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STUDIES OF RARE GAS MATRIX ISOLATED
ALKALI-IODIDE MOLECULES
S. Shamai, M. Pasternak, T. Sonnino
To cite this version:
S. Shamai, M. Pasternak, T. Sonnino. STUDIES OF RARE GAS MATRIX ISOLATED
Abstract. — Molecules of Lil, KI and Csl were isolated in a solid argon matrix at 4.2 K and 129I Mossbauer Effect studies were conducted. The spectra observed have showed broad absorp-tion lines with breadth decreasing from Lil to Csl. The broadening has been attributed to a qua-druple interaction. From the same batch of materials, spectra of crystalline samples were also recorded. From molecular and crystalline isomer shift comparisons, conclusions are derived regarding the nature of the chemical bond in the alkali-halides. The origin of the electric field gradient and intramolecular distances in the argon matrix are discussed.
1. Introduction. — Physical and chemical properties series. From previous data of dipole moments it was of alkali-halides crystals have been subjected for many expected a prion to boserve a continuous monotonic years to numerous studies. Still today they are consi- variation of the isomer shift 5 from Li to Cs, as cova-dered as « reference » species for ionic crystals and lency decreases. However, the systematics of 5 as a due to their relative structural simplicity many success- function of atomic number Z of the alkali ion revealed ful models describing both macroscopic and atomic a minimum in the vicinity of K l a n d similar values for properties were derived. However due to their perfect Csl and Lil. A year later Flygare and Hafemeister [2] high symmetry and their ionic character there could published an interpretation of the isomer shift data not be many investigations in the microscopic atomic which is based only on effects due to distortion overlap, scale to determine directly the electronic structure of Their work, based on previous models by Lowdin [3] the ions themselves. Questions such as how covalent and Kondo and Yamashita [4] invoked no transfer of
is a certain alkali-halide bond, or what is the effect of charge at all, namely all the iodide ions remain with the electrostatic interaction on the valence orbitals of close shell (5s2 5p6) however the s-electron density is
the ions and others could be answered mainly from altered due to overlap with the alkali neighbours. The
macroscopic data such as dielectric, mechanical and increase in s-density along the bounds due to the thermodynamical properties. overlap mechanism originates from Nearest
Neigh-One of the fundamental studies attempted to learn bours (NN) and Next Nearest Neighbours (NNN) about the structural properties of these ionic crystals contributions. Whereas the N N overlap increases was conducted by Hafemeister, De Pasqualli and de from Li to Cs, the N N N overlap which is larger than Waard [1] by using the 1 2 9I Mossbauer Effect in the N N ones, has a minimum around KI so that the alkali-iodide crystals. Their main purpose was to sum of the two contributions could qualitatively obtain information, in the local aspect scale, of charge reproduce the experimental results. This interpretation transfer (covalency) in the Li, Na, K, Rb, and Cs was subjected to controversial opinions, for one reason, it could not cope with the dipole moments data. The (*) This work was supported in parts from grants 1446/R2 of c r u c i a I Po i n t a n d cornerstone of Flygare and Hafemeis-the International Atomic Energy Agency and Hafemeis-the Israeli National t e r model was the need to verify experimentally the Commission for Basic Research. NNN contribution, without which the wole
interpre-STUDIES OF RARE GAS MATRIX ISOLATED ALKALI-IODIDE
MOLECULES (*)
S. SHAMAI and M. PASTERNAK Department of Physics and Astronomy Tel Aviv University, Ramat Aviv, Israel
and
T. SONNINO, Soreq Nuclear Research Center Yavne, Israel
JOURNAL DE PHYSIQUE Colloque C6, supplément au n° 12, Tome 37, Décembre 1976, page C6-525
Résumé. — Des molécules de Lil, Kl et Csl sont isolées dans une matrice d'argon solide a 4,2 K et des études d'effet Môssbauer 129I sont effectuées. Le spectre observé révèle de larges raies d'absorption, avec une largeur décroissante de Lil à Csl. L'élargissement des raies d'absorp-tion est attribué à une interacd'absorp-tion quadrupolaire. Le spectre d'échantillons cristallins de ces mêmes matériaux sont aussi relevés. La comparaison entre des déplacements isomériques dans les cristaux et les molécules conduit à la nature des liaisons chimiques dans les halogénures alcalins. L'origine du gradient du champ électrique et les distances intramoléculaires dans la matrice d'argon sont étudiées.
C6-526 S. SHAMAI, M. PASTERNAK AND T. SONNINO
tation would be worthless. In other words if 6(Z) of the alkali-iodides in the absence of NNN influence would have shown the same trend as in the real crystal, one would have to look .for an interpretation based on charge transfer instead.
Motivated by this fundamental problem, we undertook to measure the isomer shift of isolated molecules of alkali iodides. As will be shown, the
6(Z) of the molecules do differ from that of the crystal, thus indicating the existence of the distortion overlap mechanism in S-electron density.
3. Experimental.
-
The molecules were isolated by trapping them from the vapour phase into a solid- argon-matrix. This technique calledare- as-hiatrix-
Isolation (RGMI) has been extensively used in other spectroscopies and recently also in Mossbauer Effects [5]. Samples of LilZ9I, KlZ9I and CslZ9I were pre- pared directly from lZ9I-' solution supplied by Oak Ridge. The policrystalline material was thoroughly dried and kept in dry condition. Part of the material was used to make regular powdered absorbers (5 mg cm-2 of lZ9I) and the rest were pressed into pellets of 2 mm diameter. The evaporation system, gas entrance and the cold substrate were incorporated into a regular vertical motion cryostat modified for RGMI work. A simplified diagram is shown in figure 1.
FIG. 1. - A schematic view of the RGMI system : 1) source at L.He ; 2) aluminium window used as substrate ; 3) rotatable shutter used as radiation shield at L.N2 after evaporation ; 4) crucible and heater with thermocouple and radiation shield ;
5) gas nozzle entrance ; 6) photomultiplier.
In this system the z n l Z 9 ~ e source is at liquid helium and the absorber is formed upon condensation of
Ar : alkali-iodide on a thin aluminium substrate in direct contact with liquid helium. A 2.5 mm diameter ceramic crucible was used, heated with tantalum wire
and properly shielded to prevent overhaeting. The argon gas, supplied from a low pressure large reservoir was allowed to flow through a thin tube and the flow was precisely controlled and kept constant with a special needle valve. Both the rate, concentration and total thickness were monitored by observing the absorption of the 14.4 keV y-rays of a "CO source. TO avoid polimers formation the rate was kept fairly low (- 20 L% S-') with molecular concentration of 10-'. With the present RGMI system the evaporation effi- ciency was around 8
%
and typical absorber thickness was 3 mg cmM2 of 1 2 9 ~ . A Li1291 spectrum is shown in figure 2. The isomer shift values of the-50 -40 -30 -20 -10 0 0 10 20 30 40 50
Velocity [mm.s-lI
FIG. 2. - A 1291 absorption spectrum of LiI(Ar) at 4.2 K. The solid line is a least-squares-fit to an axial quadrupole interaction with negative ez qQ, using the transmission integral method.
polycrystalline absorbers were deduced from the least squares fitting to a single lorentzion line. The RGMI spectra were analyzed using the transmission integral method applied to an axial symmetric quadrupole interaction. The pertinent parameters of the fitting are given in table I. The upper part of figure 3 shows the results of isomer-shift as a function of the alkali atomic number. In the same figure we present the results of Hafemeister et al. [l]. As can be seen, their
values differ from ours though the trend of 6(Z) is still preserved.
STUDIES OF RARE GAS MATRIX ISOLATED ALKALI-IODIDE MOLECULES C6-527
Pertinent results of present work and others. Data of vapour phase is from ref. [7]. The quadrupole
coupling data of present work was scaled to Q of 1 2 7 ~ g r ~ ~ n d state. Isomer shift results designated (a), (b)
avid (c) are from references [l], (l), and [8] respectively. Numbers in parantheses correspond to errors
in last figure.
(1) Friedt, H. M., and collaborators, private communication.
LiI NaI K1 RbI CsI
given to the behaviour of 6(Z) and in particular one cannot explain consistently this case for the isolated molecules. In fact since covalency increases from CS to LiI and charge transfer is via the outer p-electrons one would have expected in the case of the molecules, a reverse trend, namely an increase of 6(Z) towards LiI. As will be shown, the approach of Flygare and Hafemeister is totally consistent with the results in both phases of the alkali-iodides.
Lowdin [3] introduced the symmetric orthogonali- zation technique which describes the atomic orbital cpj in terms of the free-ion atomic orbitals cp, as :
where p, satisfies the free-ion Hartree-Fock equations and S is the overlap matrix in the free-ion basis per bond summed over all neighbours in the crystal ;
Crystal
where daj is the Kronecker delta.
From this approach one can show that the S-electron density at the iodide nucleus can be written as :
Molecules
where the last sum is over all nieghbours. Flygare and Hafemeister have summed over all nearest and next nearest neighbors instead of calculating the squares of overlap per bond. This expression (3) will take the simplified version of :
1'
= const+
I
q5s(0)l2
11.0+
s?rn+
S&](4) where
Values of S2 were calculated from tables of Hafemeis- ter and Flygare [6] using crystalline interatomic dis-
tances. The plot of S2(Z) for NN, NNN and their sum is shown in the lower part of figure 2. As can be seen, the plot of (NN
+
NNN) and that of NN follow the same trend as 6(Z) of crystalline and molecular phases, respectively. In figure 3 we plot the dependence of 6 on S2 of the crystals and here, the linearity is well preserv- ed as expected from expression ( 4 , however the intramolecular distances are unknown, a priori, thus, one cannot calculate s2(mol.) In figure 4 the values of 6(mol) are given in the vertical axis and as can be seen it is possible to extrapolate the values of S2 for CSI and K1 though not for LiI which is beyond the appli- cability of (4). And in fact from the S2 of K1 and CsI we calculate the values of the intermolecular distance r from the following expression for the overlap integralC6-528 S. SHAMAI, M. PASTERNAK AND T. SONNINO
FIG. 3. -Systematics of isomer shift for the alkali-iodide RGMI molecules and crystals (upper part), and the variation of the square of the overlap integral per bond of nearest neigh- bours (NN), next nearest neighbours (NNN), and their sum. The S-electron density increases in the positive direction of the vertical axis. Note the similarity of trend in SZ(Z)(NN+NNN) and
6(Z) (Crystal) and SZ(Z)NN and G(Z) (molecules). 6 (mm 5-11 - 0 3 5 -O40 -Oa5 - 0 5 0 s2[ XIO-3)
'
where A and p are taken from [6]. The values obtained for r are smaller than that in the crystals yet larger than in the molecules in the vapour phase as calculated from vibrational spectra [7]. These values of r are useful in interpreting the data for
e2 qQ of the trapped molecules. The electric field gradient (efg) originates from two sources with oppo- site signs, namely : 1) The efg due to the positive alkali point charge at a distance r and2) the asymme- trical distribution of p-electrons, due to the distortion
LI Na K Rb CS I I IYsIOllJ -
,-
l I X 42 - :\+,, l L<> '? 1-,/F
X Hafernelster e t al.-
I 7 molecules Present work crystal i:- 6 ( m m 5-1) -0.51
LI Na K Rb X - c r y s t a l m - moleculeFIG. 4.
-
The linear dependence of the isomer shift on S 2 forthe crystalline samples. The molecular values of 6 are on the vertical scale since S 2 is not known due to the questionable
intramolecular distances.
overlap, introduces an effective p-density contributing a negative efg. We calculated the quadrupole coupling values for K1 and CsI using only the point charge contribution with the proper value of the Sternheimer anti-shielding factor, and both contributions. It was found that, to get values close to the experimental ones, it was necessary to include the distortion overlap effect on the 5p density.
4. Conclusion. - The combined data of RGMI and crystalline alkali-iodide invoke a bonding mecha- nism by which the S-electrons density is enhanced by virtue of the distortion overlap. The probability of charge transfer between the ions is non-detectable by the isomer-shift and also by the quadrupole interaction.
References [l] HAFEMEISTER, P. W., DE PASQUALI, G., and DE WAARD, H.,
Phys. Rev. 135 (1964) B 1084.
[2] FLYGARE, W. H. and HAFEMEISTER, P. W., J. Chem. Phys.
43 (1965) 789.
[3] LOWDIN, P. O., J. Chem. Phys. 18 (1950) 365.
[4] KONDO, J. and YAMASHITA, J., J. Phys. Chem. Solids 10
(1954) 245.
[5] BARRET, P. H. and MICKLITZ, H. in Perspectives in Mossbauer
Spectroscopy, edited by S. G. Cohen and M. Pasternak (Plenum Press) 1973 117.
[6] HAFEMEISTER, D. W. and FLYGARE, W. H., J. Chem. Phys.
43 (1965) 795.
[7] HONING, A., MANDEL, M., STITCH, M. L. and TOWNES, C . H.,
Phys. Rev. 96 (1954) 629.
[S] REDDY, K. R., DE BARROS, F. S. and DE BENEDETI?, S.,
