Polar intermetallic compounds of the silicon and arsenic family elements and their ternary hydrides and fluorides

Abstract

Our ability to understand, and possibly to predict, the properties of intennetallic compounds on the basis of their structural Information is many times limited by uncertainties about their correct compositions. The unsuspected presence of adventitious impurities in a compound may lead one to draw inaccurate, or equivocal, conclusions about their unexplained behaviors. The occurrence of such situations in the published literature has been quite frequent. For instance, the inadvertent presence of hydrogen impurities in what was thought to be pure calcium prompted Smith and Bernstein^ to assign a hexagonal allotropic form to calcium. Subsequent investigation by Peterson and Fattore^ showed the hexagonal phase was an Intermediate in the binary calcium-hydrogen system. In similar circumstances, an hexagonal allotrope of ytterbium metal was later established as an ytterbium hydride compound.^ Adventitious impurities in an intermetallic compound may stabilize a phase in a particular stnjcture that would otherwise not form in the binary system. Such stabilization processes are probably electronically driven. Examples in literature of impurity-stabilized compounds are vast. Thus, many of the so-called Nowotny phases are, or were thought to be, compounds stabilized by non-metallic impurities."® The presence and stabilizing effects of impurities were not recognized until higher quality reagents and more accurate and thoughtful experimental techniques were available, and there was need to reconsider conflictive information already reported. As an illustration, arguments of dimorphism between a hexagonal and a tetragonal structure for LagSng were clarified by Kwon and Corbett.® They demonstrated that the hexagonal phase was an oxygenstabilized compound, LagSnaO. The structures of intermetallic compounds formed by the combinations of preand post-transition elements can frequently be rationalized by the ZintI prindples from the chemist's point of view. Schafer and coworkers^-® ® have particularly demonstrated that a great many favorable combinations of structures and compositions can be classified as Zinti phases. The ZintI concept,'" in a very simple way, is based on the assumption that the electropositive element, of say a binary phase A^B^, transfers all Its electrons to the electronegative element (B) which uses these extra electrons to form H2(g) Matheson 99.999% •2(9) Matheson 99.5% Ca

β-Phase Yb5Sb3Hx: Magnetic and Thermoelectric Properties Traversing from an Electride to a Semiconductor

“…5 Previous work has shown that all Yb cations are Yb 2+ , giving one excess electron per formula unit (5 × Yb 2+ = 10 electrons donated, 3 × Sb 3− = 9 electrons accepted) and resulting in a formal valence state of (Yb 5 Sb 3 ) + (e) − unless H is incorporated, in which case the compound is charge-balanced as Yb 5 Sb 3 H. 5 The orthorhombic phase is known to be stabilized by interstitial H. When synthesized with dehydrogenated metals, the hexagonal phase was produced, and when synthesized using YbH 2 , the orthorhombic phase was produced, and heating under vacuum at 1100 °C for 3 weeks caused a transition from the orthorhombic to the hexagonal phase. 7,8 Because commercially available rare-earth metals are known to contain hydrogen impurities 38 and experimental methods of quantifying H content in solid-state materials are limited, it is difficult to experimentally determine how much H is required to drive the structural transition from the hexagonal to the orthorhombic phase. Here, we employed total energy calculations to confirm that the orthorhombic phase is Hstabilized and to elucidate the minimum amount of H necessary to produce the orthorhombic structure.…”

“…However, this result supports previous findings of a decreased cell volume with H content 28,5 in orthorhombic Yb 5 Sb 3 H and the unexpectedly small lattice constants in other Mn 5 Si 3 -type phases due to interstitial H impurities. 6,7 Other studies on the A 5 Pn 3 (A = Ca, Sr, Ba, Eu, Yb; Pn = Sb, Bi) compounds in the Ca 5 Sb 3 F structure type have used unit cell volume to estimate the amount of H in each sample. 6 Figure 5 depicts typical SEM micrographs and EDS elemental maps of Yb and Sb for the x = 0.75 sample and in SI, Figure S3, for the remaining samples.…”

“…4,5 With 5 Yb 2+ ions and 3 Sb 3− ions, the composition can be written as (Yb 5 Sb 3 + )(e − ), 6−8 and recent theoretical studies have confirmed that they are electrides. 4,5 Both structure types are known to host interstitial impurities that can accept the extra electron, making them highly tunable, 6,7 and it has been shown that the orthorhombic β- Yb 5 Sb 3 is only stable when an interstitial H or F atom is present. 7 Given this information, the β-Yb 5 Sb 3 phase is better described according to composition as Yb 5 Sb 3 H x , and the structure type was reclassified as Ca 5 Sb 3 F. This finding went unknown for many years due to the presence of H as an impurity in commercial alkaline and rare earth metals and the experimental difficulty in identifying H atoms.…”

Exploratory Synthesis in the Solid State. Endless Wonders

“…For example, the "passion" with which Sn-Sn bonding generates sometimes complex solutions to the octet rule is noteworthy. 62,63,97,98 example is also found in chains of square-planar tin in Ca 6.2 -Mg 3.8 Sn 7 . 99 ) Of course, the earlier triels would be expected to generate networks built from more delocalized, electrondeficient clusters distantly parallel to the boron hydrides, etc., and this seems to be the case without exception.…”

“…50 In fact, all eight reported examples of this structure are hydrides with the composition (M II ) 5 (Pn) 3 H x (Pn ) pnictogen Sb, Bi). 57,58 All also occur as binary Mn 5 Si 3 -type phases, most of which take up some H as a typical Z component as well (see above). At some intermediate level of hydrogen activity, nine of these (Ca, Sr, Eu, and Yb with Sb or Bi, plus the new Sm 5 Bi 3 H x ) transform to the ternary "β-Yb 5 Sb 3 " structure, which type should now be identified as that of the earlier recognized Ca 5 Sb 3 F type.…”