T. Zayarnyuk

Growth and properties of potassium holmium double tungstate KHo(WO4)2

Potassium holmium tungstate KHo(WO 4 ) 2 crystal, as other tungstates, shows the high temperature irreversible structural phase transition. Owing to this, KHo(WO 4 ) 2 single crystals were grown due to spontaneous crystallisation with use of the High Temperature Solution Growth technique, which allows to lower the temperature of crystallisation below the temperature of the phase transition. K 2 W 2 O 7 was used as a solvent. It provides a very wide temperature range of crystallisation and does not introduce additional impurities into the melt. The starting flux contained 20 mol% of KHo(WO 4 ) 2 dissolved in K 2 W 2 O 7 . It was found that potassium holmium double tungstate is pleochroic. The two different optical spectra: one spectrum for electrical vector of linearly polarised light parallel to optical Y axis (main spectrum) and second one for electrical vector perpendicular to Y axis were measured.

Crystal structure and magnetic properties of potassium erbium double tungstate KEr(WO4)2

Results of structural, magnetic and specific heat investigations of the potassium erbium double tungstate, KEr(WO 4 ) 2 , are presented. Potassium erbium double-tungstate KEr(WO 4 ) 2 single crystals have been grown by the top-seeded solution growth method (TSSG) and modified Czochralski techniques. It crystallizes in the monoclinic crystal structure (C2/c space group). The unit cell contains four formula units and is described by parameters a = 10.615(2) A, b = 10.316(2) A, c = 7.534(2) A, β = 130.73(3)°. From the x-ray diffraction measurements the fractional atomic coordinates, displacement parameters and interatomic distances have been determined. The specific heat C(T) of the KEr(WO 4 ) 2 crystal has been measured over a temperature range of 0.6-300 K. The susceptibility has been studied at T = 0.25-4.0 K. The magnetic phase transition was observed at a temperature of 0.48 K. The magnetization has been measured in the temperature region from 4.2 to 60 K and in magnetic field up to 1.6 T. A strong anisotropy of magnetic properties was found. The temperature and field dependences of susceptibility and magnetization data were used for both elucidation of character of the magnetic ordering and calculation of the exchange and dipole-dipole interaction energies as well as for determination of the possible magnetic structure of KEr(WO 4 ) 2 .

Magnetostriction of rare earth double tungstates

The isothermal, longitudal and transverse magnetostriction of rare earth double tungstates KDy(WO4)2, KHo(WO4)2 and RbNd(WO4)2 has been measured at low temperatures along the crystallographic axes in a magnetic field up to 12 T. The measurements have been performed by the strain gauge method on single crystals. For all three axes, we observed anomalies connected to the structural phase transitions induced by a collective Jahn–Teller effect or by strong external field at temperatures near a spontaneous structural phase transition of Jahn–Teller nature. It was shown that all the studied phase transitions are of first order. The magnetostriction measurements in rare earth double tungstates are important and interesting, since: (i) in these objects, there is a Jahn–Teller spontaneous structural transition at low temperatures; (ii) information on the behaviour of an elastic subsystem in these samples is necessary for understanding the mechanism of this transition under the influence of an external magnetic field applied along the crystallographic axes; (iii) it allows us to form a conclusion on the anisotropy of the studied phenomenon.

Highly photoconducting sillenite single crystals

Abstract Single crystals of Bi 12 TiO 20 , and Bi 40 Ga 2 O 63 (titanium, and gallium sillenite) were grown with use of Top Seeded Solution Growth (TSSG) technique. Mixed sillenites described by the formula Bi 12 Ti 1–x M x O 20 (M=V, Ga, Pb), some of them doped additionally with Cu were also obtained. Photoconductivity under permanent, chopped, and pulse illumination and photochromism of obtained crystals were investigated and discussed. Some characteristic common features were found for photoconductivity obtained under conditions of chopped and constant illumination. Comparing the best known sillenite photoconductors we managed to obtain mixed titanium sillenites having photoconductivity four to five orders of magnitude higher.

The magnetic properties of potassium holmium double tungstate

The magnetic investigations of potassium holmium double tungstate KHo(WO4)2 have been performed. The results of measurements of magnetic susceptibility and magnetization as a function of temperature (T from 0.3 K up to 100 K) and magnetic field (up to 1.5 T) are presented. A strong anisotropy of magnetic properties was found. The magnetic measurements data were used to calculate the interaction energy. It was shown that the interactions between nearest neighbors Ho3+ ions have antiferromagnetic character.

The low temperature magnetic properties of potassium holmium double tungstate

The magnetic investigations of potassium holmium double tungstate KHo(WO4)2 have been performed. The results of measurements of magnetic susceptibility and magnetization as a function of both temperature (T = 0.5–100 K) and magnetic field (up to 2 T) are presented.

Magnetic phase transition in KGd(WO4)2 double tungstate

Investigations of the specific heat of the potassium gadolinium double tungstate KGd(WO4)2 have been performed over the temperature range from 0.05 K up to 4 K in zero magnetic field. The λ -type anomaly observed at T = 0.42 K was interpreted as an indication of a second order magnetic phase transition within the Gadolinium sublattice. The critical exponent α was determined from experimental data.

The specific heat of potassium holmium double tungstate

The results of measurements of thermal properties (specific heat) of potassium holmium double tungstate KHo(WO4)2 as a function of temperature (from 0.5 to 300 K) and magnetic field (up to 2 T) are presented. The total specific heat without the phonon and Schottky contributions is found to have the anomaly with maximum at T SPT ∼ 5 K. This anomaly is likely related with the structural phase transition (SPT) caused by the cooperative Jahn–Teller effect. The increase of specific heat at very low temperatures and its shift towards high temperatures with increasing magnetic field are observed. The origin of this behaviour can be connected with possible magnetic phase transition induced by magnetic field.

First-order phase transition in potassium dysprosium tungstate induced by the field renormalization and softening of the elastic moduli in the vicinity of a structural Jahn-Teller-type transition

The magnetostriction of KDy(WO4)2 single crystals is measured in an external magnetic field at temperatures below the temperature of a structural phase transition of the Jahn-Teller type. A steplike irreversible variation in the elastic strain is observed to occur with an increase in the magnetic field applied along the a or b axis of the monoclinic cell of the crystal. The residual change in the strain is retained after changing the sign of the magnetic field. The return to the initial state characterized by field-induced jumps in the strain is possible only after thermal cycling well above the structural phase transition temperature. The theory of this phenomenon is developed using a phenomenologically derived thermodynamic potential of the elastic sub-system that takes into account the crystal symmetry and the field renormalization of the elastic moduli. The jumplike transitions are interpreted as being due to the magnetic softening of the elastic moduli in the vicinity of the structural phase transition temperature.

Optical properties of potassium erbium double-tungstate KEr(WO4)2

Results of spectroscopic investigations of crystalline potassium-erbium double tungstate Ker(WO4)2 are reported. The single crystal of Ker(WO4)2 were grown by Top Seeded Solution Growth technique. They belong to the chain-layered group of materials crystallizing in monoclinic syngony. The investigations were carried out in the broad spectral and temperature range. It was found that the potassium erbium double tungstate belong to the class of the diaxial crystals and it is a pleochroic. The main optical absorption spectra are determined.