The structural classification of the highly disordered crystal phases of [Nn][BF4], [Nn][PF6], [Pn][BF4], and [Pn][PF6] salts (Nn+ = tetraalkylammonium and Pn+ = tetraalkylphosphonium)

Article abstract of DOI:10.1039/c4cp03391d

The structures of 16 symmetric tetraalkylammonium (N_n⁺) and tetraalkylphosphonium (P_n⁺) salts ([N_n][BF₄], [N_n][PF₆], [P_n][BF₄], and [P_n][PF₆], where n = 1 to 4, and denotes the number of carbon atoms in each alkyl chain) have been investigated by X-ray diffraction in order to elucidate the effect of ion size on the disordered structure of organic salts. All the salts exhibit one or more solid-solid phase transitions in differential scanning calorimetric curves. Powder X-ray diffraction revealed that the highest temperature solid phase of these salts belongs to a crystal system with a high cubic or hexagonal symmetry. The structures are classified into 5 different types: CsCl′, NaCl, NaCl′, inverse NiAs, and TBPPF₆. The CsCl′-type whose octant corresponds to the original CsCl unit cell is observed for [N₁][PF₆] owing to the orientational difference for the cation or the anion. The NaCl-type structure is observed for the N₂⁺ and P₂⁺ salts while the NaCl′-type structure is observed for [N₃][PF₆], where the configuration of ions is based on the NaCltype but the four equivalent positions in the original NaCl lattice split into two sets of equivalent positions (three and one). The inverse NiAs structure is observed for [P₃][PF₆]. Single-crystal X-ray diffraction reveals that the disordering of ions in [P₄][PF₆] becomes more significant with increasing temperature. The new structure of a cubic phase, the TBPPF₆-type structure, is found for the salts with long alkyl chains. The structure is roughly determined at 333 K and the ions therein are highly disordered but not rotating. The validity of the radius ratio rule is confirmed through appropriate assessment of the ion size.

Full text of DOI:10.1039/c4cp03391d

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The structural classification of the highly disordered
crystal phases of [Nn][BF4], [Nn][PF6], [Pn][BF4], and
Cite this: Phys.Chem.Chem.Phys.,
2014, 16, 23616
+
[Pn][PF6] salts (Nn = tetraalkylammonium and
+
Pn = tetraalkylphosphonium)†
Kazuhiko Matsumoto,*a Ukyo Harinaga,a Ryo Tanaka,a Akira Koyama,a
Rika Hagiwaraa and Katsuhiko Tsunashimab
+
+
The structures of 16 symmetric tetraalkylammonium (Nn
) and tetraalkylphosphonium (Pn ) salts
([Nn][BF4], [Nn][PF6], [Pn][BF4], and [Pn][PF6], where n = 1 to 4, and denotes the number of carbon atoms
in each alkyl chain) have been investigated by X-ray diffraction in order to elucidate the effect of ion size
on the disordered structure of organic salts. All the salts exhibit one or more solid–solid phase transitions
in differential scanning calorimetric curves. Powder X-ray diffraction revealed that the highest temperature
solid phase of these salts belongs to a crystal system with a high cubic or hexagonal symmetry. The
structures are classified into 5 different types: CsCl0, NaCl, NaCl0, inverse NiAs, and TBPPF6. The CsCl0-type
whose octant corresponds to the original CsCl unit cell is observed for [N1][PF6] owing to the orientational
+
+
difference for the cation or the anion. The NaCl-type structure is observed for the N2 and P2 salts while
the NaCl0-type structure is observed for [N3][PF6], where the configuration of ions is based on the NaCl-
type but the four equivalent positions in the original NaCl lattice split into two sets of equivalent
positions (three and one). The inverse NiAs structure is observed for [P3][PF6]. Single-crystal X-ray
diffraction reveals that the disordering of ions in [P4][PF6] becomes more significant with increasing
temperature. The new structure of a cubic phase, the TBPPF6-type structure, is found for the salts with long
alkyl chains. The structure is roughly determined at 333 K and the ions therein are highly disordered but not
rotating. The validity of the radius ratio rule is confirmed through appropriate assessment of the ion size.
Received 30th July 2014,
Accepted 8th September 2014
DOI: 10.1039/c4cp03391d
www.rsc.org/pccp
safe and reliable electrochemical devices.11–16 Nuclear magnetic
resonance studies on various alkylammonium salts have suggested
Introduction
Alkylammonium and alkylphosphonium salts have been the that the constituent ions essentially rotate in the IPC phase.17–22
topics of structural studies because they exhibit a variety of Recent work on diethyl(methyl)(isobutyl)phosphonium hexafluoro-
structural changes during thermal transitions.1–10 Their highest phosphate revealed that the terminal group of the cation rotates
temperature solid phase is particularly interesting and is char- even at low temperature and rotation of the entire cation starts first
acterized by disordering of the globular or pseudo-globular com- as uniaxial rotation, and then tumbles isotropically at high
ponent ions. These phases are often referred to as ionic plastic temperatures.23 Although isotropic rotational disordering in
crystals (IPCs) and have been attracting attention recently because IPC phꢀases is achieved by using relatively spherical anions such
they can be used as solid-state electrolytes in the construction of as BF4 and PF6ꢀ, previous literature clearly shows that even
nonspherical anions such as N(SO2CF3)2ꢀ also form IPC phases if
combined with an appropriate cation.24–26
One of the prominent features of IPCs is the small entropy
change of melting (DSm). This originates from the highly dis-
ordered structure of the IPC whose entropy differs by only a small
amount from that of the liquid phase.1,27,28 Although IPCs do not
always follow Timmerman’s criterion for molecular plastic crystals
(DSm o 20 J Kꢀ1 molꢀ1),29 such a small DSm is often observed for
tetraalkylammonium and dialkylpyrrolidinium-based IPCs.
Despite such studies on IPCs, few X-ray diffraction (XRD)
studies have been performed so far for the highly disordered
a Department of Fundamental Energy Science, Graduate School of Energy Science,
Kyoto University Sakyo-ku, Kyoto 606-8501, Japan.
E-mail: k-matsumoto@energy.kyoto-u.ac.jp
b Department of Materials Science, Wakayama National College of Technology,
77 Noshima, Nada-cho, Gobo 644-0023, Japan
† Electronic supplementary information (ESI) available: Comments on determi-
nation of space groups, calculations of ionic radii and the radius ratio for the
highly disordered phases, details on some synthetic procedures, structural data of
powder and single-crystal X-ray diffraction, Raman spectroscopic data, and an
X-ray diffraction image. CCDC 1010901–1010903. For ESI and crystallographic
data in CIF or other electronic format see DOI: 10.1039/c4cp03391d
23616 | Phys. Chem. Chem. Phys., 2014, 16, 23616--23626
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phase of alkylammonium salts. This may be because of the (Daikin Industries) was dried over K2NiF6 prior to use. The
difficulty in making a detailed structural analysis through tetramethylphosphonium salts, [P1][BF4] and [P1][PF6], were
single-crystal or powder X-ray diffraction owing to the significant prepared by the reaction of [P1][(FH)nF] prepared from [P1]Br
disordering of the component ions and large atomic displace- and HF and binary fluoride BF3 (Nippon Sanso, purity 99.99%)
ment factors, whereas an understanding of the basic structural and PF5 (Nippon Sanso, purity 99%), respectively, in anhydrous
properties is important in applications of these materials such HF (Stella Chemifa Corp., purity 99.9%). The salts, [N3][PF6],
as their use as electrolytes. Several structural types including [P3][BF4], and [P3][PF6], were synthesized by the metathesis
NaCl- and CsCl-types were identified by indexing powder dif- reaction of [N3]Br (Aldrich, purity 98%) or [P3]Br (Nippon
fraction patterns in the range of the highest temperature solid Chemical Industrial, purity 98%) and the potassium fluoro-
phase for alkylammonium salts with halide or simple inorganic complex salts (K[BF4] (Aldrich, purity 99.99%) and K[PF6]
anions.20,30–34 Such simplification of the configuration of the (Aldrich, purity 99.5%), respectively) (see ESI† for the details of
component species was also observed for molecular compounds the synthesis of [P1][BF4], [P1][PF6], [N3][PF6], [P3][BF4], and [P3][PF6]).
(body- or face-centered cubic) and is one of the unique char-
acteristics of plastic crystals.35 Although alkylpyrrolidinium and
Analysis
ꢀ
alkylpiperidinium salts of N(SO2CF3)2 do not exhibit highly Thermogravimetric and DSC analysis was performed under a
symmetric crystal lattices such as cubic or hexagonal, significant dry Ar gas flow using Shimadzu DTG-60H and Rigaku Thermo-
disordering in the high-temperature phases results in very Plus EVO II DSC 8230, respectively, at the scanning rate of
similar and relatively simple X-ray diffraction patterns regardless 5 K minꢀ1. The sample was placed in a Ni open cell for TG and
of the cationic structure.26,36–38 Single-crystal XRD of the low- in an Al sealed cell for DSC. Enthalpy changes were calculated
temperature phases provides some insights into the structural by integrating the heat flow during each transition on the DSC
disordering in the high temperature solid phases. For example, curve and entropy changes were calculated by dividing the
alkylammonium and alkylpyrrolidinium cations show positional enthalpy changes by the transition temperature. Density was
ꢀ
disordering in their low-temperature phases (e.g. N(SO2CF3)2
measured using the pycnometer method. The powder sample
(ref. 25, 26 and 37) and BF4ꢀ salts32,39–41), which can be regarded was weighed in a pycnometer first and tetradecane was intro-
as the prototypical motion in high-temperature phases. It should duced onto it. The entire pycnometer was evacuated to remove
be noted that these disordered structures observed using XRD gases around the powder and tetradecane was added up to the
are averaged structures and do not guarantee full and indepen- marked line at the capillary of the pycnometer. By measuring
dent rotation of ions as indicated for molecular plastic crystals.42 the weight of tetradecane, the density of the powder was
The present study provides a systematic understanding calculated from the weights of the powder and tetradecane,
of the phase behavior of 16 symmetric tetraalkylammonium the total volume, and the density of tetradecane. The density of
(Nn+, where n denotes the number of carbon atoms in the alkyl the powder was calculated from the weight of the powder
chain, n = 1 = methyl, n = 2 = ethyl, n = 3 = propyl, and n = 4 = butyl) sample and its volume. Raman spectra were recorded (Nano-
+
and tetraalkylphosphonium (Pn , n = 1–4) salts with BF4ꢀand finder 30, Tokyo Instruments, Inc.) at room temperature using
PF6ꢀ, which fills the missing relation between the ion size and the 632.8 nm line of a He–Ne laser as the excitation line. The
crystal structure of the highly disordered highest-temperature samples for Raman spectroscopy were loaded in Pyrex glass
solid phase. The phase transition and structure of each salt is tubes under dry Ar and sealed with a plastic cap. Elemental
investigated through a combination of differential scanning analysis was performed at Organic Elemental Analysis Research
calorimetry (DSC) and single-crystal and powder XRD.
Center, Kyoto University. Powder X-ray diffraction patterns were
recorded using Ultima IV X-ray diffractometer (Rigaku Corp.)
equipped with a scintillation counter detector and graphite-
monochromated CuKa irradiation (1.5418 Å; 40 kV–40 mA). The
well-ground sample spread on a Cu holder was placed in a
temperature controlling apparatus. The measurement was
Experimental
Chemicals
Volatile materials were handled in a vacuum line constructed of performed under vacuum with a scanning rate of 0.11 minꢀ1
.
SS316 stainless steel and PFA (tetrafluoroethylene-perfluoroalkyl- The obtained data was indexed by DICVOL06 with zero-point
vinylether copolymer). Nonvolatile materials were handled under correction.43 Single-crystal X-ray diffraction data were collected
a dry Ar atmosphere in a glovebox. Methanol (Wako Chemicals, on an R-axis Rapid II diffractometer (Rigaku Corp.) equipped
99.8%, water content o 10 ppm) for crystal growth was used as with an imaging plate area detector (using the program RAPID
received. The tetraalkylammonium and tetraalkylphosphonium XRD 2.3.355) and graphite-monochromated MoKa radiation
salts, [N1][BF4] (Alfa Aesar, purity 97%), [N2][BF4] (TCI, purity (0.71073 Å; 50 kV–40 mA). Crystals of [P4][PF6] for low tempera-
98%), [N3][BF4] (Aldrich, purity 98%), [N4][BF4] (Strem Chemi- ture data collection were grown by slow recrystallization of
cals, purity 99%), [N1][PF6] (Alfa Aesar, purity 99%), [N2][PF6] a saturated methanol solution cooled down from 258 to 233 K.
(Aldrich, purity 99%), [N4][PF6] (Fluka, purity 99%), [P2][BF4] The supernatant methanol solution was removed using a pipette
(TCI, purity 95%), [P4][BF4] (Fluka, purity 99%), [P2][PF6] and the residual methanol was removed by dry Ar flow for 2 h at
(TCI, purity 99%), and [P4][PF6] (TCI, purity 98%) were pur- 233 K. Suitable crystals were selected under a cold stream of
chased and dried under vacuum at 80 1C. Anhydrous HF nitrogen (o223 K) to avoid possible solid–solid phase transition.
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A selected crystal measuring 0.2 ꢁ 0.2 ꢁ 0.6 mm3 was picked up
using a glass pin under a dry nitrogen flow (below 223 K) and
mounted on a goniometer head, using the cryotongs. A crystal of
[P4][PF6] measuring 0.1 ꢁ 0.1 ꢁ 0.1 mm3 for high temperature
data collection was grown in a quartz capillary by slow solidifica-
tion from the liquid phase. Data collection at 113, 243, and 273 K
consisted of 12 o scans (130–1901 and 51 frameꢀ1) at fixed j (01)
and w (451) angles and 32 o scans (0–1601 and 51 frameꢀ1) at
fixed j (1801) and w (451) angles. Data collection at 333 K
consisted of 36 o scans (0–1801 and 51 frameꢀ1) at fixed j
(301) and w (451) angles and 36 o scans (0–1801 and 51 frameꢀ1
)
at fixed j (2101) and w (451) angles. The exposure times were
500 s degꢀ1 at 113, 243, and 273 K and 900 s degꢀ1 at 333 K.
Integration, scaling, and absorption corrections were performed
using RAPID AUTO 2.40.44 The structure was solved using
SIR-9245 and refined by SHELXL-9746 linked to Win-GX.47 Aniso-
tropic displacement factors were introduced for all atoms for the
refinement of the data at 113, 243, and 273 K. Displacement
factors were isotropically determined in the case of the data at
333 K because of the highly disordered structure. Hydrogen atoms
were determined using an appropriate riding model for the data at
113, 243, and 273 K and were not determined for the data at 333 K.
Results and discussion
Thermal properties
Thermogravimetric curves of the Nn+ and Pn+ salts are shown in
Fig. 1. For [Nn][BF4], the introduction of a long alkyl chain
makes the salt thermally less stable, which agrees with the
trend shown in previous reports,2,48 which also applies to
[Nn][PF6]. The [Nn][BF4] salt is thermally less stable than the
Fig. 1 Thermogravimetric curves for (a) [Nn][BF4] and [Pn][BF4] and
(b) [Nn][PF6] and [Pn][PF6]. Scan rate: 5 K minꢀ1
.
[Pn][BF4] salt with the same alkyl chain lengths. Some of the phases with high-symmetry crystal lattices could not be con-
[Pn][BF4] salts, [P2][BF4] and [P3][BF4] exhibit two step decom- firmed by powder XRD below their decomposition tempera-
position curves, whereas a similar but weak trend was observed tures. Although the crystal structure of the highest-temperature
for [N2][BF4]. The distortion of the TG curve for [N1][BF4] arises solid phase for [N1][BF4] was also not determined, it is called
from its highly exothermic decomposition reaction. The Phase I by analogy with [N1][PF6] (see the section on structural
[P1][PF6], [P2][PF6], and [P3][PF6] salts exhibit distinct two-step analysis below for a discussion).
decomposition curves, whereas the decomposition of [P4][PF6]
A comparison with previous reports on the phase transitions
follows a one-step pathway similar to that of [P4][BF4]. Accord- of [Nn][BF4],1,2,49 showed the following two observations. First
ing to the results of differential thermal analysis combined with the weak transition peak at 289 K for [N3][BF4] reported in the
+
+
thermogravimetric analysis, the N1+, N2+, P1 , and P2 salts previous studies is not observed in the present study, possibly
exhibit signs of decomposition without melting or around their because of the difference in thermal history. Second, two transi-
melting temperature in the heating scan (see Fig. S1, ESI,† for tion peaks at 256 and 314 K in the DSC curve of [N4][BF4], which
the results of differential thermal analysis).
were not reported in previous studies, are observed in the
Details of the thermal transitions were investigated using present work. The DSC curve of [N1][PF6] is similar to that of
DSC. The DSC curves obtained during the heating scan for the [N1][BF4] but the solid–solid phase transition temperature is 61 K
+
+
Nn and Pn salts are shown in Fig. 2 (see Fig. S2, ESI,† for the lower. The [N2][BF4] and [N2][PF6] salts show very similar thermal
full DSC curves). The transition temperature, DH, and DS of behaviors and only one solid–solid transition (338 K for [N2][BF4]
+
+
each transition are listed in Table 1. All the Nn and Pn salts and 342 K for [N2][PF6]). The phosphonium analogues show
show one or more solid–solid phase transitions. In this paper, different phase behaviors. Four solid phases are observed for
the solid phases observed are called Phases I, II, III, and IV, [P1][BF4], whereas only two solid phases are found for [P1][PF6]
from the highest-temperature phase to the lowest. The excep- (the imaginary phase is not included here). The thermal behavior
tions are [P1][BF4] and [P1][PF6]; in these cases, the highest of [P2][BF4] is similar to that of [P2][PF6]. Both the solid–solid
temperature-solid phase is called Phase II, and an imaginary phase transition temperatures from Phase III to Phase II and from
Phase I is introduced because the structure of the highest solid Phase II to Phase I of [P2][BF4] (338 and 425 K, respectively) are
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Table 1 Summary of thermal behavior for [Nn][BF4], [Nn][PF6], [Pn][BF4],
and [Pn][PF6]a
Transition
T/K
DH/kJ molꢀ1
DS/J Kꢀ1 molꢀ1
[N1][BF4]
[P1][BF4]
II - I
Td
606
651
0.18
—
0.30
—
IV - III
III - II
II - I
Td
256
439
—
1.5
4.9
—
5.9
11.1
—
685
[N2][BF4]
[P2][BF4]
II - I
Td
342
606
8.2
—
123.9
—
III - II
II - I
I - L
Td
338
425
577
616
1.6
7.7
4.0
4.6
18.2
6.9
[N3][BF4]
[P3][BF4]
[N4][BF4]
II - I
I - L
Td
402
521
575
9.3
8.9
—
23.1
17.1
—
II - I
I - L
Td
343
509
582
1.6
12.1
—
4.6
23.6
—
IV - III
III - II
II - I
I - L
Td
256
314
340
432
551
1.6
1.2
2.1
8.7
—
6.3
3.7
6.1
20.2
—
[P4][BF4]
III - II
II - I
I - L
Td
284
329
368
612
24.1
6.9
6.4
—
84.7
20.8
17.4
—
Fig. 2 Differential scanning calorimetric curves obtained during the
heating scan for (a) [Nn][BF4] and [Pn][BF4] and (b) [Nn][PF6] and [Pn][PF6].
Scan rate: 5 K minꢀ1
.
[N1][PF6]
[P1][PF6]
II - I
Td
545
657
0.13
0.24
lower than those of [P2][PF6] (373 and 474 K, respectively). All the
III - II
II - I
Td
302
—
599
10.12
—
—
0.39
—
+
+
+
N3 , P3 , N4 , and P4+ salts exhibit two or more solid–solid phase
transitions due to the large conformational freedom of the alkyl
chains, and melt below their decomposition temperatures.
—
[N2][PF6]
[P2][PF6]
II - I
Td
338
627
7.2
—
21.3
—
Structural analysis by powder X-ray diffraction
III - II
II - I
Td
373
474
606
11.0
12.9
—
2.8
27.3
—
+
+
The powder X-ray diffraction patterns of the Nn and Pn salts
were measured at the temperature corresponding to each phase
in the DSC results. The temperature dependence of the powder
XRD patterns for [P2][PF6] is shown in Fig. 3 as an example and
the others are shown in Fig. S3–S16 (ESI†). In most cases,
changes in the diffraction patterns correspond to the phase
transitions observed in the DSC curves in Fig. 2. The general
trend observed is the decrease in the number of diffraction
peaks with increasing temperature because the higher tem-
perature phase tends to belong to a lattice with higher sym-
[N3][PF6]
[P3][PF6]
[N4][PF6]
II - I
I - L
Td
473
516
563
10.8
8.6
—
22.8
16.6
—
II - I
I - L
Td
445
504
616
5.7
5.8
—
12.8
11.5
—
III - II
II - I
I - L
Td
307
360
522
573
5.2
9.5
15.7
—
16.9
26.5
30.1
—
metry. The powder XRD patterns of Phase I for the Nn+ and Pn
+
(n = 2, 3, and 4) salts are shown in Fig. 4. The observed
diffraction peaks are listed in Table S1 (ESI†). The structural
types obtained by indexing the XRD patterns are summarized
in Table 2 and the calculated diffraction peaks are listed in
Table S1 (comparison between the observed and calculated
peak positions are shown in Fig. S17–S19, ESI†). Detailed
structural refinement of Phase I for the present salts was
impossible except for the TBPPF6-type structure described
[P4][PF6]
III - II
(II- I)b
I - L
Td
258
—
490
627
0.91
—
10.3
—
3.3
—
21.0
—
a
Solid–solid and solid–liquid transition temperatures were determined
by DSC. Decomposition temperature (Td) was determined as that at 5%
weight loss in TG curves. Roman numerals (I, II, III, and IV) and L in the
transition column denote each solid phase and liquid phase in Fig. 2.
b
This transition was observed only by single-crystal XRD.
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Fig. 3 X-ray diffraction patterns of [P2][PF6] at 323 K (Phase III), 423 K
(Phase II), and 493 K (Phase I).
below due to the significant disordering. Thus, the structural
types are discussed based on powder diffraction only by indexing.
The extinction rules and formula unit volume (FUV) for the
present salts allow only the simple structures known for
inorganic salts by considering the multiplicity of each site.
The structures of Phase I for the present salts are classified into
5 different structures including one body-centered cubic, one
face-centered cubic, two primitive cubic, and one hexagonal
lattice. The details of these structures are discussed below.
The X-ray diffraction patterns of [N1][BF4] (Phase II, Fig. S3,
ESI†), [P1][BF4] (Phase III, Fig. S4, ESI†), [N1][PF6] (Phase II,
Fig. 5), and [P1][PF6] (Phase III, Fig. S5, ESI†) at 298 K resemble
each other, suggesting their structures are similar to each other
and considered to be based on the ion configuration in the
previously known [N1][PF6] structure as described below. Phase I
of [N1][PF6] has a cubic lattice, whereas [P1][BF4] and [P1][PF6]
decompose at elevated temperatures without forming a high-
symmetry crystal lattice.50 The temperature range of Phase I for
[N1][BF4] was out of the range of our temperature controller for
XRD, and its structure was not investigated in the present study.
The crystal structure of [N1][PF6] determined at 170 K in a
previous study belongs to a tetragonal system (space group
P4/nmm, a = 8.436 Å, c = 6.089 Å, Z = 2).51 The powder XRD
patterns of [N1][PF6] at 563, 473, 373, and 298 K are shown in
Fig. 5 together with the simulated XRD pattern at 170 K.
Although the XRD pattern of [N1][PF6] at 298 K can be indexed
as a tetragonal lattice (a = 8.625(10) Å, c = 6.227(8) Å, Z = 2) and
there are some differences between this XRD pattern and the
simulated XRD pattern for the structure at 170 K such as the
peak intensities of the three relatively strong peaks in the 2y
region between 14 and 211 and the new peaks in the observed
pattern at 30.82 and 34.641 (Table S2, ESI†), suggesting that a
Fig. 4 X-ray diffraction patterns of Phase I for (a) [N2][BF4] at 373 K,
[P2][BF4] at 473 K,[N2][PF6] at 373 K, and [P2][PF6] at 498 K, (b) [N3][BF4] at
453 K, [P3][BF4] at 423 K,[N3][PF6] at 493 K, and [P3][PF6] at 473 K, and (c)
[N4][BF4] at 413 K, [P4][BF4] at 348 K, [N4][PF6] at 413 K, and [P4][PF6] at 338 K.
structural transformation occurs between 170 and 298 K. This alternately up and down along the (110) direction to minimize
+
transformation may correspond to the small transition observed the approach of the methyl group of N1 . Some XRD peaks
in the DSC curve (Fig. S2, ESI†). The unit cell of the low- splitting at 298 K gradually get closer to each other with increas-
temperature phase for [N1][PF6] is shown in Fig. 6. Although ing temperature and unite at 563 K in Phase I. The XRD pattern at
the ordered cation and anion roughly adopt a CsCl-type configu- 563 K is indexed as a cubic lattice with a = 12.59(2) Å and V =
ration in this structure, the two ordered PF6ꢀ anions are located 1997(5) Å3, and the extinction rule is consistent with the
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Table 2 Structural types (crystal system, space group, and lattice parameters) of Phase I for [Nn][BF4], [Pn][BF4], [Nn][PF6], and [Pn][PF6]a
PCCP
ꢀ
ꢀ
BF4
—
PF6
CsCl0 at 563 K, a = 12.59(2), V = 1997(5) Å3, Z = 8
+
N1+
N2+ NaCl at 373 K, a = 10.827(6) Å, V = 1269.2(12) Å3, Z = 4 NaCl at 373 K, a = 11.309(2) Å, V = 1446.3(4) Å3, Z = 4
N3+ TBPPF6 at 453 K, a = 14.129(6) Å, V = 2820(2) Å3, Z = 6
NaCl0 at 493 K, a = 12.047(16) Å, V = 1748(4) Å3, Z = 4
TBPPF6 at 413 K, a = 15.3538(13) Å, V = 3619.5(5) Å3, Z = 6
—
N4
TBPPF6 at 413 K, a = 14.971(8) Å, V = 3353(3) Å3, Z = 6
—
+
P1+
P2+
P3+
P4
NaCl at 473 K, a = 11.331(11) Å, V = 1455(2) Å3, Z = 4
TBPPF6 at 423 K, a = 14.298(6) Å, V = 2923(2) Å3, Z = 6
TBPPF6 at 348 K, a = 15.097(8) Å, V = 3440(3) Å3, Z = 6
NaCl at 498 K, a = 11.845(2) Å, V = 1661.7(5) Å3, Z = 4
Inverse NiAs at 473 K, a = 9.702(4) Å, c = 12.9888(11) Å, V = 1050.7(11) Å3, Z = 2
TBPPF6 at 338 K, a = 15.315(7) Å, V = 3592(3) Å3, Z = 6
a
CsCl0 (cubic): CsCl-type structure but the ions have different orientation and the cell is eight times larger than that for the typical CsCl type
structure. NaCl (cubic): NaCl-type structure, NaCl0 (cubic): NaCl-type structure but the ions have different orientation, inverse-NiAs (hexagonal):
inverse NiAs-type structure, TBPPF6 (cubic): a cubic structure represented by Phase I of tetrabutylphosphonium hexafluorophosphate, [P4][PF6].
Fig. 5 X-ray diffraction patterns of [N1][PF6] at 298, 373, 473, and 563 K.
The simulated pattern at 170 K from the single-crystal XRD data is also
shown.51
%
centrosymmetric space group Im3m and non-centrosymmetric
%
space groups I43m and I432. Considering the FUV of [N1][PF6] at
170 K (216.7 Å3) and 298 K (231.6 Å3), Z at 563 K is determined to
be eight. The lattice parameter a of 12.59(2) Å in the structure at
563 K is slightly larger than twice the Nꢂ ꢂ ꢂN distances in the
distorted CsCl structure at 170 K (5.965 Å and 6.089 Å (these
values correspond to a/21/2 and c of the original tetragonal cell)),
which suggests that the crystal structure of [N1][PF6] at 563 K is a
Fig. 6 Crystal structure of [N1][PF6] at 170 K:51 (a) unit cell and (b)
superlattice with a doubled cell constant of the original CsCl-type
pseudoCsCl-type cell. The Nꢂ ꢂ ꢂN direction in the pseudo CsCl-type cell
corresponds to the (110) direction in the unit cell. The z values in the
atomic coordinates for the P atoms in the unit cell are 0.4119 and 0.5881.
Hydrogen atoms are omitted for clarity.
lattice, which is now an octant of the unit cell. This can occur for
%
the centrosymmetric space group Im3m when the 2a [(0,0,0) and
(1/2,1/2,1/2)] and 6b [(1/2,1/2,0), (1/2,0,1/2), (0,1/2,1/2), (0,0,1/2),
(0,1/2,0), and (1/2,0,0)] sites of the like ions are distinguished
from each other owing to orientational differences. The other
%
ions are located at the 8a site [(1/4,1/4,1/4), (1/4,1/4,3/4), space group Fm3m or non-centrosymmetric groups F432 and
%
(1/4,3/4,1/4), (3/4,1/4,1/4), and their (1/2,1/2,1/2) translated F43m. The FUVs determined for [N2][BF4] at 298 K and [N2][PF6]
positions]. Similar discussions are possible for the non- at 220 K above are 294.5 and 316 Å3, respectively, whereas the V
%
centrosymmetric space groups, I432 and I43m (see ESI,† S-1). parameters of [N2][BF4] and [N2][PF6] at 373 K are 1269.2(12)
This structural type is called the CsCl0-type in this study (Fig. 7). and 1446.3(4) Å3, respectively. Considering the lattice expansion
The X-ray diffraction patterns of Phase I for [N2][BF4], in forming plastic crystal lattices as in the case of [N1][PF6], Z of
[P2][BF4], [N2][PF6], and [P2][PF6] are shown in Fig. 4(a), and Phase I for [N2][BF4], [P2][BF4], [N2][PF6], and [P2][PF6] is deter-
are indexed as a cubic lattice. The extinction rule suggests that mined to be 4, and their basic ion configuration is the NaCl-type
they belong to a face-centered lattice with the centrosymmetric structure in any of the possible three space groups, with the
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Fig. 7 Schematic drawings of Phase I structures, CsCl0, NaCl, NaCl0, inverse NiAs, and TBPPF6. The central positions of the cation and anion are shown in
these structures. For the TBPPF6-type structure, the ions are regarded to be highly disordered but not pseudo-spherical as shown in Fig. 8.
cations and anions occupying 4a [(0,0,0), (1/2,1/2,0), (1/2,0,1/2), X-ray diffraction pattern of [P3][PF6] is indexed as a hexagonal
and (0,1/2,1/2)] and 4b [(1/2,1/2,1/2), (1/2,0,0), (0,1/2,0), and lattice with a = 9.702(4) Å, c = 12.9888(11) Å, and V = 1050.7(11) Å3.
(0,0,1/2)] sites, respectively (Fig. 7).
The Z value is estimated to be 2 through comparison with the FUV
A variety of structures are observed for Phase I of the N3+ and of the low-temperature phase of [N3][BF4] (397 Å3).39 Two
P3+ salts, whose XRD patterns are shown in Fig. 4(b). The X-ray hexagonal lattices with Z = 2 are known as typical structural
diffraction pattern of [N3][PF6] is indexed as a cubic lattice with types for 1 : 1 salts : wurtzite (space group: P63mc) and NiAs
a = 12.047(16) Å, which is greater than that of [N2][PF6] (space group: P63/mmc) types, whereas the present NiAs struc-
(11.309(2) Å) with a smaller cation. The FUV of the low ture should be called the inverse NiAs-type because the cation is
temperature phase of [N3][BF4] is 397 Å3;39 therefore, Z of this larger than the anion. The tetrahedral site in the wurtzite
lattice is estimated to be 4 (note that the difference between structure has a smaller space than the octahedral site in the
ꢀ
ꢀ
BF4 and PF6 is negligible in the rough estimation of Z). inverse NiAs structure. The extinction rule of the X-ray dif-
However, the observed indices violate the extinction rule for the fraction indices matches both the space groups. Christe et al.
NaCl-type structure, and suggest the centrosymmetric space mentioned that the c/a ratio could be an indicator for distin-
%
%
group Pm3m and the non-centrosymmetric space groups P43m guishing the two structures; the c/a values are 1.63 for the
and P432. In these space groups, the cation occupies the 1a wurtzite structure and 1.39 for the NiAs structure.52 The c/a
[(0,0,0)] and 3c sites [(1/2,1/2,0), (1/2,0,1/2), and (0,1/2,1/2)], and ratio of [P3][PF6] is 1.33, which is close to the value for the NiAs
the anion occupies the 1b [(1/2,1/2,1/2)] and 3d sites [(1/2,0,0), structure. Thus, [P3][PF6] is considered to belong to the inverse
(0,1/2,0), and (1/2,1/2,0)] (or vice versa). In other words, the 4a NiAs-type structure (Fig. 7) (the radius ratio rule discussed
%
and 4b sites in Fm3m are divided into 1a and 3c and into 1b and below also confirms the inverse NiAs structure). To our know-
3d sites, respectively, unlike in the NaCl-type structure. This is ledge, there are two examples of organic salts with this structural
probably caused by the difference in orientation of the ion at type: [P2][(FH)2F]53 and [N1]F.52 The [N3][BF4] and [P3][BF4] salts
these sites. The difference between the centrosymmetric and belong to a primitive cubic lattice as in the cases of the N4+ and
non-centrosymmetric space groups is not clear in the present P4+ salts; the details of this structure are discussed below. In the
analysis, but arises from the difference in the site symmetry; NaCl0-type and inverse NiAs-type structures, the thermal motions
the configuration of alkyl chains or F atoms may lower the of the ions are considered to be very large and the ions are highly
molecular symmetry and result in lower site symmetries that do disordered at the least to satisfy the high site symmetry.
not satisfy the one in the centrosymmetric space group. This
The X-ray diffraction patterns of [N4][BF4], [P4][BF4], [N4][PF6],
structural type is called the NaCl0-type in this study (Fig. 7). The and [P4][PF6] shown in Fig. 4(c) are indexed as cubic lattices.
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The extinction rule suggests that they belong to a primitive cubic discrepancy in the DSC and single-crystal XRD behaviors is not
%
lattice with the centrosymmetric space group Pm3n or the non- yet clear, one of the most probable reasons is the difference in
%
centrosymmetric space group P43n. Although a similar pattern the behaviors of the powder and single-crystal samples. The
was reported in a previous work for [N4]I at 400 K indexed as powder XRD pattern at 298 K is similar to that of Phase I (even
a cubic cell,54 there has been no further discussion so far. after cooling to 233 K) and does not match the results of single-
The details of this structure are provided in the next section.
crystal XRD either. Fig. 9 shows the changes in lattice parameters
(a, b, c, b, and V) with increasing temperature. Parameters a and
c increase and b decreases with increasing temperature.
The parameter b decreases as the temperature approaches
Structural changes of [P4][PF6] along the phase transitions
The structural changes of [P4][PF6] along the phase transitions
were studied using single-crystal XRD as an example of the salts the transition temperature and drops to 90 near the tempera-
that have Phase I with the primitive cubic lattice mentioned ture of the transition to the orthorhombic phase. The change in
above ([N3][BF4], [P3][BF4], [N4][BF4], [P4][BF4], [N4][PF6], and V is relatively monotonous in the temperature range of the
[P4][PF6]) (see Table S3, ESI,† for detailed crystallographic data). monoclinic cell, and a gap is observed at the transition tem-
The ORTEP diagrams of the structures at 113, 243, 273, and perature to the orthorhombic cell. The thermal expansion in
333 K are shown in Fig. 8. The structure determined at 113 K the monoclinic cell is 4.7% during the temperature change
belongs to a monoclinic space group of P21/c, and is isostructural from 113 to 298 K, which is in the range 3–7% proposed
with the known [N4][PF6] structure at 110 K.55 The asymmetric recently for organic salts.56
unit of [P4][PF6] at 113 K contains a cation–anion pair. Three of
The crystal structure of the cubic phase was investigated at
the four butyl chains in P4 adopt an all-trans conformation, 333 K by a crystal grown by slow solidification from the liquid
+
while the one with P–C–C–C and C–C–C–C torsion angles of state. Extinction rules suggest that the crystal belongs to the cubic
173.01(11)1 and ꢀ68.09(18)1 does not. The PF6ꢀ anion is ordered space group P43n or Pm3n, which agrees with the powder XRD
at this temperature with a nearly ideal octahedral geometry. results. The diffraction image has only a few spots in the low 2y
Although ꢀthe unit cell at 243 K still belongs to the space group angle region (see Fig. S25, ESI,† for an example of the diffraction
P21/c, PF6 is disordered around two F–P–F axes. On the other images during the o scan of 51 per frame). This observation
%
%
+
hand, P4 is ordered at this temperature with larger atomic suggests that the peaks in the region of high 2y angle disappear
displacement factors than at 113 K. Upon warming to 273 K, because of the large atomic displacement factors of the constitu-
the solid–solid transition occurs as shown in the DSC curve, and ent atoms. Although the structure was not fully solved and refined
the crystal transforms from monoclinic to orthorhombic (space because of the highly disordered ions, the basic cation–anion
group: Pmcn), that is, the b angle in the monoclinic cell reaches configuration was obtained even in a rough form (see Table S4,
901 at the tranꢀsition temperature. The three-dimensional dis- ESI,† for detailed crystallographic data).
ordering of PF6 leads to its almost spherical behavior. Signifi-
By considering the FUV at 273 K (563 Å3), V at 333 K
cant disordering with large atomic displacement factors is also (3472 Å3), and the lattice expansion, we determine Z of Phase I
+
observed for P4 , but it does not look like a spherical rotation of for [P4][PF6] to be 6. However, a cubic structure with Z = 6 is not
the entire ion. Although the crystal lattice of [P4][PF6] at 298 K known for a simple inorganic 1 : 1 salt. When the centrosymmetric
%
belongs to the orthorhombic cell with slightly larger cell lengths space group Pm3n is chosen, the three Wyckoff positions (6b, 6c,
than at 273 K, the crystal broke into pieces at 323 K, suggesting a and 6d) have a multiplicity of 6. If the 6b site [(0,1/2,1/2), (1/2,0,1/2),
+
phase transition to the cubic phase. Although the reason for the (1/2,1/2,0), (0,1/2,0), (1/2,0,0), and (0,0,1/2)] is occupied by P4 or
Fig. 8 ORTEP diagrams of the unit cell for [P4][PF6] at 113, 243, 273, and 333 K. Hydrogen atoms are omitted for clarity. Thermal ellipsoids are shown at
the 50% probability level for the structures at 113, 243, and 273 K, and at the 5% probability level for the structure at 333 K. The structure at 333 K was not
fully refined owing to the significant disordering of the component ions.
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and inverse NiAs-type) is examined here (see ESI† for details of
the following calculations). According to the radius ratio rule,
a small rꢀ/r+ results in the formation of a structure with a low
CN such as the zinc blende-type if the ions behave spherically.
However, the cations in the TBPPF6-type structure are no longer
regarded as spheres as shown above and thus the applicability
is not examined here for this structural type.
In order to estimate the radii of the ions in IPCs, we started from
the assessment of the radius of PF6ꢀ, r(PF6ꢀ), in the highly
disordered structure. First, the r(PF6ꢀ) value at 298 K in the highly
ꢀ
disordered structure, r(PF6ꢀ, 298 K), is calculated from the PF6
salts K[PF6], Rb[PF6], and Cs[PF6].58,59 These three salts belong to the
same Cs[PF6]-type structure having the NaCl-type arrangement of
ions, where the fluorine atoms in PF6ꢀ are disordered into 24 sites.
The radii of the three alkali metal cations are available from the list
of crystal radii by Shannon,60 and the values for CN = 6 are chosen
here as the NaCl-type structure (r(K+) = 1.52 Å, r(Rb+) = 1.66 Å, and
r(Cs+) = 1.81 Å). Using this procedure, r(PF6ꢀ, 298 K) in IPC is
estimated from lattice parameter a of the three salts (a = 7.7891(7) Å
for K[PF6], a = 7.887(8) Å for Rb[PF6] and a = 8.197(2) Å for Cs[PF6])
according to eqn (1):
Fig. 9 Temperature dependence of the a, b, c, b, and V lattice parameters
for [P4][PF6] determined by single-crystal XRD.
PF6ꢀ, the counterion cannot occupy the 6c [(1/4,0,1/2), (3/4,0,1/2),
(1/2,1/4,0), (1/2,3/4,0), (0,1/2,1/4), and (0,1/2,3/4)] or 6d [(1/4,1/2,0),
(3/4,1/2,0), (0,1/4,1/2), (0,3/4,1/2), (1/2,0,1/4), and (1/2,0,3/4)] sites,
because the 6b and 6c or 6d sites are too close to each other.
Consequently, the only possible ion configuration is the one in
which one ion occupies the 6c site and the other occupies the 6d
site (Fig. 7). The unit cell of Phase I for [P4][PF6] is shown in Fig. 8
(note that the ORTEP diagram is drawn at 5% probability). All
the atoms have very large atomic displacement factors and the
terminal carbon atom in the butyl group could not be deter-
mined. However, the unit cell diagram provides a clear idea of
this structure in which the cation and anion are located on each
face of the unit cell and the butyl chain is extended toward the
(0,0,0) and (1/2,1/2,1/2) sites. This structure is called TBPPF6-type
here. As is clear from this model, isotropic rotation of the cation
is ruled out in this structure.
r(PF6ꢀ) = a/2 ꢀ r(M+) (M+ = K+, Rb+, or Cs+)
(1)
The three cases afford similar r(PF6ꢀ, 298 K) values (2.37 Å
from K[PF6], 2.28 Å from Rb[PF6], and 2.29 Å for Cs[PF6]),
resulting in an averaged r(PF6ꢀ) value of 2.31 Å.61
+
+
The radii of the other cations (r(Nn ) and r(Pn )) are calculated
from the lattice parameter a and r(PF6ꢀ) according to eqn (2)–(4):
ꢀ
r(Nn+) [or r(Pn+)] = a/2 ꢀ r(PF6 ) (NaCl- and NaCl0-type lattice)
(2)
ꢀ
r(Nn+) [or r(Pn+)] = 31/2ꢂa/2 ꢀ r(PF6 ) (CsCl0-type lattice)
(3)
r(Nn+) [or r(Pn+)] = [(a/31/2)2 + (c/4)2]1/2
ꢀ r(PF6ꢀ) (inverse NiAs-type lattice) (4)
It should be noted that parameter a in eqn (2) is half that for
the CsCl0-type structure, as described above.
Validity of the radius ratio rule
The radius of BF4ꢀ, r(BF4ꢀ), is then calculated from the
lattice parameter a and r(Nn+) [or r(Pn+)] by analogy with eqn (2).
The calculated radii and resulting radius ratios of the
present salts are listed in Table 3. The radius ratio rule described
The radius ratio rules are introduced by considering the colli-
sion of the larger ions in ionic crystals.57 Although the applic-
ability of the radius ratio rules is limited from the viewpoint of
structure prediction (especially for alkali metal halides), it is
still useful for a rough estimation of the effect of ion size on the
crystal structure. According to these rules, the coordination
number (CN) of a 1 : 1 salt is determined by the ratio of the radii
of the cation and anion (r+/rꢀ, where r+ and rꢀ are the radii of
the cation and anion, respectively (r+ o rꢀ)); CN = 3 for 0.155 o
r+/rꢀ o 0.225, CN = 4 for 0.225 o r+/rꢀ o 0.414, CN = 6 for
0.414 o r+/rꢀ o 0.732, and CN = 8 for 0.732 o r+/rꢀ. Although
the salts with non-spherical ions such as a complicated organic
cation do not follow this rule, the ions in highly disordered
structures often behave isotropically (or nearly isotropically),
and can be treated as spherical. Therefore, the applicability of
the radius ratio rule (note that rꢀ o r+ in the present cases) to
Table 3 Structural types, radii of the ions and the radius ratio of the cation
and anion for Phase I of selected [Nn][BF4], [Nn][PF6], [Pn][BF4], and [Pn][PF6]a
Structural type
r+/Å
rꢀ/Å
rꢀ/r+
[N1][PF6]
[N2][BF4]
[N2][PF6]
[P2][BF4]
[P2][PF6]
[N3][PF6]
[P3][PF6]
CsCl0 at 563 K (CN = 8)
NaCl at 373 K (CN = 6)
NaCl at 373 K (CN = 6)
NaCl at 473 K (CN = 6)
NaCl at 498 K (CN = 6)
NaCl0 at 493 K (CN = 6)
Inverse-NiAs at 473 K (CN = 6)
3.14
3.34
3.34
3.61
3.61
3.71
4.16
2.37
2.07
2.37
2.07
2.37
2.37
2.37
0.74
0.62
0.69
0.57
0.64
0.62
0.56
a
r+, rꢀ, and CN denote the radii of the cation and anion and coordination
Phase I of the present salts (CsCl0-type, NaCl-type, NaCl0-type, number, respectively.
23624 | Phys. Chem. Chem. Phys., 2014, 16, 23616--23626
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above is applied appropriately to all the salts with the CsCl0-type
structure ([N1][PF6]), NaCl-type structure ([N2][BF4], [N2][PF6],
[P2][BF4], and [P2][PF6]), NaCl0-type structure ([N3][PF6]) and
inverse NiAs-type structure ([P3][PF6]), although attention should
be paid to the CsCl0-type and NaCl0-type structures owing to the
limitation in the spherical model for the ions therein.
12 J. M. Pringle, P. C. Howlett, D. R. MacFarlane and M. Forsyth,
J. Mater. Chem., 2010, 20, 2056–2062.
13 J. M. Pringle, Phys. Chem. Chem. Phys., 2013, 15, 1339–1351.
14 D. R. MacFarlane, J. H. Huang and M. Forsyth, Nature, 1999,
402, 792–794.
15 E. I. Cooper and C. A. Angell, Solid State Ionics, 1986, 18–19,
570–576.
16 Y. Abu-Lebdeh, P. J. Alarco and M. Armand, Angew. Chem.,
Int. Ed., 2003, 42, 4499–4501.
Conclusions
17 S. Iwai, M. Hattori, D. Nakamura and R. Ikeda, J. Chem. Soc.,
Faraday Trans., 1993, 89, 827–831.
18 T. Shimizu, S. Tanaka, N. OnodaYamamuro, S. Ishimaru and
R. Ikeda, J. Chem. Soc., Faraday Trans., 1997, 93, 321–326.
19 T. Tanabe, D. Nakamura and R. Ikeda, J. Chem. Soc., Faraday
Trans., 1991, 87, 987–990.
20 K. Kuchitsu, H. Ono, S. Ishimaru, R. Ikeda and H. Ishida,
Phys. Chem. Chem. Phys., 2000, 2, 3883–3885.
21 P. J. Alarco, Y. Abu-Lebdeh and M. Armand, Solid State
Ionics, 2004, 175, 717–720.
22 P. J. Alarco, Y. Abu-Lebdeh, N. Ravet and M. Armand, Solid
State Ionics, 2004, 172, 53–56.
The present systematic study using X-ray diffraction techniques
revealed that the highly disordered structures of symmetric
+
tetraalkylammonium (Nn+) and tetraalkylphosphonium (Pn
)
salts ([Nn][BF4], [Nn][PF6], [Pn][BF4], and [Pn][PF6], where n = 1
to 4) are classified into 5 different types: CsCl0, NaCl, NaCl0,
inverse NiAs, and TBPPF6. The effect of ion size on the
structural type is interpreted by estimating the radius ratio of
the cation and anion. Validity of the radius ratio rules was
shown in the cases of the highly symmetric cations, whereas
the cations with long alkyl chains cannot be regarded as
spheres even in a highly disordered cubic structure. The
information obtained in this study should aid in experimental
and computational modeling of organic IPC materials.
23 L. Y. Jin, K. M. Nairn, C. M. Forsyth, A. J. Seeber, D. R.
MacFarlane, P. C. Howlett, M. Forsyth and J. M. Pringle,
J. Am. Chem. Soc., 2012, 134, 9688–9697.
24 D. R. MacFarlane, P. Meakin, J. Sun, N. Amini and M. Forsyth,
J. Phys. Chem. B, 1999, 103, 4164–4170.
25 W. A. Henderson, M. Herstedt, V. G. Young, S. Passerini,
H. C. De Long and P. C. Trulove, Inorg. Chem., 2006, 45,
1412–1414.
Acknowledgements
The authors would like to thank the Kyoto Technoscience
Center for their financial support of this work.
26 W. A. Henderson, V. G. Young, S. Passerini, P. C. Trulove
and H. C. De Long, Chem. Mater., 2006, 18, 934–938.
27 S. Forsyth, J. Golding, D. R. MacFarlane and M. Forsyth,
Electrochim. Acta, 2001, 46, 1753–1757.
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