A 69Ga, 115In NQR study of polytypes of GaS, GaSe and InSe

Solid State Communications,Vo1.39, pp.307-311. Pergamon Press Ltd. 1981. Printed in Great Britain.

0038-1098/81/260307-05$02.00/O

A 6qGa, '151n NQR STUDY OF POLYTYPES OF Gas, GaSe AND InSe T.J. Bastow, I.D. Campbell and H.J. Whitfield Division of Chemical Physics, CSIRO, P.O. Box 160, Clayton, Victoria, Australia 3168.

(Received 11 March 1981 A. G. Chynoweth) Pulsed Fourier transform NQR spectroscopy has been used to study bolvtvnism in the monochalconenidesGas, GaSe and InSe at 77 K. The spectrum of GaS was consistent with a major contribution from the 6 phase but showed evidence of higher polytypes, possibly due to E or y type faulting. The GaSe spectrum is simply interpreted as an equal mixture of E and y types. The four InSe spectra (I = 9/Z for I151n), showed a multiplicity of lines. The sharpness of the NQR lines of all spectra suggests the existence of regions with a hitherto overlooked high degree of order. r--,

A '%a,

,I

~~

"%I NQR Study of Polytypes of Gas, GaSe and InSe

T.J. Bastow, I.D. Campbell and H.J. Whitfield Introduction A number of structural polytypes of the Group III met21 chalcogenides Gas, GaSe and InSe hsve been reported in recent years as the result of both X-ray and electron diffraction studies. These compounds exhibit polytype formation due to variation in the stacking sequences of the basic tightly bound four-fold layer containing metal (M) and chalcogen (X) in the sequence XM-M-X. This basic structural unit is shown in fig. 1. Four rlyt pes of Case have been described, namely 6 , y 8 E 2 2nd 6 394. The R phase, type 2H, has thl spac)egroup P63/mmc (fig.la) ; E, type 2H, has space group P6m2 (Fig. lb); y, type T)R,has the space group R3m (Fig. lb), and 6, type 4H, has the space group P63mc. GaS has the structure of R-GaSe 5g6 as do solid solutions of GaS with GaSe 7 . For GaSe itself, however, the existence o$ the B polytype has recently been discounted , as the evidence for its existence is based solely on X-ray diffraction patterns from powdered material. The most commonly occurring f8"srn of Case iS a mixture of E and y polytypes '_. Furthermore, as the introduction of very low energy glide stacking faults transforms either of these polytypes into each other, any real crystal containing regions of E and y polytypes can be described 2s being of either modificaiion with a large number of stacking faults . The dominant polytype of InSe appears to be the rhombohedral y type 3R 11.12, altpugh a 6 type 2H phase has also been reported . electron diffraction Both X-ray and techniques, however, are of limited use as bulk survey technlaues, since they concentrate on selected, perfect 2nd very small crystals. In

the measurements reported below sample volumes of 1 - 2 cm3 were used. The resulting spectra are basically consistent with the presence of the reported polytypes, but reveal in addition the presence of structures of greater complexity 2nd profusion than previously suspected. An earlier NQR examination of the title compounds I4 using super-regenerative a oscillator spectrometer, yielded spectra which suggested, by their multiplet structure, the presence of polytypes in all three compounds. However an accurate determination of the lineshapes was not possible due to the saturation, distortion and overlap phenomena inseparable from super-regenerative operation. In the present work we report more detailed data on the resonance line shapes 2nd frequencies observed in various specimens of these compounds by pulsed Fourier transform NQR. Experimental The two compounds were prepared by reacting stoichiometric amounts of the elements together in evacuated quartz ampules in a two-zone furnace. Subsequent crystal growth and anneals were carried out in the temperature range gOO95ooc. A single crystal of grown Gas, originally by Dr. E. Mooser by an iodine transport technique, was kindly provided by Professor E.A.C. Lucken.

The NQR spectra were taken with a phase coherent pulsed spectrometer consisting essentially of a Matec 525 gated amplifier and a Matec 615 tunable receiver with a Wavetek 3000 signal generator as the master oscillator. Off resonance free-induction decay traces were digitized by a Datalab 905 transient recorder and signal averaged in an HP 2114B computer. The same computer was used to Fourier transform the data after approximately 1000 traces had been taken. We established the performance of the system at a frequency close to where most of the spectra were recorded by measuring the

307

A 6gGa,

308

*151n

NQR STUDY OF POLYTYPES OF Gas,

GaSe AND InSe

Vol.

69Ga lineshape of the resonance A single = 21.590 MHz at 77 K). (VP recorded of full width at half maximum The Fourier transform program kHz.

39,

No.

in GaI, line was 6v = 1.5 was also

tested on the 54 kHz doublet in CHCl3 at 77 K, and gave a clean two line spectrum at precisely McCalfr;z;encies the reported by Gutowsky and The presented Do]“* is the

spectra for all these compounds are as magnitude spectra, M(v) = [A(v)’ + (where A(v) is the absorption and D(v) dispersion) in order to circumvent the

problems created by frequency dependent phase shifts arising from the large frequency range If the lines are in the transform. spanned Lorentzian then M(v)’ z A(v), and the true absorption line width is in fact the magnitude width at the l/J2 points.

Figure

la

6%

Metal

0

C halcogen

6 phase

coordination

monochalcogenide

of

the

two

layers.

We were concerned to establish that lines from other compounds were not interfering in the 14, A measurements. NOR ?$pectra for CL- atS3 & have been and B type In ?Ser and InrSel reported previously. Furthermore during the course of this work we observed the ‘St 7’Ga resonances in an ordered form of GatSes cs9v~ = 11 .254 MHz at 77 K, 6v =: 30 kHz; single line), which appears on the basis of scaling the NQR coupling constants, to be isostructural with the 16. In no case was there any B form of InzSe 3 overlap between these and the spectra displayed MX compounds . for the corresponding below Because of the complex nature of the spectra, it was checked that the essential features were preserved when the offset frequency was changed As expected, only a on the master oscillator. transform was in the Fourier uniform shift observed, To eliminate the possibilty of spurious piezoelectric) transients affecting the (e.g. data we also established decay was obliterated magnetic field of about Results

that the free induction by the presence of a 200 G.

and Discussion

GaS both lines the common to Three polycrystalline and single crystal specimens are clearly resolved at 77 K (Figs. 2a and 2b). Two additional lines were observed for the single 19.308 MHz is strongest at crystal. The assigned to the B-polytype (Fig. la) which has The other lines, one Ga site in the unit cell. at 19.320 and 19.326 MHz, may be evidence e.g. of so far unreported polytype formation in GaS. An electron diffraction study by Basinski et al. l7 of specimens taken from single crystals grown reveal

Figure

lb

@

Metal

0

Chalcogen

E or y phase coordination monochalcogenide layers.

from the melt appreciable

which the strong areas faulted fault stacking

of

two

and deformed during cleavage areas of faulted crystal in

curvature of the edges of the indicated a relatively high compound as energy of this

The fault type that suggests compared to GaSe. itself most readily is a 60” rotation between two successive layer units yielding E or y It is possible that a systematic stacking. arrangement of such faults with long range coherence is responsible for the additional

2

Vol.

39,

No.

A 6gGa,

2

‘151n

NQR STUDY OF POLYTYPES OF GaS, GaSe AND InSe

309

GaS

Go S polycrystal

9.308

single crystal

19,320 19,326

19,326

\

i; I

I

19.30

9.28 FREQUENCY Figure

2a

I

FREQUENCY

(MHZ)

6gGa spectrum of polycrystalline Gas, (l/2, 312) transition.

Figure

2b

I

19.32

(MHz)

6gGa spectrum

of

of

3/Z)

Gas,

(l/2,

19.34

a single

lines. However, the possibility of a very slight lowering of the conventionally accepted symmetry of the S-phase, leading to an enlarged unit cell with extra Ca site inequivalencies, has been removed, regions, by a recent

at least convergent

Go Se

for submicron beam electron

19.395

diffraction study ’ which has established that the space group of a “good crystal” is in fact P6l/mmc to high accuracy. It is notable that the main three lines appear at the same frequencies, but with the same relative intensities for both specimens. The signal to noise ratio, was sufficiently high that good spectra with only slightly worsened resolution were obtained at 293 K. The only noticeable change from 77 K is that the spectra are shifted bodily to lower frequencies with virtually no change in shape; the main peak now occurs at 19.053 MHz.

:: Gi

E -Z

GaSe for two and

crystal

transition.

i The principal features of the GaSe spectrum our polycrystalline specimen at 77 K are strong doublets at 19.395, 19.404, 19.447 19.458 MHz (Fig. 3). These are assigned to

the E and y polytypes, each of which contributes one doublet. The structures of both E and polytypes distinguishable

are

characterized metal sites within

by the

I

19.35

I

I

I

19.40

19.45

19.50

FREQUENCY

(MHz)

y

two unit

cell. The most immediate difference in the coordination of the chalcogens around the two sites is indicated in Fig. Ib, a geometry which is the same for both E and y phases; differences occur between the phases only in atomic coordination beyond the second layer. Therefore, considering contributions to the electric field gradient up to and including the second layer,

Figure

3

6gGa spectrum of GaSe, (l/2, 3/2)

the frequency differences Ga(2) resonances should

polycrystalline transition.

between the Ga(1) and be equal for both o and

y phases. This equality will be removed when the relatively smaller contributions for subsequent (third, fourth and more distant) layers are considered. With these purely

A 6gGa,

310 geometric reasonable

considerations to deduce from

l151n

NQR STUDY OF POLYTYPES OF Gas,

in mind it seems the spectrum that the

basic frequency difference between a Ga( 1) and Ca(2) resonance in either phase is approximately 50 kHz. This still leaves four possible ways to assign a pair of resonances to each phase but there seems no unambiguous assignment. We note that the yield a patterns

four line and crystal

clear

way

4H polytype

to

make would

an also

spectrum, but X-ray powder morphology support an (e,y)

mixture. Subsidiary weaker lines that were also consistently observed are tentatively ascribed to systematically faulted regions of the crystallites 17. For our (polycrystalline) GaSe sample the signal permit

to noise a spectrum

that at identical

was good enough at 293 K to with resolution comparable to The spectrum was virtually 77 K. in shape and line separation, but

shifted bodily down in frequency by 277 kHz, shift frequency comparable to the 255 kHz observed in GaS. InSe In the polycrystalline sample of were four (ml ++ (m + 1 ( transitions for ‘IsIn (I = 9/2) near 10.5, 21.0, Each transition 42 MHz at 77 K. multiplicity cf a number

can

GaSe AND InSe

The X-ray powder pattern be completely indexed

Vol.

for our specimen equally well for

either the 8 or the y form, but does not distinguish between the two. However, it does set an upper limit of approximatleBly 10% for the possible presence of InsSeT . The well characterized rhombohedral type 3R y-ln.Se 11*12 has two In sites per unit cell as discussed above for GaSe. It therefore seems natural to assign the two most intense lines, marked Al,2 in Fig. 4a, and B~,z in Fig. 4b to the In (1) and In (2) sites in the y-phase. However both the In (1) and In(2) point group symmetries are jm (as is that of the In site in the B-phase) so that r~ = 0. would reouire An (AZ, 82) pairing a small positive n while (Al), (B,) cannot even be fitted in this manner since 2v(A1) < I. accuracy and consistency of our The measurements, fl kHz for the spectra in Figs. 4a and 4b, rules out the possibility of this discrepancy being due to random experimental While exact 1:2 ratios can be found error. among the conviction. ratio for

minor peaks, the assignments lack We stress that the signal-to-noise the spectra in Figs. 4a and 4b were

comparable good, attempt to fit

31.5 and shows a

hexadecapole interaction was unsuccessful and was considered unlikely to succeed at the outset following the null result of a recent high precisionlzearch for hexadecapole interactions - for nuclei with I 1 5/2 in a number by Segel

(l/2, 3/2) and (j/2, 5/2) transitions are shown These spectra, while in Figs. 4a and 4b. certain similar, present superficially difficulties in interpretation, as discussed prevented a satisfactory below, which have analysis.

to

that found for psirs the line

InSe

(3/Bq5/2)

Figure

4a

l151n InSe,

, 21.107 BZ

6,

FREQUENCY

GaSe. with

IO.60

(MHz)

spectrum of polycrystalline (l/2, 3/2) transition.

An a

Spectra taken at 293 K show the of solids. whole group of lines shifted bodily down in frequency, For the (j/2, 5/2) group the shift is 440 kHz with a frequency separation between

20,934,

IO.50

No.

InSe all observed

of lines indicating the existence of polytypes. The spectra for the

IO.40

39,

FREQUENCY Figure

4b

l151n InSe,

(MHz)

spectrum of polycrystalline (3/2, 5/2) transition.

2

Vol.

39,

No.

1151,

A 6gGa,

Z

NQR STUDY OF POLYTYPES

the outermost peaks of the two 177 kHz, corresponding separation being 173 kHz at 77 K. The appearance of the spectra in Figs. 4a and 4b, with the relatively intense and sharply defined outer limits to the multiplets, suggests the possible existence of a quasi-incommensurate periodic lattice distortion in the crystal. However this is admittedly less evident in the ‘IsIn transitions. As yet no electron higher which might confirm such a diffraction study, view, has been made.

Conclusion In general the sharpness of the spectral lines which have not been accounted for suggests the existence of regions in the specimens with 2 high degree of order. These regions appear to have been overlooked or rejected in previous studies as crystallographic apparently disordered or poorly ordered. Furthermore the strength of these lines indicates that they represent an appreciable volume of the crystal, 25% in CaS. e.g. The similarity between the spectra at 77 K and 293 K for all three compounds indicates the

311

OF Gas, GaSe AND InSe

absence of temperature

any phase changes in this range. The dependence of NQR lines generally

is due to the excitation of short-wavelength lattice vibrational modes which rock the electric field gradient axes about the nuclear direction. bodily shift with spin The temperature of the entire NOR spectrum of both GaS and GaSe seems to indicate that the short wavelength part of the phonon spectrum is essentially the same for all polytypes of the This is not unexpected since one compound. part of the phonon contributions to this spectrum X-M-M-X

come layer

mainly from the and are essentially

tightly bound independent 20 . of the weakly bound neighbouring layers Furthermore there is only an 8% difference between the temperature shifts in GaS and GaSe despite a considerable difference in atomic weights of the cations. Further work is in progress to isolate the individual polytypes and to characterize their and structures electron X-ray both by diffraction.

REFERENCES

1.

F. Jellinek 713

2. 3. 4.

5.

K. Schubert, 2. Metallkunde, A. Kuhn, Phys. Stat.

E. D&rre s, 216

A. Chevy Sol. (a),

P. Goodman and H.J. 219,

J.C.J.M. Terhell Phys. Stat. Sol.

9.

J . L . Brebner

11.

12.

J. Rigoult, Cryst., x,

and

M. Kluge,

(1955).

13.

and B. Celustka J. Phys. Chem, Solids , 35,

and 460

R. Chevalier, (1975).

14.

T. J. Eastow J. Mag. Res.,

A. Rimsky,

Z. Anorg.

Chem.,

Acta

z,

15.

Solid

State

J.L.

Brebner 274

Whitfield,

Acta

(a),

and 2,

, S. Jandl Commun., 2, and

Lieth,

R.M.A. Lieth, 529 (1972). and B.M. Powell, 1555 (1973).

E. Mooser,

Phys.

Letters,

(1967).

A. Likforman, D. Car&, B. Bachet, Acta Cryst.,

J. Etienne

m,

1252

and

(1975).

H.J.

Whitfield,

1 (1975).

and 32,

548

D.W. McCall, (1960).

17.

Z.S. Basinski, D.B. Helv. Phys. Acta, ?‘(,

18.

A. Chevy, J. Cryst.

719 (1971).

2,

H.S. Gutowsky J. Chem. Phys.,

Acta

S. Popovic, 287 (1974).

and 20,

A. Kuhn,

T. J . Bestow and H.J. Whitfield, International Symposium on Proc . Second Nuclear Quadrupole Resonance Spectroscopy, Italy, September 1973. Viareggio,

Cryst.,

R.M.A.

and (a),

and A. Rimsky 1916 (1980).

16.

(1980).

J.C.J.M. Terhell Phys. Stat. Sol.

a,

2,

(1955).

8.

10.

2. Naturf.,

3l,

H. Hahn and G. Frank,

_g&, 7.

H. Hahn,

and A. Kuhn, R. Chevalier Cryst., _B31, 2841 (1975). 340

6.

and

(1961).

19.

S.L.

A. Kuhn Growth,

Segel,

38,

Dove and E. Mooser, 373 (1961). and

M.-S.

Martin,

118 (1977).

J. Chem. Phys.,

2434

69,

(1978). 20.

T. J. Wieting and Phonons Reidel,

and M. Schliiter, in Layered Crystal

Dordrecht,

Holland,

“Electrons Structures”, 1979,

p.321.