Falk ; Lissner

Dr Thomas Schleid
email: schleid@iac.uni-stuttgart.de

Bei Der Redaktion

Dr Gerd Meyer

Geburtstag

___________________________________________________________________________

Die nicht-isotypen Nitridselenide Dy 3 NSe 3 und Ho 3 NSe 3 : Ketten und Dimere The Non-Isotypical Nitride Selenides Dy 3 NSe 3 and Ho 3 NSe 3 : Chains and Dimers

Keywords: Lanthanides, Nitride selenides, Crystal Structure Lanthanides, Nitride Selenides, Crystal Structure

Abstract. The non-isotypical lanthanoid(III) nitride selenides M 3 NSe 3 of dysprosium (Dy 3 NSe 3 ) and holmium (Ho 3 NSe 3 ) are formed by the reaction of the respective rare-earth metal (M = Dy and Ho) with sodium azide (NaN 3 ), selenium and an excess of iodine at 900 °C from torch-sealed evacuated silica ampoules within seven days. Dy 3 NSe 3 crystallizes orthorhombically (a = 1245.38 (9), b = 393.69 (3), c = 1303.74 (9) pm) in space group Pnma with Z = 4, whereas monoclinic Ho 3 NSe 3 (a = 1152.93 (8), b = 1006.61 (7), c = 1248.76 (9) pm, β = 112.847(3)°) adopts the space group C2/c with Z = 8. Both crystal structures comprise three crystallographically different M 3+ cations along with one N 3-and three independent Se 2- anions, but they differ fundamentally in the linkage of their structure dominating N 3--centered (M 3+ ) 4 tetrahedra. In Dy 3 NSe 3 , these [NDy 4 ] 9+ units are vertex-linked to form

1 ∞ {[N(Dy1) t 1 / 1 (Dy2) t 1 / 1 (Dy3) v 2 / 2
] 6+ } chains (t = terminal, v = vertex-shared), while just two show coordination numbers of six, six plus one and seven, whereas in the case of Ho 3 NSe 3 only coordination numbers of six occur with respect to the anions (N 3-and Se 2-). In both crystal structures the three independent sorts of Se 2-anions provide the three-dimensional linkage as well as the charge balance of the 1 ∞ {[NDy 3 ] 6+ } chains and the 0 ∞ {[N 2 Ho 6 ] 12+ } dimers, respectively.

Einleitung

Von ternären Nitridchalkogeniden der Lanthanide (M = La -Dy) [1] kennt man bislang drei unterschiedliche Formeltypen. Schon 1993 berichteten wir mit Sm 3 NS 3 über das erste Lanthanoid(III)-Nitridsulfid [2], dem nach systematischen Untersuchungen weitere Vertreter dieses Formeltyps M 3 NS 3 (M = La -Nd, Sm, Gd -Dy) [3] und auch das Cer(III)-Nitridselenid Ce 3 NSe 3 [4] Formeltypen Cs 2 M 6 N 2 Te 7 (M = Gd, Tb) [12], M 4 NCh 3 X 3 (M = La -Gd; Ch = S, Se; X = Cl, Br) [13] oder auch M 6 N 3 S 4 X (M = La -Nd; X = Cl, Br) [14] sowie M 5 N 3 S 2 Cl 2 (M = La -Pr) [15], M 5 N 2 Se 4 Br (M = Ce) [16] und M 3 N 2 SeBr (M = Nd) [17]

Experimentelles

Bei der Umsetzung von Dysprosium-oder Holmium-Metall (jeweils ChemPur; 99,9 %) mit Selen (Aldrich; 99,99 %), Natriumazid (E. Merck; 99,9 %) und elementarem Iod (ChemPur; 99 %) in evakuierten Kieselglasampullen erhält man unter Verwendung von Natriumiodid

Strukturbeschreibungen, Ergebnisse und Diskussion

Die Kristallstruktur des Dysprosiumnitridselenids Dy 3 NSe 3 lässt sich isotyp zu den Nitridsulfiden M 3 NS 3 (M = La -Nd, Sm, Gd -Dy) [3]

Molvolumen a) , V m /cm

Se2 68( 5) 57( 5) 79

[

  NHo 4 ] 9+ tetrahedra have one edge in common to join as isolated bitetrahedra 0 ∞ {([N(Ho1) ) 2 } (e = edge-connecting, t = terminal) in Ho 3 NSe 3 . So the alternative formula Ho 6 N 2 Se 6 with Z = 4 is likewisely valid. The M 3+ cations in Dy 3 NSe 3

1 ∞

 1 folgten. Die Kristallstrukturen all dieser Nitridchalkogenide werden von stickstoffzentrierten M 3+ -Tetraedern [NM 4 ] 9+ dominiert, deren eindimensionale Eckenverknüpfung zu Ketten 1 ∞ {[NM 3 ] 6+ } führt. In den Nitridchalkogeniden des nitridreicheren Formeltyps M 4 N 2 Ch 3 (Ch = S, Se, Te) liegt zwangsweise ein höherer Kondensationsgrad der beteiligten [NM 4 ] 9+ -Tetraeder vor. Dies ist in den Kristallstrukturen von Sm 4 N 2 S 3 [5] und Tb 4 N 2 Se 3 [6, 7] so realisiert, dass die [NM 4 ] 9+ -Tetraeder zwar ebenfalls noch Endlosketten bilden, die hier jedoch der Zusammensetzung 1 ∞ {[NM 2 ] 3+ } folgen und einer cis-Kantenverknüpfung der Baueinheiten ihren Ursprung verdanken. Eine trans-Kantenverknüpfung beobachtet man dagegen in den Kristallstrukturen der Nitridtelluride M 4 N 2 Te 3 (M = La -Nd) [8], in denen die Stränge allerdings einer Wellung unterliegen. Eindimensionalität bezüglich der Tetraederkondensation tritt auch in dem erst kürzlich beschriebenen La 4 N 2 S 3 -Typ [9] auf. Hier liegen neben kanten-auch eckenverknüpfte [NM 4 ] 9+ -Tetraeder vor, indem Tetraederdoppel [N 2 M 6 ] 12+ aus zwei [NM 4 ] 9+ -Einheiten mit gemeinsamer Kante über die verbleibenden vier freien Ecken einen Doppelstrang {[N 2 M 4 ] 6+ } ausbilden. Strukturell lassen sich davon die miteinander nicht isotyp kristallisierenden Verbindungen Pr 4 N 2 S 3 [10] und M 4 N 2 Se 3 (M = Pr, Nd) [7, 10] vom Nd 4 N 2 Se 3 -Typ ableiten. Durch die nicht-lineare Verknüpfung von über vier Ecken kondensierten Tetraederdoppeln [N 2 M 6 ] 12+ wird hier erstmals auch die Bildung von aufgelockerten Schichten beobachtet, die sich in beiden Strukturtypen lediglich durch ihre Verknüpfungsmotive und Porenweiten (nämlich (4+8)er-Maschen im Pr 4 N 2 S 3 -gegenüber (6+6)er-Maschen im Nd 4 N 2 Se 3 -Typ) unterscheiden. Strukturell nicht auf den ersten Blick ableitbar, stellt sich die Kristallstruktur des Nitridselenids Pr 5 NSe 6 [11] dar, das über Kantenverknüpfung der tetraedrischen [NPr 4 ] 9+ -Einheiten erstmals diskrete Dimere [N 2 Pr 6 ] 12+ aufweist. Als aussagekräftiger erweist sich die Formulierung [Pr 6 N 2 ]Pr 4 Se 12 , wonach neben solchen Dimeren auch Pr 3+ -Kationen in reiner Selenid-Koordination die Struktur mit gestalten und somit nicht an den [NPr 4 ] 9+ -Baueinheiten beteiligt sein können. Darüber hinaus ergeben sich durch kationische und anionische Derivatisierungen viele weitere Beispiele mit gleichen, ähnlichen oder anderen Motiven der Tetraederkondensation wie die Verbindungen mit den

  eindrucksvoll zeigen. Auffällig scheint jedoch bei allen genannten Verbindungen die Tatsache, dass vorwiegend Lanthanoid(III)-Nitridchalkogenide (samt ihrer kationischen und anionischen Derivate) mit den Elementen Lanthan bis Neodym existieren und bislang spätestens das Element Dysprosium mit dem letzten Vertreter dieser Verbindungsklassen das Endglied bildet. Tiefergehende Untersuchungen im Bereich von Nitridchalkogeniden der schweren Lanthanoide (M = Gd -Lu) waren uns daher von großem Interesse.

( 2 [ 18 ]

 218 NaI: ChemPur; 99,9 %) als Flussmittel nach sieben Tagen bei 900 °C Nitridselenide vom Formeltyp M 3 NSe 3 . Erstmals setzten wir bei unseren Präparationen elementares Iod ein, um die Umsetzung des aus NaN 3 entstehenden Natriums zu NaI gemäß 18 M + 18 Se + 2 NaN 3 + I 2 → 6 M 3 NSe 3 + 2 NaI zu provozieren. Das Iod wurde in beiden Versuchen wohlweislich im großzügigen Überschuss hinzugefügt, eingedenk der Tatsache, dass das Abschmelzen der befüllten Reaktionsampullen trotz Kühlung mit flüssigem Stickstoff zu Halogen-Verlusten führen kann. Die Bildung von anionisch derivatisierten Verbindungen wie z. B. vom Formeltyp M 6 N 3 Se 4 I nach dem Vorbild der leichteren Homologen M 6 N 3 S 4 X (M = La -Nd; X = Cl, Br) [14] oder auch Pr 19 ON 7 Se 16 I haben wir dabei billigend in Kauf genommen. Das Auftreten von langen, braunen Nadeln der Zielzusammensetzung Dy 3 NSe 3 deutete schon gleich nach dem Sichten der abgekühlten Glasampulle auf die wohlbekannte orthorhombische Kristallstruktur des Formeltyps M 3 NS 3 hin, wohingegen das analoge Holmiumexperiment gelbe, flächenreiche Kristalle mit einer monoklinen, basiszentrierten Metrik hervorbrachte. Beide Produkte blieben beim Versetzen mit Wasser beständig, wodurch das Auswählen von Einkristallen für die Röntgenstrukturanalyse erleichtert wurde. Ein jeweils unter flüssigem Paraffin selektiertes Exemplar von Dy 3 NSe 3 und Ho 3 NSe 3 wurde auf einem Kappa-CCD-Diffraktometer der Fa. Bruker-Nonius (Karlsruhe) vermessen. Ausführliche Datenauflistungen zu diesen Messungen finden sich in den Tabellen 1 bis 3 wieder.

]∞Tabelle 1

 1 orthorhombisch in der Raumgruppe Pnma mit vier Formeleinheiten pro Elementarzelle (a = 1245,38(9), b = 393,69(3), c = 1303,74(9) pm) beschreiben, wobei sich alle Teilchen auf der kristallographischen Lage 4c befinden, die jeweils eine Spiegelebene (m) beinhaltet. Bei den homologen Nitridsulfiden hatten wir aufgrund signifikanter Änderungen in der Koordinationssphäre der M 3+ -Kationen die M 3 NS 3 -Reihe von Lanthan bis Dysprosium in drei unterschiedliche pseudo-Strukturtypen unterteilt. Für die Verbindungen La 3 NS 3 bis Pr 3 NS 3 konnten wir als Umgebung von (M1) 3+ eine siebenfache Koordination durch die Anionen S 2-(6×) und N 3-(1×) feststellen (La 3 NS 3 -Typ), die sich durch Verlust eines S 2--Anions (S2) in der Koordinationssphäre für die nun folgenden Lanthanoide (M = Nd -Dy) zu einem verzerrten trigonalen Antiprisma reduziert (Sm 3 NS 3 -Typ). Beim Sprung von Sm 3 NS 3 zu Gd 3 NS 3 verhält es sich ähnlich, was in den Kristallstrukturen der Nitridsulfide M 3 NS 3 mit den schwersten Lanthanoiden (M = Gd -Dy) in einer Sechserkoordination für (M3) 3+ resultiert (Dy 3 NS 3 -Typ). Bei dem neu erhaltenen orthorhombischen Nitridselenid Dy 3 NSe 3 sollte es sich aufgrund der Koordinationsverhältnisse um den Dy 3 NS 3 -Typ handeln, wenn man die Abstände M 3+ -Se 2- mit denen der analogen Cerverbindung Ce 3 NSe 3 [4] vergleicht. So zeigt Abbildung 1 die Koordinationssphären der drei kristallographisch unterschiedlichen Dy 3+ -Kationen als verzerrtes Oktaeder (trigonales Antiprisma) mit einem N 3--und fünf Se 2--Anionen um (Dy1) 3+ , als überkapptes trigonales Prisma mit einem N 3--und sechs Se 2--Anionen um (Dy2) 3+ und genau wie bereits für (Dy1) 3+ beschrieben als gestauchtes Oktaeder um (Dy3) 3+ . Für (Dy3) 3+ liegen jedoch zwei N 3--und vier Se 2--Anionen als Primärliganden sowie ein zusätzliches, deutlich weiter entferntes Se 2--Anion (Se2) in einem Abstand von 333 pm als Kappe vor. Da sich alle übrigen Abstände d(Dy 3+ -Se 2-) zwischen 282 und 319 pm bewegen, hat die Formulierung als 6+1-Koordination um (Dy3) 3+ hier durchaus seine Berechtigung (vgl. hierzu auch die Tabellen 4 und 6). Selbst in der Kristallstruktur von Ce 3 NSe 3 [4] liegt der betreffende Abstand entgegen dem allgemeinen Trend der Lanthanoidenkontraktion mit 328 pm deutlich unterhalb des Wertes der Dysprosiumverbindung, was als überzeugendes Indiz für den Verlust eines Liganden in der Koordinationssphäre von (Dy3) 3+ zu werten ist. Die Tatsache, dass das Verhältnis Dy 3+ : N 3-einen Wert kleiner als vier aufweist und dass (Dy3) 3+ von zwei N 3--Anionen umgeben ist, führt zu dem Schluss, dass in der Kristallstruktur von Dy 3 NSe 3 keine isolierten [NDy 4 ] 9+ -Tetraeder anzutreffen sein werden. Wie schon so oft in der Kristallchemie von Nitridchalkogeniden der Lanthanoide liegen eindimensionale Stränge vor, die in diesem Fall gemäß 1 ∞ {[N(Dy1) 6+ } (t = terminal, e = eckenverknüpfend) aus über zwei (Dy3) 3+ -Ecken verknüpften Tetraederbaueinheiten [NDy 4 ] 9+ resultieren (vgl. Abb. 2). Diese Ketten 1 ∞ {[NDy 3 ] 6+ } verlaufen in der vorliegenden orthorhombischen Struktur entlang [010] und sind wie in Abbildung 3 verdeutlicht nach Art einer hexagonalen Stabpackung angeordnet. Der dreidimensionale Zusammenhalt und die Gewährleistung der Elektroneutralität in der Kristallstruktur von Dy 3 NSe 3 werden durch die drei kristallographisch unterschiedlichen Se 2--Anionen gesichert, von denen (Se2) 2-, wie bereits erläutert und in der Abbildung durch gestrichelte Bindungen hervorgehoben, nur schwach an (Dy3) 3+ koordiniert ist. Trotz identischer physikalischer und chemischer Bedingungen zeigte sich nach Reaktionsende beim Blick durch ein Stereomikroskop, dass sich das Produkt des Holmiumexperiments bei gleicher Zielvorgabe maßgeblich vom analogen Dysprosiumfall unterschied. Anstatt dunkelbrauner Nadeln (Dy 3 NSe 3 ) lagen hier nach Reaktionsende gelbe, flächenreiche, isometrische Einkristalle vor. Die durch Röntgenbeugungsmethoden ermittelte monokline und basiszentrierte Metrik (a = 1152,93(8), b = 1006,61(7), c = 1248,76(9) pm, β = 112,847(3)°) wies auf keine bislang bekannte Verbindung auf dem Gebiet von Nitridchalkogeniden der Lanthanoide hin, so dass erst die Auswertung des vollständigen Datensatzes ein neues, obschon formelgleiches, Nitridselenid des Holmiums (Ho 3 NSe 3 ) offenbarte. Die Kristallstruktur umfasst, wie schon für Dy 3 NSe 3 , erneut drei kristallographisch unterschiedliche M 3+ -Kationen sowie ein N 3--und drei Se 2--Anionen, welche allesamt die allgemeinen Punklage 8f besetzen und somit im Gegensatz zur Dysprosiumverbindung (Z = 4) einen doppelt so großen Zellinhalt (Z = 8) bewirken. Bei den in Abbildung 4 gezeigten Koordinationspolyedern handelt es sich ausnahmslos um verzerrte Oktaeder, deren Ecken aus zwei N 3-und vier Se 2-(für (Ho1) 3+ ) sowie aus jeweils einem N 3--und fünf Se 2--Anionen (für (Ho2) 3+ und (Ho3) 3+ ) bestehen. Hier steht die allgemein gültige Abnahme der Koordinationszahl für die drei in Dy 3 NSe 3 (Abb. 1) und Ho 3 NSe 3 (Abb. 4) betrachteten Polyeder um Dy 3+ und Ho 3+ im guten Einklang mit dem beim Übergang von Dy 3+ zu Ho 3+ absinkenden Ionenradius von 91 auf 90 pm (jeweils für CN = 6) [22]. Den in Tabelle 5 aufgelisteten Abständen d(Ho 3+ -Se 2-) von 281 bis 302 pm folgt als nächst kürzester Ho 3+ -Se 2--Kontakt ein Wert von bereits 406 pm (= d(Ho3) 3+ •••(Se2′′) 2-). In ihrem grundlegenden Strukturprinzip, nämlich der Kondensation von [NM 4 ] 9+ -Tetraedern, unterscheidet sich die formelgleichen Dysprosiumverbindung. Die Verknüpfung der [NHo 4 ] 9+ -Baueinheiten wird zwar wiederum über nur eine Kationensorte (hier: Ho1), die über zwei N 3--Nachbarn verfügt, gewährleistet, doch diesmal werden anstatt (eindimensional) unendlicher Ketten durch Kantenverknüpfung nur (nulldimensionale) diskrete Dimere [N 2 Ho 6 ] 12+ gemäß 0 ) 2 } (k = kantenverknüpfend, t = terminal) gebildet. Als alternative Formelschreibweise zu Ho 3 NSe 3 mit acht Formeleinheiten hat demnach Ho 6 N 2 Se 6 mit nur derer vier pro Elementarzelle eine durchaus stärkere Berechtigung. Solche diskreten 0 ∞ {[N 2 M 6 ] 12+ }-Einheiten wurden seinerzeit schon im Praseodymnitriselenid Pr 5 NSe 6 [11] beschrieben, in dessen Kristallstruktur allerdings gemäß der Schreibweise [Pr 6 N 2 ]Pr 4 Se 12 auch Pr 3+ -Kationen vorliegen, die nur von Se 2--Anionen koordiniert und somit nicht am Aufbau der [NM 4 ] 9+ -Tetraeder beteiligt sind. Zudem liegt dort das Tetraederdoppel [N 2 M 6 ] 12+ , bedingt durch überbrückende Se 2--Anionen, nicht ideal gestreckt vor, was im Gegensatz zu Pr 5 NSe 6 in der Kristallstruktur von Ho 3 NSe 3 durch die Lage des Inversionszentrums (1 ) auf der Kantenmitte (Ho1•••Ho1) unabdingbar ist (vgl. Abb. 5). Stets konnten wir bei allen bisher erhaltenen Kristallstrukturen von Lanthanoid(III)-Nitridchalkogeniden, die eine Kantenverknüpfung von [NM 4 ] 9+ -Tetraedern aufweisen, die kürzeste der sechs Tetraederkanten (M•••M) auch als die verknüpfende zuordnen. Hier zeichnet sich ebenfalls dieser Abstand d(Ho1•••Ho1) mit 333 pm als kleinster innerhalb der sechs Tetraederkanten (333 -416 pm) aus. Auf dieser Grundlage lässt sich sogar der recht kleine Winkel (∡(Ho1-N-Ho1) von 94° in Richtung der verknüpfenden Kante erklären, der sich durch einen sehr großen Gegenwinkel (∡(Ho1-N-Ho3′) von 136° kompensieren lässt (vgl. Tab. 5). Für die Dysprosiumverbindung Dy 3 NSe 3 mit Kettenstruktur werden diesbezüglich aufgrund der fehlenden Kantenverknüpfung mit Werten zwischen 102 und 116° deutlich geringere Abweichungen vom idealen Tetraederwinkel (109,5°) beobachtet. Gegensätzlich verhalten sich aber die tetraederrelevanten Abstände d(N 3--M 3+ ), die in der Kristallstruktur von Dy 3 NSe 3 eine recht große Spanne (217 -232 pm) aufweisen und durch die Bildung einer Dy3-N-Dy3-Kettenlinie, die mit d(Dy3•••Dy3) ≈ 394 pm gleichzeitig auch die kürzeste Repetitionsperiode (≡ b-Achse) der gesamten Kristallstruktur darstellt, verursacht wird. Dieser Befund spielt in der Kristallstruktur von Ho 3 NSe 3 keine Rolle, was sich in dem viel homogeneren Abstandsintervall d(N 3--Ho 3+ ) von 222 bis 227 pm widerspiegelt. Abbildung 6 zeigt die Projektion der Kristallstruktur von Ho 3 NSe 3 in die (010)-Ebene (oben) sowie mit Blick entlang [110] (unten). In beiden Ansichten sind zwar die isoliert vorliegenden Tetraederdoppel 0 ∞ {[N 2 Ho 6 ] 6+ } gut erkennbar, doch werden durch die drei kristallographisch Anionen, die wiederum den Ladungsausgleich und die dreidimensionale Verknüpfung bewirken, in der standardmäßig gezeigten monoklinen ac-Ebene (oben) die Tetraederecken (≡ Ho 3+ -Kationen) vollständig verdeckt. Einen besseren Einblick gewährt dagegen der untere Teil von Abbildung 6, der darüber hinaus zusammen mit dem oberen Bild die Verknüpfung der Tetraederdoppel [N 2 Ho 6 ] 12+ durch die Se 2--Anionen in alle drei Raumrichtungen verdeutlicht. Die für die Nitridulfide M 3 NS 3 (M = La -Nd, Sm, Gd -Dy) [3] von uns beschriebene Unterteilung in die drei pseudo-Strukturtypen (La 3 NS 3 -, Sm 3 NS 3 -und Dy 3 NS 3 -Typ), bei denen sich die Koordinationszahlen vom größeren La 3+ -bis hin zum kleineren Dy 3+ -Kation teils kontinuierlich, teils diskontinuierlich erniedrigen, spiegelt sich zwar in der nicht-linearen Schrumpfung von a-und c-Achse wider, nicht jedoch im molaren Volumen der Elementarzellen. Hier beobachtet man in der Reihe von La 3 NS 3 (V m = 100,59 cm 3 /mol) bis hin zu Dy 3 NS 3 (V m = 87,33 cm 3 /mol) eine stetige mittlere Abnahme des Wertes für V m von rund 1,5 % zwischen zwei benachbarten Lanthanoid(III)-Kationen. Im krassen Gegensatz dazu stehen die hier vorgestellten Nitridselenide Dy 3 NSe 3 und Ho 3 NSe 3 : Die Werte von 96,24 cm 3 /mol für die Dysprosium-und 100,53 cm 3 /mol für die in der Reihe der Lanthanoide direkt nachfolgende Holmiumverbindung zeigen sogar einen nicht zu erwartenden Anstieg des molaren Volumens um etwa 4,5 %, womit sich die neuartige Holmiumverbindung (Ho 3 NSe 3 ) wohl als potentielle Niederdruck-oder Hochtemperaturform der gesamten M 3 NSe 3 -Serie, auf alle Fälle aber als Phase mit der niedrigeren Dichte (D x (Dy 3 NSe 3 ) = 7,673 g/cm 3 vs. D x (Ho 3 NSe 3 ) = 7,417 g/cm 3 ) präsentiert. Ob es letztendlich gelingt, mit verdichtenden Hochdruckpäparationstechniken diese monokline Phase in die orthorhombische Struktur zu überführen, bleibt genauso ein lohnendes Ziel für zukünftige Experimente, wie Versuche zur Darstellung der übrigen noch fehlenden Nitridselenide M 3 NSe 3 der Selten-Erd-Elemente (M = Sc, Y, La, Pr, Nd, Sm, Gd, Tb, Er, Tm, Lu), wenn man bedenkt, dass für die Elemente Europium und Ytterbium gegenüber den reduzierend wirkenden Selenid-Anionen wohl eher die Zweiwertigkeit wahrscheinlich ist. Da wir nach der röntgenographischen Charakterisierung des ersten Lanthanoid(III)-Nitridsulfids Sm 3 NS 3 [2] in der Literatur sogleich auf Zintl-Phasen mit anti-isotyper Kristallstruktur wie z. B. Ca 3 AlAs 3 [23] gestoßen sind, haben wir im Laufe der Zeit nach erfolgter Strukturbestimmung an neuen Nitridchalkogeniden und deren Derivaten stets versucht, Analogieschlüsse zu diesen Verbindungsklassen oder auch zu Thio-und Selenoferraten herzustellen. So konnten wir für die Nitridselenide M 4 N 2 Se 3 (M = Pr, Nd; monoklin, C2/c) [7, 10], die Nitridtelluride M 4 N 2 Te 3 (M = La -Nd; orthorhombisch, Pnma) Die nicht-isotypen Nitridselenide Dy 3 NSe 3 und Ho 3 NSe 3 : Kristallographische Daten und ihre Bestimmung a) Dy 3 NSe 3 Ho 3 NSe 3

1 NSe 3 Abb. 2 ] 3 Abb. 3 3 Abb 3 Abb. 6 Page 3

 132333363 Abbildungen

  3

  klU 23 + 2a * c * hlU 13 + 2a * b * hkU 12 )]; U ij in pm 2 . Ho 3 NSe 3 : Atomlagen und anisotrope thermische Auslenkungsparameter

	•mol -1 Messbereich (Θ max /grd) Dichte a) , D x /g•cm -3 Z Kristallsystem Raumgruppe Messgerät Strahlung Indizierungsbereich, ±h max ±l max ±k max F(000) Tabelle 1 (Fortsetzung) Restelektronendichten (e -⋅10 6 pm 3 , max./min.) a) weitere Einzelheiten zu den Kristallstrukturbestimmungen können beim 96,235(8) 100,534(8) 7,673 7,417 4 8 orthorhombisch monoklin Pnma (Nr. 62) C2/c (Nr. 15) -Kappa-CCD --Mo-Kα (Graphitmonochromator; λ = 71,07 pm) -28,11 28,13 16 15 5 13 17 16 1228 2480 ZAAC 3,06 / -2,82 2,22 / -2,19 Fachinformationszentrum (FIZ) Karlsruhe, D-76344 Eggenstein-Leopoldshafen, unter Angabe der Hinterlegungsnummern CSD-420156 (Dy 3 NSe 3 ) und CSD-420157 (Ho 3 NSe 3 ) angefordert werden. Page 14 of 25 ZAAC Tabelle 2 Dy 3 NSe 3 : Atomlagen und anisotrope thermische Auslenkungsparameter Lageparameter x/a y/b z/c Dy1 (4c) 0,06423(5) 1 / 4 0,63204(5) Dy2 (4c) 0,70552(5) 1 / 4 0,64162(5) Dy3 (4c) 0,54295(5) 1 / 4 0,35074(5) N (4c) 0,0369(9) 1 / 4 0,7961(9) Se1 (4c) 0,86986(11) 1 / 4 0,00473(11) Se2 (4c) 0,09769(11) 1 / 4 0,39952(11) Se3 (4c) 0,77550(11) 1 / 4 0,31198(11) Koeffizienten der anisotropen Auslenkungsparameter a) U 11 U 22 U 33 U 23 U 13 U 12 Dy1 174(4) 83(3) 61(3) 0 -4(2) 0 Dy2 148(4) 82(3) 81(3) 0 1(2) 0 Dy3 124(4) 76(3) 90(3) 0 -4(2) 0 N 194(66) 123(60) 68(48) 0 26(43) 0 Se1 164(7) 86(6) 67(6) 0 8(5) 0 Se2 143(7) 107(7) 105(6) 0 8(5) 0 Se3 145(7) 84(6) 90(6) 0 5(5) 0 a) definiert als Temperaturfaktor in der Form: exp[-2π 2 (a *2 h 2 U 11 + b *2 k 2 U 22 + c *2 l 2 U 33 + ZAAC Tabelle 3 Lageparameter x/a y/b z/c Ho1 (8f) 0,31996(5) 0,34112(5) 0,11881(4) Ho2 (8f) 0,07384(5) 0,13766(5) 0,12254(4) Ho3 (8f) 0,17923(5) 0,42743(5) 0,38710(4) N (8f) 0,2424(9) 0,1317(9) 0,0772(8) Se1 (8f) 0,17379(11) 0,13336(11) 0,37172(9) Se2 (8f) 0,08080(11) 0,41568(11) 0,13996(9) Se3 (8f) 0,42711(11) 0,35313(11) 0,37316(9) Koeffizienten der anisotropen Auslenkungsparameter a) U 11 U 22 U 33 U 23 U 13 U 12 Ho1 48(3) 46(3) 39(3) -10(2) 10(2) -14(2) Ho2 48(3) 55(3) 61(3) -5(2) 26(2) -10(2) Ho3 61(3) 43(3) 68(3) -4(2) 4(2) -14(2) N 71(45) 56(43) 77(46) -5(35) 22(38) -12(36) 2b Page 15 of 25 Page 13 of 25 Se1 59(5) 42(5) 47
	µ/mm -1	51,66	51,42
	Datenkorrekturen	-Untergrund, Polarisations-und Lorentzfaktoren -
	Absorption	-Programm X-SHAPE [19] -
	Extinktionskoeffizient (g/10 -4 )	6(1)	2(1)
	gemessene Reflexe	9964	12294
	symmetrieunabhängige Reflexe	891	1639
	R int / R σ	0,095 / 0,032	0,095 / 0,056
	Strukturbestimmung und -verfeinerung	-Programmsystem SHELX-97 [20] -
	Streufaktoren	-International Tables, Vol. C [21] -
	Goodness-of-Fit	1,078	1,092
	R 1 (für alle Reflexe)	0,039	0,053
	R 1 (mit 4σ(F o )-Schranke)	0,034	0,038
	Reflexe oberhalb der 4σ-Schranke	818	1332
	wR 2 (für alle Reflexe)	0,081	0,084
	Wiley-VCH Wiley-VCH Wiley-VCH

* c *

Page 17 of 25 Wiley-VCH ZAAC Tabelle 6

  ) definiert als Temperaturfaktor in der Form: exp[-2π 2 (a *2 h 2 U 11 + b *2 k 2 U 22 + c *2 l 2 U 33 + 2b * c * klU 23 + 2a * c * hlU 13 + 2a * b * hkU 12 )]; U ij in pm 2 . Dy 3 NSe 3 vs. Ho 3 NSe 3 : Motive der gegenseitigen Zuordnung

				ZAAC				Page 16 of 25
	Tabelle 4	Dy 3 NSe 3 : Ausgewählte interatomare Abstände (d/pm) und Winkel (∡/grd)
	Dy1 -N (1×) 216,6	N	Dy2 -N (1×) 225,1 Se1 Se2	Se3	Dy3 -N (2×) CN	231,8
	-Se2 (2×) 284,8 Dy1	1/1	-Se1 (2×) 281,8 0/0 3/3	2/2	-Se1 (1×) 6	286,3
	-Se3 (2×) 289,7 Dy2	1/1	-Se3 (2×) 297,7 2/2 2/2	2/2	-Se3 (1×) 7	294,0
	-Se2′ (1×) 306,0 Dy3	2/2	-Se2′ (2×) 318,9 3/3 0+1/0+1	1/1	-Se1′ (2×) 6+1	301,4
		CN	4	5	5+1	5	-Se2 (1×)	333,3
	N -Dy1 (1×) 216,6 Ho1	2/2	Dy1 -N -Dy2 1/1	(1×) 102,1 1/1 2/2	6
	-Dy2 (1×) 225,1 Ho2	1/1	Dy2 -N -Dy3 2/2	(2×) 106,8 1/1 2/2	6
	-Dy3 (2×) 231,8 Ho3	1/1	Dy1 -N -Dy3 2/2	(2×) 111,8 2/2 1/1	6
				Dy3 -N -Dy3	(1×) 116,3
	Tabelle 5	Ho 3 NSe 3 : Ausgewählte interatomare Abstände (d/pm) und Winkel (∡/grd)
	Ho1 -N (1×) 227,0		Ho2-N (1×) 222,6		Ho3-N (1×)	222,0
	-N′ (1×) 228,4		-Se2 (1×) 280,6		-Se2 (1×)	284,7
	-Se3 (1×) 288,1		-Se3 (1×) 286,5		-Se2′ (1×)	288,6
	-Se3′ (1×) 293,0		-Se1 (1×) 286,8		-Se1 (1×)	289,0
	-Se1 (1×) 294,4		-Se1′ (1×) 288,4		-Se1′ (1×)	296,5
	-Se2 (1×) 296,7		-Se3′ (1×) 291,2		-Se3 (1×)	302,4
	N -Ho3 (1×) 222,0		Ho1 -N -Ho1	(1×)	93,8
	-Ho2 (1×) 222,6		Ho1 -N -Ho2	(1×) 102,8
	-Ho1 (1×) 227,0		Ho1 -N -Ho3	(1×) 105,1
	-Ho1′ (1×) 228,4		Ho2 -N -Ho3	(1×) 107,5
				Ho1 -N -Ho2′ (1×) 109,1
				Ho1 -N -Ho3′ (1×) 136,1
				Wiley-VCH

a

Wiley-VCH ZAAC


Tanja Graf

Frederick Casper

Jürgen Winterlik

Benjamin Balke

Gerhard H Fecher

Prof. Dr Claudia Felser
email: felser@uni-mainz.de

Dr Gerd Meyer

Crystal Structure of New Heusler Compounds

Keywords: Heusler compounds, intermetallic phases, crystal structure, X-ray diffraction Compound Structure

Heusler compounds are promising materials in many fields of contemporary research. The spectrum of their possible applications ranges from magnetic and magneto-mechanical materials over semiconductors and thermoelectrics to superconductors. An important feature of the Heusler compounds is the possibility of controlling the valence electron concentration by partial substitution of elements. On the other hand, the properties also depend on the degree of ordering of the the crystal structure. In general, Heusler compounds crystallize in the Cu 2 MnAl-type structure but in many cases certain types of disorder are observed. In this paper a detailed description of the different types of disordered structures is given. Additionally, the synthesis of new Heusler compounds is reported. The prepared compounds contain ternary Cr-, Rh-, Ru-, Ni-, Pd-, and Pt-based Heusler systems as well as quaternary Co 2 -based compounds and compounds of the general formula XX'YZ. The crystal structure was determined by XRD measurements and the structural ordering of the compounds is discussed.

Introduction

Heusler compounds are ternary intermetallics with a 2:1:1 stoichiometry [1].

They first attracted interest of the magnetism community when Heusler et al. [2] had shown that the compound Cu 2 MnAl becomes ferromagnetic, even though none of its constituents is ferromagnetic by itself. However, it took three decades before their structure was explained to be that of an ordered compound with a face centred cubic structure [3,4].

Interest in Heusler compounds persists since it has been established that the properties of these intermetallic compounds can be altered by changing the degree and kind of chemical order as well as by substituting one element by another. Co-based compounds were synthesized and investigated in the 1970s [5]. They attracted a great deal of interest due to their high Curie temperatures [6,7] and being predicted to be half metallic ferromagnets [8,9]. Many of the Co 2 -based compounds are now used in magnetic tunnel junctions [10,11]. Today Heusler compounds have various applications, for example they are investigated due to their properties as half-metallic ferromagnets in the field of spintronics, thermoelectrics and superconductors [12][13][14][15].

Experimental Section

Polycrystalline ingots of the compounds were prepared by arc melting of stoichiometric amounts of high purity elements in an argon atmosphere to avoid oxygen contamination at a pressure of 10 -4 mbar. Additionally, a Ti sponge was used to bind remaining oxygen.

The samples were melted three times. They were turned over after each melting process to yield a homogeneous sample. The weight loss after the whole melting procedure was less than 1 %. The polycrystalline ingots were annealed in an evacuated quartz tube for one to three weeks.

The crystallographic structure was investigated by X-ray powder diffraction using excitation by Mo K α radiation (λ = 0.7093165 nm; Bruker, AXS D8) in reflection geometry or Cu K α radiation (λ = 1.540598 nm; Bruker, D5000) in transmission geometry in a θ-θ scanning mode. The experimental diffraction patterns were refined using the FULLPROF program [16].

Results and Discussion

Crystal Structure

The properties of Heusler compounds depend significantly upon the degree and type of chemical order. In general, X 2 YZ Heusler compounds (Insert figure 1) Fig. 1 shows the transition from the ordered to the most prominent disordered structures as will be explained in the following: If the Y and the Z atoms are evenly distributed, the 4a and 4b positions become equivalent. This leads to a CsCl-like structure, also known as B2-type disorder. The symmetry is reduced and the resulting space group is P m 3 m.

The random distribution of the X and the Y or the X and the Z atoms results in the BiF 3type disorder (Space group no. 216: F m 3 m, DO 3 ). The NaTl-type structure is observed very rarely. In this structure type the X-atoms which occupy one fcc sub-lattice are mixed with the Y atoms whereas the X-atoms on the other sub-lattice are mixed with the Z atoms. This kind of disorder is also known as B32a disorder (Space group no. 227, F d 3 m). The X-atoms are placed at the Wyckoff position 8a (0, 0, 0), the Y and Z randomly distributed at position 8b (1/2, 1/2, 1/2). In contrast to these partial disorder phenomena all positions become equivalent in the tungsten-type structure with a bcc lattice and reduced symmetry I m 3 m (A2).

The cubic X 2 YZ compounds are also found in the CuHg 2 Ti-type structure as illustrated in Fig. 2 (note that the classification of the cubic X 2 YZ compounds is sometimes not uniquely given in Pearson's Handbook [START_REF] Villars | Pearson's Handbook of Crystallographic Data for Intermetallic Phases[END_REF]). In the CuHg 2 Ti-type structure (space group no. 216: F 4 3m, X) the X atoms do not build a simple cubic lattice but occupy the 4a and 4c Wyckoff positions at (0, 0, 0) and (1/4, 1/4, 1/4). The Y and Z atoms are located on 4b (1/2, 1/2, 1/2) and 4d (3/4, 3/4, 3/4) positions, respectively. All four positions have T d symmetry. This structure is frequently observed if the nuclear charge of the Y element is larger than the one of the X element from the same period (Z(Y)>Z(X)). The structure may also appear in compounds with transition metals from different periods. Space group no. 216 is also observed in quaternary noncentrosymmetric XX'YZ Heusler compounds. Every element occupies one of the fcc sublattices. The X atoms are located on the 4a sites whereas the X' atoms occupy the 4b positions. The Y and Z atoms are situated at the 4c and 4d positions, respectively. The prototype is LiMgPdSn (compare Fig. 2). The reviewing articles of Bacon and Plant [START_REF] Bacon | [END_REF] as well as Neumann and Ziebeck [START_REF] Webster | Landolt-Brönstein-Group III Condensed Matter[END_REF][START_REF] Ziebeck | Landolt-Brönstein-Group III Condensed Matter[END_REF] summarize different types of disorder in Heusler compounds.

(Insert figure 2)

Fig. 3 shows the group -subgroup relationship between all types of simple disorder that are possible in the Heusler structure. The numbers after t and k indicate the index of the reduction in symmetry. One should notice that there are 2 atoms per unit cell for the tungsten and the CsCl structure. For all other structure types there are 16 atoms per unit cell due the doubling of all cell axes.

(Insert figure 3)

Table 1 summarizes the different ordering variants of Heusler compounds. The site occupancy is correlated with the corresponding general formula. Different notations of the crystal structures according to the ISCD, the Strukturberichte (SB) and to Pearson as well as the space group are given.

(Insert table 1)

Theoretical X-ray diffraction patterns of Co 2 MnSi which were simulated under the assumption of different crystal structures are displayed in Fig. 4. Table 2 provides 200) reflexions vanish not only in the tungsten-type structure but also if all contained elements are from the same row of the periodic table due the nearly equal scattering factors. A small amount of disorder in an ordered compound or a low degree of order in a disordered structure may occur. This is not detectable by standard XRD. Thus, small amounts of partial disorder cannot be excluded without detailed analysis of further structural investigations, for example by neutron diffraction, Mössbauer spectroscopy, NMR or EXAFS, even if the XRD data indicate the Cu 2 MnAl structure.

(Insert figure 4)

(Insert table 2)

Structural Characterization

We report on the crystal structure of newly synthesized Heusler compounds. Tables 3,4, and 5 provide an overview of the different types of Heusler compounds and their crystal structure.

Table 3 summarizes the prepared ternary Heusler compounds. Fig. 5 shows the XRD results of Cr 2 CoGa. The compound fulfils the criteria of Z(X) < Z(Y). Since the (200) reflexion is not present but the (311) reflexion is detectable, it is concluded that the compound exhibits the CuHg 2 Ti structure. The (111) reflexion cannot be detected due to the fluorescence in that θ range. For comparison the calculated diffraction pattern for the Cu 2 MnAl-type structure given as an inset in Fig. 5.

(Insert figure 5) (Insert table 3)

The On the other hand it should be noted that it is hard to distinguish between some disorder types with regular XRD methods. As the magnetic and electronic properties of Heusler compounds depend on the crystalline structure, these compounds should be investigated by many structure sensitive analysing methods. Although the diffraction pattern shows the typical features of the tungsten-type structure, the degree of order cannot be determined since the intensity of the fcc-typical (111) and (220) reflexions is too weak.

Figure Captions

Table captions

Table 1 Relation between ordered -disordered Heusler structures and different forms of notations.

Table 2 Relative intensities of the reflexions for Co 2 MnSi in different structures. Table 3 Structural properties ternary X 2 YZ Heusler compounds. 2 ) The Cu 2 MnAl-type structure is possible but the (111) reflexion is not detectable due to the low scattering intensity. 3 ) The XRD pattern of the structure LiMgPdSn does not differ significantly from one of the Cu 2 MnAl structure. Therefore it is not possible to determine whether the compounds are ordered at X and X' positions.

Table 5 Page

  crystallize in the cubic Cu 2 MnAl structure (space group no. 225: F m 3 m, L2 1 ). The X and Y atoms are transition metals and Z is a main group element. In some cases Y is replaced by a rare earth element or an earth alkali metal. The cubic Cu 2 MnAl structure consists of four interpenetrating fcc sub-lattices, two of which are equally occupied by X. These two X-site fcc sub-lattices combine to form a simple cubic lattice. The Y and Z atoms occupy alternating the centers of the simple cubic lattice. This results in a CsCl-like superstructure. The X atoms are placed on the Wyckoff position 8c (1/4, 1/4, 1/4). The Y and Z atoms are located on 4a (0, 0, 0) and 4b (1/2, 1/2, 1/2) positions, respectively. The 4a and 4b positions have O h symmetry whereas the 8c position has T d symmetry. The crystal structure is displayed in Fig. 1. The illustration is shifted by (1/4, 1/4, 1/4) with respect to the standard illustration of the F m 3 m cell to show the CsCl-like superstructure.

  a survey of the relative intensities of the reflexions for the XRD patterns displayed in Fig. 4. The Cu 2 MnAl-type structure is identified by the occurrence of the fcc-typical (111) and (200) reflexions and their relation to the (220) reflexion. However, the intensity of the two fcc-typical reflexions is very low in many of the investigated compounds. If the Cu 2 MnAl-type and the BiF 3 -type structure are compared, a difference in the intensities of the (111) and (200) reflexions is evident. The Cu 2 MnAl and the CuHg 2 Ti structure are hardly distinguishable by X-ray diffraction. Much care has to be taken in the structural analysis, as both have general fcc-like symmetry. In the CsCl-type structure only the (111) reflexion vanishes whereas in the NaTl-type structure the (200) reflexion disappears. Both, the (111) and (

Fig. 7

 7 figure. The diffraction pattern exhibits only the (220) reflexion. This is caused by the

Acknowledgements.

  Financial support by the Deutsche Forschungsgemeinschaft DFG (Research unit FOR 559 (projects P 01 and P 07) and Collaborative Research Center Condensed Matter Systems with Variable Many-Body Interactions (TRR 49)) is gratefully acknowledged.

Fig. 1 Crystal

 1 Fig.1

Fig. 2 Crystal

 2 Fig.2

Fig. 3 Bärninghaus

 3 Fig.3

Fig. 4 SimulatedFig. 5 X

 45 Fig.4

Fig. 6 XRD

 6 Fig.6

Fig. 7 XRD

 7 Fig. 7

.

  Fig. 1

Fig. 5 Page

 5 Fig. 5

  the X2YZ Heusler compounds. The Cu2MnAl (L21) structure and preferential types of disorder: The BiF3 (DO3), CsCl (B2) and Tungsten (A2) structure. Note that all positions are shifted by (1/4, 1/4, 1/4) with respect to the standard F m m cell to make the CsCl superstructure visible. 254x190mm (600 x 600 DPI) of the Heusler compounds in space group F 3m. The CuHg2Ti-structure and the noncentrosymmertic LiMgPdSn-structure of XX'YZ compounds. 196x91mm (600 x 600 DPI) the group-subgroup relationship of the Heusler structure and its disorder variants. The indices of klassengleiche (k) and translationsgleiche (t) as well as the unit cell transformations and the origin shifts are given. 247x233mm (600 x 600 DPI) ray diffraction patterns measured with Mo Kα radiation for Co2MnSi in different crystal structures: a) Cu2MnAl-type (L21), b) CsCl-type (B2), c) BiF3-type (DO3), d) tungsten-type (A2), e) CuHg2Ti-type (X) and f) NaTl-type (B32a) structure. 81x115mm (600 x 600 DPI) diffraction pattern of Cr2CoGa measured with Cu Kα radiation. The compound exhibits the CuHg2Ti-type structure. For comparison the theoretical diffraction pattern of the Cu2MnAl-type structure is displayed as an inset. 43x56mm (600 x 600 DPI) Co2Mn0.95Sc0.05Ga measured with Mo Kα radiation. Although the diffraction pattern shows the typical features of the tungsten-type structure, the degree of order cannot be determined since the intensity of the fcc-typical (111) and (220) reflexions is too weak. 86x111mm (600 x 600 DPI) NiCuScIn measured with Mo Kα radiation. The diffraction pattern shows the typical features of the Cu2MnAl structure, the superlattice reflexions (111) and (200) are present. A differentiation between the Cu2MnAl and the LiMgPdSn structure is not possible since the calculated XRD patterns of these two structures are almost identical. 86x111mm (600 x 600 DPI)

Table 4

 4 Structural properties of Co 2 -based quaternary Heusler compounds.

Table 5

 5 Structural properties quaternary XX'YZ Heusler compounds.

Table

Table 3

 3

	Page 26 of 36

Table 4

 4

	Page 28 of 36


Dragan Pitzschke

Jan Curda

Dr Martin Jansen
email: m.jansen@fkf.mpg.de

Synthesis, Crystal Structure and Ionic Conductivity of Ag 8 I 2 (CrO 4 ) 3

Keywords: Silver, Iodine, Chromate, Ionic conductivity, Crystal structure Silver, Iodine, Chromate, Ionic conductivity, Crystal structure

Ag 8 I 2 (CrO 4 ) 3 has been synthesized by solid state reaction, starting from stoichiometric mixtures of Ag 2 O, AgI and Cr 2 O 3 , at elevated oxygen pressures. The compound crystallizes in the hexagonal space group P6 3 /m, with the unit cell dimensions a = 9.4474(4) Å, c = 10.2672(4) Å, γ = 120°, V = 793.61(6) Å 3 , and Z = 6. The crystal structure was solved by direct methods and refined, basing on single crystal diffraction data (815 independent reflections, R1 = 2.45 %). The structure is fully ordered. The CrO 4

2-anions are arranged in the mode of an hcp packing. Such a building principle is providing channels of face sharing octahedral voids. In the case of the title compound, there are three of such channels in the unit cell. Two of them accommodate two iodine atoms and one silver each, with iodine occupying the octahedral voids, and silver centering the triangular faces connecting the octahedra. Thus, for silver a trigonal bipyramidal coordination by three oxygen and two iodine ions result. In the third column of face sharing octahedra, silver is in the centre of the octahedra. The remaining silver atoms are located in the tetrahedral voids, between the CrO 4 2-anions. According to the results of impedance measurements, Ag 8 I 2 (CrO 4 ) 3 is a silver ion conductor. The compound shows an increase in the ionic conductivity in the temperature range from 25 to 175 °C, and has a silver ion conductivity of 6.5 X 10 -4 Ω -1 cm -1 at 30 °C. The activation energy for silver ion conduction is 0.21 eV, in the temperature range from 25 to 50 °.

____________

Introduction

Fast-ion electrolytes based on Ag-containing solids have been of interest in that they often show high ionic conductivity already at room temperature [1,2]. The search for new silver-ion conductors continues, considering various chemical systems, which include halides, chalcogenides and others [3][4][5]. The most famous members in this class of electrolytes are AgI and RbAg 4 I 5 . These materials are characterized by their high ionic conductivities, comparable to those observed in molten salts [5][6][7]. RbAg 4 I 5 shows the highest ionic conductivity ever observed for a solid at ambient temperature (σ(i) = 0.27 Ω -1 cm -1 ) [8]. The structures of both, AgI and RbAg 4 I 5 , have been studied thoroughly, in particular with respect to the disorder of the cation partial structure [7]. Among the good silver ion conductors, there is a class of compounds with compositelike topologies, containing silver iodide, and silver oxyacid salts, like Ag 3 PO 4 , Ag 2 WO 4 , Ag 3 AsO 4 etc., at the same time [9][10][11][12][13][14][15][16]. The incorporation of silver ions in an immobile framework of complex oxoanions has proven to be an effective route to raise the silver ion conductivity. Recently, we expanded this field to composites containing AgI and silver selenates and tellurates, this way identifying Ag 3 ITeO 4 , Ag 4 I 2 SeO 4 and Ag 9 I 3 (SeO 4 ) 2 (IO 3 ) 2 as new silver solid electrolytes [17,18]. In the course of our structural investigation of crystalline analogues of the ion-conducting silver iodide silver oxyacid salt glasses, we have synthesized and characterized the compound Ag 8 I 2 (CrO 4 ) 3 .

Experimental Section

Synthesis

Starting materials for the preparation of Ag 8 I 2 (CrO 4 ) 3 were Ag 2 O (freshly precipitated), AgI (Fluka, p.a.) and Cr 2 O 3 (Fluka, > 98 %). The synthesis was performed by solid state reaction in stainless steel autoclaves at elevated oxygen pressures and temperatures. Stoichiometric amounts of the starting materials were intimately mixed and placed into gold tubes which were sealed from one side and mechanically (not gas-tight) closed from the other side. Polycrystalline samples of Ag 8 I 2 (CrO 4 ) 3 were obtained under an oxygen pressure of 160 MPa, at a temperature of 450°C and a reaction time of 60 hours. The single crystals as obtained were of red-brown colour, and of irregular habitus. In order to prepare single crystals suitable for X-ray diffraction analysis, 0.2 ml H 2 O was added to the starting mixture as a mineraliser. The crystalline product was filtered off, washed with deionised water and dried in air. The compound is stable towards air and water. The compositions of the samples were confirmed by EDX (Philips ESEM XL 30 equipped with an EDAX detector) analysis (Ag/I/Cr ratio 8.0 : 1.83 : 3.09).

X-ray investigations

X-ray investigations on powder samples were performed using high-resolution X-ray powder diffraction data (D8, Bruker, Cu Kα 1 radiation from primary Ge(111) Johannson-type monochromator) at room temperature. The powder diffraction data were collected in the range of 5 to 90 degrees in 2 Theta. Experimental d values and relative intensities are given in Table 1. The single crystal diffraction data were collected on a Bruker AXS Smart-CCD diffractometer (Mo Kα radiation, graphite monochromator). The intensities were corrected for Lorentz, polarisation effects, and a face-indexed absorption correction was applied. The structure was solved and refined using the program packages SHELXS-97 and SHELXL-97 [START_REF] Sheldrick | SHELXS-97[END_REF][START_REF] Sheldrick | SHELXL-97[END_REF]. The final refinement included anisotropic displacement parameters for all atoms. For the technical details of data acquisition and further crystallographic data see Tables 234. Impedance Spectroscopy Ion-blocking gold electrodes were used to measure ionic conductivity of compact samples (diameter 6 mm, thickness 0.85 mm, pressed with 200 MPa). The samples were placed into a quartz glass cell in air during the measurement [START_REF] Köhler | [END_REF]. The temperature dependent ac impedance spectra were measured with an Novocontrol Alpha-A 4.2 Analyzer combined with the impedance interface ZG4 in a 2-wire arrangement in the frequency range of ν = 1 Hz to ν = 20 MHz. Measurements and data recording were performed with the WinDeta program [START_REF][END_REF]. The bulk conductivity was determined by non linear mean square deviation curve fitting of the impedance spectrum using the WinFit program [23].

Results and Discussion

Crystal Structure

As a characteristic feature, Ag 8 I 2 (CrO 4 ) 3 exhibits the uncommon coexistence of I -and CrO 4 2-ions, within the same compound. The aesthetically appealing crystal structure (see Figure 1) contains one crystallographically independent site for chromium, which is in fairly regular tetrahedral coordination by four oxygen atoms. The Cr-O bond distances cover the range from 1.634 -1.674 Å, and thus are in good agreement with typical values found for chromates(VI) [START_REF] Curda | [END_REF]26]. The average O-Cr-O angle of 109.5° is very close to the regular tetrahedral value and only the O2-Cr1-O2 angle (112.4°) differs more than 2° from this ideal case. The isolated CrO 4 2-tetrahedra are arranged in the sense of an hcp packing scheme (Fig. 1 and2). Table 2. Details of the data collection and selected refinement results for Ag 8 I 2 (CrO 4 ) 3 .

Ag8I2(CrO4)3

Crystal system hexagonal Formula weight / g mol -1 488. In accordance with the number of six CrO The structure thus corresponds in part to the anti-TiI 3 structure type, with CrO 4 2-anions in place of the iodine atoms. The coordination environment of the silver atoms follows from their positions within the hcp of the chromate(VI) anions, at the triangular faces, and the tetrahedral and octahedral voids. Positioned at a center of symmetry, the coordination of Ag1 can be well described as a trigonal bipyramid with three oxygen atoms in the equatorial plane and two iodine atoms in apical positions, with Ag-O bonds of 2.426 Å and apical Ag-I distances of 2.809 Å, respectively. The corresponding angles are ideal (90.0, 120.0 and 180.0°), indicating the perfect trigonal bipyramidal environment of Ag1 (Fig. 3). Ag2 exhibits a ψtrigonal-bipyramidal coordination in respect of the oxygen atoms, with bond lengths in a narrow range between 2.384 and 2.541 Å. At some longer distances of 3.042 and 3.144 Å there are two additional bonds to iodine atoms, forming together with the oxygen atoms a heavily distorted octahedral surrounding of Ag2 (angles: 77.2 to 150.9°). The X-ray single-crystal structure determination establishes that the coordination sphere of Ag3 simply consists of six oxygen atoms, forming a moderately distorted octahedron with equidistant Ag-O bond lengths of 2.439 Å and angles between 81.6 and 180.0° (Fig. 3). Table 4. Selected interatomic distances/Å and angles/° for Ag 8 I 2 (CrO 4 ) 3 . Estimated standard deviations are given in parentheses.

Similar silver coordination numbers and the corresponding bonding distances have been frequently observed in various silveroxide and silveriodine materials [7][8][9][10][11][12][13][14][15][16][17][18]. The shortest Ag-Ag distance observed is 2.880(1) Å (Ag2-Ag2). This is shorter than the sum of the van-der-Waals contacts of 3.4 Å and comparable to the interatomic distance in metallic silver (2.89 Å). Such short silver-silver distances have been frequently observed in silver rich oxides [27]. The remaining Ag-Ag distances are between 3.043 and 3.812 Å.

Ionic conductivity

The temperature dependence of the ionic conductivity for The Arrhenius plot of the temperature-dependent ionic conductivity is displayed in Fig. 4. The corresponding activation energy (E a ) has been derived from the slopes of the conductivity curves by using the Arrhenius equation, and the conductivities at selected temperatures are listed in Table 5. Between RT and 175 °C, the conductivity of Ag 8 I 2 (CrO 4 ) 3 increases gradually from 6.5 X 10 -4 to 5.7 X 10 - 2 Ω -1 cm -1 in two steps. The curve of the temperaturedependent ionic conductivity can be divided into two different steps. During the first step between RT and 120°C a flat increase is noticeable. Within this range, the conductivity increases by one order of magnitude. Between 120 and 175 °C a jump occurs, with a further increase by one order of magnitude in the conductivity. During the heating process, no colour change was observable. The calculated activation energies by applying the Arrhenius model yield similar values for the measurements in both, the heating and cooling mode (Table 5). Table 5. Ionic conductivities (σ) and activation energy (E a ) for Ag 8 I 2 (CrO 4 ) 3 .

σ/Ω -1 cm -1 E a /eV heating 30°C: 6.5 X 10 -4 Ω -1 cm -1 110°C: 8.3 X 10 -3 Ω -1 cm -1 175°C: 5.7 X 10 -2 Ω -1 cm -1 25-50°C: 0.21 90-120°C: 0.36 150-175°C: 0.25 cooling 30°C: 7.3 X 10 -4 Ω -1 cm -1 110°C: 8.7 X 10 -3 Ω -1 cm -1 175°C: 5.6 X 10 -2 Ω -1 cm -1 25-50°C: 0.25 90-120°C: 0.36 150-175°C: 0.15

The change in conductivity values upon the heating and cooling cycles is completely reversible. In order to check for possible phase transformations occurring during the heating process, powder and single crystalline samples of Ag 8 I 2 (CrO 4 ) 3 were investigated on respective diffractometers at a temperature of 120 and 180°C. Unfortunately, the crystallinity of the samples decreased significantly upon heating, presumably due to the melting of the silver sublattice. Therefore no conclusive evidence for a structural phase transitions to occur, and being the reason for the steep increase in conductivity, was obtained.

Conclusions

The new solid electrolyte Ag 8 I 2 (CrO 4 ) 3 is showing pure silver ionic conductivity as high as 6.5 X 10 -4 Ω -1 cm -1 at RT, and an activation energy for conduction of 0.21 eV between 25 and 50 °C. In analogy to Ag 4 I 2 SeO 4 , also Ag 8 I 2 (CrO 4 ) 3 is consisting of isolated building units. In contrast to former silver iodine materials the jump in the absolute value of the ionic conductivity is only moderate in Ag

  4 2-anions per unit cell, there are six face sharing octahedral vacancies in this packing, which generate three columns of face sharing octahedra. Two of the resulting channels extending along [001] accommodate two iodine and one silver atom each, with iodine occupying the octahedral voids. The triangular faces connecting the octahedra in these latter rods are occupied by Ag + (3 + 2 coordination). The octahedral sites of the third column, passing the origin, are solely occupied by silver ions. Because of the larger diameter of I -as compared to Ag + the former two columns are expanded. The remaining silver atoms are occupying all the tetrahedral voids of the hcp arrangement of the CrO 4 2-anions.

Figure 1 .

 1 Figure 1. View at the crystal structure of Ag 8 I 2 (CrO 4 ) 3 along [001].

Figure 2 .

 2 Figure 2. "Channel" of face sharing octahedral voids of the chromate hcp packing, occupied by iodide and silver.

Figure 3 .

 3 Figure 3. Coordination environments of Cr and Ag in Ag 8 I 2 (CrO 4 ) 3 .

Figure 4 .

 4 Figure 4. Temperature dependence of bulk ionic conductivity for Ag 8 I 2 (CrO 4 ) 3 . Filled symbols represent heating, open symbols cooling.

Table 1 .

 1 Experimental X-ray powder data for Ag 8 I 2 (CrO 4 ) 3 with d > 1.5 Å (I > 1%).

	dobs / Å	I / %	h k l	dobs / Å	I / %	h k l	dobs / Å	I / %	h k l
	8.1816	4.2	0 1 0	2.2157	2.7	3 1 1	1.7106	5.5	2 1 5
	6.3985	2.7	0 1 1	2.1742	1.4	0 2 4	1.7106	5.5	1 2 5
	5.1336	7.8	0 0 2	2.1456	1.6	2 2 2	1.6863	1.9	1 4 2
	4.2913	1.8	1 1 1	2.0755	2.1	1 3 2	1.6863	1.9	4 1 2
	3.8003	1.5	0 2 1	2.0800	2.1	3 1 2	1.6457	3.8	3 2 3
	3.4760	6.3	1 1 2	2.0454	3.6	0 4 0	1.6457	3.8	2 3 3
	3.1993	8.3	0 2 2	2.0060	1.7	0 4 1	1.6363	6.5	0 5 0
	3.1600	5.8	0 1 3	1.9917	1.2	0 1 5	1.6159	1.2	0 5 1
	3.0924	1.9	1 2 0	1.9439	38.8	2 2 3	1.6089	24.3	1 1 6
	3.0924	1.9	2 1 0	1.9001	5.1	0 4 2	1.5996	2.7	0 4 4
	2.9610	7.1	1 2 1	1.8912	12.4	1 3 3	1.5800	18.1	4 1 3
	2.9610	7.1	2 1 1	1.8912	12.4	3 1 3	1.5800	18.1	1 4 3
	2.7714	68.0	1 1 3	1.8832	8.4	1 1 5	1.5787	1.0	0 2 6
	2.7272	100.0	0 3 0	1.8770	2.1	3 2 0	1.5700	31.8	3 3 0
	2.6498	1.1	1 2 2	1.8770	2.1	2 3 0	1.5590	10.8	0 5 2
	2.6489	1.1	2 1 2	1.8691	14.9	0 3 4	1.5600	1.6	3 3 1
	2.6249	14.7	0 2 3	1.8464	3.2	3 2 1	1.5496	6.5	2 2 5
	2.5668	4.3	0 0 4	1.8464	3.2	2 3 1	1.5462	4.1	4 2 0
	2.4085	38.1	0 3 2	1.7854	4.4	1 4 0	1.5462	4.1	2 4 0
	2.3618	6.6	2 2 0	1.7854	4.4	4 1 0	1.5289	5.9	4 2 1
	2.3017	5.9	2 2 1	1.7629	8.2	3 2 2	1.5289	5.9	2 4 1
	2.2945	9.8	2 1 3	1.7629	8.2	2 3 2	1.5226	4.7	1 3 5
	2.2945	9.8	1 2 3	1.7590	1.3	1 4 1	1.5226	4.7	3 1 5
	2.2692	3.9	1 3 0	1.7590	1.3	4 1 1	1.5200	5.3	3 2 4
	2.2692	3.9	3 1 0	1.7557	4.6	0 4 3	1.5151	5.3	2 3 4
	2.2553	28.7	1 1 4	1.7400	23.3	2 2 4	1.5053	18.4	3 3 2
	2.2157	2.7	1 3 1	1.7112	4.8	0 0 6	1.5000	2.5	1 2 6

Wiley-VCH ZAAC

Acknowledgments

We thank Dr. J. Nuss for collecting of the single crystal X-ray data, and Güliz Cakmak for recording the ionic conductivity measurements.

____________


Peter Lorenz

Dr Walter Jung
email: walter.jung@uni-koeln.de

New Scandium Platinum Silicides

Keywords:

Two new scandium platinum silicides, ScPt 2 Si (orthorhombic, Cmcm, a = 3.6653(6) Å, b = 9.831(1) Å, c = 6.967(1) Å, Z = 4) and Sc 5 Pt 9 Si 7 (hexagonal, P 6, a = 9.9418(5) Å, c = 3.8832(2) Å, Z = 1) were prepared by reaction of the elements in an electric arc furnace and subsequent annealing. The crystal structures were solved and refined from single crystal diffraction data. ScPt 2 Si crystallizes with an 'inverse' MgCuAl 2 structure while Sc 5 Pt 9 Si 7 forms a variant of the Zr 2 Fe 12 P 7 type.

____________

Introduction

While ternary silicides of the platinum metals with a havier rare earth metal (Y, La-Lu) as a third component have been studied to a larger extent [1], less is known about corresponding compounds with scandium. Former phase analytical work done by Braun et al. [START_REF] Braun | [END_REF] in the system Sc-Rh-Si revealed the existence of 9 ternary phases. However, later on only ScRhSi [3], ScRhSi 2 [4], ScRh 3 Si 7 [5] and Sc 5 Rh 4 Si 10 [6] were structurally characterized, the latter only from powder data. Four ternary phases have been reported in the System Sc-Ir-Si, but only the structures of ScIr 3 Si 7 [5] and Sc 5 Ir 4 Si 10 [6] are known from single crystal work. The structures of ScIrSi [3] and Sc 4 Ir 7 Si 6 [7] were assigned on the basis of powder patterns. This holds also for the structures of ScPdSi and ScRuSi [3]. The structure of Sc 3 Os 2 Si 6 is not yet known [6]. In the system Sc-Pt-Si for a long time only ScPtSi with TiNiSi type structure was known [3], until recently Sc 3 PtSi 3 together with Sc 2 RuSi 2 was presented by Harmening et al. at a conference [8]. Here we report on ScPt 2 Si and Sc 5 Pt 9 Si 7 crystallizing with variants of the MgCuAl 2 [9] and the Zr 2 Fe 12 P 7 type [10], respectively.

Results and Discussion

ScPt 2 Si ScPt 2 Si crystallizes in space group Cmcm and may be formally assigned to the MgCuAl 2 type of structure [9], an ordered variant of the Re 3 B type [11]. This structure type is frequently observed, especially with alkaline earth or rare earth transition metal gallides, indides, and stannides like CaPtGa 2 [12], CePdIn 2 [13], or EuIrSn 2 [14]. In these compounds transition metal and post-transition metal atoms occupy the sitesets 4c and 8f, respectively. In ScPt 2 Si, however, these sitesets are interchanged. Platinum atoms are now the majority component and reside on 8f, while silicon atoms are found on 4c. Therefore, the ScPt 2 Si structure may be called an 'inverse' MgCuAl 2 type. The MgCuAl 2 structure has been described in a somewhat formal way [13], aiming at rows of trigonal prisms running along the a axis and shifted against each other by a/2. In the case of ScPt 2 Si the prisms are formed by platinum and scandium atoms and they are centered by silicon atoms. In Figure 1a, a projection of the structure along [100], these prisms are emphasized by bold lines. In Figure 1b they are compared to corresponding prisms in the structure of CaPtGa 2 . This view of the structure originates from the archetype Re 3 B, because trigonal transition metal prisms centered by boron atoms are typical building elements of metal rich borides. To ScPt 2 Si a different description is more adequate. According to the differences between the electronegativities of scandium on one side and platinum and silicon on the other side, the commonly accepted bonding model regards positively charged scandium atoms and a polyanion built of platinum and silicon atoms. In Figure 2 showing perspective views of the structure, this polyanion is emphasized by drawing Pt-Pt and Pt-Si bonds. It contains channels with pentagonal and with hexagonal cross sections running along the a-and the c-axis, respectively. The scandium atoms are located at the points of intersection inside of these channels. Their environment is shown in Figure 3a. It consists of three silicon atoms (d(Sc-Si): 1x2.766(5) Ǻ, 2x2.825(4) Ǻ) and 10 platinum atoms (d(Sc-Pt): 4x2.913(1) Ǻ, 4x2.937(2) Ǻ, 2x3.402(3) Ǻ). From the latter only 8 are close enough for bonding interactions. Their distances correspond to the sum of the metallic radii for coordination number (CN) 12 [START_REF] Pauling | Die Natur der Chemischen Bindung[END_REF]. The silicon atoms are surrounded by 6 platinum atoms (d(Si-Pt): 2x2.430(2) Ǻ, 4x2.496(2) Ǻ) forming a distorted trigonal prism and three scandium atoms capping the side-faces of the prism (Figure 3b). The coordination polyhedron around the platinum atom is quite irregular. It is built up by four scandium, four platinum, and three silicon atoms. The shortest interatomic distances in the structure are those between platinum and silicon atoms with a mean value of 2.474 Ǻ which is nearly the same as the sum of Pauling's single bond radii for platinum and silicon of 2.468 Ǻ [START_REF] Pauling | Die Natur der Chemischen Bindung[END_REF]. Strong bonding interactions may thus be assumed, supporting the model of a Pt-Si-polyanion. All the interatomic distances considered above compare well with those found in ScPtSi [3]. ScPt 2 Si is the first silicide crystallizing with an ordered variant of the Re 3 B type. The structure of Re 3 B is closely related to that of Fe 3 C [START_REF] Herbstein | [END_REF]. An ordered variant of the latter is already known with the YPd 2 Si structure adopted by two series of ternary rare earth silicides of palladium and platinum [17]. Therefore the existence of a ternary silicide with Re 3 B derived structure was formerly predicted by Parthé and Chabot [1b].

Sc 5 Pt 9 Si 7

The crystal structure of Sc 5 Pt 9 Si 7 , shown as a projection along the c-axis in Figure 4, is a substitution variant of the Zr 2 Fe 12 P 7 structure [10], a well known structure type, which is adopted by numerous ternary transition metal phosphides preferently with a rare earth metal as the third component. Sc 5 Pt 9 Si 7 and Zr 2 Fe 12 P 7 both crystallize in space group P6 with similar lattice constants and positional parameters. The phosphorus positions are taken by the silicon atoms. Three of the four iron sitesets (2x3j + 3k) are occupied by platinum atoms, while the remaining one (3k) is occupied by scandium atoms. The other scandium atoms reside on the zirconium positions. Sc 5 Pt 9 Si 7 is not the first compound, in which a substitution of transition metal atoms on this special 3k site by the more electropositive component is observed, but it is the first example of a complete substitution, and it is the first silicide with a Zr 2 Fe 12 P 7 derivative structure. Examples of substitution to a small extent are the compounds Mg 2.5 Ni 11.5 P 7 , Ca 2.1 Ni 11.9 P 7 , and Ca 2.3 Ni 11.7 As 7 [18], while in Yb 2+3-x Pd 12-3+x P 7 with (x=0.40) the substitution is nearly complete [19]. With regard to these examples, a statistical distribution of scandium and platinum atoms on the 3k site of Sc 5 Pt 9 Si 7 could also be possible. However, in samples with different compositions no significant deviations of the lattice parameters were observed. Therefore a constant composition is assumed.

As stated above for ScPt 2 Si, the structure of Sc 5 Pt 9 Si 7 is best discussed considering a 3D-polyanion built from platinum and silicon atoms in which positively charged scandium atoms are inserted (Figure 4). This view is again supported by the short distances between platinum and silicon atoms (Table 4) indicating strong bonding interactions. The three kinds of platinum atoms are tetrahedrally coordinated by four silicon atoms. The Pt-Si distances range from 2.336(6) to 2.761(5) Ǻ with a mean value of 2.486 Ǻ which again is nearly equal to the sum of Pauling's single bond radii [START_REF] Pauling | Die Natur der Chemischen Bindung[END_REF]. Moreover three platinum atoms (d(Pt-Pt): 2.815(1) Ǻ -2.900(1) Ǻ) and four scandium atoms (d(Pt-Sc): 2.913(4) Ǻ -3.402(1) Ǻ) contribute to the first coordination spheres of the platinum atoms. The silicon atoms Si1, Si2, and Si3 are connected to three, five, and six platinum atoms, respectively.

The three crystallographically distinct scandium atoms are located in channels with pentagonal and hexagonal crosssections running in the direction of the c-axis (Figure 4). Their environments are shown in Figure 5. The pentagonal channels containing the Sc1 atoms are arranged in groups of three. This results in a clustering of the Sc1 atoms (Figure 5a) and Sc1-Sc1 distances of 3.396(8) Ǻ, which are the shortest distances between scandium atoms in the structure. They are, however, still larger than those observed in hcp scandium (3.254 and 3.309 Ǻ [20]). All other Sc-Sc distances are larger than 3.78 Ǻ indicating the absence of bonding interactions. Depending on the cross-sections of the channels, the first coordination spheres of the scandium atoms are either pentagonal or hexagonal prisms. They are completed by additional atoms in front of the rectangular side-faces. The prisms, highlighted by bold lines in Figure 5, are composed of six platinum and four silicon atoms in the case of Sc1 and of six platinum and six silicon atoms in the case of Sc2 and Sc3. Atoms capping the side-faces are 1Pt+1Si for Sc1, 3Pt for Sc2, and 6Pt for Sc3. The Sc-Pt distances range from 2.913(4) to 3.200(1) Ǻ, the Sc-Si distances from 2.759(3) to 2.990(5) Ǻ. The former are thus close to, the latter smaller than the sum of the metallic radii for CN 12 [START_REF] Pauling | Die Natur der Chemischen Bindung[END_REF].

Experimental Section

Synthesis

Starting materials for the preparation of the title compounds were the elements: scandium metal, cut into small pieces (99.9% Johnson Matthey), platinum powder (99.9%, Degussa), and silicon pieces (m8N, Johnson Matthey). Mixtures of the elements in the atomic ratios according to the formulas were pressed into pellets of about 700 mg weight and arc melted under purified argon. To achieve homogeneity, the buttons were turned around and remelted two times. Weigt loss during the melting process was negligible. Subsequently the buttons were placed in alumina crucibles, sealed in silica tubes under argon atmosphere and annealed four days at 1200 °C. Finally they were cooled to room temperature at a rate of 20 °C/h. From the crushed samples single crystals of irregular shape were isolated. They are stable to moist air for long periods of time. Semi-quantitative EDX measurements (Philips XL40 scanning electron microscope with EDAX detector) confirmed the compositions of the crystals as obtained from the structure analysis and excluded the presence of other elements.

X-ray diffraction and crystal structure

X-ray powder data were taken with a Guinier diffractometer (Huber G645, CuKα 1 , Si as external standard). For correct indexing of the patterns they were compared to theoretical ones calculated on the basis of the structure refinements. While ScPt 2 Si was single phase according to the accuracy of the method, the Sc 5 Pt 9 Si 7 samples contained ScPtSi as a by-product.

The quality of the single crystals was checked by means of Laue photographs taken on a Buerger precession camera. Single crystal intensity data were collected at room temperature on a STADI-I diffractometer. Numerical absorption corrections were performed after optimizing the crystal faces with the program X-SHAPE [21]. The structures were solved by means of direct methods with SHELXS-97 [START_REF] Sheldrick | SHELXS-97 -Program for the Determination of Crystal Structures[END_REF] and refined on F 2 values using SHELXL-97 [START_REF] Sheldrick | SHELXL-97 -Program for Crystal Structure Refinement[END_REF] with anisotropic displacement parameters for all atoms. No significant peaks were found in the difference Fourier syntheses. Crystallographic data and details of the data collection and the structure refinement are listed in Table 1. Table 2 contains the positional parameters and the equivalent isotropic displacement parameters, Table 3 the anisotropic displacement parameters. Selected interatomic distances are listed in Table 4.

Figure 1 .

 1 Figure 1. Projections along [100] of the structures of a) ScPt 2 Si ('inverse' MgCuAl 2 type) and b) CaPtGa 2 (MgCuAl 2 type) (Pt: black; Si, Ga: gray; Sc, Ca: white; this holds for all drawings; numbers indicate x parameters of the atoms; unit cells outlined). The lines emphasizing the trigonal prisms do not all represent chemical bonds.

Figure 2 .

 2 Figure 2. Perspektive views of the ScPt 2 Si structure approximately along [100] and [001].

Figure 3 .

 3 Figure 3. Environments of a) Sc-and b) Si-atoms in the structure of ScPt 2 Si.

Figure 4 .

 4 Figure 4. Projection of the Sc 5 Pt 9 Si 7 structure along [001].

Figure 5 .

 5 Figure 5. Environments of the scandium atoms in the structure of Sc 5 Pt 9 Si 7 .

FurtherFigure 1 .Figure 2 .Figure 3 .Figure 4 .Figure 5 .

 12345 Figure 1. Projections along [100] of the structures of a) ScPt2Si ('inverse' MgCuAl2 type) and b) CaPtGa2 (MgCuAl2 type) (Pt: black; Si, Ga: gray; Sc, Ca: white; this holds for all drawings; numbers indicate x parameters of the atoms; unit cells outlined). The lines emphasizing the trigonal prisms do not all represent chemical bonds. 162x104mm (600 x 600 DPI)

Table 1 .

 1 Crystallograhic data and structure refinement of ScPt 2 Si and Sc 5 Pt 9 Si 7

	Formula	ScPt 2 Si	Sc 5 Pt 9 Si 7
	Crystal system	orthorhombic	hexagonal
	Space group	Cmcm	P 6
	Lattice parameters
		V = 251.05 Å 3	V = 332.39 Å 3
	Fomula units/cell, Z	4	1
	Calculated density	12.256 g/cm 3	10.877 g/cm 3
	Absorption coefficient	113.9 mm -1	97.23 mm -1
	Crystal size	20x40x50 µm 3 30x40x50 µm 3
	Transmission ratio (max/min) 2.72	4.31
	Diffractometer	STOE IPDS	STOE IPDS
	Radiation	MoKα	MoKα
	φ Range, increment	0-200°, 2.0°	0-200°, 2.0°
	Exposure time	9 min	10 min
	θ max for data collection	33°	33°
	Ranges for h, k, l	±5, ±13, ±10	±15, ±15, ±5
	Reflections measured	1805	4787
	unique	281	922
	I>2σ(I)	258	829
	R int	0.0566	0.1165

a = 3.665(6) Å a = 9.9418(5) Å (powder data) b = 9.831(1) Å c = 3.8832(2) Å c = 6.967(1) Å

Table 2 .

 2 Atomic coordinates and equivalent isotropic displacement parameters (pm 2 ) for ScPt 2 Si (space group Cmcm) and Sc 5 Pt 9 Si 7 (space group P 6). U eq is defined as one third of the trace of the orthogonalized U ij tensor.

	Atom	Wykoff	x	y	z	U eq
		position
	ScPt 2 Si:
	Sc	4c	0	0.0469(3) 1/4	64(6)
	Pt	8f	0	0.36214(4) 0.04860(6) 62(2)
	Si	4c	0	0.7656(4) 1/4	58(8)
	Sc 5 Pt 9 Si 7 :
	Sc1	3k	0.2274(5) 0.1244(6) 1/2	76(7)
	Sc2	1f	2/3	1/3	1/2	85(14)
	Sc3	1c	1/3	23	0	107(23)
	Pt1	3j	0.1229(1) 0.2854(1) 0	68(2)
	Pt2	3j	0.4338(1) 0.3531(1) 0	77(2)
	Pt3	3k	0.0517(1) 0.4487(1) 1/2	71(2)
	Si1	1a	0	0	0	76(7)
	Si2	3j	0.4298(1) 01148(7)	0	47(11)
	Si3	3k	0.2844(7) 0.4275(8) 1/2	58(12)

Table 3 .

 3 Anisotropic displacement parameters (pm 2 ) for ScPt 2 Si and Sc 5 Pt 9 Si 7 .

	Atom	Site	U 11	U 22	U 33	U 23	U 12
	ScPt 2 Si
	Sc	4c	48(11) 30(11) 112(15 0	0
	Pt	8f	81(2)	46(2)	57(3)	-2(1)	0
	Si	4c	82(1)	36(17) 54(21) 0	0
	Sc 5 Pt 9 Si 7
	Sc1	3k	43(18) 114(20) 60(19) 0	32(16)
	Sc2	1f	99(21) U 11	56(35) 0	49(10)
	Sc3	1c	101(23) U 11	127(40) 0	51(12)
	Pt1	3j	68(4)	73(4)	63(4)	0	35(3)
	Pt2	3j	54(4)	67(4)	100(4) 0	23(3)
	Pt3	3k	87(4)	73(4)	60(4)	0	46(4)
	Si1	1a	85(33) U 11	151(60) 0	42(17)
	Si2	3j	37(25) 19(24) 70(30) 0	3(21)
	Si3	3k	47(26) 65(29) 49(30) 0	19(24)

Table 4 .

 4 Selected interatomic distances (Å) for ScPt 2 Si and Sc 5 Pt 9 Si 7

	Atoms	d	Atoms	d
	ScPt 2 Si
	Sc -1Si	2.766(5)	Pt -1Si	2.430(2)
	2Si	2.825(4)	2Si	2.496(2)
	4Pt	2.913(1)	1Pt	2.794(1)
	4 Pt	2.937(2)	1Pt	2.806(1)
	Si -2Pt	2.430(2)	2Sc	2.913(1)
	4Pt	2.496(2)	2Sc	2.937(2)
	1Sc	2.766(5)	2Pt	2.946(1)
	2Sc	2.826(4)
	Sc 5 Pt 9 Si 7
	Sc1 -2Si1 2.759(3)	Pt3 -1Si3 2.336(6)
	1Si3 2.774(9)	1Si3	2.426(7)
	2Si2 2.827(6)	2Si2	2.466(4)
	2Pt2 2.913(4)	2Pt1	2.839(1)
	2Pt1 2.940(4)	2Pt2	2.900(1)
	1Pt3 2.976(5)	1Sc1 2.976(5)
	2Pt1 3.012(4)	2Sc3 3.200(1)
	Sc2 -6Si2 2.990(5)	1Sc2 3.402(1)
	6Pt2 3.102(1)	Si1 -3Pt1 2.465(1)
	1Pt3 3.402(1)	6Sc1 2.759(3)
	Sc3 -6Si3 2.916(5)	Si2 -1Pt2 2.346(6)
	6Pt3 3.200(1)	1Pt2	2.353(7)
	Pt1 -2Si3 2.465(4)	2Pt3	2.466(4)
	1Si1	2.465(1)	1Pt1	2.520(6)
	1Si2	2.520(6)	2Sc1 2.827(6)
	1Pt2	2.815(1)	2Sc2 2.990(5)
	2Pt3	2.839(1)	Si3 -1Pt3 2.336(6)
	2Sc1 2.940(4)	1Pt3	2.426(7)
	2Sc1 3.012(4)	2Pt1	2.465(4)
	Pt2 -1Si2 2.346(6)	2Pt2	2.761(5)
	1Si2	2.353(7)	1Sc1 2.774(9)
	2Si3	2.761(5)	2Sc3 2.916(5)
	1Pt1	2.815(1)
	2Pt3	2.900(1)
	2Sc1 2.913(4)
	2Sc2 3.102(1)

Acknowledgments

Financial support by the Deutsche Forschungsgemeinschaft is gratefully acknowledged.