New high-pressure polymorphs in sodium halides

I Phw C-hem. Sohds Vol 44 No 2. PP. 135-140. 1983 Printed m Great Brilsin

O!!-3697!83/0!0132~2$03.00~0 Pergamon Precr Lrd

NEW HIGH-PRESSURE POLYMORPHS IN SODIUM HALIDES TAKEHIKOYAGI,TOSHIHIRO SUZUKIand SYUN-ITIAKIMOTO Institute for Solid State Physics, University of Tokyo, Roppongi, Minato-ku, Tokyo 106,Japan (Received 22 February 1982;accepted in revised form 5 May 1982) Abstract-Static compression experiments to 50 GPa, employing X-ray diffraction through a diamond cell, were made pn NaF, NaBr and NaI. NaF was found to transform from its initial Bl (NaCI-type) to the B2 (CsCl-type) structure at 272 1 GPa on the ruby fluorescence scale with a volume change at the transition of -8.9%. New high-pressure polymorphs showing birefringence under microscope were found both for NaBr and NaI at 29 t 1GPa and 26 2 1 GPa, respectively. X-ray diffraction patterns of these high-pressure polymorphs could not be indexed as the B2 structure. The GeS-type structure (the distorted NaCl structure) was tentatively assigned to the high-pressure polymorph of NaI. _.

INTRODUCTION Alkali halides are the simplest and most typical ionic crystals, and a number of theoretical and experimental investigations have been conducted on their thermodynamical stability, phase diagrams, and equations of state. Only two structure types, Bl (the NaCl structure) and B2 (the CsCl structure) are known in alkali halides at pressure and temperature conditions so far studied. At ambient condition, among twenty alkali halides, only three Cs-halides, CsCl, CsBr, and CsI, crystallize in the B2 structure and all others crystallize in the Bl structure. At elevated pressure, CsF and all the Rbhalides transform from the Bl phase to the B2 phase in the pressure range OS-3 GPa. All the K-halides transform from the Bl phase to the B2 phase at pressures between 2 and 4 GPa. In Na-halides, NaCl undergoes the B l-B2 transformation at approximately 30 GPa[ 11. Shock compression on NaF indicates discontinuous volume reduction at pressures around 24 GPa[2]. Although this discontinuity is most likely to be the Bl-B2 transformation, no X-ray work has been done and the real nature is not known yet. In NaBr and NaI, and in all the Li-halides, no phase transformations are known yet. In the present experiments, high pressure behavior of three Na-halides, NaF, NaBr, and NaI, is studied up to 50 GPa using high pressure X-ray diffraction technique at room temperature. The purpose of the present study is: (I) to study the nature of the discontinuous volume reduction of NaF at around 24GPa, and (2) to find possible phase transformations in NaBr and NaI up to 50 GPa at room temperature. EXPERIMENTAL.

Diamond-anvil type high pressure apparatus combined with powder diffraction camera and energy dispersive X-ray diffraction system[3] was used in the present study. The diamond cell is a modified version of the lever and spring type developed by Mao and Be11[4],and it is capable of generating very high pressures up to at least 120GPa for X-ray work[5]. Powdered specimen of Na halides is placed in a gasket hole (250 pm in dia.) and compressed directly between two diamonds with a flat

surface of 450 pm in dia. Small chips of ruby (-10 pm size) are mixed with the specimen as pressure markers. Pressure values were determined by a ruby fluorescence method[6] using the pressure scale proposed by Mao et al.[7]. This pressure scale gives approximately 5% higher value than a linear extrapolation of the scale proposed by Piermarini et al.[8] in a pressure range around 50GPa. The difference is reduced to less than 3% below 30 GPa. Pressures in a very small area (as small as 10 pm in dia.) can be measured in our system, and the pressure distribution in the specimen area can be measured easily. Since alkali halides are soft materials, the pressure gradient across the X-rayed area was less than 5% of the peak pressure in a center of the specimen, although no pressure transmitting fluid was used in the present experiments. An X-ray from a rotating anode type high power MO target (55 kV and 160mA) was collimated to a narrow beam, approximately 140 Km in diameter, by a double pinhole collimator, and the beam was directed on a center of the specimen. In a film technique, Zr-filtered MO Ka! radiation was used, and diffracted X-rays were recorded on two separate films simultaneously. Two films are located approximately 22.7 mm and 57.3 mm apart from the specimen. A flat film closer to the specimen is used to see the whole diffraction pattern in a short exposure time, and a large curved film is used to measure the d-values accurately. Since a distance between two films is fixed and well known, sample to film distance can be calculated accurately without using internal standard material. Accuracy of the d-values determined in the present study is 20.2%. Typical exposure time was 212hr. In an energy dispersive technique, a conical slit combined with pure germanium solid-state detector[3] was used, and exposure time was only 20-60min. In this technique, transformation to a new phase can be detected very rapidly but the resolution of the diffraction pattern is inferior to film technique. Besides, relative intensity of the diffraction peaks obtained in this technique is complicated, and this produces some difficulty in comparing the observed profile with the calculated in135

T.YACIrf al.

136

tensities. For these reasons the analysis of a new phase was performed using film technique. The Na-halide samples used in this study were commercial reagent-grade powder of 99.5% purity provided from Wako Pure Chemical Industries. NaF sample was ground in an agate mortar and placed in a hole of a gasket. NaBr and NaI form hydrates very rapidly by absorbing water vapor from the air. In order to prevent the formation of hydrates, these samples were fired at 500°C for an hour in the air and then placed in a gasket hole while they were hot. Grain size of these materials was large compared with NaF, and spotty lines were observed at the beginning of compression. When pressure was elevated, however, samples were crushed into small grains and smooth diffraction lines were observed. RFSULTS

(a) NaF Sodium fluoride was found to transform from the BI structure to the B2 structure at 27.0 2 l.OGPa in the increasing pressure cycle (Fig. I). The volume change accompanied with the transition was 8.9%. These results are in good agreement with shock compression data[2]. Least squares fitting of the second-order Birch-Murnaghan equation of state to the volume compression data of the Bl phase gives the following values for the isothermal bulk modulus and its pressure derivative: K,, = 47.8 GPa and K:) = 4.47. In order to represent the compression behavior of the B2 phase completely. determination of three parameters, Ko, K;, and a virtual volume at one bar, Vo, are required. The accuracy of the present data, however, is not good enough to determine all these three parameters simultaneously, and Kh of the B? phase was assumed to be equal to that of the Bl phase. Results are as follows: K,, = 70.9 GPa

and

Vo(B2)/ V,,(B 1) = 0.859.

Fig. 2. Photomicrograph indicating phase transformation of Nal. Central dark region indicates high-pressure phases. Transformation pressure was determined to be approximately %GPa based on the shift of R, line of the ruby chips on the rim of the dark region.

Volume compression data obtained experiment is compared in Fig. I obtained before by high pressure X-ray by a shock compression experiment[2]. are in substantial agreement with each

by the present with the results studies [9-l I] and All these results other.

(b) NaBr and Nal When pressure was elevated above 25 GPa, transformation to new phases were observed clearly under microscope in both NaBr and NaI (Fig. 2). From the observation of Becke line, these new high-pressure phases were found to have higher refractive index than that of the corresponding Bl phase. It was also found that these new phases show birefringence. This indicates that the crystal symmetry is not cubic, and consequently the B? structure is ruled out for the new high-pressure phase

1

Pressure/GPo Fig.

I. Isothermal

compression

of NaF

at room temperature

New high-pressure polymorphs in sodium halides

137

r

Pressure vs d-values of the low and high pressure phases obtained by X-ray measurements are summarized in Figs. 3 and 4. It is clear from these figures that the new phases are stable up to at least 50 GPa. Transition pressures measured in the increasing and decreasing pressure cycle are listed in Table 1. The transitions are rapid and reversible at room temperature. No quenching of the high pressure phase to the ambient condition was successful. X-ray diffraction patterns of NaBr and NaI were analyzed, and possible unit cells were picked out by a computer program developed by Takemura et al. [12]. The details of this analysis are discussed in a later section. It should be emphasized here that the X-ray diffraction patterns of the high pressure phases of NaBr and NaI cannot be explained by the CsCl-type structure.

I

NaBr

3t-

DISCUSSION

(a) Volume change accompanied with the Bl-B2 transformations All the pressure-induced phase transformations so far known in alkali halides are from the Bl structure to the B2 structure. It is also known that some of the Cs-halides with the B2 structure transform into the Bl structure at high temperature. Not any structures other than the Bl or B2 structure were known to exist in alkali halides. From this point of view, the behavior of NaBr and NaI is quite different from other alkali halides. This strange behavior of NaBr and NaI can be understood by considering the ionic radius ratio of constituent ions and the packing of the structure. When the rigid-sphere ion model is adopted, the unit cell dimensions of the Bl and B2 structures can be calculated as a function of ionic radius ratio of constituent ions. In Fig. 5, cell dimensions of the Bl and B2 phase, which are normalized by ionic radius of anions, are plotted as a function of ionic radius ratio of cation to anion, RA/Rx. The density ratio of the Bl and B2 phase calculated from the unit cells are also shown. It is clear from this figure that when RA/Rx is larger than 0.73, the

IO

30

NaI

. I.

!

“9 -075

l

(2201

20

.

l

m

f3lil * -.-* I2221

15

I

.

1

0

IO

+-

P

UP

'down

-

PrFssure/GPa

Fig. 4. Change in observed d-spacings with pressure for NaI.

NaF

NaBr

NaI

(GPa)

27.Otl.O

29,0*1.0

26.Oel.O

(@a)

21.0+1.0

23.5el.O

22.5tl.O

Pressure

I 50

Pressure/GPa

Table 1. Transition pressure in Na-halides Transition

40

Fig. 3. Change in observed d-spacings with pressure for NaBr.

B2 phase is 23% denser than the Bl phase. When RA/Rx

becomes smaller than 0.73, however, the difference of density becomes smaller with decreasing ionic radius ratio and finally the Bl phase becomes denser than the B2 phase. The critical value of RnlRx, where the density of the Bl and B2 phase becomes equal, is 0.59 in this rigid-sphere ion model, and hence the B2 phase is not necessarily denser than the Bl phase. This can be understood by considering the coordination number and the packing of ions. In the Bl and B2 structure the coordination number of cations is six and eight, respec-

20

T. YACI

I

4@cr

BI(I

0‘ I’

20’

0

et al.

,, ’

IO) /’

,A’

_/,”_

1

Y ~mm_i____

@J\\ ’ ” ____ ----

@(‘OO)

_

~~~~~_____

+.-.-_‘--

_ ----

,lo

---

A

0

15 073 RA/Rx

Fig. 5. Cell dimensions and volumes of the Bi and B2 structure calculated from the rigid-sphere ion model. Cell dimensions are normalized by ionic radius of anions. When ionic radius ratio. R*/Rx is smaller than 0.41. cell dimension of the Bl structure is regulated by contact of ions in the (I IO)direction. Similarly, cell dimension of the B? structure is regulated by contact of ions in the (100) direction, when RJR., is smaller than 0.73. Note that the B2 structure is not necessarily denser than the Bl structure.

tively. Geometrical consideration tells us that when R,JRx is smaller than 0.73, eight-fold coordination cannot be a close packing structure. The lower critical value for six-fold coordination is 0.41. Therefore, when Ra/Rx is smaller than these critical values, it is natural that the structure with the higher coordination number is not stable. In Fig. 6, ionic radius ratio at the transition vs volume change, AV/V associated with the Bl-B2 phase transformation in ten alkali halides is shown. A similar plot has already been reported by Demarest et a1.[13] using ionic radius ratio at one bar. In the present plot, ionic radius ratio at the transition is calculated from the ionic radius at one bar[l4] and the compressibility of individual ions in alkali halides[l5]. Data for the values of A V/ V at the B l-B2 transition are the same as those used by Demarest et al. except for NaF and NaCI. For NaF the present determination of 8.9% was used, and for NaCl the revised value of 4.7%[ 161 was adopted. All these results clearly indicate that AV/V at the Bl-B2 transformation decreases with decreasing ionic radius ratio of cation to anion. Demarest et af.[13] also successfully interpreted this relation based on an ionic model with power law repulsion term.

NaCl has the smallest value of ionic radius ratio among many alkali halides known to undergo the B l-B2 phase transformation. The radius ratio of NaBr and NaI

010

> 3 a

I

.NaF

Nol 0

1 04

Fig. 6. Ionic

i

OK:

NaBr

I 06

__L_

08

80

I?

HA/RX radius ratio, R,~/Rx vs volume change at the Bl-B? phase transformation in .Jkali halides.

Newhigh-pressurepolymorphsin sodiumhalides

139

at the transition are estimated to be 0.57 and 0.51, conditions for the computer search of the unit cell are as follows: (1) discrepancy of d-values
NaF (30GPa)

NaCl (31 GPa)

NaBr (35GPa)

NaI

L__L

132GPaIl 3.5

L 3.0

I 25

, 2.0

d/it Fig, 7. X-Raydevotion patterns of the ~~-pressure phase of Na-aides.

I 1.5

140 Table 2. Observed

351

h

k

and calculated d-values for NaI at 32 GPa 1

110 0

21

111 13

25

NIB type

0

d

ohs(')

d

3.369

3.366

1.226

3.226

2.613

2.612

1.469

2.471

i

iI

2.121

2.122

0

0

L

2.071

2.071

1

5

0

1.782

1.782

-

3.562(2);

=

10.288(4)i

=

4.143(1)X 151.78Cl3)i3

REFERENCES

C/Cl Fig. 8. Axial ratio plot, b/a vs c/a, for AX compounds with orthorhombic unit cell. Solid circles are possible unit cells for the high-pressure phase of Nal. A candidate unit cell shownin Table 2 is represented by an open circle. al.[22] and Kafalas and Mariano[23], respectively. Although the nature of the bond in these chalcogenides is quite different from that of alkali halides, it seems reasonable to conclude that the density of NaI increases 5-7% by the transition from the NaCl type to the GeS type structure. As shown in Fig. 8, in the present analyses, many unit cells, which can explain the observed lines equally well, are found in an area where b/a = 2.7-2.9 and c/a = l.l1.3. Although the volumes in these cells are similar, indexing of the peaks are different from solution to solution. It is difficult to determine the space group from these analyses alone. Detailed structure analyses including intensity calculation is in progress. Acknowledgements-The authors are grateful to Prof. J. C. Jamieson for helpful comments and discussions, and to Dr. K. Takemura for providing the computer program used for the present analyses. This research was partially supported by Grantin-Aid, 56740155and 56460034from the Ministry of Education, Science and Culture, Japan.

Bassett W. A., Takahashi T.. Mao H. K. and Weaver J. S.. J. Appl. Phys. 39, 319 (1968). Carter W. J., High Temp.-High Pressures 5, 313 (1973). Yagi T. and Akimoto S., High Pressure Research in Geophysics (Edited by S. Akimoto and M. H. Manghnani), p. 81. Center Acad. Publ. Japan, Tokyo (1982). Mao H. K. and Bell P. M.. Carnegie Inst. Wash. Yearb. 74, 402 (1975). Yagi T., Suzuki T. and Akimoto S., High Temp.-High Pressures, to be submitted (1982). Barnett J. D.. Block S. and Piermarini G. J.. Reo. Sci. Instrum. 44, I (1973). Mao H. K., Bell P. M., Shaner J. W. and Steinberg D. J., J. Appl. Phys. 49, 3276(1978). Piermarini G. J., Block S., Barnett J. D. and Forman R. A., J. Appl. Phys. 46, 2774(1975). Spieglan M. and Jamieson J. C.. High Temp.-High Pressures 6,479 (1974). Sato Y., In High-Pressure Research: Applications in Geophysics (Edited by M. H. Manghnani and S. Akimoto). p. 307. Academic Press, New York (1977). Il. Yagi T., Carnegie Inst. Wash. Yearbook 76, 528 (1977). 12. Takemura K., Minomura S.. Shimomura 0. and Fujii Y.. Phys. Rev. Lett. 45. 1881(1980). 13. Demarest H. H. Jr.. Cassell C. R. and Jamieson J. C.. J. Phys. Chem. Solids 39, I21I (1978). 14. Shannon R. D. and Prewitt C. T., Acta Cryst. B25,925 (1%9). 15. Ida Y., Phys. Earth Planet. Inter. 13, 97 (1976). 16. Liu L. and Bassett W. A.. .I. Appl. Phys. 44, 1475(1973). 17. Sato Y. and Jeanloz R., J. Geophys. Res. 86, 11773(1981). 18. Jeanloz R., Ahrens T. J., Mao H. K. and Bell P. M., Science 206, 829 (1979). 19. Jeanloz R. and Ahrens T. J.. Geophps. 1. R. Astr. Sot. 62,505 (1980).

20. Wyckoff R. W. G., Crystal Structures, 2nd Edn, Vol. 1, p. 85. Interscience, New York (1%3). 21. Bassett W. A., Takahashi T. and Stook P. W., Reo. Sci. Instrum. 38, 37 (1967). 22. Kabalkina S. S., Serbryanaya N. R. and Vereshchagin L. F.. Sou. Phys.-Solid St. 10, 574 (1%8). 23. Kafalas J. A. and Mariano A. N.. Science 143, 952 (1964).