A. Wurth

Neue Arsenide mit ThCr2Si2- oder einer damit verwandten Struktur: Die Verbindungen ARh2As2 (A: Eu, Sr, Ba) und BaZn2As2 / New Arsenides with ThCr2Si2-type or Related Structures: The Compounds ARh2As2 (A: Eu, Sr, Ba) and BaZn2As2

Four new arsenides of rhodium and zinc were prepared by heating mixtures of the elements at high temperatures (1000 - 1200 °C) and investigated by single crystal X-ray methods. EuRh2As2 (a = 4.067(1), c = 11.319(2) Å ) and BaRh2As2 (a = 4.053(1), c = 12.770(3) Å ) crystallize with the well-known ThCr2Si2-type (I4/mmm; Z = 2). Due to the rigid layers of RhAs4 tetrahedra, and to the atomic size of europium and barium, the As-As distances between the layers with values of 2.97 and 3.66 Å, respectively, are very long. SrRh2As2 is polymorphic and undergoes two phase transitions at about 190 and 282 °C. Main features of the three crystal structures are also layers of RhAs4 tetrahedra. At room temperature α-SrRh2As2 (a = 5.676(1), b = 6.178(2), c = 11.052(2) Å ) probably crystallizes with the BaNi2Si2-type (Cmcm; Z = 4), whereas β -SrRh2As2 (a = 5.760(3), b = 6.067(4), c = 11.264(5) A° , Fmmm, Z = 4) forms a new orthorhombically distorted variant of the ThCr2Si2-type. Single crystals grown in a flux of lead and quenched at high temperature show that the γ -phase (a = 4.112(1), c = 11.431(6) Å ) crystallizes with the ThCr2Si2-type. The same is true for the high temperature modification of BaZn2As2 (β -phase; a = 4.120(1), c = 13.578(1) Å ), whereas the already known α-BaZn2As2 forms the α-BaCu2S2-type (Pnma; Z = 4) consisting of a 3D-network of edge- and vertex-sharing ZnAs4 tetrahedra with Ba atoms in the voids of this network.

First- and second-order phase transitions in ternary europium phosphides with ThCr2Si2-type structure

Abstract The ThCr2Si2- type compounds EuFe2P2 and EuRu2P2 exhibit as a function of pressure continuous phase transitions which are accompanied by extremely strong changes of the lattice parameters and particularly of the P–P distance dP−P along the tetragonal c-axis. In the isostructural integralvalent LaT2P2 compounds similar phase transitions of second (T=Fe) and first order (T=Co) occur as a function of pressure indicating that the T element essentially determines the nature of the phase transition. The different types of phase transitions are explained phenomenologically.

First-order phase transitions in the ThCr2Si2-type phosphides ARh2P2 (A = Sr, Eu)

Abstract We observed for the first time a first-order phase transition with strong and extremely anisotropic changes of the lattice parameters in compounds crystallizing in the ThCr2Si2 structure type. The phase transition occurs SrRh2P2 with increasing pressure at 6 GPa (300 K) and in EuRh2P2 with increasing temperature at 810 K (ambient pressure). On the basis of single-crystal data of ARh2P2 (A = Ca, Sr, Ba, Eu) at ambient pressure and temperature we discuss the PP distance in the framework of band-structure calculations at the first-order phase transition the PP state changes from a ‘no-bond’ to a ‘single-bond’ state.

Neue ternäre Phosphide und Arsenide mit einem Metall : Nichtmetall‐Verhältnis im Bereich von 2 : 1

Durch Erhitzen entsprechend zusammengesetzter Elementgemenge bzw. deren Umsetzung in Sn- oder Pb-Schmelzen als Reaktionsmedium wurden sechs neue Verbindungen dargestellt, die rontgenographisch mit Einkristall-Methoden untersucht wurden. Sc2Ni12P7 (a = 9,013(1), c = 3,590(1) A) kristallisiert im Zr2Fe12P7-Typ (P6; Z = 1); diesen bilden im wesentlichen auch die Verbindungen Eu2Pd12As7 (a = 10,040(1), c = 4,100(1) A) und Sr2Rh12P7 (a = 9,626(1), c = 3,844(1) A), bei denen aber eins von sieben Nichtmetallatomen eine etwas andere Umgebung besitzt und langs [001] fehlgeordnet ist. Ihre Kristallstrukturen entsprechen dem Ho2Rh12As7-Typ (P63/m; Z = 1). Ca2Ni7P4 (a = 3,703(1), b = 9,209(1), c = 10,378(1) A) kristallisiert im Nd2Ni7P4-Typ (Pmn21; Z = 2), wahrend die Kristallstrukturen von Ca4Rh13As9 (a = 3,903(3), b = 11,221(1), c = 19,411(4) A) und Sm4Rh13As9 (a = 3,913(3), b = 11,242(6), c = 19,440(6) A) dem Ho4Ir13Ge9-Typ (Pmmn; Z = 2) entsprechen; allerdings mussen bei ihnen die Positionen einer Rh-Lage mit einer Splitlage beschrieben werden. Bei allen sechs Verbindungen bilden die Ubergangsmetallatome, die drei, vier oder funf P- bzw. As-Atome zu ihren nachsten Nachbarn zahlen, mit diesen jeweils ein dreidimensionales kovalent gebundenes Gerust, dessen Hohlraume von den elektropositiven Metallatomen besetzt werden. Die Umgebung der P- bzw. As-Atome besteht meist aus trigonalen Metallatomprismen, deren Rechteckflachen von weiteren Metallatomen uberdacht sind und die charakteristisch ist fur ternare Phosphide und Arsenide mit einem Metall : Nichtmetall-Verhaltnis im Bereich von 2 : 1. New Ternary Phosphides and Arsenides with a Metal : Non-Metal Ratio in the Range of 2 : 1 Six new compounds were prepared by heating mixtures of the elements or by reaction of them in a tin(lead) flux. They were investigated by single crystal X-ray methods. Sc2Ni12P7 (a = 9.013(1), c = 3.590(1) A) crystallizes in the Zr2Fe12P7 type structure (P6; Z = 1), which is basically built up likewise by Eu2Pd12As7 (a = 10.040(1), c = 4.100(1) A) and Sr2Rh12P7 (a = 9.626(1), c = 3.844(1) A), but one of seven non-metal atoms has a somewhat modified environment and is disordered along [001]. Therefore their crystal structure corresponds to the Ho2Rh12As7 type structure (P63/m; Z = 1). Ca2Ni7P4 (a = 3.703(1), b = 9.209(1), c = 10.378(1) A) forms the Nd2Ni7P4 type structure (Pmn21; Z = 2), whereas the atomic arrangements of Ca4Rh13As9 (a = 3.903(2), b = 11.221(1), c = 19.411(4) A) and Sm4Rh13As9 (a = 3.913(2), b = 11.242(6), c = 19.440(6) A) correspond basically to the Ho4Ir13Ge9 type structure (Pmmn; Z = 2), but the disorder of Rh8 required the occupation of splitting positions. The transition metals have three, four or five neighbouring atoms of phosphorus or arsenic and form together with them three-dimensional covalent frameworks, of which holes are occupied by the atoms of the electropositive metal. Most of the polyhedra around the P and As atoms respectively consist of trigonal prisms of metal atoms with additional metal atoms capping the rectangular faces of the prisms. This environment ist characteristic for ternary phosphides and arsenides with a metal : non-metal ratio in the range of 2 : 1.

Neue ternäre Rhodium‐ und Iridium‐Phosphide und ‐Arsenide mit U4Re7Si6‐Struktur

Einkristalle von Mg4Rh7P6 (a = 7,841(1) A), Mg4Rh7As6 (a = 8,066(1) A), Yb4Rh7As6 (a = 8,254(1) A) sowie von Mg4Ir7As6 (a = 8,082(2) A) wurden durch Umsetzen der Elemente in einer Bleischmelze erhalten und rontgenographisch untersucht. Die Verbindungen sind isotyp und kristallisieren im U4Re7Si6-Strukturtyp (Im 3 m; Z = 2). Dieser wird von CeMg2Si2-analogen Bausteinen aufgebaut, die um 90° gegeneinander verdreht sind. Die am Aufbau dieses Bauelements beteiligten Rh- bzw. Ir-Atome werden dadurch verzerrt tetraedrisch vom Nichtmetall koordiniert. Als Folge der Verknupfung bilden die P- bzw. As-Atome von jeweils sechs Bausteinen ein regulares Oktaeder, das von einem weiteren Rh(Ir)-Atom zentriert wird. Dieser zweite Baustein entspricht im wesentlichen dem Perowskit-Typ. New Ternary Rhodium- and Iridium-Phosphides and -Arsenides with U4Re7Si6 Type Structure Single crystals of Mg4Rh7P6 (a = 7.841(1) A), Mg4Rh7As6 (a = 8.066(1) A), Yb4Rh7As6 (a = 8.254(1) A) and Mg4Ir7As6 (a = 8.082(2) A) were prepared by heating mixtures of the elements in a lead flux and were investigated by means of X-ray methods. The compounds are isotypic and they crystallize in the U4Re7Si6 type structure (Im 3 m; Z = 2), which is formed by CeMg2Si2 analogous units, which are twisted against each other. The Rh(Ir) atoms building these units are coordinated tetrahedrally by the non-metal. The P(As) atoms of six units form a regular octahedron, which is centred by an additional Rh(Ir) atom. This second structural segment corresponds to the perovskit type structure.

SYNTHESE UND KRISTALLSTRUKTUR VON SR2RH7P6

Einkristalle von Sr2Rh7P6 wurden durch Umsetzung der Elemente in einer Bleischmelze bei 1100 °C erhalten und rontgenographisch untersucht. Die tetragonal kristallisierende Verbindung (a = 11,080(2), c = 4,098(1) A) bildet einen Strukturtyp (P 4 21m; Z = 2), in dem ThCr2Si2-analoge Baueinheiten in einer bislang unbekannten Art miteinander verknupft sind. Dadurch sind die RhP4-Tetraeder nicht in Schichten, sondern in Strangen aus je drei kantenverknupften Tetraederketten angeordnet, die entlang [001] verlaufen. Das Verknupfungsmuster ermoglicht einem Teil der P-Atome die Bildung kurzer P–P-Abstande von 2,26 A und bietet weiteren Rh-Atomen Platz, die ebenfalls verzerrt tetraedrisch von P-Atomen koordiniert werden. Synthesis and Crystal Structure of Sr2Rh7P6 Single crystals of Sr2Rh7P6 were obtained by reaction of the elements in molten lead at 1100 °C and investigated by X-ray methods. The compound crystallizes tetragonally (a = 11.080(2), c = 4.098(1) A) and forms a crystal structure (P 4 21m; Z = 2) with ThCr2Si2 analogous units, which are linked with each other in a new way. Therefore the RhP4 tetrahedra form bands of edge sharing chains parallel to [001] anstead of layers as in the ThCr2Si2 type structure. The arrangement enables a part of the P atoms to form short P–P distances of 2,26 A and space for additional Rh atoms with a likewise distorted tetrahedral coordination of P atoms is obtained.