Ln₃Au₂Al₉ (Ln  Dy, Tb): Heteronuclear versus Homonuclear Bonding in Intermetallic Phases

Abstract

press. I81 Due to the strongly elongated bonds thecompounds modeled here are the first examples of cobalt(ti1) hexaamines for which an anharmonic potential is required, and for which this may be accurately parameterized. The quality of the parameters is based on structural data, including that of the yellow isomer and a set of cobalt(rn) amines with moderately long bonds, and on the computed ligand field and redox properties (see below and ref. (71). [9] a) P. Ill] CAMMAG [I21 was used for the AOM calculations. 1121 a) M. Gerloch, M . CAMMAG, a Fortran Program for AOM Calculations, University ofcambridge, U. K.; b) H. Stratemeier, M. A. Hitchman, P. Comba, P V. Bernhardt, M. J. Riley, Inorg. Chern. 1991,30,4088; the parameterization is described in ref. [9]. [13] Ligand-induced steric strain is not the only contribution to the redox potentla1 of coodination compounds, and other effects have been discussed in detail [5,7]. The admittedly nmplistic approach used here was shown to lead to valuable results for hexaaminecobalt(m/ri) couplesMain-group intermetallic phases can be classified into three groups based on the valence electron concentration (VEC) per atom: Hume-Rothery phases with VEC<2, Zintl phases with VEC > 4, and compounds with 2 < VEC < 4 whose structures cannot be rationalized by any simple electron-counting rules. Compounds of these three classes display a rich variety o f structures and compositions ranging from close-packed structures with wide phase widths to valence compounds containing covalent two-center, two-electron bonds."' Many of these intermetallic phases offer chemists the challenge of complex structural features such as fractional and mixed site occupancies, site preferences, vacancy ordering, and structural phase transitions.Ostenson for magnetic susceptibility measurements.In addition to careful crystallography, calculations of electronic structure are essential for understanding the structures of these compounds. To explore the connection between VEC and siructure type for the compounds between the Hume-Rothery phases and the Zintl phases, we focused on the synthesis, characterization, and determination o f electronic structure of new ternary aluminum-rich intermetallic phases with 2 <VEC <4.The electronic structures of several binary aluminides-for example BaA14,r2"1 CaAl, ,[2b1 and LaA1,r2'1-were investigated recently. These studies suggest phase stability within narrow ranges of VEC, and optimization of Al-A1 bonding according to their calculated overlap populations. The ternary compound BaCu,Al,,-, ( 5 < x < 6 ) has the NaZn,, structure, in which Ba atoms are surrounded by a ~[Cu(Cu,-Al,,-,)] (4 < y < 5) cubic framework of Cu-stuffed icosahedra with randomly arranged Cu (ca. 33 %) and A1 atoms (ca. 67 YO). This is an example of a ternary aluminide whose VEC maximizes the bonding within the ~[Cu,AI,] network, as observed in its overlap populations. These theoretical results provide justification for attractive Al-Al, Cu-Al, and Cu-Cu interactions in these intermetallic phases. When...

Synthesis and Structure of the Alkali–Metal Amalgams A3Hg20 (A=Rb, Cs), K3Hg11, Cs5Hg19, and A7Hg31 (A=K, Rb)

“…This structural type includes primarily rear-earth compounds with aluminum and zinc (5,8). The structure can be also related to the BaAl -type structure as another defect version of the latter (Fig.…”

“…Several factors, among which the size of the cation and the number of available electrons, are thought to in#uence the relative stability of the two structures, and therefore they de"ne which one is preferred (8). The compound K Hg provides not only the "rst example of an amalgam with the -La Al structure but also the "rst one with alkali metal instead of rear-earth element.…”

Ternary phases in the Yb–Zn–Al system with La3Al11- and Yb8Cu17Al49-type structures

“…Besides the two cited Tb 3 (Zn,Al) 11 compounds, we refer to Dy 3 Au 2.1 Al 8.9 [18] and other phases in the systems ReZneAl (R ¼ Y, Gd, Dy, Ho) [3], ReCueAl (R ¼ Y [19], Gd [20], Tb [21], Dy [22], Ho [22]) and ReAgeAl (R ¼ Y [23], Gd [24], Tb [25], Dy [22], Ho [22]). Here called X1 and X4 sites show a sharply prevailing or a total occupation by the aluminium atoms; X2 is mainly filled by Zn in the Zn-containing phases and by a mixture in the others; X3, generally with mixed occupation, is sometimes practically filled by aluminium only.…”

On the crystal chemistry of new ternary rare earth gold aluminides

“…Rare earth -transition metal -aluminium systems have been intensively investigated in recent years [1] with respect to their phase relations, crystal structures and widely varying magnetic and transport properties. Among them, the R-Au-Al systems have been poorly studied; only the existence of a certain number of isostructural ternary compounds has been reported [2][3][4][5][6][7][8][9]. Investigations of ternary Gd 2 Au x Al 1-x phases with the Co 2 Si structure type (space group Pnma) were performed in [2].…”

“…The authors of [4][5] reported the existence of RAu 2-x Al 2+x phases (x ~ 0-1) (R = La-Yb) with the tetragonal BaAl 4 (space group I4/mmm), CaBe 2 Ge 2 (space group P4/nmm) and BaNiSn 3 (space group I4mm) structure types. Examples of novel R 3 Au 2 Al 9 compounds (R = Tb and Dy) with the orthorhombic La 3 Al 11 structure type (space group Immm) can be found in [8]. New representatives of the ScRh 3 Si 7 structure type (space group R-3c) in R-Au-Al (R = Ce-Nd, Sm, Gd-Lu) systems were discovered in [9].…”