Annealing induced transformations in structural and optical properties of Ge30Se70−xBix thin films

ABSTRACT

Thin films of Ge₃₀Se_70?xBi_x (x?=?5, 15, 20) were prepared by thermal evaporation method on glass substrates with thickness 800?nm. The films were annealed at 250°C and 320°C for 2?h to study the annealing-induced structural and optical change. The X-ray diffraction characterization revealed the amorphous to crystalline phase transformation with annealing. The indirect optical band gap decreased with annealing which is explained on the basis of phase transformation and density of localized states. The formation of surface dangling bonds around the crystallites during crystallization process reduced the band gap. The Tauc parameter and Urbach energy change show the degree of chemical disorderness in the films. The transmitivity decreased while the absorption coefficient increased with the annealing process. The microstructural study done by Field emission scanning electron microscopy shows the formation of crystallites upon annealing. Atomic force microscopy investigation on these films shows the influence of annealing on surface topography.     

Elastic matching of phases and domains in KCN-type crystals

The present work contains a new information on the role of elastic matching in formation and rearrangement of complex domain structures as well as at phase coexistence in KCN, (KBr)_1-x (KCN)_x and (KCl)_1-x (KCN)_x crystals.     

Elastic and quasielastic neutron scattering studies in KBr:KCN mixed crystals

Neutron scattering studies in (KBr)_1–x(KCN)_x mixed crystals are presented utilizing powder diffraction, single crystal diffraction and time-of-flight techniques. Forx>0.6 (KBr)_1–x(KCN)_x crystals exhibit ferroelastic and ferroelectric low-temperature phases. Crystals withx<0.6 undergo transitions into an orientational glass state. Here we present a detailed phase diagram including new results for x=0.85 and x=0.65. For the latter system a stable rhombohedral low-temperature phase has been detected where the orientational disorder of the plastic phase is only partly removed and quadrupolar relaxations between three body diagonals are still possible. From the powder diffraction experiments we determined further the concentration dependence of the static Debye-Waller factors which can be explained by an interplay of the rotation-translation and the rotation-random strain coupling. With single crystal diffraction techniques we studied the diffuse scattered intensities which are directly related to the order parameter of the glass state. The temperature dependence of the quasielastic intensities near the critical concentration shows a strong increase forT<110K indicative for a freezing-in of shear fluctuations which is a characteristic feature of a non-ergodic instability. This phenomenon appears for ordering (x=0.65) and for non-ordering, glassy compounds. A further anomaly in (KBr)_0.43(KCN)_0.57 at 75 K is interpreted in terms of a residual elastic ordering process. With high-resolution time-of-flight techniques we analysed the dynamic structure factor for x=0.57. We demonstrate that the central peak consists of a static and a dynamic component. The results are compared with mode coupling theories which describe the glass transition in supercooled liquids.     

Comparative analysis of the effect of La and Co on the superconductivity and energyband spectrum of YBa2Cu3Oy for different oxygen contents

Temperature dependences of the resistivity and Seebeck coefficient of Y(Ba_1−x La_x)₂Cu₃O_y and YBa₂Cu_3−x Co_xO_y samples (x=0–0.25) have been measured under maximum sample saturation with oxygen, as well as following their anneal in an oxygen-deficient atmosphere. The T _c (x) dependences for as-prepared samples were found to pass through a maximum at x=0.05, which persists after annealing for Y(Ba_1−xLa_x)₂Cu₃O_y and disappears for YBa₂Cu_3−x Co_xO_y. A phenomenological model of the band spectrum in normal phase has been used to determine the parameters of the conduction band and of the carrier system, and to analyze their variation with the dopant type and content, as well as with annealing. Despite the differences observed in the T _c (x) dependence, the critical temperatures for all the sample series studied were found to correlate with the conduction-band effective width. The mechanism of the effect of impurities on the band-structure parameters and the reasons for the different influence of annealing on the properties of Y(Ba_1−x La_x)₂Cu₃O_y and YBa₂Cu_3−x Co_xO_y are discussed. Fiz. Tverd. Tela (St. Petersburg) 41, 389–394 (March 1997)     

The Influence of the Annealing Temperature on the Properties of Sn<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Fe<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>O<Subscript>2</Subscript> Powders Evidenced by EMR Spectroscopy

Electron magnetic resonance (EMR) investigations of Sn_{1− x} Fe _x O₂ (x = 0.07) powders annealed at different temperatures are reported. EMR spectra show the presence of both the isolated paramagnetic Fe³⁺ ions incorporated into the SnO₂ lattice and a ferromagnetically ordered component. By increasing the annealing temperature, the EMR spectrum attributed to the ferromagnetically coupled Fe³⁺ ions diminishes probably due to the migration of Fe ions and the formation of the second phase. The presence of the second phase was evidenced by X-ray diffraction measurements.     

Effect of Li doping on the critical temperature and glass formation in the Bi-Sr-Ca-Cu-O system

A study has been made of the glass-forming ability, structure, and superconducting properties of Bi_2.2Sr_1.8Ca_1.05Cu_2.15Li_xO_y and Bi_2.2Sr_1.8Ca_1.05Cu_2.15−x Li_xO_y (x=0;0.3;0.5;0.7). The compounds were melted by rf at T=1300–1500 °C. Rapid quenching produces glassy alloys whose glass-forming ability is the highest when lithium is substituted for copper. Glass annealing at 700–800 °C results in the formation of the HTSC phase 2212 with a critical temperature of up to 91 K. In lithium-doped samples the HTSC phase forms at lower temperatures and shorter anneals and it depends on the cooling rate following the anneal. The composition and properties of the 2212 phase depend nonmonotonically on the anneal time. The lattice parameter C of the 2212 phase increases with increasing lithium content. Fiz. Tverd. Tela (St. Petersburg) 41, 18–21 (January 1999)     

Shift in elastic phase boundaries due to nanoscale phase separation in network glasses: the case of Ge x As x S1 − 2 x

Binary and ternary sulphide glasses, in contrast to their selenium counterparts, are usually not fully polymerized. This circumstance provides a means to examine the role of nanoscale phase separation effects on global elastic phase diagrams of disordered networks. In bulk Ge _x As _x S_{1 ? ?2 x} glasses, the non-reversing enthalpy (ΔH _nr) near T _g is found to display a global minimum (～0) in the 0.11<x<0.15 range, the reversibility window. Furthermore, the ΔH _nr term is found to age for glass compositions below (x<0.11) and above (x>0.15) the window, but not in the window. In analogy to corresponding selenides, glass compositions in the window represent the intermediate phase, those at x<0.11 are floppy, and those at x>0.15 stressed-rigid. Raman scattering shows floppy and stressed-rigid networks to consist of S₈, and As₄S₄ and As₄S₃ monomers, respectively, aspects of structure that contribute to a narrowing of the intermediate phase and to suppression of the ΔH _nr term in S-rich glasses qualitatively in relation to corresponding Se-rich glasses that are fully polymerized.     

Texture in a ternary Nd16.2Fe78.2B5.6 powder using a modified hydrogenation–disproportionation–desorption–recombination process

A modified hydrogenation–disproportionation desorption-recombination (HDDR) process consisting of (i) solid disproportionation and (ii) slow recombination under partial hydrogen pressure has been applied to a Nd_16.2Fe_78.2B_5.6 alloy. Scanning electron microscopy shows that an initially fine rod-like structure of NdH_2±x and Fe observed after 15 min of hydrogenation at 900°C is transformed into a granular morphology with prolonged annealing. Simultaneously, finely dispersed tetragonal Fe₃B particles of 10–50 nm diameter exist. XRD studies show that this metastable Fe₃B phase is transformed to Fe₂B and Fe on further annealing. Short solid-disproportionation times result in a higher degree of anisotropy after recombination, whereas long annealing times and conventional processing lead to isotropic material. It is concluded that the formation of the intermediate tetragonal Fe₃B phase after solid disproportionation is pivotal for the inducement of texture in HDDR processed ternary NdFeB-type alloys.     

Hard mode infrared spectroscopy of CaTiO3—CaFeO2.5 perovskites

Abstract

Powder-absorption infrared (IR) spectra of perovskites CaFe_xTi_1?x O_3?x/2 (0≤x≤1) annealed at different temperatures were investigated at room temperature in the range 135–2000 cm^?1. The spectra change as a function of composition, annealing temperature and structural state (order-disorder of oxygen vacancies). Autocorrelation analysis has been used to determine variations of average line widths of groups of peaks in the primary IR spectra. The band widths increase on increasing Fe content in the region of the structures with disordered oxygen vacancies and they decrease on going through the order-disorder boundary. High degrees of local structural heterogeneity are suggested by the effective line widths of the phases at intermediate compositions. The intensity of bands at ～150 and ～443 cm^?1 decreases with increasing Fe content in the compositional range of the disordered structures. Finally, for every annealing temperature, the frequency of the band at ～600cm^?1 systematically shifts to higher values on increasing Fe content, these values decreasing again for the fully ordered structures.     

Low-temperature specific heat of orientational glasses

This review summarizes specific heat data measured at low temperatures (T<1 K) on orientational glasses. Three species of mixed molecular crystals exhibiting orientational disorder are considered, namely (KBr)_1–x (KCN) _x , (NaCN)_1–x (KCN) _x (Rb)_1–x (NH₄) _x H₂PO₄. For intermediate concentrations of the anisotropic components, glass-like excitations have been observed. It is demonstrated that with respect to thermal properties, orientational disorder leads to the same universal behaviours than for structural disorder, i.e. a specific heat which varies below 1 K and for times 10^–4 s–10 s as:C _p(T,t)T ¹×ln(t). The variation of the glass-like anomaly with compositional disorder is also discussed. It is concluded that the low-energy excitations are related to orientational degrees of freedom which are frozen at low temperatures but still perform reorientations via quantum tunneling motions.     

Buried (Fe1-xCox)Si2 layers with variable band gap formed by ion beam synthesis

Buried layers of (Fe_{1 –x} Co _x )Si₂ were prepared by sequent implantation of iron and cobalt into (100) silicon. The depth distributions of iron and cobalt and the atomic concentration ratio silicon/metal were determined by Auger Electron Spectroscopy (AES) and Rutherford Backscattering Spectrometry (RBS). The phase composition and the microstructure of the silicide layer were studied by X-ray diffraction and electron microscopy. The band gap energy was evaluated from Infrared (IR) reflection and transmission experiments. The semiconducting-FeSi₂ structure remains stable up to a cobalt fraction ofx = 0.2 if the iron silicide is stabilized by an intermediate annealing between iron and cobalt implantation. With increasing cobalt content, the electrical resistivity as well as the energy of the direct band gap of the (Fe_{1 –x} Co _x )Si₂ layer decrease. In this way, a band gap tuning between 0.84 and 0.70 eV is possible. The gap energy is found to vary quadratically with the Co content in the composition range 0 <x < 0.15.     

Brillouin scattering study of elastic anomaly in (KCN)x(KBr)1−x system

Brillouin scattering is used to study concentration and temperature dependence of transverse acoustic phonon frequency ω_TA (110) in (KCN)_x(KBr)_1?x mixed crystals. Unlike pure KCN, mixed xrystals with x < 0.75 show no phase transition upon cooling, however, in these crystals we find a minimum in ω_TA as a function of temperature. This minimum can be understood in terms of dynamical theory of rotation-translation coupling in molecular crystals.     

Phases and phase transitions in the mixed molecular system (NaCN)1−x(KCN)x

The phase diagram of (NaCN)_1–x(KCN)_x was examined by neutron powder diffraction in the temperature range 5K T300 K. Several non-cubic low-temperature phases were identified for concentrationsx<x _c1=0.15 andxx _c2=0.89. Lattice parameters and ferroelastic deformations were determined from the observed powder patterns. The phase transformations were characterized following the temperature dependence of the appropriate order parameters.     

Local configurations and atomic intermixing in as-quenched and annealed Fe1−xCrx and Fe1−xMox ribbons

Local atomic configuration, phase composition and atomic intermixing in Fe-rich Fe_1?xCr_x and Fe_1?xMo_x ribbons (x = 0.05, 0.10, 0.15), of potential interest for high-temperature applications and nuclear devices, are investigated in this study in relation to specific processing and annealing routes. The Fe-based thin ribbons have been prepared by induction melting, followed by melt spinning and further annealed in He at temperatures up to 1250 °C. The complex structural, compositional and atomic configuration characterisation has been performed by means of X-ray diffraction (XRD), transmission Mössbauer spectroscopy and differential scanning calorimetry (TG-DSC). The XRD analysis indicates the formation of the desired solid solutions with body-centred cubic (bcc) structure in the as-quenched state. The Mössbauer spectroscopy results have been analysed in terms of the two-shell model. The distribution of Cr/Mo atoms in the first two coordination spheres is not homogeneous, especially after annealing, as supported by the short-range order parameters. In addition, high-temperature annealing treatments give rise to oxidation of Fe (to haematite, maghemite and magnetite) at the surface of the ribbons. Fe_1?xCr_x alloys are structurally more stable than the Mo counterpart under annealing at 700 °C. Annealing at 1250 °C in He enhances drastically the Cr clustering around Fe nuclei.     

XRD,Raman and magnetic studies of Bi1−xLaxFeO3 solid solution obtained by mechanochemical synthesis

ABSTRACT

Bi_{1 ? x}La_xFeO₃ single-phase solid solution with 0 ≤ x ≤ 0.3 was obtained by mechanochemical synthesis and characterized by X-ray diffraction, FT NIR Raman spectroscopy and magnetic studies. The nanocrystalline powders of rhombohedrally distorted perovskite structure R3c were studied ‘as synthesized’ by mechanosynthesis and after annealing at 500 °C for 1 h. The annealing, which resulted in recrystallization of the amorphous shell of the nanograins, was found to modify the external lattice modes of Bi_1?xLa_xFeO₃ and resulted in changes in the temperature variation of the magnetization.     

Frozen magnetic states in quasi-two-dimensional Cr1/3−x NixTaS2 compounds with competing exchange interactions

A study of the dynamic and static magnetic susceptibility of quasi-two-dimensional, doubly intercalated compounds Cr_1/3−x Ni_xTaS₂ (0.08⩽x⩽0.23) is reported. It is shown that cooling from paramagnetic phase in this concentration range produces a spin-glass-type frozen magnetic state. In contrast to the classical 3D spin glasses, however, this state sets in within a broad temperature range rather than at a phase transition. A magnetic phase diagram of the studied compounds has been constructed. Fiz. Tverd. Tela (St. Petersburg) 39, 1801–1805 (October 1997)     

Aspects of magnetic ordering in the system (FexCr1−x )2O3

Anomalon in temperature dependence of the magnetic moment (FexCr_1−x )₂O₃ single crystal has been observed which can be explained by antiparallel orientation of the mean weakly-ferromagnetic moments of the d subsystems (weak ferromagnetism). The disappearance of the linear magneto-electric effect for x=0.1 below 45 K is explained by a spontaneous phase transition from a conical spiral to a cycloidal spin structure, which is found to agree with a thermodynamic treatment. Fiz. Tverd. Tela (St. Petersburg) 39, 112–114 (January 1997)     

Concentration dependence of the compositional order-disorder phase-transition temperature in the solid solution PbSc0.5Nb0.5O3-PbSc0.5Ta0.5O3

The degree of compositional order S of crystals of the complete series of solid solutions (1−x)PbSc_0.5Nb_0.5O₃-xPbSc_0.5Ta_0.5O₃ grown under identical conditions is determined by x-ray examination. The form of the dependence S(x) and the variations of S for different compositions, after high-temperature annealing, indicate that the concentration dependence of the compositional order-disorder phase-transition temperature is close to linear in the given system of solid solutions. Fiz. Tverd. Tela (St. Petersburg) 41, 1828–1830 (October 1999)     

Effects of low-level La doping and ageing on TlPb-1223 high Tc superconductors

Bulk samples of the nominal composition of (Tl_0.6Pb_0.5)(Sr_0.8Ba_0.2)₂Ca₂Cu₃O_8+δ−xLaO_1.5 (x = 0–0.1) were prepared by using two-step process and their microstructure, T _c values, and magnetization were studied. The samples consist of the Tl-1223 dominant phase with small Tl-1212 admixture, which increases with a rise of La content. Five years ageing and following oxygen annealing at 450°C and subsequently at 750°C have only a modest effect on T _c values of the studied samples. Low-level La doping (x = 0:04) leads to an increase of T _c values by about 2 K in comparison with undoped samples. Oxygen annealing at 750°C results in an increase in the volume magnetization hysteresis in low applied magnetic fields and rise of critical current density at zero magnetic field and 77 K. This effect is most pronounced for the low La doped sample with x = 0.04. Changes of the induced voltage, U originating in the Meissner effect and of its harmonics in dependence on temperature were measured and used for characterization of the temperature distribution of inter-grain junctions.     

Phase transitions in RbxK1−x LiSO4 mixed crystals

Structural phase transitions in Rb_xK_1−x LiSO₄ mixed crystals (with x varying from 0 to 1) have been studied from the melting point to liquid-nitrogen temperature. Calorimetric (DTA and DSM) data, birefringence and optical polarization measurements were used to construct the full phase (T-x) diagram. It has been established that crystals of most compositions (x⩽0.95) grow in the hexagonal-trigonal KLiSO₄ structure. Replacement of K by the larger Rb atom results in a considerable increase of the region of existence of the P31c phase and expulsion of the high-temperature hexagonal phase. Fiz. Tverd. Tela (St. Petersburg) 40, 1341–1344 (July 1998)