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This paper investigates the possibilities of creating magnetic field sensors using the direct magnetoelectric (ME) effect in a monolithic heterostructure of amorphous ferromagnetic material/langatate. Layers of 1.5 μm-thick FeCoSiB amorphous ferromagnetic material were deposited on the surface of the langatate single crystal using magnetron sputtering. At the resonance frequency of the structure, 107 kHz, the ME coefficient of linear conversion of 76.6 V/(Oe∙cm) was obtained. Furthermore, the nonlinear ME effect of voltage harmonic generation was observed with an increasing excitation magnetic field. The efficiency of generating the second and third harmonics was about 6.3 V/(Oe2∙cm) and 1.8 V/(Oe3∙cm), respectively. A hysteresis dependence of ME voltage on a permanent magnetic field was observed due to the presence of α-Fe iron crystalline phases in the magnetic layer. At the resonance frequency, the monolithic heterostructure had a sensitivity to the AC magnetic field of 4.6 V/Oe, a minimum detectable magnetic field of ~70 pT, and a low level of magnetic noise of 0.36 pT/Hz1/2, which allows it to be used in ME magnetic field sensors. [ABSTRACT FROM AUTHOR]

The luminescent properties of Ca3TaGa3Si2O14 (CTGS, catangasite) single crystals have been studied by means of the vacuum ultraviolet excitation spectroscopy utilizing synchrotron radiation from 1.5 GeV storage ring of MAX IV synchrotron facility. Two emission bands at 320 nm (3.87 eV) and 445 nm (2.78 eV) have been detected. Examining excitation spectra in vacuum ultraviolet spectral range, the 320 nm emission band was explained as the emission band of self-trapped exciton in CTGS single crystal. Its atomic structure is discussed. It is also proposed that the 445 nm (2.78 eV) emission in the CTGS is due to the F centers, which have shown a well-resolved excitation (absorption) band at 5.1 eV (243 nm). [ABSTRACT FROM AUTHOR]

We performed a comprehensive study of the properties of La 3 Ga 5.5 Ta 0·5 O 14 crystals grown in different atmospheres before and after the post-growth treatments (annealing in air and vacuum) using different methods to clarify the origin of defects. The optical properties and elemental composition of the crystals depend on the growth atmosphere and the atmosphere of annealing. The absorption spectra of the colored crystals contain two strong broad bands in the UV spectral region (λ max ∼290 and λ max ∼370 nm), one broad band in the visible range (λ max ∼480 nm). The spectra of the colorless crystals show a weak band at λ max ∼290 nm. The data obtained by X-ray photoelectron spectroscopy and energy dispersive X-ray spectroscopy indicate that there is a difference in the ratio of the main elements in dependence on the growth atmosphere and lack of gallium is observed at any growth atmosphere. An analysis of X-ray diffuse scattering indicates that the predominant type of defects is vacancy-type. There are more vacancy-type defects in crystals grown in Ar + O 2 atmosphere than in Ar. On the base of the results of our study, we propose a self-consistent model of the defect origin in LGT crystals. The main defects are the Schottky-type defects. With add of oxygen in the growth atmosphere, F-centers are formed. During annealing in air, the concentration of the F-centers increases, and annealing in a vacuum leads to the destruction of F-centers. • La 3 Ga 5.5 Ta 0·5 O 14 optical properties depend on growth and annealing atmospheres. • Defect structure and composition of La 3 Ga 5.5 Ta 0·5 O 14 depend on growth atmosphere. • The model of defect formation in La 3 Ga 5.5 Ta 0·5 O 14 is presented. • The main defects are the Schottky-type defects, coloration is referred to F-centers. [ABSTRACT FROM AUTHOR]

The ultrasound attenuation in langasite crystals increases rapidly at about 800 K with increasing temperature for reasons that are not well understood. In this paper, the attenuation quantified as $Q^{-1}$ of the langasite-type materials La3Ga5SiO14 (LGS) and La3Ta0.5Ga5.5O14 (LGT) was studied from room temperature to 1653 and 1608 K, respectively, using resonant ultrasound spectroscopy. Two to three attenuation peaks can be seen. A change of the magnitudes of the largest two attenuation peaks in LGT was correlated with the changing color of an LGT sample, which is related to its oxygen vacancy concentration. Thus, the attenuation likely involves oxygen vacancies. The observed $Q^{-1}$ can be explained well by a model based on the anelastic relaxation of two to three noninteracting point defects causing Debye peak-like attenuation maxima. The activation energies of the largest two relaxation peaks match the activation energies for different conductivity mechanisms in LGS and LGT. Thus, the oxygen movement-based conductivity and the relaxation processes seem to involve the exchange of ions and vacancies on the same positions. The largest two attenuation peaks are probably caused by the movement of ions induced by two different phenomena, the deformation of the lattice (point-defect relaxation) on the one hand and the electric field via the piezoelectric effect (piezoelectric/carrier relaxation) on the other hand. [ABSTRACT FROM AUTHOR]

LaGaTaO crystals were grown in pure argon and in an argon + 2% oxygen atmosphere. The growth atmosphere significantly effects the elemental composition of the crystals, their color, and the genesis and intensity of absorption bands in the ultraviolet, visible, and infrared spectral regions. Two types of defects were identified in the crystals: planar interstitial defects and nearly spherical vacancy-type defects. [ABSTRACT FROM AUTHOR]

This contribution reports the hydration and electrical transport properties of effectively acceptor doped single crystalline and polycrystalline langatate, La 3 Ga 5.5 Ta 0.5 O 14 . The electrical properties are investigated over wide ranges of p H 2 O , p D 2 O and p O 2 in the temperature range 400 to 1000 ° C. Acceptor doped langatate is dominated by oxygen vacancies in dry atmospheres and at high temperatures, and by protonic defects in wet atmospheres and at lower temperatures. The corresponding standard hydration enthalpy and entropy are − 90 ± 5 kJ/mol and − 130 ± 5 J/mol K, respectively. Further, all compositions display pure proton conductivity in wet atmospheres below 700 ° C with a proton mobility enthalpy in the range of 70–75 kJ/mol, depending on doping level and crystallographic direction. Hence, protons are important for the physiochemical properties of langatate even at 1000 ° C, and could therefore influence the behavior of langatate-based resonator devices. The proton conductivity is slightly anisotropic, being higher in the X- and Y- than in the Z-direction. At high temperatures and under dry conditions, electron holes and oxide ions dominate the conductivity, and the enthalpy of mobility of vacancies is 140 ± 5 kJ/mol. [ABSTRACT FROM AUTHOR]

A Judd–Ofelt analysis is performed on ceramic samples of erbium-doped langatate (La 3 Ga 5.5 Ta 0.5 O 14 – LGT). The intensity parameters are calibrated using the lifetime of the radiative-decaying 4 I 13/2 level. The influence of the light scattering on the absorption spectra of Er:LGT and implicitly on the intensity parameters is investigated by changing the sample thickness. The value of the Ω 2 parameter was found to be more sensitive to light scattering effects, while Ω 6 is the most insensitive to such effects. The quantum efficiency of the ( 2 H 11/2 , 4 S 3/2 ) and 4 F 9/2 levels was calculated; their values are only slightly distorted in the presence of light scattering. The possibility of erroneous interpretations of the Judd–Ofelt parameters due to light scattering effects is pointed out. [ABSTRACT FROM AUTHOR]

La 3 Ga 5,5 Ta 0,5 O 14 (LGT) langatate crystals were grown along Z-axis by Czochralski technique in argon and mixed argon with 0.1%O 2 atmosphere. The coloration and the performance of langatate crystals were strongly connected to the starting chemical composition, the gas atmosphere and the growth parameters. Any deviation from the optimal LGT generate macroscopic defects such cracks and grains boundary causing a deterioration of the crystals performance. [ABSTRACT FROM AUTHOR]

A Judd–Ofelt analysis was performed on a langatate (La 3 Ga 5.5 Ta 0.5 O 14 ) translucent ceramic sample doped with 1 at% Tm 3+ . Due to the scattering of the transmitted light, the effective thickness of the sample is unknown and the absorption spectrum of Tm 3+ in langatate ceramic sample is obtained in relative units. The absorption spectrum was calibrated using the magnetic-dipole contribution to the 3 H 6 → 3 H 5 transition. The Judd–Ofelt parameters obtained are: Ω 2 =2.65×10 –20 cm 2 , Ω 4 =0.80×10 –20 cm 2 , Ω 6 =1.86×10 –20 cm 2 . The electric-dipole and magnetic-dipole transition probabilities, radiative lifetimes and luminescence branching ratios were calculated. The calibration allowed the estimation of the ‘effective’ thickness of the ceramic sample which was approximately four times larger than the geometric thickness. [ABSTRACT FROM AUTHOR]

Ultraviolet (365 nm, ¹D2→³H6), blue (480 nm, ¹G4→³H6), red (650-660 nm, ¹G4→³F4+³F2,3→³H6), and infrared (800 nm, ³H4→³H6) upconversion luminescence, were obtained for the first time in La3Ga5.5Ta0.5O14:Tm:Yb ceramic sample, synthesized by solid-state reaction, pumped at 973 nm. Yb3+→Tm3+ energy transfer processes accounting for population of the emitting levels of Tm3+ are discussed. Also, the back-transfer process Tm3+→Yb3+ was observed. The decay curves of the metastable levels ¹D2, ¹G4, ³H4, and ³F4 of Tm3+ and ²F5/2 of Yb3+ were analyzed. The cross-relaxation microparameters of Tm3+ were found. The efficiency of the Yb3+→Tm3+ transfer was estimated. [ABSTRACT FROM AUTHOR]

Lanthanum-gallium tantalate single crystal (La3Ta0.5Ga5.5O14, langatate, LGT) is a perspective piezoelectric material as an active component of pressure sensors. An investigation of the growth conditions influence (the growth atmosphere) on the electrophysical characterization of LGT, obtained in different atmospheres (Ar, Ar + O2) was carried out. The frequency dependences of the relative dielectric constant (ε11/ε0) and of the admittance depend on the growth atmosphere. The langatate electrophysical ?haracterization in alternating electric fields were analyzed by means of the impedance spectroscopy method. The behavior of short circuit currents in specimens of polar cuts of LGT single crystals with the same material electrodes without preliminary polarization is described. [ABSTRACT FROM AUTHOR]

• A large LTG crystal was successfully grown with the Czochralski method in air. • The as-grown LTG crystal possesses great qualities of crystalline perfection, piezoelectric performance and thermal properties. • The electrical resistivity of LTG was improved to 2.45 *108 Ω•cm at 500°C after being annealed in Ar at 1100°C for 10 h. • The obtained high-quality large LGT crystal can be used in the field of high-temperature piezoelectric sensors. Large, 60 mm in diameter langatate (La 3 Ta 0.5 Ga 5.5 O 14 , LTG) crystals were grown with the Czochralski method in air. The full width at half maximum (FWHM) value of the rocking curve is 39.6″, and the piezoelectric coefficient d 11 value is 7.1 pC/N. The as-grown LTG crystal possesses excellent thermal properties, and the electrical resistivity at high temperatures (400∼600°C) was measured. After being heat-treated in Ar at 900, 1100, and 1300°C, the electrical resistivity increases evidently, and a La-rich and Ga-poor layer is observed on the surface of LTG. The LTG sample annealed in Ar at 1100°C for 10 h shows the highest electrical resistivity, which is up to 2.45*108 Ω⋅cm at 500°C. The content of oxygen vacancies of LTG increased after the annealing procedure, and the defect model was used to explain the increase in high-temperature electrical resistivity. The obtained high-quality large LTG crystal can be used in the field of high-temperature piezoelectric sensors. [Display omitted] [ABSTRACT FROM AUTHOR]

Langatate (LGT) crystals of La 3 Ga 5.5 Ta 0 · 5 O 14 composition of diameter 50 mm were grown from the melt by Czochralski technique. Using (1–2 wt %) Ga 2 O 3 excess in the starting charge and growing crystal in mixture argon (0.1–1%O 2) gas atmosphere are a good condition to crystallize LGT under stationary stable regime. The LGT crystals grown along Z-axis exhibit strong faceting. The grown crystals were exempt of inclusions, cracks and secondary phases. The presence of oxygen in the growth chamber is necessary to limit gallium oxide evaporation and strongly affect the crystals coloration and the transmission spectra in the range (200–500 nm). The electrical resistivity is sensitive to the oxygen content in the growth environment. • Under stationary stable regime, large LGT (f=50 mm) crystals were grown from the melt by Czochralski technique. • The Ga 2 O 3 losses from the melt and high oxygen concentration (≥1%) in the growth chamber are source of defects. • Crystals coloration is strongly affected by the amount of oxygen in the growth environment. • The transmission spectra and the electrical resistivity depend on the oxygen concentration. [ABSTRACT FROM AUTHOR]

Abstract: Violet (410nm, 2 H 9/2→4 I 15/2) and near-ultraviolet (380nm, 4 G 11/2→4 I 15/2) upconversion luminescence, were obtained for the first time in Er:Yb:La3Ga5.5Ta0.5O14 ceramic sample, synthesized by solid-state reaction, by pumping at 973nm. Yb3+–Er3+ energy transfer processes accounting for population of the emitting levels are discussed. The lifetime of the emitting level 2 H 9/2, responsible for violet luminescence, was measured and its quantum efficiency was estimated. [Copyright &y& Elsevier]

Abstract: Bright green (at 525 and 550nm) and red (at 660nm) luminescence in Er:Yb:La3Ga5.5Ta0.5O14 (LGT) powder synthesized by solid state reaction was obtained by pumping at 936nm. Yb3+–Er3+ energy transfer processes accounting for population of the 2H211/2, 4S3/2 and 4F9/2 Er3+ levels are discussed. The dependence of ratio between the intensities of the green and red luminescence on pump intensity is analyzed. The rather high quantum efficiency (58%) of the (4S3/2, 2H211/2) Er3+ emitting level recommends LGT doped with erbium and ytterbium for upconversion applications. [Copyright &y& Elsevier]

Abstract: Single crystalline langatate (La3Ta0.5Ga5.5O14, LTG) has been widely used in piezoelectric sensors for high temperature applications because of its structural stability at high temperature. Ionic conduction originated from oxygen vacancy diffusion has been reserved at high temperature regime (over 700°C). Similarly, an increase of electrical conductivity has been also observed at the intermediate temperature region ranges from 300 to 700°C. Thus, it can be presumed that spontaneously generated point defects influence resistivity degradation of the crystal. In this study, to elucidate the oxygen vacancy effects on electronic structures and transport properties of LTG, formation energy curves of three different states of oxygen vacancy (V•• O, V• O and V× O) and their electronic structures and transport properties, such as carrier concentration and electrical conductivity were calculated by utilizing first principles calculations. The calculated band gaps by the GW method were 5.36eV, 4.34eV and 4.66eV for defect-free LTG, LTG with V•• O and V× O, respectively. Also, the GW result showed higher transition level of oxygen vacancy compared with the transition level calculated from the conventional DFT method. Although, in a previous experimental result, Ga loss was observed in Langasite (LGS) crystal, however, the formation energy of VO was much lower than the formation energy of VGa in LTG. At 900K, the calculated electrical conductivity of LTG with two stable oxygen vacancy (V•• O and V× O) were ten times higher than the conductivity of defect-free LTG around the Fermi levels. Therefore, at intermediate temperature, oxygen vacancy generation can cause an increase of electrical conductivity. [Copyright &y& Elsevier]

The optical transmission spectra and electrical properties of langasite (LaGaSiO) and langatate (LaTaGaO) single crystals are studied experimentally in relation to the oxygen concentration in an argon-oxygen atmosphere of growth. The variation of langatate conductivity with temperature under a constant electric field is examined over the range 20 to 500°C. Its dependence on current-carrying material is investigated for an Ir, Au/Ti, or Ag/Cr coating. [ABSTRACT FROM AUTHOR]

Abstract: The spectroscopic properties of two partially disordered crystals langasite (LGS) and langatate (LGT) doped with Eu3+ were investigated. The fluorescence lifetimes of 5D0 and 5D1 excited levels are measured. The spectroscopic figures of merit R 0, R 2 (the areas of the electric-dipole transitions 5D0 → 7F0 and, respectively, 5D0 → 7F2 divided by the area of the magnetic-dipole one –5D0 → 7F1) and the maximum splitting of the 7F1 level (ΔE) are calculated from the luminescence spectra. The spectral overlapping between the luminescence from 5D0 and 5D1 levels was eliminated using pulsed excitation at 532nm and suitable delays. We obtained R 0(LGS)> R 0(LGT), R 2(LGS)< R 2(LGT) and ΔE (LGS)>ΔE (LGS). Possible reasons (covalency, J-mixing effects) for these inequalities are discussed. [Copyright &y& Elsevier]

Abstract: The nature of luminescence centers formed by Eu3+ in the partially disordered crystal langatate (La3Ga5.5Ta0.5O14) is investigated using fluorescence spectroscopy. In the low-temperature fluorescence spectra three nonequivalent luminescent centers can be evidenced. This fact can be explained considering the perturbations induced at the Eu3+ site by the nonequivalent configurations produced by Ga3+ and Ta5+ randomly placed in the nearest two octahedral positions. [Copyright &y& Elsevier]

This paper reports predicted and measured properties of the pure shear horizontal (SH) mode for the LGX family of crystals, which includes langasite (LGS), langanite (LGN), and langatate (LGT). These crystals are of the trigonal class 32 group, as quartz, and they exhibit the SH symmetry type uncoupling for the Euler angles (0°, θ, 90°). This surface acoustic mode, also known as surface transverse wave (STW), is especially attractive for liquid sensing due to the moderate damping observed in liquid or viscous environments. Numerical and experimental propagation data presented for the SH mode on LGX (0°, θ, 90°) includes phase velocity (vp), electromechanical coupling coefficient (K2), temperature coefficient of delay (TCD), fractional change in frequency with respect to temperature (Δf/fo), penetration depth, metal strip reflectivity, and excitation of spurious plate modes as a function of θ. High electromechanical coupling and zero temperature coefficient of delay (TCD) along LGX Euler angles (0°, θ, 90°), θ between 10° and 25°, with penetration depths comparable to surface acoustic wave (SAW) devices are disclosed. In particular, along LGT (0°, 13.5°, 90°), the experimental results reported with resonators and delay line structures verify the high electromechanical coupling (0.8%) for a SH SAW mode, about 10 times stronger than the 36° Y rotated quartz SH orientation, and the existence of zero TCD around 140°C. The phase velocity of 2660 m/s is within 0.2% of the calculated value, which is about 55% below the phase velocity of 36° Y quartz, thus leading to smaller STW devices. The penetration depth of 6.5 wavelengths is eight times more shallow than 36° Y quartz, thus providing significant SH mode energy trapping close to the surface. With such positive predicted and measured coupling and propagation characteristics, these orientations are appropriate for the fabrication of high coupling, zero TCD, smaller, and highly sensitive STW devices for filtering, frequency control, and liquid sensor applications. [ABSTRACT FROM PUBLISHER]