3860
J. Phys. Chem. 1984,88, 3860-3877
Structures and Properties of Metal-Ammonia Compounds on the Trail of a New Ammonia Geometry W. S. Glaunsinger,*t R. B. Von Dreele,+R. F. Marzke,x R. C. Hamon,$ Department of Chemistry and Department of Physics, Arizona State University, Tempe, Arizona 85287
P. Chieux, Institut Max von Laue-Paul Langevin, 38042 Grenoble, Cedex, France
P. Damay, Laboratoire de Chimie Physique, 59046 Lille, Cedex, France
and R. Catterall Department of Chemistry, University of Salford, Salford M5 4WT, U.K. (Received: August 24, 1983;
In Final Form: January 17, 1984)
The ammonia molecules in metallic metal-hexaammine compounds apparently adopt a nearly planar, distorted structure containing one short and two extremely long bonds, rather than its normal pyramidal geometry containing equivalent protons. The experimental research on the structures and properties of metal-ammonia compounds related to this highly distorted structure is reviewed in detail, and the possible origin of this novel geometry is discussed.
I. Introduction Ammonia is a unique solvent in terms of its ability to dissolve all the alkali metals, the alkaline earth metals Ca, Sr, and Ba, and the lanthanide metals Eu and Yb. The resulting metal-ammonia (M-NH3) solutions have been the subject of numerous investigations in the past,'-5 primarily because they exhibit a fascinating variety of electronic behavior as the metallic concentration is increased. They have also served as most useful model systems for testing concepts in chemistry. In the dilute region, the solutions are blue and insulating, with the valence electron(s) of the metal apparently residing in cavities surrounded by several preferentially oriented ammonia molecules. These "solvated" electrons are of great fundamental interest and also provide chemists with a powerful reducing agent. At intermediate concentrations extensive electron spin pairing occurs, and there is a transition to the metallic state. The variation from insulating to metallic properties within a single-phase region in these solutions provides an attractive alternative to using solids, which are frequently plagued by phase transformations,6 to study this transitional region. Finally, in the concentrated region the solutions are bronze and metallic due to the large overlap of electronic wave functions. The interest in concentrated solutions in part stems from the presence of polar ammonia molecules, which dilute the metallic concentration sufficiently for these liquids to display some of the unusual properties associated with electronically dilute metals.' In spite of all the research in this area during the past century, the precise nature of solvated electrons and of associated species in ammonia remain unsolved problems, and as yet there is no completely satisfactory theory of the metal-insulator, or Mott, transition. Upon freezing concentrated solutions containing Li, Ca, Sr, Ba, Eu, and Yb, metallic compounds are formed which have the approximate compositions Li(NH3)4and where A = Ca, Sr, Ba, Eu, or Yb.S In analogy to their solution counterparts, these "expanded-metal" compounds exhibit a fascinating variety of properties. In particular, the hexaammines crystallize in a cubic structure above about 50 K and have molecular complexes, A(NH3)6, located a t each lattice sites9 However, the unique structural aspect of these compounds is the observation of a highly distorted, nearly planar ammonia geometry having two inequiv+Department of Chemistry. $Department of Physics.
0022-3654/84/2088-3860$01.50/0
alent sets of protons with one N-H distance being rather short ( ~ 0 . A), 9 whereas the other two are extremely long (-1.4 A).9 Moreover, as the temperature is increased, the molecular motions of the ammonia molecules in these materials range from tunneling to classical rotation to diffusion above about 100 K, which is only about one-third the melting temperature of these material^.^ The metallic nature of the hexaammines is believed to originate from the loss of up to two valence electrons from each complex to a conduction band, and it is anticipated that the rather large radius of the resulting molecular ions, coupled with possible Brillouinzone-contact effects, may result in a low-electron-density metal. This expectation is supported by detailed electrical transport,1° magnetic susceptibility,' and electron paramagnetic resonance (EPR)12measurements in the related compound Li(NH3)4,as well as m a g r ~ e t i c ' ~ and * ' ~ EPR15 studies of the hexaammines. In summary, the electronic properties of metal-ammonia compounds, in addition to the molecular motions and structures of the metal-hexaammines, have proven to be highly unusual and have had important implications for theories of electron-electron interac(1) "Solutions Mttal-Ammoniac", Colloque Weyl I, G. Lepoutre and M. J. Sienko, Eds., Benjamin, New York, 1964. (2) "Metal-Ammonia Solutions", Colloque Weyl 11, J. J. Lagowski and M. J. Sienko, Eds., Butterworth, London, 1970. (3) "Electrons in Fluids", Colloque Weyl 111, J. Jortner and N. R. Kestner, Eds. Springer-Verlag, New York, 1973. (4) "Electrons in Fluids-The Nature of Metal-Ammonia Solutions", Colloque Weyl IV, J . Phys. Chem., 79, 2789 (1975). (5) "The Fifth International Conference on Excess Electrons and MetalAmmonia Solutions", Colloque Weyl V, J . Phys. Chem., 84, 1065 (1980). (6) See, for example, N. F. Mott, Proc. Phys. SOC.London, Ser. A, 62,416 (1949). (7) See, for example, M. H. Cohen and J. C. Thompson, Ado. Phys., 17, 857 (1968). (8) N. Mammano, ref 2, p 367. (9) W. S. Glaunsinger, J . Phys. Chem., 84, 1163 (1980), and references therein. (10) R. C. Cate and J. C. Thompson, J. Phys. Chem. Solids, 32, 443 (1971). (11) W. S. Glaunsinger, S. Zolotov, and M. J. Sienko, J. Chem. Phys., 56, 4756 (1972). (12) W. S. Glaunsinger and M. J. Sienko, J. Chem. Phys., 62, 1873 (1975). (13) M. J. Mobley, Ph.D. Thesis, Arizona State University, 1979. (14) M. J. Mobley and W. S. Glaunsinger, Solid Sfate Commun.,40, 357 (1981). (15) F. Y. Robb and W. S. Glaunsinger, J. Magn. Reson., 46, 98 (1982).
0 1984 American Chemical Society
Structures and Properties of Metal-Ammonia Compounds tions, spin-lattice relaxation, and molecular electronic structure in the metallic state. However, none of the properties of these materials is more intriguing than the discovery of a novel ammonia geometry in the hexaammines in which the ammonia molecule adopts a nearly planar, distorted structure containing one short and two extremely long bonds, rather than its normal pyramidal geometry containing equivalent protons. Although the origin of this molecular structure is unresolved at present, an impressive and growing body of evidence provides strong support for its existence. This evidence includes the determination of the metal, nitrogen, and deuterium positions in completely deuterated compounds using powder neutron diffraction (PND);9 the dramatic low-frequency shift, relative to dipositive octahedral metal-hexaammine complexes, and large splitting of the asymmetric ammonia bending vibrations observed in inelastic neutron scattering experiment^;'^,'^ the reasonable agreement between the low-temperature proton nuclear magnetic resonance (NMR) line width and that calculated from P N D structural parameters;ls the extremely long spin-lattice relaxation times and exceedingly narrow deuteron resonances in Ca(ND3)6, which are consistent with a nearly planar ammonia geometry;19the large average intramolecular interproton distance for N H 3 in Ca("3)6 estimated from proton N M R spin-relaxation measurements;20and the observation of imide formation as the initial decomposition product of these compounds in a diamond-anvil high-pressure which perhaps results from breaking the two long bonds in the distorted ammonia molecule. This molecule has the most unusual structure of any small molecule in chemistry and is providing a stiff challenge for theoretical structural predictions. In view of the highly unusual nature of the ammonia structure in M-NH3 compounds and its potential importance in chemistry, this paper is devoted primarily to a detailed view of the experimental research on the structures and properties of M-NH3 compounds related to this geometry and its interpretation. The possible origin of this geometry is also discussed schematically. 11. Experimental Evidence In this section the physical and chemical evidence related to the ammonia structure is reviewed and discussed. Most of the more recent research has been performed either in our own laboratories at Arizona State University or in collaboration with scientists at other laboratories. Since the synthesis of high-quality materials is of crucial importance to the success of detailed studies of their properties, some important aspects of the synthesis and stoichiometry of these reactive expanded-metal compounds will be covered prior to a consideration of their properties. A . Materials Preparation. Since, Li, Ca, Sr, Ba, Eu, and Yb dissolve rapidly in liquid ammonia, in principle the preparation of their respective compounds is straightforward. In most cases these compounds have been prepared by condensing sufficient dried ammonia into tubes containing the pure metal to produce the desired concentration. The tubes are normally fabricated into a configuration best suited to the particular study to be undertaken, then sealed, allowed to warm to ambient temperature to ensure homogenization and sample stability, and finally transferred to a dewar filled with liquid nitrogen for long-term ~ t 0 r a g e . lThese ~ materials can also be synthesized by condensing excess dried ammonia into large tubes (1-2 cm diameter) containing the pure metal to form an 8-10 mole percent metal (MPM) solution, (16) R. B. Von Dreele, W. S . Glaunsinger, P. Chieux, and P. Damay, J . Phys. Chem., 84, 1172 (1980). (17) R. B. Von Dreele, W. S . Glaunsinger, J. Eckert, and J. Goldstone, Bull. Am. Phys. SOC.,26, 291 (1981). (18) T. R. White, D. A. Gordon, R. F. Marzke, R. B. Von Dreele, J. L. Yarnell, A. L. Bowman, and W. S . Glaunsinger, Nature (London), 271,414 (1978). (19) M. J. Mobley, W. S . Glaunsinger, and R. F. Marzke, Chem. Phys. Lett., 81, 159 (1981). (20) K. W. Rawlings, R. F. Marzke, and W. S . Glaunsinger, Chem. Phys. Lett., in press. (21) T. R . White, W. S . Glaunsinger, J. Gill, and R. C. Hanson, unpublished results.
The Journal of Physical Chemistry, Vol. 88, No. 17, 1984 3861 followed by the slow removal of ammonia. The latter method, although very tedious, is most convenient for the preparation of large sample volumes or thick metal films required for neutron diffraction experiments.16*22 Such samples are also useful for measurements on a single material with a variety of techniques, since these compounds can be handled and transferred into various sample containers under liquid nitrogen. Both synthetic methods result in polycrystalline products. The compounds retain their polycrystalline nature up to their melting temperature or the temperature a t which decomposition becomes evident. However, rather large grains (=l mm) are observed occasionally in the case of Li(NH3)4near its melting point (89 K).9 In some cases, measurements have been performed on the same compound prepared by both techniques, and we have found that the results are identical within experimental error. These compounds have unknown melting points, except for Li(NH3)4(89 K)23 and Ca("3)6 (=265 K).24 They are also relatively unstable and have a pronounced tendency to decompose into ammonia, hydrogen, and the metal amide, which further catalyzes the decomposition reaction. In order to reduce decomposition, it is of the utmost importance to (1) scrupulously clean the sample tubes prior to sample preparation, (2) have the highest-purity metals possible, and (3) make sure that the ammonia is thoroughly dry before condensing it on the metal. Under these conditions the compounds are quite stable below 240 K and relatively stable at temperatures as high as 330 K, above which they tend to decompose rapidly. However, if these precautions are not taken, then these materials can be relatively unstable even at temperatures as low as 240 K. In order to further reduce the extent of sample decomposition, measurements on these materials were conducted as soon as possible after their preparation, and the temperature was not raised above 240 K for extended periods of time. In general, samples were discarded if they showed any visual signs of decomposition either prior to or during the course of an experimental study. B. Stoichiometry. Although samples can be prepared by the direct reaction of metal with ammonia, corresponding to the commonly assumed formulations Li(NH3)4and such preparations probably do not correspond to the exact equilibrium stoichiometry for these compounds. Unfortunately, the determination of the precise stoichiometry of these compounds is very difficult and subject to considerable experimental error. In principle, compound formation and stoichiometry can be determined from precise measurements of the phase diagram of these systems by thermal techniques, but such determinations have only been possible for the Li-NH, system due to the instability of the compounds at higher temperatures. Previous stoichiometry studies for the A-NH3 systems have involved measurements of the composition dependence of the equilibrium vapor pressure of ammonia over the metal. In particular, upon removal of NH3 from a saturated solution, one anticipates that the vapor pressure should drop abruptly and become invariant upon precipitation of the A(NH3), compound, with the high-concentration end of the invariant region yielding the stoichiometry of the compound. However, the vapor pressures of the A-NH3 systems do not conform to this simple behavior. We first consider the lanthanide-hexaammines. Hagedorn and LelieurZ5have recently performed a detailed vapor-pressure study of the Yb-NH, system, and some of their results are shown in Figure 1. Although it is observed that the vapor pressure decreases exponentially rather than abruptly near 13 MPM, it is unclear whether this behavior is an experimental artifact or an intrinsic property of the Yb-NH, system, in which case it could be due to absorption of NH3 on the metal or perhaps a solid solution of NH, with the metal. The observation of the same ~,*~ phenomenon for the Yb-NH, system by other w o r k e r ~ ~suggests (22) R. B. Von Dreele, W. S. Glaunsinger, A. L. Bowman, and J. L. Yarnell, J . Phys. Chem., 79, 2992 (1975). (23) N. Mammano and L. V. Coulter, J. Chem. Phys., 47, 1564 (1967). (24) M. J. Mobley, W. S. Glaunsinger, and J C. Thompson, ref 5 , p 1168. (25) R. Hagedorn and J. P. Lelieur, J. Phys. Chem., 85, 275 (1981). (26) R. H. Frisbee and N. M. Senozan, J . Chem. Phys., 57, 1248 (1972).
3862 The Journal of Physical Chemistry, Vol. 88, No. 17, 1984
TABLE I: Stoichiometry of Metal-Ammonia Compounds M(NH,). M T,K n Fi"
600 500 -
0
Glaunsinger et al.
I 5
IO 15 concentration (MPM)
Ca
209.2 227.7 240 213 293
5.67b 5.19b 5.900" 5.869e 5.82SC
5.8 f 0.1
Sr
209.2 227.1 213 250 213
4.876 4.92b 6.3gd 6.1 5d 6.01d
5.1 f 0.7
Ba
209.2 227 223 250 273
7.496 7.55b 6.97e 6.30e 6.10e
6.9 f 0.7
Eu
197.1
6.3f
6.3
Yb
197.1 213 213 243
6.4' 6.69 6.5h 6.4g
6.5 f 0.1
Li
89 89 89
4.0' 4.1Y 4.00k
4.0
-243 K --
228 K ----213 K
20
Figure 1. Ammonia vapor pressure vs. ytterbium concentration in YbN H 3 solutions at 213,228, and 243 K. The plateaus at lower and higher concentrations correspond to equilibria between the Yb-NH, compound and a saturated Yb-NH3 solution and between the Yb-NH3 compound and Yb metal, respectively. Data from ref 25.
that it may indeed be real. The vapor pressure begins to be invariant at 13.2 f 0.2 M P M at 213 K and 13.6 0.2 M P M at 243 K, which corresponds to the compositions Yb(NH3), 6+o and Yb("3)64+0 respectively. Hence, it appears that the composition of the compound is independent of temperature within experimental error and is definitely not Yb(NH3),. Our recent vapor-pressure measurements on the Yb-NH3 system28indicated 5+0 at 21 3 K, which is in good that the composition is Yb(",), agreement with the results of Hagedorn and LelieurSz5In addition, Thompson, Stone, and WaughZ9found the compositions Yb(NH3)64and E u ( N H ~ at ) ~197 ~ K by extrapolating the N H 3 vapor pressure to zero. In analogy to the Yb-NH3 system, vapor pressure measurements on the alkaline earth-hexaammines also indicate that the vapor pressure drop on precipitation of A(",), is not sharp and exhibits a pronounced curvature at the lowest pressure where the last traces of NH, are removed from the metal,30which again may be due to NH3 absorption on the metal or to solid-solution formation. It has been reported that the composition of these compounds is nonintegral and varies with the t e m p e r a t ~ r ealthough ,~~ it is difficult to access the experimental errors in these measurements. The compositions of the A-NH3 compounds determined by vapor-pressure measurements are summarized in Table I. It can be seen that the measurements in different laboratories frequently do not agree, which illustrates the difficulty in making reliable stoichiometry measurements on the systems. In particular, it is unclear whether the temperature dependence of the NH, content in the alkaline earth compounds is real and not influenced by decomposition, although qualitatively it appears that the N H 3 content of the Ca-, Sr-, and Ba-NH3 compounds declines above about 240, 213, and 223 K, respectively. The results for the Ca-NH331,32and Yb-NH3Z5,28,29 systems appear to be the most internally consistent, with the Ca-NH, compound being slightly NH3 deficient (CaNH,), 8*0 and the Yb-NH, compound having a substantial excess of NH, [Yb(",), 5+0 l]. A single measurement of the stoichiometry of the Eu-NH3 compoundz9in-
*
(27) s. Dickman, N. M. Senozan, and R. L. Hunt, J . Chem. Phys., 52, 2657 (1970). (28) T. R. White and W. S. Glaunsinger, unpublished results. (29) D. S. Thompson, M. J. Stove, and J. S. Waugh, J . Phys. Chem., 70, 934 (1966). (30) R. Catterall, ref 1 , p 40, and references therein. (31) P. R. Marshall and H. Hunt, J. Phys. Chem., 60, 732 (1956). (32) C. A. Kraus, J . Am. Chem. SOC.,30, 653 (1908).
"Values given are the average values of n A their standard deviation. bFrom ref 31. CFromref 32. dFrom ref 33. eFrom ref 34. fFrom ref 29. gFrom ref 25. From ref 28. 'From ref 35. 'From ref 23. kFrom ref 31.
dicates that its composition is nearly the same as the Yb-NH, compound, although Eu2+ and Yb2+ have quite different radii (Eu2+ = 1.09 A, Yb2+ = 0.93 A). Although the uncertainty in ~~,~~ composition is large (&lo%),the Sr-NH3 c o m p o ~ n dappears to be stoichiometric [Sr(NH3)5.7+0.7], whereas the Ba-NH3 compound31*34 seems to have an excess of NH3 over the ideal hexa9+0,7]. Since the ionic size inammine composition [Ba(",), creases in going down the alkaline earth series (rca2t = 0.99 A, rSr2+= 1.13 A, rBa2+= 1.35), it is possible that Ba is large enough to coordinate more than six N H 3 molecules or to perhaps accommodate an excess of N H 3 in interstitial positions. There is also the possibility that the nonstoichiometry originates from cation vacancies in these compounds. Hence, although the observation of excess N H 3 in the Ba-, Yb-, and Eu-NH, compounds is apparently real, the mechanism by which excess N H 3 is accommodated in these structures is unresolved (see Section 11). It is also possible that the nonstoichiometry observed for some of these compounds may have an important influence on their physical properties. Clearly, it would be highly desirable to have more precise and detailed stoichiometry determinations for the Ca-, Sr-, Ba-, and Eu-NH3 compounds to compliment existing and future studies of their physical properties. For simplicity, the formulation will be used hereafter, with the implicit understanding that some of these compounds may contain up to 10% excess NH,. In contrast to the A-NH3 compounds, the vapor pressure of the Li-NH3 system36varies continuously with concentration and shows no evidence of compound formation. However, these measurements were all made at temperatures well above the melting point of the compound (89 K). Fortunately, this compound is sufficiently stable near its melting point to permit precise thermal measurements, and the results of these studies are summarized in Table I. The heat capacity measurements of Mammano and Coulterz3 indicate that the composition corresponds to Li(NH3)4,15at the eutectic at 89 K, which occurs very close to (33) G. Roederer, C. R . Acad. Sci., 140, 1252 (1905). (34) R. C. Mentree, C.R. Acad. Sci., 135,790 (1902); Bull. SOC.Chim. Fr., 29, 493 (1903). (35) T. David, W. Glaunsinger, S. Zolotov, and M. J. Sienko, ref 3, p 323. (36) C. A. Kraus and W. C. Johnson, J . Am. Chem. SOC.,47, 727 (1925).
Structures and Properties of Metal-Ammonia Compounds TABLE 11: Structures of Metal-Ammonia Compounds Determined by X-Ray Diffraction compd T,K structure cell constants, 8, Li(NH3h 17 hCP a = 7.0a c = 11.1 a = 7.12b c = 11.29 fcc 9.55" Ca("3)6
77 233
bcc bcc
9.2OC 9.12d
Sr("3)6
77 233
bcc bcc
9.45= 9.576
Ba("3)6
77 233
bcc bcc
9.71' 9.976
Eu("3)6
198
bcc
9.55"
W"3h
198
bcc
9.30'
@Fromref 40. ref 39.
From ref 41.
From ref 8. dFrom ref 38.
The Journal of Physical Chemistry, Vol. 88, No. 17, 1984
3863
CRLCIUM[OIHEXaMMINE 9000
1
8000 7000
6000
(0
k-
z
2 5000 0
4000
3000 2000
IOOC 0
15 e From
the melting point of the compound. Differential thermal analysis (DTA) m e a s ~ r e r n e n t shave ~ ~ also revealed that the eutectic composition lies very close to Li(NH,)4. Recently, a careful calorimetric study by Coulter et al. has resolved two eutectics separated by only 0.11 K very close to 20 MPM so that the phase diagram is apparently normal for Li(NHJ4 compound formation. C. Physical Evidence. 1 . Structural Research. The instability and striking properties of M-NH3 compounds suggest that they may crystallize in unusual structural arrangements. The structures of these materials have been studied by both X-ray and neutron diffraction, and the application of the latter technique has led to the discovery of a highly unusual ammonia geometry. In this section the results of these diffraction studies are reviewed and discussed. a. X-ray Diffraction. Previous powder X-ray diffraction studies at 778 and 233 K38for the alkaline earth-hexaammines and at 198 K39 for the lanthanide-hexaammines have indicated that crystallizes in a My-centered-cubic (bcc) structure and that Li(NH3)4 probably solidifies at 89 K into a face-centered cubic (fcc) structure, which transforms into another structure, possibly hexagonal closest packed (hcp), below 82 K.40!41 The suggested structures and cell constants for these compounds are given in Table 11. In general, the data obtained in these studies is of poor quality due to the low scattering power of most of the atoms as well as sample displacement problems during the course of the measurements. Also, the precise composition of the sample was uncertain due to the ease of sample decomposition in the small-diameter capillary tubes used in these experiments. The data on Li(NH,), is most difficult to interpret because (1) the experiments were performed very near the phase transition at 82 K with inadequate temperature control, (2) the data contained reflections from both phases, and (3) very few reflections were observed due to the low scattering power of the Li, N , and H atoms. Hence, only the positions of the heavy metals A can be reliably located in these studies. It is surprising that Sr("!)6 and Ba(NH3)6show the normal increase in cell constant (ao) with apparently increasing temperature, whereas a. for Ca("3)6 decreases with increasing temperature. However, the interpretation of these results is complicated because the expected thermal expansion of these compounds with increasing temperature is opposed by the possible decrease in NH, content (see Table I) (37) L. V. Coulter, S. H. Lee-Bechtold, V. Madhvaraja, and J. K. Gibson, J . Chem. Thermodyn., 13, 815 (1981). (38) F. W. Cagle and H. J. Holland, presented at the 145th National Meeting of the American Chemical Society, New York, Sept, 1963. (39) H. Oesterreicher, N. Mammano, and M. J. Sienko, J . Solid State Chem., 1, 10 (1969). (40) N. Mammano and M. J. Sienko, J . Am. Chem. Soc., 90,6322 (1968). (41) L. Kleinman, S. B. Hyder, C. M. Thompson, and J. C . Thompson, ref 2, p 229.
20
25
TWO-THETG,
DE;.
30
35
40
45
53
Figure 2. Observed and calculated neutron powder diffraction profile from a least-squares refinement of Ca(ND3)6. The points are the observed intensities corrected for background scattering from the quartz sample tube, and the solid line is the calculated diffraction profile. A difference curve is also shown. Data from ref 22. TABLE III: Sequential Refinement of the Best Initial Structural Model for Ca(ND&O profile trial ND3 structure parameters results residual 0.248 1 normal pyramidal, Ca-N 2.45 8, with threefold axis along Ca-N bond
LI
2
symmetrical
Ca-N N-D Ca-N-D
2.18 8, 1.18 8, 83.8O
0.155
3
unsymmetrical
Ca-N N-DI N-D2 N-D3 DI-N-D, Dz-N-D, Ca-N-D, Ca-N-D2
2.69 8, 0.94 8, 1.39 8, 1.39 8, 122' 115' 106'
0.109
86O
From ref 22.
and increase in H2 pressure resulting from decomposition as the temperature is increased. Probably the most significant result of the X-ray experiments is that the A("3)6 compounds retain a bcc structure down to 77 K, in spite of possible variations in stoichiometry and various degrees of sample decomposition. This structure should be preferred over the more densely packed fcc structure in order to reduce the strong electron-electron repulsions in these low-electron-density metals. Due to the relatively weak X-ray scattering power of N and H, the X-ray studies provide no information on the ammonia structure itself. However, it has been possible to determine the ammonia geometry by powder neutron diffraction. b. Neutron Diffraction. ( 1 ) Elastic Scattering. Powder neutron diffraction (PND) has been used to determine the structures of all the A(ND3)6 compounds and their component molecular complexes except Eu(ND,),, which is plagued by the large neutron absorption of E U . ~Deuterated samples must be used to avoid the large incoherent scattering of hydrogen for neutrons. In many respects, P N D is ideally suited for determining the structures of these compounds. In contrast to X-ray diffraction, by a leastsquares fitting of the P N D profile, it is possible to determine precise positions for the metal, nitrogen, and deuterium nuclei in these compounds due to their substantial, yet different, coherent scattering cross sections for neutrons. Moreover, the large powder samples ("2 X 4 cm) required for such studies helps minimize sample decomposition and greatly simplifies sample alignment.
3864 The Journal of Physical Chemistry, Vol. 88, No. 17, 1984
Glaunsinger et al.
TABLE I V Atomic Positions for Ca(ND&'
atom Ca N Dl DZ
X
V
Z
0 0 0 0.130 (2)
0 0 0.100 (4) -0.082 (2)
0 0.298 (2) 0.328 ( 6 ) 0.286 (2)
aThe value in parentheses is the estimated standard deviation in the last significant figure. From ref 22.
I
\\
Figure 4. Computer-generated representation of one of the nearly 46
possible structures of the molecular complex Ca(ND,)6 in metallic Ca(ND& The central calcium atom is octahedrally coordinated to the nitrogens of the six surrounding ammonia molecules. Reprinted with permission from ref 18. Copyright 1978, Nature (London). ammonia molecule has a fourfold rotational disorder. It follows from Figure 3 that a pair of adjacent ND3 molecules within the Ca(ND3)6 complex are in contact (rD- = 1.98 A) relative to the van der Waals contact distance (2.2 for only one of the four positions allowed for each. This one restriction in the relative positions of the ND3 molecules prevents completely free rotation about the Ca-N bond, so that the deuterium atoms are found in fixed, but disordered, positions. Hence, there are a total of nearly 46 possible ammonia orientations in each complex. A computer-generated representation of one of the possible structures of this Ca(ND,)6 complex in metallic Ca(ND,)6 is shown in Figure 4. The intermolecular contacts between Ca(ND3)6 complexes are much greater than the van der Waals distances regardless of the orientation of any of the ND, molecules. Also, there is no hydrogen bonding in this structure, since the closest nonbonded N-D distance is 3.08 A. This structure permits a high degree of thermal motion, which is evidenced by rapid decrease in the intensity of the neutron pattern with increasing scattering angle (see Figure 2) as well as the large temperature factor (=8 Az). In support of this structure, an independent analysis of the neutron scattering data for Ca(ND3)6 by scientists at Los Alamos4' was in excellent agreement with the above results. A detailed series of temperaturedependent PND studies have shown that Ca(ND3)6 retains the same structure in the range of 4-180 K, so that there is apparently no proton ordering at low temperatures:, which is again consistent with the disorder required by the space group Im3m. P N D has been employed to investigate the structures of the other M-ND, compounds,44 except Eu(ND,)~,in the range of 4-100 K and the structural results are summarized in Table V. All these compounds are isomorphous with Ca(ND,), above about 65 K, and the corresponding cell constants for the alkaline earth-hexaammines show qualitatively the expected increase with increasing cationic size. However, the possible increase in N H 3 content in going down the alkaline earth series can also contribute to the observed increase in ao. In fact, excess NH, in the Yb(ND3)6structure is quite possibly the reason why its a. is larger than for Ca(ND&, since Ca2+ and Ybz+ have nearly the same ionic size. P N D refinements of the ammonia site occupancy of the cubic A("3)6 compounds indicated that they were stoichiometric within about 5-IO%, which is within the stoichiometry limits for these compounds (see Table I).
1)
Figure 3. A perspective representation of the structure c :Ca(ND3)6 complex in metallic Ca(ND,)6. One disordered ND3 molecule is shown with four adjacent ND, molecules each in one of their four possible orientations. Several interatomic distances and angles are also shown. From ref 22.
Sample positional stability is excellent and variable-temperature studies in the range 4-300 K can be performed in a straightforward manner. Typical observed and calculated PND profile intensities as well as the difference curve for Ca(ND& at 75 KZ2are shown in Figure 2. The structure is bcc with Ca(ND,)6 complexes located on each lattice site. The space group is Zm3m, and the cell constant is 9.0137 ( 4 ) A. The results of the sequential refinement of the best initial structural model for the arrangement of ND3 molecules in Ca(ND3)6are summarized in Table 111. The atomic positions obtained for the best refinement are given in Table IV along with their estimated standard deviations given by the least-squares analysis. The most intriguing result of this analysis is the most unusual structure of the Ca(ND3)6complex. The relative positions of the ND3 molecules about the Ca atom as well as the pertinent bond lengths and angles are shown in Figure 3 . As expected, the six nitrogens are arranged in an exact octahedron around Ca, but the Ca-N distance is rather long. However, the most striking feature of the structure of Ca(ND,)6 is the dramatic distortion of the ammonia geometry. The ammonia molecules in Ca(ND3)6 have an entirely different geometry from that of the normal pyramidal ND,, in which the N-D bond angle and distance are llOo and 1.00 A, respectively. In this metal the ammonia molecules are nearly planar (D-N-D bond angles of 122' and 1 1 5') and have two inequivalent sets of deuterons, with one N-D distance being rather short (0.94 A), whereas the other two are extremely long (1.39 A). This makes the D-D distances in each molecule much larger (2.05 and 2.35 A) than in normal ND3 (1.65 A). Another unusual feature of this structure is that, although each ammonia molecule is coordinated to the metal by the nitrogen, their pseudotrigonal axes are not coincident with the Ca-N bond, but make an angle of about 1 3 O with it. Furthermore, each
and J. L. Yarnell, private communication. (43) R. B. Von Dreele and W. S. Glaunsinger, unpublished results. (44) P. Damay, P. Chieux, R. B. Von Dreele, and W. S. Glaunsinger, unpublished results. (42) A. L. Bowman
The Journal of Physical Chemistry, Vol. 88, No. 17, 1984 3865
Structures and Properties of Metal-Ammonia Compounds TABLE V: Structures of Metal-Ammonia Compounds Determined by Powder Neutron Diffraction
T range, K
compd Ca(ND3)6n*b Sr(ND,),b Ba(ND3)6b
structure
54 260 560
bcc bcc tetragonal
275
bcc
N
Li(NH3)4c
!I r? a 2 0 0
1
50-75d
550d 41' Yb(ND&" Li(ND3)4C
cell parameters8 9.01 (100) 9.50 (100) a = b = 13.22 (4.8), c = 9.19 9.73 (100)
monoclinic
54 530 30-85 60 85
a = 19.18 (47), b = 9.55, c = 19.17, y = 93.90' 0. 0
bcc
bcd
14.80 (3) 15.03 (85) 14.90 (60)
bcc bcc
80
180
2 8 (dag)
From ref 43. From ref 44. From ref 45. dThe sample was cooled slowly from 100 K. eThe sample was cooled rapidly from 100 K. /Several new low-intensity peaks appear below about 30 K which correspond to the formation of a superstructure with period 2a according to ref 46. gThe value in parentheses is the Kelvin temperature at which the cell constant was determined. (I
i
3 0
u
u)
c z a n
-
2000
l
0
4.75 K
m
/b
A
*
c z
.
a 1000
0
0
80
0
I
180
lJ I
I
0
80
I
U 160
2 8 (dag)
Figure 5. Powder neutron diffraction profiles of Sr(ND3)6: (a) the high-temperature bcc phase; and (b) the low-temperature tetragonal phase, which transforms to the high-temperature bcc phase near 60 K. 0
Like Ca(ND&, there are no observable phase changes in Yb(NH3), down to 4 K. However, Sr(ND3), and Ba(ND3), both exhibit phase transitions at low temperatures. As shown in Figure 5, Sr(ND,), is isomorphous with Ca(ND,), above about 63 K, but the P N D pattern becomes more complex at lower temperatures. The P N D pattern at 4.2 K can be indexed to a primitive tetragonal cell containin four S T ( N D ~units ) ~ and having cell constants a = b = 13.22 and c = 9.19 A. We have found that the transition temperature is exceedingly sensitive to the pressure caused by sample decomposition. For slightly decomposed samples (P (l~~H)12)
(7)
where N , is the number of A(",), complexes per unit volume, xvais the volume conduction-electron susceptibility, and ( l\k(H)12) is the total conduction-electron probability density per complex averaged over the Fermi electronic states, with @ ( r ) being normalized to the volume of a complex. With the experimental values of xva and K(H) in Table VIII, the resulting values of ( l@(H)12) at 200 K are also given in Table VIII. It has been generally accepted that the existence of a very small negative proton spin density in metal-ammonia solutions is a characteristic property of solvated electrons. In fact, the prediction of a negative spin density has been used as a criterion for reliable calculations on solvated electrons in NH3.63 However, since in this study there is effectively no solvation of electrons as in dilute M-NH3 solutions, it follows that the observation of a very small negative proton spin density is not a unique property of electron solvation. High-resolution deuteron N M R experiments have also been performed on Ca(ND&, in order to take advantage of the quadrupole moment of deuterium to further probe the ammonia geometry and molecular motions in these materials.19 As shown in Figure 11, the deuteron resonances are very narrow. Consistent with proton N M R studies of Ca(",),, above about 100 K a (63) M. Newton, ref 4, p 2795.
rapid narrowing of the N M R line is observed. The line width is nearly constant at 280 Hz in the range 70-100 K and broadens between 70 and 45 K to a value of 857 Hz. These extremely narrow lines are in sharp contrast to the observed deuteron N M R lines in solid and liquid ND3 shown in Figure 12,respectively. The deuteron N M R spectrum in solid ND, at 185 K exhibits a characteristic first-order quadrupole-split pattern with an average line separation of 54.3 f 0.7 kHz. Although no quadrupole splitting is observed in liquid ND3 at 298 K, the line width is 5.5 kHz, which indicates a considerable quadrupole broadening due to incomplete averaging of the electric field gradient in this highly associated liquid. The origin of the greatly reduced quadrupole interaction in Ca(ND3), probably involves the nearly planar ammonia geometry found in metal-ammonia compounds. In particular, rapid C3 rotation of normal pyramidal ND3 will average out the nonaxial electric field gradient; however, as evidenced in both solid and liquid ND,, for this motion there remains an appreciable axial field gradient and quadrupole interaction because the nitrogen atom lies on the axis of rotation and outside the plane of the deuterons. Now in Ca(ND3)6,in which there is no hydrogen bonding, if the nitrogen atom lies in the plane of the deuterons and ND3 is rotating rapidly, then both the axial and nonaxial field gradients, and hence the quadrupole interaction, will be effectively reduced. The absence of an appreciable quadrupole interaction in Ca(ND& provides strong evidence in support of the nearly planar ammonia geometry deduced from structural studies of these materials. c. Pulsed NMR Studies. The first measurements of the proton spin-lattice relaxation time ( T , ) in M-NH, compounds were conducted on the alkaline earth-hexaammines at 8 MHz in the range 40-200 K by using a 18Oo-r9O0 pulse sequence, and these measurements have provided additional motional information for these compounds.64 Absorption lines were obtained by Fourier transforming the free-induction decay following a 90' pulse as the magnetic field was swept through resonance. In order to increase the signal-to-noise ratio and avoid eddy-current ringing, samples were dispersed in anhydrous alumina powder to form a dispersion in which the metal grains were isolated. Consistent with previous broad-line work, all spectra consisted of a single asymmetrical line. The temperature dependences of the proton spin-spin and spin-lattice relaxation times are shown in Figure 13 for ST(",)^. Ca("3)6 and Ba(NH,), display similar behavior, except that a high-temperature T1 minimum was not observed in Ca(NH,),, presumably due to its larger diffusional (64)
M. J. Mobley, W. S. Glaunsinger, and R. F. Marzke, ref 5, p
1129.
The Journal of Physical Chemistry, V O ~88, . No. 17, 1984 3871
Structures and Properties of Metal-Ammonia Compounds
t
t
t
,.-
-2
L . .
**
**.
Sr
/" Tl
H
1
i
8 . 0 1 MHz I
1
.
I
A
i
-
!
P
IO+
"
i
Ca(NH,)*
\ i
w
. 0
0
1
1
.
25
"
.
~
.
~
,
'
~
. , 75 IoYTrK ' I
.
'
~ . lo0
I '
' 1 ,
~
I
300
i
Figure 14. Temperature dependence of the proton spin-lattice relaxation time for Ca("3)6 at low temperatures. The dashed and solid lines depict experimental trends and do not represent theoretical fits to the data. From ref 20.
Figure 13. Temperature dependence of the spin-lattice (TI) and spinspin (T2*)relaxation times for Tz* was evaluated from the full-width at half-maximumabsorption ( A v ) by using the equation T2* = In 2/(2'/277Au). From ref 64. TABLE IX: Rotational and Diffusional Activation Energies for the Alkaline Earth Hexaammines" compd ,cot, kcal mol-' E>ff, kcal mol-' Ca("& 1.1 0.2 0.44 i 0.04 2.7 f 0.3 Sr(NH3)6 0.55 f 0.06 2.0 & 0.3 Ba(NH3)6
parable to those found for OD groups in liquid alcohols, such as CD3CDODCD3.65 Spin-lattice relaxation of deuterium is almost always dominated by the interaction between the quadrupole moment and the electric field gradient at the nucleus, with the latter being modulated by molecular motions.65 In the limit of rapid isotropic motion
