Optical waveguides in LiNbO<Subscript>3</Subscript> and stoichiometric LiNbO<Subscript>3</Subscript> crystals by proton exchange

The formation of optical planar waveguides in LiNbO₃ and stoichiometric LiNbO₃ crystals by proton exchange was reported. The prism-coupling method was used to characterize the dark-line spectroscopy at the wavelength of 633 and 1539 nm, respectively. The mode optical near-field outputs from proton-exchanged LiNbO₃ and SLN waveguides at 633 nm were presented. The mode field from stoichiometric LiNbO₃ (SLN) waveguide is lighter and more uniform than that from LiNbO₃ waveguide, which means the quality of the waveguide in SLN crystal is better than that of the LiNbO₃ waveguide. For proton-exchanged LiNbO₃ waveguides, the evolution of the refractive index profile with annealing was presented. The disorder profiles of Nb atoms in proton-exchanged LiNbO₃ waveguides were obtained by Rutherford backscattering/channeling technique. It is shown that the longer the exchange time, the larger the displacement of Nb atoms. Supported by the National Natural Science Foundation of China (Grant No. 10475052) and the Scientific Research Start-up Financing of Qufu Normal University     

High-power high beam-quality multi-waveguide CO<Subscript>2</Subscript> amplifiers

The results from modeling the energy characteristics of a multi-waveguide power amplifier are presented. The optical schemes and calculations for the most promising circuits of multichannel waveguide CO₂ amplifiers are given. The amplifying system itself removes the problem of phase locking in individual channels of multichannel systems. The experimental results from the synchronization of arrays of multichannel waveguide CO₂ lasers allow the production of high-power (up to 15 kW) high beam-quality multibeam lasers. Technological lasers with such properties have yet to be produced anywhere in the world.     

High-temperature heat capacity of the Al<Subscript>63</Subscript>Cu<Subscript>25</Subscript>Fe<Subscript>12</Subscript> quasicrystal

The mechanisms of energy absorption by metallic alloys with long-range aperiodic lattice order and electronic properties of marginal metals are studied. The heat capacity and linear expansion coefficient of the Al₆₃Cu₂₅Fe₁₂ icosahedral phase in the temperature range 300–1000 K are measured for the first time. Disagreement between the measured heat capacity and predictions made from the Debye model is found and analyzed. It is shown that the excess heat capacity observed at the temperatures of the experiment is fitted by Einstein’s function in the approximation T

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Θ_E.

     

Multiple magnetic transitions in single crystals of Ce<Subscript>12</Subscript>Fe<Subscript>57.5</Subscript>As<Subscript>41</Subscript> and La<Subscript>12</Subscript>Fe<Subscript>57.5</Subscript>As<Subscript>41</Subscript>

Measurements of magnetic and transport properties were performed on needle-shaped single crystals of Ce₁₂Fe_57.5As₄₁ and La₁₂Fe_57.5As₄₁. The availability of a complete set of data enabled a side-by-side comparison between these two rare earth compounds. Both compounds exhibited multiple magnetic orders within 2–300 K and metamagnetic transitions at various fields. Ferromagnetic transitions with Curie temperatures of 100 and 125 K were found for Ce₁₂Fe_57.5As₄₁ and La₁₂Fe_57.5As₄₁, respectively, followed by antiferromagnetic type spin reorientations near Curie temperatures. The magnetic properties underwent complex evolution in the magnetic field for both compounds. An antiferromagnetic phase transition at about 60 K and 0.2 T was observed merely for Ce₁₂Fe_57.5As₄₁. The field-induced magnetic phase transition occurred from antiferromagnetic to ferromagnetic structure. A strong magnetocrystalline anisotropy was evident from magnetization measurements of Ce₁₂Fe_57.5As₄₁. A temperature-field phase diagram was present for these two rare earth systems. In addition, a logarithmic temperature dependence of electrical resistivity was observed in the two compounds within a large temperature range of 150–300 K, which is rarely found in 3D-based compounds. It may be related to Kondo scattering described by independent localized Fe 3d moments interacting with conduction electrons.     

Microwave dielectric properties of Ba(Zn<Subscript>1/3</Subscript>Ta<Subscript>2 / 3</Subscript>)O<Subscript>3</Subscript> for application in high power waveguide window

Higher dielectric constant, low dielectric loss and good transmission characteristicshave been the goal for developing the ceramic waveguide window for high power windowapplications. The choice of materials having high k with low dielectric lossand reduced window size is key parameters to achieve maximum microwave transmissionwithout unleashing microwave dissipation. The microwave dielectric properties ofsynthesized Ba(Zn_1/3Ta_{2 /3})O₃ (BZT) ceramics have been studied for high power windowapplications. The structural studies are correlated with microwave dielectric propertiesof BZT. The maximum values of dielectric constant ?_r =30, Q ×f₀ = 102 THz and near zero temperaturecoefficient of resonance frequency were obtained for BZT ceramics sintered at thetemperature of 1550 °Cfor 4 h. The measured results are used to design a tapered transition from air filledwaveguide to narrow (reduced width and height) dielectric filled waveguide using Heckenslinear taper at a specific frequency. The simulation result shows that the lowerreflection loss is obtained for the tapered transition of the narrow BZT window ascompared to the standard waveguide BZT window. The return loss of –34 dB is obtained forS-bandwaveguide window with a bandwidth of 675 MHz. The return loss observed in the narrow BZTwindow is –46 dB with a bandwidth of 570 MHz at a center frequency of 3.63 GHz. Most ofthe disadvantages in conventional windows will be rectified using the design of the tapertransion employing narrow waveguide window in high power applications.     

Magnetocaloric effect in (La<Subscript>0.6</Subscript>Ca<Subscript>0.4</Subscript>)<Subscript>0.9</Subscript>Mn<Subscript>1.1</Subscript>O<Subscript>3</Subscript>

The results of investigations of the magnetization, susceptibility, and magnetic-field-induced changes in the entropy of polycrystalline manganite (La_0.6Ca_0.4)_0.9Mn_1.1O₃ near the magnetic phase transition have been presented. Magnetic measurements have been carried out at temperatures in the range from 210 to 310 K in magnetic fields of up to 9 T. The magnetocaloric effect has been revealed by measuring the magnetic-field dependences of magnetization. The magnitude of the magnetocaloric effect is compared with similar results obtained for other manganites.     

Magnetic structure and phase diagram of Sr<Subscript>5</Subscript>Rh<Subscript>4</Subscript>O<Subscript>12</Subscript> frustrated Ising magnet

The magnetic structure of Sr₅Rh₄O₁₂ is based on Ising chains of rhodium ions with a variable valence, Rh³⁺-Rh⁴⁺. The ordering in the chains is assumed to be ferromagnetic. It has been shown that the magnetic structure and phase diagram of Sr₅Rh₄O₁₂ are well described in a model taking into account weak antiferromagnetic interactions between the nearest and next-nearest neighbors on the triangular lattice of ferromagnetic Ising chains. The ground state at low temperatures is the two-sublattice stripe phase; this phase in the magnetic field is transformed to the ferrimagnetic phase and, then, to the ferromagnetic phase. Small plateaus can be observed in the region of the transition from the ferrimagnetic phase to the ferromagnetic one.     

Elastic properties of La<Subscript>0.82</Subscript>Ca<Subscript>0.18</Subscript>MnO<Subscript>3</Subscript> single crystal

The temperature dependences of the velocity of longitudinal sound waves and the internal friction in a La_0.82Ca_0.18MnO₃ single crystal with the Curie temperature T _C = 181 K have been studied. As temperature decreases, the single crystal is shown to undergo the transition from the pseudocubic O* to the Jahn–Teller O’ phase at T ~ 254 K and the reverse transition from O’ to O* phase at T ~ 84 K. The velocity of sound and the internal friction in the O’ phase are found to be significantly smaller than those in the O* phase.     

Phase transitions in the (NH<Subscript>4</Subscript>)<Subscript>2</Subscript>NbOF<Subscript>5</Subscript> oxyfluoride

The thermal and dielectric properties of the (NH₄)₂NbOF₅ oxyfluoride have been investigated. It has been established that the structural phase transitions Cmc2₁ → C2 → Ia observed at the temperatures T ₁ = 258.0 K and T ₂ = 218.9 K exhibit a nonferroelectric nature. The hydrostatic pressure, which stabilizes the initial phase and destabilizes the low-temperature phase, hardly affects the temperature range of stability of the intermediate phase. The model of sequential ordering of the structural elements due to phase transitions has been analyzed using experimental data on the entropies of the phase transitions.     

Synthesis,thermal and electrical behavior of the superprotonic conductor phase in Rb<Subscript>3</Subscript>(HSeO<Subscript>4</Subscript>)<Subscript>2.5</Subscript>(H<Subscript>2</Subscript>PO<Subscript>4</Subscript>)<Subscript>0.5</Subscript> compound

The Rb₃(HSeO₄)_2.5(H₂PO₄)_0.5 compound was prepared and its thermal behavior and electric properties were investigated. The thermogravimetry (TGA) analysis and the differential scanning calorimetric (DSC) show the presence of a structural phase transition of the title compounds at 374 K which is confirmed by the variation of fp and σ_dc as a function of temperature. The complex impedance of the Rb₃(HSeO₄)_2.5(H₂PO₄)_0.5 compound has been investigated in the temperature range of 295–453 K and in the frequency range 209 Hz–1 MHz. The impedance plots show semicircle arcs at different temperatures, and an electrical equivalent circuit has been proposed to explain the impedance results. The circuits consist of the parallel combination of bulk resistance Rp and constant phase elements CPE₁ in series with fractal capacity CPE₂. The frequency dependence of the conductivity is interpreted in terms of Jonscher’s law. The conductivity dc follows the Arrhenius relation. The near value of activation energies obtained from the analysis of modulus, conductivity data, and circuit equivalent confirm that the transport is through the ion hopping mechanism, dominated by the motion of the H⁺ proton in the structure of the investigated materials.     

Phase transitions of SC(NH<Subscript>2</Subscript>)<Subscript>2</Subscript> ferroelectrics in Al<Subscript>2</Subscript>O<Subscript>3</Subscript>-based nanoporous matrices

Temperature dependences of the linear permittivity ε' and the third harmonic amplitude γ_3ω of composites prepared by introducing ferroelectrics SC(NH₂)₂ into matrices of porous aluminum oxide Al₂O₃ with pore sizes 60 and 100 nm are determined. It is found that temperature T _c of the ferroelectric phase transition and the temperature T _i of the phase transition from incommensurable phase to the paraphrase increase significantly. The transition shifts increase as pore diameters decrease.     

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.     

Effect of deuteration on the thermal properties and structural parameters of the (NH<Subscript>4</Subscript>)<Subscript>2</Subscript>WO<Subscript>2</Subscript>F<Subscript>4</Subscript> oxyfluoride

The thermal properties and structure of (ND₄)₂WO₂F₄ crystals are investigated. It is established that deuteration does not lead to a change in the symmetry of the initial phase Cmcm but considerably decreases the extent of its disordering, which, in turn, brings about a substantial decrease in the phase transition entropy. Apart from the anomalies associated with phase transitions characteristic of the protonic compound, the heat capacity exhibits two additional anomalies. Analysis of the phase diagram of the deuterated crystal reveals a triple point at a pressure p = 0.18 GPa, which is predicted for (NH₄)₂WO₂F₄ at about 0.7 GPa.     

Pressure-induced superconductivity and structural transitions in Ba(Fe<Subscript>0.9</Subscript>Ru<Subscript>0.1</Subscript>)<Subscript>2</Subscript>As<Subscript>2</Subscript>

Electrical transport and structural characterizations of isoelectronically substituted Ba(Fe_0.9Ru_0.1)₂As₂ have been performed as a function of pressure up to ～ 30 GPa and temperature down to ～ 10 K using designer diamond anvil cell. Similar to undoped members of the AFe₂As₂ (A = Ca, Sr, Ba) family, Ba(Fe_0.9Ru_0.1)₂As₂ shows anomalous a-lattice parameter expansion with increasing pressure and a concurrent ThCr₂Si₂ type isostructural (I4/mmm) phase transition from tetragonal (T) phase to a collapsed tetragonal (cT) phase occurring between 12 and 17 GPa where the a is maximum. Above 17 GPa, the material remains in the cT phase up to 30 GPa at 200 K. The resistance measurements show evidence of pressure-induced zero resistance that may be indicative of high-temperature superconductivity for pressures above 3.9 GPa. The onset of the resistive transition temperature decreases gradually with increasing pressure before completely disappearing for pressures above ～ 10.6 GPa near the T-cT transition. We have determined the crystal structure of the high-T _c phase of Ru-doped BaFe₂As₂ to remain as tetragonal (I4/mmm) by analyzing the X-ray diffraction pattern obtained at 10 K and 9.7 ± 0.7 GPa, as opposed to inferring the structural transition from electrical resistance measurement, as in a previous report [S.K. Kim, M.S. Torikachvili, E. Colombier, A. Thaler, S.L. Bud’ko, P.C. Canfield, Phys. Rev. B 84, 134525 (2011)].     

Manifestation of a ferroelectric-antiferroelectric phase transition in Li<Subscript>0.12</Subscript>Na<Subscript>0.88</Subscript>Ta<Subscript>0.4</Subscript>Nb<Subscript>0.6</Subscript>O<Subscript>3</Subscript> in its Raman scattering spectra

The ferroelectric-antiferroelectric phase transition in the Li_0.12Na_0.88Ta_0.4Nb_0.6O₃ ceramic solid solution has been studied by the Raman scattering technique. As the temperature approached the transition point from below, we observed an appreciable broadening of the lines associated with the vibrations of the cations occupying octahedral and cubooctahedral cavities of the structure and with the oxygen network vibrations (which implies a substantial increase in disorder on the cation sublattices), as well as a decrease to zero intensity of the 875-cm^?1 line corresponding to stretch vibrations of the bridging oxygen in the BO₆ octahedral anion in the vicinity of the transition. The temperature dependence of the 875-cm_?1 line intensity near the transition was used to study the behavior of the phase transition order parameter η. The behavior of η was found to disagree markedly with the Landau theory of second-order phase transitions. It is shown that discrepancies originate from the increase in disorder in the niobium and tantalum sublattices in the Li_0.12Na_0.88Ta _y Nb_1-y O₃ solid solution system with increasing y. The order of the transition is lowered.     

A new commensurate phase on the theoretical phase diagram of the [N(CH<Subscript>3</Subscript>)<Subscript>4</Subscript>]<Subscript>2</Subscript>CuCl<Subscript>4</Subscript> crystal

A theoretical phase diagram of the [N(CH₃)₄]₂CuCl₄ crystal with a new commensurate phase characterized by a dimensionless wavenumber q = 2/5 is constructed on a plane specified by two coefficients of the thermodynamic potential. This diagram is used as the basis for the construction of a theoretical pressure-temperature (P-T) phase diagram. The theoretical P-T phase diagram thus obtained is compared with the experimental P-T phase diagram.     

Mechanical alloying of Ni<Subscript>80</Subscript>Ta<Subscript>20</Subscript> and Ni<Subscript>80</Subscript>Nb<Subscript>20</Subscript>. Crystallization of the amorphous phase

The sequence and crystallization kinetics of the amorphous phase have been analyzed by differential scanning calorimetry and X-ray diffraction for mechanochemically activated Ni₈₀Ta₂₀ and Ni₈₀Nb₂₀ samples. The formation of equilibrium products from the amorphous phase occurs through the formation of metastable products of the A3 type. The kinetic parameters (activation energy and reaction order) and thermodynamic characteristics of this process are determined. Crystallization of the amorphous phase for the Ni₈₀Ta₂₀ and Ni₈₀Nb₂₀ systems occurs due to the growth of existing nuclei through the polymorphic and eutectic mechanisms, respectively. The thermal effects of the synthesis of equilibrium products from a mechanochemically activated mixture of components are identified.     

Phase transition in Pd<Subscript>40</Subscript>Ni<Subscript>10</Subscript>Cu<Subscript>30</Subscript>P<Subscript>20</Subscript> bulk metallic glass under HP &; HT

The phase transitions in Pd₄₀Ni₁₀Cu₃₀P₂₀ bulk metallic glass (BMG) have been studied under high pressure and high temperature (HP & HT) by X-ray diffaction measurements with synchrotron radiation source. We found that the BMG underwent a phase transitions of amorphous-crystalline-amorphous at 10 GPa upon heating. The parallel experiments were carried out at 7 GPa, while we did not observe the amorphous-crystalline-amorphous transitions by increasing temperature. Quenching the melted BMG at 7 GPa, it was found that the phase crystallized from the melt differed from the primary phase crystallized from the starting amorphous solid upon heating suggesting there existed a distinct mechanism in two cases.     

Cascade of phase transitions in GdFe<Subscript>3</Subscript>(BO<Subscript>3</Subscript>)<Subscript>4</Subscript>

Cascade of phase transitions in GdFe₃(BO₃)₄ at 156, 37, and 9 K has been detected by specific heat measurements and further studied by Raman scattering and Nd³⁺ spectroscopic probe method. A weakly first-order structural phase transition at 156 K is followed by a second-order antiferromagnetic ordering phase transition at 37 K and a first-order spin-reorientational phase transition at 9 K.     

Resistive switching in β-SrV<Subscript>6</Subscript>O<Subscript>15</Subscript>

We investigate the pressure and temperature behavior of current-dependent resistivity of β-SrV₆O₁₅. We observe a switching between states of different resistivities in the insulating state of β-SrV₆O₁₅. In the low pressure phase, the resistive switching appears at temperatures below the semiconductor-insulator transition. In the high pressure phase, under ~1.6 GPa, the switching appears in the temperature range of the phase transition. The existence of switching may imply an important role of strontium off-stoichiometry for the electrical transport in β-SrV₆O₁₅. No electric-field-induced enhancement of the conductivity is observed. However, the conduction is significantly nonlinear under ~1.6 GPa, indicating that the charge order pattern in the high pressure phase is considerably different from that of the low pressure phase.