A new phase transition in LiNH4SO4 : an x-ray study

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A new phase transition in LiNH4SO4 : an x-ray study

T. Simonson, F. Dénoyer, R. Moret

To cite this version:

T. Simonson, F. Dénoyer, R. Moret. A new phase transition in LiNH4SO4 : an x-ray study. Journal

de Physique, 1984, 45 (7), pp.1257-1261. �10.1051/jphys:019840045070125700�. �jpa-00209862�

1257

A new phase transition in LiNH4SO4 : ^an X-ray study

T. Simonson, ^F. Dénoyer ^{and R. Moret}

Laboratoire de Physique ^{des Solides} (*) Université de Paris-Sud, ^Bâtiment 510, ^F-91405 Orsay, ^France

(Reçu ^le 6 janvier 1984, révisé le 7 mars, accepté ^{le 26}

1984)

Résumé. 2014 Une étude _par rayons X de LiNH4SO4 ^{entre 295 K et 10 K montre} l’existence d’une transition de

phase ^{à 11 °C, et}

nouvelle transition entre 27 K et 28 K, qui semble du second ordre. Entre 284 K ^et 28 K, la symétrie ^est P21/c. Entre 27 K et 28 K, il apparaît

surstructure de vecteur d’onde q

(1/2, 1/2, 0), qui ^double

la maille primitive, ^{et donne la} symétrie ^Cc.

Abstract.

X-ray studies of LiNH4SO4 between 295 K and 10 K show

phase transition at 11 °C, ^and

transition between 27 K and 28 K, ^{which seems to} be 2nd order. Between 284 K and 28 K, ^the symmetry ^{is shown}

to be P21/c. Between 27 K and 28 K,

superstructure ^of wave-vector q

(1/2,1/2, 0) appears, doubling ^the pri-

mitive cell, ^and leading ^{to the} symmetry ^Cc.

J. Physique ⁴⁵ (1984) ^{1257-1261 JUILLET} 1984,

Classification

Physics ^Abstracts

61.60

77.80

81.30

1. Introduction.

The room temperature ^{form of} lithium and ammo-

nium sulfate is pseudo-hexagonal ^and closely ^related

to that of tridymite Si02. Its space group ^was deter- mined by ^Dollase [1] ^{to be the} polar, orthorhombic group Pc2, ^{n or} C’,, ^c being ^the macroscopic pseudo- hexagonal ^axis.

Till _now, three phase transitions were known, ^one

at 186.5 OC, ^{one at} 11 °C, ^{and one at room} tempera-

ture at 7 kbars. We shall report ^{the existence of a}

new phase transition at 27 K, ^{which had} probably

been indirectly ^observed already by ^infrared absorp-

tion measurements [2]. ^{The room} temperature phase

has been shown to be ferroelectric [31. ^The phase

below 11 OC has been variously ^{described as} polar

and non-polar. According ^to Kruglik et ^al. [4], ^the

structure is antiferroelectric, ^with space group PI 2,/cl (C). References [31 ^and [5] also find the structure to be non-polar. However, references [2] ^and [61 ^both report detection of either a pyroelectric ^{or a} piezo-

electric signal below 11 °C.

To clarify this, and search for other phase transitions,

we did X-ray experiments ^{between room} temperature and 10 K on LiNH4SO4. ^We shall describe _separa-

tely ^the experiments and results relative ,to the 11 °C and 27 K transitions.

(*) ^Associe

^CNRS.

2. Experimental : ^{the 11} OC transition.

Between 295 K and 95 K, X-ray experiments ^were

done using ^the Burger precession technique, ^which gives ^an undeformed picture ^{of a} plane ^of reciprocal

space.

Monochromatic MoKa radiation (A

^0.709 A)

was selected by ^the (002) reflection of a pyrolithic graphite ^mono chromator. We were supplied ^with pre-oriented single-crystals by ^{X. Gerbaux} (1).

These were cooled by ^{a flow of} temperature-

regulated nitrogen gas. We ^were able thereby ^to

examine temperatures ^{between 295 K and 95 K} with a stability ^of approximately ^{± 0.5 OC.}

Differential thermal analysis ^was also done between 480 K and 120 K. It revealed no phase transitions other than those at 186.5 OC and 11 "C.

Figure ^{la shows} highly overexposed pictures ^of

the reciprocal planes (hko), (hol) ^and (okl) ^{above the} 11°C transition. We can verify the extinction rules of Pc2l ^{n. We} also observe diffuse intensity, ^and particularly around the points (h

± 3, k

0,

1

± 1/5) ^and (h = 0, arbitrary k, ^{1 =} 2 p + I ^± 1/5).

Microdensitometer measurements show the maxi-

mum of these diffuse spots ^to be located at G + qo, qo

0.18 C* (G ^{is the nearest} reciprocal ^lattice

(’) Laboratoire Infrarouge Lointain, Universite de

Nancy I, C.O. 140, ⁵⁴⁰³⁷ Nancy Cedex, ^France.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphys:019840045070125700

1258

Fig. 1.

Precession photographs ^of reciprocal planes ^(hol), (okl), (hko) ^{at room} temperature (ax ^{and at various} temperatures below 11 °C (b) : (okl) ^{at 3} OC, (hol) ^{at - 36 °C and} (hko) ^{at - 20 OC.} Monochromatic Moka radiation

used (A

^0.709 A).

The small dots at (h

^± 3, 1 = 0) ^and (h

0, odd) ⁱⁿ (a) ^{are due to} contamination by the first harmonic A/2 ^{of the beam.}

vector). Their intrinsic width is approximately c*/5 by a* /3.

Figure lb shows the same planes below the transi- tion. The c glide plane ^has disappeared ^{and the} (0, k, ^odd I ) points ^{are now} visible; ^what’s mo.re,

new points ^have appeared in between these, showing

that the unit cell has doubled along ^{the c direction.}

The (k

0, ^odd 1) reflections are invisible, ^{as well}

as the (h ^{= 1 =} 0, ^odd k) ^{ones. Therefore the} 2, ^axis

has been preserved ^{and a c} glide plane ^has appeared,

1259

orthogonal ^{to it. The} symmetry group is thus non-

polar : successive pseudo-hexagonal layers ^bear opposite dipole ^moments along the b axis. Nor is the group ^a sub-group ^{of the} high-temperature ^{one :}

the transition has no order parameter. ^{The exact} space group is easily ^{found to be} PI 21/cl, ⁱⁿ agree- ment with Kruglik et ^al.

3. Discussion.

Since the transition is without an order parameter,

we cannot relate the fluctuations responsible ^{for the}

diffuse spots ⁱⁿ figure ^{1 to a} simple transition mecha- nism. They may be critical fluctuations relative to a

transition which could be induced by ^{an external} parameter, like mechanical strain. Or they may be due to a transition to a phase ^so limited in tempera-

ture as to be impossible ^{for us to} detect. We have not yet studied the variations of the diffuse intensity

with temperature.

In the latter _case, despite the absence of dynamical data, ^one might ^associate tentatively the fluctuations with a vibrational mode Wj(qo) ^{of very low frequency.}

This mode would thus pilot ^a displacive ^transition

to a modulated phase ^of wave-vector not far from qo.

Several compounds ^{of the} tridymite family present such a transition : RbLiS04 [7] ^and K2SeO4 [8], ^for

instance. A group analysis ^{of the} dispersion ^branches

at the Brillouin ^zone boundary points shows that they

c* c*

are degenerate ^at k1 = 2 ^{+ yb* and k -} 2 ^{+ za*.}

Therefore they ^{are not} required by symmetry ^{to have}

zero slopes, and the Lifschitz criterion has no reason to be verified. A continuous transition is thus theo-

retically possible ^{with a} wave-vector of the form ac* + yb* ^{or ac* + za*.}

4. Low temperature experiments.

Preliminary photos ^were taken with the fixed-film

fixed-crystal method, ^{with a} cryocooler, ^between

100 K and 20 K. Between 30 K and 20 K, additional intense reflections appeared ^{on the films, and we were}

led to do a more elaborate experiment, using ^a Displex cryocooler ^{on a 3-circle} X-ray diffractometer in the normal beam geometry. ^The crystal ^{was mounted in}

a vacuum, ^{on a} copper support.

The temperature ^{was measured} by ^a chromel-Au-Fe

thermocouple ^{attached to the} sample holder. The actual crystal temperatures ^{are estimated to differ} from the temperature readings by ^{less than 1} degree.

Copper ^{radiation was used} (A

^1.54 A ) ^{and we were}

able to observe the planes k

0, k

^{1 and k}

1,

down to 10 K.

At 100 K, the space group is still PI 21/cl. ^At

10 K, however, superstructure reflectians of wave

vector q

(1/2,1/2, 0) ^have appeared. ^Their intensity

is much smaller than that of basic reflections : at 10 K, I(1/2,1/2, - 7)

² 290, 1(3/2, 3/2, - 9)

²¹⁰

and I(l, 1, - 7)

^{12 120} (arbitrary units). ^{The new} primitive translations are a + b, ^{a - b and c.}

The c glide plane ^{is still} present, ^{since the} (k=O,

odd I) reflections are invisible. The cell is thus mono-

clinic, and the conventional basic cell is quadruple : (2a, 2b, c), the C face being ^centred (Fig. 2). ^{We found}

the monoclinic angle ^{to be} fl

^90.180.

In order to locate the critical temperature, ^{we did} series of ^scans around the reciprocal points (1/2, 1/2,

-

7) ^and (3/2, 3/2, - 9) ^with varying temperature.

The intensities and widths of these reflections are

shown as functions of temperature ⁱⁿ figure ^{3. The}

transition takes place ^{between 27 K and 28 K. The} intensity ^at (1/2, 1/2, - 7) goes continuously ^{to zero,}

and we found the same intensities and widths on

raising ^{and on} lowering ^the temperature. ^{This indi-} cates, within the precision ^{of our} measurements, ^a second-order transition.

Our geometry didn’t enable us to examine the 21

extinctions. However, the only subgroup ^of P2,/c

which explains ^{the c} extinctions and the centring ^of

the C face is Cl cl or Cf : ^the disappearance ^{of the}

b translation has done _away with the 21 ^{axis. We}

cannot observe the symmetry ^{of the H} atoms, nearly

invisible to X-rays. Except ^for this, successive pseudo- hexagonal layers ^{are still} exchanged by ^{the c} glide plane, ^{so that} there should be no resultant electric moment along the b axis.

Aside from the transition discussed, ^a striking

feature of figure ^{3 is the} sharp decrease in intensity ^at (1/2, 1/2, - 7) ^and (3/2, 3/2, - 9) ^{between 13.2 K}

and 16.4 K, ^{as well as the} slight, ^but distinct, ^broaden- ing of the latter reflection. These features require

further and more specific investigation.

Fig. ^2.

^The change ^of primitive translations at the 27 K transition. A pseudo-hexagonal layer is shown. The tetra- hedra centres

occupied by ^sulfer (small tetrahedra) ^and

lithium (large tetrahedra) ^atoms alternately. ^{The room} temperature parameters

^9.14 A, b

^5.28 A,

c ⁼

8.79 A( 1 ). ^{The low} temperature ^{cell is C} centred. 0 and +

represent vertices which are

longer equivalent below 27 K.

1260

Fig. ^3.

(2) represents ^the intensity ^{of the} (1/2,1/2, - 7) ^reflexion

function of temperature.

(3) represents ^the intensity ^{of the} (3/2, 3/2, - 9) ^reflexion.

The arbitrary scale used is the number of counts per second.

a* b*

(1) ^and (4)

^{the widths} of (1/2, 1/2, - 7) ^for

along q = 2 + 2 ^{and along c*} respectively, ^for increasing temperature.

2 2

((4)’ corresponds ^to diminishing temperature.)

(5) is the width of (3/2, 3/2, - 9) ^scanned along q. The unit widths

I a* ; It* I = ₂ ^{1.38 A-’ and I c* = 0.73} Å -1 respecti- vely.

The widths measured at 10 K

simply the instrumental resolution.

5. Conclusion.

We have done an X-ray analysis ^{of the} symmetry ^of

LiNH4SO4 between 295 K and 10 K, ^{as well as}

thermal analysis between 480 K and 120 K. This

analysis confirms the centrosymmetrical ^character

of LiNH4SO4 between 284 K and 28 K,. ^{and its}

space group P2,/c. The 11 OC transition is of the first order, ^{and has no order} parameter. ^{We have} also observed important fluctuations at room tempe- rature, in the form of diffuse spots ^of wave-vector qo

0.18 C*, ^which may be associated with a criti- cal value of a parameter other than temperature, ^like mechanical strain or electric field. The system may thus present ^{a modulated} phase under the right

conditions.

At a temperature ^{between 27 K and 28} K, ^we

observed a transition to a phase ^of symmetry ^{Cl cl} and quadruple basic cell. The transition corresponds

to the appearance of a superstructure ^of wave-vector q

(1/2, 1/2, 0).

However, microscopic studies, ^or complete ^struc-

tural studies are required ^{if we wish to understand}

the intimate mechanism of both high (11°C) ^and

low temperature transitions.

Aeknowlectgments.

We are particularly grateful ^{to X.} Gerbaux and A. Hadni for drawing ^{our attention to the} problem

of LiNH4SO4 ^{and for} supplying ^us with abundant

samples, ^{as well as for} interesting discussions. We

also owe thanks ^{to M.} Lambert for much fruitful

discussion.

1261

References

[1] ^{DOLLASE, W. A., Acta} Cryst. ^{B 25} (1969) ^2298.

[2] ^GERBAUX, X., MANGIN, J., HADNI, A., PERRIN, D., TRAN, C. D., Ferroelectrics 40 (1982) ^53.

[3] MITSUI, T., OKA, T., SHIROISHI, Y., TAKASHIGE, M., IION, K., J. Phys. ^Soc. Japan 39, ³ (1975) ^845.

[4] KRUGLIK, A., SIMONOV, M., ALEKSANDROV, K., Sov.

Phys. Cryst. 23, ³ (1978) ^274.

[5] ALEKSANDROVA, I., CHEKMASOVA, T., Ferroelectrics 20

(1978) 283.

[6] POULET, H., MATHIEU, ^J. P., Solid State Commun. 21

(1977) 421.

[7] MASHYAMA, H., HASEBA, K., TANISAKI, S., SHOROISHI, Y., SAWADA, S., J. Phys. ^Soc. Japan 47, ⁴ (1979) ^1198.

[8] IIZUMI, M., AXE, J. D., SHIRANE, G., SHIMAOKA, K.,

Phys. ^{Rev. B} 15, ⁹ (1976) ^4392.