Ewa Mielniczek-Brzóska

Effect of Cu(II) ions on the growth of ammonium oxalate monohydrate crystals from aqueous solutions: growth kinetics, segregation coefficient and characterisation of incorporation sites

Abstract The experimental results of a study of the effect of concentration of Cu(II) ions and solution supersaturation on growth rates of ammonium oxalate monohydrate crystals, impurity segregation coefficient and sites of incorporation of cations in the crystal lattice are described and discussed. It is found that the effects of impurities on growth kinetics and the value of the segregation coefficient are related processes and involve adsorption of impurity particles on growing surfaces. Examination of EPR spectra shows that high incorporations of Cu(II) ions in the crystal lattice favour the substitution of ammonium sites while at low incorporations copper(II) ions enter the lattice both in interstitial and substitutional lattice positions.

Study of segregation coefficient of cationic impurities in ammonium oxalate monohydrate crystals during growth from aqueous solutions

Abstract The segregation coefficient k eff of Cu(II), Fe(III) and Cr(III) ions in ammonium oxalate monohydrate single crystals during growth from aqueous solutions at a constant temperature was investigated as a function of solution supersaturation σ and impurity concentration c i . It was observed that: (1) irrespective of the impurity concentration, there is a threshold supersaturation of about 0.03 above which Cr(III) is captured in the crystals but there is no threshold supersaturation in the case of Cu(II) and Fe(III) ions, (2) above the threshold supersaturation, k eff increases with increasing σ and decreasing c i , and (3) at a given σ eff and c i , k eff decreases in the sequence: k eff [Cu(II)] ⪢ k eff [Fe(III)] > k eff [Cr(III)]. The dependence of effective segregation coefficient k eff on supersaturation and impurity concentration is in agreement with the predictions of the model involving surface coverage due to impurity adsorption and dependence of accumulation and depletion of solvated host molecules at kinks of steps on the F faces of crystals on supersaturation [J. Crystal Growth 212 (2000) 522]. Analysis of the effective segregation coefficient k eff of different impurities suggests that the dehydration energies of cations mainly determine the capability of capture of impurity species by the growing crystal.

Study of segregation coefficient of Mn(II) impurity in ammonium oxalate monohydrate crystals and the relationship between segregation coefficient and growth kinetics

Abstract The experimental results of the dependence of effective segregation coefficient k eff of Mn(II) impurity in ammonium oxalate monohydrate single crystals during growth from aqueous solutions at a constant temperature on solution supersaturation σ and impurity concentration and the relationship between k eff and face growth rates R are described and discussed. Analysis of the results revealed that: (1) the segregation of Mn(II) impurity during crystal growth from solutions is mainly associated with the time-dependent adsorption of impurities and occurs when there is a sudden increase in the growth rate, (2) the time-dependent adsorption takes place when surface coverage by the adsorbed impurity approaches a particular value, (3) k eff of an impurity is related with the face growth rate R of the crystals, and (4) the relationship between k eff and R can be described by an equation based on the model involving surface coverage due to impurity adsorption and dependence of accumulation and depletion of solvated host molecules at kinks of steps on the F faces of crystals on supersaturation.

Electron spin echo and spin relaxation of low-symmetry Mn2+-complexes in ammonium oxalate monohydrate single crystal

Pulse EPR experiments were performed on low concentration Mn(2+) ions in ammonium oxalate monohydrate single crystals at X-band, in the temperature range 4.2-60K at crystal orientation close to the D-tensor z-axis. Hyperfine lines of the resolved spin transitions were selectively excited by short nanosecond pulses. Electron spin echo signal was not observed for the low spin transition (+5/2↔+3/2) suggesting a magnetic field threshold for the echo excitation. Echo appears for higher spin transitions with amplitude, which grows with magnetic field. Opposite behavior displays amplitude of echo decay modulations, which is maximal at low field and negligible for high field spin transitions. Electron spin-lattice relaxation was measured by the pulse saturation method. After the critical analysis of possible relaxation processes it was concluded that the relaxation is governed by Raman T(7)-process. The relaxation is the same for all spin transitions except the lowest temperatures (below 20K) where the high field transitions (-3/2↔-1/2) and (-5/2↔-3/2) have a slower relaxation rate. Electron spin echo dephasing is produced by electron spectral diffusion mainly, with a small contribution from instantaneous diffusion for all spin transitions. For the highest field transition (-5/2↔-3/2) an additional contribution from nuclear spectral diffusion appears with resonance type enhancement at low temperatures.

Study of the nature of Cu(II) complexes in aqueous ammonium oxalate solutions by ultraviolet-visible spectroscopy

Experimental results of an investigation of aqueous ammonium oxalate solutions containing Cu(II) impurity by ultraviolet–visible spectroscopy are described and discussed from the standpoint of speciation of complexes. The results show that absorption of light by aqueous ammonium oxalate solutions containing Cu(II) impurity in the range −5 < ln(ci/c) < 2.5 of the ratio of concentrations ci and c of impurity and solute, respectively, leads to decrease or increase in the intensity of bands of the ultraviolet–visible spectral regions, and these changes may be expressed by full width at half maximum, molar extinction coefficient, peak wavelength and oscillator strength. The changes are caused by the coordination of C2O42− ligand with Cu(H2O)62+ aquocomplex, and are related with the impurity–solute concentration ratio ci/c. The coordination of C2O42− ligand with Cu(H2O)62+ aquocomplex in the range 0 < ln(ci/c) < 2.5 leads to the formation of Cu(C2O4) complex, but the coordination of the C2O42− ligand with Cu(C2O4) complex in the concentration ratio range −5 < ln(ci/c) < 0 results in the formation of predominantly Cu(C2O4)22− complex. The effect of successive coordination of the C2O42− ligand is well-defined in the ultraviolet spectral region but poor in the visible region.