From 3D to 2D: Structural, Spectroscopic and Theoretical Investigations of the Dimensionality Reduction in the [PtAl₂]^δ− Polyanions of the Isotypic MPtAl₂ Series (M=Ca–Ba, Eu)

Abstract

Four new MPtAl2 (M=Ca, Sr, Ba, Eu) compounds, adopting the orthorhombic MgCuAl2‐type structure, have been synthesized from the elements using tantalum ampoules. All compounds are obtained as platelet‐shaped crystallites and exhibit an increasing moisture sensitivity with increasing size of the formal M cation. Structural investigations indicate a pronounced elongation of the crystallographic b‐axis, which results in a significant distortion of the [PtAl2]δ− polyanion. Within the polyanion, layer‐like arrangements can be found with bonding Pt−Al interactions within the slab; the increase of the b‐axis can be attributed to increasing Al−Al distances and therefore decreasing interactions between the slabs, caused by the differently‐sized formal M cations. While the alkaline earth (M=Ca, Sr) representatives exhibit Pauli paramagnetism, BaPtAl2 shows diamagnetic behavior, finally EuPtAl2 is ferromagnetic with TC=54.0(5) K. The effective magnetic moment indicates that the Eu atoms are in a divalent oxidation state, which is confirmed by 151Eu Mössbauer spectroscopic investigations. Measurements below the Curie‐temperature show a full magnetic hyperfine field splitting with Bhf=21.7(1) T. 27Al and 195Pt magic‐angle spinning NMR spectroscopy corroborates the presence of single crystallographic sites for the Pt and Al atoms. The large 27Al nuclear electric quadrupolar coupling constants confirm unusually strong electric field gradients, in agreement with the structural distortions and the respective theoretical calculations. X‐ray photoelectron spectroscopy has been utilized to investigate the charge transfer within the polyanion. The Pt 4f binding energy decreases with decreasing electronegativity / ionization energy of the alkaline earth elements, suggesting an increasing electron density at the Pt atoms. Theoretical investigations underline the platinide character of the investigated compounds by Bader charge calculations. The analysis of the integrated crystal orbital Hamilton population (ICOHP) values, electron localization function (ELF) and isosurface analyses lead to a consistent structural picture, indicating stable layer‐like arrangements of the [PtAl2]δ− polyanion.

Extending the knowledge on the quaternary rare earth nickel aluminum germanides of the RENiAl4Ge2 series (RE=Y, Sm, Gd–Tm, Lu) – structural, magnetic and NMR-spectroscopic investigations

“…Furthermore, the significant difference in the isotropic magnetic (de-)shielding contributions, δ iso ms , between Al1 and Al2 signifies a large difference between the s-density of states (s-DOS) at the Fermi level for these two aluminum species. While the values measured for Al1 are comparable to those obtained in numerous other intermetallic compounds [18,20,22,75], resulting in a substantial Knight shift contribution, Simulation of the spectra of Figure 9 right (method 1). b F1/F2 Center of gravity analysis of the TQ spectra (method 2).…”

“…Due to our interest in the crystal structures and the physical and NMR spectroscopic properties [18] of intermetallic aluminum compounds of the alkali (Na 2 Au 3 Al [19]), alkaline earth (e.g. MAu 2 Al 2 with M = Ca, Sr [20]; MAuX with M = Ca-Ba and X = Al-In [21]; MPtAl 2 with M = Ca-Ba [22]) and the rare earth elements (e.g. RET 5 Al 2 with RE = Y, Gd-Tm, Lu and T = Pd, Pt [23], RE 2 TAl 3 with RE = Y, La-Nd, Sm, Gd-Lu and T = Ru, Rh, Ir [24], Eu 2 Pt 6 Al 15 [25], Eu 2 Ir 3 Al 9 [26] or YbPd 2 Al 3 [27]), the interest of extending the knowledge of quaternary aluminum containing compounds has sparked.…”

Structural, Physical, Theoretical and Spectroscopic Investigations of Mixed‐Valent Eu2Ni8Si3 and Its Structural Anti‐Type Sr2Pt3Al8

“…The polyanionic [Pt 3 Al 8 ] δ − network contains [Pt−Al] tubes along the c axis, which are linked with Al3−Al3 and Pt2−Al bonds to form a 3D substructure (Figure 10). The strength of −2.50 eV is observed for the Pt2−Al2 bond and significantly exceeds the strength of all Pt−Al interactions observed in Ba 3 Pt 4 Al 4 (−1.47 to −2.01 eV) [24] and SrPtAl 2 (−1.94 and −2.02 eV) [28] . Even higher bond strengths of −3.09 eV (Pt4−Al9) have been observed in Ba 6 Pt 22 Al 53 [72] …”

“…In the past few years we have started to investigate the ternary EA − T −Al ( EA =Ca, Sr, Ba; T =Pd, Pt, Au) systems ourselves in more detail and extended the knowledge of the few known compounds. Several compounds were synthesized and structurally characterized, e. g. Ca 2 Pt 6 Al 15 [27] (Sc 2 Pt 6 Al 15 type [27] ), CaPtAl 2 [28] (MgCuAl 2 type [29] ), CaAu 2 Al 2 [19b] ( anti ‐ThRu 2 P 2 type [30] ), Sr 2 Pd 2 Al [31] (Ca 2 Pd 2 Ge type [3] ), Sr 2 Pd 4 Al 5 [32] (own type), the incommensurately (3+2)D modulated SrPt 2 Al 2 [33] (CaBe 2 Ge 2 type [20] ), SrPt 3 Al 2 [31] (CeCo 3 B 2 type [34] ), SrPtAl 2 [28] (MgCuAl 2 type [29] ) or BaPtAl 2 [28] (MgCuAl 2 type [29] ), BaAuAl [35] (LaIrSi type [36] ) and Ba 3 Au 5+ x Al 6– x [19b] (own type). All of these compounds exhibit [ T x Al y ] δ− polyanionic networks with the alkaline earth metals occupying voids.…”

“…We have continued our endeavors in the ternary Sr−Pt−Al system and initially found tetragonal Sr 2 Pt 3 Al 8 ( anti ‐Eu 2 Ni 8 Si 3 type) as a side product during attempts to synthesize SrPtAl 2 [28] . The compound was subsequently synthesized as phase‐pure material and characterized by powder and single crystal X‐ray diffraction experiments, 27 Al and 195 Pt solid‐state NMR spectroscopy and first‐principles electronic structure calculations.…”