaries. As a result, a new stable state forms, which is characterized by (i) a series of diffuse maxima from the R-phase imposed on lines assigned to the main phase, (ii) the presence of the ECR line with g = 2 with the line with g = 1.875 retained, and (iii) the transition of the sample to a low-resistivity state (ρ ～ 100 Ω cm).     

EXAFS investigation of amorphous-to-crystal transition in Ge

The structure of a-Ge as a function of the deposition temperature T_s has been studied by the EXAFS technique for the first time. The results demonstrate the sensitivity of EXAFS to structural differences in the medium range. The data provide strong evidence for a continuous transition from the amorphous to the crystalline phase over a temperature interval form 130°C to 300°C. The analysis of the data is in line with the microcrystal model of amorphous Ge films and allows an estimate of the average grain size for the lower range T_s used.     

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.     

Linear expansion, phase separation, and magnetic inhomogeneities in La0.92Ca0.08MnO3

A relation of the thermal expansion with magnetic and magnetotransport properties has been revealed in La_0.92Ca_0.08MnO₃ single crystals in the paramagnetic state. The magnetotransport and lattice properties and the specific features in the neutron scattering characteristics of the La_0.92Ca_0.08MnO₃ single crystals have been explained by the phase separation in the paramagnetic state into magnetic inhomogeneities (clusters) with short-range (～10 Å) and long-range (>10² Å) orders. The performed investigations have demonstrated that the clusters are closely related to the crystal lattice and that the magnetic inhomogeneities in the paramagnetic region are correlated to T ～ 250–300 K ? TC.     

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.     

Superionic behavior in the xAgI–(1−x)CsAg2I3 polycrystalline system

A superionic phase behavior (with DC ionic conductivities higher than 0.01 S/cm) has been observed in xAgI–(1−x)CsAg₂I₃ (x≈0.67) polycrystalline system grown by slow evaporation using AgI and CsI powders (molar ratio Cs/Ag=0.25) as starting salts and an aqueous solution of HI as solvent. The transition from the normal-to- the superionic state is first-order with a hysteretic behavior in temperature centered at about 116 °C as reflected by thermal (DSC) and electrical conductivity measurements. This mixture is composed of CsAg₂I₃ and AgI crystalline phases and an additional amorphous AgI phase that explains the glassy-type behavior observed in the superionic phase transition.     

Crystallization behaviour in a new multicomponent Ti16.6Zr16.6Hf16.6Ni20Cu20Al10 metallic glass developed by the equiatomic substitution technique

A new amorphous Ti_16.6Zr_16.6Hf_16.6Ni₂₀Cu₂₀A1₁₀ alloy has been developed using the novel equiatomic substitution technique. Melt spinning Ti_16.6Zr_16.6Hf_16.6Ni₂₀Cu₂₀A1₁₀ forms an amorphous phase with a large supercooled liquid region, ΔT=70°C. After isothermal annealing within the supercooled liquid region for 3 h at 470°C, the amorphous alloy crystallizes to form a fine-scale distribution of 2–5 nm nanocrystals, and the supercooled liquid region increases to ΔT=108°C. Atomic-scale compositional analysis of this partially crystalline material using a three-dimensional atom probe (3DAP) is unable to detect any compositional difference between the nanocrystals and the remaining amorphous phase. After annealing for 1 hr at 620°C, the amorphous alloy crystallizes to form 20–50nm equiaxed grains of a hexagonal-type C14 Laves phase with lattice parameters a = 5.2Å and c = 9.0 Å. 3DAP analysis shows that this Laves phase has a composition very close to that of the initial amorphous phase, suggesting that the alloy crystallizes via a polymorphic rather than a primary crystallization mechanism, despite the complexity of the alloy composition.     

3D dependence of the dielectric dispersion in a BaTiO3 single crystal

The 3D dependences ?′(log f, T) and tanδ(logf, T) of a perfect BaTiO₃ single crystal grown by the Remeika method have been studied in the ranges f = 1–2 × 10⁷ Hz and T = ?80–130°C. These dependences characterize a transition from the paraelectric phase (121.5°C) as a near-antiferroelectric transition followed by the transition to the tetragonal phase at ～79.5°C. According to a number of signs, the range 121.5–79.5°C corresponds to a metastable phase typical of first-order phase transitions. The unexpected result of this work has been discussed with invoking the hypothesis on the BaTiO₃ structure in the paraelectric phase, according to which it consists of three antiferroelectric states oriented along the crystallographic axes. Using the dielectric properties of BaTiO₃ as an example, the method of direct correct determination of the temperatures of the structural transformations from the anomaly of tanδ(logf, T) has also been demonstrated.     

Evolution of the spectral response of amorphous europium molybdate under annealing

The phase transformations occurring in amorphous europium molybdate Eu₂(MoO₄)₃ during annealing at atmospheric pressure are studied using optical spectroscopy and x-ray diffractometry. It is established that the metastable β phase is formed at a temperature of ～550°C, whereas the transition to the stable equilibrium α phase takes place at higher temperatures T ≥ 700°C. The spectral characteristics of the α phase, which differ substantially from those of the amorphous state and the β phase, are measured for the first time.     

Amorphous germanium II. Structural properties

The coherent X-ray scattering for momentum transfer, k, between 0·025 and 15·0 Å^?1 has been measured for a series of sputtered amorphous Ge films prepared at various substrate temperatures, T _s, between 0 and 350°C. Differences in the radial distribution function (RDF) of films of different T _s have been determined by an accurate differential scattering technique. The small angle scattering (SAS) of the films is less than 100 electron units for k < 1 Å^?1. From a combination of SAS, RDF and scanning electron microscope studies, it is concluded that an observed increase in film density with increasing T _s occurs through a reduction in the number of voids about 7 Å or less in diameter. No variation of bond length with T _s is found. With increasing T _s, there is an increase in first and second-neighbour coordination and a reduction in bond angle distortion.

The rate of change of coordination, C, with density, ρ₀, is found to be d ln C/d ln ρ₀ = 0·6±0·2. Using a new, general theory of the dependence of the RDF on the dihedral angle distribution, P(θ), it is shown that with increasing T _s there is an increased probability of dihedral angles corresponding to the staggered configuration. For all films, the experimental RDF between r = 4·5 and 6·2 Å agrees with a nearly random P(θ) distribution. Comparison of experimental RDF's of crystalline and amorphous Ge indicates the static distortion of the first-neighbour bond length has a standard deviation of only about 0·04 Å.     

Pulse laser deposition of vanadium dioxide films

Thin amorphous and crystalline films of VO₂ are obtained on (0001) sapphire substrates via pulsed laser deposition with the speed separation of particles under a variety of deposition conditions. The electrical and optical properties of the films in the vicinity of the phase transition in the temperature range of 20 to 100°C are studied. The temperature of transition (T_c) and the width of the hysteresis are found to be 67.5 and 3°C, respectively.     

Low-temperature conduction of a system of tunnel-coupled quantum dots in YBaCuO and LaSrMnO dielectric films

This paper reports on new results of experimental investigations into the nature of the electrical resistivity ρ(T) ? const at T<T _crit for YBaCuO and LaSrMnO dielectric films. The films are prepared by pulsed laser deposition and contain nanocrystalline clusters with metallic conductivity. Dependences of the electrical resistivity ρ(T) ? const are observed for the epitaxial films YBaCuO (T _crit=10 K) with a tetragonal structure after exposure to KrF excimer laser radiation and for the as-prepared amorphous films LaSrMnO (T _crit?160 K). The effect of interest (ρ(T) ? const) manifests itself in the case when the optical spectra of the studied samples contain portions attributed to absorption by free charge carriers. The inference is made that this effect can be associated with tunneling conduction in a system of quantum dots.     

Influence of electromagnetic radiation on the phase composition of clustered CN<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> films

The effect of white and UV radiation on the phase composition of amorphous CN _x films are studied by X-ray diffraction analysis and visible-range spectroscopy. The films have variable-range atomic order and consist of amorphous graphite clusters (30 Å) crystalline clusters (50–100 Å) of graphite, diamond, and carbon nitride phases; and intercluster medium with long-range (1–2 Å) atomic order. It is shown that irradiation of the films by white light facilitates the growth of fine graphite clusters. Irradiation by UV light suppresses the growth of the graphite and carbon nitride phases, favoring the growth of the diamond phase (1.5%). It is demonstrated that a change in the mesoscopic phase composition of the CN _x films causes a change in the energy gap width in the visible range from E _g = 0.75 eV for the films irradiated by white light to E _g = 1.75 eV for those exposed to UV radiation.