Structural Characterization of Intermetallic Compounds by ²⁷Al Solid State NMR Spectroscopy

Abstract

Intermetallic compounds are of broad interest for solid state chemists, condensed matter physicists, and material scientists due to their intriguing crystal chemistry, their physical properties, and their potential applications, ranging from lab curiosities to everyday objects. To characterize and understand the properties of new compounds and novel materials, the availability of structural information, particularly single-crystal X-ray diffraction data, is a mandatory prerequisite. Especially when it comes to the formation of compounds with deficient or mixed site occupancies, superstructures, or representatives crystallizing in other, thus far unknown structure types, a complementary method for structural analysis is of great value. Solid state nuclear magnetic resonance spectroscopy has been a valuable tool in many areas of chemistry, being an element-selective, site-specific, and inherently quantitative tool for detailed structural characterization. Magic-angle spinning conditions eliminate or reduce the effect of anisotropic interactions in the solid state, producing high-resolution spectra. Until recently, Al NMR studies of intermetallic aluminum compounds have been relatively sparse and mostly limited to binary systems. In this Account, we will summarize the current state of the art of high-resolutionAl NMR in intermetallic compounds focusing on recent research efforts in our laboratories and the interpretation of NMR parameters in terms of the structural details of the compounds investigated. Besides theoretical aspects of Al NMR spectroscopy, short paragraphs on experimental details and the crystal chemistry of the discussed compounds are given. In the main part of this Account, we focus on three key aspects: (i) crystal structure validation, (ii) structural disorder and mixed site occupancies, and (iii) the electronic structure, all of which can be investigated by spectroscopic means. For the first part, we have chosen the ternary equiatomic compounds CaAuAl (TiNiSi type), BaAuAl (LaIrSi type), and BaPtAl (own type). Structural disorder and mixed site occupancies have been probed in the ScTAl series (T = Cr, Ru, Ag, Re) crystallizing in the TiNiSi, HfRhSn, and MgZn-type structures. Also NaAuAl and the Heusler compounds, Sc(TT')Al (T = T' = Ni, Pd, Pt, Cu, Ag, Au), have been used for structure validation purposes, based on the number and signal area ratios of the resonances observed and on the comparison between experimental and theoretically calculated nuclear electric quadrupolar interaction parameters. Electronic structure information available from Al magnetic shielding will be discussed based on experimental data obtained for the RETAl series (RE = Y, Lu; T = Pd, Pt), the extended RETAl series (RE = Y, Lu; T = Fe, Ru, Os, Co, Rh, Ir, Ni, Pd, Pt), and the ordered Heusler compounds ScTAl (T = Ni, Pd, Pt, Cu, Ag, Au).

DFT-GIPAW 27Al NMR Simulations for Intermetallics: Accuracy Issues and Magnetic Screening Mechanisms

“…One is the so-called Knight shift MSM which comes from hyperfine interactions with spins of itinerant electrons existing in those compounds having a rather metallic character. This mechanism is normally expected to deshield the 27 Al nuclei, and given the high PDOS with s -character at E F in CuAl 2 and AlB 2 (see Figure S1), it is reasonable to state that it prevails in these two intermetallics as well as in elemental Al (see ref ), which is in agreement with the respective computed shieldings depicted in Figure . In the second accounted MSM, a supposed concurrent shielding effect comes from hyperfine interactions with local magnetic moments at transition-metal sites ( m TM ).…”

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).…”

“…Quaternary intermetallic compounds, however, have not been studied in this great detail yet. 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.…”

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)

“…In the platinum‐rich representatives YPt 5 Al 2 and LuPt 5 Al 2 shifts of δ iso ms =277 and 273 ppm were measured. Additional 27 Al NMR data can be found in a review article . It must be pointed out, however, that Knight shifts in intermetallic compounds are affected by multiple aspects of the electronic structure and that generally simple concepts such as electronegativities can only be invoked when comparing data obtained for series of isostructural compounds.…”