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AMMONIUM HYDROGEN SULPHATE, ITS STRUCTURE AND THE FERROELECTRIC
TRANSITION
R. Nelmes
To cite this version:
R. Nelmes. AMMONIUM HYDROGEN SULPHATE, ITS STRUCTURE AND THE FER- ROELECTRIC TRANSITION. Journal de Physique Colloques, 1972, 33 (C2), pp.C2-85-C2-87.
�10.1051/jphyscol:1972225�. �jpa-00214961�
JOURNAL DE PHYSIQUE Colloque C2, supplkment au no 4, Tome 33, Avril 1972, page C2-85
AMMONIUM HYDROGEN SULPHATE,
ITS STRUCTURE AND THE FERROELECTRIC TRANSITION
R. J. NELMES
Department of Physics, Edinburgh University, The Kings Buildings, Mayfield Road Edinburgh EH9 3JZ, Scotland
R6sumC. - L'affinement par les moindres car& de la structure de NH4HSO 4 (A. H. S.) B la temp&
rature ambiante, indique que cette phase est desordonnke. Le dkpouillement de clichks obtenus par diffraction des rayons X B - 80 O C , a permis de determiner la structure de la phase ferroklectrique dans la projection parallele a b. Nous concluons que la structure ferroklectrique resulte de I'enlkve- ment du desordre dans la structure I'ambiante.
Abstract. - Further refinement of the room-temperature structure of ammonium hydrogen sulphate (A. H. S.) has shown it to be disordered. Photographic X-ray data has been collected at
- 80 OC from which the structure of A. H. S. in its ferroelectric phase has been solved in a b-axis projection. It is concluded that the ferroelectric structure corresponds to an ordering of the room- temperature phase.
The structure of ammonium hydrogen sulphate (A. H. S.) has been determined in its room-temperature non-ferroelectric phase from X-ray and neutron data by Nelmes [I]. In this phase A. H. S. has a pseudo- orthorhombic unit cell (a = 24.66, b = 4.60,
c = 14.82 A,
P
= 89.870) of space group B 2,la. The cell contains sixteen formula units, with two in each asymmetric unit. The published structural parameters (see Table I1 of [I]) show anomalously high values for some thermal parameters of one of the two symmetri- cally inequivalent sulphate groups (see Table I). This effect was thought to be not demonstrably significant.More recently the structure of the equivalent phase of an isomorphous salt, rubidium hydrogen sulphate (R. H. S.), has been independently determined by Ashmore [2]. This structure has parameters closely similar to A. H. S., including the appearance of high thermal parameters in the S 1 group (S(l), 0(1), 0(2), O(3) and O(4) for A. H. S.). The thermal parameters for A. H. S. given in Table I are the mean-square displacements along the crystallographic axes - in order x, y, z for each atom. The absolute values are uncertain because of absorption and other systematic effects in the observed X-ray intensities ; but compa- rison with R. H. S. strongly suggests the appearance of the relatively large values t o be a significant feature.
Using the same data as in [I], except that (( acciden- tally absent )) reflections were now omitted, a model of A. H. S. was refined in which the sulphate ions were treated as rigid units 131, [4]. This and subsequent refinements made use of the techniques of constrained refinement [4] and the full matrix least-squares pro- gram developed by Dr. G. S. Pawley of this Depart- ment. The S 1 group showed pronounced libration about a direction close to L TO^]. The oxygen atoms
Thermal parameters of the ordered room-temperature model [l]
Temperature factor
= exp [- 2 n2(~,,(lza*)2
+
2 U12(ha* kb") -t.-.)I
Uij given in units of 0.001 x
A2.
Uij omitted.i+ j
of S 1 were now separated into two (( half atoms )) by rotation of the group in opposite directions about [102], to give sites equally displaced from the ordered model coordinates [I]. The magnitude of the separation for this disorder model was estimated from the libration parameters. For an unconstrained disorder model the conventional R-factor
fell to 5.8
%
from the 6.9%
found in [I]. Hamilton's R-factor ratio test [5] showed the improvement to be highly significant. The S 1 group is thus found t o be disordered between two sites in the room-temperature phase - to form S 1, comprising S 1 and S 1" (S(l), 0(1), 0(2), O(3) and O(4) ; and S(l)", 0(1)", 0(2)", O(3)" and O(4)") ; the S 2 group (S(2), 0(5), 0(6), O(7)Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1972225
C2-86 R. J. NELMES
and O(8)) shows no indication of disorder within the resolution of the experiment.
The disorder model was first refined omitting the bonding hydrogen atoms, H(9) and H(10) (see [I]). A difference-Fourier map clearly showed both of these, and furthermore showed H(9) split between two sites.
A completely stable least-squares refinement with H(9) disordered was not attained and the accuracy of the hydrogen parameters given in Table I1 is somewhat uncertain. However it remains clear that the hydrogen atoms are ordered onto off-centre sites in the 0-H
...
0 bonds as reported before [I] - a conclusion now supported by Ashmore's results [2].Positional parameters for the disordered room-temperature model
Coordinates are given in
A
referred to orthogonal axes X, Y and Z where Xis parallel to a, Yis parallel to b* and Z is parallel to a x b*.Standard deviations ( x lo3) are given in brackets.
The positional coordinates found for the unconstrain- ed disorder model are given in Table 11. Other models were refined in which various constraints were applied to the shape of the sulphate ions and to their thermal motions. The conclusions reached and a full discussion will be published shortly. Here it is reported only that the occupancies of the two disorder sites were not significantly different.
A. H. S. undergoes a second-order transition to a ferroelectric phase at - 3 OC [6]. The cell dimensions are very similar to those given above. Weissenberg photographs taken at - 700C yielded the values a = 24.48, b = 4.58, c = 14.75
A, B
=i= 900. The space group becomes Ba.A specimen of A. H. S. was cut and then mounted inside a thin-walled quartz bulb as described in [I], [7].
A chromel-alumel thermocouple was sealed in with its junction close to the specimen. (h01) intensity data for CuKoc were recorded with double packs of Ilford Industrial G and B film on a Nonius Integrating Weissenberg camera. The temperature recorded from the thermocouple was - 81 f 5 OC. Intensities were measured on a Joyce-Loebl double-beam microdensito- meter, scaled between films and corrected for the Lp factor. Intensity errors were estimated by repeated measurements, comparison of equivalent reflections, and comparison of values obtained for individual reflections from different films. The measurements were combined to give a single set of 226 independent reflections, of which seventeen were (( accidentally absent
)>.
The b-axis projection of the structure of A. H. S. in its ferroelectric phase was refined quite straight- forwardly from this data. The asymmetric unit contains twenty-four atoms (neglecting hydrogen). Only iso- tropic temperature factors were applied. For the final refinement the (( accidentally absent )) reflections were omitted ; so also were the eight strongest reflections - which earlier refinements had indicated to be affected by extinction. The positional parameters obtained are given in Table 111. The R-factor was 6.3
%.
Positional parameters for the, ferroelectric phase
Coordinates are given in
A
referred to orthogonal axes (see Table 11).Standard deviations ( x lo2) are given in brackets.
Origin taken at S(1).
In the room-temperature phase the sulphate groups S 1, and S 2 are related to S 1; and S 2' respectively by a centre of symmetry. In the ferroelectric phase these four groups become symmetrically independent.
Comparison of Tables I1 and I11 shows that the pro- jected ferroelectric structure is closely similar to a b-axis projection of the room-temperature phase with
AMMONIUM HYDROGEN SULPHATE, ITS STRUCTURE C2-87
S 1, placed entirely in one (S 1) and S 1; in the other (S 1") of the two disordered configurations. (To compare, for example, the coordinates of O(1)' with those of O(1)" (Table 11) substract the X and Z coordi- nates of O(1)" from 12.33 and 7.41
A
respectively.) The relative displacements of the other sulphur and oxygen atoms, and of the ammonium ions, ale small but have been shown to be significant.It is concluded that A. H. S. is disordered in its
room-temperature phase and that the transition to the ferroelectric phase at - 3 OC principally involves an ordering of the S 1, sulphate ions.
The author wishes to thank Prof. W. Cochran and Dr. G. S. Pawley for their interest and generous assistance, and Dr. P. Ashmore for kindly sending his unpublished results. The support of a Science Research Council Research Fellowship is gratefully acknowledged.
References
Tll NELMES (R. J.), Acta Cryst., 1971. B 27, 272. Diffraction Methods. Vol. 4. Eds : W. Home and - - . ,. . .
[2] ASHMORE (P.), Private Communication, 1970. R. Mason. Pergamon Press, 1972.
[5] HAMILTON (W. C.), Acta Cryst., 1965, 18, 502.
[3] SCHOMAKER (V.) and TRUEBLOOD (K. N.), Acfa Cryst., [6] P E ~ I N S K Y (R,), VEDAM (K.), HOSHINO (S.) and
1968, B 24, 63. OKAYA (Y.), Phys. Rev., 1958,111,1508.
[4] PAWLEY (G. S.), Advances in Structure Research by [7] NELMES (R. J.), J. Appl. Cryst., 1970,3,422.
