The structural inhomogeneity of lead-cadmium fluoride systems and its implications.

In this paper we present a detailed study of structural and dynamical properties of the CdF(2)-PbF(2) systems. Particular attention is devoted to the processes involving the phase separation, a phenomenon of fundamental importance in the correct description of some dynamical properties, as introduced in our previous works. We show here, that the phase separation trend is observed in the undercooled melt, whether by cooling the liquid below its melting point, or by heating a homogeneous glass at temperatures above T(g). The degree of phase separation depends on the procedure employed to obtain the crystalline materials, and the simplest way to determine this property is by performing an isothermal treatment in the undercooled melt region. The local fluorine environments Q((n)), corresponding to fluorine surrounded by nPb and 4-nCd neighbors, is used as a probe in the average local composition. We demonstrate that the increase in the structural inhomogeneity around the composition with x = 0.35 is the factor which leads to a better fluorine conducting property for the Cd(0.35)Pb(0.65)F(2) solid solution, in addition to providing a correct determination of the melting temperature of the Cd(x)Pb(1-x)F(2) compositions.

Local order around rare earth ions during the devitrification of oxyfluoride glasses.

Erbium L(3)-edge extended x-ray absorption fine structure (EXAFS) measurements were performed on rare earth doped fluorosilicate and fluoroborate glasses and glass ceramics. The well known nucleating effects of erbium ions for the crystallization of cubic lead fluoride (based on x-ray diffraction measurements) and the fact that the rare earth ions are present in the crystalline phase (as indicated by Er(3+) emission spectra) seem in contradiction with the present EXAFS analysis, which indicates a lack of medium range structural ordering around the Er(3+) ions and suggests that the lead fluoride crystallization does not occur in the nearest neighbor distance of the rare earth ion. Molecular dynamics simulations of the devitrification process of a lead fluoride glass doped with Er(3+) ions were performed, and results indicate that Er(3+) ions lower the devitrification temperature of PbF(2), in good agreement with the experimental results. The genuine role of Er(3+) ions in the devitrification process of PbF(2) has been investigated. Although Er(3+) ions could indeed act as seeds for crystallization, as experiments suggest, molecular dynamics simulation results corroborate the experimental EXAFS observation that the devitrification does not occur at its nearest neighbor distance.

Molecular dynamics simulations on the local order of liquid and amorphous ZnTe.

Molecular dynamics studies of structural and dynamical correlations of molten and vitreous states under several conditions of density and temperature were performed. We use an effective recently proposed interatomic potential, consisting of two- and three-body covalent interactions which has successfully described the structural, dynamical, and structural phase transformation induced by pressure in ZnTe [D. S. Borges and J. P. Rino, Phys. Rev. B 72, 014107 (2005)]. The two-body term of the interaction potential consists of Coulomb interaction resulting from charge transfer, steric repulsion due to atomic sizes, charge-dipole interaction to include the effect of electronic polarizability of anions, and dipole-dipole (van der Waals) interactions. The three-body covalent term is a modification of the Stillinger-Weber potential. Molecular dynamics simulations in isobaric-isenthalpic ensemble have been performed for systems amounting to 4096 and 64 000 particles. Starting from a crystalline zinc-blende (ZB) structure, the system is initially heated until a very homogeneous liquid is obtained. The vitreous zinc telluride phase is attained by cooling the liquid at sufficiently fast cooling rates, while slower cooling rates lead to a disordered ZB crystalline structure. Two- and three-body correlations for the liquid and vitreous phases are analyzed through pair distribution functions, static structure factors, and bond angle distributions. In particular, the neutron static structure factor for the liquid phase is in very good agreement with both the reported experimental data and first-principles simulations.

Structural ordering in Cd(x)Pb(1-x)F2 alloys: a combined molecular dynamics and solid state NMR study.

Molecular dynamics (MD) simulations of binary Cd(x)Pb(1-x)F(2) alloys have been carried out, using a two-body Buckingham interaction potential, leading to a correct description of structural properties as a function of composition and pointing towards an understanding of the eutectic phenomenon. The simulation data can be analyzed in terms of five local fluorine environments Q((n)) (4> or =n> or =0), where n is the number of Pb nearest-neighbor environments. The results suggest a highly nonstatistical population distribution, suggesting an intrinsic phase segregation tendency in the undercooled melt, during the cooling process. This prediction has been tested experimentally for six representative compositions (0.2< or =x< or =0.7) on the basis of high-resolution (19)F solid state NMR data, revealing important similarities between theory and experiment. While the NMR data confirm that the population distribution is, indeed, nonstatistical for all compositions, the results are only found to be consistent with an intrinsic segregation tendency of PbF(2)-rich domains. This tendency manifests itself in substatistical populations of Q((3)) units, resulting in preferred Q((2)) and Q((4)) formations.

New insight on the structural trends of polyphosphate coacervation processes.

In this study new compositions of polyphosphate coacervates were obtained with the ions Ni (2+) and Co (2+). Samples of the glassy systems were prepared with proportions P/M (+2) varying between 0.5 and 10. The qualitative and quantitative description of the first coordination shells of the transition metal were obtained through extended X-ray absorption fine structure spectroscopy (EXAFS) analysis, performed at the Ni (2+) and Co (2+) K-edges. An analysis of the symmetric stretching vibrations of terminal P-O t and bridging P-O b groups performed through Raman spectroscopy revealed the different phases of the coacervation process in terms of bond strengths and corroborates the EXAFS results. The results obtained permitted a detailed structural description of these materials as well as the role played by the metallic ions on the coacervation process.

Molecular dynamics simulation study of erbium induced devitrification in vitreous PbF2.

Molecular dynamics simulations of the devitrification process of a lead fluoride glass doped with Er(3+) ions were carried out. This technique appears to be a relevant way to perform systematic analysis of the system structure and to study the influence of defects on PbF2 crystallization. We modeled the total enthalpy, the radial distribution functions, and the diffracted intensities of systems containing different amounts of Er(3+) ions. We demonstrated by means of different simulations that Er(3+) ions lowered the devitrification temperature of PbF2, in good agreement with the experimental results. The genuine role of Er(3+) ions in the devitrification process of PbF2 has been investigated. Er(3+) ions have an unquestionable influence of the crystallization of PbF2. Although the latter does not start in the nearest neighborhood of Er(3+) ions, the presence of Er(3+) ions in a close environment may favor the lead fluoride crystallization.

Molecular dynamics simulation on devitrification: isothermal devitrification and thermodynamics of PbF2 glasses.

The vitrification and devitrification features of lead fluoride are investigated by means of molecular dynamic simulations. The influence of heating rate on the devitrification temperature as well as the dependence of the glass properties on its thermal history, i.e., the cooling rate employed, is identified. As expected, different glasses are obtained when the cooling rates differ. Diffusion coefficient analysis during heating of glass and crystal, indicates that the presence of defects on the glassy matrix favors the transition processes from the ionic to a superionic state, with high mobility of fluorine atoms, responsible for the high anionic conduction of lead fluoride. Nonisothermal and isothermal devitrification processes are simulated in glasses obtained at different cooling rates and structural organizations occurring during the heat treatments are clearly observed. When a fast cooling rate is employed during the glass formation, the devitrification of a single crystal (limited by the cell dimensions) is observed, while the glass obtained with slower cooling rate, allowing relaxations and organization of various regions on the glass bulk during the cooling process, devitrifies in more than one crystalline plane.