CORRELATIONS BETWEEN ELECTRONIC STRUCTURE, THERMODYNAMICS AND CRYSTAL STRUCTUREAbout the correlation between electronic configurations of actinide ions and the properties of their compounds

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CORRELATIONS BETWEEN ELECTRONIC

STRUCTURE, THERMODYNAMICS AND CRYSTAL STRUCTUREAbout the correlation between electronic

configurations of actinide ions and the properties of their compounds

G. Ionova, V. Spytsyn

To cite this version:

G. Ionova, V. Spytsyn. CORRELATIONS BETWEEN ELECTRONIC STRUCTURE, THERMO-

DYNAMICS AND CRYSTAL STRUCTUREAbout the correlation between electronic configurations

of actinide ions and the properties of their compounds. Journal de Physique Colloques, 1979, 40 (C4),

pp.C4-199-C4-206. �10.1051/jphyscol:1979464�. �jpa-00218860�

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CORRELATIONS BETWEEN ELECTRONIC STRUCTURE, THERMODY- NAMICSAND CRYSTAL STRUCTURE

About the correlation between electronic configurations of actinide ions and the properties of their compounds

G. V. Ionova and V. I. Spytsyn

Institute of Physical Chemistry, Academy of Sciences of the USSR, Moscow, Leninski prospect 31, U.S.S.R.

Résumé. — L'objet principal de cet article est de montrer l'importance des énergies de reconstruction sur les propriétés des actinides tant à l'état solide qu'en solution. En raison du dualisme spécifique de localisation- délocalisation des électrons 5f et 6d, des ondes de densité de charge peuvent se produire dans les composés cristallins à basse température. Dans un modèle de Hubbard étendu à deux bandes et prenant en compte les interactions de Coulomb intra- et inter-sites ainsi que l'énergie cinétique, on obtient les critères d'apparition des ondes de charge et d'orbitales. L'ordre orbital peut s'accompagner de la formation d'onde de densité de spin. Quelques considérations sont présentées sur la supraconductivité pour laquelle on suggère que la formation de paires d'électrons dans un état de valence intermédiaire est l'un des mécanismes importants. Le rôle de l'énergie d'excitation sur la stabilité et la géométrie des groupes actinyls est analysé. Une estimation des potentiels d'oxydation pour Cm(IV-V), Cm(V-VI) et Cm(VI-VII) est donnée. On montre que, pour les cations AnO;, il est possible d'avoir une distorsion de la structure linéaire vers une structure courbée.

Abstract. — The main purpose of this paper is to show the importance of the reconstruction energies on the actinide properties both in solid state and solutions. As a consequence of the specific dualism localization- delocalization of the 5f and 6d electrons, charge waves can occur in crystal compounds at low temperature. In an extended two-band Hubbard model which takes into account the intra- and inter-site Coulomb interactions as well as the kinetic energy, the criteria for the occurrence of the charge and orbital waves are obtained.

Orbital ordering can be accompanied by spin density wave formation. Partial attention is given to the occurrence of supraconductivity, it is proposed that electron pair formation in a mixed valence state is one of the important mechanisms of supraconductivity state formation. The influence of the excitation energy on the stability and geometry is analysed. Estimates of the Cm(IV-V), Cm(V-VI) and Cm(VI-VII) oxidation potentials are given. It is shown that distortion from the linear to the bent structure is possible for An02 cations.

I. Introduction. — It is now well known that in the beginning of the actinide serie, 6d and 7s levels are lower in energy than the 5f ones for all electronic configurations. According to our numerical Hartree- Fock calculations which take into account relativistic effects, the energy inversion between 5f levels and 6d and 7s ones begins with Pa and is completed in Np for most of the electronic configurations ( f ds2, f d2s, f" dsp, etc.)- Thus, thorium behaves as a transition element but there are hybridized f-d-s bands in U, Np and Pu, whereas Pa is intermediate between thorium and these elements. The proximity of 5f, 6d and 7s energy levels in U, Np and Pu results in a variety of valence states and therefore these elements exist in both the low and high ionicity states. In crystal compounds, a broadening of d-f bands take place and acts against f electron localization.

So the 5f electronic states possess the original dualism : localization-delocalization. As a result there is a fractional number of localized f electrons and the valence does not correspond to a stoichiometric composition in the metallic compounds of these elements, e.g. in chalcogenides. The fractional atomic magnetic moments, with a large deviation from an integer value, can serve as an indication of 5f

electron derealization. Said in other words, the f electrons have a non-zero bandwidth. Because the f, d and s levels have comparable energies, a redistribution of the electronic density on crystal sites and orbitals can take place when applying pressure, with appropriate doping or when changing the temperature [1, 2]. It can result in the formation of dynamical charge waves (DCW), static charge waves (SCW), spin density waves (SDW) or finally superconducting state (SS). The purpose here is to identify the special features and parameters charac- terizing one or another of the states and to determine the conditions for various transitions. Not only the occurrence of the cooperative effects is determined by the excitation energy from the 5f states to the 6d or 7s ones, but also the properties of isolated molecules in solutions and of the gas phase (stability, geometry, etc.). We will briefly discuss these points.

2. General background for the electronic redistribution in crystal. — The instability of a crystal lattice to the formation of various kinds of ordering is of great interest. The fundamental importance of electronic charge and orbital ordering study consists in that they will, hopefully, provide a better

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Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1979464

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C4-200 G. V. IONOVA AND V. I. SPYTSYN understanding of the solid state electronic structure

and valency theory. Crystals with charge and/or orbital ordering must have unusual specific properties caused by the transition from a state with alternating occupancy of crystal sites and orbitals t o a state with average occupancy. In some crystalline actinide compounds one part of the electrons is localized but the other part forms hybridized bands the width of which is determined by the kinetic energy. Here w e discuss the conditions for transition t o the charge or charge and orbital ordered states which are characterized by the alternate occupancy of crystal sites or the alternate hybridization of the 5f and 6d valence orbitals on adjacent sites whereas, in a disordered state, each site has the same occupancy and hybridization and all sites are equivalent.

The mechanism of ordering is due t o electron correlations. The correlations result in deformations of Hartree-Fock orbitals by mixing one-electron levels which are symmetric with respect t o the Fermi level : they reduce the total energy of the system.

Some of the simple ideas about the possibility of ordered hybridization occurrence can b e reinforced by a generalization of the Hubbard model. This model is now very popular since it represents a compromise between the band theory (reflecting the itinerant aspect) and the localized state model. In the Hubbard model, only one valence orbital o n each lattice site is considered and only intra-atomic interactions are taken into account. Considering more than o n e orbital on each lattice site and introducing intersite electron correlations is of prime importance for a system in which : (i) close-lying energy levels are present and consequently their orbitals are mixed by the crystal field and form a hybridized band, (ii) various oxidation states can exist and transitions between them are possible.

We consider a crystal lattice which consists of N identical sites and each site has a ( a = 1 , 2 . . . ) atomic orbitals xu. The following notations are used : n is the average number of electrons on the site, n , = xn and n , = yn are the respective occupancies of the orbitals X , and

x2

( x

+

y = 1). The hybridization parameter, ( x - y ) n, determines the difference in the electron occupancy of X , and X , orbitals on a crystal site. T h e Hamiltonian of this electron system is written in second quantization a s :

where i, j are crystal site indices, a is a spin variable, a and /3 are orbital indices ( a , /3 = 1 , 2 ) ,

aa

and

a,,

are intra-site, and Via,, and V ,,.,, are inter-site Coulomb interaction parameters. The kinetic energy of electrons is

We assume an arrangement of two sublattice A and B, with predominantly one state on sublattice A and the other state on sublattice B. In the Hartree- Fock approximation and considering only nearest neighbour interaction ( z ) , one obtains :

v , p ^o for nearest neighbours t ,,.I,,; v-.i.=

{

^"@. ⁰ otherwise.

We obtain a secular equation of fourth-order whose solutions determine the energy spectrum w of the crystal. The equation can be solved in the simple case :

E , ~ ( K ) -e E , ( K ) , E ~ ( K ) , ( 6 ) where ( K ) is the Fourier transforms of the kinetic energy

ti, = N -'I2 (K) e'K(R1-Rl)

.

K

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The energy spectrum is

where

2 A, =

{ ( a ,

⁺²z V l ) ( n l A

+

n,,)

+

+ 2(3 12 + zV12) (n2, + n2,)

1

(10) 2

B,

= { ( X 2 + 2 z V , ) ( n , ,

+

n,,)+

+ 2 ( a , , + z v , , ) ( n , , + n , , ) } ( 1 1 ) 2 A , = { ( a , - 2 Z V , ) ( n , , - n , , ) +

+ 2(

a,,

^-ZV,,) (.,A - n,,)

1

⁽¹²⁾

2 A , ⁼{ ( 3 , - 2 z V 2 ) (n , , - 4 , ) +

+ 2(%,2 - zV12) (n1,

-

n , , )

1

(13) and nu,, nu, are the average occupation numbers. In the disordered state ( A , = A, = 0 )

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ABOUT T H E CORRELATION BETWEEN ELECTRONIC CONFIGURATIONS C4-201

where density on adjacent sites A and B and the parameter

B, = n

I (a,

⁺^{2 zV,) Y}

⁺

^2(2,,⁺^zv,,)^x

I .

⁽¹⁶⁾

of orbital ordering q :

The relative position of these bands determines the ~ h , electron occupanc~es in the disordered state is average energy gap

n l A = n , , = x n , n,,= n,,= yn A = Z { B , - A , - 1 W , - W , ) . (I7) while in the ordered state :

where W, and W, are the bandwidths. In a one- dimensional crystal (or isotropic cubic crystal) the dependence of the band structure on A is shown in figure 1. If there is one electron on a lattice site in the disordered state, the system is a metal. For n = 2, figure l a corresponds to either an insulator or a semiconductor, figure 115 represents the semimetal, figure Ic the gapless state. The transition to the ordered state is accompanied by a splitting of the bands with the occurrence of energy gaps A , and A, (Fig. 2). For n = 1 the order-disorder transition can be viewed a s a metal-insulator (Fig. 2a, 2 b ) , metal- semiconductor (Fig. 2 a to c), and metal-semimetal (Fig. 2 d ) transition. Now we shall derive the crite- rion of charge-orbital state occurrence. For conve- nience we introduce the parameter of charge ordering m which is the difference in the electron

n l A = x n ( l

+

a,,), n,, = yn(l

+

6,,) n,, = x n ( l + 6,,) , n,, = yn(1

+

6,,)

where n,, and n,, are expressed in term of the parameter x and y and of small positive or negative coefficients 6. Some peculiar cases can be considered : (i) the pure charge ordering

S,,x -S,,y = -6,,x = -S,,y = 6 ,

m = 4 n 8 , q = O (ii) the orbital ordering i.e. the electron density is redistributed between the atomic orbitals but within the site.

S,,x = S,,y = - 6,,x = - S,,y = 6 ,

q = 4 n 6 , m = O ;

Fig. 1 . -The possible band structures of the disordered state

Fig. 2. -The possible band structures of the ordered state.

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C4-202 G . V. IONOVA AND V . I. SPYTSYN

(iii) the mixed charge and orbital redistribution when

6

= a = - a

⁼^{- 6} = f i

I A 2B 1 B 2 A 7

m = 2 n f i ( x - y ) , 77 = 2 n 6 T o investigate the stability of the ordered state we determine the average energy for a single electron a t T = O K

where i is the band index, f the Fermi function, and D comes from the two-fold account of the electron interaction in the Hartree-Fock approximation. The criteria for the ordered states are obtained by mini- mizing E with respect t o the parameter 6. If the average number of electrons on a site is one, the lower band is completely occupied at T = 0 K (Fig. 2a to c ). From self-consistent equation with respect t o 6 the criteria for various ordered states are obtained for :

(i) the charge ordering

A , , = ( ; i , - 2 z V I ) + ( & - 2 z V 2 ) +

+

4(

-

2 zV,,) C 0 ; (20) (ii) the orbital ordering

A,, =

(2,

- 2 zV,)

+ (aZ

^-^{2 zV,)}

-

- 4 ( ; i l 2 - z v l 2 ) < O . (21) If the orbitals ^aand

P

are degenerated

3 ,

=

2,

and V, = V,. In this case the ordering respective to the components of the degenerated orbitals takes place. It results in the anionic distortion ;

(iii) t h e mixed charge and orbital ordering

T h e difference between A: and A : is that the charge redistribution is essential in the first case whereas the orbital redistribution is important in the second one. The phase transition temperature can be obtained from the self-consistent equations and is given by :

where p is the density of states corresponding t o the Fermi level.

3. Discussion of the types of electronic ordering. - Here it will b e convenient t o begin with the physical interpretation of the states which could be realized in principle at low temperature. A periodic modula- tion of the electron density takes place in the SCW state and results in a lattice period change. The period can be either double, triple, commensurate or incommensurate (with a period of the disordered state), depending on the number of electron per site.

Of some interest are the half filled bands when there is one electron on a crystal site A' (ThT1', PalV, Uv, etc.). The important electron reconstruction of the whole system

is due to the fact that all three valent states A", A' and A2 (at m = 2) can exist. f.e. Th", Th"' and ThIV, UTV, UV and U v l , etc., but the disproportioned states are more stable in energy. Here the electron pair formation is of considerable interest. It is clear from eq. (20) that the collective disproportion with electron pair formation depends upon the sign of J = J,

+

J2

+

J,, a s well a s upon the relation between J and V = V,

+

V,

+

V12. In a half filled band when J < 0 and V = 0, the electron pairs have a degenerated ground state. If J > 0 but ( J - 2 zV) < 0, the pairs alternate t o minimize the energy. When there are strong couplings between the lattice vibrations and the ionic states, the charge waves occur because of the powerful increase in the electron susceptibility when the temperature decreases. For the charge wave occurrence, the condition t o be satisfied is [ I , 21 :

where G ( q ) is a matrix element of the electron interaction with the ionic vibrations, ~ ( q ) is the susceptibility of non-interacting electrons. It can result in a displacement of lattice sites. At T = 0 K , there is a regular crystal structure which is sup- pressed a t increasing temperature owing t o ionic vibrations (which increase when the nearest neighbour overlap increases), s o an instantaneous appearance of charges on sites occur. These waves have the dynamical fluctuation character and are due t o the deviation from regular structure. The charge wave amplitude is proportional t o the periodic lattice change. Here the charge waves appearan- c e mechanism is an electron-phonon interaction though their existence are again due t o the close lying energy valence states. In essence, the oxidation-reduction process takes place when the charge wave moves.

In the chemistry of transition elements, experi- ments have given conclusive evidence of both SCW and DCW occurrence. Now it is established that in the compounds of about forty elements [3] of the

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ABOUT THE CORRELATION B E m E N ELECTRONIC CONFIGURATIONS C4-203 Periodic Table the collective disproportion of an

electron density can take place under appropriate circumstances (changing the temperature, appropriate doping, defects, applying pressure). The crystal compounds,

Cs,Sb~Sb~,,Cl, ; MX,, M = Nb, Ta ; X = S , Se, etc. furnishe examples of the SCW and DCW.

Actinide compounds of this type are still not well understood. Based on the excitation energy from f n to fn-I d1 state for Ln" and An2+ [4] it is expected that the charge waves may occur in divalent compounds of Ac, Th, Pa, U , Np, Pu, Cm and Bk in which E , is not large (Table I).

Hydrides, the series of non stoichiometric oxides (analogous to Magneli phases, vanadium and tungs- ten bronzes) can apparently involve two different valence species. Based on a study by XPS and EPR, U,O, was described as UiVu; [5] and two different states, UIV and UV', were found in U30, and

u2°5

L61.

On the basis of magnetic moment measurements, it is believed that mixed valence exists in many carbides, pnictides and chalcogenides (US, US,, UP, PUS,, a-Pu,S3, PuSe, etc.). A mix2d valence was proposed for AmFe,, the time and space average configuration being : 0.95

f6 +

0.05 f7 [7]. It is reaso- nable to suppose that there are strong couplings between the ionic states and the lattice vibrations in the crystals with the hexagonal symmetry (f .e.

ThTe,, Th,S,, TbSe,,, U,Te12). Among the simple halide compounds, ThI, can be mentioned, its structure consists of four two dimensional infinite layers alternating between trigonal prismatic and triional antiprismatic layers. There exists some ana- logy between the structure and the properties of transition metal dichalcogenides (f.e. MeX, where Me = Nb, Ta and X = .S, Se) and actinide compounds with the hexagonal symmetry. They are favourable to charge wave formation at low temperature as a consequence of their specific Fermi surface. If the ligand fields around the ions of different valence are very different, it is expected that by applying pressure, the smoothing of a valence can be achieved as for the transition element compounds (InTe -+ In11n"'Te2).

Not only the excitation energy, butealso the ligands regulate the relation between the valence states changing the bandwidth and the localized f level energy. Thus SmS is a semiconductor, SmH, and SmC, are conductor and SmI, is salt-like insulator. This regulation takes place through the change of an effective charge which results ih the excitation energy change : for uranium and its' ions, U', UZ+

and U3', the excitation energy from f to d levels varies from 0 to 3.70 eV 141.

We will not discuss the fluctuating valence of rare earth compounds. Theoretical discussion of the chalcogenides has already been presented [8, 91.

Apparently the same conclusions about charge wave occurrence at low temperature can be made (LnH,, LnC,, etc.) though the correlation effects for 4f electrons are smaller than for 5f electrons.

4. About the mechanism of the superconducting state.

-

We want now to discuss the relation between the charge density waves and the superconductivity. Superconductivity requires the formation of bound electron pairs. It is generally assumed that the electron attraction is due to electron-phonon interaction, apparently this mechanism is not the only one. We believe that the superconductivity properties which occur in inorganic coordination and intermetallic compounds, are largely associated with the ability of metallic ions to give rise to two valence states, one of which has a possible electron pair [lo-121. Instability of a crystal lattice with respect to charge waves with electron pair formation can favour the origin of the conditions of superflui- dity and superconductivity since they are charged.

For this a low energy two electron spectrum must apparently exist.

There are two types of elementary excitations in the system with charge state :

a ) the one-particle excitations corresponding to unpairing of one electron pair with energy,

b ) collective excitations corresponding to a transfer of an electron pair to a neighbour vacant site, E, = 4 V. The collective excitation of the electron pair lie within the one-particle spectrum if -

2

> V.

This is possible in the case when the intra-atomic

Table I.

-

Excitation energies E,, (in lo3 cm-') from f" to fn-' d' states for the doubly ionized lanthanides and actinides [4].

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C4-204 G. V. IONOVA AND V. I. SPYTSYN electron interaction corresponds to an attraction and

is stronger than the inter-site one. In the simplest case for one-dimensional system the two states under consideration are degenerated (since the energy of unpairing or transfer of any electron pair is the same in both directions). Using N states with one excited pair as basic functions it can be shown that the account of the bandwidth removes the degenera- cy of the excited state and splits it into a band. The correction to the energy is written as

where

The operators b f and bj satisfy the boson commuta- tion relations if i # j. If i = j then

From the latter condition it follows that b: excitations are well described by the Bose statistics if the number of one-particle excitations is sniall enough.

This corresponds to the conditions

Using the Fourier transform for b f operators we have

The band of collective excitations corresponding to a movement of electron pair is separated from the ground state by the energy gap

This model supplements the Hill plot

[Ill.

For the small An-An distances, i.e. in the beginning of the actinide series the overlapping between the 6d (5f)-wave functions of adjacent atoms is large and the electron correlations are substantial. The compounds of these elements can be superconducting. All thorium compounds (formally divalent and trivalent) are considered as tetravalent ones :

Note that neither sulphur nor nitrogen have positive affinity for a second electron, to say nothing of the third. The tetravalent charge state (Th43 is unlikely to occur in practice. According to our semi-empirical molecular calculations in which relativistic effects

are taken into account, the effective charge of actinide atoms is not greater than

+

2.5. When there is about one electron on a valence orbital this state becomes unstable with respect to electron pair formation. So there are some compounds for which the electronic configuration might be sl, but these compounds have no unpaired electrons (Gas, CszSbC1,, etc.). As a rule, the disproportion takes place in them (sl + so

+

s2).. The analogous compounds (GeP, SnSb, InTe, etc.) prepared by high pressure synthesis are superconductors [IS].

We think that the mechanism of superconducting state formation is the same in actinide compounds (f.e. ThC, Th2C3, etc.). In the alloys or solid solutions, the cations having the smaller ionic radii can be considered as compressed by the cations of greater radii. This is equivalent to applying hydrosta- tic pressure on the former ones and could help in the formation of electron pairs (ThTc,, U,Fe, Th,Fe3, Th,A12, Th,Ag).

5. Stability and geometry dependence on the promotion energy. - As a consequence of their specific electronic structure, the actinides display a variety of valencies. The question as to which valencies are stable for transneptunium elements, concerns first plutonium and curium. Of some interest is the stability of the high valencies of curium.

The correlations based on the quantum mechanical theory of the chemical bond may be useful here. One way of doing this is to correlate the excitation energy E , (fn -+fn-'d) with the oxidation poten- tials. In nature this correlation is clear since the promotion of localized f electrons to d levels will help in their taking part in the chemical bond formation and in increasing the oxidation number. The

Fig. 3. -Correlation between the oxidation state potentials EO(III-IV) and the promotion energy.

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ABOUT THE CORRELATION BETWEEN ELECTRONIC CONFIGURATIONS C4-205 The promotion energies can be used to determine the geometry of a molecule. Of course such a consideration gives only a qualitative result. It is known that actinides form the stable actinyl groups AnO;' (z = 1,2) having a linear structure. However the dioxide transition metal elements (f.e. CrO; or MOOS') have a bent geometry. Thus the trans- and cis-configurations are found in Cs,UO,CI, and Cs,WO,Cl, respectively [21]. The electronic structure of the most stable uranyl ion UO? can serve as a basis for the study of other actinyls. The quantum- chemical calculations of UO: made during the last twenty years [22-251 indicate that : (i) the effective charge on an actinide ion is close t o the actinyl ion charge ; (ii) f-d hybridization is important for the An-0 bond ; (iii) the 12-electron core :

Fig. 4. - Correlation between the oxidation state potentials E0(IV-V) and the promotion energy.

available data relative to E , are obtained from atom and ionized ion spectra 116, 171, thermodynamical data for metals [4], Hartree-Fock calculations for actinide atoms and ions [IS, 191 and from Coulomb and spin-orbit parameters [20]. As shown in figures 3 t o 5, correlation between the experimental oxidation potentials (E0(I11-IV), E0(IV-V), E0(V- VI) and E0(V1-VII)) and Em exists and makes possible the determination of the oxidation potentials for curium. We have obtained that the heptavalent curium is unstable, but the hexavalent curium may exist in solution :

E0(VI-VII) = 2.7 eV and E0(V-VI) = 1.5 e V

.

The oxidation potentials

E '(IV-V) = 1.7 eV and E0(V-VI) are close to one another.

is stable. Filling the non-bonding e,, level or taking out bonding electrons from the e,, molecular orbital decreases the stability. This is the reason why UO:' is more stable than UO; and why the stability decreases with increasing atomic number alocg the actinyl series : A ~ O ? or AnO; (with An = U, Np, Pu and Am). This approach is limited to a qualitative trend. In fact PaO; and PuO:' are isoelectronic t o UO:' but they are unknown. In addition t o the number of bonding electrons, the electronic configuration of the actinide ion (An") in the AnO;' actinyls contributes also to the stability and the geometry.

In the maximum overlapping model, the linear nuclei configuration corresponds either to f-d or f-s hybridization while the bent configuration corresponds to the d-s one. Let us consider the isoelectronic serie PaO;, UO?, NpO: and PuO:'. For Pa' the promotion energy in the f Z d s state corresponding to the bent geometry is less than that in the f 3 s configuration 141. The bent geometry predilection singles out PaO: from this serie. The large promotion energy for the transition f4('1,) to f 3 ~ ( ' L J (for Np" 38 000 2 7 000 cm-I [4] or 3 1 600 cm-' [20] and 47000 cm-' for Pu43 is a serious handicap for the occurrence of N~O:' and PuO? stable cations. At the same time divalent uranium is able t o form the linear configuration without cost in energy. It is of some interest t o compare the geometry of UO: (I) with that of WO: o r MOO?. The ground state of the divalent molybdenium is d4(5D) and the first excited state d3 s('F) which has a promotion energy

Ed,

= 30 000 cm-' [26] corresponds t o a bent geometry. The promotion energy to the d3 p state corres-

Fig. 5 . -Correlation between the oxidation state potentials Eo(V-VI) and the promotion energy.

(') In crystals, the bending of UOi' in specific cases may be due to the added interaction of the oxygen atoms with ligands in the equatorial plane. It results in the partial reduction of U VI when the electonic density is transferred from these ligands to the oxygens.

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C4-206 G. V. IONOVA AND V. I. SPYTSYN ponding to the linear coordination is considerably

larger (Edp = 75 000 cm-I) [26]. In the following actinyl series (UO:, N ~ O , ~ ' and PuO;), the last cation is unstable for the same reason as for NpO;. The ground state of the monovalent uranium ion is f3 S'(~I~,~), the first excited f3 d@L,,,,) state almost coincides with the ground state (Ed, = 289 cm-' [4]).

Apparently, both the linear and bent geometries can occur for UO:. In any event, the linear geometry in UO: is less stable than that in UO?. Analogously, NpO: and CmO: have a tendency towards distortion. Note that a change of the linear geometry with

an increase of electrons is known in chemistry : NO:

has the linear structure but NO, and NO; are bent.

This is in agreement with the electron-vibronic unstability of the linear coordination of molecules with a degenerated ground state. In principle, the above mentioned flexibility of some AnO: cations can result in the tunnelling transition from one bent coordination to another which is equivalent to the first. Depending upon the characteristic time of an experiment (T), average linear ^(T

>

t ) or bent

( 7 < t ) coordinations will be observed ( t being the tunnelling time).

References [I] CHAN, S. K. and HEINE, V., J. Phys F 3 (1973) 795.

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131 ROBIN, M. B. and DAY, P., Adv. Inorg. Chem. Radiochem.

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[4] BREWER, L., J. Amer. Opt. Soc. 61 (1971) 1101 ; 61 (1971) 1666.

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DISCUSSION Pr. MULLER.

-

You indicated oxidation poten-

tials of higher valence states of Cm and mentioned their experimental confirmation. What is the experimental evidence for Cm species with oxidation numbers higher than IV ?

Dr. JOHANSSON.

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You mentioned that you pre- dicted oxidation potentials for curium. It was not quite clear to me how these predictions were per- formed. Did you base the predictions on measured ionic spectra or were they based on theoretical calculations ?

G. V. IONOVA.

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The theoretical prediction for the Cm high oxidation states appeared in print (September 1978 issue of the Russian journal 0 Dokladi Akad. Nauk, Ser. Khim). The detailed

experiment will be printed in the November number of this journal (authors V. I. Spytsyn, N. N. Krot, V. F. Peretrukhin et al.).

COMMENT BY Dr. F. WEIGEL TO Dr. IONOVA. - In connection with your work on Cs2U02C14, Cs2Mo0,C14 and Cs,W02CI,, I might mention that we prepared and studied Cs2Np02C1,. This salt forms bright orange monoclinic crystals, and it is isomor- phous with the uranium compound. The crystal structure determination is in progress, but it appears that the NpO? jon has the same configuration as the UO; ion.

G. V. IONOVA. - I am speaking of course only about the possibility of bending in UO:, NpO: and CmO: (i.e. mono-charge cations).