Collapse of sodium polyacrylate chains in calcium salt solutions

The sodium salt of polyacrylic acid (NaPA) precipitates in the presence of Ca²⁺-ions. This phase behaviour can be represented by a phase diagram where the critical NaPA concentration is plotted versus the critical Ca²⁺ concentration resulting in a straight line as a phase boundary. The location of this phase boundary is influenced by the presence of an inert monovalent salt like NaCl. The present contribution focuses on the coil dimensions of NaPA chains in dilute aqueous solution corresponding to the one phase region of such a phase diagram. A variety of parameters with which the size and shape of the polyelectrolyte chains can be modulated are revealed. Approaching the phase boundary by decreasing the NaPA concentration at a constant Ca²⁺ content leads to a collapse of the NaPA chains. Combined static and dynamic light scattering suggests a compact spherical shape as the final state of this transition, both in 0.1 M NaCl and in 0.01 M NaCl. In the lower NaCl concentration, indication is presented for the existence of a cigar or pearl necklace like intermediate. Most strikingly, the collapsed chains can be reexpanded by increasing the concentration of inert NaCl at constant content of NaPA and Ca²⁺. Clearly, excessive Na⁺-ions displace the Ca²⁺-ions from the NaPA chains. Received 18 July 2000 and Received in final form 24 August 2000     

Calcium precipitation in NaCl monitored by the Eu2+ -fluorescence

Taking into account that the optical spectroscopy of the Eu²⁺ ion is quite sensitive to the crystalline environment in which this ion is located, in the present investigation the fluorescence of a small concentration (8 ppmm) of divalent europium incorporated into a NaCl crystal which was also doped with ≈4500 ppmm of Ca²⁺ has been systematically investigated as a function of different thermal treatments in order to study the calcium-precipitation processes in the host NaCl. The data presented in this paper strongly suggest that the annealing of quenched samples at 200°C produces the incorporation of Eu²⁺ into the stable dihalide phase CaCl₂, as well as into the metastable precipitated CaCl₂-like plate zones parallel to the {111} planes of the NaCl matrix. This fact is associated with the increase in intensity of two overlapping emission bands peaking at 430 and 432 nm. On the other hand, the aging of quenched samples at room temperature produces the growth of three emission bands peaking at 400, 414, and 447 nm. The former two emission bands have been associated with Eu²⁺ embedded into calcium precipitates, the structures of these precipitates being different from that of CaCl₂, while the band at 447 nm has been ascribed to europium ions incorporated into the metastable precipitated CaCl₂-like plate zones parallel to the {310} planes of the NaCl matrix. Some of the characteristics of the different calcium second phase precipitates have been obtained by measuring the crystal field splitting (10 Dq) of the 4f⁶5d configuration of the divalent europium ions when they were located inside them. Values for the 10 Dq splitting were determined from the excitation spectrum of each of the emission bands associated with the different calcium-precipitated phases.     

Phase transitions in PbTe under quasi-hydrostatic pressure up to 50 GPa

PbTe has been investigated using synchrotron X-ray diffraction (XRD) in a diamond anvil cell under quasi-hydrostatic pressures up to 50 GPa. Upon compression to 6.6 GPa, the initial NaCl phase transforms to an intermediate phase, which is confirmed to be an orthorhombic structure with a space group Pnma. At 18.4 GPa, the intermediate Pnma phase undergoes a phase transition to the CsCl structure. The systemic analysis of the crystal structures between the NaCl and intermediate phases indicates that the structure of the Pnma phase could be derived from the distortion of the NaCl structure. The bulk modulus of the CsCl phase is B₀=52(2) GPa with V₀=60.8(4) ų and B′₀=4.0 (fixed), slightly larger than the NaCl phase (B₀=44(1) GPa) and the intermediate phase (B₀=49(3) GPa).     

Growth and luminescent properties of the Ce,Pr doped NaCl single crystals grown by the modified micro-pulling-down method

We have investigated luminescent properties of nondope, Ce and Pr doped NaCl [nondope NaCl, Ce:NaCl, Pr:NaCl] single crystals grown by a modified micro-pulling-down method with a removable chamber system. Nondope, Ce 1% and Pr 1% doped NaCl crystals with a single phase of NaCl structure were obtained and the crystals indicated general crystal quality by the X-ray rocking curve measurement. For the nondope NaCl and Pr:NaCl crystals, the transmittance spectra indicated almost more than 60% in the wavelength from 200 to 800 nm and an absorption of Ce³⁺ ion was observed in the transmittance spectrum of Ce:NaCl crystal. The emission spectrum originated from Ce³⁺ 5d–4f transition appeared around 300 nm in the photoluminescence spectrum and the decay time was 19.7 ns.     

Influence of the precipitated phases of Pb++ and Sr++ on the f-coloring behavior of NaCl

The F-coloring efficiencies of NaCl:Pb⁺⁺ and NaCl:Sr⁺⁺ have been measured during stage I as a function of lead and strontium precipitation into the metastable and stable phases, the optical absorption spectrum of the doped crystals was analyzed after each annealing treatment. On the other hand, strontium precipitation was monitored using Eu⁺⁺ as an optical probe taking advantage of the fact that this ion can be incorporated into the Sr-precipitates producing emission bands characteristic of each type of precipitate in which it is embedded. The results indicate that the metastable and stable second phase precipitates of these ions contribute in a quite different manner to the coloring processes.     

EELS of methane physisorbed on NaCl(100)

EELS spectra of CH₄, CD₄ and CH₂D₂ physisorbed on NaCl(100) at 40 K are presented. For the clean NaCl surface, increasing the incident beam energy to the 30 to 50 eV range is sufficient to overcome charging effects. However, when adsorbate is present charging prevents extended signal averaging. All methane modes appear to contribute to the EELS spectra. Loss peaks corresponding to adsorbate vibrations are very broad (400 to 600 cm⁻¹, FWHH), the low resolution being probably due to a combination of effects including surface disorder, scattering of electrons by gas phase molecules and charging effects.     

EPR spectra of Mn2+ ions in precipitated divalent impurity phases in monocrystalline NaCl

The EPR spectra of Mn²⁺ ions embedded into precipitated phases of Mg²⁺, Cd²⁺, Fe²⁺ and Ca²⁺ in NaCl single crystals have been investigated. The spectrum from samples whose major impurity is Mg, Cd or Fe corresponds to Mn²⁺ substituting some divalent cation inside the Suzuki phase (6NaCl.MCl₂). The existence of such a phase has been ascertained by means of Raman spectroscopy. On the other hand, the EPR spectrum of Ca²⁺ doped samples has been attributed to Mn²⁺ inside various CaCl₂ precipitates.     

Aggregation pathways and Suzuki phase formation in doped alkali halides

A model about Suzuki phase formation in Alkali Halides doped with divalent cationic impurities is presented. It considers rearrangement of small aggregates as dimers and trimers. Experimental data of Suzuki phase nucleation at room temperature in NaCl:Cd²⁺ corroborate this model.     

Effect of temperature and ionizing radiation on the Eu-Mn pair in NaCl

In the present paper photoluminescence measurements have been carried out in single crystals of NaCl doubly doped with Eu²⁺ and Mn²⁺ ions in order to determine the influence of annealing temperature and ionizing radiation on the Eu-Mn pairs which are formed preferentially in this material. From the results obtained it appears that the NaCl:Eu:Mn system may be employed not only as a good device for optical convertion of U.V. light into visible, being sure that its efficiency is not affected by the aggregation-precipitation processes of the manganese ions occurring in the matrix, but also as an efficient photo-luminescence dosimeter.     

Temperature dependence of EPR spectrum of Mn2+ doped in ammonium iodide single crystals

EPR of Mn²⁺ doped in ammonium iodide single crystal has been studied at X-band in the temperature range 573–577 K. The observed temperature dependence of line widths and spin Hamiltonian parameter b₂⁰ below room temperature is related to the structural transformations in the crystal. The coexistence of high temperature phase (NaCl) and low temperature phase (CsCl) is attributed to the large thermal hysteresis in line widths and b₂⁰. The dissociation of ion vacancy pairs occurs near 500 K and is reflected in the reversible change of an anisotropic EPR spectrum in an isotropic sextet near this temperature. The ion vacancy pair models for NaCl and CsCl phases are discussed along with the effects of thermal processing of the samples. Heating the crystals above 500 K leads to expulsion of Mn²⁺ impurity from the crystal.     

Microhardness studies on melt-grown single-phase mixed (NaK)Cl and solution-grown two-phase NaClKCl crystals

Microhardness studies were carried out on melt-grown (NaK)Cl crystals. The quenching strains and the difference in the ionic sizes of the cations constituting the mixed system introduced large numbers of defects, viz. dislocations, grain boundaries, etc. The etching experiments and supplementary X-ray studies clearly revealed that the Na⁺ or K⁺ ions are precipitated at the dislocation sites. These phase particles strongly interact with dislocations so as to obstruct the mobility of the latter contributing to the hardening mechanism. Results are compared with solution-grown two-phase mixed NaClKCl pure NaCl and pure KCl crystals.     

Solubility of free and associated strontium in NaCl crystals

The solubility of free and associated strontium in NaCl crystals is investigated by the method of electrical conductivity and ionic thermoconductivity (I.T.C.). The solubility of free strontium in NaCl is rather low, the enthalpy of solution is $\text{h}{}_{\mathrm{d}}{}^{\mathrm{<mtext>f.i.</mtext>}}\text{2}\text{= 0.904}\text{eV}$. The dissolution of segregated strontium in the NaCl lattice is a two-stage process: in the first stage (290–530 K) there are present the isolated I.V. dipoles (peak at 226 K) and Suzuki phase inclusions (peak at 233 K); at ageing temperatures over ～ 530 K there takes place the thermal decomposition of Suzuki phase inclusions: this is the second stage. The solubility of associated strontium is characterized by the energy of solution 0.49 ± 0.03 eV. The reorientation of I.V. dipoles is described by the relaxation mode with τ₀ ～ 10^?13 sec and the enthalpy of 0.66 ± 0.02 eV.     

Impurity induced far infrared absorption in doped NaCl and KCl

Far infrared absorption measurements with NaCl single crystals doped with AgCl and CuCl respectively show resonant band mode lines at 4.2 ?K at 52.5 cm^?1 and 23.6 cm^?1 respectively. Their measured halfwidths are 10 cm^?1 and 0.7 cm^?1 respectively. The halfwidth of the NaCl: Cu⁺ line increases strongly with increasing temperature. In NaCl:Mg⁺⁺, NaCl:Ca⁺⁺ and NaCl:Ag⁺ a temperature independent broad absorption is observed above 50 cm^?1, which behaves roughly as the density of phonon states in the unperturbed crystal. In KCl: Cu⁺ and KCl: Br^? a temperature independent absorption with a maximum at 108 cm^?1 is obtained corresponding to an expected region of high density.     

Effect of X-irradiation on impurity-vacancy dipoles in lead and calcium-doped NaCl

In the present paper the effects of the room temperature X-irradiation in calcium and lead-doped NaCl have been reinvestigated in order to get a deeper insight into the nature of the radiation induced halogen interstitial traps. Special attention has been paid to determine whether a correlation exists between the change in the concentration of Ca²⁺ (Pb²⁺-cation vacancy dipoles and the color center formation. Our results indicate that for both systems, the dipoles are the fundamental traps for the interstitial defects in agreement with the theoretical models recently developed for F-center formation in alkali halides doped with divalent impurities. This conclusion is, however, in disagreement with that reported by Ikeya et al in a previous work on calcium-doped NaCl in which it was concluded that the isolated positive ion vacancies were the dominant traps for the halogen interstitial defects. Some comments about this discrepancy are given.     

Effect of irradiation on the laws of annealing of impurity hardening of NaCl crystals

Studies were carried out on crystals containing various types of impurity: Ca²⁺, a hole-acceptor impurity, which dissolves well in NaCl; and Pb²⁺, an electronacceptor impurity, which dissolves poorly in NaCl. For the first time when doped crystals were annealed softening was observed to occur as a result of irradiation with soft x-rays prior to the annealing. The microhardness of irradiated doped NaCl crystals drops below the value that corresponds to the unannealed unirradiated sample. This effect is more pronounced in NaCl:Pb. It is postulated that the weakening effect is due to the breakup of impurity complexes. This assumption is supported by the results of studies on the laws governing the annealing of dislocation structures about the imprints of an indenter. These laws are shown to depend on the type of impurity: processes of dislocation polygonization were observed in NaCl:Ca crystals and the dislocation distribution was random in NaCl:Pb crystals. Pre-irradiation of the latter caused dislocations to be built in and polygonization to appear. Evidently, by breaking up impurity complexes in NaCl:Pb crystals x-ray irradiation brings the state of the impurity in them closer to that in NaCl:Ca.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 8, pp. 57–62, August, 1988.     

Status of an R&D project of a positron gun at “Horia Hulubei” NIPNE Bucharest

A new positron gun (PG) will enable high sensitivity measurements in applications of positron annihilation spectroscopy in Romania. Some data concerning the design of a modular system for focussing, transport and acceleration of mono-energetic positrons in the range 0.8-50 keV have been obtained and experimenting on moderators and CDBS was performed. We present a short overview of the present status of the project and preliminary results from Coincidence Doppler Broadening Spectroscopy with a ²²NaCl source, on Al samples. The entire positron gun system will be designed as a high-vacuum dedicated extension operating with two options: a 50 mCi ²²NaCl source and in-line with the NIPNE cyclotron or a new intense compact cyclotron.     

The structures of the ammonium halides

A comparison with the alkali halides suggests that all the ammonium halides should occur in the NaCl centre-of-mass structure. Experimentally, at room temperature and atmospheric pressure, only NH₄I crystallizes in this structure, while NH₄F is found in the ZnO structure, and NH₄C1 and NH₄Br occur in the CsCl structure. We show that a distributed charge on the NH₄⁺ ion can explain these structures. Taking charges of + 0.2e on each of the five atoms in NH₄⁺, as suggested by other studies, we have recomputed the Madelung energy in the cases of interest. A full ionic theory including electrostatic, van der Waals and repulsive interactions then explains the centre-of-mass structures of all the four ammonium halides. The thermal and pressure transitions are also explained reasonably well. The calculated phase diagram of NH₄F compares well with experiment. Barring the poorly understood NH₄F(II) phase, which is beyond the scope of this work, the other features are in qualitative agreement. In particular, the theory correctly predicts a pressure transition at room temperature from the ZnO structure directly to the CsCl structure without an intermediate NaCl phase. A feature of our approach is that we do not need to invoke hydrogen bonding in NH₄F.     

Conductivity relaxations in single crystals of sodium chloride containing suzuki phase precipitates

Single crystals of NaCl containing volume fractions of up to 28% of the Suzuki phase Cd□Na₆Cl₈ have been subjected to a.c. electrical measurements over a range of frequencies (10-10⁷ Hz). Complex modulus plots and modulus spectra reveal two distinct electrical relaxations arising in the matrix, and the precipitates. The Maxwell layer model and the Maxwell-Wagner model for dispersions of two phases are used to separate the two ionic conductivities. The activation energy for ionic conduction in the Suzuki phase is found to be 0.93 ± 0.05 eV, which is in agreement with a previously reported value obtained on polycrystalline Cd□Na₆Cl₈.     

Development of NaCl:K(6T) phosphor for thermoluminescence dosimetry

The present paper discusses the development of the NaCl:K (10 ^–1 molar fraction of potassium) phosphor as dosimeter material for beta-radiation thermoluminescent dosimetry (TLD). The TLD grade NaCl:K(6T) material was prepared by annealing the NaCl:K powder obtained from aqueous solution by the method of recrystalization and annealed at 600°C for two hours in open air. It is observed that thermal glow curves of NaCl:K(6T) specimen exhibit well defined glow peak around 170°C alongwith a weaker peak at lower temperature. The high-temperature peak is found to grow supralinearily with increase in beta dose in the range 10–2×10³ rad. Since the nature of glow curves under the influence of different doses remains more or less identical, it is believed that the phosphor does not undergo radiation damage and give high intrinsic TL around peak III. Examination of the system for fundamental dosimetry requirement shows that the NaCl:K(6T) material can be used as a suitable TLD material in beta radiation.     

Positron annihilation in ionic crystals

Positron lifetime measurements were made on Ca²⁺ doped NaCl, Ba²⁺ doped NaCl and X-ray- and additively-coloured KCl crystals. In the NaCl(Ca) system a correlation which could be approximated by a power equation was found between the dopant concentration and the positron annihilation parameters. In the NaCl(Ba) system the decay parameters do not change significantly as expected and no significant difference in the decay parameters of the coloured and uncoloured crystals could be found. Paper C 4 presented at 3rd Internat'l Conf. Positron Annihilation, Otaniemi, Finland (August 1973).