J. MATER. CHEM., 1991, 1(4), 555-558 Synthesis and Crystal Structures of the Layered I-Ill4 Zintl Phases, K,ln,X,, where X=As, Sb Teresa L. T. Birdwhistell," Cheryl L. Klein," Tammy Jeffries/' Edwin D. Stevensb and Charles J. O'Connor*b a Department of Chemistry, Xa vier University of Louisiana, New Orleans, Louisiana 70125,USA Department of Chemistry, University of New Orleans, New Orleans, Louisiana 70148, USA Ternary Zintl phase materials of the formula K,ln,X,, where X=As or Sb, have been prepared following a high- temperature procedure. The crystal structure of K,In,As, has been determined at low temperature and consists of [In,As:-], sheets of ca. 5 A thickness insulated by a layer of potassium ions of ca. 3 A thickness. The crystal structure of K41n,Sbs has been determined at room temperature and is isomorphous with the K,ln,Sb, compound.~=4,For K,I~,As,, a= 14.323(3) A, 6=7.106(2) A, c= 15.795(3) A, p=90.30(2)", space group P~,/c, p,=4.44 g cm-3 at T= lOO(5) K, R= 0.035 for 2688 unique observed reflections. For K,ln,Sb,, a= 15.282(5)A, b= 7.544(1) A, c= 16.788(3) A, /?=90.52(2)", space group P2,/c, Z=4, p,=4.62 g cmP3 at T=299 K, R=0.042 for 3001 unique observed reflections. Keywords: Zinti phase; Crystal structure; indium; Antimony; Arsenic; Layered structure Zintl materials are 'valence' compounds: the metals and metalloids that comprise the material will transfer or share electrons in order to achieve an octet in the outermost electron shell.' This often results in the formation of covalently bonded polyatomic anionic units, referred to as Zintl anions.The valence character of Zintl materials is often expressed as the 8-N rule (i.e., N covalent bonds are formed to complete an octet of electrons in the valence shell).' The octet valence shell of the Zintl phase arises when the more electropositive metals (ie. alkali and alkaline-earth metals) are allowed to react with the less electropositive metals and metalloids (i.e. the post-transition main-group metals). Binary Zintl alloys are usually prepared by heating a direct combination of the elements. For example, the Zintl material KSn may be prepared by mixing molten potassium with molten tin to give the KSn powder.2 Similar synthetic tech- niques produce KPb,3 K51n8,4 KSb,' K3Bi,6 KTl,7 K5Ga8,8 and others.These binary Zintl phases may be isolated and identified by powder diffraction prior to subsequent reaction. In recent years there has been a rebirth of interest in Zintl- phase materials with numerous reports of ternary materials from Corbett and co-worker~,~ von Schnering and co-work- ers," Schafer and co-workers," Cordier and co-workers,'2 Eisenmann and co-workers,13 Nespar and co-workers,14 Hau- shalter and co-~orkers,'~ among others. We have undertaken the synthesis of a I-III-V ternary Zintl phase material as a preliminary step in the investigation of new synthetic routes to the preparation of III-V-type semiconductors and semiconducting devices. The reactivity of Zintl phase materials affords the possibility of solution-phase deposition of semiconducting films and perhaps the layering of metal-semiconductor deposition.16 Several I-III-V Zintl phase materials have already appeared in the literature including KGaSb2,17 K3A12As3,18 Na7A12Sb5,19 Na2A12Sb3,20 K2A12Sb3,20 and K3Ga3A~4.2' We report here on the preparation and structural charac- terization of two additional Zintl-phase materials that have layered structures composed of In :X layers alternating with K layers.Experimental Synthesis. The same synthetic procedure was used for the preparation of K41n4As, and K41n4Sb6. Therefore, only the synthesis of K4In4AS6 is described. K41n4As6 was prepared by adding K (1.00 g, 25.6 mmol), In (0.97 g, 8.5 mmol), and As (1.27 g, 17.0 mmol) to a quartz tube that was then sealed under a vacuum.The sample was heated at 750 "C in a tube furnace for 10 h, then cooled to ambient temperature over a 40 h period. A metallic-grey crystalline product was obtained and could be isolated in ca. 80% yield. Crystal Data.? K41n4As6, M = 1065.2, monoclinic, a = 14.323(3) A, b=7.106(2)A, c= 15.795A, p =90.30(2)", U = 1607.6(7)A3 at lOO(5) K (by least-squares refinement on diffractometer setting angles for 25 automatically centred reflections in the range 54<28/" <67, A =0.71073 A), space group P2Jc (No. 14), Z=4, p,=4.44 g ~m-~. Extremely air- sensitive metallic-grey crystal of irregular shape and approxi- mate dimensions 0.15 mm x 0.25 mm x 0.50 mm was separated from melt and mounted under dodecane oil, then rapidly transferred to the diffractometer and cooled to 100 K in a stream of N2 gas [p(MoKa)= 188.2 cm-'1.K4In4Sb6, M = 1346.2, monoclinic, a = 15.288(5)A, b= 7.544( 1) A, c = 16.790(3)A, p =90.54(2)", at 296 K (by least- squares refinement on diffractometer setting angles for 25 automatically centred reflectipns in the range 28 <20/" d37, 1=0.71073 A), space group P2Jc (No. 14), Z=4, pc= 4.62 g cm- '. Air-sensitive metallic-grey crystal of irregular shape and approximate dimensions 0.22 mm x 0.34 mm x 0.1 1 mm was separated from melt in an inert atmosphere and sealed in a glass capillary tube [p(Mo-Ka)= 137.3 cm-'1. Data Collection and Processing. K41n4AS6, CAD4 diffractometer, 0-28 scan mode with scan width = 1.00" +0.35" tan 8, scan speed 0.95-6.8" min-', graphite-monochromated Mo-Ka radiation; temperature maintained to within +lo as measured by a thermocouple in N2 gas stream, 8709 reflections measured (2 <28/"<50, fh, fk, & E), 30 16 unique [merging R =0.032 after corrections for decay (linear, 1 S%) and absorption (empirical, max., min.relative transmission factors =1 .O, 0.24)], giving 2688 reflections with I>341). K41n4Sb6, CAD4 diffractometer, 0-28 scan mode with scan t Supplementary data available from the University of Bonn: see Information for Authors, J. Muter. Chem, 1991, Issue 1. K41n4Sb6,CAD4 diffractometer, 0-28 scan mode with scan width =0.90" + 0.35" tan 8, scan speed 1.4-6.8" min- ', graph-ite-monochromated Mo-Kcr radiation; 7624 reflections meas- ured (2 < 20/" < 50, h, & k, l), 3679 unique [merging R = 0.014 after corrections for decay (linear, 0.8%) and absorption (empirical, max., min.relative transmission factors = 1.O, 0.21)], giving 300 1 reflections with I > 341). Structure Analysis and Refinement. K41n4As6, direct methods yielded positions of all In, As, and K atoms. Full-matrix least- squares refinement on F magnitudes with anisotropic thermal parameters. The weighting scheme used w = l/a2(Fo)where a(F2)= [azs+ (0.02F2)2]'12,with ocsfrom counting statistics. Final R and R' values are 0.035, 0.047. All programs from the CAD4-SDP system22 and scattering factor data from ref. 23. K4In4Sb6, starting coordinates taken from the structure of K41n4As,.Full-matrix least-squares refinement of F magni-tudes with anisotropic thermal parameters and an isotropic extinction parameter. The weighting scheme used w = l/02(Fo) where a(F2)= [ozS+ (0.04F2)2]'I2. Final R and R' values are 0.042 and 0.063. Resistivity Measurements. The linear four-probe methodz4 was used on pressed pellets of the material. The current was supplied by a Keithley Model 224 programmable source and the voltage drop across sample measured with a Keithley model 18 1 digital nanovoltmeter. Results and Discussion The crystal structures of K41n4As6 and K41n4Sb, are found to be isomorphous with the previously reported structures of Na4A1,Sb620 and K4A14Sb6.20 The structure is described in terms of the K41n,&6 compound. The structure of K41n4AS6 consists of potassium ions and [In4As614- alternating in the c direction.The structure of the [In4As614- layer is quite different from that of the well known semiconductor InAs The latter has the zinc blende structure, with the In and As atoms alternating in six-membered rings in an infinite three-dimensional array. When n J. MATER. CHEM., 1991, VOL. 1 the structure is truncated and restricted to two dimensions, smaller, more highly strained ring systems are formed. The asymmetric unit of K41n4AS6 contains a polyhedron com-posed of edge-sharing four-, five-, and six-membered rings (see Fig. 1). In InAs all of the atoms are tetrahedrally coordi- nated.In K41n4AS6, each of the In atoms is four-co-ordinate, but the As atoms are only three-co-ordinate. In addition, two of the six As atoms in the asymmetric unit are bonded to each other in what may formally be viewed as an As$-fragment. The average In-As bond distance of 2.67(4)A is only slightly longer than the 2.62 8, distance observed in InAs. The As- As bond distance of 2.492( 1) A is only slightly longer than the sum of the covalent radii for As, 2.44 A. The [In4As614- fragments are linked together to form two- dimensional, covalently bonded sheets which run perpendicu- lar to c. These are insulated from one another by layers of K+ ions. The InAs sheets are not planar but 'rippled', and the K+ ions sit in the channels created by this rippling effect Fig.1 ORTEP diagram of the asymmetric unit of K,In,As, n Fig. 2 Structural diagram showing the layered nature of the indium-arsenide sheets insulated by potassium sheets J. MATER. CHEM., 1991, VOL. 1 (see Fig. 2). The thickness of the InAs layers is ca. 5 A and they are separated by ca. 3 A. In K4In4Sb6, the average In-Sb bond distance is 2.86(4) 8, compared with the distance of 2.81 8, in the structure of InSb. The Sb-Sb bond distance is 2.866(1)& compared with 2.892( 1) A in Na4A14Sb620 and 2.877( 1) A in K4A14$b6.20 The reactivity of K4In4AS6 requires handling of the sub- stance in an argon atmosphere. Attempts to dissolve the material in polar solvents without decomposition have been unsuccessful. The material decomposes in H20 and is insol- uble in ethylenediamine, liquid ammonia, and organic sol- vents.Preliminary data indicate that K4In4AS6 will dissolve in molten salt solution (e.g. NaCl at 750 "C) and we are investigating the reactivity of K4In4AS6 in this solvent system. The room-temperature resistivity measurements of both materials were unsuccessful because the specific resistivity was beyond our detection limits (p>lo7 Q cm) and both materials would therefore be classified as insulators. Substitution of other Group 13 or Group 15 atoms for the In and As has Table 1 Positional parameters atom X Y Z B/A2 0.31768(4) 0.091 80(4) 0.58959(4) -0.183 7 l(4) -0.02929(6) 0.23019(7) 0.2 1490( 7) 0.24642( 7) 0.24875( 7) 0.04 18(I) 0.1679 l(4) 0.23024(4) 0.32393(4) 0.25827(4) 0.32857(5) 0.503(9) 0.505(9) 0.523(9) 0.544(9) 0.54(1) 0.47828(6) 0.24871(6) 0.33 3 54( 6) 0.163 19(6) 0.74634(6) 0.1418( 1) 0.0834(1) 0.3440(1) 0.5624(1) 0.0448(1) 0.2054(1) 0.57 10( 1) 0.0606( 1) 0.3550(1) 0.5470(3) 0.5636(2) 0.5826(3) 0.4596(2) -0.1 5804( 5) 0.32289(5) 0.10800(5) 0.09 104( 5) 0.17277(5) 0.3940( 1) 0.4767(I) 0.0896(I) .0.0084(1) 0.54( 1) 0.57(1) 0.55( 1) 0.53(1) 0.54(1) 0.83(3) 1.07(3) 0.94(3) 1.32(3) Anisotropically refined atoms are given in the form of the isotropic equivalent thermal parameter.Table 2 K,In,As, interatomic distances and angles ~~~~ ~ atoms dis tance/A atoms distance/A AS(AS AS(6) 2.492(1) In(4)-As(2) 2.669( 1) In(1)-As(2) 2.657(1) In(4)-As(4) 2.692( 1) In(1)-As( 3) 2.650(1) In( 4)- As( 6) 2.733(I) In( 1)-As(4) 2.61 I( 1) In(4)-As( 2) 2.67 1( 1) In( 1)-As(5) 2.793(1) In(3)-As( 1) 2.648(1) In(2)-As( 1) 2.639(1) In( 3)-As(4) 2.7 1q1) In(2)-As(1) 2.656(1) In(3)-As(5) 2.621(1) In(2)-As( 3) 2.677( 1) In(3)-As(6) 2.697(1) In(2)-As(5) 2.666( 1) atoms angle/" atoms angle/" As(2)-In( 1)-As(3) 110.39(3) As( l)-In(3)-As(4) 113.58(3) As(2)-In( 1)-As(4) 1 1 1.2q3) As( 1)-In(3)-As(5) 11 1.22(3) As(2)-In( 1)-As(5) 116.45(2) As( I)-In(3)-As(6) 119.21(3) As(3)-In( l)-As(4) 115.48(3) As(4)-In(3)-As( 5) 105.99( 3) As(3)-In( l)-As(5) 94.26(3) As(4)-In(3)-As(6) 10 1.03( 3) As(4)-In( 1)-As(5) 108.24( 3) As( 5)-In(3)-As(6) 104.48(3) As( I)-In(2)-As( 1) 113.08(2) As(2)-In(4)-As(2) 120.22(2) As( l)-In(2)-As( 3) 102.67( 3) As(2)-In(4)-As(4) 116.84(3) As( l)-In(2)-As(5) 123.37( 3) As(2)-In(4)-As( 6) 110.30(3) As( 1)-In(2)-As(3) 119.61(3) As(2)-In(4)-As(4) 97.46(3) As( l)-In(2)-As( 5) 101.62(3) As( 2)-In(4)-As(6) 109.23(3) As( 3)-In(2)- As( 5) 96.64(3) As(4)-In(4)-As( 6) 100.57( 3) In(1)-As( 3)-As( 6) 94.66(3) In(4)-As(6)-As( 3) 106.60(3) In(2)-As(3)-As(6) 93.67(3) In(3)-As(6)-As(3) 104.54(3) Numbers in parentheses following values of distance or angle are estimated standard deviations in the least significant digits.Table 3 K41n4Sb6 positional parameters atom X Y Z B/A2 0.31683(5) 0.09279(5) 0.59240(5) 0.47880(5) 0.25040(5) 0.3 3262( 5) 0.16193(5) 0.74944(5) 0.8585(2) 0.6543(2) 0.0778(2) 0.5582(2) -0.18225(5) -0.02488(5) 0.2207(1) 0.21 30( 1) 0.2375( 1) 0.2360( 1) 0.0365(1) 0.0337(1) 0.2 I63( 1) 0.5638(1) 0.0550(I) 0.3 366( 1 ) 0.5336(4) 0.4492(4) 0.5352(4) 0.5562(5) 0.16490(5) 0.22865(5) 0.32674(5) 0.25969(5) 0.33 134(5) 0.15869(5) 0.32216(5) 0.10330(5) 0.08857(5) 0.173 17(5) O.OOSS(2) 0.5226(2) 0.3887( 2) 0.101 l(2) I .42(2) 1.37(1) 1.41( 1) 1.48(2) 1.34(1) 1.41(1) 1.45( 1) 1.32(1) 1.34(I) 1.44(1) 2.43(6) 2.48(6) 2.79(6) 3.41(7) Anisotropically refined atoms are given in the form of the isotropic equivalent thermal parameter.Table 4 K,In,Sb, interatomic distances and angles atoms dist ance/A atoms distance/A Sb( 3)-Sb( 6) 2.866(1) In(4)-Sb(2) 2.852(1) In( 1 )-Sb( 2) 2.853(1) In(4)-Sb(4) 2.871(1) In( 1)-Sb( 3) 2.838(1) In( 4)- 5b( 6) 2.9 18( 1) In( 1)-Sb(4) 2.798(1) In(4)-Sb(2) 2.858(1) In( 1)-Sb(5) 2.959(1) In(3)-Sb( I) 2.844(1) In(2)-Sb( 1) 2.836(1) In(3)-Sb(4) 2.904(1) In(2)-Sb( 1) 2.833(1) In( 3)-Sb( 5) 2.801( 1) In(2)-Sb(3) 2.864(1) In( 3)-Sb(6) 2.872(1) In(2)-Sb( 5) 2.849( 1) atoms angle/" atoms angle/" Sb(2)-In( 1)-Sb(3) 1 10.28(2) Sb( 1)-In(3)-Sb(4) 114.90(2) Sb(2)-In( 1)-Sb(4) 1I 1.47(2) Sb( 1)-In(3)-Sb(5) 110.63(2) Sb(2)-In( 1)-Sb( 5) 117.81(2) 5b( )-In( 3)-Sb( ) 117.61(2)16 Sb(3)-In( 1)- Sb( 4) 112.81(2) Sb(4)-In(3)-Sb(5) 105.36(2) Sb(3)-In( 1)-Sb(5) 96.08(2) 5b( 4)-In( 3)-Sb( ) 101.42(2)6 Sb(4)-In( 1)-Sb( 5) 107.5 9( 2) Sb( 5)-In( 3)-Sb( 6) 105.7 l(2) Sb( l)-In(2)-Sb( 1) 1 12.84(2) 5b( 2)-In( 4)-5b( 2) 118.24(2) Sb( 1)-In(2)-Sb(3) 101.89(2) Sb(2)-In(4)-Sb(4) 118.00(2) Sb( l)-In(2)-Sb(5) 123.22(2) 5b( 2)-In(4)-S b( ) 109.98(2)6 Sb( 1)-In(2)-Sb(3) 119.41(2) Sb(2)-In(4)-Sb(4) 97.054 2) Sb( 1)-In(2)-Sb(5) 10 1.8q2) 5b( 2)-In(4)-S b( ) 110.73(2)6 Sb( 3)-In( 2)-Sb( 5) 98.O 1(2) Sb(4)-In(4)-Sb(6) 101.09(2) In( 1)-Sb(3)-Sb(6) 92.12(2) In(4)-5b(6)-5b( 3) 104.29(2) In( 2)-S b( 3)-Sb( 6) 90.39( 2) In(3)-5b(6)-5b( 3) 103.62(2) Numbers in parentheses following values of distance or angle are estimated standard deviations in the least significant digits.produced related layered2'g2' and p~lymeric'~ structures. The related ternary I-111-V Zintl-phase material composed of K, Ga, and As (K3Ga3As4) has also been synthesized in our lab, and its structure and electrical properties have recently been reported.22 K3Ga,As4 shows a layered structure similar to the K41n4X6 materials, but K3Ga,As4 does not contain any catenated bonds.The analogue with K, Ga, and Sb (K2Ga2Sb4) has a polymeric structure with (Ga2Sb3) rings bridged by Sb atoms. K2Ga,Sb4 has also been shown to be an intrinsic semicond~ctor.~~ C.J.O. wishes to acknowledge support from a grant from the Louisiana Education Quality Support Fund administered by the Board of Regents of the state of Louisiana and the donors of the Petroleum Research Fund administered by the Amer- ican Chemical Society. 558 J. MATER. CHEM., 1991, VOL. 1 References H.Ochmann and H. Schafer, Rev. Chim. Min., 1985, 22, 58; G. Cordier, H. Ochmann and H. Schafer, Rev. Chim. Min., 1984, 1 H. Schafer, Ann. Rev. Muter.Sci., 1985, 15, 1; H. Schafer, 21, 282; G. Cordier, H. 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