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largely due to the mixtures of different metal oxides in the Partial random distribution functions (RDFs) of ion pair surrounding environment of uranyl ions. distances were calculated from the MD model systems , 2002, 44, . Structure and charge transfer dynamics of uranyl ions in boron oxide and borosilicate glasses Pair distances between U and it ligands in various uranyl phases (pm) UO2B2O3 UO2SiO UO2(NO3)(H2O)2 UO2(SO4)(H2O)Ion pair MD/EXAFS [14] MD/EXAFS [14] [20] [20] UOax 176/180 182/182 174 UOeq 248/249 222/223 249 UB (Si, N, S) 340/335 367/ 304 Borosilicate glass studied in this work.

for structure indentification and comparison with EXAFS and ab initio calculations [20]. It is also supported by experiments [13,14]. comparison between the vibrational energies measured in the laser spectra and those calculated for the simulated From the simulated glass systems, we can visualize system of uranyl in B2O3 glass [21].

the exact atomic structure in which a uranyl is centered.

Fig. 4 shows the cluster structure of uranyl in B2Oglass matrix. In addition to the six oxygen ions that are 3. The mechanism of charge directly coordinated with the core uranium ion, we see four transfer-lattice interaction boron ions in connection with additional oxygen ions in the form of four BO3 triangles. The BO3 triangle units While our MD simulations provide a model structure for that are bonded with the uranyl are slightly bent and uranyl clusters in boron oxide and borosilicate glasses that twisted, which is understood because these BO3 groups is in excellent agreement with the experimental observation, are connected with other such groups that are randomly one of the main purposes of the present work is to construct oriented in the glass matrix. The BO bond length of a theoretical model of charge transfer-lattice interaction the four BO3 groups in the uranyl cluster varies from that helps to explain the formation of uranyl clusters in 0.132 to 0.153 nm in comparison with 0.136-0.138 nm glass environments. The basic consideration is to take into for the intrinsic BO bond. It is always the case that account an ionic-covalent bonding between the six oxygen the two axial oxygen ions are not actively bonded with ions as well as between uranium and uxyden ions both with any boron (or Si) ions in the glass matrix. In agreement charge transfer-lattice interaction in the UO6 cluster (see with this observation, calculations of molecular bonding Fig. 5).

using quantum theory also suggest that no stable bonding can be established on the top of an actinyl ion along its axis [19]. The reality of the model structure is based, of course, on the agreement between the EXAFS data and the calculated RDF for the uranium ion and its ligands, and the agreement between the simulated glass matrices and that previously extablished by Raman and neutron scattering techniques. The calculated ion pair distances are listed in the Table in comparison with those obtained from EXAFS [14] Figure 5. Charge transfer channels and vibronic distortion of in UO6 cluster that contains uranyl. The lines indicate charge transfer channels and the arrows indicate distortion modes.

The mechanism of charge transfer-lattice interaction can be realized based on the following phenomena that are simultaneously active.

The first phenomenon is a self-consistent charge transferlattice distortion. There are three actual channels of the charge transfer in the UO6 cluster, namely oxygen 2poxygen 2p charge transfer (q1) between the six active oxygen ions, oxygen 2p, 2suranium 6p (q2), and oxygen 2puranium 5 f (q3) charge transfers. Such charge transfers are self-consistent with the local lattice displacements. The latter Figure 4. Model structure of uranyl cluster in B2O3 glass established using a molecular dynamics simulation. is due to the vibronic-type charge transfer-lattice interaction 3 , 2002, 44, . 1378 G.K. Liu, H.Z. Zhuang, J.V. Beitz, C.W. Williams, V.S. Vikhnin proposed recently in the framework of consideration of the microscopic nature of the above mentioned charge transferproblem of charge transfer vibronic excitons (CTVE) [7,8]. lattice interaction (direct influence of charge transfer on The CTVE is the electronic polaronhole polaron correlated the local vibrational dynamics of active ions) explains pair with strong vibronic interaction of polaronic origin [7]. the uncommonly strong increase in vibronic interaction Namely, the small pllaron nature of the electronic and hole with charge transfer. It is important to underline that states of CTVE is responsible for the high strength of the this CTVE-effect is mainly formed by the competition vibronic interaction in the case of the uranyl cluster.

between cluster energy increasing due to oxygenoxygen The important peculiarity of CTVE in the uranyl system charge transfer and cluster energy decreasing due to lattice is the charge transfer between the same type of active distortion as a result of LCI.

ions. This is mainly due to the possible strong equilibrium The uranyl ground state structure can be treated as an charge transfer between different oxygen ions within the oxygen-related CTVE-pair trapped by the central U6+ ion.

degenerated states, which significantly reduces the quadratic The tetragonal structure of such CTVE-pair leads to clusincreasing of cluster energy with charge transfer and hence, ter energy minimum due to Coulomb repulsion between produces rather soft conditions for the CTVE formation two electronic polarons (two oxygen ions with increased even in the ground charge transfer state.

negative charges and corresponding high ionicity). Another The oxygen 2p(2s)uranium 6p charge transfer should origin of the geometry with tetragonal symmetry for the also be considered. It induces additional strengthening UO6 cluster connects with specific hybridization of the of CTVE effect in the cluster under discussion. The states in the framework of PJTE. The role of the central U6+ pseudo-Jahn-Teller type mechanism of such charge transfer ion is essential in the construction of the uranyl structure.

presumably exists in the present case. The equilibrium Because of significant UO state overlapping, the indirect charge transfer in the oxygenuranium channel becomes mechanism of O(1)O(2) effective charge transfer between important when taking into account of the simultaneous two different oxygen ions via successive O(1)U and action of the charge transfer analog of the pseudo-JahnUO(2) channels of state mixing becomes important. The Teller effect (PJTE) and PJTE on the active tetragonal latter strongly decreases the electronic part of cluster energy, distortion of the cluster. Both types of PJTE are realized which is characterized by enhancement in charge transfer.

in the framework of oxygen 2p, 2suranium 6p states As a result, the energy of the localized CTVE-pair of OO mixing. Moreover, this type of charge transfer is responsible type goes down due to softening of the corresponding for the effective indirect and two step mechanism of the CTVE-pair trapping by the U6+ ion.

oxygen 2poxygen 2p charge transfer resulting from the It should be stressed that there is another important oxygen 2puranium 6poxygen 2p charge transfer via the source of the U6+ ion influence on the uranyl structure intermediate uranium 6p state.

and properties. This is the PJTE due to the mixing Last but not least, the classical [14] oxygen 2puraof the 6p-5 f uranium ion states unduced by linear nium 5 f charge transfer also phays an important role vibronic interaction with active tetragonal distortion of the in the uranyl formation. Here, the PJTE mechanism of cluster coordinate. This PJTE-mechanism of the 6p-5 f charge transfer also has actual contribution. Again the states mixing accompanied by an equilibrium distortion equilibrium charge transfer becomes significant when taking can explain the linear geometry of the uranyl structure.

into account the simultaneous action of charge transfer It should be also pointed out that the importance of the analog of PJTE width oxygen 2puranium 5 f states mixing 6p-5 f hybridization for the stability of a linear geometry as well as PJTE on structure-distortion with the same active was discussed for the uranyl ion in the review article [4].

electronic states.

We have proposed here the microscopic mechanism of such The strong local displacements of oxygen ions that were a phenomenon.

detected in the experiments on the uranyl cluster need very The CTVE-effect under discussion can be described in definite conditions for charge transfer-lattice interaction even the framework of a semiphenomenological approach. The in the frame of the CTVE-model. This situation takes latter is based on the cluster potential expansion into the place within the second phenomenon mentioned above.

powers of active variables. It corresponds to the strong PJTE Namely, there is an increase in the ionicity of the two case, and to the two successive adiabatic approximations active oxygen ions of UO2+ due to charge transfer under within the hierarchy of the characteristic rates of active consideration (tending to O2- final states where the ionic subsystems. Namely, the latter hierarchy is realized within radius is decreased). Correspondingly, a decrease of the the interacted very fast subsystem of electronic degrees of BornMayer repulsion radius with an increasing ionicity will freedom of ionic states, intermediately fast subsystem of lead to significant softening of the quasi-local vibration for these two oxygen ions. The latter induces local configu- charge transfer degrees of freedom, and slow subsystem of quasi-local vibrations of the ionic cluster under considerarational instability (LCI) caused by charge transfer-lattice interaction under discussion. Such LCI is accompanied by tion. A solution for the problem can be found from analysis strong equilibrium tetragonal lattice distortion for the UO6 of successive potential energy minimization in accord with cluster in the glass matrix (with displacements of two the subsystem hierarchy mentioned above. Intermediate axial oxygen ions towards to each others). Therefore, the adiabatic potential of a UO6 cluster embedded in the matrix , 2002, 44, . Structure and charge transfer dynamics of uranyl ions in boron oxide and borosilicate glasses of B2O3 (or SiO2) contains the leading charge transfer- charge transfer channels in the uranyl ion (UO2) and a lattice interaction, and depends on three types of charge larger cluster (UO6) are considered important in formation transfer variables {q1, q2, q3} as well as the slow vibration of a crystalline-like cluster. The predicted structural and variable x. We obtain the adiabatic potential for the slow electronic properties for the uranyl cluster are in agreement subsystem after minimization of the intermediate potential with the spectroscopic experiments and MD simulation.

on the {q1, q2, q3} variables. Its expansion on the powers In addition, the long lifetime of the uranyl fluorescence of x-displacements can be presented in the usual form but is also explained based on the vibronic reduction of the with strongly renormalized coefficients by charge transfer recombination process in the electronic transitions between states with strongly defferent equilibrium positions of cluster U = D(x)2 + (x)4 + (x)6 +.... (1) ions.

Here two different types of the solutions are realized.

References Namely, in the case of D < 0, > 0 (LCI of the second order) we have to deal with equilibrium cluster distortion [1] C.K. Jrgensen. Acta Chem. Scand. 11, 166 (1957).

accompanied by correspondent equilibrium charge transfer [2] C. Gorller-Walrand, L.G. Van Quikenborne. J. Chem. Phys.

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case, in the situation of D > 0, but with <0, >0, pure [4] R.G. Denning. In: Complexes, Clusters and Crystal ChemCTVE-type solution is realized (with equilibrium charge istry. Springer-Verlag, Berlin, Heidelberg (1992). V. 79. P. 215.

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The model proposed in the present work is in agreement [9] R.G. Denning, T.R. Snellgrove, D.R. Woodwark. Mol. Phys.

with EXAFS and MD simulation, and can explain the 30, 1819 (1976).

stability of the uranyl cluster on the basis of uncommonly [10] R.G. Denning, C.N. Ironside, J.R.G. Thorne, D.R. Woodwark.

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[11] G.K. Liu, V.V. Zhorin, M.R. Antonio, S.T. Li, C.W. Williams, lifetime of the uranyl fluorescence also finds the explanation L. Soderholm. J. Chem. Phys. 112, 1489 (2000).

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states with strongly different equilibrium positions of cluster [13] B.W. Veal, J.N. Mundy, D.J. Lam. In: Handbook on the ions. The latter results from strong lattice distortion in the Physics and Chemistry of Actinides / Ed. A.J. Freeman and framework of CTVE-effect.

G.H. Lander. North Holland (1987). P. 271.

Thus, we have investigated experimentally and theo[14] G.K. Liu, H.Z. Zhuang, M.R. Antoio, L. Soderholm. To be retically the formation of a locally ordered structure of published.

uranyl ions in B2O3 glass and borosilicate glass. Our [15] D. Frenkel, B. Smith. Understanding Molecular Simulation.

experimental results show a ordered local structure in Academic Press, N.Y. (1996).

[16] D.J. Evans, G.P. Morris. Computer Phys. Rep. 1, 297 (1984).

which a UO2+ ion is coordinated with its surrounding [17] D.C. Rapaport. The art of molecular dynamics simulation.

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