Physica B 156 & 157 (1089) 118-120

North-Holland. Amsterdam

NEUTRON DIFFRACTION REFINEMENT AND HIGH RESOLUTION X-RAY STUDY OF CRYSTAL STRUCTURE OF LiND,SO, (DLAS)

P. FISCHER’, I. SOSNOWSKA’ and T. WROBLEWSKI’ ‘Labor fiir Neutronenstreuung, ETH Ziirich, CH-5303 Wiirenlingen, Switzerland ‘Institut fiir Festkdrperforschung, KFA Jiilich, D-51 70 Jiilich, Fed. Rep. Germuny and Institute of Experimental Physics. Warsaw University*, PL-MI-681 Warsaw, Poland

‘DESY, HASYLAB, D-2000 Hamburg, Fed. Rep. Germany

The crystal structure of polycrystalline DLAS at room temperature was investigated using neutron and high resolution

(Ad/d = 8 x 10 -“) synchrotron radiation diffraction. Maxima which are expected to be single lines are found to be split.

1. Introduction

The room temperature form of lithium am- monium sulfide, when it crystallized above 4O”C, is pseudo-hexagonal and closely related to that of tridymite SiO,. Its space group was deter- mined by Dollase [l] to be the polar, ortho- rhombic group Pc2,n. When crystallization takes place at lo”C, the c*-modification of LAS with the space group P2,ca appears [2].

At atmospheric pressure LAS has three phase transitions: at 459.5 K, 284 K, and at 27 K [3]. It is known [4] that DLAS also exhibits a sequence of phase transitions similar to that of LAS, with the transition temperatures shifted by several degrees from those of LAS.

In order to get information concerning the crystal structure of DLAS and its phase trans- formations we investigated polycrystalline DLAS using neutron diffraction and high resolution synchrotron radiation.

2. Experimental

Polycrystalline samples of LAS and DLAS were obtained by powdering monocrystals in a helium atmosphere.

The samples for neutron diffraction studies were placed in a helium atmosphere in a cylindri- cal aluminium container. In the synchrotron

* Partially supported by project CPBP 01.12.

radiation experiment polycrystalline materials were placed on a silicon single crystal ((711) surface) using a high vacuum grease.

With the help of the multidetector diffrac- tometer DMC placed at the Saphir reactor, at Wiirenlingen, and by using wavelength A = 2.329 A, neutron diffraction patterns of poly- crystalline DLAS were obtained at T = 20 K, 240 K and 295 K, for the three previously re- ported phases. The data were collected for val- ues of the scattering angle 28 in the range 3-130 degrees. Recently neutron diffraction data, using wavelength A = 1.7067 A at T = 100 K and 323 K, were also obtained.

Using synchrotron radiation and the high res- olution diffractometer (Ad/d = 8 x lo-’ [5]) we obtained high resolution X-ray diffraction pat- terns of DLAS and LAS at RT. Data were collected for scattering angles 28 from 12.6” to 50”, using wavelength A = 1.4358 A.

3. Results and interpretation

Fig. 1 shows observed neutron diffraction pat- terns of DLAS at 323 K (A = 1.7067 A), along with the pattern calculated neglecting the weak additional peaks possibly due to small amounts of a-phase. Data were analyzed by the Rietveld profile method [6,7]. Profile analysis, excluding background points, based on the space group Pc2,n proposed by Dollase [l] and applied by Hildmann [2], gives a intensity reliability factor

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P. Fischer et al. I Crystal structure of LiND,SO, (DLAS) 119

LiND,SO,, 323 K, 1.7067 A

25.

20.

? 0 c

2, 15. .;

: E

E 10. L 3

z”

5.

0. -l-

0 2

1 :, :I L

- obs

____- talc

__ diff

60 80

P-Theta (Degrees)

0.

-5.

? 0 c

-10. 3 -”

E

-15.

-20.

Fig. 1. Calculated and observed neutron diffraction patterns of DLAS at 323 K (A = 1.7067A). Peaks probably coming from alpha phase are shadowed.

R=16% and lattice parameters: a= 9.1310(15) A, b = 5.2786(9) A and c=

8.7710(8) A. Parameters obtained from the fit are summarized in Table I. Atomic positions in LAS given by Hildmann [2] for DLAS lead to an R-factor of 36%. The extra maxima observed in neutron diffraction are present also in our high

Table I Crystal structure of DLAS at 323 K.

Space group Pc2,n.

a = 9.1310(15) A, b = 5.2786(9) A, c = 8.7710(8) A.

Atom x Y z B (A’)

s 0.082(5) 0.0000 0.196(l) 1.3(8) 01 0.087(2) 0.074(7) 0.038(2) 3.2(2) 02 0.961(4) -0.062(l) 0.257(4) 3.2(2) 03 0.037(3) 0.330(9) 0.256(3) 3.2(2) 04 0.233(3) -0.011(8) 0.238(3) 3.2(2) Dl 0.374(3) 0.588(9) 0.124(4) 9.9(6) D2 0.346(3) 0.841(8) -0.008(S) 9.9(6) D3 0.185(5) 0.502(8) 0.000(5) 9.9(6) D4 0.311(4) 0.545( 1) -0.087(4) 9.9(6) N 0.274(2) 0.629(7) -0.002(3) 4.0(3) Li 0.561(6) 0.156(2) 0.235(9) 3 (1)

R ,NT = 0.16, R,, = 0.19, Rerp = 0.07.

resolution X-ray pattern. Some of them can be ascribed to the RT o-phase of DLAS for the lattice constants given for LAS by Hildmann [2]. Nevertheless it is very unlikely that our single crystal, which was powdered, contained two dif- ferent phases. The extra maxima remain also at 323 K, therefore they cannot be ascribed to the low temperature phase of DLAS [2]. The in- fluence of the extra peaks on the final crystal parameters is difficult to estimate.

Fig. 2 shows a part of a synchrotron radiation diffraction pattern. Due to the very good quality of our powder the line widths are very close to those one can expect from the theoretical resolu- tion of the high resolution X-ray diffractometer. The h01 reflections are split (see e.g. 101 reflec- tion in fig. 2) while Ok1 reflections show no splitting (see e.g. 012 reflection in fig. 2), which is inconsistent with orthorhombic symmetry [l, 21. The LAS high resolution X-ray pattern also shows the splitting of the h01 reflections. Therefore we can not attribute the splittings to inhomogenities caused by nonuniform deuter- ation of our sample. A large preferred orienta-

120 P. Fischer et al. / Crystal structure of LiND,SO, (DLAS)

n ; 30

r LITHIUM AMMONIUM SULFIDE (DMS). SYNCHR.-DESY

14 16 16 20

SCATTERING ANGLf 29 IDEGREESI

A .,

22 24

Fig. 2. Section of synchrotron radiation diffraction pattern of DLAS at RT. Maxima probably coming from alpha phase are

shadowed.

tion of the grains is also unlikely as our sample was rotated during the data collection.

Therefore we can conclude that the room tem- perature phase of DLAS is certainly more com- plicated than was previously believed and needs further studies.

References

PI

[31

]41

[51

Fl 171

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B.O. Hildmann. Ph.D. Thesis, Technical University

Aachen, Fed. Rep. Germany, 1980.

T. Simonson, F. Denoyer and R. Moret, J. Physique 45

(1984) 1257.

V.I. Torgashev, Yu. Yuzyuk, F. Smutny and M. Polom-

ska, Phys. Stat. Solidi 135 (1986) 93.

T. Wroblewski, J. Ihringer and J. Maichele, Nucl. Instr.

Meth. A 266 (1988) 664.

H.M. Rietveld, J. Appl. Cryst. 2 (1969) 65.

A.W. Hewat, Harwell Report AERE-R7350 (1973).