KBr Single Crystals Research Articles

Exciton-Like luminescence in a KBr crystal exposed to uniaxial elastic deformation

The effect of a low-temperature uniaxial deformation (85 K, along the 〈110〉 or 〈100〉 directions) on the radiative decay of electronic excitations has been studied in pure KBr single crystals doped with sodium ions. As the deformation increases (up to approximately ε≈1.0 %), the X-ray luminescence bands of both, self-trapped excitons (the σ and π emission bands peaked at 4.4 and 2.3 eV, respectively) and exciton-like formations associated with sodium impurity ions increase linearly. In KBr:Na, the recombination formation of bound excitons near single Na+ and pair Na+-Na+ impurity centers leads to luminescence bands at 2.85 and 3.1 eV, respectively.

The Effect of Fast Kr Ion Irradiation on the Optical Absorption, Luminescence, and Raman Spectra of BaFBr Crystals

In this work, using photoluminescence (PL), optical absorption (OA), Raman spectroscopy (RS), and atomic force microscopy (AFM), the radiation damage of BaFBr crystals irradiated with 147 MeV 84Kr ions to fluences (1010–1014) cm2 was investigated. The manifestations of the oxygen impurity contained in the studied crystals on the effects associated with ion irradiation are also considered. In unirradiated crystals, the PL spectra exhibited bands related to the oxygen impurity. Moreover, it was found that quenching and a shift of the PL maximum occur, which is due to the fact that, with increasing dose, aggregation of defects occurs. Electronic and hole aggregate color centers appear mainly in the bromide sublattice. A detailed study of the Raman spectra and comparison with the corresponding data for KBr single crystals made it possible to reveal the corresponding manifestations of the Raman modes of complex Br3−-type hole centers.

Bulk and surface processes in KBr single crystals examined by thermostimulated luminescence and exo-electron emission

This article belongs to a three-part series dedicated to the simultaneous measurements of thermally stimulated exo-electron emission (TSEE), desorption (TSD) and luminescence (TSL). These works aim is to verify the ability of the applied experimental setup to characterize and analyze at the same time all the three above-mentioned stimulated processes. In agreement with the two preceding publications that deal TSD and TSEE, this article proves the consistency of the results of the individual modes of joint measurements of TSL and TSEE (as well as bulk and surface, the corresponding sources of emission, the recombination processes and their interactions). The whole experimental apparatus was tested using a well-known material, high-purity monocrystalline KBr, in the temperature range ΔT = −196 °C–430 °C (ΔT = 77 K–703 K) and in the chamber with base pressure p < 1 × 10−6 Pa to obtain a better comparison of results. Our in-house developed software for decomposing of the measured curves into the individual components and the subsequent calculation of the corresponding activation energies and frequency factors clearly confirms these results.

Surface processes on KBr single crystals examined by thermostimulated exo-electron emission and desorption

Potassium bromide (KBr) is a material with many potential uses in various branches of physics and technology (in scintillators, for a Fourier spectrometer, electro-optical elements such as IR laser windows, sensors and optical fibers). We conducted a series of combined measurements of thermally stimulated exo-electron emission (TSEE) and thermally stimulated desorption (TSD). At low doses of UV irradiation, the most prominent process is the combined emission of negative charges with desorption of the components of O, H2O and mainly K, KOH and Br, as described below. At higher irradiation doses, the KBr F-center plays the most important role (Tm = 160 °C) and at temperatures above +200 °C also the decomposition of chemically bonded atoms and other components occurs. This process is accompanied by the corresponding emission of electrons and at the same time some of the already chemically bonded structures such as HBr3 are released. TSD maxima are defined and the same temperature maxima for TSD and TSEE are shown, mainly at the already mentioned higher temperatures. These maxima are linked to the defects and interactions in the surface layer. The obtained results fully confirm the applicability of our apparatus for combined measurements of TSD, TSEE, thermally stimulated luminescence (TSL) and possibly also thermally stimulated conductivity (TSC).

Elaboration and study of structural and optical properties of AuBr nanocrystalsdispersed in KBr single crystals

The structural and optical properties of the AuBr nanocrystals embedded in the KBr single crystals grown by the Czochralski method were studied using several techniques. The X-ray diffraction reveals their formation and incorporation. The crystallite average radii calculated from XRD using the Scherrer's formula was found to vary between 19 and 39 nm. The photoluminescence spectra obtained on these same samples present an emission band located at 1.91 eV (650 nm), which can be attributed to the orange luminescence of AuBr nanocrystals. However, the photoluminescence excitation measurements of these nanocrystals exhibit an excitation band located at 3.44 eV (360 nm), slightly shifted towards higher energies in comparison with the massive crystal.

Luminescence and structural characterization on praseodymium (Pr3+) doped potassium bromide (KBr) single crystals.

The luminescence properties of praseodymium (Pr3+ ) ion in potassium bromide (KBr) host have been investigated. The excitation bands observed in the region 220-300nm are attributed to the transitions from 4f level to the 5d crystal field splitting levels of Pr3+ ion. Emission bands originated due to transition of 3 P2 →3 H6 (564nm), 3 P0 →3 H6 (604nm), 1 D2 →3 P4 (721nm) and 3 P0 →3 F2 (659nm) have been observed. Optical absorption, photoluminescence (PL) and thermoluminescence (TL) studies confirms the trivalent state of Pr3+ ion in KBr host. Trap parameters for the TL glow curve have been calculated and the values are tabulated by using Chen's peak shape method. The lattice cell parameters of Pr3+ doped KBr single crystals are a=b=c=6.596Å, α=β=γ=90°. From the scanning electron microscopy (SEM) micrograph analysis, the particle size ranges from a few micrometres to 5μm. Electron paramagnetic resonance (EPR) spectrum exhibits two resonance signals at magnetic fields of 2895 and 3106 gauss with g-values of 2.32 and 2.16, respectively, calculated using Lande's formula.

Dynamics of radio-photoluminescence and thermally-stimulated luminescence in KBr:Sm

Sm-doped KBr single crystals were prepared by the Bridgman–Stockbarger technique. The samples show both radio-photoluminescence (RPL) and thermally-stimulated luminescence (TSL). The RPL is due to the valence change of the Sm ion (Sm3+ → Sm2+), induced by X-rays, so the photoluminescence emission by Sm2+ appears as a result of irradiation and the intensity increases with the irradiation dose. The emission of Sm2+ is due to the 5d–4f transitions. The RPL response seems to be related with the thermally-stimulated luminescence (TSL) on the same samples. The TSL emission is predominantly due to Sm2+, instead of Sm3+, indicating that capture of a hole by Sm2+ takes place. As a result of TSL (or heat treatment), Sm2+ reverts to Sm3+ as the original state. The thermal activation energies have been derived from TSL isothermal analyses, and the obtained values are 1.42, 1.55, 1.65, and 1.74 eV.

X-ray-induced defects as obstacles to dislocation motion in alkali halide crystals

NaBr and KBr single crystals were exposed to X-ray (W-target, 30 kV, 20 mA) at room temperature and strain rate cycling tests associated with ultrasonic oscillation were carried out for the crystals at 77–293 K. The stress decrement (Δτ) due to oscillation and the stress change due to strain rate cycling have been measured during plastic deformation. The relative curve of ∆τ and strain rate sensitivity (λ) of flow stress has a stair-like shape for the two kinds of crystals. That is to say, the curve has two bending points and is divided into three regions: two plateau regions and the region between the two bending points, where λ decreases gradually with increasing ∆τ. The first region is a plateau within the small ∆τ. This implies that X-ray-induced defects have weak interaction with dislocation and act as obstacles to dislocation motion. Furthermore, the dependence of stress decrement (τ p) at the first bending point on the activation volume (V) obtained from the difference between λ in the first and second plateau regions reflects the interaction between dislocation and defects induced by the X-irradiation. The activation energy for the break-away of a dislocation from the defect could be obtained on the basis of τ p–V curve fitting the Barnett model to the experimental results. Then, the values of activation energy are 0.76 and 0.81 eV for NaBr and for KBr, respectively.

Study of neutron diffraction at a single crystal of KBr

The diffraction of neutrons at a single crystal of potassium bromide in experiments performed at the 50-m-long baseline of the RADEX facility at the Institute for Nuclear Research in February 2012 is studied. The neutron-beam parameters are determined. The effect of splitting of the 200 reflection in the region of the p-wave resonance of 81Br is found. The mosaicity of the studied sample is estimated. The estimate is indicative of the fine quality of the single crystal of potassium bromide.

Surface three-dimensional velocity map imaging: A new technique for the study of photodesorption dynamics

The authors describe the application of a combination of velocity map imaging and time-of-flight (TOF) techniques to obtain three-dimensional velocity distributions for surface photodesorption. They have established a systematic alignment procedure to achieve correct and reproducible experimental conditions. It includes four steps: (1) optimization of the velocity map imaging ion optics’ voltages to achieve optimum velocity map imaging conditions; (2) alignment of the surface normal with the symmetry axis (ion flight axis) of the ion optics; (3) determination of TOF distance between the surface and the ionizing laser beam; (4) alignment of the position of the ionizing laser beam with respect to the ion optics. They applied this set of alignment procedures and then measured Br(P23/2) (Br) and Br(P21/2) (Br∗) atoms photodesorbing from a single crystal of KBr after exposure to 193 nm light. They analyzed the velocity flux and energy flux distributions for motion normal to the surface. The Br∗ normal energy distribution shows two clearly resolved peaks at approximately 0.017 and 0.39 eV, respectively. The former is slightly faster than expected for thermal desorption at the surface temperature and the latter is hyperthermal. The Br normal energy distribution shows a single broad peak that is likely composed of two hyperthermal components. The capability that surface three-dimensional velocity map imaging provides for measuring state-specific velocity distributions in all three dimensions separately and simultaneously for the products of surface photodesorption or surface reactions holds great promise to contribute to our understanding of these processes.

Thermoluminescence, photoluminescence, photo-stimulated luminescence and optical studies on X-ray irradiated KBr:Ce3+ crystals

Abstract This paper reports thermoluminescence (TL) glow and emission, optical absorption and optically stimulated emission studies made on Ce3+ doped KBr single crystals irradiated at room temperature. Optical absorption and photoluminescence studies confirm the presence of cerium ions in the trivalent state. A broad and intense blue emission around at 290 and 390 nm, attributable to the transition from 5d excited state to the 4f1 ground state of Ce3+ ions could be observed in the photoluminescence emission. Two TL glow peaks were observed at 374 and 422 K. Presence of characteristic emission due to Ce3+ ions in the optically stimulated emission at the F-band confirms the participations of the Ce3+ ions in the defect production and recombination processes.

Interaction between dislocation and divalent impurity in KBr single crystals

Interaction between dislocation and divalent impurity has been investigated in KBr single crystals doped with various impurities by strain rate cycling tests under ultrasonic oscillation. The interaction energy between dislocation and impurity-vacancy dipole (I-V dipole) has been obtained by fitting Barnett model to experimental results. The interaction energies between dislocation and I-V dipole were determined to be 0.51, 0.30, 0.23 and 0.36eV for Ba2+, Sr2+, Ca2+ and Mg2+, respectively. The energy increases with increasing ionic radius of divalent impurity except Mg2+. This may be because the I-V dipole is made of a divalent impurity and a nearest vacancy for Ba2+, Sr2+ and Ca2+ but of a divalent one and a next nearest vacancy for Mg2+.

A kelvin probe force microscopy of charged indentation-induced dislocation structures inKBr

The incipient stages of plasticity in KBr single crystals have been examined in ultra-highvacuum by means of atomic force microscopy and Kelvin probe force microscopy (KPFM).Conducting diamond-coated tips have been used to both indent the crystals and image theresulting plastic deformation. KPFM reveals that edge dislocations intersecting the surfacecarry a negative charge similarly to kinks in surface steps, while screw dislocations show nocontrast. The charges are attributed to trapped cation vacancies which compensatethe charge of divalent impurities. Furthermore, the site of indentation has beenfound to carry a large positive charge. Weak topographic features extending in the⟨110⟩ direction from the indentation are identified by atomic-resolution imagingto be pairs of edge dislocations of opposite sign, separated by a distancesimilar to the indenter radius. They indicate the glide of two parallel{110} planes perpendicular to the surface, a process which allows for a slice of KBr to be pushedaway from the indentation site.

Decay kinetics of the luminescence of Ag– centers doped in alkali halide crystals

AbstractDecay kinetics of the luminescence of Ag– centers doped in NaCl, KBr, and CsBr single crystals have been investigated in the temperature range from 4 K to 260 K. At low temperature, the radiative transitions from the 1T1u states in the Ag– centers exhibit the luminescence with the decay time constants of about 1.3 ns. With increasing temperature, the decay time constants decrease, reflecting a thermally activated non‐radiative transition process from the 1T1u to 3T1u states. On the other hand, the luminescence due to the radiative transitions from the 3T1u states have the decay time constants of about 1.0 ms at low temperature and about 10 μs above 80 K. From the analysis of the temperature dependence of the decay time constants, the parameters of a three‐level model and a thermally activated non‐radiative process from the 1T1u to 3T1u states are obtained. The parameters are discussed as compared with those of other Tl+‐type centers doped in alkali halide crystals. (© 2009 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

CdTe aggregates in KBr crystalline matrix

In this work, we report the experimental results on the fabrication and optical characterization of Czochralski (Cz) grown KBr single crystals doped with CdTe crystallites. The results of the optical absorption have shown two bands, the first one located at 250 nm demonstrates the incorporation of cadmium atoms in the KBr host followed by a partial chemical decomposition of CdTe, the second band located at 585 nm shows an optical response of CdTe aggregates. Photoluminescence spectra at room temperature before annealing showed a band located at 520 nm (2.38 eV), with a blue shift from the bulk gap of 0.82 eV ( E g (CdTe)=1.56 eV). While the photoluminescence spectra after annealing at 600 °C showed a band situated at 640 nm (1.93 eV), these bands are due to band-to-band transitions of CdTe nanocrystals with a blue shift from the bulk gap at 0.38 eV. Blue shift in optical absorption and photoluminescence spectra confirm nanometric size of dopant. X-ray diffraction (XRD) spectra have shown the incorporation of CdTe aggregates in KBr.

Nano-meter scale plasticity in KBr studied by nanoindenter and force microscopy

AbstractThe early stages of plasticity in KBr single crystals have been studied by means of nanometer-scale indentation in complementary experiments using both a nanoindenter and an atomic force microscope. Nanoindentation experiments precisely correlate indentation depth and forces, while force microscopy provides high-resolution force measurements and images of the surface revealing dislocation activity. The two methods provide very similar results for the onset of plasticity in KBr. Upon loading we observe yield of the surface in atomic layer units which we attribute to the nucleation of single dislocations. Unloading is accompanied by plastic recovery as evident from a non-linear force distance unloading curve and delayed discrete plasticity events.

Three-dimensional velocity map imaging of KBr surface photochemistry

We have combined the velocity map imaging technique with time-of-flight measurements to study the surface photochemistry of KBr single crystals. This approach yields 3-dimensional velocity distributions of Br atoms resulting from 193 nm photodesorption. The velocity distributions indicate that at least two non-thermal mechanisms contribute to the photodesorption dynamics. Our experimental geometry also allows us to measure the Br(2P3/2):Br(2P1/2) branching ratio, which is found to be 24:1.

Effect of microwave heating on diffusion in KCl–KBr single crystals

The interdiffusion in single-crystal KCl-KBr diffusion couples, which form continuous solid solutions, is examined under microwave heating (24 GHz). The results are compared with data obtained under conventional heating. Electron microprobe analysis is used to examine the composition of the diffusion zone and microscopic analysis is used to examine its microstructure. It is shown that ion interdiffusion occurs faster under microwave heating than under conventional heating. The ascending diffusion of Cl and Br ions is revealed under microwave heating. The microstructure of the diffusion zone between KCl and KBr single crystals is highly heterogeneous after microwave heating and markedly different from that after conventional heating.

Analysis of temperature and concentration dependence of flow stress in KCl–KBr single crystals with special reference to solute distribution

Available data on the temperature and concentration dependence of critical resolved shear stress (CRSS) of KCl–KBr solid-solution crystals containing 9, 17, 27 and 45 mol% KBr in the temperature range 77–230 K have been analyzed within the framework of the kink-pair nucleation model of plastic flow in solid- solution crystals. It is found that CRSS τ decreases with increasing temperature T in accordance with the model relation lnτ = A − BT, where A and B are positive constants. The CRSS τ at a given temperature depends on solute concentration c as τ ∝ cp , where exponent p has a value between 0.33 and 0.57 as temperature T rises from 0 to 230 K. The model parameter W o, i.e. binding energy between the edge-dislocation segment L o involved in the unit activation process and the solute atoms close to it (T → 0 K), which is inversely proportional to B, increases with solute concentration c monotonically as W o ∝ c 0.33 up to a critical value c m = 35 mol% KBr, which is in reasonable agreement with the model prediction W o ∝ c 0.25. However, W o decreases with an increase in c beyond c m, which indicates somewhat ordered distribution of solute in the host lattice of concentrated KCl–KBr solid solutions with c > c m.

Color center formation in KCl and KBr single crystals with femtosecond laser pulses

Because of their extremely high instantaneous powers, femtosecond lasers can color many nominally transparent materials. Although the excitations responsible for this defect formation occur on subpicosecond time scales, subsequent interactions between the resulting electronic and lattice defects complicate the evolution of color center formation and decay. These interactions must be understood in order to account for the long-term behavior of coloration. In this work, we probe the evolution of color centers generated by femtosecond laser radiation in potassium chloride and potassium bromide single crystals on time scales from microseconds to hundreds of seconds. By using an appropriately chosen probe laser focused through the femtosecond laser spot, we follow the changes in coloration due to individual or multiple femtosecond pulses and the evolution of that coloration for long times after femtosecond laser radiation is terminated.