Unannealed Crystals Research Articles

Enhancement of the piezocatalytic properties of BaTiO3 dendrite by annealing modulation

Annealing treatment is believed to improve the catalytic performance of crystal structure and alter the ferroelectric properties of piezoelectric catalysts. However, nano-catalysts easily agglomerate during the calcination process, which lead to a sharp drop in catalytic performance. Herein, we synthesized BaTiO3 dendrite catalyst and modulated its crystal structure and ferroelectric properties via annealing treatment. A diagram of the crystal structure change mechanism at different annealing temperatures was drawn. The results show compared with unannealed BaTiO3 crystals, BaTiO3 crystals exhibit the improved piezocatalytic performance when the annealing temperature is 700–800 ℃, respectively. However, the piezocatalytic activities of the samples decreased with the increase of the annealing temperature over 800 ℃, which is mainly attributed to the random distribution of electric domains in BaTiO3 polycrystalline crystals. This work provides the references for improving the piezocatalyst’s activity via a simple method.

Bridgman growth and luminescence properties of CdMoO4 single crystal

CdMoO4 single crystal was grown successfully by vertical Bridgman method. The as-grown CdMoO4 crystal presents blue-green color due to the existence of oxygen vacancies, while the color will become light even disappeared by annealing in air atmosphere as a result of oxygen introduction. The transmissivity of unannealed and annealed CdMoO4 crystals reached 60% and 78%, respectively. The photoluminescence peak of annealed CdMoO4 is located at 535 nm, and the emission intensity is obviously higher than that of unannealed sample, indicating that the annealing is favorable to improve the photoluminescence intensity. The X-ray excited luminescence of unannealed CdMoO4 crystal is very weak with low signal-noise ratio due to the strong self-absorption phenomena in crystal. For annealed crystal, the X-ray excited luminescence peak is located around 525 nm. The peak deviates about 10 nm wavelength from the peak of photoluminescence spectra due to the much deeper X-ray penetration.

Third-harmonic-generation nonlinear absorption coefficient of 70% deuterated DKDP crystal

The nonlinear optical absorption (NLA) of 70% deuterated DKDP crystals that were cut along different directions and annealed under different temperatures was measured at the third-harmonic-generation (THG) wavelength (355 nm) of a nanosecond Nd:YAG laser by using the Z-scan method. The nonlinear absorption (NLA) coefficient β was obtained by fitting the experimental data. According to the fitting result, the nonlinear absorption at 355 nm is identified to two-photon absorption. Results indicate that the β value of the type I THG direction (5.6 × 10−2 cm/GW) was close to that of the type II THG direction (5.2 × 10−2 cm/GW). Moreover, the β values of both types were obviously below the data of the Z-axis direction (7.2 × 10−2 cm/GW). Our experiments showed that thermal annealing can effectively decrease NLA coefficients. At the optimum annealing temperature of 140 °C, the β of the type II THG sample was 2.4 × 10−2 cm/GW, which was only 46% that of the unannealed crystal. This work will aid the THG application of DKDP crystals in inertial confinement fusion systems.

A comparative study of the electrical properties of reduced and unreduced LiTaO3 crystals

We have carried out a comparative study of the electrical properties of lithium tantalate (LiTaO3) crystals in a wide temperature range (300–900 K) before and after reductive treatment in H2O vapor and subsequent oxidative annealing. The results demonstrate that, in the temperature range of Li+ ion conduction (550–900 K), the activation enthalpy for ionic conduction in the reduced lithium tantalate crystal is H a = 1.37 eV, which slightly exceeds that in the initial state of the crystal (1.34 eV). In the temperature range 390–450 K, the σ(T) data for the unannealed crystal are well represented by the Arrhenius law in the presence of two carrier types, with activation energies E 1 = 1.03 eV and E 2 = 0.29 eV, characteristic of proton and electron hopping conduction, respectively. After reductive annealing, the activation energy for conduction is ~0.65 eV, characteristic of the activation energy for bipolaron conduction. After subsequent oxidative annealing of the reduced crystals in dry air, the activation energy is ~1.2 eV. It seems likely that the presence of oxygen vacancies in the reduced LiTaO3 crystal stimulates hydrogen release from the crystal during oxidative annealing.

Effect of annealing on the electrical properties of SnTe crystals

We have studied the effect of annealing on the electrical conductivity σ and thermoelectric power α of SnTe crystals. The results indicate that the temperature dependences of σ and α for both the unannealed and annealed SnTe crystals, as well as the effect of annealing on these parameters, are adequately accounted for in terms of a model that considers two valence bands separated by an energy gap.

Influence of annealing on the photodeposition of silver on periodically poled lithium niobate

The preferential deposition of metal nanoparticles onto periodically poled lithium niobate surfaces, whereby photogenerated electrons accumulate in accordance with local electric fields and reduce metal ions from solution, is known to depend on the intensity and wavelength of the illumination and the concentration of the solution used. Here, it is shown that for identical deposition conditions (wavelength, intensity, concentration), post-poling annealing for 10 h at 200 °C modifies the surface reactivity through the reorientation of internal defect fields. Whereas silver nanoparticles deposit preferentially on the +z domains on unannealed crystals, the deposition occurs preferentially along 180° domain walls for annealed crystals. In neither case is the deposition selective; limited deposition occurs also on the unannealed –z domain surface and on both annealed domain surfaces. The observed behavior is attributed to a relaxation of the poling-induced defect frustration mediated by Li+ ion mobility during annealing, which affects the accumulation of electrons, thereby changing the surface reactivity. The evolution of the defect field with temperature is corroborated using Raman spectroscopy.

Resistivity of Ba(Fe1−xCox)2As2: Evidence for a broad composition range of non-Fermi-liquid behavior

The resistivity vs temperature of annealed and unannealed single crystals of Ba(Fe${}_{1\ensuremath{-}x}$Co${}_{x}$)${}_{2}$As${}_{2}$ over a broad range of nominal compositions between $x$ $=$ 0.05 and 0.30, with special focus on compositions around that (${x}_{\mathrm{opt}}$) of the optimal superconducting transition temperature, is reported. Above any transition temperature, either superconducting (${T}_{c}$) or structural (${T}_{S}$) for $x$ ${x}_{\mathrm{opt}}$, the low temperature resistivity, \ensuremath{\rho}, behaves approximately linearly with temperature from $x$ $=$ 0.05 up to at least $x$ $=$ 0.095, with $\ensuremath{\rho}\ensuremath{\propto}{T}^{2}$ starting for $x$ $=$ 0.14. The apparent constancy of the temperature dependence over a broad composition range argues strongly for the presence of a quantum critical point (QCP) underneath the superconducting dome. However, the temperature dependences measured down to ${T}_{c}$ leave the exact location of the QCP open. Interestingly, although the superconducting transition temperature rises by \ensuremath{\approx}1.5 K with annealing, the temperature dependence of \ensuremath{\rho} appears unaffected by annealing, i.e., the QCP appears insensitive to small amounts of lattice disorder.

Kinetics of nucleation during the bcc-hcp structural transition in solid helium

The nucleation kinetics of the hcp phase in the supercooled bcc phase of solid 4He and a 3He-4He solution containing 1% 3He is studied experimentally. Precision measurements of the pressure in the crystal are made as it is cooled along an isochore near the bcc-hcp phase equilibrium curve. The lifetime of the metastable bcc phase in 4He is found to have a complicated dependence on the level of supercooling which can be described in terms of two nucleation mechanisms—homogeneous and heterogeneous. The homogeneous mechanism occurs for the highly supercooled bcc phase in carefully annealed crystals, while the heterogeneous mechanism, for which the activation coefficients are several orders of magnitude lower than for the homogeneous mechanism, occurs with low supercooling. The influence of defects and impurities on the nucleation process is also studied. In unannealed (defective) 4He crystals the nucleation rate is found to have an enormous scatter, which is indicative of poor reproducibility from one crystal to another. For annealed samples of 3He-4He solution the nucleation kinetics data can be described only by the heterogeneous nucleation mechanism.

Improving the Laser-Induced-Damage Tolerance Characteristics of 4-Dimethylamino-N-methyl-4-stilbazoliumtosylate Crystals for THz Wave Generation by Annealing

We demonstrated that annealing 4-dimethylamino-N-methyl-4-stilbazoliumtosylate (DAST) crystals near their melting point improves their tolerance to laser-induced damage. Their laser-induced-damage tolerance characteristics were evaluated and confirmed using difference-frequency THz generation. The DAST crystals employed in this study were all approximately the same size. THz wave output did not decrease in DAST crystals that were about 80% annealed. Microscopy images revealed that the crystallinity and orientation of the DAST crystal were improved by annealing. Moreover, THz wave output in the DAST crystal plane was uniformized by annealing. Further, annealed DAST crystals with superior laser-induced-damage tolerance produced 10 times greater THz wave output at a power density of 1.5 GW/cm2 than did unannealed crystals at a much greater power density. Therefore, the laser-induced-damage tolerance characteristics of DAST crystals were successfully improved by annealing.

Low temperature nuclear orientation observations of severe neutron activation damage in ( 160 Tb)TbNiAl 4

An unusually large attenuation (approximately 500%) in the low temperature nuclear orientation directional gamma anisotropy from unannealed thermal neutron irradiated single crystal (160Tb)TbNiAl4 has been observed in the region of saturation of the second order nuclear orientation coefficient, B2. It is suggested that the form of the remanent neutron activation recoil damage in unannealed (160Tb)TbNiAl4 is fundamentally electric quadrupolar in origin, as opposed to magnetic dipolar.

X-ray Diffraction Study of the Structure and Defect System of Nominally Pure and Er3+- and Ce3+-Activated (Na0.5La0.5)MoO4 Crystals

Nominally pure and activated (Er3+ and Er3+ + Ce3+) sodium lanthanum molybdate single crystals grown by the Czochralski technique from the (Na0.5La0.5)MoO4 melt in different environments and then oxidized in air at 100°C are characterized in detail by x-ray diffraction (lattice parameters and structure refinement by the Rietveld method and single-crystal techniques). The results confirm that (Na0.5La0.5)MoO4 crystallizes in a tetragonal scheelite structure (sp. gr. I 1/a). The crystals (especially unannealed crystals grown in neutral atmosphere) are shown to contain oxygen vacancies. In addition, some of the samples contain Mo vacancies. The Er3+ distribution over some of the activated crystals is highly inhomogeneous. As a result, the crystals contain Er-enriched zones with a distorted scheelite structure.

Dynamics of green light-induced infrared absorption in KTiOPO4 and periodically poled KTiOPO4

Induced thermal-lens spectroscopy with a common-path interferometer has been used for high-sensitivity measurements of green-light induced absorption dynamics in single-domain and periodically poled KTiOPO4. Although the linear infrared absorption in both materials is the same, the saturated green-light induced absorption is consistently higher in periodically poled crystals. Initial relaxation of the induced absorption is also different in these materials where periodically poled KTP does not exhibit as fast initial relaxation characteristic as the single domain KTP. These differences in the absorption are explained by higher concentration of native defects associated with weakly bonded K+ ions, which stabilize electron and hole traps forming color centers in the material. Unannealed as-poled crystals also exhibit characteristic overshoot dynamics in the induced thermal lens signal, which signifies remnant stress in the domain walls. Investigations showed that the lattice vibration energy generated by the dissociation of induced color centers is sufficient for complete relaxation of this remaining stress. The sensitivity of the measurement technique is demonstrated by the measurement of small signal absorption in the blue spectral region arising during continuous wave quasi-phase-matched second harmonic generation in periodically poled KTP.

CHEVKINITE-(Ce): CRYSTAL STRUCTURE AND THE EFFECT OF MODERATE RADIATION-INDUCED DAMAGE ON SITE-OCCUPANCY REFINEMENT

The crystal structure of annealed chevkinite-(Ce), ideally (REE,Ca)4 Fe 2+ (Ti,Fe 3+ ,Nb)4 Si4 O22, monoclinic, C2/m, crystal (2 an ) a 13.400(1), b 5.7232(4), c 11.0573(9) A, � 100.537(2), V 833.7(2) A 3 and crystal (3 an ) a 13.368(2), b 5.7243(6), c 11.041(1) A, � 100.516(1), V 830.7(2) A 3 ; Z = 2, from Mongolia, has been solved by direct methods and refined to R1 values of 6.3 and 5.7% using 1287 and 1180 unique (|Fo| > 4� F) reflections collected with a single-crystal diffractometer and MoK� radiation. Electron-microprobe analysis gave: crystal (2 an ) SiO2 18.81, TiO2 15.66, FeO 11.33, MnO 0.24, CaO 1.61, Nb2O5 1.55, ThO2 0.40, Y2O3 0.30, Ce2O3 23.45, La2O3 13.92, Nd2O3 6.88, Pr2O3 1.95, Sm2O3 0.51, Gd2O3 0.42, HfO2 0.01, ZrO2 0.28, sum 97.32 wt.%, and crystal (3 an ) SiO2 18.53, TiO2 16.11, FeO 12.62, MnO 0.31, BaO 0.02, CaO 1.21, Nb2O5 1.69, ThO2 0.10, Y2O3 0.30, Ce2O3 24.30, La2O3 12.94, Nd2O3 8.13, Pr2O3 2.44, Sm2O3 0.82, Gd2O3 0.36, HfO2 0.16, ZrO2 0.08, Ta2O5 0.03, WO3 0.02, sum 100.17 wt.%. The corresponding chemical formulae are: (2 an ): (Ce1.85 La1.11 Nd0.53 Pr0.15 Sm0.04 Gd0.03 Ca0.32 Th0.02)� 4.05 (Fe 2+ 0.85 Mn 2+ 0.04 Zr0.03 Y0.03 Ca0.05)� 1.00 (Ti2.54 Fe 3+ 1.19 Nb0.15)� 3.88 Si4.06 O22 and (3 an ): (Ce1.88 La1.01 Nd0.61 Pr0.19 Sm0.06 Gd0.03 Ca0.27 Y0.03 Th0.01)� 4.08 (Fe 2+ 0.93 Mn0.06 Zr0.01)� 1.00 (Ti2.55 Fe 3+ 1.29 Nb0.16)� 4.00 Si3.91 O22 based on 13 cations per formula unit. Infrared spectra of unannealed crystals suggest significant radiation-induced damage. The structures of two unannealed crystals were refined, but there are major discrepancies (~13%) between the sums of the site-scattering values at the octahedrally coordinated M sites and the analogous values calculated from the chemical compositions determined by electron-microprobe analysis. These discrepancies were not present when data were collected on annealed crystals, indicating that the original crystals are partly metamict and that this can significantly affect the results of site-scattering refinement. In the crystal structure of chevkinite, there are two distinct (SiO4) tetrahedra that share one vertex to form an [Si2O7] group: = 1.614 (2 an ) and 1.612 (3 an ) A, = 172.0 (2 an ) and 170.6 (3 an ). There are four octahedrally coordinated M sites that are occupied by small- to medium-sized divalent to pentavalent cations. The M(1) site is occupied dominantly by Fe 2+ with minor Mn 2+ , and the M(2), M(3) and M(4) sites are occupied by Ti, Fe 3+ and Nb. There are two A-sites that are occupied by REE and minor Ca and Th, with = 2.56 (2 an ) and 2.55 (3 an ) ([8]-coordinated) and 2.66 (2 an ) and 2.55 (3 an ) A ([10]-coordinated). There are two crystallographically distinct rutile-like chains of octahedra ([M(3) + M(4)] and [M(2) + M(2)]) that extend in the b direction and link to form a layer of octahedra parallel to (001). The layers of octahedra link through [Si2O7] groups to form a framework with two interstitial A sites, CN = [8] and [10], in which Ce is dominant over other REE, Ca and Th.

Donor–acceptor pair photoluminescence spectra analysis in CdTe:Ag

Photoluminescence (PL) spectra of CdTe single crystals and films, undoped and Ag-doped, were studied in the range of the photon energy (hν): 1.5–1.6 eV, at 10 K. Films were doped during the growth process. Thin layers of Ag were vacuum evaporated onto the surface of undoped CdTe crystals, then were submitted to thermal annealing in a N2+2% H2 ambient at 400 °C for 3 h. PL spectra of crystals exhibit a donor–acceptor pair (DAP) emission at hν=1.491 eV (named here DAPAg2) with the highest intensity with respect to other signals. The PL of films is characterized by a peak at 1.5 eV followed by several phonon replicas. In both cases, the DAP emissions are due to the Ag-impurity presence and are accompanied by phonon replicas. Unannealed intrinsic crystals exhibit a unique broad DAPAg1 peak at hν=1.501 eV, and one month aged at room temperature Ag-doped crystals only display a wide DAPAg2 peak at hν=1.491 eV with 65% of its initial intensity, that reflects a reduced density of Ag diffused-doping levels. The origin of DAP lines in a PL spectra of CdTe:Ag has not been completely well defined until now, and we have introduced some explanatory conclusions about and a simple model for electronic transitions. This issue is important because Ag is an everpresent residual impurity in CdTe.

The influence of annealing at different temperatures and atmospheres on the optical rotation of Bi12SiO20 crystals

Abstract The optical activity changes in Bi 12 SiO 20 (BSO) crystals induced by thermal annealing treatments are studied. It is found that the annealing in vacuum and hydrogen atmosphere causes decreasing of the optical rotation. Thermal treatment annealing induces reduction of Bi-ions to metallic bismuth on the surface as well as oxygen and bismuth diffusion and evaporation is supposed. Annealing in oxygen leads to partial recovery of the oxygen content in the crystals and the optical activity values tends to approach the level of un-annealed BSO crystal.

Typical Electron Beam Doping (Superdiffusion) of Impurity Atoms in Damage-Free Regions of Semiconductors by the Kick-Out Mechanism

Experiments involving 750 keV electron beam doping (EBD) of Si and Zn atoms into GaAs were performed for systems of GaAs (layer 3)/Si(Zn)//Si(Zn)/GaAs (layer 1). The Si(Zn)/GaAs consists of Si(Zn) evaporation-deposited layers on GaAs wafers. The overlying layer is in contact only with one other Si(Zn) layer. The surface of layer 3 (GaAs) was irradiated with a fluence of (∼3.7–5.0)×1017 electrons·cm-2 at 750 keV and at 100°C in a N2 gas atmosphere at a mean current density of 8.1 µA·cm-2 using a Van de Graaff accelerator. After irradiation, the remaining unreacted Si and Zn layers were carefully removed by chemical etching. It was confirmed by Auger electron spectroscopy that no Si or Zn layer remained on the wafer after etching. The unannealed GaAs crystals were measured by secondary ion mass spectrometry (SIMS) and photoluminescence (PL). The diffusion depth of Zn atoms increased with the thicknesses of the deposited layers.

SIMS analysis and optical study of annealed iron-doped LiNbO 3 crystal

The distribution of iron content along the depth of annealed LiNbO 3 substrates has been measured by the secondary ion mass spectrometry (SIMS). It is found that Fe out-diffuses to the surface of annealed LiNbO 3 crystals, causing a high content of Fe in a thin surface layer. Planar waveguides have been fabricated from both unannealed and annealed crystals by the proton exchanged method, and two-wave mixing experiments have revealed an enhanced photorefractive effect in the waveguide of the annealed LiNbO 3 crystal.

A new bound-exciton emission band in ZnSe crystals and multiplasmon optical transitions

We have investigated cathodoluminescence both in unannealed ZnSe crystals and in crystals annealed in a Bi melt at a temperature of 1200K for 120 h with subsequent quenching. In the wavelength range 450–480 nm we have detected a new line series I i s -nLO-mPl consisting of the bound-exciton emission line I i s with wavelength λ=455.9 nm and its plasmon and LO-phonon echoes I i s -LO (λ1=461.3 nm), I i s -2LO (λ2=466.8 nm), I i s -3LO (λ3=472.4 nm), and I i s -4LO (λ4=478.3 nm). We have determined the mean number of emitted LO phonons NLO=2.2±0.1 per photon. It is shown that the observed finer structure of the band may be due to multiphonon optical transitions. At low plasma densities (ω p ≪ω LO ) the Coulomb interaction causes broadening of the I i s -nLO series. In samples with denser plasma, in which the condition ω p ⩽ω LO is met, multiplasmon satellites of the series I i s -nLO-mPl are observed. Theoretical calculations of the shape of the emission band agree with experiment.

Annealing effects on organic ferromagnet C 60 - TDAE

The annealing effects on the magnetic and structural properties for organic ferromagnet C 60 - TDAE are reported. The ESR line-width for unannealed single crystal shows a large hysteresis behavior around 130K. For the well-annealed crystal, on the other hand, the line-width continuously changed with temperature. In addition, the room temperature structural analysis of the single crystal indicated that all the crystal axes, especially c-axis, shrunk after annealing.

Formation of 4H and 8H polytypes in bulk Zn 1− xMg xSe crystals

X-ray diffraction, photoluminescence and high-resolution transmission electron microscopy (HRTEM) were used to show that several as-grown Zn 1− x Mg x Se crystals prepared by the high-pressure Bridgman method exhibit polytypism for x in the range from 0.17 to 0.21. The largest contribution of polytypes is found in two unannealed crystal plates characterised by x=0.17 (polytype 8H) and 0.18 (polytype 4H). Crystal domains with disordered stacking were also found. A small quantity of 4H or 8H is detected in other sphalerite type crystals of various x values lower than 0.17 and for wurtzite type crystals for x from 0.18 up to about 0.25. Exciton transition energy for the given Mg content depends on the polytype. With Mg content increasing along the crystal, the polytype sequence observed at diffractograms (in agreement with HRTEM observations and with luminescence spectra) is 3C→8H→4H→2H. The presence of higher polytypes (4H and 8H) depends on thermal history of the crystal growth. Annealing of the as-grown crystals containing these two polytypes results in full or almost full transformation to 3C, 2H or their mixture, depending on the Mg concentration..