Crossover effects in scheelite DyLiF4

The Zeeman effect, magnetization, and differential susceptibility of a DyLiF₄ crystal in a pulsed magnetic field are studied experimentally and theoretically. It is found that Dy³⁺ ion levels in DyLiF₄ approach each other and a crossover occurs in a magnetic field H ‖ [001], which leads to a smearing of peaks in the differential magnetic susceptibility dM/dH and to inflection points in the magnetization curves M(H) at low temperatures. It is demonstrated that magnetic anomalies that accompany the crossover in DyLiF₄ in a field H ‖[001] are sensitive to the electronic structure of the Dy³⁺ ion. Therefore, these anomalies can be used to refine the crystal-field parameters. The effects of variations in the crystal field and temperature and of a deviation of the direction of the magnetic field from the symmetry axis on the magnitude and character of the magnetic anomalies associated with the crossover are investigated. The crystal field and crossover effects in the scheelite structure are compared with those in the zircon structure.     

Low-temperature group velocity and elasticity in CaWO4

The CRESST experiment looks for evidence of dark matter particles colliding with nuclei in CaWO₄, using cryogenic bolometers sensitive to energy deposition (～10 keV) with high accuracy. Calibration of the energy deposited in the phonon system depends upon the details of the evolution of the non-equilibrium energy in the CaWO₄ absorber. Phonon images sensitively reveal variations in angular phonon flux in a single-crystal sample and provide accurate data for the group velocity along non-symmetry directions. Our measurements, over a wide range of propagation angles, provide new and precise values for phonon group velocities and uncover significant discrepancies between the experimentally observed transport of non-equilibrium energy and that predicted using values of the low-temperature elastic constants widely cited in the literature.     

Synthesis and tunable luminescence properties of monodispersed sphere-like CaWO4 and CaWO4:Mo/Eu,Tb

Monodispersed CaWO₄ and CaWO₄:Mo/Eu, Tb microspheres have been obtained hydrothermally with excellent reproducibility. X-ray diffraction, field-emission scanning electron microscopy and photoluminescence were used to characterize the as-synthesized samples. The results uncover the formation process of the microspheres and the influence of ethylenediamine. Doping proper concentration of Mo into CaWO₄ micropheres can extend the excitation range as the incorporation induces obvious redshift. We have also tuned the color from blue to pink blue, white and green only by varying the relative content of Eu–Tb in the CaWO₄ host.     

PhotoleitfÄhigkeit von CaWO4-Einkristallen

Photoconductivity has been detected in CaWO₄-single crystals. By means of a pulse method the mobility of the charge carriers could be determined. The very small mobilities (Μ ⁺=0.1 cm²/Vs,Μ ^?≈ 5 cm²/Vs) are explained in the picture of “small polarons”.     

Optisch stimulierte elektrische Leitfähigkeit in CaWO4-Kristallen

The thermally stimulated conductivity of a pure CaWO₄-crystal was investigated after irradiation with X-rays at low temperature. The spectral behaviour of the conductivity correlated to the EPR-results gives information about the trap depth and the charge carriers which are responsable for the conductivity. The conductivity and luminescence phenomena are discussed in a simple model.     

Monazite (monoclinic LaPO4) slip systems at room temperature

Polycrystalline monazite (monoclinic LaPO₄) was deformed by spherical indentation at room temperature. Slip systems were identified using TEM of thin sections prepared parallel and close to the indented surface. Dislocation Burgers vectors (b) were identified by Burgers circuit closure in high resolution TEM images, supplemented by diffraction contrast where possible. A total of 441 b determinations were made in 97 grains. The most common slip systems were [001]/(010), [100]/(010) and [010]/(100). Slip on (001) was less common. Many other less common slip systems and Burgers vectors were also identified, including b = [101], [101], [011], [110] and [111]. b = [101] dislocations dissociate into ½[101] partials, and b = [101] dislocations are inferred to dissociate to ½[101] partials, with a low energy stacking fault of ～30 mJ/m². b = [100] dislocations may dissociate into ¼[210] + ¼[210] partials. b = [010] may sometimes dissociate to ½[010] + ½[010] partials. Other types of partial dislocations were also observed and discussed. All partial dislocations were climb dissociated. The line energies of monazite dislocations and their partials were calculated, and stacking fault structures for partial dislocations are analyzed. Satisfaction of the Von Mises criterion for full ductility most likely involves [101]/(111) and ?011?/{011} or {111} slip, but other combinations that require both b = [101] and ?011? or ?110? are possible. If deformation twinning is active, slip systems with b = ?011? or ?110? may not be necessary for full ductility.     

Luminescence of CaWO4:Pr3+ and CaWO4:Tb3+ at ambient and high hydrostatic pressures

Photoluminescence spectra of CaWO₄ doped with Pr³⁺ and Tb³⁺ obtained at high hydrostatic pressures up to 315 kbar applied in a diamond anvil cell (DAC) are presented. The intensities of the luminescence from the ³P₀ state of Pr³⁺ and from the ⁵D₃ state of Tb³⁺ decreased with increasing pressure. At pressures greater than 50 kbar, the ¹D₂ → ³H_J transitions in Pr³⁺ and the ⁵D₄ → ⁷F_J transitions in Tb³⁺ dominated the spectra. At pressures greater than 100 kbar, only emissions from the lower excited states were observed. At pressures greater than 150 kbar, luminescence from the ¹D₂ and ⁵D₄ states also decreased with increasing pressure, and at a pressure of 315 kbar for CaWO₄:Pr³⁺ and 190 kbar for CaWO₄:Tb³⁺, the emissions related to the Pr³⁺ and Tb³⁺ were quenched. These effects were related to the influence of impurity trapped excitons (ITEs) on the efficiency of the f–f emission in the Pr³⁺ and Tb³⁺ ions. Analysis of the emission spectra collected at different pressures allowed the energies of the ground states of the Pr³⁺ and Tb³⁺ ions with respect to the band edges of the CaWO₄ host to be estimated.     

Luminescence and energy transfer mechanisms in CaWO4 single crystals

The processes of the excitation energy transfer to the emission centers have been investigated for calcium tungstate crystals taking into account features of the electronic structure of valence band and conduction band. The calculations of the electronic structure of host lattice CaWO₄ were performed in the framework of density functional theory. The underestimation of the bandgap value in the calculations has been corrected according to the experimental data. Luminescence of two samples grown using Czochralski (cz) and hydrothermal (ht) techniques were studied. Intrinsic emission band related to excitons, self-trapped on WO₄ complexes has been observed for the both samples while the additional low-energy emission band related to the defects of crystal structure has been observed only for (ht) sample indicating the enhanced concentration of the defects in the sample. It was shown that the features of the conduction band electronic structure are reproduced in the excitation spectrum of intrinsic luminescence only for the (ht) sample while for (cz) sample the correlation is absent. The enhanced role of the competitive channels in the process of excitation energy transfer to intrinsic emission centers in (ht) sample is responsible for the observed difference.     

Lumineszenz- und Leitfähigkeitsuntersuchungen an CaWO4-Einkristallen

Luminescence decay times of CaWO₄ single crystals after excitation with UV-light and electrons and thermoluminescence after irradiation with X-rays and electrons have been investigated. Under certain conditions thermally stimulated and photoconductivity is observed. The experimental results lead to a modification of the usual model of the luminescence centre in CaWO₄.     

Absorption and luminescence spectra of Pr3+ in CaWO4 monocrystals

A complete diagram of the Stark sublevels of Pr³⁺ for the P_0,1;¹D₂;³F_2,3 terms has been constructed on the basis of a study of the absorption and luminescence spectra of CaWO₄ crystals containing Pr³⁺. Strong splitting of the Pr³⁺ terms in the environment examined has been established.The authors would like to thank Assistant Professor L. Ya. Shekun for suggesting the subject of this work and for his constant interest.     

Energy transfer in rare earth-doped CaWO4 after red edge excitation

Host sensitized energy transfer in CaWO₄ doped with Eu³⁺ and Sm³⁺ is investigated using pulsed laser time-resolved spectroscopy techniques. The excitation is into the long wavelength tail of the absorption edge and the results indicate that this selectively excites tungstate ions which are located next to activator impurity ions. The subsequent energy transfer is found to be consistent with a single step, nearest neighbor process. Both electric dipole-dipole and exchange interactions can predict strong enough transfer to account for the observed rates. These results are compared to those obtained previously for broad band excitation into the highest energy absorption bands of CaWO₄ crystals.     

Pb3+-ions in CaWO4: Temperature dependence of hyperfine interaction

The hyperfine interaction of the (6s)¹ electronic configuration of Pb³⁺ with Pb-207-nuclei (I=1/2) shows a temperature dependent Fermi-contact-term obeying aT ⁴ × Debyeintegral law. This effect is correlated to increasing delocalisation with temperature.Dedicated to Prof. Dr. Dr.-Ing. E.h. W. Hanle on occasion of his 75th birthday.     

Effect of electron bombardment on the luminescence properties of CaWO4

Journal of Applied Spectroscopy -     

Pb2+: CaWO4晶体电子结构研究

运用局域密度泛函理论的离散变分方法（DV-Xa）模拟计算了掺Pb2+后CaWO4晶体的电子结构。计算结果表明,掺Pb2+后晶体的带隙明显变窄;晶体中可能存在Pb的6 s态到W的5 d态的电子跃迁过程,用过渡态的方法计算得到其光学跃迁能为3.86 eV（对应322 nm吸收带）。并解释了晶体内440 nm发光带起源于W的5 d态到Pb的6 s的金属离子间电子转移过程。     

Relaxation dynamics of electronic excitations in CaWO4 and CdWO4 crystals studied by femtosecond interferometry technique

The relaxation of electronic excitations in CdWO₄ and CaWO₄ crystals was studied using the method of time-resolved interferometry with 100-fs temporal resolution at temperatures 15–295 K. The electronic system was excited in the one-photon and two-photon regime within the excitonic band in CaWO₄ and in the electron-hole continuum in CdWO₄. Immediate trapping of charge carriers was detected under pumping in the excitonic band of CaWO₄. This result is in agreement with decay kinetics measurements with nanosecond time resolution under direct creation of excitons by 100-fs laser pulses. Fast relaxation of charge carriers followed by formation of excitons was observed in CdWO₄. The comparison with previous work allows suggesting the formation of bulk excitons and surface-perturbed excitons in the multi-photon and one-photon regime. The corresponding models of self-trapped exciton creation in tungstate crystals are discussed.     

Two modes of slip systems in iron silicon crystals

Single crystals of iron 11·6 wt.% silicon were deformed in compression and slip bands were observed by optical microscope in specimens of different orientations. The deformation took place mostly by crystallographic slip along {110} planes, however, non-crystallographic slip in specimens of particular orientations was observed, too. On the basis of the present and previous results a simple qualitative model is proposed which explains which slip planes should be observed in crystals of iron with different silicon content deformed under various conditions. The main idea of the model is the eixstence of two different modes of slip systems in these crystals. The model predicts the conditions at which one slip mode prevails.The author would like to thank Dr. F. Kroupa, Dr. A. Gemperle, Dr. B. esták and Dr. J. Blahovec for useful discussions.     

Effect of a Mo impurity on the luminescence properties of CdWO4, ZnWO4, and CaWO4

The thermoluminescence, the spectral composition of the x-ray luminescence and the thermoluminescence, the short-wave absorption, and the luminescence-excitation spectra were studied for ZnWO₄, CdWO₄, and CaWO₄ single crystals with and without molybdenum impurities. The thermal-activation energies were found for radiationless transitions for all these crystals from the temperature dependence of the steady-state x-ray luminescence. A qualitative explanation for the results is offered.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii Fizika, No. 10, pp. 72–77, October, 1970.Deceased.