Luminescence of defect centres in Hg2Cl2

Abstract

Luminescence in Hg₂Cl₂, crystals excited with UV light is investigated in the spectral region 0.8–2.25 eV. Measurements are performed on as-grown samples and on samples previously exposed to UV light at RT. Six emission bands are found which depend on the concentration of the photochemical entities produced by irradiation of the crystals at RT. It is concluded that infra-red (IR) luminescence of Hg₂Cl₂ originates from crystal defects. The observed emission bands are tentatively attributed to the emission of (HgCI_xBr_3?x)^? and (HgCl_xBr_4?x)^2? complexes formed with residual Br impurities. Centres responsible for IR Hg₂Cl₂ emissions are excited: (i) via excitons of Hg₂Cl₂, (ii) via excited states of isolated Hg₂Br₂ molecules, and (iii) resonantly through the excitation bands of defect centres.     

A Refinement of the Hg2F2 Structure by the Use of NMR Data

Abstract

The crystal structure of Hg₂F₂ was studied in works.^1,2 The Hg₂F₂elementary cell of the tetragonal syngony with a=3.66 Å and c=10.89 Å contains two formula units, the space-group being 14/mmm. Hg₂F₂ belongs to those types of structures which possess one non-fixed coordinate (z_F and zHg); hence, the position of the fluorine nuclei may be determined from the value of the second moment of the NMR spectrum of ¹⁹F for powder sample.³ In this case it is ostensibly important, inasmuch as determining the positions of fluorine nuclei near the heavy mercury atoms presents a difficulty for X-ray analysis.     

Effects of the phase transition in Hg2(Br,I)2 crystals

The effects of the phase transition in Hg₂(Br,I)₂ crystals have been investigated over a wide range of temperatures by the Raman scattering spectroscopy and X-ray diffraction analysis. The overtones (at the X point of the Brillouin zone boundary) and the fundamental tones (at the center of Brillouin zone) of soft modes are found in the Raman spectra of these crystals and studied in detail. The density of one-phonon states of the soft TA branch manifests itself in the Raman spectra of mixed crystals. The potentialities of the soft-mode spectroscopy are realized in full measure. Analysis of the ratio between intensities of overtones and fundamental tones of the soft modes has demonstrated the applicability of the Landau phenomenological theory of phase transitions. The orthorhombic splitting of the reflections corresponding to the basal plane is revealed in the X-ray diffraction patterns and thoroughly explored. The temperature dependences of the isotropic and shear spontaneous strains are obtained. It is shown that the shear spontaneous strain plays a decisive role. The critical indices are determined and the model of the improper ferroelastic phase transition D _4h ¹⁷ → D _2h ¹⁷ in the vicinity of the tricritical point is corroborated.     

Simultaneous removal of NO and Hg0 from flue gas over MnSmCo/Ti catalyst at low temperature

The MnSmCo/Ti catalyst was designed for the simultaneous removal of NO and Hg⁰ at low temperature. The MnSmCo/Ti catalyst exhibited 80% NO conversion, almost 100% N₂ selectivity and 100% Hg⁰ removal in the absence of HCl within the temperature range of 150–250 °C with a gas hourly space velocity of 100,000 h^?1. The influence of flue gas components on Hg⁰ oxidation was investigated and the NO and O₂ are mainly responsible for Hg⁰ oxidation. Hg balance analysis revealed that the Hg⁰ removal was achieved through chemisorption and catalytic oxidation. Furthermore, the Hg⁰ oxidation mechanism was explored using transient reaction along with temperature-programmed desorption of mercury and X-ray photoelectron spectroscopy. Hg⁰ oxidation proceeds through Mars–Maessen mechanism in which adsorbed Hg⁰ is bound to MnO_x to form weakly bonded HgMnO_x+1 species and follows Langmuir?Hinshelwood mechanism, where adsorbed Hg⁰ reacts with active NO₂ to generate HgO.     

Pressure dependence of the soft modes in the model ferroelastics Hg2I2

The pressure dependence of the soft modes in the paraphase (boundary of the Brillouin zone, X point) and the ferrophase (center of the Brillouin zone, Γ point) is studied in the model ferroelastic crystals Hg₂I₂. An anomalous linear pressure dependence of the soft-mode frequency in the paraphase (p<p _c ) is observed. The results obtained and the nature of the ferroelastic phase transition induced in Hg₂I₂ crystals by a high hydrostatic pressure are discussed. Pis’ma Zh. éksp. Teor. Fiz. 63, No. 4, 227–231 (25 February 1996)     

Phase transition, order parameter, and their manifestation in optical spectra of Hg2Cl2

This paper reports on a high-precision study of the effects induced by the phase transition in Raman spectra of Hg₂Cl₂ crystals. The odd (acoustic and IR-active) phonons, including the soft modes, from the Brillouin zone (BZ) X points of the paraphase of these crystals, which are induced by the phase transition, unit cell doubling, and the X ?? ?? BZ folding, have been investigated both theoretically and experimentally. The temperature dependences of the intensities of these phonons, complemented by a theoretical analysis, have yielded the first information on the behavior of the transition order parameter. The corresponding critical exponents have been derived, which correlate in magnitude with the data obtained from X-ray diffraction measurements and, thus, imply the closeness of the phase transition in these crystals to the tricritical point.     

Novel Pyrazoline-Based Selective Fluorescent Sensor for Hg2+

This paper presents the preparation of a pyrazoline compound and the properties of its UV–Vis absorption and fluorescence emission. Moreover, this compound can be used to determine Hg²⁺ ion with selectivity and sensitivity in the EtOH:H₂O?=?9:1 (v/v) solution. This sensor forms a 1:1 complex with Hg²⁺ and shows a fluorescent enhancement with good tolerance of other metal ions. This sensor is very sensitive with fluorometric detection limit of 3.85?×?10^?10 M. In addition, the fluorescent probe has practical application in cells imaging.     

Surface analysis of Hg2Cl2

A surface composition of (110) face of Hg₂Cl₂ in high vacuum is studied for the room temperature. The impact of primary electrons with energy of 2·5 keV causes a decomposition of the surface layer which is described in detail.The single crystals of Hg₂Cl₂ used in this work were prepared by Dr. Barta in Institute of Solid State Physics of Czechosl. Acad. Sci., Prague.     

Indole-based Fluorescent Sensors for Selective Detection of Hg2+

A series of indole-based fluorescent chemosensors 1–4 were prepared and investigated characteristick features with transition metal ions. Sensors 1 and 2 were selective for Hg²⁺ ion among a series of metal ions in H₂O–DMSO with association constants of 4.60×10⁴ and 5.90×10⁴?M^?1 and detection limits of 140 and 101.6 μM, respectively.

Resonant Raman studies of surface composition and structural ordering in laser-irradiated Hg1-xCdxTe

1-x Cd_xTe for x=0.165 and x=0.18. The annealing was performed in air with a Nd:YAG laser tuned to the first harmonic. A pulse duration of 72 ns and irradiation energy densities of 270 and 400 mJ/cm² were used. Information on both the structural ordering and the alloy composition in irradiated Hg_1-xCd_xTe was obtained by resonance Raman spectroscopy with laser photon energies of between 2.41 and 2.54 eV. The presented Raman spectra from annealed samples indicate structural disorder and a decrease in the degree of alloying as a result of Hg outdiffusion and segregation caused by laser-induced nonequilibrium melting and solidification. Moreover, a shift of the maximum resonant enhancement for the TO₂, LO₂, A, and LO₁&TO₁ modes in the case of a Hg_0.82Cd_0.18Te sample, annealed by 400 mJ/cm² laser pulse, to higher photon energy points to an increase in the composition from x=0.18 to x?0.3. Received: 14 July 1997/Accepted: 10 October 1997     

The order parameter, its fluctuations, and the debye-waller factor of Hg2Cl2 model Ferroelastics

Data on the temperature behavior of the order parameter and its fluctuations were obtained by studying the X-ray reflexes and acoustic soft modes generated from X points on the boundary of the Brillouin zone in the paraphase of Hg₂Cl₂ crystals induced by phase transition and the doubling of unit cells. The corresponding critical exponents, the values of which coincide within the experimental error and testify to how close the phase transition in these crystals is to the tricritical point, are determined by means of X-ray structural analysis and spectroscopic investigations.     

Der Zerfall des Au195m,des Hg195m und des Hg195

Theγ-radiation of Au^195m , Hg^195m and Hg¹⁹⁵ was measured with a Ge(Li)-semiconductor detector. Conversion coefficients of most transitions could be determined from the measuredγ-intensities and from already known electron intensities. Spin and parity of the corresponding niveaus and logft-values for the decay of Hg¹⁹⁵ and Hg^195m were determined. Newγ-energies (698 keV, 754 keV, 852 keV) were found in the decay of Hg^195m . With a coincidence method a value of α_tot=13±3 for the 61,5 keV-radiation of Au^195m was measured.     

Band structure and optical transitions in the Hg3Se2Cl2 crystals

First-principles calculations of the band structure for Hg₃Se₂Cl₂ crystal were performed by means of density functional theory (DFT). The exchange and correlation potential was described within a framework of the local density approximation based on exchange-correlation energy optimization to calculate the total energy. DOS/PDOS and valence charge distribution were studied in details. Absorption near the fundamental edge was found to be due to indirect and direct allowed transitions. For device applications based on Hg₃Se₂Cl₂ crystal, understanding these fundamental issues is highly important and essential. It should be noted that optoelectronic applications of Hg₃Se₂Cl₂ are caused by coexistence of the large polarized Hg cations and a huge contribution of an harmonic phonon subsystem caused by anions.     

Two-photon absorption and energy band structure of orthorhombic Hg2Cl2 crystals

Dependence of two-photon absorption (TPA) rate on the state of polarization of a laser beam is investigated in the low-temperature orthorhombic modification of Hg₂Cl₂ crystals. Theoretical calculations of the dependence of TPA rate on the direction of polarization vector of the beam are performed for centrosymmetric points, Y, Z, T, R andS of the Brillouin zone. The domain structure of real crystals is taken into consideration and it is shown that periodicity of the polarization dependence may indicate whether TPA is due to transitions either at, Y, Z, T orR, S points. The polarization dependence of TPA cannot, however, distinguish between points inside these two groups. Comparison of theory with a low-temperature (T 8·5 K) experimental curve of polarization dependence is discussed. It is shown that the experiment can be explained in main features by a model of noninteracting oriented linear dipoles. Further, on the basis of TPA measurements, a simple energy band structure of Hg₂Cl₂ is proposed regarding Hg₂Cl₂ crystal as a linear chain of molecules.The authors express thanks to Dr. . Barta for supplying the Hg₂Cl₂ crystals, Dr. Z. Bryknar for critical comments and Dr. B. Velický for stimulating discussion. We thank also Dr. V. Kohlová for assistance during measurement.     

Quadrupole interaction studies of Hg in Sb

Time differential perturbed angular correlation and nuclear orientation studies of the electric quadrupole interaction for Hg in Sb have been performed. The effective field gradients |V _zz ^eff (HgSb)|=1.43(18)×10¹⁷V cm^–2 at room temperature andV _zz ^eff (HgSb)=+1.8(2) × 10¹⁷V cm^–2 below 0.05 K have been derived. These two values are no indication for an anomalous temperature dependence of the effective field gradient for Hg in Sb. The value of the electric field gradient fits well into the systematics for Hg in other hosts. It is shown that the electronic enhancements of the field gradients are correlated to the valence of the impurities and are rather insensitive to the host properties.On leave of absence from: University of Lisboa, Portugal     

199Hg Mössbauer measurements on mercury alloys and Hg-fluorides

The Mössbauer effect on the 158 keV 5/2^??1/2^? transition in¹⁹⁹Hg, of the order of 10 ppm, has been studied using the current integration technique. The isomer shift between the Hg(I)- and Hg(II)-fluorides as well as the quadrupole splitting in Hg₂Pt and Hg₂F₂ are interpreted in terms of relativistic Hartree-Fock-Slater and Molecular Orbital calculations. The following nuclear parameters could be derived: δ2<=(3.2± l.l)10^?3fm² andQ(5/2^?)=(-0.8±0.4)b. Evidence for an oblate triaxially deformed¹⁹⁹Hg nucleus is derived from particle plus rotor calculations.     

Characterization of the structure and dynamics of an aqueous Hg solution by an ab initio molecular dynamics study

Quantum mechanical charge field (QMCF) MD simulation has been performed to investigate the structure and dynamics of Hg²⁺ hydrate. The first-shell hexacoordinated [Hg(H₂O)₆]²⁺ complex with an average Hg²⁺-O distance of 2.40 Å is dominantly found, which corresponds to the neutron diffraction and extended X-ray absorption fine structure (EXAFS) experiments. Other species, in particular the 7-fold coordinated complexes, can be formed transiently, according to the water exchange processes with an associative interchange (I_a) mechanism. The second hydration shell exhibits a Hg²⁺-O distance of 4.6 Å with a coordination number of ~ 14. The mean residence times (MRTs) of first- and second-shell waters clearly indicate a strong “structure-forming” ability of Hg²⁺ in aqueous solution.     

Low-temperature Raman spectra of Hg<Subscript>2</Subscript>(Br,I)<Subscript>2</Subscript> mixed crystals

Raman spectra of Hg₂(Br,I)₂ mixed crystals were studied. The spectra revealed multimode behavior of optical vibrations, which were assigned to the existence in these crystals of molecules of three types, namely, Hg₂Br₂, Hg₂I₂, and Hg₂BrI. The spectra exhibit a manifestation of phase transition effects associated with soft modes, the density of states of IR-active vibrational branches, and of nanoclusters, whose nucleation is induced by the Br-Hg-Hg-I dipole molecules.     

Radiative Lifetimes of Some 5d10n2F and 5d10n2P Levels of Hg+

Radiative lifetimes of 8p²P_3/1 (15.3 ns) and nf²F (7f²F_5/2 - 24.7 m. 7f²F_7/2 - 5.5 ns, 8f²F_7/2 - 48.5 ns, 8f²F_5/2 - 47.0 ns) levels of Hg⁺ have been measured employing delayed coincidence method. The dependencies of radiative lifetimes vs effective principal quantum number for np²P_3/2 and np²F_7/2 series are non linear.