J. Phys. Chem. 1980, 84, 1206-1211
1206
Species Present in M-NH3 near the Metal-Insulator Transition J. V. Acrlvos,' K. Hathaway, Department of Chemistty, San Jos6 State Universky, San Jos6, California 95 792
A. Robertson, A. Thompson, and M. P. Klein Lawrence Biodynamics. Berkeley, California 94720 (Received July 17, 1979) Publication costs assisted by the National Science Foundation
We have investigated the M-NH3 system (M = Rb and Sr) for the purpose of ascertaining the nature of the species responsible for the metal-insulator transition (MI transition) which amorphous metals undergo. We have found that (i) the K-edge shifts with temperature and the oxidation state of M in metal-ammonia solutions (MAS);(ii) the Rb-N distance in MAS is 3.098, and it remains constant with temperature and concentration in the range t = 10 to -60 "C and mole ratio R = [NH,]/[Rb] = 8-160; (iii) the metal number density ascertained spectroscopicallyshows a temperature dependence similar to that measured gravimetrically;and (iv) the effects of phase separation have been detected ( R = 25, T = 197 K) in the density as well as in the Rb-N distance as a spread of K-edge values for the two coexisting liquid phases. Transitions (1) in Figure 5 (Table I) appear to be very sensitive to the potential in the neighborhood of the absorber. These are absent in the pure metals and increase in intensity near the MI transition suggesting that the final state levels for (1) are broadened in the metal. These transitions could be used in the future to develop theoretical models for the potential in the neighborhood of the absorber near the MI transition but all models must be consistent with a solvated species where the metal-nitrogen distance is constant to within 0.03 A.
1. Introduction
Metal-ammonia solutions (MAS) are not completely understood to this date even though investigators have worked on the subject for over a century, using all the possible techniques available. One reason for this intractability may be due to lack of structure information within times shorter than the lifetime of the species present in MAS, which may be as short as their resistivity in the s (i.e., esu unit of time used by Drudela of 7 = Q m in SI units). X-ray and neutron diffraction studies have given a limited insight into the structure of the M(ND,), compounds,14 and now a structure determination of the MAS is possible by X-ray absorption spectroscopy (XAS). This work is based on the following fundamental information: (1.1) the lifetime of an excitation in the metallic state obeys the uncertainty principle, Le., 7 > h/2EF, where EFis the Fermi energy; (1.2) for MAS with [MI > 1 mol/L of a monovalent metal, the nearly free electron model (NFE) s and; (1.3) X-ray gives an estimate of 7 = [M]-2/3X absorption edge spectra (XAES) and extended X-ray absorption fine structure (EXAFS) measurements probe molecular structure within the lifetime of the photoelectron s, Le., within (limited mainly by Auger processes), 7p < times shorter than 7. The above techniques have been used in this work with the primary objective to extract the following type of information: (1.4.1) the radial distribution of a solvent shell around the M species as a function of temperature and composition and (1.4.2) the nature of the final state levels in the neighborhood of the Fermi level for MAS. The techniques used in this investigation are described in section 2. Section 3 discusses the data obtained to date for Rb-NH, and SI-", and in section 4 we present a new insight for MAS suggested by the data. Here the MAS concentration is varied and the system goes through the following transitions in the conductivity Pmetal
Q
m
>
poor
metal
=
Qm
Pinsulstor
i=
Qm 0022-3654/80/2084-1206$0 1.OO/O
the nature of the final state levels for a photoexcited 1s electron is expected to vary in a definite manner which depends essentially on the potential in the neighborhood of the species M,4 i.e., whether the potential is screened or unscreened by conduction electrons. Also if the photoexcited electrons sense more than one type of potential within times long compared to 10-l6 s the edge should show some type of structure. 2. Techniques 2.1. Chemical Preparation. The Stanford Synchrotron Radiation Laboratory source with the Dewar used for this study (under the auspices of proposals 96 and 312) is outlined in Figure 1.2 A sample mount (Figure 1) has been constructed to control temperatures to within f l "C by using a Varian V-4343 temperature control. The samples were prepared as described earlier., They were contained in cells made from Wilmad precision rectangular Pyrex glass tubing (aged in HF). Two parallel sides of the Pyrex cells (3 by 0.3 cm) were ground or etched with HF to a cm. The two walls give a combined thickness of =3 X absorbance of one for X-ray photons near the K-edge of Rb and Sr. The sample path length (established by the dimensions of the container) was varied from to 1 cm to achieve an optimum ratio of intensities near the edge to give an absorbance of A = In lo/lx 2 in Figure 1. The inverse linear absorption coefficients near the K edge of M for Pyrex 1 ( l / ~ )vs. the atomic number Z(M) and of MAS vs R = [NH,]/[M] given in Figure 2 were evaluated from the tables of mass absorption coefficients and the estimated density of the materials in the CRC Handbook. The metal number densities dM determined experimentally are given in Figure 3 vs. 1/T. 2.2. X A S . The terminology in XAS is reviewed here for the benefit of those unfamiliar in the field. However, those interested in the results only should go to section 4. The absorption cross section for the photoexcitation of a K-shell electron of a species M by an X-ray photon of energy E is given by the relation4 AM@') = AMO(E)[~ + XMS(k)l 0 1980 American Chemical Society
(1)
The Journal of Physical Chemistry, Vol. 84, No. IO, 1980 1207
M-NH, near the Metal-Insulator Transition Go s
-0.:
-0.680
W-Hrola ,
-0
.?ZO
-0
.?SO
P I R9s.
I I Tharmomalcf
-0.560
1-Ll
"
114" Cu
0
O
D
Flgure 1. Schematic of instrumental details for XAS of MAS. -0.600 ( NH31/IMI
0
IO
20
30
40
50
60
70
1 -0.640
0
-0.680
l
I
lb
I
I
I
I
1
1
I
I
t
t
Io-:!
20
30
40
50
z (M) Figure 2. Inverse linear absorption coefficient ( 1 1 ~for ) Pyrex 1 near the K edge vs. atomic number Z(M) and for MAS vs. mole ratio R.
where AMo is an atomlike contribution and xm is the part contributed by the EXAFS produced by the local environment (S). The nomenclature used separates the spectral regions relative to the K-edge photon energy as outlined in Figure 4, i.e. preedge < IK-edge < EXAFS < postedge (lo3eV, 0) (0, lo2 eV) (lo2, lo3 eV) (lo3, 1.5 X lo3 eV) The photon mergy where the second derivative of the absorbance shown in Figure 5 vanishes near the K edge (EJ is used to identify transitions to states in the neighborhood of thie Fermi level in metals. The microscopic origin of EXAFS is sufficiently well understood to allow its successful application by using the relati~n~-~
k x ~ s ( k=) S 2 J m d r r-2P~s,&[Li~~,e2ik'](2) L
O
Figure 3. Metal density dM vs. I/Tfor Rb-NH,: 25.
(a) R = 20, (b) R
=
where PMS,(r) is the radial distribution of species Siin the neighborhood of M (i.e., So"& P m equals the total number of species S within the sample). The angular distribution of neighbors does not enter into PMs except for single crystals. Ik) is the final state of the excited electron with energy (in atomic units) k2/2 = El,+ E where E l , C 0 is the core binding energy and the scattering of M phlotoelectrons by S is written as4-7 AMS = (--2i7r2)ts+(-k,k)exp[-2r/h(k)
+ 2iSM(k)]
(3)
where ts+ is the back-scattering matrix element for ,S, X is the photoelectron mean free path, and 8M is the p-wave phase shift due to the potential of the excited M.
The Journal of Physical Chemistry, Vol. 84, No.
1208
Acrivos et ai.
IO, 1980
M and, if multiple scattering may be neglected, (2) reduces to4
2.036
W
V
z
sin [2kRi
Q
m
+ ai(k)]exp(-2[Ri/X + k2A2]) (2')
LT
0
m
m Q
1.501 I 15.2690
I
I
I
I
I
I
I
I
17.0395
16.1543 keV
Figure 4. Absorbance (arbitrary scale)vs. X-ray the Sr K edge.
16,060
photon energy near
16,085
16,110
eV
Here the subindex i identifies the MSi terms and ai(k)= 26&) + -ys,(k)where ys, is the phase of tsr+. Both 6 and y can be expanded in a Taylor series retaining only the first few powers of k, Le., a = a. + alk+ azk2+ .... Here the data analyses of EXAFS have been carried out by graphical and Fourier transform (FT)techniques*Jand the limitations and errors pertinent to this work will be described in the next section. 2.3. Advantages. The advantage of XAS over other types of diffraction measurements for obtaining PMsis contained in the Debye-Waller factor exp(-2kZb2)which decreases the scattered signal as k increases. In convenmeasures the mean square deviation tional diffraction bb2 of an atom from the origin while, in the EXAFS relation (29, Az measures the mean square deviation of the atom at the origin relative to neighboring atoms. If the displacements of the atoms are independent of each other for say Cu metal then Az = 2A02 but if the motions are completely correlated then A2 = 0. The diffraction and EXAFS data for Cu indicate the motions are partially correlateda6Therefore whenever this is true the EXAFS data will give more accurate information on PMs than the diffraction data. The decomposition of samples by the Xradiation flux has been ruled out by the reproducibility of results at a given temperature for different runs made on the same sample during different temperature cycles in Figure 3. 3. Treatment of Data The various steps in the data treatment are summarized as follows (those interested in the results only should go to section 4): (3.1) Experimentally the absorbance determined is the sum of contributions from a slow varying background (Abg) and the K-edge contributions A & ! ) . The background is subtracted by various methods of polynomial fits to preand postedge data giving AM(E) in Figure 6 to be used in relation 2. (3.2) The magnitude of the change in AMonear the edge (Figure 5) allows us to determine the number density, Le.
A A M O = Aa~Otd, 15.160
15.220 keV
c
Vacuum Level E, P-Like
lT\bJ T
Localized States
11 2~
,s
-
Figure 5. Normalized K edge absorbance ( A M ) : (a) Sr compounds; (b) Rb compounds. The arrows on the RbN3 XAES indicate the Rydberg series for transitions I s n p (n L 5) discussed in Table I. (c)Typical
transitions observed.
For the special case when the distribution contains a single Gaussian peak:
where A2 is the mean square deviation of r from RMs and N , is the total number of S atoms at a distance R,, from
(5)
where AaM0is the change in the atomic mas8 absorption coefficient (CRC Tables), t is the cell thickness in Figure 1,and the number density dM = x(MW)/P (where x is the metal mole fraction and MW i@ weight in dalton) allows to measure the molar volume V of the MAS (Figure 3). (3.3) The energy in the neighborhood of the edge where the second derivative of the absorbance AMovanishes is determined for the MAS and standards RbN3, RbBr, SrBrz.6H20. (Errors in Eiarise mainly from the slipping of monochromator steps in Figure 1 but this can be reduced by placing a standard behind the detector I and behind this a third counter to measure I' which acts as a reference.) Here Ei depends on the oxidation state of M and this dependence is similar to that already observed for Mo and Mn corn pound^.^^^ Figure 5 shows the edge region for Sr and Rb compounds. As the coordination charge increases the edge inflection point moves to higher energy in going from Sr metal to SrBr2.6H20 or Rb to
M-NH, near the Metal-Insulator Transition
The Journal of Physical Chemistry, Vol. 84, No. 10, 1980
1209
from the data by the relation
I
15 .zoo
14 .BOO
15.600
16 .OOO
E(KEU! 1.500
I
I
b
f a 1
0
1.000
0,
n
a
.500
I
a
where (k-, k-) = (4,14 kl)are experimental limits" and W is an apodization window used to reduce the side lobes which usually appear in truncated FT@. There are several semiempirical methods to evaluate ai(k). A Taylor series expansion, retaining only linear terms, allows us to estimate the Ri from @ by using standard compounds with known crystalline structure and assuming a transferability of ai,but this does not allow for many body effects which appear in the back-scattering matrix element t,'. The theory of these effects is poorly understood but the ]presence of many body interactions may be surmised when the EXAFS amplitudes decay rapidly for high k . This has recently been explained by the fact that as k increases the relaxation of nonexcited one electron states from initial states li) to li') reduces the K shell transition probability in the one electron approximation by the product of overlap matrix elements (ili').lon This is similar to the effect of conduction electron states in exciton transitions explained by Nozieres et al.Iobi.e., auger processes. Also, the effect may be stronger in odd electron compounds with large contact hyperfine terms at the nucleus of M because the contact term is a result of the mixing of Ins) atomic states in the unpaired electron wave function by the exchange interaction. This suggests that relaxed unpaired electron state li') should show a different contact term in say a Knight shift measured as a function of E in the E M S region which suggests a method for detecting such effects. The magnitude of RMN estimated for Rb-NH3 ([R= 8-160 and T = -10 to -60 OC) by using relation 6 is 3.09 A independent of temperature and concentration within the uncertainty of the measurement and a (A2)1/2I0.15
A.
u
0.000
-0 .so0
I
I I
14.800
15 .zoo
:L5.600
16 .OOO
E(KEU1 Figure 6. Absorbance for Rb-NH, ( d M= 0.4310f 0.002, t = -70 O C ) : (a) A M AM, (b) AM.
+
RbN@ However, in Figure 5a the shoulder appearing at lower energies near the edge for saturated Sr-NH3 indicates the existence of additional bound final state levels. The dependence of Ei vs. T for Rb-NH3 (Figure 7, x = 0.05) shows tlhat as T increases EI moves to higher energies close to valuw observed for the reference standards. This information may be used to ascertain the nature of the metal-insulator transition in section 4 because E j is affected by transitions in the neighborhood of the edge. These are tr,ansitions to P-like atomic states. Transitions of type (1) are absent in the metal and increase in intensity as the mole ratio of R increases from 10 to 20 and 50 as indicated in Table I by the absorbance amplitude A(1) relative to the edge jump, AAMO. The edge jump was measured by the difference in the preedge and postedge regions in Figure 4 when extrapolated to the edge region. (3.4) In the EXAFS region information on PMs in eq 4 is deduced from the FI'of knXM(k).The FT are estimated
4. Microscopic View of MAS Since there are as many models for MAS as there are researchers in this field, this section is an attempt to discuss the above results in light of the different motdels. (4.1) The edge shift vs. concentration in Sr-NH3 indicates that as R decreases E j decreases to the value observed for the metal. In saturated Sr-NH3 the appearance of a shoulder at energies lower than observed for Sr suggests transitions to bound exciton states and/or the effects of a lower valence than SrO. The transitions (1)to P-like excited atomic states are not observed for Rb or Sr mstals, but A(l) increases in intensity as R increases and is present in RbBr and RbN3. In the metal this transition may not be observed either because it is symmetry forbidden or because of broadening of the excited state by interactions with extended states near the Fermi level. Since Rb is a bccub metal which melts near 38 " C the first reason is unlikely to be true but more work needs to be carried out with say mixed solutions of (Rb-Na) in NH3 to establish the effect of symmetry in transition (1). The continuous change of Ei vs. T (Figure 7) rules out an inhomogeneous description of Rb-MAS. (4.2) The R b N distance measured by EXAFS does not vary appreciably with temperature and concentration in Rb-NH3 indicating that a true model for MAS inust consider that a solvated element species remains invariant as the conductivity varies by over one order of magnitude.ll Here the Rb-N distance can be compared with the hardsphere radius R(hs) deduced by Thompsonlb for other alkali M-NH3 from compressibility data shown in Figure
1210
The Journal of Physical C h e m M y , Vol. 84,
Acrivos et at.
No. 10, 1980
TABLE I: XAES Data Near the Rb K Edge Giving the Number Density d~ and the Parameters for Transition 1 in Figure 5,a and the Number of NH, Molecules Deduced from EXAFS sample mole ratio no. of no. Rf2 temp, K runs d ~ g/cm3 , ~ A ( ~ ) / A A M ' ~E , ~ - E,,b eV N(NH,) Rb-NH,, A 2 8 228(1) 4 0.4206(32) 0.211(9) 5.92(6) 25 228(1) 4 0.1165(3) 0.301(1) 5.42(1) (5.2) (3.09(7)) 1(D) 2 30 228(1) 4 0.0941(3) 0.297(3) 5.39(2) 2 8 199(2) 9 0.4299(39) 0.231(15) 5.66(16) 5.6 3.07(3) 1 25 197(1) 4 0.0866(4)c 0.244(2) 5.78(3) 3.23(17)' 3 30 197(1) 8 0.0964(7) 0.303(4 ) 5.24(6) 3.10(3) 5 160 197(1) 8 0.0180(1) 0.303(2) 5.37(3) 5.7 3.10(3) If in Figure 5 transitions 1 may be represented by: Rb'(1s)l.. . (4~)~:' S Rb+(ls).. . ( 4 ~ ) ~ n p :with ' P n > 5, then according to (Parratsb)the spacing between the transitions 1 may be estimated from the observed spacing between the n p spin S' = S t 1 / 2 and from ref 12 it follows that the spacing between (n,n')= (5,6), atomic levels of Sr+(ls)'. . . ( 4 ~ ) ~ with (6,7), (7,8) . . . are respectively 4, 1.4, and 0.8 eV. Estimate of error in last place in parentheses. Phase separation has occurred here showing the sensitivity of the method for determining the parameters for critical phenomena, The large error in the Rb-NH, distance is a result of phase separation. +
I
Il15194.5 5 194.0 5
1
I
l
9
i
I
4
.
I
I
.
5
V
I
/a
0
Ei(eV) 15193.5
200
220
240
260
T (K)
Figure 7. ,Els vs. Tfor RbNH, (5 mpm).
I
I
I
0
Figure 8. Rb-N distance in Rb-NH3 compared with the hard-sphere radius deduced from compressibility b Thompsonlb for other alkali metal MAS. Here R(hs) = Rb-N -t 0.8 fits the plot.
1
8 vs. the element ionic radius Ri and its ionization potential (IP). Let R(hs) = RMN
+C
(7)
where the value of the constant C is determined (by interpolating the Rb-Rm data in Figure 8) to be 0.8 .& The basis for this interpolation is as follows: If the structure of the solvated species remains invariant as both the mole ratio and the conductivity of MAS varies by two orders of magnitude, then this species must be a cation and therefore R(hs) must increase with the ionic radius Ri and consequently decrease as the ionization potential (IP) increases. For C = 0.8 A it follows that RMN- Ri E 1.5 A
is constant to within 5% for all alkali element MAS but = 0.98 if the solvated species is not ionized RMN - Ratomic f 0.1 A for the alkali MAS. Both (RMN - Ri) or (RMN Ratomic)are constant as one goes through the alkali MAS and cannot decide on the presence of solvated cation species or solvated atoms, but the fact that RMN remains constant independent of T and concentration favors the presence of solvated cations with N atoms at a distance equal to the N van der Waal radius of 1.5 A. The root mean square deviation (AZ)lI2 I 0.15 A (which arises mainly from the Rb-N bond vibration and/or fluctuations) suggests that there is some motion of the ammonia molecules about the central ion. Also, a cation species is favored by the electrostriction observed. Here the volume occupied by the Rb species plus six ammonia molecules is 148.5 A3 whereas in pure ammonia six molecules occupy 150.4 A3 giving rise to an electrostriction of -1.1 cm3/mol. Now this data should also be compared with the large volume variations vs. Na mole fraction in MAS measured by Schindewolf and Werner [paper in this issue] from chemical potentials. Our data suggest that solvated Rb(NH&+ ions are passive species and that the MI transition is controlled by the electrons in the condensed phase. Also, since a chemical equilibrium for the association of species using Debye-Huckel activity coefficients can be invoked only for concentrations below MAS data above these concentrations M (x < must be treated from the point of view of a band model [Mott, paper in this issue]. (4.3) The T dependence of dM (R = 20 and 25 for RbNH3) in Figure 3 can be fitted to a semiempirical relation indicated by the solid lines where a least-square fit (31 points for R = 20 and 20 points for R = 25) gives a l = d(ln V)/d(l/T) = -6.02 X lo2 K + 2a2T a2 =
d2(ln V)/d(l/n2 = 6.48 X lo4 K
which is in apparent agreement with the parametrization of the density data for all M-NH, [Acrivos, 1973 and P. Rusch and J. C. Thompson, 1973 unpublished]. (4.4) The fact that the effects of phase separation" are made evident by a lower average number density (measured at the edge) and a greater error in the Rb-N distance (measured from the EXAFS) than the respective values for the same sample at higher temperatures (R = 25, 197 K in Table I) indicates the sensitivity of the method for establishing that the Rb MAS are not inhomogeneous above T,. We surmise that the lower number density observed near the phase separation is due to a spread of K-edge values which in turn introduces a large error in the Rb-N distance.
J. Phys. Chem. 1980, 84, 1211-1215
1211
References and Notes
Thus, a sunmary of work to date indicates that the XAS method is useful to elucidate the properties of MAS. Here Ei, El, A(l), and AAMo are a more sensitive measure of fluctuations near critical points and for this Rb-M-NHB (M = Li, Na) needs to be investigated near X,, T,. The valence of other compounds (alkaline earths and lanthanoids in MAS) could be established. Acknowledgment. This work was carried out under the auspices of IJSF Grant DMR 78 20577 and SSRL Proposals 76 and 312 a t SJSU and the Biomedical and Environmental Research Division of DOE under contract W-7405-Eng-48at the Lawrence Berkeley Laboratory. The facilities at S8RL are supported under NSF contract DMR 77-27489 in cooperation with the U S . Department of Energy. We also acknowledge the help and enthusiasm of fellow workers J. Kirby, L. Esparza, J. Code, J. Reynolds, S. Bro'wn, Y. Yokoka, R. Ott, and C. Forney. J. V. is gr'Lteful for the interest shown in this work by N. F. Mott, J. C. Thompson, J. J. Lagowski, J. Dye, and D. E. Bowen,
(1) (a) P. Drude, "The Theory of Optics", Dover Publication, New York, 1965. (b) J. C. Thompson, "Electrons in Liquid Ammonh", Clarondon Press, London, 1976. (2) J. A. Kirby, "Manual for Data Collection Rogram for Use on the :May Lines at SSRL", UCID-4017, 1978. (3) J. V. Acrivos and K. S. Pitzer, J. Phys. Chem., 86, 1693 (1962). (4) E. A. Stern, D. E. Sayers, and F. W. Lytle, Phys. Rev. 6 , 15, 4836 (1975). and references therein. (5) k. de'L. Kronig, Z . Phys., 75, 191, 468 (1932); D. R. Hartroe, R. de L. Kronig, and H. Petersen, Physica, 1, 895 (1934). (6) S. P. Cramer, T. K. Eccles, F. Kutzler, K. 0. Hodgson, and S. Mach, J. Am. Chem. Soc., 98. 8059 (1976). (7) T. M. Hayes and P. N. Sen, Phys. dev, Lett., 15, 956 (1975). (8) (a) R. 0. Shulman, Y. Yafet, P. Elsenberger, and W. E. Blurmberg, Proc. Natl. Acad. Sci. U . S . A . ,73, 1384 (1976). (b)L. G. Purratt, Phys. Rev., 58, 295 (1939); (c) F. K. Richtmyer, S. W. Barnes, and E. Ramborg, Phys. Rev., 48, 843 (1934); (d) B. K. Teo and P. A. Lee, J. Am. Chem. Soc., 101, 2815 (1979). (9) J. A. Kirby, unpublished. (10) (a) E. A. Stern, S. M. Heald, and B. Bunker, Phys. Rev. Lett., 42, 1372 (1979); (b) P. Nozieres and C. T. DeDominicis, ibid., 178, 1097 (1969). (1 1) A. C. Sharp, R. L. Davis, J. A. Van der Hoff, E. W. Le Master, and J. C. Thompson, Phys. Rev. A , 4, 414 (1971). (12) C. E. Moore, Nafl. Bur. Stand. U.S., Clrc., No. 487 (195211.
A Comparative Study of the Concentration Fluctuations in Metal-Ammonia (Li and Na in Liquid NDS)and Metal in Molten Salts (K in KBr) P. Chieux," Institut Laue-Langevin, 156X, 38042 Grenoble Cedex, France
P. Damay, Laboratoire des Surfaces et Interfaces, Ecole des Hautes Etudes Industrielles, 59046 L i b , France
J. Dupuy, and J. F. Jal apatfement de Physique des Malriaux, Universl, Claude Bernard, 89000 Lyon- Villeurbanne, France (Received Juiy 17, 1979)
The solutions of metals in liquid ammonia and of metals in molten salts are very similar. They display nonmetal-to-metaltransition and analogous phase diagrams with liquid-liquid immiscibility. The concentration fluctuations have been measured in both systems in the vicinity of the liquid-liquid critical point by small-angle neutron scattering. The most recent results are presented arid compared. The differences observed in the values of the critical indices which are of mean field type for metal-ammonia except quite near T,arid of tridimensional Ising model type for potassium in potassium bromide correspond to what might be expected from the Ginzburg criterion, since the metal-ammonia solutions have much longer Debye correlation lengths. The question of the possible interaction of these fluctuations with the electronic localization processes is comparatively discussed for the two systems but the problem remains largely open. While a coinsiderable amount of work has already been devoted to the nonmetal-to-metal transition in disordered systems' one is still far from a proper understanding of the transition at the microscopic level. In particular the detail of the intenaction between electronic and structural properties,2 the possibility of electron localization within well-identified chemical structures or by disorder effects, and the exact role of the concentration fluctuations related to the liquid--liquid phase separation generally observed in the vicinity of the electronic transition remain an area open to investigation. A few years ago we started a structural investigation by small-angle neutron scattering of the concentration fluctuations near the liquid-liquid critical point in solutions of Li in ND3.3 We present here a comparative study of the most recent results obtained on concentration fluctuations in two analogous systems, solutions of alkali metals in ND3 and solutions of alkali 0022-3654/80/2084-1211$01 .OO/O
metals in their molten salts, more specifically Li and Na in ND,4 and K in We present in Figures 1 and 2 phase diagrams of the two ~ y s t e m s . ~One J notices the difference in the concentration scales since in the metalammonia system saturation of the solution at room itemperature is obtained for concentrations near 15-20% mole fraction, which is roughly the solvated ion concentration and might be taken as the upper end of diagram. Howlever, if one considers the differences in the two solvents the analogy is quite surprising. One can compare in Figure 3 the electrical c o n d u c t i ~ i t ywhere ~ ~ ~ A is the position of the Mott criterion for nonmetal-to-metal transition (300 9-' cm-l) and B the accepted lower limit for the metallic regime (about 1000 Q-' cm-'). We have also marked1 the location of the critical point for liquid-liquid immiscibility. As compared to the M-NH3 system the M-MX system has certainly been much less investigated (see the reviews in 0 1980 American Chemical Society
