M.T. Borowiec

Light induced charge transfer processes in Cr doped Bi12GeO20 and Bi12SiO20 single crystals

Abstract The optical absorption and ESR spectra of Bi 12 GeO 20 and B 12 SiO 20 doped with Mn have been measured before and after illumination with visible light. Uniaxial stress measurements on a sharp line observed at 8026 cm −1 were performed. The observed ESR spectrum is a superposition of six lines resulting from the hyperfine interaction of manganese ions in tetrahedral positions. The g -factor and hyperfine constant are g = 1.999 ± 0.003 and A = 78 Gs. Analysis of the light-induced absorption spectrum leads to the conclusion that a small hole polaron bound to an Mn impurity at a tetrahedral site is responsible for the very broad absorption band which appears after illumination. The sharp line is interpreted as due to a transition inside the Mn + center in tetrahedral coordination. Bands in the region 10,000–16,000 cm −1 are due to Mn 3+ centers in interstitial positions, whose symmetry can be treated to a first approximation as tetragonal. The following crystal field parameters for this center were found: B = 565 cm −1 , Dq = 1400 cm −1 , Dt = −330 cm −1 , Ds = 4170 cm −1 and C = 2260 cm −1 . The illumination conditions which are needed for homogeneous coloration of the sample are also discussed.

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.

Electron paramagnetic resonance spectra of Er3+ in the monoclinic KY(WO4)2 crystal

Electron paramagnetic resonance (EPR) studies of Er3+ of single-crystal KY(WO4)2 have been carried out at X-band frequencies over a temperature range from 4.2 to 300 K. The main x- and z-axes of the g-tensor in the ac plane do not coincide with the crystallographic a-and c-axes, while the y- and b-axes are parallel. The EPR spectrum is characterized by a strong anisotropy of the g-factors (gz = 13.25 ± 0.05, gx = 0 ± 0.1, gy = 3.378 ± 0.005) and of the hyperfine interaction parameters (Az = −469 ± 3, Ax = 0 ± 3.5, Ay = −120.75 ± 0.8 in 10−4 cm−1). The temperature dependence of the g-factors is caused by an anharmonic part of the spin–phonon Hamiltonian as well as by a rotational contribution to this Hamiltonian. A spin–lattice relaxation is shown to result from two Orbach processes through the first and second excited levels of an electronic spectrum.

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 .

Jahn-Teller type structural transition in KDy(WO4)2

Abstract The low temperature structural phase transition in the KDy(WO 4 ) 2 single crystal has been first studied by means of both ESR method and specific heat C ( T ). A broad maximum of C ( T ) with peak at T c = 6.38 K and λ-shaped temperature dependence of the ESR line width ΔH ( T ) in the vicinity of transition may be explained as being due to the structural phase transition caused by the interaction of the electronic states of Dy ions with the lattice vibrations. An analysis of C ( T ) and ΔH ( T ) dependencies show that the second order structural phase transition is the transition of Jahn-Teller type and does not occur with a change of point symmetry.

Magnetic Ordering of Dy3+ Ions in RbDy(WO4)2 Single Crystal

The specific heat C(T) of the monoclinic RbDy(WO4)2crystal has been studied at very low temperatures 0.2≤T≤1.9 K and in magnetic fields 0≤H≤0.38 T. The Neel temperature was shown to be equal to TN = 0.818 ± 0.005 K. The experimental value of the effective exchange parameter was obtained to be equal to J/k = − 0.798 K. The C T) dependence below Neel temperature 0.5TN<T<0.99TN) is well described by 2D Ising model, whereas in the temperature region close above TN(1.01 TN<T< 2TN) it can be described by neither 2D, nor 3D Ising model. The experimental and theoretical H-TNdiagrams for fieldH ‖ aare in a reasonable agreement for simple quadratic lattice.

Spin-spin interaction of Dy3+ ions in KY(WO4)2

The spin-spin interaction of Dy3+ ions in a KY(WO4)2 single crystal is investigated by electron paramagnetic resonance (EPR) spectroscopy at a temperature of 4.2 K and a frequency of 9.2 GHz. The EPR spectra of ion pairs located in different coordination shells are analyzed. It is revealed that the considerable contribution to the spin-spin interaction of the nearest neighbor ion pair nn is made not only by the magnetic dipole-dipole interaction but also by the isotropic exchange interaction with the parameter Inn = (+601 ± 17) × 10−4cm−1. The exchange interaction in pairs of more widely spaced ions is substantially weaker: I5n = (−38 ± 3) × 10−4cm−1 and I9n = (+18 ± 4) × 10−4cm−1. For the other ion pairs, the magnetic dipole-dipole interaction dominates. It is found that the EPR spectra of single ions and ion pairs exhibit a superhyperfine structure associated with tungsten nuclei.

Pressure Effect on the Gd3+ Ion Ground State in the KY(WO4)2 + 0.5%Gd Crystal

Pressure effect on the ground state of Gd 3+ ions in the low symmetry crystal KY(WO 4 ) 2 + 0.5% Gd has been studied by means of EPR method. The parameters of the spin-Hamiltonian of monoclinic symmetry describing the EPR spectrum and their changes under pressure were determined. All four structural positions of the Gd 3+ ion are magnetically equivalent. The direction of maximum splitting of the EPR spectrum has a deviation of about 20° from the c-axis. The initial splitting of Gd 3+ ion ground state is established to increase under pressure.

Spin-spin interaction and the EPR spectrum of KDy(WO4)2

The EPR spectrum of a KDy(WO4)2 monoclinic crystal is investigated. It is found that the EPR spectrum of magnetically concentrated materials at a low frequency (9.2 GHz) undergoes a substantial transformation in addition to the well-known broadening of the EPR lines. At low Dy3+ concentrations (x<10−2), the EPR spectrum of an isomorphic crystal, namely, KY(1−x)Dyx(WO4)2, is characterized by the parameters gx=0, gy=1.54, and gz=14.6. For a magnetically concentrated crystal KDy(WO4)2, the g values are as follows: gx=0, gy=0.82, and gz=2.52. It is demonstrated that the difference in the parameters is associated with the specific spin-spin interaction between Dy3+ ions, including the Dzyaloshinski interaction, which is not observed at high frequencies.

ESR-study of the low-temperature phase transition in KDy(WO4)2

Abstract The low-temperature structural phase transition of the Jahn-Teller type in a KDy(WO4)2 single crystal has been studied by means of the ESR method. The ESR line width near the point of transition (Tspt≈7 K) is shown to increase from 0.17 T up to a maximum value of 0.24T and then it decreases to 0.19 T. The width of transition area is about 8 K. The symmetry of g-tensor does not change at the point of the phase transition while the values of the g-tensor components change considerably: gz=3.13; gx≈0; gy=0.82 at T > Tspt; gz=1.98; gx≈0; gy=1.19 at T