Infrared spectra of the ammonium ion in ammonium metavanadate NH4VO3

Spacrrmhimica Am. Vol. 46A. No. II, Printed in Great Britain

pp. 1639-1648.

0584~8539/90 s3.00+0.00 @I 1990 Pergamon Press plc

1990

Infrared spectra of the ammonium ion in ammonium metavanadate NHJO, D. DE WAAL and A. M. HEYNS* Department

of Chemistry, University of Pretoria, 0002 Pretoria, South Africa

and K.-J. RANGE* and C. EGLMEIER Institute of Inorganic Chemistry, University of Regensburg, (Received

13 February 1990;

Universitltstr.

31, D-8400 Regensburg,

F.R.G.

in final form 3 June 1990; accepted 5 June 1990)

Abstract-The N-D stretching modes of isotopically dilute NH3D+ ions in NI&V03 are in agreement with the predicted splitting into C,, C, and C,(2) components under C, site symmetry for the NH,+ ion. The three bands observed represent the three N-H bonding distances in the crystal, and the position, shape and low temperature behaviour of each band confirms the existence of two types of hydrogen bonding in NH4V0,. The low temperature infrared modes of NH: and ND: in N&VOX and ND,VOJ, respectively, can be assigned under space group Pbcm. Temperature dependence of these modes also reflects the presence of both normal and bifurcated hydrogen bonds in NHYO3.

INTRODUCTION INFRARED and Raman spectroscopy have been used in the past to investigate the properties of the ammonium ion in ammonium metavanadate [l-3]. In the room temperature i.r. spectrum of NH4V03, however, the assignment of individual components like Y, and ~1~in the broad N-H stretching region is virtually impossible [l]. Low temperature spectra of isotopically dilute NH,D+ are particularly useful in this respect as its vibrations are not complicated by vibrational coupling and Fermi resonance between the various possible vibrational levels, e.g. IQ, y3, 2v4, 2v2 and vz+ v4, as may be the case for NH: and ND:. The procedure of using isolated NHP+ ions as a probe to investigate the NH: environment in the crystal is performed by incorporating a low percentage of deuterium into the crystal so that factor group effects are negligible [4]. The frequency of the sharp N-D stretching fundamental v,(NH3D+) is also far removed from the spectral region where NH: ions absorb thus enabling the unambiguous assignment [5] of this band. This technique made it possible to investigate the properties of the ammonium ion in various crystal structures [6-171, and was also employed here to study the ammonium ion in NH4V03. The results showed no evidence of a low temperature transition in NH4V03 and spectra recorded over the entire temperature range studied can be interpreted in terms of the room temperature crystal structure. NH4V03 belongs to spacegroup Pbcm(DE) with Z=4 [18]. The N, V, O(l), O(2), H(l), H(2) atoms all occupy sites 4(d) with site symmetry C,, O(3) atoms 4(c) sites with symmetry C, and H(3) atoms 8(e) sites with C, symmetry. The structure of the metavanadates is shown in Fig. 1, including hydrogen bonds in NH4V03.

EXPERIMENTAL

Two different methods were used to prepare deuterated NH4VOS. Samples containing between 1 and 75% deuterium were prepared by substituting Hz0 with stoichiometric mixtures of H20/D20 and using ND&I as a substitute for NH&l in the preparation of NH4V03 [19]. In the second method VzOs was reacted with ND3 in D20 under inert conditions to obtain NDIVOJ. Infrared spectra were recorded both at room temperature and 90 K on a Bomem Michelson-100 FTIR spectrometer with a resolution of 4 cm- ‘. All the samples were recorded in the form of KBr * Authors to whom correspondence

should be addressed. 1639

D.

1640

DE WAAL

et al.

Fig. 1. Recommended atomic nomenclature for metavanadates and the hydrogen bonding arrangement in NH,VO+

pellets. To ensure that frequency shifts observed were not due to ion exchange, the 1% deuterated sample was also recorded in the form of a CsCl pellet and identical results have been obtained with KBr and CsCI. During the temperature dependence studies the spectra were recorded at various temperatures between 80 and 291 K on a Bruker IFS 113 V spectrometer with a resolution of 1 cm-‘. For the low temperature measurements a continuous flow cryostat (Oxford Instruments Model CF 1100) with liquid nitrogen as coolant to obtain spectra at different temperatures between 80 K and room temperature.

RESULTS AND DISCUSSION

Infrared spectra

The fundamental vibrational modes of the NH: tetrahedron are v,(A,) the totally symmetric stretching mode, v*(E) the doubly degenerate bending mode I+(&) the triply degenerate asymmetrical stretching mode and y4(Fz) another triply degenerate bending mode. Under the site group of C, all the modes of Nz become both i.r. and Raman active. v,(A,) is predicted to remain a single mode (A) while Q(E) should split into two bands (A’ and A”) with each of Y~(FJ and v4(Fz) splitting into three bands (2A’ +A”). Factor group splittings will cause A’ type modes to split into A,(R), B,,(R), B,,(i.r.), B,,(i.r.) and A” type modes into L&,(R), B&R), B&r.), B,,,(i.r.). It can be expected, however, that only site group splittings will possibly be resolved in the vibrational spectra. It has been reported [l] that an unambiguous assignment of ammonium bands between 2500 and 3500 cm-’ in the room temperature vibrational spectra of NH4V03 proves to be difficult if not impossible because of complications resulting from vibrational coupling and Fermi resonance in this region. This also seems to hold true for spectra of NH4VOX recorded at low temperatures down to 80 K. Broad bands in the room temperature spectrum split into several bands at 80 K as is shown in Fig. 2. Full assignment of the low

Infrared spectra of NH: in NH,VOx

I

3400

I

I

3000

2600 WAVENUMBERS

Fig. 2. The N-H stretching region in NH4VOJ at room temperature

IN cm-’

(top) and 80 K (bottom).

temperature spectrum of NH4V03 is included in Tables 1 and 2. Three broad bands and some possible shoulders in the N-H stretching region of the room temperature spectrum split into nine bands and four shoulders of strong to medium intensity at low temperatures. Although high intensity bands can usually be associated with fundamental vibrations rather than overtones and combination bands it is possible that coupling in this region might be responsible for changes in intensity. The yI fundamental was expected to remain single under site group C, and to split into two i.r. and two Raman active bands under D2,,, and a single band at 2925 cm-’ in the room temperature spectrum was assigned to this mode. This band occurs at 2926 cm-’ in the low temperature spectrum and a second band appearing as a shoulder at 2899 cm-’ could possibly be assigned to the second band expected for Y, under factor group splitting. This band could, however, also be assigned to one of the components of 2v., in addition to five other bands at 2790,2810, 2830, 2839 and 2858cm-‘. The bending fundamental v4 is expected to split into three bands under (2A’+A”) C, site symmetry and into six i.r. and six Raman active modes under the unit cell symmetry, as has already been shown. Three components were in fact reported for v4 in the room temperature Raman spectrum at 1416, 1438 and 1462 cm-’ [l]. One high intensity band is observed in the room temperature i.r. spectrum at 1417 cm-’ that splits into at least three bands at 1406, 1414 and 1422 cm-’ at 80 K and probably represent the site group components under CT. The band is broad and asymmetrical on the low frequency side, and could possibly reveal further splitting at liquid helium temperatures. It can be assumed that the highest frequency vibration in Fig. 2 corresponds to a stretching mode involving the longest N-H bond, N-H(3) [l]. This mode occurs at 3190 cm-’ in the room temperature i.r. spectrum and is assigned to v+ The three bands occuring at the highest frequencies in the low temperature spectrum, viz. the ones at 3207, 3135 and 3122 cm-’ are tentatively assigned to the three modes expected for vj under C, symmetry. Although the v2 bending mode is expected to split into two components under C, symmetry it is observed as a single band in both the room and low WA)46:11-G

1642

D. DEWAAL~~

Table 1. Assignment of low temperature between 3450 and 2000 cm-’

i.r. active

modes

al.

of NH4V02

Wavenumbers

and variously

deuterated

samples

50% D

75% D

90% D

3208s -

3202m -

3201~ 3179sh -

3124m -

3123~ -

3122~ -

2984sh -

3009sh -

2935m -

2934~ -

in cm-’

100% 1% D

Assignment

NH,VO,

+(NHq+).VW-W) VW-W’)

3207s -

3215s -

3209s -

3211~s -

WW

3135s 3122s 308&h 306Om 3019m -

3142sh -

3140sh -

3088s 306Om 3025m -

3086sh 306lsh 3020sh -

3134sh -

2926s -

2927m -

2928s -

2934s -

2836s -

2835m -

2793s -

2810s 2795s -

2370w -

2371~ -

2812m 2798s 2729vw 2669vw 2478vvw 2409vw 2386~ 2370m -

2409sh 2389m 2377m -

2348~ -

2345~~ -

2352~ -

2354~ 2337~ -

21%m -

2248~~ 2199m 2185sh -

2012bw -

2001bw -

2249w 2202m 2184m 2134~~ -

2254sh 219Om 2167sh 2134m 2115~ -

v,(NW), v,(NHD:) vr + vq(NH:) vz + vd(NH:) VI + v,(NH:) v,(NHrD;) v,(NH:), ~+(NHsD+) Q(NHzD;) VI, 2vq(NH:) 2v,(NH: ) v,(NH,D;) 2v,(NH: ) 2v,(NH:) 2v,(NH: ) 2v,(NHD;) 2MNHzD: ) vz + v,(NHD;) v,(ND: ) v,(ND: ) v,(NHsD+) v,(NHD:) v,(NHrD+) v,(NH,D:) 2v,(ND;) vr + v,(ND:) v,(NHD: ) 2v,(ND:) v,(NH,D+) v,(ND:) 2v,(ND: ) 2v,(ND: ) vz + vb(NH:) vz + v,(NH:) s = strong,

2899sh 2858m 2839~ 2830sh 2810m 2790s 2000bw 1944bw

w = weak,

sh = shoulder,

2857m -

2273~s -

m = medium,

5% D

30% D

3027sh -

2946m 2919m 2834~ 2798~ 2728~ 2669w -

2833m 2725m 2472~~ 2404sh 2385m 2377m 2369s -

2866wI 2861~ 2838sh 2781~ 2728~~ 2669~~ 2407~s 2384~s 2354sh -

2250w -

2309w 2271sh 2219sh -

2186s 2127s 2110m -

2192s 2125s 2110s -

2316sh 2272sh -

v = very, b = broad.

temperature spectra at 1659 and 1663 cm-‘, respectively. Combination modes involving the librational mode vh, vi + vg (i = 2,4) are identified at 1737, 1770, 1795 (vq + v6) and 1944, 2004 cm-’ (v2 + VJ at low temperature. These bands are easily identified, even at room temperature where they appear as single bands at 1737 and 2000 cm-‘. These frequencies indicate that the hydrogen bonds in NH4V03 are most probably not fluxional as has also been observed in an earlier study [l]. The V~mode is calculated to occur between 330 and 380cm-’ which is in close agreement to the reported position of this band at 362 cm-’ in the far i.r. spectrum [20]. The low temperature spectra of the various deuterated samples of N&V03 are shown in Figs 3 and 4. A full assignment of the spectra appear in Tables 1 and 2. With an effective symmetry of C,Yfor NH: in NHIVOJ the v, (N-D) stretching mode of NH3D+ should split into C,, C, and C,(2) components, depending on whether the N-D bond coincides with the u-plane of the crystal (C symmetry) or C, symmetry with the N-D bond pointing in the opposite direction but still in the o-plane of the crystal, or having two equivalent positions of C, symmetry. This is, of course, in agreement with the three bond distances [21] of N-H(l) =0.82 A, N-H(2) =0.94 8, and N-H(3) =0.97 A (2 bonds). The bending mode v4 splits into v~~(A,) and v4,JE) for “free” NHjD+ groups

Infrared spectra of NH: in NH,VO,

1643

Table 2. Assignment of low temperature i.r. active modes of NH4V03 and variously deuterated samples between 2000 and 1000 cm-’ Wavenumbers in cm-’ 100% NH,VO,

Assignment

1795b 1770b l737b 1663sp

v, + V&W) v, + v,(NH:)

v4+v&‘W) vz(NH:1 v2WW”) MWD; 1

-

~2 + v,W’:) VW-G

1422~s 1414vs 1406sh

1

v~NH:) 4W+

)

v,PHW+ v,

+

-

)

NW)

v&JH~D;

)

vsWW+) vaWW+) vOH,D+) v&-W

+1

v2WX) GWW v2UW) v4NJW v,,(NHD; v,WX

1, )

v,,(NHD;

1

-

1% D

5% D

30% D

50% D

75% D

-

-

-

-

-

1776m 1664m 1611~

1776~ 1650w 1611~ 1433sh 142th~~ 1410vs 1344w 1325~ 1276m 1267m 1257m 1250m 1194vw 1187~~ 1182~~ 1124~ 1078vvw

1778~~ 1655~~ 1608~~ 1564vw 1445sh 1432sh 1419vs 141lvs 1344w 1325~ 1276m 1268m 1258m 1250m 1192~~ l188w 1182~ 1124~ 1091w 1078~~

1781~ 1608~~ 1563~ 1414m 1403sh 1385sh 1344w 1325~ 1278~ 1268w 126&v 1252sh 1190w 1189sh 1182~ 1123m 108Ow lO77w

1788~ 1610w 1559m 1455w 1435sh 1415m 14OOw 1384sh 1344m 1325m 1277m 1269m 1260m 1251m 1190m 1183m 1123m 1089m 1078m

1422~s 141lvs

1276m 1267m 1257m 1250m

1144w 1127~~ 105Ovvw

90% D

1646vW 1558m 1456m 1417m 1410m 13%m 1344m 1325m 1272~ 1268w 1257~ 1192~ 1184m 1126m 1078~s

s = strong, w = weak, sh = shoulder, m = medium, v = very, b = broad.

and the latter mode into six components under C, symmetry. Isotopically dilute NH3D+ gives three sharp bands in the N-D stretching region at 2370,2348 and 2196 cm-‘. These bands can be considered to represent at least two types of hydrogen bond interactions in solid NH4V03. The two weaker bands at 2370 and 2348 cm-’ are likely to represent the bifurcated hydrogen bonds, that is N-H-O(l) and N-H-O(3) [l], while the lower frequency component at 2196cm-’ is likely to represent very strong normal hydrogen bonding as might be expected to be present in the almost straight line N-H-O(2) bond [l]. The difference in strength between the two types of hydrogen bonds must be considerable as the wavenumbers of the bands that represent these are separated by more than 150 cm-’ and the lower frequency band has a high relative intensity that can also be an indication of hydrogen bond strength. The bending mode, v4bc(NH3D+), was expected to split into six components under C,, but only four components are observed at 1276,1267, 1257 and 1250 cm-‘. A mode at 1611 cm-’ in the low temperature is assigned to the y2 fundamental of NH3D+. Very weak bands of the fully deuterated species ND: are present in Fig. 4: v2 at 1144cm-’ and v4 at 1127 and 1050cm-‘. In the low temperature spectrum of 5% deuterated NH4V03 the vl(NHJD+) bands are of higher intensity than before as would be expected and two shoulders are now visible towards lower wavenumbers on the 2199 cm-’ band at 2185 and 2166 cm-‘. These can be assigned to the v, and 2v4 modes of ND:, respectively. A very weak band at 2248 cm-’ can also be assigned to the latter mode. Of the two bands appearing as shoulders at 1344 and 1325 cm-’ the first can be attributed to the combination mode v4 + vg of ND: while the other is assigned to v& (NH,D:). Four of the five low intensity bands between 1050 and 1200 cm-’ seem to originate from ND:. Two bands are observed for each of the v2 and v4 fundamentals at 1194, 1182 and 1124, 1078 cm-‘, respectively. At 30% deuteration the band that was present at 3088 cm-’ in the spectrum of pure NH4V03 has disappeared. This supports the assignment of this band to the combination

1644

D. DE WAAL et al.

Y b

3170

,5,.°

2890

2610

2530

2050

WAVENUMBER5 iN cm -~

Fig. 3. Low temperature i.r. spectra of various percentage deuterated samples of NH4VO~ between 3380 and 1980cm =.

z

z

1450

1350

1250

1150

1050

WAVENUMBERS IN cm -~

Fig. 4. Low temperature i.r. spectra of various percentage deuterated samples of NH~VO~ between 15(X)and 1(125cm

Infrared spectra of NH:

in NH4V03

1645

mode v2 + vq (NH:). The highest relative intensity for the bands assigned to NH3D+ vibrations is found in this spectrum as would be expected from the statistical distribution of the NH,_,D, species. In the 2100-24OOcm-’ region new bands appear that can be assigned to ND: modes. Two bands of medium intensity at 2202 and 2184cm-’ are tentatively assigned to v, as two i.r. active bands are expected for this fundamental under factor group splitting. Weak bands at 2409 and 2386 cm-’ are similarly assigned to the v3 fundamental while the overtones 2v4 are present at 2134 and 2249 cm-‘. Towards lower wavenumbers N-D bands have higher ‘intensities than before. A new shoulder at 1445 cm-’ is assigned to the v2 + v6 combination mode of ND: as V~has been observed at 267 cm-’ [20]. A new weak band at 1091 cm-’ is attributed to a bending mode in the NHD: species. With less NH: ions in the lattice at 50% deuteration, two N-H stretching modes of the NH2Dz species become visible at 2946 and 2919 cm-‘. In the N-D stretching region NH*DT is represented by a new band at 2337 cm-‘. There is a change in the relative intensity of v3(ND4+) and vl(NH$+) at 2389 and 2377 cm-’ with the former increasing relative to the latter from 30 to 50% D. In the spectrum of 75% deuterated NH.,V03, all N-D modes of ND: are of higher intensity than the remaining N-H bands. N-D stretching vibrations in the 2000-2400 cm-’ region are taking a similar shape to that of N-H vibrations between 2500 and 3400 cm-’ in pure NH4V03. Two new combination modes for ND:, 2v, and v2+ v.,, appear as shoulders at 2316 and 2272 cm-‘. The 2000-2500 cm-’ region in the spectrum of NH4V03 [90% D NI&V03 (Fig. 3)] can now be related to the 2500-3500 cm-’ region in the undeuterated compound. In both, three broad bands occur in the room temperature spectrum which split into several bands and shoulders at low temperature. Unambiguous assignment of the various modes remains impossible as various fundamentals, combination modes and overtones are expected in this region (e.g. vl, v3, v2+ v4, 2v2 and 2v4), but the bands at 2407 and 2384 cm-’ are tentatively assigned to v3 and the single mode at 2192 cm-’ to vl and ND:. The shoulder at 2354 cm-’ which was assigned to vl(NHJD+) possibly overlaps with the third mode expected for v,(ND:). The combination band v2+ v4 is observed at 2271 cm-‘. Two bands at 2125 and 2110 cm-’ are attributed to 2v4 with 2v2 at 2309 cm-’ as two sharp bands at lower wavenumbers (Fig. 4), 1078 and 1126cm-‘, have been assigned to v4 and a band and a shoulder at 1192 and 1184cm-’ to v2. The two combination modes v2 + vg and v4 + vg are observed at 1456 and 1344 cm-‘. Temperature dependence

The behaviour of some i.r. active NH3D+, NH: and ND: modes between 80 K and room temperature is shown in Figs 5-7. The dependence of some N-H and N-D Raman active bands on high temperatures were already reported [l]. At low temperatures, further information on the nature of hydrogen bonding in the crystal can be obtained as more bands are observed, especially those of isotopically dilute NH3D+ that are of very low intensity at room temperature. The temperature dependence of NHD: modes in NH4V03 shown in Fig. 5 are reported for the first time. It was difficult to follow the very weak band at 2348 cm-’ above 80 K but the ones at 2370 and 2196 cm-’ remained visible up to room temperature (Fig. 5). Of these two, the N-D stretch at 2370 cm-’ probably represents the N-H(3) band which forms a bifurcated hydrogen bond as it moves towards lower frequencies with an increase in temperature at a rate of -0.05 cm-’ K-‘. The opposite effect is observed for the mode at 2196 cm-’ that probably represents N-H(l) with normal, almost straight line hydrogen bonding; This band moves upwards at a rate of 0.05 cm-’ K-‘. Results obtained for the NH: and ND: species show similar behaviour to that obtained for NHD;. It was assumed that the highest frequency vibration in the spectrum corresponds to a stretching mode involving N-H(3) in NH: while the one assigned to vl(NH4+) can be associated with either N-H(l) or N-H(2) in the almost straight line hydrogen bond [ 11. The highest frequency mode at 3207 cm-’ in pure NH4V03 shows a

D. DEWAAL

1646

2!d

50

s 100

etal.

’ 200

’ 150 TMPERAXAE

Fig. 5. Temperature NHD+inNHVO. 3 4

dependence of two of the N-D

250

300

IN K

stretching modes of isotopically dilute

3

decrease in wavenumbers with increasing temperature [Fig. 6(a)]. Similar results were obtained for this mode in the samples with various percentages of deuterium. This shows that the hydrogen atom H(3) becomes increasingly more associated with a particular oxygen atom with a concomitant increase in hydrogen bond strength. The mode associated with N-H(l) and N-H(2) shows a downwards frequency shift in pure N&V03 but changes in the opposite direction in all deuterated samples, showing that strong coupling probably exists between the Y, and 2v4 modes in the N-H frequency range in pure NH4V03. The stretching modes in ND: which represent the three different bond lengths are most likely the bands assigned to v3(ND4+) [N-D(3) and N-D(2)] and v,(ND4+) [N-D(l)]. Of these three, the highest wavenumber band shifts downwards at a rate of -0.05 cm-’ K-’ upon an increase in temperature while the other two that represent the almost straight line hydrogen bonds shift upwards at respective rates of

1790 (b)

1740.

.

.

.

aoj{i -L ______._1 11901 n3L----A 50

100

150 200 TEMPERATURE IN K

300

Fig. 6. The temperature dependence of some NH; (b) v,+ vh (two modes), Y?and v, (2 modes).

50

!OO

50 200 XHPERATURE IN K

250

modes in NH4V03: (a) v3, Y,, 2v, and v?+ v,,;

300

Infrared spectra of NH; in NH4VOJ

1647

242Or

.

...

-1-1

50

100

150

200

250

300

TEMPERATURE IN K

Fig. 7. The temperature (2 modes).

dependence

of some ND: modesin ND4V03: vj (2 modes), Y, and 2v,

0.02and 0.06cm-’ K -’ (Fig. 7). This, together with results obtained for the temperature

dependence of NH: and NH3D+ modes, is in agreement with the reported results for Raman mode behaviour [l] between room temperature and 473 K, showing that both normal and bifurcated hydrogen bonds are present in NH4V03 with the weak bifurcated bond increasing in strength at higher temperatures while the normal bonds decrease in strength at ambient conditions. This is also reflected in the bending vibrations where the two components of Y~(NH$) at 1422 and 1416cm-’ become a single band above 120K [Fig. 6(b)]. Th e former band shows a decrease in frequency of -0.04 cm-’ K-’ while little overall change occurs for the second between 80 K (1416 cm-‘) and room temperature (1415 cm-‘). Acknowledgemenr-The financial support given by the Fonds der Chemischen Industrie, the University of Pretoria and the Foundation of Research Development is gratefully acknowledged.

REFERENCES

[l] [2] [3] [4] [5] [6] [7]

A. M. Heyns, M. W. Venter and K.-J. Range, Z. Nafurforsch. 428, 843 (1987). S. D. Hamman, Auf. J. Chem. 31, 11 (1978). Y. S. Park and H. F. Shurvell, /. Raman Specrrosc. 20, 673 (1989). G. J. Kearly and I. A. Oxton, Adu. Infrared Raman Spectrosc. 10, I I I (1963). I. A. Oxton, 0. Knop and M. Falk, Can. J. Chem. 53(18), 2675 (1975). I. A. Oxton, 0. Knop and M. Falk, Can. J. Chem. 53(22), 3394 (1975). I. A. Oxton, 0. Knop and M. Falk, Can. J. Chem. 54(6), 892 (1976). [S] I. A. Oxton, 0. Knop and M. Falk, /. Mofec. Strucr. 37, 9 (1977). [9] I. A. Oxton and 0. Knop, J. Molec. Srrucr. 49, 309 (1978). [lo] I. A. Oxton, 0. Knop and M. Falk, Can. J. Chem. 57,404 (1979). [ll] 0. Knop, W. J. Westerhaus and M. Falk, Can. J. Chem. 58(3), 270 (1980). (121 0. Knop, W. J. Westerhaus and M. Falk, Can. J. Chem. f%(9), 867 (1980). [13] 0. Knop, T. S. Cameron, M. A. James and M. Falk, Can. J. Chem. 59(16), 2250 (1981). 114) W. J. Westerhaus, 0. Knop and M. Falk, Can. J. Chem. 58, 1355 (1980). [15] 0. Knop, I. A. Oxton, W. J. Westerhaus and M. Falk, J. Chem. Sot. Faraday Trans. 2. 77,309 (1981).

1648

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[ 161 0. Knop, I. A. Oxton, W. J. Westerhaus and M. Falk, J. Chem. Sot. Faraday Trans. 2, 77, 811 (1981). (171 0. Knop, W. J. Westerhaus, M. Falk and W. Massa, Can. J. Chem. 63, 3328 (1985). [ 181 F. C. Hawthorne and C. Calvo, J. Solid State Chem. 22, 157 (1977). [ 191 R. Zintl, PhD Dissertation, University of Regensburg, Regensburg, F.R.G. (1984). 1201 S. Onodera and Y. Ikegami, Inorg. Chem. 18,466 (1979). [21] F. C. Hawthorne and C. Calvo, J. Solid State Chem. 22, 157 (1977).