Spectroscopic properties of alexandrite crystals II

Several aspects of the optical spectroscopic properties of alexandrite crystals not previously investigated are reported here. For Cr³⁺ ions occupying the Al³⁺ lattice sites with mirror symmetry in BeAl₂O₄, the positions of the zero-phonon lines for absorption transitions to the ²T_1g and ⁴T_2g levels are identified, and vibronic transition peaks in the fluorescence spectrum are compared to transitions appearing in the Raman spectrum and Stokes excitation spectrum. In addition, the effects of radiation trapping are shown to lengthen the fluorescence lifetime of the ²E_g - ⁴A_2g transition for ions in these sites at low temperatures. For Cr³⁺ ions occupying the Al³⁺ lattice with inversion symmetry, the ground state splitting of the ²E_g - ⁴A_2g transition is reported and the decrease of the fluorescence lifetime with temperature is shown to be due to the increase in vibronic emission probability as well as increased probability of direct radiationless decay.     

dc ionic conductivity in single crystals of lead nitrate

dc ionic conductivity measurements on solution grown single crystals of lead nitrate were made in the temperature range of 100 to 410°C. Activation energies corresponding to both extrinsic and intrinsic regions have been calculated.     

Thermal conductivity and expansion of PbF2 single crystals

In order to check a phenomenon of the negative correlation between ionic and thermal conductivities of solid substances, we studied the thermal conductivity and expansion of cubic PbF₂ single crystals at 50–300 and 5.6–317 K, respectively. We found that lead difluoride had a thermal expansion coefficient α that was equal to (28.5 ± 0.3)10⁻⁶ K⁻¹ at 300 K, and a thermal conductivity coefficient k(T) was equal to 1.40 ± 0.07 W/(m K) at the same temperature. Thus, the thermal conductivity for PbF₂ is the lowest among fluorite-type MF₂ (M = Ca, Sr, Ba, Cd, Pb) thermal conductivities, whereas its fluoride-ion conductivity is the highest one among MF₂ (M = Ca, Sr, Ba, Cd, Pb) ionic conductors.

     

Ionic and electronic conductivity of nitrogen-doped YSZ single crystals

The ionic and electronic charge transport was studied for single crystals of 9.5 mol% yttria-stabilized zirconia with additional nitrogen doping (YSZ:N) of up to 7.5 at.% (referred to the anion sublattice and formula unit Zr_0.83Y_0.17O_1.91) as a function of temperature and nitrogen content. The total conductivity being almost equivalent to the oxygen ion conductivity has been measured by AC impedance spectroscopy under vacuum conditions in order to prevent re-oxidation and loss of nitrogen. The electronic conductivity has been determined by Hebb–Wagner polarization using ion-blocking Pt microelectrodes in N₂ atmosphere. The ionic conductivity of YSZ:N decreases in the presence of nitrogen at intermediate temperatures up to 1000 °C. The mean activation energy of ionic conduction strongly increases with increasing nitrogen content, from 1.0 eV for nitrogen-free YSZ up to 1.9 eV for YSZ containing 7.3 at.% N. Compared to nitrogen-free YSZ, the electronic conductivity first decreases at nitrogen contents of 2.17 and 5.80 at.%, but then increases again for a sample with 7.53 at.%. At temperatures of 850 °C and above, the presence of the N^3? dopant fixes the electrode potential and thus the oxygen partial pressure at the Pt electrode to very low values. This corresponds to a pinning of the Fermi level at a relatively high energy in the upper half of the band gap. At 7.53 at.% N and 950 °C, the oxygen partial pressure in YSZ:N corresponds to p_O2 = 3 × 10^? 18 bar. At temperatures above 850 °C, even in the presence of a very small oxygen concentration in the surrounding gas phase, the nitrogen ion dopant becomes highly mobile and thus diffuses to the surface where it is oxidized to gaseous N₂. The results are discussed in terms of the ionic and electronic defect structures and the defect mobilities in YSZ:N.     

Evolution of the dielectric and acoustic parameters of chromium-doped SBN single crystals with variations in temperature

The temperature dependences of the dielectric characteristics of Sr_0.61Ba_0.39Nb_{2 − x} O₆: Cr _x ³⁺ single crystals with x = 0.0, 0.005, and 0.010 are investigated in the low-frequency and infrared ranges. It is revealed that the behavior of the dielectric response exhibits specific features in the vicinity of the transition from the ferroelectric phase to the paraelectric phase and in the low-temperature range. The measurements are performed along the [001] (polar axis) and [100] directions. The temperature dependences of the relative sound velocity and the attenuation of elastic waves are also studied, and the temperature ranges of the anomalous behavior of these parameters are determined. The specific features revealed in the behavior of the dielectric and acoustic properties are considered from the viewpoint of the structural transformations occurring in the SBN system upon doping with a chromium impurity. Original Russian Text ? A.S. Pilipenko, A.I. Burkhanov, L.I. Ivleva, 2009, published in Fizika Tverdogo Tela, 2009, Vol. 51, No. 3, pp. 543–546.     

Thermally stimulated and photo-stimulated conductivity in ZnS-Cu single crystals

The development of thermally stimulated and photo-stimulated conductivity in zinc-sulfide phosphors has been studied. The sign of the charge carriers in the single crystal samples inspected has been determined. The effect of infrared light in the region 1–3.5 m upon previously excited crystals has been investigated and the temperature dependence of the photostimulated conductivity established.In conclusion, the authors express their deep gratitude to E. S. Bespalovaya for the samples and also to K. Arun for his help in measuring the Hall constant.     

Electrical conductivity and photoconductivity of single crystals of lithium hydride

Nonactivated LiH crystals and crystals of LiH with Mg, In, Tl, Sn, Sb, and Bi impurities are investigated. Photoconductivity is found in luminescent crystals. The temperature dependence of the electrical conductivity and photoconductivity in the temperature range of 100–570 ° K is measured. Volt-ampere characteristics of the dark current and photocurrent in fields up to 10⁴ V/cm at various temperatures are obtained. Data is presented on the inertia of the photocurrent and its dependence on the intensity of the exciting light. Conclusions are drawn concerning some connection of LiH photoconductivity with the type of activating impurity, the absence of a connection with the brightness of the luminescence, and the specific role of photoconductivity in color centers.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii Fizika, No. 10, pp. 94–98, October, 1971.     

Electric conductivity of electron-bombarded single crystals of KCl-RbCl solid solutions

We present the results of investigations of the influence of divalent cation impurities on the radiative changes in the electric conductivity of RbCl crystals and KCl-RbCl solid solutions bombarded by electrons. It was observed that a temperature dependence of the electic conductivity of the bombarded crystals has a complicated form in this case. In the case of crystals containing no cation impurities of different valence, there is no radiative decrease of the electric conductivity due to the action of the Smoluchowski mechanism, and a sharp increase is observed instead. We consider physical properties that lead to the appearance of a maximum in the temperature dependence of the electric conductivity of the bombarded crystals.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 12, pp. 92–96, December, 1970.     

Isotope effect in the thermal conductivity of germanium single crystals

The thermal conductivity of chemically, structurally, and isotopically highly pure germanium single crystals is investigated experimentally in the temperature range from 2 to 300 K. It is found that the thermal conductivity of germanium enriched to 99.99% ⁷⁰Ge is 8 times higher at the maximum than the thermal conductivity of germanium with the natural isotopic composition. Pis’ma Zh. éksp. Teor. Fiz. 63, No. 6, 463–467 (25 March 1996)     

Ionic conductivity study of CaF2: Nd single crystals

The electrical ionic conductivity of unirradiated and irradiated CaF₂: Nd crystals in the range of 60 to 800°C has been measured. The conductivity plot is basically divided into four parts, i.e., intrinsic and extrinsic unassociated, extrinsic associated, and extrinsic segregated regions. Activation energy (for unirradiated samples) in the extrinsic unassociated region is in the range of 0.69 to 1.20 eV depending on the doping concentration while for the intrinsic region, it is of the order of 1.89 eV. The conductivity in the extrinsic unassociated region increases with increase of Nd content in the sample. Also, the conductivity in the extrinsic region forγ-irradiated sample is higher than that for unirradiated one. In the intrinsic region, however, the conductivity is independent of dopant concentration orγ-irradiation. From these results it is surmised thatF ⁻ interstitials are the charge carriers in this region for CaF₂: Nd³⁺ system.     

Frequency dependance of the conductivity in trigonal selenium single crystals

An analysis of the frequency dependences of a barrier-limited conductivity model is presented. Theoretical results are in good accordance with previous experimental results. At low temperature the model predicts the same variation σ (ω) ∝ ω^s with s < 1 as a hopping conduction process.     

Thermal conductivity of USb2 and UBi2 single crystals

The thermal conductivity (κ) of single crystals of tetragonal uniaxial antiferromagnets USb₂ (T _N = 202 K) and UBi₂ (T _N = 180.8 K) has been measured along the a-axis (κ_a ) over the temperature range from 0.5 to 300 K and along the c-axis (κ_c ) from 0.5 to 70 K. The as-grown samples have residual resistivity ratio (RRR) values of about 500–600 and 100–150 for UBi₂ and USb₂, respectively. The anisotropy of the thermal conductivity (κ_a (T)/κ_c (T) ～ 5) and the low-T Lorenz ratios are discussed in relation to Fermi surface topology for both compounds.     

Anisotropy of the hopping conductivity in TlGaSe2 single crystals

The temperature dependences of the conductivities parallel and perpendicular to the layers in layered TlGaSe₂ single crystals are investigated in the temperature range from 10 K to 293 K. It is shown that hopping conduction with a variable hopping length among localized states near the Fermi level takes place in TlGaSe₂ single crystals in the low-temperature range, both along and across the layers. Hopping conduction along the layers begins to prevail over conduction in an allowed band only at very low temperatures (10–30 K), whereas hopping conduction across the layers is observed at fairly high temperatures (T?210 K) and spans a broader temperature range. The density of states near the Fermi level is determined, N _F=1.3×10¹⁹eV·cm³)^?1, along with the energy scatter of these states J=0.011 eV and the hopping lengths at various temperatures. The hopping length R along the layers of TlGaSe₂ single crystals increases from 130 Å to 170 Å as the temperature is lowered from 30 K to 10 K. The temperature dependence of the degree of anisotropy of the conductivity of TlGaSe₂ single crystals is investigated.     

Crystal growth and conductivity investigations on BiVO4 single crystals

Some oxides with the scheelite structure are well known mixed ionic and electronic conductors. This structure family, with the general composition ABO₄, can be described as a layer structure derivative of the fluorite structure. The relations between the structural and the electrical transport properties of these materials have been investigated. The model system used in this work was BiVO₄, which shows both electronic and ionic conduction. Single crystals of bismuth vanadate (BiVO₄) were grown using the Czochralski technique. Single crystal boules up to 55 mm in diameter and up to 30 mm long were produced. The crystallographic orientation was determined by the Laue method, and they were cleaved along [001] planes and polished to optically perfect cubes with an edge length of 5 mm. Two-point ac impedance measurements in the temperature range between 550°C and 700°C were performed. The conductivities were measured as a function of the crystal orientation and showed strong anisotropy as expected from the crystal structure. Paper presented at the 2nd Euroconference on Solid State Ionics, Funchal, Madeira, Portugal, Sept. 10–16, 1995     

Heat capacity study of β-FeSi2 single crystals

Heat capacity of needle-like (length = 5 mm, diameter = 1 mm) β-FeS₂ single crystal grown by chemical vapor transport has been measured. Two anomalies are found; a broad deviation centered around 160 K and a clear deviation at a temperature of (approximately) 255 K. We have attempted to relate these to the anomalies previously reported in the case of the resistivity data. The transient thermoelectric effect (TTE) results lead us to the inference that the system under-goes a transition from a single carrier system to at least a two carrier system at 220 K. Our heat capacity results seem to provide further independent evidence for this transition in this system.     

Study of dc ionic conductivity in single crystals of RbNO3

dc ionic conductivity has been studied on single crystals of RbNO₃ grown by slow evaporation method. The range of temperature covered is from room temperature to its melting point atmospheric pressure. An attempt is made to correlate the known phase transitions with the variation of ionic conductivity with temperature.