Solid State Sciences 4 (2002) 1077–1081www.elsevier.com/locate/ssscie

Antifluorite to Ni2In-type phase transition in K2S at high pressures

Angel Vegasa,∗, Andrzej Grzechnikb, Michael Hanflandc, Claus Mühleb, Martin Jansenb

a Instituto de Química Física “Rocasolano”, CSIC, Serrano 119, 28006 Madrid, Spainb Max-Planck Institut für Festkörperforschung, Heisenbergstrasse 1, 70569 Stuttgart, Germany

c European Synchrotron Radiation Facility, B.P. 220, 38000 Grenoble, France

Received 29 April 2002; accepted 6 May 2002

Abstract

The high-pressure behaviour of dipotassium sulfide K2S with the antifluorite structure (Fm̄3m, Z = 4) has been studied up to 23.0 GPaby in situ synchrotron angle-dispersive X-ray powder diffraction in a diamond anvil cell at room temperature. There occurs a transformationinto a distorted Ni2In-type structure (Pmma,Z = 4) at about 6 GPa upon compression. A mixture of several phases is observed at lowerpressures. Unlike in the case of pressure-induced modifications of Li2S and Na2S, none of them is of the anticotunnite-type (Pnma,Z = 4).The Ni2In-related structure of K2S is compared with the cationic arrays found in different polymorphs of K2SO3 and K2SO4. 2002Éditions scientifiques et médicales Elsevier SAS. All rights reserved.

Keywords: High pressure; X-ray powder diffraction; Crystal structure; Dipotassium sulfide

1. Introduction

Dipotassium sulfide, K2S, first synthesized by Zintl etal. [1] and by West [2], has the antifluorite structure (Fm3̄m,Z = 4) at ambient conditions. Pressure-induced phase tran-sitions in the fluorite compounds into the PbCl2 cotunnitestructure (Pnma,Z = 4) were first reported for CaF2, SrF2,BaF2, PbF2 and EuF2 [3,4]. The orthorhombic structuresof CaF2 and BaF2 were refined from X-ray data and neu-tron diffraction data by Gerward et al. [5] and by Legeret al. [6], respectively. Subsequently, the fluorite–cotunnitetransition was found to take place in several dihalogenidesand dioxides [7–9]. Some of the dihalogenides further trans-form into the “post” cotunnite (P21/a, Z = 8) or Ni2In(P63/mmc, Z = 2) structures. The sequence of pressureinduced transitions Fm̄3m→Pnma→P21/a or Fm̄3m→Pnma→P63/mmc involves an increase of the cation co-ordination numbers from 8 (fluorite), 9 (cotunnite) and10 (“post” cotunnite) to 11 (Ni2In).

We have recently reported two high-pressure studies onLi2S [10] and Na2S antifluorites [11]. In the case of Li2Sup to about 20 GPa, only the antifluorite→anticotunnitetransition is observed at 12 Gpa [10]. In the pressure range

* Correspondence and reprints.E-mail addresses: avegas@iqfr.csic.es (A. Vegas),

m.jansen@fkf.mpg.de (M. Jansen).

to 22 GPa, a sequence of structural transformations alongthe path antifluorite→anticotunnite→Ni2In-type occurs inNa2S at pressures of about 7 and 16 GPa, respectively [11].In this paper, we report on the high-pressure behaviour ofK2S studied by in situ angle-dispersive X-ray diffraction ina diamond anvil cell (DAC) at pressures up to 23 GPa atroom temperature.

2. Experimental

K2S was synthesized from the elements, by heating themin stoichiometric amounts in a quartz ampoule at 623 Kfor 88 hours. The obtained white powder was identified tobe pure K2S using X-ray powder diffraction. The diagramwas recorded with a STOE diffractometer in the 2θ rangeof 10–110◦ using monochromatic CuKα1 radiation. A least-squares refinement of the 2θ values of 20 reflexions led tothe unit cell parametera = 7.40482(9) Å.

Angle-dispersive X-ray powder diffractograms were mea-sured on the ID9 beamline at the European Synchrotron Ra-diation Facility. Monochromatic radiation atλ = 0.41417 Åwas used for pattern collection on image plates. The im-ages were integrated using the program FIT2D [12] to yieldintensity versus 2θ diagrams. The instrumental resolution(minimum full-width at half height of diffraction peaks) was0.03◦. To improve powder averaging, the DAC was rotated

1293-2558/02/$ – see front matter 2002 Éditions scientifiques et médicales Elsevier SAS. All rights reserved.PII: S1293-2558(02 )01360-2

1078 A. Vegas et al. / Solid State Sciences 4 (2002) 1077–1081

Fig. 1. Selected X-ray powder diffraction patterns of K2S at differentpressures. Backgrounds are subtracted. The intensity of the pattern for thesample decompressed from 23 GPa is multiplied by 4.

by ±3◦. The ruby luminescence method [13] was used forpressure calibration. The K2S fine powder was always han-dled in glove boxes. The sensitivity of K2S precluded anyuse of a pressure medium since there is no fully hydrosta-tic medium that could be loaded together with such a sam-ple into a DAC. Two series of measurements were carriedout. In the first one, the pressure was slowly increased upto 10.2 GPa (see Fig. 1). In the second one, the pressure wasincreased at larger intervals up to 23.0 GPa and then the sam-ple was decompressed down to atmospheric conditions.

3. Results

Diffraction diagrams measured at different pressures areshown in Fig. 1. At about 0.6 GPa, the lines corresponding tothe antifluorite structure have almost disappeared, indicatingthe existence of a phase transition at near atmospheric pres-sure. In the pressure range 6.4 to 23 GPa upon compression,the diagrams correspond to a unique phase which remainsstable on decompression down to 4.4 GPa. As seen from thepattern of the sample recovered to ambient conditions, K2Sdoes not fully transform back to the antifluorite phase andthe X-ray pattern includes peaks due to the high-pressurepolymorph.

Fig. 2. Observed, calculated and difference X-ray powder diffraction pro-files for K2S (Pmma,Z = 4). Vertical markers indicate Bragg reflections.TheRwp, Rp, andR(F2) residuals, whose definitions are given in the GSAS

manual [15], are 0.57%, 0.49% and 14.6%, respectively. The residualsRwpandRp are calculated only for the Bragg contribution to the diffraction pat-terns.

Attempts to index the lines of the diagrams upon com-pression up to about 6 GPa and of the diagram for the decom-pressed sample from 23 GPa were unsuccessful, suggestingthat they in fact correspond to a mixture of different poly-morphs. It should be noted, however, that the anticotunnitephase (Pnam,Z = 4), occurring for Li2S and Na2S at highpressures, is not present. It is quite possible that the onsetof structural changes in this material is already induced bygrinding of the sample during preparations prior to its load-ing into a diamond anvil cell.

The pattern for the sample at 4.4 GPa upon decompres-sion (Fig. 2) could be indexed by means of the programDICVOL [14] on the basis of an orthorhombic cell witha = 6.529(2), b = 5.0940(2), c = 8.772(2) Å (M17 = 5.1,F17 = 6.9). The volumeV = 291.74 Å3 indicates that thenumber of K2S formula units in the unit cell isZ = 4. Thisdiagram is in fact very similar to that of the Ni2In-typephase in Na2S (P63/mmc,Z = 2) [11]. This observation to-gether with the fact that the lattice parameters are relatedasc ≈ b

√3 suggest that the new phase is an orthorhombic

distortion of the hexagonal Ni2In structure. The systematicabsences are consistent with the space group Pmma whichis related to P63/mmc by the group–subgroup relationship,P63/mmc→Cmcm→Pmma. Accordingly, a set of atomicparameters can be derived from the ideal Ni2In structure us-ing the above relationship.

The powder diffraction diagram of K2S at 4.4 GPa upondecompression (Fig. 2) was refined with the Rietveld methodusing the program GSAS [15]. The background was fittedwith the shifted Chebyshev function (10 points) [16] andnot refined. The isotropicUi/U∗

e 100 thermal parameters forall the atoms were fixed at 0.025. The refined parameterswere: the fractional atomic coordinates, Stephens profilefunction with terms to account for the reflection anisotropic

A. Vegas et al. / Solid State Sciences 4 (2002) 1077–1081 1079

Table 1Final atomic coordinates for the Ni2In-type phase of K2S upon decompres-sion at 4.4 GPa

Atom x y z

K(1) 0 0 0K(2) 0 1/2 1/2K(3) 1/4 0 0.343(5)K(4) 1/4 1/2 0.843(5)S(1) 1/4 1/2 0.159(6)S(2) 1/4 0 0.659(6)

Fig. 3. The Ni2In-type structure of K2S upon decompression at 4.4 GPa.The structure is projected alongb and shows the walls of trigonal prisms ofK atoms which are occupied by S atoms. Alternate walls are shifted byb/2.The atoms are labelled as in Table 1.

broadening [17], an overall scaling factor, March–Dollasepreferred orientation correction (the 0 1 0 plane) and thecell parameters. The final atomic coordinates are given inTable 1 and the interatomic distances are listed in Table 2.The structure is represented in Fig. 3. From the refinedcoordinates (space group Pmma), it can be noticed that thefour independent K atoms, on one hand, and the two Satoms, on the other, are related by a lattice translation of(b/2+ c/2), in a way that the structure can also be describedin the supergroup Amma (standard setting Cmcm). However,the existence of very weak reflections at low angles (Fig. 2),which are not compatible with Amma space group, indicatesthat although the structure with nearly the A centered cell israther Pmma.

4. Discussion

The Ni2In-type structure of K2S in a projection onto the(a, b) plane is shown in Fig. 3. It can be described as straightchains of K atom triangular prisms. The prisms are sharinglateral edges and form walls parallel to the(a, b) plane. Allatoms in the structure are situated aty = 0 and 1

2, so that

Table 2Interatomic distances (in Å) in the Ni2In-type phase of K2S at 4.4 GPa

K–K prisms

K(1)i–K(1)iii 3.265(1)

K(1)ii –K(1)iv 3.265(1)

K(3)i–K(1)i 3.42(4)

K(3)ii –K(1)ii 3.42(4)

K(3)i–K(1)iii 3.42(4)

K(3)ii –K(1)iv 3.42(4)

K(2)i–K(2)iii 3.265(1)

K(2)ii –K(2)iv 3.265(1)

K(4)i–K(2)i 3.42(4)

K(4)ii –K(2)ii 3.42(4)

K(4)i–K(2)iii 3.42(4)

K(4)ii –K(2)iv 3.42(4)

K–K interprisms

K(3)i–K(2)i 3.33(2)

–K(2)ii 3.33(2)

–K(2)iii 3.33(2)

–K(2)iv 3.33(2)

K(4)i–K(1)x 3.32(2)

–K(1)xiv 3.32(2)

–K(1)xi 3.32(2)

–K(1)xv 3.32(2)

S–K

S(1)–K(1)i 3.33(2)

–K(1)ii 3.33(2)

–K(1)iii 3.33(2)

–K(1)iv 3.33(2)

–K(2)i 3.41(5)

–K(2)iii 3.41(5)

–K(3)i 3.01(4)

–K(3)ii 3.01(4)

–K(4)v 2.77(7)

–K(4)vi 3.265(1)

–K(4)vi 3.265(1)

S(2)–K(2)i 3.33(2)

–K(2)viii 3.33(2)

–K(2)ix 3.33(2)

–K(2)iii 3.33(2)

–K(1)x 3.41(5)

–K(1)xi 3.41(5)

–K(4)i 3.02(4)

–K(4)viii 3.02(4)

–K(3)i 2.77(7)

–K(3)xii 3.265(1)

–K(3)xiii 3.265(1)

Symmetry code:i x, y, z; ii x, 1+ y, z; iii 1

2 − x, y, z; iv 12 − x,y + 1, z; v x,

y, z − 1; vi −x, 1− y, 1− z; vii 1− x, 1− y, 1− z; viii x, y − 1, z;ix 1

2 − x, y − 1, z; x x, y, z + 1; xi 12 − x, y, z + 1; xii −x, −y,

1− z; xiii 1− x, −y, 1− z; xiv x, y + 1, z + 1; xv 12 − x, y + 1, z + 1.

1080 A. Vegas et al. / Solid State Sciences 4 (2002) 1077–1081

adjacent walls are shifted byb/2. The S atoms are lodgedinto these trigonal prisms and are coordinated by eleven Katoms, six of them forming the prisms and with the otherfive situated at the samey coordinate as that of the S atoms,forming a pentagon around it.

The K–K distances are distributed rather uniformly, withvalues ranging from 3.26 to 3.42 Å in the prisms and withvalues of 3.33 Å between the atoms of prisms belonging toadjacent chains (Table 2). The S atoms are not situated atthe centre of the prisms but are shifted towards the K(3) andK(4) atoms (Fig. 3), resulting in four distances of 3.33 Åwith the K(1) and K(2) atoms, i.e., with those at the(0 0 1)and (0 0 2) planes, respectively, and in two distances of3.01 Å with the K(3) and K(4) atoms. These deviations arenecessary to occur to avoid too short distances with the K(4)and K(3) atoms of adjacent chains (2.77 Å), which, in fact,are the shortest distances in the structure. In Na2S with theideal Ni2In structure (P63/mmc), these three distances areequal to 2.557 Å [11].

The results of this study compare well with those ob-tained for the homologous sulfides Li2S and Na2S. Follow-ing the general trend, the phase transitions upon compressionoccur at lower pressures with increasing the atomic numberwithin the group. Thus, the Ni2In-type structure does not ap-pear in Li2S up to 20.8 GPa [10]. It appears at 16 GPa inNa2S [11] and is present at 0.6 GPa in K2S. On the otherhand, the anticotunnite structure in Cs2S occurs already atambient pressure.

An unexpected anomaly observed in dipotassium sulfideis that the anticotunnite structure has not been observed inthe pressure range studied here. If the behaviour of K2S weresimilar to both lithium and sodium sulfides, the anticotunnitestructure would be expected to occur at pressures lowerthan 0.6 GPa. However, K2S might behave anomalouslyand follow the inverse sequence antifluorite→Ni2In→anticotunnite. It should be recalled that such a behaviourtakes place in Ni2Si [18], which adopts the anticotunnitestructure at ambient conditions and transforms to the Ni2In-type structure at high temperature (just contrary to the effectof high pressure).

Another interesting aspect of this study is the comparisonof the HP phase of K2S with the cation array in the corre-sponding oxides K2SO3 and K2SO4, similarly to Na2S andNa2SO4 [11]. At ambient conditions, K2SO4 is orthorhom-bic (Pnma) and its cation array is of the anticotunnite-typeas shown by O’Keeffe and Hyde [19]. O’Keeffe and Hydealso pointed out how antifluorite K2S, following the behav-iour of the other fluorite-type compounds, might transformunder pressure into the anticotunnite-type structure to attainthe same cation array as that of the ambient-pressure phaseof K2SO4 [19]. At high temperatures of about 1000 K, potas-sium sulphate undergoes a phase transition to a hexagonalpolymorph (P63/mmc) [20]. The cation array of this newphase is of the Ni2In-type [19] (Fig. 4), just like the arrange-ment found in the HP phase of K2S described here.

Fig. 4. The crystal structure of the high-temperature phase of K2SO4(P63/mmc, Z = 2) projected along thea axis. The K2S subarray(Ni2In-type) has been outlined by drawing the K–K contacts forming thetrigonal prisms where the S atoms are inserted (compare with Fig. 3). Adja-cent chains of trigonal prisms are shifted by half the projection axis. Large,medium and small circles represent K, S and O atoms, respectively. Thefigure shows the disorder observed in the O atoms.

Fig. 5. The crystal structure of K2SO3 (P3̄m1,Z = 2) projected along thea axis. TheK2S subarray (very close to the Ni2In-type) has been outlinedby drawing the K–K contacts forming the distorted trigonal prisms. Notethat the S atoms are shifted 0.5 Å from the centre of the prisms (comparewith Figs. 3 and 4). Large, medium and small circles represent K, S and Oatoms, respectively.

For K2SO3, a unique phase at ambient conditions hasbeen reported [21]. It is a trigonal structure (P3̄, Z = 2)with three crystallographically independent K atoms, K(1) at(0,0,0), K(2) at (0,0,1/2) and K(3) at(1/3,2/3,0.6746).The S atom is situated at(1/3,2/3,0.1835). This structureis a distortion of the Ni2In-type (Fig. 5). The distortion isproduced by a displacement (approximately by 0.5 Å) of theK(3) and S atoms along thez axis, in such a way that theS atoms are no longer at the centre of the irregular trigonalprisms depicted in Fig. 5. In the ideal Ni2In-type structure,they are situated atz = 3/4 andz = 1/4, respectively.

These results can be interpreted according to the modelreported earlier [22] establishing a formal equivalence be-tween oxidation and pressure. Our experiments describedhere have failed to detect the anticotunnite-type phase forK2S. However, if this phase does exist at higher pressures,following the path antifluorite→Ni2In→anticotunnite, thestructural relations could be explained in the following way:the insertion of four oxygens per K2S formula to form

A. Vegas et al. / Solid State Sciences 4 (2002) 1077–1081 1081

K2SO4 would produce an internal pressure which is suffi-cient to stabilize the higher pressure phase of the alloy. Whenthis phase is heated, which is equivalent to a release of the in-ternal pressure, the lower pressure phase of the alloy (Ni2In-type) is formed, as it occurs in HT-K2SO4.

These arguments can also be applied to K2SO3. It seemsthat the insertion of only three O atoms does not producethe pressure which is needed to stabilize the Ni2In-typestructure and the cations arrange in a way which is veryclose to it. It is quite possible that the application of asmall external pressure to K2SO3 could lead to the formationof a cationic array of the Ni2In-type. Such an experimentwould be desirable. The results discussed here provide newexamples of how cation arrays in the oxides are intimatelyrelated to the structures of their corresponding alloys [22].

Supplementary material

Supplementary material has been sent to Fachinforma-tionszentrum Karlsruhe, Abt. PROKA, D-76344 Eggenstein-Leopoldshafen, Germany, as supplementary material No.SUP 412535 (5 pages) and can be obtained by contacting theFIZ (quoting the article details and the corresponding SUPnumber).

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