V. N. Shlegel

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.

Specific features of the phase formation, synthesis, and growth of ZnMoO4 crystals

The ZnO-MoO3 phase diagram in the range of ZnO compositions from 0.95 to 1.05 mol % is refined by differential scanning calorimetry. Data on crystal stoichiometry are obtained using the weighting method by reducing ZnMoO4 in a hydrogen atmosphere. The specific features of solid-phase synthesis of ZnMoO4 are studied, and its heat of fusion is measured. The modes of solid-phase synthesis and growth of ZnMoO4 crystals are optimized. Some experimental data on the ZnMoO4 crystal growth in the [001] direction by the low-thermal gradient Czochralski method are presented. Crystals with a cross section of ∼50 × 50 mm2, a length of 160 mm, and a weight up to 1 kg have been obtained.

Li2MoO4 Crystals Grown by Low-Thermal-Gradient Czochralski Technique

For the first time Li2MoO4 crystals, including samples enriched with isotope molybdenum-100 (Mo), were grown by Czochralski technique at low-thermal-gradient conditions (LTG Cz), which allowed obtaining crystals of record size and quality. Dependence between growth rate, growth mechanism and crystal faceting was established. Low temperature tests of the produced Li2MoO4 and Li2MoO4 scintillating bolometers have demonstrated high performance of the detectors in terms of energy resolution and particle discrimination ability. Radioactive contamination of the Li2MoO4 and Li2MoO4 crystal scintillators produced from selected initial materials after double crystallization was tested in low background underground conditions. The achieved radiopurity level satisfies the requirements of the next generation large scale double beta decay experiments.

ESR and luminescence of ZnWO4 crystals activated by gadolinium ions

The crystals of zinc tungstate (ZTO) are a radiation-hardened matrix and are widely used as scintillators for high energy radiation. Therefore, it is interesting to study the possibility of introducing gadolinium ions into this structure to obtain the lasing properties. In order to activate ZTO crystals by gadolinium ions, 0.5 mol.% of Gd2O3 is added to the load. High-quality large crystals of ZTO are produced. The spectra of optical transmission, luminescence excitation, and luminescence are measured at room temperature. It is shown that the introduction of gadolinium ions does not result in a shift of the main luminescence band of the ZTO crystals. The analysis of the ESR spectra and their modeling enables the calculation of spin-Hamiltonian parameters. It is shown that the observed spectrum depends on the state of Gd3+ ions with S = 7/2 and is well described by the spin-Hamiltonian parameters gx = 1.9835, gy = 1.9685, gz = 1.9688 and D = 644.88 Gs, E = 161.49 Gs. Directions of the principal values of the D tensor are determined; they reflect a strong distortion of the nearest-neighbor oxygen environment.

Specific features in shaping Bi12GeO20 crystals grown by low thermal gradient Czochralski technique

Germanosillenite (BGO) crystals have been grown by the low thermal gradient Czochralski technique [1] at crystallization rates of v = 0.05–4 mm/h. The evolution regularities of the faceted front forms have been studied taking into consideration their growth conditions (crystallization rate and thermal conditions). The orientations of the faces forming the crystallization front during crystal growth in the 〈111〉 direction have been determined. The relationship between the front morphology (and, therefore, growth conditions) and the quality of crystals formed is established. The quality of the BGO crystals grown is evaluated by X-ray topography.

Growth of Bi12GeO20 and Bi12SiO20 crystals by the low-thermal gradient Czochralski technique

Bi12SiO20 crystals have been grown for the first time by the low-thermal gradient Czochralski technique in the 〈111〉 and 〈110〉 directions. The conditions for reproducible crystal growth with a high-quality polyhedral faceted front are found. The systematic features of shaping Bi12SiO20 and Bi12GeO20 crystals, grown by the low-thermal gradient Czochralski technique, are compared. The defect formation in these crystals is studied and their optical homogeneity is analyzed by interferometry.

Investigation of the Thermal Conductivity of Tungstate Crystals

Thermal conductivities of MWO4 (M = Ca, Cd, or Ba), NaGd(WO4)2:1 at % Er, NaGd(WO4)2:2 at % Yb, and NaLa0.5Gd0.5(WO4)2:2 at % Nd single crystals have been experimentally investigated in the temperature range of 50–300 K.

EPR of copper ions in Pb2MoO5 crystals

The article presents the results of EPR studies of Pb2MoO5 crystals containing a copper impurity. Based on the analysis of angular dependences of the EPR spectra, it is found that copper ions incorporate into the structure of the Pb2MoO5 crystal in the Cu2+ state and occupy the molybdenum site with the formation of a linear extended Cu(II)–V(O)–Pb(IV) defect along the а axis of the crystal. An oxygen vacancy appears in the structure of the defect to compensate the charge and the lead ion acquires the Pb4+ charge state. According to the structure of this center, one magnetically non-equivalent position with the direction of main values of А and g of A(Cu)zz and gzz tensors parallel to the а axis is observed in the EPR spectra. Moreover, the EPR spectra exhibit an addition hyperfine structure from one lead atom on which the unpaired electron density is 0.061%. The obtained data on the structure of the defect formed when the copper impurity incorporates into the Pb2MoO5 crystal provided the assumption that the observed light scattering when the light beam is directed perpendicular to the а axis may be due to the cooperative effect of the presence of di- and tetravalent ions substituting for molybdenum in the linear configuration of Pb–O–Mo bonds.

Study of the possibility of growing germanium single crystals under low temperature gradients

The possibility of growing germanium single crystals under low temperature gradients in order to produce a dislocation-free material has been studied. Germanium crystals with a dislocation density of about 100–200 cm−2 have been grown in a system with a weight control of crystal growth at maximum axial gradients of about 1.5 K/cm.

Optimization of the crystallization rate in growing Bi4Ge3O12 crystals by the LTG CZ technique

The influence of the growth rate on the shaping and quality of Bi4Ge3O12 (BGO) crystals during their growth by the low-thermal-gradient Czochralski technique has been studied. Several series of BGO crystals have been grown with the rate varying from 0.3 to 15 mm/h. The limited growing rates at which the crystal quality is not deteriorated are established. The results of the study make it possible to increase the growth rate in commercial systems by a factor of 1.5–2 and thus significantly increase the growth efficiency.