n-Pentylammonium dihydrogenarsenate, C5H14N+·H2AsO4- (C5ADA), is ferroelastic at room temperature. The structure is monoclinic (P21/n) and isostructural with n-pentylammonium dihydrogenphosphate (C5ADP). In contrast to C7ADA and C9ADA, as well as to C7ADP and C9ADP, two independent dihydrogenarsenates in the present structure are not disordered. Similarly, as in other known members of the series (C2ADP-C10ADP and C6ADA-C9ADA), there are strong hydrogen bonds between the anions, and moderate hydrogen bonds between the anions and the cations. The hydrogen-bond distances correspond well to those observed in the dihydrogenphosphates. There are two H atoms in the structure which are involved in asymmetric hydrogen bonds between respective oxygen pairs. These H atoms jump from the donor to the acceptor O atoms during ferroelastic switching. A phase transition was observed at about 380 and 371 K during heating and cooling, respectively.
Supporting information
CCDC reference: 162826
Key indicators
- Single-crystal X-ray study
- T = 290 K
- Mean (C-C) = 0.005 Å
- R factor = 0.036
- wR factor = 0.045
- Data-to-parameter ratio = 13.3
checkCIF results
No syntax errors found
Alert Level B:
PLAT_112 Alert B ADDSYM detects additional (pseudo)symmetry ... ?
| Author response: The structure is ferroelastic, i. e. twinned.
|
PLAT_113 Alert B ADDSYM suggests Pseudo/New Spacegroup ........ Pbcn
| Author response: The suggested space-group is a space-group
of a prototypic phase.
|
Alert Level C:
GOODF_01 Alert C The least squares goodness of fit parameter lies
outside the range 0.80 <> 2.00
Goodness of fit given = 2.020
0 Alert Level A = Potentially serious problem
2 Alert Level B = Potential problem
1 Alert Level C = Please check
Precipitation of n-pentylamine and H3AsO4. The precipitate was filtered off,
dried and dissolved in 96% ethanol from which the single crystals were grown
by slow evaporation at room temperature. The crystal which appeared single
domained under the polarization microscope was selected for a diffractometer
measurement.
The structure is ferroelastic. It can be related to the prototypic space group
P2/b21/n21/a. Therefore, the sample was expected to be twinned but the
twinning turned out to be insignificant. The disorder was taken into account
by the refinement on separate scales for h even and odd, respectively, but it
was irrelevant either. The extinction correction was negligible and for the
final refinement not used. No maxima which would belong to the disordered
dihydrogenarsenates were detected. All H atoms except those which are
pertinent to the OH groups could be distinguished on the Fourier maps. The
bond distances as well as the angles in which the H atoms were involved were
restrained. The O—H, N—H and C—H bond lengths were restrained to
0.90 (3), 0.90 (2) and 0.95 (1) Å, respectively. The H—C—H and H—N—H
angles were restrained to 109 (1)°.
Data collection: COLLECT (Nonius, 1997-2000); cell refinement: HKL SCALEPACK (Otwinowski & Minor, 1997); data reduction: HKL DENZO and SCALEPACK (Otwinowski & Minor, 1997); program(s) used to solve structure: JANA2000 (Petříček & Dušek, 2000); program(s) used to refine structure: JANA2000; molecular graphics: ORTEPIII (Burnett & Johnson, 1996); software used to prepare material for publication: JANA2000.
n-pentylammonium dihydrogenarsenate
top
Crystal data top
C5H11NH3+·H2AsO4− | F(000) = 944 |
Mr = 229.11 | Dx = 1.595 Mg m−3 |
Monoclinic, P21/n | Mo Kα radiation, λ = 0.71073 Å |
a = 9.3380 (3) Å | Cell parameters from 54194 reflections |
b = 27.3950 (8) Å | θ = 1.0–27.5° |
c = 7.4570 (8) Å | µ = 3.54 mm−1 |
β = 90.568 (1)° | T = 290 K |
V = 1907.5 (1) Å3 | Prism, colourless |
Z = 8 | 0.32 × 0.10 × 0.04 mm |
Data collection top
Nonius KappaCCD diffractometer | 4366 independent reflections |
Radiation source: fine-focus sealed X-ray tube | 2740 reflections with I > 3σ(I) |
Graphite monochromator | Rint = 0.053 |
ϕ and ω scans | θmax = 27.5°, θmin = 1.5° |
Absorption correction: integration Gaussian integration (Coppens, 1970) | h = −12→12 |
Tmin = 0.461, Tmax = 0.868 | k = −35→35 |
27031 measured reflections | l = 0→9 |
Refinement top
Refinement on F | 52 restraints |
Least-squares matrix: full with fixed elements per cycle | 0 constraints |
R[F2 > 2σ(F2)] = 0.036 | All H-atom parameters refined |
wR(F2) = 0.045 | Weighting scheme based on measured s.u.'s w = 1/[σ2(Fo) + 0.0001(Fo)2] |
S = 2.02 | (Δ/σ)max = 0.001 |
4365 reflections | Δρmax = 0.67 e Å−3 |
327 parameters | Δρmin = −0.83 e Å−3 |
Crystal data top
C5H11NH3+·H2AsO4− | V = 1907.5 (1) Å3 |
Mr = 229.11 | Z = 8 |
Monoclinic, P21/n | Mo Kα radiation |
a = 9.3380 (3) Å | µ = 3.54 mm−1 |
b = 27.3950 (8) Å | T = 290 K |
c = 7.4570 (8) Å | 0.32 × 0.10 × 0.04 mm |
β = 90.568 (1)° | |
Data collection top
Nonius KappaCCD diffractometer | 4366 independent reflections |
Absorption correction: integration Gaussian integration (Coppens, 1970) | 2740 reflections with I > 3σ(I) |
Tmin = 0.461, Tmax = 0.868 | Rint = 0.053 |
27031 measured reflections | |
Refinement top
R[F2 > 2σ(F2)] = 0.036 | 52 restraints |
wR(F2) = 0.045 | All H-atom parameters refined |
S = 2.02 | Δρmax = 0.67 e Å−3 |
4365 reflections | Δρmin = −0.83 e Å−3 |
327 parameters | |
Special details top
Experimental. rotation scans:10 images collected ω scans, rotation per image 10°, 202 s
exposure, crystal to detector distance 35 mm. |
Refinement. The structure was intended to be refined as a twin, however the domain fraction
f was refined to negative values; the trial measurement on a 4-dircle
diffractometer with a point detector did not indicate existence of a second
domain. >From the similarity with other compounds of the series the twinning matrix is
analogous as in other related sompounds. The components for the twinning
matrix is given in _diffrn_reflns_transf_matrix_ items The H atoms were restrained by the distfix and anglefix functions of JANA2000:
The values for distfix were 0.90(0.021) A ng. for O—H distances. The values
for distfix were 0.90(0.022) A ng. for N—H distances. The values for distfix
were 0.9(0.022) A ng. for methyl-H distances. The values for distfix were
0.95(0.013) A ng. for methylene-H distances. The values for anglefix were
109.47(1.00) ° for all H—N—H; H—C—H angles. |
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å2) top | x | y | z | Uiso*/Ueq | |
As1 | 0.31307 (3) | 0.307442 (10) | 0.25283 (3) | 0.02457 (11) | |
O11 | 0.1862 (2) | 0.35199 (8) | 0.2262 (3) | 0.0434 (9) | |
O21 | 0.3460 (2) | 0.29710 (7) | 0.4683 (2) | 0.0384 (8) | |
O31 | 0.4509 (2) | 0.32376 (7) | 0.1276 (2) | 0.0353 (8) | |
O41 | 0.2413 (2) | 0.25415 (7) | 0.1745 (2) | 0.0372 (8) | |
As2 | 0.80930 (3) | 0.304597 (10) | 0.26180 (3) | 0.02493 (12) | |
O12 | 0.6922 (3) | 0.35176 (8) | 0.2620 (3) | 0.0430 (9) | |
O22 | 0.8455 (2) | 0.29388 (8) | 0.0409 (2) | 0.0405 (9) | |
O32 | 0.9521 (2) | 0.32228 (7) | 0.3745 (2) | 0.0363 (8) | |
O42 | 0.7361 (2) | 0.25369 (6) | 0.3420 (2) | 0.0350 (8) | |
N1 | 0.9655 (3) | 0.68617 (9) | 0.2684 (3) | 0.0323 (10) | |
C11 | 0.8867 (4) | 0.63951 (12) | 0.2472 (4) | 0.0363 (13) | |
C21 | 0.9866 (4) | 0.59597 (12) | 0.2549 (4) | 0.0390 (13) | |
C31 | 0.9051 (4) | 0.54822 (14) | 0.2469 (5) | 0.0446 (15) | |
C41 | 1.0003 (5) | 0.50296 (13) | 0.2490 (5) | 0.0472 (15) | |
C51 | 0.9154 (6) | 0.45551 (15) | 0.2440 (6) | 0.069 (2) | |
N2 | 0.4732 (3) | 0.68587 (10) | 0.2308 (3) | 0.0326 (10) | |
C12 | 0.3916 (4) | 0.63946 (12) | 0.2493 (4) | 0.0343 (12) | |
C22 | 0.4886 (4) | 0.59559 (12) | 0.2450 (4) | 0.0355 (12) | |
C32 | 0.4038 (4) | 0.54833 (13) | 0.2519 (5) | 0.0436 (14) | |
C42 | 0.4983 (5) | 0.50296 (13) | 0.2507 (5) | 0.0480 (15) | |
C52 | 0.4136 (6) | 0.45545 (15) | 0.2524 (6) | 0.064 (2) | |
H1n1 | 1.002 (2) | 0.6862 (9) | 0.380 (2) | 0.080 (14)* | |
H2n1 | 0.908 (2) | 0.7118 (8) | 0.256 (2) | 0.033 (10)* | |
H3n1 | 1.035 (2) | 0.6882 (9) | 0.190 (3) | 0.032 (10)* | |
H1c11 | 0.818 (2) | 0.6360 (11) | 0.338 (2) | 0.045 (10)* | |
H2c11 | 0.840 (2) | 0.6403 (12) | 0.135 (2) | 0.070 (12)* | |
H1c21 | 1.049 (2) | 0.6013 (12) | 0.157 (2) | 0.057 (11)* | |
H2c21 | 1.042 (2) | 0.5950 (12) | 0.362 (2) | 0.067 (12)* | |
H1c31 | 0.841 (2) | 0.5480 (12) | 0.344 (2) | 0.056 (11)* | |
H2c31 | 0.852 (2) | 0.5475 (11) | 0.137 (2) | 0.051 (11)* | |
H1c41 | 1.058 (2) | 0.5036 (10) | 0.144 (2) | 0.038 (10)* | |
H2c41 | 1.061 (2) | 0.5056 (9) | 0.351 (2) | 0.028 (9)* | |
H1c51 | 0.856 (2) | 0.4533 (12) | 0.343 (3) | 0.074 (15)* | |
H2c51 | 0.985 (3) | 0.4303 (12) | 0.249 (3) | 0.095 (17)* | |
H3c51 | 0.859 (3) | 0.4522 (13) | 0.136 (3) | 0.111 (18)* | |
H1n2 | 0.539 (2) | 0.6849 (8) | 0.320 (2) | 0.025 (8)* | |
H2n2 | 0.421 (3) | 0.7120 (9) | 0.239 (2) | 0.080 (16)* | |
H3n2 | 0.517 (2) | 0.6851 (8) | 0.126 (2) | 0.021 (8)* | |
H1c12 | 0.325 (2) | 0.6390 (9) | 0.152 (2) | 0.021 (8)* | |
H2c12 | 0.342 (2) | 0.6390 (10) | 0.358 (2) | 0.043 (10)* | |
H1c22 | 0.539 (2) | 0.5985 (11) | 0.136 (2) | 0.050 (10)* | |
H2c22 | 0.554 (2) | 0.5989 (11) | 0.342 (2) | 0.043 (10)* | |
H1c32 | 0.351 (2) | 0.5509 (10) | 0.360 (2) | 0.042 (10)* | |
H2c32 | 0.339 (2) | 0.5498 (13) | 0.153 (2) | 0.075 (14)* | |
H1c42 | 0.552 (2) | 0.5040 (10) | 0.143 (2) | 0.041 (10)* | |
H2c42 | 0.562 (2) | 0.5054 (10) | 0.351 (2) | 0.034 (9)* | |
H1c52 | 0.359 (2) | 0.4523 (11) | 0.356 (3) | 0.067 (13)* | |
H2c52 | 0.485 (3) | 0.4295 (11) | 0.250 (3) | 0.074 (14)* | |
H3c52 | 0.351 (2) | 0.4518 (11) | 0.150 (3) | 0.078 (14)* | |
HO11 | 0.104 (3) | 0.3383 (13) | 0.284 (4) | 0.067 (13)* | |
HO41 | 0.250 (4) | 0.2488 (12) | 0.049 (3) | 0.096 (14)* | |
HO12 | 0.603 (3) | 0.3402 (14) | 0.239 (4) | 0.070 (15)* | |
HO22 | 0.857 (5) | 0.2615 (9) | 0.027 (5) | 0.15 (2)* | |
Atomic displacement parameters (Å2) top | U11 | U22 | U33 | U12 | U13 | U23 |
As1 | 0.0251 (2) | 0.02775 (19) | 0.02088 (18) | −0.00111 (16) | 0.00106 (14) | 0.00268 (12) |
O11 | 0.0326 (18) | 0.0381 (14) | 0.0596 (16) | 0.0062 (13) | 0.0076 (13) | 0.0161 (11) |
O21 | 0.0528 (19) | 0.0398 (13) | 0.0226 (11) | −0.0086 (12) | −0.0046 (11) | 0.0059 (8) |
O31 | 0.0278 (15) | 0.0520 (14) | 0.0263 (11) | −0.0058 (12) | 0.0065 (10) | 0.0026 (9) |
O41 | 0.0495 (18) | 0.0381 (13) | 0.0242 (12) | −0.0159 (12) | 0.0048 (11) | −0.0041 (9) |
As2 | 0.0252 (2) | 0.02890 (19) | 0.02063 (18) | −0.00158 (16) | −0.00088 (14) | −0.00086 (12) |
O12 | 0.0326 (18) | 0.0347 (14) | 0.0617 (16) | 0.0022 (13) | −0.0051 (13) | −0.0066 (10) |
O22 | 0.062 (2) | 0.0356 (14) | 0.0235 (12) | −0.0058 (13) | 0.0051 (11) | 0.0005 (9) |
O32 | 0.0276 (15) | 0.0534 (14) | 0.0278 (11) | −0.0065 (12) | −0.0020 (10) | 0.0005 (9) |
O42 | 0.0490 (17) | 0.0334 (12) | 0.0226 (11) | −0.0079 (11) | −0.0041 (10) | 0.0027 (8) |
N1 | 0.028 (2) | 0.0341 (17) | 0.0350 (17) | 0.0041 (15) | 0.0023 (15) | 0.0010 (12) |
C11 | 0.038 (2) | 0.034 (2) | 0.037 (2) | −0.0064 (19) | −0.003 (2) | −0.0010 (15) |
C21 | 0.036 (2) | 0.036 (2) | 0.045 (2) | 0.001 (2) | 0.002 (2) | −0.0003 (16) |
C31 | 0.047 (3) | 0.041 (2) | 0.046 (2) | −0.004 (2) | −0.002 (2) | −0.0032 (17) |
C41 | 0.057 (3) | 0.039 (2) | 0.045 (2) | 0.004 (2) | −0.003 (2) | −0.0006 (17) |
C51 | 0.105 (5) | 0.031 (2) | 0.070 (3) | −0.009 (2) | −0.005 (3) | 0.0051 (19) |
N2 | 0.035 (2) | 0.0341 (17) | 0.0285 (17) | −0.0004 (16) | −0.0011 (15) | 0.0015 (11) |
C12 | 0.029 (2) | 0.035 (2) | 0.038 (2) | 0.0019 (18) | 0.0044 (18) | −0.0009 (15) |
C22 | 0.033 (2) | 0.033 (2) | 0.041 (2) | −0.0012 (19) | −0.0003 (19) | −0.0011 (15) |
C32 | 0.048 (3) | 0.032 (2) | 0.051 (2) | −0.003 (2) | −0.001 (2) | 0.0012 (17) |
C42 | 0.060 (3) | 0.040 (2) | 0.044 (2) | 0.004 (2) | 0.000 (2) | −0.0031 (17) |
C52 | 0.087 (4) | 0.039 (2) | 0.065 (3) | −0.005 (2) | 0.000 (3) | −0.0006 (19) |
Geometric parameters (Å, º) top
As1—O11 | 1.711 (2) | C11—H1c11 | 0.94 (2) |
As1—O21 | 1.6575 (19) | C11—H2c11 | 0.939 (18) |
As1—O31 | 1.659 (2) | C21—H1c21 | 0.95 (2) |
As1—O41 | 1.707 (2) | C21—H2c21 | 0.95 (2) |
As2—O12 | 1.693 (2) | C31—H1c31 | 0.95 (2) |
As2—O22 | 1.710 (2) | C31—H2c31 | 0.950 (18) |
As2—O32 | 1.642 (2) | C41—H1c41 | 0.953 (19) |
As2—O42 | 1.667 (2) | C41—H2c41 | 0.948 (18) |
O11—HO11 | 0.96 (3) | C51—H1c51 | 0.93 (2) |
O41—HO41 | 0.95 (2) | C51—H2c51 | 0.95 (3) |
O12—HO12 | 0.90 (3) | C51—H3c51 | 0.96 (2) |
O22—HO22 | 0.90 (2) | N2—H1n2 | 0.90 (2) |
N1—C11 | 1.483 (4) | N2—H2n2 | 0.87 (2) |
C11—C21 | 1.515 (5) | N2—H3n2 | 0.890 (18) |
C21—C31 | 1.514 (5) | C12—H1c12 | 0.953 (17) |
C31—C41 | 1.526 (5) | C12—H2c12 | 0.941 (17) |
C41—C51 | 1.523 (6) | C22—H1c22 | 0.949 (18) |
N2—C12 | 1.489 (4) | C22—H2c22 | 0.95 (2) |
C12—C22 | 1.505 (5) | C32—H1c32 | 0.949 (19) |
C22—C32 | 1.519 (5) | C32—H2c32 | 0.95 (2) |
C32—C42 | 1.525 (5) | C42—H1c42 | 0.950 (19) |
C42—C52 | 1.523 (6) | C42—H2c42 | 0.95 (2) |
N1—H1n1 | 0.90 (2) | C52—H1c52 | 0.93 (2) |
N1—H2n1 | 0.89 (2) | C52—H2c52 | 0.98 (3) |
N1—H3n1 | 0.88 (2) | C52—H3c52 | 0.97 (2) |
| | | |
O11—As1—O21 | 110.86 (11) | O12—As2—O22 | 105.42 (11) |
O11—As1—O31 | 106.41 (11) | O12—As2—O32 | 107.20 (11) |
O11—As1—O41 | 107.53 (12) | O12—As2—O42 | 111.77 (12) |
O21—As1—O31 | 117.02 (11) | O22—As2—O32 | 112.10 (11) |
O21—As1—O41 | 104.71 (10) | O22—As2—O42 | 106.71 (10) |
O31—As1—O41 | 110.01 (10) | O32—As2—O42 | 113.41 (10) |
Hydrogen-bond geometry (Å, º) top
D—H···A | D—H | D···A | D—H···A |
O11—HO11···O32i | 0.96 (3) | 2.591 (3) | 172 (3) |
O41—HO41···O42ii | 0.95 (2) | 2.489 (3) | 168 (4) |
O12—HO12···O31 | 0.90 (3) | 2.574 (3) | 161 (3) |
O22—HO22···O21iii | 0.90 (3) | 2.550 (3) | 167 (5) |
N1—H3n1···O22iv | 0.88 (2) | 2.969 (4) | 162 (2) |
N1—H1n1···O32v | 0.90 (2) | 2.774 (3) | 168 (2) |
N1—H2n1···O42vi | 0.88 (2) | 2.759 (4) | 160 (2) |
N2—H1n2···O21vii | 0.90 (2) | 2.833 (4) | 162 (2) |
N2—H3n2···O31viii | 0.89 (2) | 2.785 (3) | 161 (2) |
N2—H2n2···O41ix | 0.87 (3) | 2.835 (4) | 157 (2) |
Symmetry codes: (i) x−1, y, z; (ii) x−1/2, −y+1/2, z−1/2; (iii) x+1/2, −y+1/2, z−1/2; (iv) −x+2, −y+1, −z; (v) −x+2, −y+1, −z+1; (vi) −x+3/2, y+1/2, −z+1/2; (vii) −x+1, −y+1, −z+1; (viii) −x+1, −y+1, −z; (ix) −x+1/2, y+1/2, −z+1/2. |
Experimental details
Crystal data |
Chemical formula | C5H11NH3+·H2AsO4− |
Mr | 229.11 |
Crystal system, space group | Monoclinic, P21/n |
Temperature (K) | 290 |
a, b, c (Å) | 9.3380 (3), 27.3950 (8), 7.4570 (8) |
β (°) | 90.568 (1) |
V (Å3) | 1907.5 (1) |
Z | 8 |
Radiation type | Mo Kα |
µ (mm−1) | 3.54 |
Crystal size (mm) | 0.32 × 0.10 × 0.04 |
|
Data collection |
Diffractometer | Nonius KappaCCD diffractometer |
Absorption correction | Integration Gaussian integration (Coppens, 1970) |
Tmin, Tmax | 0.461, 0.868 |
No. of measured, independent and observed [I > 3σ(I)] reflections | 27031, 4366, 2740 |
Rint | 0.053 |
(sin θ/λ)max (Å−1) | 0.649 |
|
Refinement |
R[F2 > 2σ(F2)], wR(F2), S | 0.036, 0.045, 2.02 |
No. of reflections | 4365 |
No. of parameters | 327 |
No. of restraints | 52 |
H-atom treatment | All H-atom parameters refined |
Δρmax, Δρmin (e Å−3) | 0.67, −0.83 |
Selected bond lengths (Å) topAs1—O11 | 1.711 (2) | O22—HO22 | 0.90 (2) |
As1—O21 | 1.6575 (19) | N1—C11 | 1.483 (4) |
As1—O31 | 1.659 (2) | C11—C21 | 1.515 (5) |
As1—O41 | 1.707 (2) | C21—C31 | 1.514 (5) |
As2—O12 | 1.693 (2) | C31—C41 | 1.526 (5) |
As2—O22 | 1.710 (2) | C41—C51 | 1.523 (6) |
As2—O32 | 1.642 (2) | N2—C12 | 1.489 (4) |
As2—O42 | 1.667 (2) | C12—C22 | 1.505 (5) |
O11—HO11 | 0.96 (3) | C22—C32 | 1.519 (5) |
O41—HO41 | 0.95 (2) | C32—C42 | 1.525 (5) |
O12—HO12 | 0.90 (3) | C42—C52 | 1.523 (6) |
Hydrogen-bond geometry (Å, º) top
D—H···A | D—H | D···A | D—H···A |
O11—HO11···O32i | 0.96 (3) | 2.591 (3) | 172 (3) |
O41—HO41···O42ii | 0.95 (2) | 2.489 (3) | 168 (4) |
O12—HO12···O31 | 0.90 (3) | 2.574 (3) | 161 (3) |
O22—HO22···O21iii | 0.90 (3) | 2.550 (3) | 167 (5) |
N1—H3n1···O22iv | 0.88 (2) | 2.969 (4) | 162 (2) |
N1—H1n1···O32v | 0.90 (2) | 2.774 (3) | 168 (2) |
N1—H2n1···O42vi | 0.88 (2) | 2.759 (4) | 160 (2) |
N2—H1n2···O21vii | 0.90 (2) | 2.833 (4) | 162 (2) |
N2—H3n2···O31viii | 0.89 (2) | 2.785 (3) | 161 (2) |
N2—H2n2···O41ix | 0.87 (3) | 2.835 (4) | 157 (2) |
Symmetry codes: (i) x−1, y, z; (ii) x−1/2, −y+1/2, z−1/2; (iii) x+1/2, −y+1/2, z−1/2; (iv) −x+2, −y+1, −z; (v) −x+2, −y+1, −z+1; (vi) −x+3/2, y+1/2, −z+1/2; (vii) −x+1, −y+1, −z+1; (viii) −x+1, −y+1, −z; (ix) −x+1/2, y+1/2, −z+1/2. |
Ferroelasticity and a phase transition in the n-alkylammoniumdihydrogenphosphates (CnADP) and dihydrogenarsenates (CnADA) were discovered by Kroupa & Fuith (1993, 1994). Until now, however, the presence of phase transitions in C5ADA was not investigated. The related dihydrogenphosphate (C2ADP—C10ADP) and dihydrogenarsenate (C6ADA—C8ADA) structures have been studied previously. C2ADP, C3ADP and C4ADP were studied by Kasatani et al. (1998), C3ADP by Fábry et al. (2000a), C4ADP by Fábry et al. (2000b), C5ADP and C6ADP by Kasatani et al. (1999), C5ADP, C6ADP and C9ADP by Fábry et al. (2000), C7ADP and C8ADP by Fábry et al. (1997), and C10ADP by Oliver et al. (1998). C6ADA and C8ADA were determined by Fábry, Kroupa & Císařová (2001), while C7ADA was determined by Fábry, Krupková & Císařová (2001). All the structures are monoclinic (P21/n). The prototypic phases are orthorhombic (P2/b21/n21/a).
The ferroelastic switching is concomitant with the hydrogen jumps of two H atoms within the hydrogen bridges from the donor to the acceptor O atoms: O41—HO41···O42 and O22—HO22···O21.
Each structure contains a pair of symmetry independent anion molecules (dihydrogenphosphates or dihydrogenarsenates) and a pair of symmetry independent n-alkylammonium molecules. The latter ones are almost exactly displaced by (1/2, 0, 0).
It was found that C3ADP and C5ADP belong to a different structure type than C7ADP and C9ADP (Kasatani et al., 1999; Fábry et al. 2000). The preference for the different structure type, either C3ADP or C5ADP, seems to be related to the smaller number of intermolecular contacts between the n-alkylammonium chains in C3ADP and C5ADP (Fábry et al., 2000) than in C7ADP and C9ADP. This means that these chains in C3ADP and C5ADP are more loosely packed than in C7ADP and C9ADP.
Disorder of the anionic molecules was first observed in C7ADP and C9ADP. Even more prominent disorder (the proportion was more than 10°) was observed in C7ADA. This means that until now the disorder was observed only in the compounds which belong to the structure type of C7ADP. This disorder can be envisaged as stacking faults (1/2, 0, 0) or, alternatively, as the co-existence of both structure types, i.e. of C5ADP and of C7ADP. The greater prominence of the disorder in the dihydrogenarsenates is in accordance with its view as co-existence of both structure types of C5ADP and C7ADP since the structure type of C5ADP seems to be supported by a lesser number of contacts between n-alkylammonium chains. The lesser number of contacts between the n-alkylammonium chains in C3ADP and C5ADP implies more space between these molecules. On the other hand in the dihydrogenarsenates the larger size of the anions would cause the n-alkylammonium chains to be more separated, and therefore these chains would have more space around. For these reasons, it may be expected that C5ADA, (I), would be isomorphous with C5ADP and no disorder would exist in it.
The non-existence of the disorder of the dihydrogenarsenate molecules was confirmed. Figs. 1 and 2 depict the studied structure (Burnett \& Johnson, 1996). The structure is isostructural with C5ADP.
The presence of phase transition in C5ADA was also investigated by DSC [Perkin Elmer DSC 7, software PYRIS (1997); 10.7 mg of the powdered capsule were put into an Al-capsule, rate 10 K min-1, temperature range 298–423 K], as well as by observation of the samples in the polarized light during heating and cooling. It was found by the DSC experiments that a phase transition occurs during heating at ~380 K and during cooling at ~371 K. Observation of the sample between crossed polarizers revealed that above 373 K the structure becomes gradually optically homogeneous though it does not turn into liquid. After repeated cooling, the domain pattern develops again. The domains partially recover if the temperature would not exceed 393 K. If the temperature reaches at least 393 K than the cooling results in a more dense domain pattern.