ZnWO4 Single Crystals Research Articles

Complex nature of the interaction of AFM tip with the freshly splitted (010) surface of CdWO4 and ZnWO4 single crystals

The discovery of new low-dimensional materials formed by splitting known bulk crystals into separate elementary layers has enabled future realization of still undiscovered potential of long-studied materials. High-resistivity two-dimensional films obtained by splitting ionic layered crystals are of great interest for creating new vertical van der Waals heterostructures. For unveiling the potential of using two-dimensional monocrystalline films of CdWO4 and ZnWO4, it is necessary to gain a more penetrating insight into the processes that occur on the freshly splitted surface of the crystals. In the present study, for the first time we investigated the effect of AFM probe on the (010) surfaces of freshly splitted CdWO4 and ZnWO4 single crystals. It is shown that the observed “protruding areas” on the splitted surfaces of these crystals are of a complex nature, and they are mainly related with the transfer of charges, as well as with the formation of complete bonds on the surface of the broken crystal lattice of examined crystals. The charge spots observed on the splitted surface of the crystals arise as a result of local charging by AFM probe. As a result of charge diffusion, as well as under the influence of ambient medium, the complete dissipation of charges occurs in less than 2 h.

Effect of Li+ codoping on the mechanical strength of YB:ZnWO4 single crystals

ZnWO4 single crystals both undoped and doped with various concentrations of Yb3+ and Li+ ions have been grown. The actual concentrations of dopants had been measured by atomic emission spectroscopy with inductively coupled plasma, as well as by polarized optical absorption spectroscopy. The segregation coefficients of the dopants between the crystal and melt have been evaluated. Hardness and fracture toughness of the crystals were measured by indentation method. The dependencies of the measured values on the dopants concentrations were analyzed. The optimal dopants concentrations, providing the best mechanical strength of Yb,Li:ZnWO4 crystal have been found.

New Self‐Organization Route to Tunable Narrowband Optical Filters and Polarizers Demonstrated with ZnO–ZnWO4 Eutectic Composite

AbstractElectromagnetic fields interacting with microscopic structural features in a composite material provide emerging optical properties that surpass those offered by the individual components. However, composite materials can be generally lossy due to the scattering effects induced by inhomogeneities at the interfaces between different compounds. To overcome such problems, complicated and costly manufacturing procedures, such as top‐down approaches, are generally required. In contrast, here ZnO–ZnWO4 eutectic self‐organized composites grown by the micropulling method are considered, displaying sharp and strongly polarized transmission at 397 nm. Such an optical response is notable because it is not observed in either ZnO or ZnWO4 single crystals. The optical response is due to the refractive index matching of the two constituents, which self‐organize into ordered structures via a micropulling down method. The optical behavior reported here can directly lead to applications, such as tunable narrowband filters with bandpass of 3 nm and polarizers, paving the way to a new self‐organization route for manufacturing optical components.

Influence of chemical purity of initial reagents ZnO, WO3 and Yb2O3 on spectral-absorbing quality of ZnWO4 single crystals

Influence of chemical purity of initial reagents ZnO, WO3 and Yb2O3 on spectral-absorbing quality of ZnWO4 single crystals

Radioluminescence, low temperature thermoluminescence and scintillation properties of Ca and Eu doped ZnWO4 single crystals

In this work, radioluminescence, low temperature thermoluminescence, and light yield of ZnWO4, ZnWO4:Ca, and ZnWO4:Ca, Eu single crystals is reported. It is shown that although the single doping with Ca improves, in a limited scope, the scintillation properties of the studied material, it is not the case with the double doping with Ca and Eu. The values of light yield and energy resolution of the investigated ZnWO4:Ca, Eu samples are clearly worse compared to either ZnWO4 or ZnWO4:Ca. The X-ray excited spectra of all the crystals, dominated by the intrinsic luminescence, unveil a strong decrease of intensity from 10 to 350 K; the addition of Eu does not improve this property. Furthermore, the presence of Eu strongly influences the low temperature glow curves, i. a. by introducing a distinct peak close to 150 K.

Luminescence of the tungsten-activated MgF2 ceramics synthesized under the electron beam

Abstract For the first time the possibility of synthesizing of an optical ceramics MgF2 using a powerful electron beam as a heater is shown. In the process of synthesis, the charges consisting of magnesium fluoride powder with different ratio between the dopants WO3 and LiOH were exposed to an electron beam with 1.4 MeV electron energy and power of 18 kW on an area of 1 cm2. Investigations of the elemental composition and structure of the synthesized ceramics showed the presence of crystalline phases MgF2, W, WO3, and Mg in the samples. The size of the crystalline grains and efficiency of their formation depend on the composition of the charge. It was found that the synthesized ceramics samples luminesce in the 470 nm region at 300 K. Excitation spectrum of this emission in ceramics consists of two bands with maxima at 250 nm and about 200 nm. The spectral properties of MgF2 ceramics in general are similar to those in ZnWO4 single crystals and LiF crystals doped with tungsten oxide.

Growth and optical properties of ZnWO4 single crystals pure and doped with Ca and Eu

In frames of this work a series of ZnWO4 single crystals, pure and doped with either Ca (5at%) or Ca (4at%) and Eu (1at%), has been grown by the Czochralski method. Phase analysis and structural refinement was performed X-ray powder diffraction and the powder diffraction and the patterns were analyzed by the Rietveld refinement method to analyze the lattice parameters of the crystal structure. Additionally, the absorbance, photoluminescence emission (PL) and photoluminescence excitation (PLE) and EPR spectra were measured and compared to investigate influence of Ca and Eu co-doping on optical properties of the obtained crystals. Furthermore the optical band-gap of the crystals has been calculated. The crystal structure of all mentioned ZnWO4 samples has also discussed in the scope of environment symmetry and possible sites of dopant ions.

Facile growth of centimeter-order, highly crystalline ZnWO4single crystals by the flux evaporation technique using molten NaCl

Wolframite-type ZnWO4 crystals were grown from molten NaCl using the flux method at temperatures between 700 and 1100 °C. Centimeter-order, idiomorphic single crystals of ZnWO4 were easily obtained from a solute concentration of 40 mol% at 1100 °C, with well-defined facets at {010}, {101}, {−101}, {130}, and {041}. As the heating temperature decreases, the size of the ZnWO4 crystals also decreases. However, millimeter-order ZnWO4 single crystals were still grown from a NaCl flux at 700 °C, whereas solid-state reactions gave only indefinite, micron-order particles even at 1100 °C. It is assumed that NaCl and ZnWO4 form a eutectic composition, resulting in effective crystal growth through a liquid-phase process even below the melting point of NaCl (∼800 °C). In addition, the large-scale evaporation of molten NaCl should facilitate the supersaturation of ZnWO4 crystals, resulting in centimeter-order growth at 1100 °C. The crystallographic characteristics of the flux-grown ZnWO4 crystals were also evaluated; elemental analysis and microscopic observations revealed a highly crystalline nature with respect to chemical stoichiometry, impurities, crystal defects, and regularity of the lattice.

Electronic properties of ZnWO4 based on ab initio FP-LAPW band-structure calculations and X-ray spectroscopy data

Total and partial densities of states of the atoms constituting zinc tungstate, ZnWO4, have been calculated using the ab initio full potential linearized augmented plane wave (FP-LAPW) method. The theoretical data reveal that main contributors in the valence band of ZnWO4 are the Zn 3d-, W 5d- and O 2p-like states: the Zn 3d- and W 5d-like states contribute mainly at the bottom, whilst the O 2p-like states at the top of the valence band, with also significant portions of contributions of the above states throughout the whole valence-band region of the tungstate under study. In addition, data of our band-structure FP-LAPW calculations indicate that the conduction band of ZnWO4 is dominated by contributions of the W 5d-like states. To verify the theoretical findings, high-quality inclusion-free ZnWO4 single crystals were specially grown along the [010] direction for the present experimental studies employing the low thermal gradient Czochralski technique. It has been established that, comparison on a common energy scale of the X-ray photoelectron valence-band spectrum and the X-ray emission bands representing the energy distribution of mainly the Zn 3d-, W 5d- and O 2p-like states of ZnWO4 confirm experimentally the present FP-LAPW theoretical data regarding the occupations of the valence band of zinc tungstate.

First-principles calculations of the structural, electronic and elastic properties of ZnWO4 and CdWO4 single crystals at the ambient and elevated pressure

Two technologically important tungstate crystals – CdWO4 and ZnWO4 – are studied theoretically using the plane wave based first-principles calculations. The optimized crystal structures were used to calculate the structural, electronic and elastic properties of both materials under varying hydrostatic pressure. It was shown that the band gaps, which are direct at ambient pressure, turn into indirect ones in both compounds in the pressure range from 5 to 10 GPa, whereas the values of the band gaps themselves depend only slightly on the applied pressure. Differences in compressibility along the crystallographic axes and the W–O, Cd–O, Zn–O chemical bonds were revealed and quantified by calculating the pressure coefficients for all these characteristic distances. The first estimations of the complete elastic tensor constants for both materials are reported.

Ab-initio studies of the electronic and optical properties of ZnWO4 and CdWO4 single crystals

Two technologically important materials – cadmium tungstate CdWO4 and zinc tungstate ZnWO4 – are studied in details using the plane wave based first-principles calculations. The optimized crystal structures were used to calculate the electronic and optical properties of both materials. Comparison of the calculated results with the available in the literature experimental X-ray photoelectron spectra (XPS) and reflectivity spectra yielded good agreement and allowed for assignment of the experimentally observed features, including those induced by difference between the Cd 4d and Zn 3d states. In addition, the variations in the electron density distributions related to two crystallographically in equivalent oxygen positions were revealed.

The growth of ZnWO4 and CdWO4 single crystals from melt by the low thermal gradient Czochralski technique

The features of growth of single crystals of cadmium and zinc tungstates by the low thermal gradient Czochralski technique have been investigated. The effect of change in the growth conditions on the crystal morphology and the capture of macroinclusions is studied. Crystals 45 mm in diameter and up to 150 mm long (cylindrical part) are obtained. Their optical and scintillation characteristics are measured.

Impurity Sn-related paramagnetic defects in UV-illuminated ZnWO4 single crystals

Electron spin resonance methods are used to characterize a new Sn-impurity-perturbed W5+ centre with C1 symmetry produced in ZnWO4 single crystals illuminated at 77 K with 365 nm UV light. The new centre is electron-like with its highest spin density at a W site, but also with a substantial spin density at an Sn impurity substituting for a neighbouring Zn cation. It is denoted as a W5+ - SnZn++ centre. The concentration of this defect is estimated to be less than 1 µmol mol-1.

Point Defects Identified by Spectroscopic Methods in ZnWO4 Single Crystals

The characterisation of a number of defects in ZnWO 4 , e.g. Fe, Cr, Pt, OH and Mo impurities, as well as intrinsic and impurity-related colour centres, is presented using results from optical and magnetic resonance spectroscopies.

PHOTOLUMINESCENCE PROPERTIES OF TUNABLE LASER CRYSTAL Cr3+:ZnWO4

We investigate the photoluminescence properties of Cr3+:ZnWO4 single crystals which channels all the fluorescence into the broadband 4T2(t22e) to 4A2(t32) transition at about 990nm wavelength. Experiment results of the absorption spectra and the excitation spectrum, temperature dependence of the emission spectra, decay curves of Cr3+ ions are presented and discussed. The most efficient excitation wavelength is in the red region. Radiative decay from the luminescence level to the ground level is characterized by broad emission bands as a consequence of a strong electron-phonon interaction ——Hua-Rhys factor S(300K)=4.22, S(12.7K) = 7.09, the average pho-non energy hv(300K)=405cm-1, hv(12.7K)=213cm-1. The zero-phonon transition line probably is at wavelength of 856.7nm. If there are two kinds of Cr3+ center, one is compensated by the adjacent vacancy(1.8μs), and the other is compersated nonl-ocally (5.45μs).

ESR of Nd1ions in ZnWo4single crystals

ESR of Nd¹ions in ZnWo₄single crystals

OPTICAL CHARACTERISTICS OF ZnWO4 SINGLE CRYSTALS

The transmittance curves of ZnWO4 single crystals grown by Czochralski method have been measured. We suggest that the existense of oxygen vacancy is the basic reason for the rosy color of the crystal. The position of the optical indicatrix, the refractive indices in the visible wavelength region and the Sellmeier's equations have been determined more accurately.

Analysis of Raman spectra of ZnWO4 single crystals

The first-order Raman spectra of ZnWO4 single crystals have been obtained. All the vibrational frequencies are assigned by group theory. Among them the Bg modes at 269, 166, and 147 cm−1 and Ag mode at 124 cm−1 are reported for the first time. In view of x-ray structure data of ZnWO4, we suggest that the frequencies of vibrational modes can be assigned on the basis of analysis with octahedral WO6 as a starting point, instead of tetrahedral WO4.

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.