The Zintl Phase Eu2Si

The Zintl phase Eu₂Si was synthesized from elemental europium and silicon in a sealed tantalum tube in a high‐frequency furnace at 1270 K and subsequent annealing at 970 K. Investigation of the sample by X‐ray powder and single crystal techniques revealed: Co₂Si (anti‐PbCl₂) type, space group Pnma, a = 783.0(1), b = 504.71(9), c = 937.8(1) pm, wR2 = 0.1193, 459 F² values and 20 variables. The structure contains two europium and one silicon site. ¹⁵¹Eu Mössbauer spectroscopic data show a single signal at an isomer shift of −9.63(3) mm/s, compatible with divalent europium. Within the Zintl concept electron counting can be written as (2Eu²⁺)⁴⁺Si⁴⁻, in agreement with the absence of Si‐Si bonding. Each silicon atom has nine europium neighbors in the form of a tri‐capped trigonal prism. The silicon coordinations of the Zintl phases Eu₂Si, Eu₅Si₃, EuSi, and EuSi₂ are compared.     

4‐Chloro­aceto­phenone [1‐(4‐methoxyphenyl)­ethyl­idene]­hydra­zone

The crystal structure of the title mixed azine, C₁₇H₁₇ClN₂O, contains four independent mol­ecules, A–D, and mol­ecule B is disordered. All four mol­ecules have an N—N gauche conformation, with C—N—N—C torsion angles of 136.5 (4), 137.0 (4), ?134.7 (4) and ?134.7 (4)°, respectively. The phenyl rings are also somewhat twisted with respect to the plane defined by C_ipso and the imine bond. On average, the combined effect of these twists results in an angle of 64.7° between the best planes of the two phenyl rings. Arene–arene double T‐contacts are the dominant intermolecular inter­action. The methoxy‐substituted phenyl ring of one azine mol­ecule interacts to form a T‐contact with the methoxy‐substituted phenyl ring of an adjacent mol­ecule and, similarly, two chloro‐substituted phenyl rings of neighboring mol­ecules interact to form another T‐contact. The only exception is for mol­ecule B, for which the disorder leads to the formation of T‐­contacts between methoxy‐ and chloro‐substituted phenyl rings. The prevailing structural motif of T‐contact formation between like‐substituted arene rings results in a highly dipole‐parallel‐aligned crystal structure.     

Flow equations for electron-phonon interactions: phonon damping

We report Nuclear Magnetic Resonance studies of CeAuAl₃ and LaAuAl₃ at frequencies between 1.30 and 76.91 MHz and at temperatures from up to . CeAuAl₃ is a new heavy-electron compound which orders antiferromagnetically at . For the two inequivalent Al sites of CeAuAl₃, the transferred hyperfine couplings and are relatively small compared to those in other Ce compounds. The nuclear quadrupolar coupling constants e2qQ / h are 2.9 MHz and 7.8 MHz, respectively. We suggest that the magnetic structure of the ordered state below is of a simple spiral type with the ordered Ce moments arranged ferromagnetically within the ( ab ) planes of the tetragonal crystal lattice. Based on this magnetic structure we estimate a 25% reduction of the Ce moments, most likely due to Kondo screening. At high temperatures, in the paramagnetic state of CeAuAl₃, the spin-lattice relaxation rate T1 ^-1 is dominated by the fluctuations of the localised Ce moments. At , T1 ^-1 is , more than two orders of magnitude larger than for the reference compound LaAuAl₃. For temperatures lower than T1 ^-1 decreases as a function of ( T / H ) and below , displays a linear-in-T behaviour, strongly enhanced compared to the reference compound LaAuAl₃. Received: 5 May 1998 / Received in final form: 16 July 1998 / Accepted: 20 July 1998     

Neubestimmung der Kristallstrukturen der Hexahydroxometallate Na2Sn(OH)6, K2Sn(OH)6 und K2Pb(OH)6

Redetermination of the Crystal Structures of the Hexahydroxometallates Na₂Sn(OH)₆, K₂Sn(OH)₆, and K₂Pb(OH)₆ Slow cooling down of hot saturated hydroxo stannate‐ resp. ‐plumbate solutions gives crystals of Na₂Sn(OH)₆, K₂Sn(OH)₆, and K₂Pb(OH)₆ well suited for an X‐ray structure determination. With these crystals the so far known crystal data were verified, determined more precisely and H‐positions found for the first time. The compounds crystallize rhombohedral in the space group R 3. The hexagonal unit cells contain three formula units with Na₂Sn(OH)₆: a = 5.951(1) Å, c = 14.191(2) Å, c/a = 2.384 K₂Sn(OH)₆: a = 6.541(1) Å, c = 12.813(4) Å, c/a = 1.959 K₂Pb(OH)₆: a = 6.625(1) Å, c = 12.998(2) Å, c/a = 1.962 The compounds are not isotypic whereas the atoms occupy in all three cases the same Wyckoff positions. Na₂Sn(OH)₆ has with an hcp packing of O a CdI₂ like superstructure with Na and Sn in octahedral interstices. Hydrogen bonds O–H…O–H play a role in solid K₂Sn(OH)₆ and K₂Pb(OH)₆. In these compounds the potassium ions are shifted from an octahedral coordination in an hcp packing of O. They have nine nearest O‐neighbours. The hydrogen bonds are investigated by Raman spectroscopy.     

Intermetallic Gold Compounds REAuMg (RE = Y,La–Nd,Sm, Eu,Gd–Yb)

New intermetallic rare earth compounds REAuMg (RE = Y, La–Nd, Sm, Eu, Gd–Yb) were synthesized by reaction of the elements in sealed tantalum tubes in a high‐frequency furnace. The compounds were investigated by X‐ray diffraction both on powders and single crystals. Some structures were refined on the basis of single crystal data. The compounds with Y, La–Nd, Sm, and Gd–Tm adopt the ZrNiAl type structure with space group P62m: a = 770.8(2), c = 419.5(1) pm, wR2 = 0.0269, 261 F² values for PrAuMg, a = 750.9(2), c = 407.7(1) pm, wR2 = 0.0561, 649 F² values for HoAuMg with 15 variables for each refinement. Geometrical motifs in HoAuMg are two types of gold centered trigonal prisms: [Au1Mg₃Ho₆] and [Au2Mg₆Ho₃]. The gold and magnesium atoms form a three‐dimensional [AuMg] polyanion in which the holmium atoms fill distorted hexagonal channels. The magnesium positions show a small degree of magnesium/gold mixing resulting in the refined compositions PrAu_1.012(2)Mg_0.988(2) and HoAu_1.026(3)Mg_0.974(3). EuAuMg and YbAuMg contain divalent europium and ytterbium, respectively. Both compounds crystallize with the TiNiSi type structure, space group Pnma: a = 760.6(3), b = 448.8(2), c = 875.8(2) pm, wR2 = 0.0491, 702 F² values, 22 variables for EuAuMg, and a = 738.4(1), b = 436.2(1), c = 864.6(2) pm, wR2 = 0.0442, 451 F² values, and 20 variables for YbAuMg. The europium position shows a small degree of europium/magnesium mixing, and the magnesium site a slight magnesium/gold mixing leading to the refined composition Eu_0.962(3)Au_1.012(3)Mg_1.026(3). No mixed occupancies were found in YbAuMg where all sites are fully occupied. In these structures the europium(ytterbium) and magnesium atoms form zig‐zag chains of egde‐sharing trigonal prisms which are centered by the gold atoms. As is typical for TiNiSi type compounds, also in EuAuMg and YbAuMg a three‐dimensional [AuMg] polyanion occurs in which the europium(ytterbium) atoms are embedded. The degree of distortion of the two polyanions, however, is different.