Structural aspects of solid-state amorphization in Eu<Subscript>2</Subscript>(MoO<Subscript>4</Subscript>)<Subscript>3</Subscript> single crystals

X-ray diffraction studies of Eu₂(MoO₄)₃ single crystals were performed, which demonstrate that, in contrast to polycrystalline samples, these crystals do not exhibit amorphous-like diffraction patterns during the reverse transition from the high-pressure phase into the initial β phase; rather, the diffracted intensity in their diffraction patterns decreases significantly to the background. Such a diffraction pattern can be explained under the assumption that a single crystal is divided into small (nanoscopic) regions inside which the lattice parameters of the high-pressure phase and the initial β phase change continuously. The simultaneous recovery of the single-crystal state of the β phase from this intermediate state in all nanoscopic regions as the annealing temperature increases indicates that nanocrystals in this state are structurally correlated with each other. This result suggests that the halo-type diffraction patterns of polycrystalline samples reflect an intermediate state between the high-pressure phase and the β phase in every initial crystallite (as in the single crystals) rather than being caused by an amorphous structure of the sample. In this case, the total diffraction pattern of differently oriented crystallites gives an amorphous-like diffraction pattern reflecting the contributions from numerous various crystallographic planes involved in diffraction.     

Features of the pressure-induced phase transitions in Eu<Subscript>2</Subscript>(MoO<Subscript>4</Subscript>)<Subscript>3</Subscript> single crystals

The structural changes induced by a 9-GPa pressure in Eu₂(MoO₄)₃ single crystals at room temperature have been studied using x-ray diffraction. It is established that a structural phase transition from the initial tetragonal phase to the new high-pressure tetragonal phase occurs rather than solid-phase amorphization that was observed previously in polycrystalline samples. The samples in the observed transition remain single-crystalline despite a significant difference (ΔV ～ 18%) between the specific volumes of the initial and final phases. It is shown that the transition from the initial state to the high-pressure phase occurs via the formation of broad transition zones featuring a continuous and smooth change of the crystal lattice parameters.     

Microstructure,dielectric and piezoelectric properties of (K<Subscript>0.5</Subscript>Na<Subscript>0.5</Subscript>)NbO<Subscript>3</Subscript>-Ba(Ti<Subscript>0.95</Subscript>Zr<Subscript>0.05</Subscript>)O<Subscript>3</Subscript> lead-free ceramics with CuO sintering aid

Using an ordinary ceramic fabrication technique, we fabricated lead-free (1-x)(K_0.5Na_0.5)NbO₃-xBa(Ti_0.95Zr_0.05)O₃ ceramics with CuO sintering aid . Ba(Ti_0.95Zr_0.05)O₃ diffuses into (K_0.5Na_0.5)NbO₃ to form a new solid solution. The ceramics with perovskite structure possess orthorhombic phase at x≤0.04 and become tetragonal phase at x≥0.06. Both the paraelectric cubic–ferroelectric tetragonal and the ferroelectric tetragonal–ferroelectric orthorhombic phase transition temperatures decrease with increasing the concentration of Ba(Ti_0.95Zr_0.05)O₃. The doping of CuO effectively promotes the densification of the ceramics. The coexistence of the orthorhombic and tetragonal phases at 0.04<x<0.06 and the improvement in sintering performances of the ceramics significantly enhance the piezoelectric and dielectric properties at room temperature. The ceramics with x=0.04–0.06 and y=0.75–1.50 possess excellent properties: d₃₃=119–185 pC/N, k_P=37–44%, k_t=35–49%, ε=341–1129, cosδ=0.7–4.4% and T_c=312–346 °C. PACS 77.65.-j; 77.84.Dy; 77.84.-s     

Thermophysical study of structural phase transitions in Na<Subscript>0.95</Subscript>Li<Subscript>0.05</Subscript>NbO<Subscript>3</Subscript> solid solution

Temperature dependences of specific heat C_p(T) and coefficient of thermal expansion ;(T) for Na_0.95Li_0.05NbO₃ sodium-lithium niobate ceramic samples are investigated in the temperature range of 100–800 K. The C_p(T) and α(T) anomalies at T₃ = 310 ± 3 K, T₂ = 630 ± 8 K, and T₁ = 710 ± 10 K are observed, which correspond to the sequence of phase transitions N ? Q ? S(R) ? T2(S). The effect of heat treatment of the samples on the sequence of structural distortions was established. It is demonstrated that annealing of the samples at 603 K leads to splitting of the anomaly corresponding to the phase transition Q → R/S in two anomalies. After sample heating to 800 K, the only anomaly is observed in both the C_p(T) and ;(T) dependence. Possible mechanisms of the observed phenomena are discussed.     

Gas sensor based on nanosize In<Subscript>2</Subscript>O<Subscript>3</Subscript>:Ga<Subscript>2</Subscript>O<Subscript>3</Subscript> film

Nanosize films of In₂O₃:Ga₂O₃ (96:4 weight %) have been deposited on a glassceramic substrate by the method of rf magnetron sputtering. The surfaces of fabricated films were studied with use of a scanning electron microscope; sizes of grains were determined and the thicknesses of films were measured. In order to prepare a gas-sensitive structure, a thin catalytic palladium layer and ohmic comb contacts were deposited on the In₂O₃:Ga₂O₃ film surface by the method of ion-plasma sputtering. The sensitivity of sensors based on the glassceramic/In₂O₃:Ga₂O₃ (96:4 weight %)/Pd structure to different concentrations of propane and butane gas mixture, as well as to methane was investigated at temperatures of working substance from 250 to 300°C.     

Temperature evolution of the dielectric response function of Pb(Fe<Subscript>0.95</Subscript>Sc<Subscript>0.05</Subscript>)<Subscript>2/3</Subscript>W<Subscript>1/3</Subscript>O<Subscript>3</Subscript> relaxor ceramics in a wide frequency range

Electrodynamic properties of Pb(Fe_0.95Sc_0.05)_2/3W_1/3O₃ solid solution belonging to A(B'B'')O₃ perovskite structural family have been investigated by broadband dielectric spectroscopy in a wave-number range of (4 × 10^–9–4 × 10³) cm^–1 and a temperature range of 100–600 K. The influence of low-frequency relaxations on the vibrational spectrum is determined within the four-parameter factorized dispersion model. Anomalies in the behavior of the dielectric response function are found near the temperature-diffuse maximum of permittivity.     

Anti-stokes luminescence of solid AgCl<Subscript>0.95</Subscript>I<Subscript>0.05</Subscript> solutions

An anti-Stokes luminescence band with λ_max = 515 nm of microcrystals of solid AgCl_0.95I_0.05 solutions excited by a radiation flux of density 10¹³–10¹⁵ quanta/cm²·sec in the range 600–800 nm at 77 K was detected. It is shown that the intensity of this luminescence and the frequency of its excitation depend on the prior UV-irradiation of samples. Analysis of the stimulated-photoluminescence spectra and the anti-Stokes luminescence excitation spectra of the indicated microcrystals has shown that to the centers of anti-Stokes luminescence excitation correspond local levels in the forbidden band of the crystals. These states are apparently due to the atomic and molecular disperse silver particles that can be inherent in character or formed as a result of a low-temperature photochemical process. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 72, No. 6, pp. 738–742, November–December, 2005.     

Catalytic growth and characterization of Ga<Subscript>2</Subscript>O<Subscript>3</Subscript> nanowires

β-Ga₂O₃ nanowires have been synthesized using Ga metal and H₂O vapor at 800 °C in the presence of Ni catalyst on the substrate. Remarkable reduction of the diameter and increase of the length of the Ga₂O₃ nanowires are achieved by separation of Ga metal and H₂O vapor before they reach the substrate. Transmission electron microscopy analyses indicate that the β-Ga₂O₃ nanowires possess a single-crystalline structure. Photoluminescence measurements show two broad emission bands centered at 290 nm and 390 nm at room temperature. Received: 27 June 2002 / Accepted: 7 October 2002 / Published online: 17 December 2002 RID="*" ID="*"Corresponding author. Fax: +886-6/234-4496, E-mail: wujj@mail.ncku.edu.tw     

Temperature-controlled synthesis of Ga<Subscript>2</Subscript>O<Subscript>3</Subscript> nanobelts and nanosheets

We have investigated the effect of growth temperature on structural morphology and photoluminescence (PL) properties of as-synthesized gallium oxide (Ga₂O₃) nanostructures. The products consisted of Ga₂O₃ nanobelts and nanosheets (i.e. wider nanobelts), which had monoclinic crystalline structures. The average width of structures grown at 1000 °C was relatively greater than those at 800 °C, revealing that higher temperature favored the formation of nanosheets. PL measurements of 800 °C- and 1000 °C-grown samples indicated that both samples exhibited a broad emission band peaked around the blue-light region, while only the 800 °C-grown sample showed a red peak. PACS 81.07.-b; 81.05.Je; 61.10.Nz; 68.37.Hk; 68.37.Lp     

The memory effect in Eu<Subscript>2</Subscript>(MoO<Subscript>4</Subscript>)<Subscript>3</Subscript> single crystals exposed to high hydrostatic pressure

The influence of high-pressure hydrostatic compression and subsequent annealing on the structural properties of β’-phase single crystals of europium molybd ate was studied by IR spectroscopy and X-ray diffraction. It was demonstrated that after compression, the IR spectra exhibit drastic changes and do not depend on the initial crystallographic orientation of a single crystal. The high-pressure compression of single crystals followed by their annealing was found to result in the formation of β’-Eu₂(MoO₄)₃ single crystals with the same crystallographic orientation as that of the initial samples.     

Acoustic emission and thermal expansion of Pb(Mg<Subscript>1/3</Subscript>Nb<Subscript>2/3</Subscript>)O<Subscript>3</Subscript> and Pb(Mg<Subscript>1/3</Subscript>Nb<Subscript>2/3</Subscript>)O<Subscript>3</Subscript>-PbTiO<Subscript>3</Subscript> crystals

The temperature dependence of the elongation per unit length for Pb(Mg_1/3Nb_2/3)O₃ crystals unannealed after growth and mechanical treatment is investigated in the course of thermocycling. It is revealed that this dependence deviates from linear behavior at temperatures below 350°C. The observed deviation is characteristic of relaxors, is very small in the first cycle, increases with increasing number n of thermocycles, and reaches saturation at n≥3. In the first cycle, a narrow maximum of the acoustic emission activity is observed in the vicinity of 350°C. In the course of thermocycling, the intensity of this maximum decreases and becomes zero at n>3. For (1?x)Pb(Mg_1/3Nb_2/3)O₃-xPbTiO₃ crystals, the dependence of the temperature of this acoustic emission maximum on x exhibits a minimum. It is assumed that the phenomena observed are associated with the phase strain hardening due to local phase transitions occurring in compositionally ordered and polar nanoregions.     

Investigations of dielectric enhancement in (Ta<Subscript>2</Subscript>O<Subscript>5</Subscript>)<Subscript>1-x</Subscript>(TiO<Subscript>2</Subscript>)<Subscript>x</Subscript> ceramics prepared by laser-sintering technique

The maximum dielectric permittivity of Ti-doped Ta₂O₅ ceramics may reach 450 by a laser-sintering technique. The aim of this study is to investigate the mechanisms of the dielectric enhancement based on the unique structural and morphological properties of the laser-sintered ceramics. The reason for the dielectric enhancement is due to the crystal structure distortion in the high-temperature phase, the oriented grain growth taking place in a direction deviating from [001] in the laser-sintered ceramics. The concurrent nature of quenching effects, a sharp temperature gradient and mass transfer in liquid phase originated from laser high energy irradiation with strict directivity leads to the structural and morphological properties. PACS 81.40.Tv; 61.80.Ba; 77.22.-d; 77.22.Ch     

Optical studies of phase transitions in the (NH<Subscript>4</Subscript>)<Subscript>3</Subscript>Ti(O<Subscript>2</Subscript>)F<Subscript>5</Subscript> crystal

Polarization-optical study of twinning and measurements of the Raman spectra and birefringence in oxyfluoride (NH₄)₃Ti(O₂)F₅ were carried out over the temperature range 90–350 K. Phase transitions were detected at temperatures T ₀₁ = 266 K (second-order transition) and T ₀₂ = 225 K (first order). It is assumed that the crystal symmetry is changed as follows: Fm3m ? I4/mmm ? I4/m. Anomalies of the spectral parameters are established in the frequency range of internal vibrations of ammonium ions and Ti(O₂)F₅ complexes. An analysis of the results shows that the transition at T ₀₁ is likely due to small shifts of the tetrahedral groups from their position on the triad axis and that the transition at T ₀₂ is due to fluorine-oxygen ordering of Ti(O₂)F₅ complexes.     

Luminescence in Dy<Superscript>3+</Superscript> and Eu<Superscript>3+</Superscript> Activated K<Subscript>3</Subscript>Al<Subscript>2</Subscript>(PO<Subscript>4</Subscript>)<Subscript>3</Subscript>

The Dy³⁺ and Eu³⁺ activated K₃Al₂ (PO₄)₃ phosphors were prepared by a combustion synthesis. From a powder X-ray diffraction (XRD) analysis the formation of K₃Al₂ (PO₄)₃ was confirmed. In the photoluminescence emission spectra, the K₃Al₂(PO₄)₃:Dy³⁺ phosphor emits two distinctive colors: blue and yellow whereas K₃Al₂(PO₄)₃:Eu³⁺ emits red color. Thus the combination of colors gives BYR (blue–yellow–red) emissions can produce white light. These phosphors exhibit a strong absorption between 340 and 400 nm which suggest that present phosphor is a promising candidate for producing white light-emitting diodes (LED).     

Magneto-elastic properties of Tb<Subscript>3</Subscript>Ga<Subscript>5</Subscript>O<Subscript>12</Subscript> (TGG)

The temperature dependence of the elastic constants in Tb₃Ga₅O₁₂ was measured and analysed with a simple crystal field model. The magneto-elastic coupling constants have been deduced from this experiment. The coupling constant g_Γ5, related to the c₄₄ mode, is anomalously large. These coupling constants are important for the interpretation of the phonon Hall effect.     

Multinuclear MAS NMR Investigation of Sol-Gel and Ball-Milled Nanocrystalline Ga<Subscript>2</Subscript>O<Subscript>3</Subscript>

⁷¹Ga magic-angle spinning (MAS) nuclear magnetic resonance (NMR) has been used to characterize the structural evolution of nanocrystalline Ga₂O₃ samples prepared by sol-gel and ball-milling techniques. ²⁹Si and ²⁷Al MAS NMR have also been used to characterize silica and alumina Zener pinning phases. ⁷¹Ga NMR parameters are reported for the α- and β-Ga₂O₃ phases, and more tentatively for the δ-Ga₂O₃ phase. By simulating the octahedrally coordinated gallium NMR line of β-Ga₂O₃ using Gaussian distributions in χ_Q, the extent of disorder in the Ga₂O₃ crystallites has been quantified. The ball-milled samples contain much more inherent disorder than the sol-gel samples in the nano-phase, which was observed from simulations of the ⁷¹Ga MAS NMR spectra. The silica pinning phase produced highly crystalline and densely aggregated nanocrystalline Ga₂O₃, as well as the smallest nanocrystal sizes. Authors' address: Mark E. Smith, Department of Physics, University of Warwick, Coventry CV4 7AL, UK     

Comparative Study of the Magnetoelectric Effect in HoAl<Subscript>3</Subscript>(BO<Subscript>3</Subscript>)<Subscript>4</Subscript> and HoGa<Subscript>3</Subscript>(BO<Subscript>3</Subscript>)<Subscript>4</Subscript> Single Crystals

The comparative study of the magnetoelectric properties and magnetostriction of HoGa₃(BO₃)₄ and HoAl₃(BO₃)₄ single crystals has been carried out. The investigated compounds exhibit qualitatively similar magnetodielectric and inverse magnetoelectric ME _E effects with the close absolute values, which is indicative of the weak effect of a nonmagnetic metal ion. On the contrary, the magnetostriction of the galloborate has been found to be threefold higher than that of the alumoborate. In addition, the difference between the qualitative behaviors of magnetostriction has been established: the magnetic-field dependence of magnetostriction for the alumoborate has the maximum near 70 kOe at T = 4.2 K, while the galloborate magnetostriction has no maximum and does not saturate in a field of 140 kOe.     

Photoluminescence,thermoluminescence and electron spin resonance studies on Eu<Superscript>3+</Superscript> doped Ca<Subscript>3</Subscript>Al<Subscript>2</Subscript>O<Subscript>6</Subscript> red phosphors

Tricalcium aluminate doped with Eu³⁺ was prepared at furnace temperatures as low as 500°C by using the convenient combustion route and examined using powder X-ray diffraction, scanning electron microscope and photoluminescence techniques. A room-temperature photoluminescence study showed that the phosphors can be efficiently excited by UV/Visible region, emitting a red light with a peak wavelength of 616 nm corresponding to the ⁵D₀–⁷F₂ transition of Eu³⁺ ions. The phosphor exhibits three thermoluminescence (TL) peaks at 195°C, 325°C and 390°C. Electron Spin Resonance (ESR) studies were carried out to study the defect centres induced in the phosphor by gamma irradiation and also to identify the defect centres responsible for the TL process. Room-temperature ESR spectrum of irradiated phosphor appears to be a superposition of three distinct centres. One of the centres (centre I) with principal g-value 2.0130 is identified as O⁻ ion while centre II with an axially symmetric principal values g _∥=2.0030 and g _⊥=2.0072 is assigned to an F⁺ centre (singly ionized oxygen vacancy). O⁻ ion (hole centre) correlates with the TL peak at 195°C and the F⁺ centre (electron centre), which acts as a recombination centre, is also correlated to the 195°C TL peak. F⁺ centre further appears to be related to the high temperature peak at 390°C. Centre III is also assigned to an F⁺ centre and seems to be the recombination centre for the TL peak at 325°C.     

Magnetic properties of DyFe<Subscript>3</Subscript>(BO<Subscript>3</Subscript>)<Subscript>4</Subscript>

The magnetic properties of an easy-axis trigonal DyFe₃(BO₃)₄ antiferromagnetic crystal have been theoretically studied. On this basis, recent experimental data [1] on the field and temperature dependences of magnetization and the temperature dependence of the initial magnetic susceptibility for three crystallographic directions in this antiferromagnet have been interpreted. The characteristics of the trigonal crystal field for the rare earth ion and the parameters of the Fe-Fe and Fe-Dy exchange interactions are determined. Limitations imposed by features of the magnetic characteristics (anisotropic magnetization in the three crystallographic directions, Schottky-type anomalies in the magnetic susceptibility, etc.) on the possible splitting of the ground-state multiplet in the crystal field and the splitting of the lowest doublet due to the f-d interaction for Dy³⁺ ions are established.     

Photoluminescence of CaGa<Subscript>2</Subscript>S<Subscript>4</Subscript>:Pr Polycrystals

The photoluminescence (PL) and PL excitation spectra of CaGa₂S₄ polycrystals doped with praseodymium are studied in the regions of the activator absorption and the fundamental absorption of the host. It is found that the PL excitation spectrum consists of two regions: broadband absorption in the range of 200-380 nm corresponding to the fundamental absorption of the host and the narrow-band absorption of the dopant in the range of 430–515 nm. The luminescence spectra are different for different excitation wave-lengths, which occurs because Pr³⁺ ions substitute divalent cations occupying different crystallographic positions in the host crystal lattice.