Crystal structures of nitronium tetranitratogallate and its reversible solid-state phase transition mediated by nonmerohedral twinning

Article abstract of DOI:10.1021/ic0002418

Single-crystal X-ray crystallographic analyses of [NO₂][Ga(NO₃)₄] reveal that it undergoes a reversible phase transition without any apparent damage to the crystal during repeated temperature cyclings. The room-temperature, noncentrosymmetric, body-centered tetragonal (I 4), polymorph 1 (a = 9.2774(3) A, c = 6.1149(2) A, Z = 2) consists of well-separated nitronium and tetranitratogallate ions. The [Ga(NO₃)₄]^- units exhibit a slightly squashed tetrahedral geometry in which all of the ligands are monodentate. Below approximately 250 K, distortions lower the symmetry to the chiral, body-centered monoclinic nonstandard space group I2. Both components (2a: a = 9.5857(2) A, b = 5.9399(1) A, c = 8.9759(2) A, β = 90.409(1)°, Z = 2. 2b: a = 9.5898(2) A, b = 5.9376(1) A, c = 8.9784(1) A, β = 90.420(1)°, Z = 2) of the nonmerohedrally twinned structure are independently refined and found to be enantiomeric with nearly identical distance and angle parameters. As in the high-temperature polymorph, the cations and anions are well separated. The most notable change involves two of the nitrato ligands in the [Ga(NO₃)₄]^- ions that have become bidendate, causing the molecular structure to distort toward octahedral geometry.

Full text of DOI:10.1021/ic0002418

Inorg. Chem. 2000, 39, 4621-4624
4621
Crystal Structures of Nitronium Tetranitratogallate and Its Reversible Solid-State Phase
Transition Mediated by Nonmerohedral Twinning
Daniel G. Colombo, Victor G. Young, Jr.,* and Wayne L. Gladfelter*
Department of Chemistry, University of Minnesota, Minneapolis, Minnesota 55455
ReceiVed March 3, 2000
Single-crystal X-ray crystallographic analyses of [NO2][Ga(NO3)4] reveal that it undergoes a reversible phase
transition without any apparent damage to the crystal during repeated temperature cyclings. The room-temperature,
noncentrosymmetric, body-centered tetragonal (I 4h), polymorph 1 (a ) 9.2774(3) Å, c ) 6.1149(2) Å, Z ) 2)
consists of well-separated nitronium and tetranitratogallate ions. The [Ga(NO3)4]- units exhibit a slightly squashed
tetrahedral geometry in which all of the ligands are monodentate. Below approximately 250 K, distortions lower
the symmetry to the chiral, body-centered monoclinic nonstandard space group I2. Both components (2a: a )
9.5857(2) Å, b ) 5.9399(1) Å, c ) 8.9759(2) Å, â ) 90.409(1)°, Z ) 2. 2b: a ) 9.5898(2) Å, b ) 5.9376(1)
Å, c ) 8.9784(1) Å, â ) 90.420(1)°, Z ) 2) of the nonmerohedrally twinned structure are independently refined
and found to be enantiomeric with nearly identical distance and angle parameters. As in the high-temperature
polymorph, the cations and anions are well separated. The most notable change involves two of the nitrato ligands
in the [Ga(NO3)4]- ions that have become bidendate, causing the molecular structure to distort toward octahedral
geometry.
reactivity of anhydrous metal nitrates toward organic substrates, all IR
spectra were obtained for Fluorolube mulls using NaCl salt plates.
Elemental analyses were performed by Schwartzkopf Microanalytical
Laboratory (Woodside, NY).
Introduction
The covalent nature of the bonding in anhydrous metal nitrates
increases their volatility and chemical reactivity,1 rendering these
compounds attractive as carbon- and hydrogen-free single-source
precursor molecules for the chemical vapor deposition (CVD)
of metal oxide films.2-4 We recently demonstrated their use in
the CVD of high-κ dielectrics in microelectronic devices.2 In
the course of this research, we have found that [NO2][Ga-
(NO3)4]5 produces amorphous films of gallium oxide.3 The
current study reports the single-crystal X-ray diffraction study
of [NO2][Ga(NO3)4] and the observation that nitronium tetrani-
tratogallate undergoes a reversible phase transition from a room-
temperature tetragonal phase to a lower temperature nonmero-
hedrally twinned monoclinic phase.
Synthesis of Dinitrogen Pentoxide. CAUTION! Fuming nitric acid
and dinitrogen pentoxide are both powerful nitrating and oxidizing
agents. Contact of these compounds with any organic substrate should
be aVoided. All ground-glass joints should be greased with Krytox,
and only Teflon stopcocks and ValVes should be used. Through its
central neck, a flame-dried and nitrogen-purged three-neck, round-
bottom 1000 mL flask was charged with 60 mL (94.44 g, 1.50 mol) of
fuming nitric acid. The flask was flushed with nitrogen, after which
the open neck was sealed using a glass stopper fitted with a Teflon
sleeve. The HNO3 was solidified by cooling with a liquid-nitrogen bath,
and 425 g (1.50 mol) of P4O10 was then quickly added to the flask,
which was again flushed with nitrogen and sealed. Throughout the
experiment, the entire reaction apparatus was protected from direct light.
The gas supply was changed from nitrogen to oxygen, the liquid-
nitrogen bath was removed, and the contents of the reaction flask were
allowed to warm slowly to ambient temperature. As the flask contents
were warming, a stream (5 bubbles/s) of oxygen (dried by bubbling
the gas through a H2SO4 bubbler and a 40 cm column of Drierite and
4 Å Linde sieves) was flushed through the entire reaction apparatus
into a silicone oil bubbler. The receiver flask used as a trap for N2O5
was cooled using a dry ice/2-propanol bath. After approximately 45
min, a large volume of reddish gas rapidly evolved from the reaction
flask; this was swept by O2 through the apparatus to condense in the
trap as a white, crystalline solid. The O2 is important both for enhancing
the efficiency of mass transport of gaseous N2O5 to the trap and for
stabilizing the N2O5. After gas evolution had ceased, the reaction flask
was gently warmed to 40 °C using an oil bath. Further gas evolution
occurred at this temperature. After gas evolution was complete, the
temperature was maintained at 40 °C for 6-8 h under the stream of
O2. The reaction flask was then allowed to cool to ambient temperature,
and all stopcocks were closed to isolate the receiver trap. The receiver
flask was immediately removed and attached to another reaction
apparatus setup to synthesize [NO2][Ga(NO3)4]. Although crude N2O5
yields of up to 80% have been reported, no attempt to recover pure
N2O5 was made; the crude material was used as synthesized.
Experimental Section
General Procedures. All reactions and reagent preparations, unless
otherwise noted, were carried out under dry nitrogen using standard
Schlenk or drybox techniques. Gallium(III) chloride (Strem) was
sublimed in vacuo and stored in sealed glass ampules prior to use.
Fuming nitric acid (Aldrich or Fisher, >90% HNO3) was used as
received but was stored with desiccant and out of contact with direct
light to minimize decomposition. Anhydrous phosphorus pentoxide
(Aldrich) was used as received. All IR spectra were recorded using a
Nicolet Magna 560 FTIR spectrometer. Because of the intrinsic
(1) Addison, C. C.; Logan, N. AdV. Inorg. Chem. Radiochem. 1964, 6,
71-142.
(2) Gilmer, D. C.; Colombo, D. G.; Taylor, C. J.; Roberts, J.; Haugstad,
G.; Campbell, S. A.; Kim, H.-S.; Wilk, G. D.; Gribelyuk, M. A.;
Gladfelter, W. L. Chem. Vap. Deposition 1998, 4, 9-11.
(3) Colombo, D. G.; Gilmer, D. C.; Young, V. G., Jr.; Campbell, S. A.;
Gladfelter, W. L. Chem. Vap. Deposition 1998, 4, 220-222.
(4) Taylor, C. J.; Gilmer, D. C.; Colombo, D. G.; Wilk, G. D.; Campbell,
S. A.; Roberts, J.; Gladfelter, W. L. J. Am. Chem. Soc. 1999, 121,
5220-5229.
(5) Bowler, D.; Logan, N. J. Chem. Soc., Chem. Commun. 1971, 582-
583.
10.1021/ic0002418 CCC: $19.00 © 2000 American Chemical Society
Published on Web 09/09/2000

4622 Inorganic Chemistry, Vol. 39, No. 20, 2000
Colombo et al.
Table 2. Atomic Fractional Coordinates for 1, 2a, and 2b
Table 1. Crystallographic Data for Compounds 1, 2a, and 2ba
1
2a/2b
atom
x
y
z
empirical formula
fw
space group
a, Å
b, Å
GaN5O14
363.77
I4h
9.2774(3)
9.2774(3)
6.1149(2)
GaN5O14
363.77
I2
9.5857(2)/9.5898(2)
5.9399(1)/5.9376(1)
8.9759(2)/8.9784(1)
90.409(1)/90.420(1)
511.06(2)/511.22(2)
2
0.710 73
2.364/2.363
27.99/27.98
173(2)
Complex 1
Ga(1)
O(1)
O(2)
O(3)
O(4)
N(1)
N(2)
1.0000
0.5000
0.7500
0.8286(2)
0.7345(2)
0.9613(3)
0.5000
0.8425(2)
0.5000
0.5378(2)
0.6903(3)
0.7056(3)
0.5000
0.6511(2)
0.5000
0.9062(3)
1.1287(4)
1.0477(4)
1.1818(16)
1.0350(3)
1.0000
c, Å
â, deg
V, Å3
526.31(3)
2
0.710 73
2.295
27.18
298(2)
Z
λ, Å
Complex 2a
Ga(1)
O(1)
O(2)
O(3)
O(4)
O(5)
O(6)
O(7)
O(8)
N(1)
N(2)
N(3)
0.0000
0.0000
0.0000
-0.0302(3)
-0.2288(3)
-0.1856(3)
0.1918(3)
0.0212(4)
0.2552(4)
0.0000
F(calcd), g cm-3
µ, cm-1
T, K
0.1607(3)
0.0568(3)
0.2771(3)
0.0442(3)
0.1621(5)
0.1899(4)
0.5000
0.1774(5)
0.3173(5)
0.3796(5)
-0.1292(5)
-0.2925(6)
-0.3938(6)
0.1530(12)
0.5280(18)
0.2976(5)
-0.2822(5)
0.3387(15)
R1, wR2 [I > 2σ(I)]b 0.0228, 0.0570 0.0384/0.0470, 0.0990/0.1284
a The double values listed are for the independently refined twins
2a and 2b, respectively. b R1 ) ∑||Fo| - |Fc||/∑|Fo|; wR2 ) [∑w(Fo
2
- Fc2)2/∑w(Fo )2]1/2, where w ) 1/[σ2(Fo ) + (aP)2 + bP], P ) (Fo
+
2
2
2
2Fc2)/3, and a and b are constants given in the Supporting Information.
0.5000
0.0000
0.1642(3)
0.1391(3)
0.5000
-0.1559(3)
0.1583(4)
0.0000
Synthesis of [NO2][Ga(NO3)4]. Caution! Nitronium tetranitrato-
gallate is a nitrating agent. Precautions similar to those mentioned
aboVe should be employed. For this synthesis, modifications of
published procedures5,6 were used. In a drybox, 5.00 g (28.8 mmol) of
anhydrous GaCl3 was placed in a flame-dried three-neck, round-bottom
250 mL reaction flask equipped with two gas adapters, a condenser,
and a Teflon-coated magnetic stirbar. The reaction flask was cooled
with liquid nitrogen, and the freshly prepared N2O5 was condensed in
vacuo onto the GaCl3. After the vacuum transfer of N2O5 was complete,
the receiver trap was removed, a gas adapter was attached to the reaction
flask on top of the condenser assembly, and a silicone oil bubbler was
attached to the gas adapter. The reaction flask was allowed to warm to
ambient temperature over the course of 1.5 h while direct exposure to
light was avoided. The volatile byproduct ClNO2 was allowed to vent
through the bubbler. Cooling the reaction flask with an ice bath
moderated the rate of gas evolution. The biphasic reaction mixture was
stirred for an additional 20 h, during which the red color deepened
substantially and NO2 gas was visible in the headspace of the reaction
apparatus. The N2O4 generated during the reaction and any residual
volatiles were removed by flushing the reaction flask with dry N2 until
a crude crystalline, yellow mass remained. The solid mass was
outgassed in vacuo for several hours at 15 °C. The crude material was
recovered in the drybox and placed in a sublimator as a finely ground
powder. The crude pale yellow product was purified by sublimation at
80 °C and 0.05 mmHg, yielding 2.91 g (8.02 mmol, 27.8% yield) of
air-sensitive, translucent white crystalline blocks; the sublimed material
was indefinitely stable toward decomposition while stored under N2 or
Ar. Crystals suitable for single-crystal X-ray analysis were obtained
by slow sublimation over 5-7 d. FTIR: ν(N-O), nitryl cation,
antisymmetric, 2369 cm-1 (Fluorolube mull on NaCl plates).
Crystallographic Structure Determinations of the Polymorphs
of [NO2][Ga(NO3)4]. Diffraction data were collected using a Siemens
SMART platform CCD with specimens mounted in a Lindeman
capillary (for studies at 298 K) or frozen in a Krytox film on a glass
fiber (for data collection at 173 K). The 298 K phase crystallized in
the tetragonal in space group I 4h. All four nitrate ligands were equivalent
in this noncentrosymmetric space group. The crystal data and X-ray
experimental details are summarized in Table 1. Lowering the tem-
perature below ca. 250 K caused a phase change where the crystal
system was lowered to monoclinic. Possible space group choices were
the conventional C2 and the nonstandard I2. The latter was chosen to
facilitate direct comparisons between the phases. An additional
complication in the phase change was the appearance of a nonmero-
hedral twin with both components present in nearly equal amounts.
Dirax7 was used to index the twin components from a set of 50
Complex 2b
Ga(1)
O(1)
O(2)
O(3)
O(4)
O(5)
O(6)
O(7)
O(8)
N(1)
N(2)
N(3)
0.0000
0.1606(4)
0.0572(4)
0.2768(4)
-0.0441(4)
-0.1633(7)
-0.1901(5)
0.5000
0.0000
0.0000
-0.1781(6)
-0.3177(7)
-0.3793(7)
0.1291(7)
0.2912(8)
0.3942(8)
0.8474(15)
0.4711(20)
-0.2971(7)
-0.2823(7)
0.6605(19)
-0.0302(4)
-0.2295(5)
-0.1861(4)
-0.1919(4)
-0.0215(5)
-0.2567(6)
0.0000
0.5000
0.0000
0.1640(4)
-0.1393(4)
0.5000
-0.1558(4)
-0.1581(5)
0.0000
randomly selected reflections, and Gemini8 was used to determine the
putative twin laws. Both twin components were integrated using
SAINT.9 The cell constants of the components were derived using 4038
and 4002 reflections, respectively, from the separate integrations.
Absorption corrections were applied by using SADABS.10 The struc-
tures of both twin components were independently solved by direct
methods, and refinements indicated that they were enantiomeric twins.
UNTWIN11 was used to correct the data for partially and exactly
overlapped reflections between the twin components; approximately
one-third of the data required correction. The crystal data and details
of the intensity collection and refinement procedures for both twin
components are also included in Table 1. Refinements uncorrected for
twinning produced R1 residuals of 0.0735 and 0.0942, but with
corrections, these improved to 0.0384 and 0.0470, respectively. All
reflections were retained throughout the refinements. No refinement
was attempted with data excluding the overlapping reflections. Both
twin components (2a and 2b) are believed to have the correct
handedness on the basis of the Flack absolute structure parameter. The
atomic coordinates for 1, 2a, and 2b are listed in Table 2, and complete
crystallographic details are provided in the Supporting Information.
Results and Discussion
Excess dinitrogen pentoxide and gallium(III) chloride, onto
which it was condensed in vacuo at -198 °C, reacted to yield
(8) Software for Indexing and Correcting Data Affected by Non-mero-
hedral Twinning; Bruker-AXS, Inc.: Madison, WI, 1999.
(9) Area Detector Integration Software, Version 4.05; Bruker-AXS,
Inc.: Madison, WI, 1995.
(10) Blessing, R. H. Acta Crystallogr. 1995, A51, 33.
(11) Young, V. G., Jr. University of Minnesota. Unpublished work, 1997.
The metric tensor, the twin law, and an algorithm for partiallly
overlapped reflections were used to output a corrected HKLF 5 SHELX
reflection file. Partially overlapped reflections were grouped in separate
(6) Ivanov-Emin, B. N.; Odinets, Z. K.; Yushchenko, S. F.; Zaitsev, B.
E.; Ezhov, A. I. Russ. J. Inorg. Chem. (Engl. Transl.) 1975, 20, 843-
846.
(7) Duisenberg, A. J. M. J. Appl. Crystallogr. 1992, 25, 92.
batches in ascending 0.005 Å-1 sets in the range 0.00-0.025 Å-1
.

Nitronium Tetranitratogallate Polymorphs
Inorganic Chemistry, Vol. 39, No. 20, 2000 4623
Figure 1. Thermal ellipsoid drawing of the crystal structure of
polymorph 1 at the 50% probability level.
Figure 2. Thermal ellipsoid drawing of the crystal structure of
polymorph 2a at the 50% probability level.
Table 3. Selected Bond Lengths (Å) and Angles (deg) for
Polymorph 1 of [NO2][Ga(NO3)4]
of 1 are consistent with the compound synthesized by Bowler
and Logan.5
Ga(1)-O(1)
N(1)-O(2)
N(2)-O(4)
1.888(2)
1.210(3)
1.112(9)
N(1)-O(1)
N(1)-O(3)
1.320(3)
1.214(3)
The four, equivalent, nitrates in 1 are monodentate. The
Ga(1)-O(1) bond distance is 1.888(2) Å, while the next nearest
Ga-O interaction, Ga(1)-O(3), is 2.657(3) Å. The geometry
of the complex ion can best be described as a tetrahedron that
is slightly squashed along the 4h axis. A simple measure of this
distortion can be obtained by comparing the O(1)-Ga(1)-
O(1B) angle, where the nitrates are related by a 2-fold rotation,
to the angle for the 4h relationship between the nitrates expressed
as O(1)-Ga(1)-O(1A). These values are 119.21(3) and
104.84(6)°, respectively. At 1.320(3) Å, the N(1)-O(1) bond
is longer than both N(1)-O(2) and N(1)-O(3) (1.210(3) and
1.214(3) Å, respectively). Although the nitrate ligand remains
planar, these distortions in bond lengths accompany an enlarge-
ment of the O(2)-N(1)-O(3) angle relative to the other
O-N-O angles.
The bond distances in the independently refined twins of the
low-temperature polymorph, 2a/2b, are equivalent within two
standard deviations; therefore, only one set of the numbers (from
2a) will be quoted in this discussion (both sets appear in Table
4). The important change in the geometry involves the motion
of two of the nitrate ligands to allow a weak, bidentate
interaction. The lowered symmetry gives rise to three indepen-
dent Ga-O bond distances. The two equivalent monodentate
nitrates (Ga(1)-O(1)) exhibit the shortest bonds at 1.887(3) Å,
indistinguishable from those found for the monodentate nitrates
in 1. At 1.929(3) Å, the stronger interactions of the bidentate
ligands are significantly longer than the monodentate Ga-O
distances. Finally, the weaker interactions of the bidentate
ligands, Ga(1)-O(5), are 2.338(4) Å. The existence of asym-
metric bidentate nitrates has been demonstrated for several
transition and main group metal complexes.12
O(1)-Ga(1)-O(1A) 104.84(6) O(1)-Ga(1)-O(1B) 119.21(13)
N(1)-O(1)-Ga(1)
O(2)-N(1)-O(1)
115.59(14) O(2)-N(1)-O(3)
116.2(2) O(3)-N(1)-O(1)
126.6(3)
117.2(2)
O(4)-N(2)-O(4A) 180
[NO2][Ga(NO3)4] during gradual warming to 25 °C. A crude
white solid was recovered after the removal of all volatiles at
15 °C and 0.05 Torr. Slow vacuum sublimation of the crude
white solid resulted in the formation of air-sensitive, translucent
white crystalline plates; the sublimed material was stable toward
decomposition while stored under N2 or Ar. The IR spectrum
of the sublimed material in a fluorolube mull had a discernible
band of moderate intensity at 2369 cm-1 that was assigned to
the nitronium ion.
Single-crystal structural analyses revealed that [NO2][Ga-
(NO3)4] underwent a reversible phase transition with no apparent
damage to the crystal. The room-temperature polymorph 1
(Figure 1 and Table 3) was indexed to a tetragonal cell in the
noncentrosymmetric space group I 4h. The low-temperature
nonmerohedrally twinned monoclinic phase 2a/2b (Figure 2 and
Table 4) was indexed in the nonstandard space group I2. The
phase transition was estimated to occur near 250 K. Comparison
of the a lattice constants of 2a/2b with a of 1 shows an increase
of 0.31 Å. This increase is matched by a corresponding decrease
(0.30 Å) in c of 2a/2b relative to a of 1. The twin components
in 2a/2b were present in a 1:1 ratio. The difference in unit cell
volumes at the two different temperatures would seem reason-
able even if no phase transition occurred. In the report by
Ivanov-Emin et al. describing the reaction of gallium halides
with N2O5, the formula of the sublimed white crystalline
compound was Ga(NO3)3.6 Comparison of the experimental
powder diffraction pattern with that calculated for 1 establishes
that they are not the same compound. The formula and structure
As the motion toward chelation progresses, the changes in
the O-Ga-O angles are characteristic of a change from a four-
(12) Sisler, H. H. In Encyclopedia of Inorganic Chemistry; King, R. B.,
Ed.; Wiley: New York, 1994; Vol. 5, p 2544.

4624 Inorganic Chemistry, Vol. 39, No. 20, 2000
Colombo et al.
Table 4. Selected Bond Lengths (Å) and Angles (deg) for the Low-Temperature Polymorph of [NO2][Ga(NO3)4]a
Ga(1)-O(1)
Ga(1)-O(4)
Ga(1)-O(5)
1.887(3)/1.889(3)
1.929(3)/1.929(4)
2.338(4)/2.340(6)
O(1)-N(1)
O(2)-N(1)
O(3)-N(1)
1.336(4)/1.331(5)
1.222(4)/1.221(6)
1.218(4)/1.220(6)
O(6)-N(2)
N(2)-O(5)
N(3)-O(8)
1.195(5)/1.207(6)
1.253(5)/1.251(6)
1.12(1)/1.12(2)
N(2)-O(4)
N(3)-O(7)
1.332(4)/1.325(5)
1.10(1)/1.11(1)
O(1)-Ga(1)-O(1A)
112.1(2)/111.9(2)
O(1)-Ga(1)-O(4)
101.1(1)/105.5(2)
O(1A)-Ga(1)-O(4)
O(4)-Ga(1)-O(5A)
O(4)-Ga(1)-O(5)
O(5A)-Ga(1)-O(5)
O(3)-N(1)-O(2)
O(2)-N(1)-O(1)
O(6)-N(2)-O(4)
N(2)-O(4)-Ga(1)
O(7)-N(3)-O(8)
105.6(1)/100.2(2)
85.3(1)/85.5(2)
59.3(1)/59.4(2)
84.0(2)/84.7(3)
126.3(3)125.5(4)
118.5(3)/119.1(4)
119.6(3)/119.0(5)
102.6(2)/102.4(3)
180/180
O(4)-Ga(1)-O(4A)
O(1)-Ga(1)-O(5A)
O(1)-Ga(1)-O(5)
N(1)-O(1)-Ga(1)
O(3)-N(1)-O(1)
O(6)-N(2)-O(5)
O(5)-N(2)-O(4)
N(2)-O(5)-Ga(1)
133.1(2)/133.2(3)
161.1(1)/83.1(2)
83.3(1)/161.4(2)
116.4(2)/116.4(3)
115.2(3)/115.4(4)
127.9(4)/128.3(5)
112.5(4)/112.7(4)
85.5(3)/85.5(3)
a The double values listed are for the independently refined twins 2a and 2b, respectively.
Figure 3. Relationship among the structures of the high-temperature tetragonal polymorph (central figure) and the two enantiomeric twins of the
low-temperature polymorph. The orientations of the cells are arranged to emphasize the similarities among the three structures. The scales of the
drawings are identical, and the thermal ellipsoids of the high- and low-temperature polymorphs are shown at 20% and 50% probability levels,
respectively.
coordinate, tetrahedral geometry to an octahedral structure. One
of the two equivalent O-Ga-O angles bisected by the c axis
in 1 (O(1)-Ga(1)-O(1B)) closes from 119.21(13) to 112.1(2)°
for O(1)-Ga(1)-O(1A) in 2a/2b. The equivalent angle on the
opposite side of the gallium in 1 opens to 133.1(2)° in 2a/2b.
Relatively smaller changes occur in the remaining O-Ga-O
angles.
Figure 3 focuses on the changes in the [Ga(NO3)4]- ion during
the phase transition. The central drawing shows the high-
temperature, tetragonal, polymorph. As the temperature is
lowered, the formation of the weak Ga-O interactions drives
the phase transition to the lower symmetry structures located
on either side in Figure 3. There is an equal probability for this
to occur along [001] or [001h] in 1. Due to the polar nature of
the low-temperature structure (I2 space group), as soon as the
contents of one unit cell change, a local dipole moment is
formed. Considering the cooperative nature of these interactions,
this initial dipole may influence neighboring unit cells, leading
to one of the twin domains. Although we have no information
that reveals the sizes of the twin domains, from the lack of
broadening seen in the X-ray diffraction reflections, they must
each be at least 20 nm in diameter. What is perhaps most
surprising is that the phase transition is reversible without
noticeable destruction of a single crystal. This would be less
unusual in the absence of twinning.
The nitrate ligands also reflect the additional interactions with
the gallium, although the changes are small. In 2a/2b, each
monodentate nitrate exhibits parameters similar to those found
in 1. Each bidentate nitrate shows a shortening of N(2)-O(6)
to 1.195(5) Å and a lengthening of N(2)-O(5) to 1.253(5) Å.
Each O(4)-N(2)-O(5) angle, which is part of a developing
ring incorporating the gallium, is decreased to 112.5(4)°.
The nitronium ions occupy special positions in both poly-
morphs, extending along channels parallel to the crystallographic
c axis for 1 and along channels parallel to the crystallographic
b axis for 2a/2b. There are no unusually close contacts with
the tetranitratogallate anion in either polymorph. The relative
positions of the [NO2]+ and [Ga(NO3)4]- ions change only
slightly during the phase transition. The four symmetry-related
Ga(1)-N(2) nearest-neighbor distances of 4.884 Å in 1 change
to two distances each of 4.589 and 5.198 Å (note that the
average is 4.894 Å). Considering the relatively open channels
along which the nitronium ions are located in 1, it is not
surprising to find that N(2) and O(4) exhibit relatively large
anisotropic displacements parallel to the c axis.
Acknowledgment. This research was supported by a grant
from the National Science Foundation (CHE-9616501).
Supporting Information Available: X-ray crystallographic files,
in CIF format, for the structure determinations of 1 and both twins of
2a/2b. This material is available free of charge via the Internet at
http://pubs.acs.org.
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