From Metallic LnTt (Ln = La, Nd; Tt = Si, Ge, Sn) to Electron‐precise Zintl Phase Hydrides LnTtH

Abstract

LnTt (Ln = La, Nd; Tt = Si, Ge, Sn) compounds crystallize in the CrB‐ or the FeB‐structure type with 1∞[Tt2–]‐polyanionic zigzag chains and can be rationalized as Ln3+Tt2–e–. They take up hydrogen at gas pressures from 10 to 160 bar and temperatures up to 750 K. The hydrides LnTtH1–x (0.04 ≤ x ≤ 0.39) crystallize either in a hydrogen filled CrB‐structure type (ZrNiH type, C‐phase, Cmcm: LaSiD0.92(1), LaGeD0.88(1), NdSiD, NdGeD0.95(1), LaSnHx) or in a hydrogen filled FeB‐structure type (LaGeH type, P‐phase, Pnma: LaSiD0.61(3), LaGeD0.96(4), NdSiD0.88(6)). In both cases, hydrogen atoms partially fill tetrahedral Ln4 voids as proven by neutron powder diffraction. While CrB‐structure type LnTt reacts to C‐phase only, FeB‐structure type LnTt hydrogenates to mixtures of C‐ (major phase) and P‐type hydrides. Electronic structure calculations reveal the depopulation of Ln‐d states, which cause the metallic conductivity in LnTt to be the main electronic effect upon hydrogenation. They further suggest Zintl phase hydrides LnTtH to be still poor metallic conductors with a pseudo bandgap at the Fermi level. The hydrogenation reaction from LnTt to LnTtH may thus be regarded as an incomplete metal to semiconductor transition.

Size Matters: New Zintl Phase Hydrides of REGa (RE = Y, La, Tm) and RESi (RE = Y, Er, Tm) with Large and Small Cations

“…The hydrogenation of RESi (RE = Y, Er, Tm) yielded compounds with CrB type structure with a reduced a and an expanded b lattice parameter, while c shows almost no change (Supplementary Materials Figures S1-S3, Tables S1-S3). These results are similar to those in LnTtH x [8] and comparable to LaGa. Thermal analysis (DSC) of TmGa under hydrogen gas pressure shows an exothermic signal at 430 K ( Figure S4).…”

“…Zintl phases form various hydrides with two structural motifs highlighted [6]. The hydrogen atoms can be bound covalently to the polyanion (e.g., in KSiH 3 ), incorporated inside a polyhedral void with strong ionic interactions (e.g., LaSiH, Ca 3 SnH 2 ) and even show both structural motifs at the same time (e.g., BaSiH 2 ) [7][8][9][10]. Despite the ionic or covalent bonding pattern, some of the hydrides keep their metallic character (CaSiH 4/3 , SrAl 2 H 2 ) [11,12].…”

“…Rare earth containing compounds offer the opportunity to vary the cation radius while maintaining the valence electron concentration thus enabling the investigation of the crystal chemistry as a function of the ion size. There are many examples for the hydrogenation of compounds with a CrB type structure, e.g., ZrNi or TbNi forming the metallic hydrides ZrNiH x (x = 1-3) and TbNiH 3.3 and the Zintl phases AeTt (Ae = Ca, Sr, Ba; Tt = Si, Ge, Sn), LnTt (Ln = La, Nd; Tt = Si, Ge, Sn), and LnGa (Ln = Nd, Gd) [8,10,[16][17][18][19][20][21]. AeTt (Ae = Ca, Sr, Ba; Tt = Si, Ge, Sn) form polyanionic hydrides with different hydrogen content and polyanions with different degrees of condensation (chains, ribbons) determined by the atomic radius ratio r Ae /r Tt .…”

Hydrogenation Properties of LnAl2 (Ln = La, Eu, Yb), LaGa2, LaSi2 and the Crystal Structure of LaGa2H0.71(2)

“…Hydrogenation thus transforms the electronically imbalanced (metallic) Zintl phase SrAlSi to an electron-precise Zintl phase hydride SrAlSiH. Metallic Zintl phases LnTt (Ln = La, Nd; Tt = Si, Ge, Sn) react in the same manner and take up hydrogen to form electron-precise Zintl phase hydrides LnTtH [24]. The metallic Zintl phase LaSi 2 adopts the ThSi 2 structure type, which is characterized by a trigonal prismatic surrounding of silicon atoms by lanthanum atoms and a three-dimensional silicon network ( Figure 1) [25].…”

Crystal Structure and Luminescence Properties of the First Hydride Oxide Chloride with Divalent Europium: LiEu2HOCl2

“…Rare‐earth metal compounds with both hydride and chalcogenide anions are uncommon, but not unknown, as the RE HSe and RE HTe ( RE = rare‐earth elements) series demonstrates. The ternary hydride oxides RE HO for RE = La – Nd, Sm, Gd, Ho, Er as well as the quaternary lithium rare‐earth metal hydride oxides Li RE 2 HO 3 with the trivalent rare‐earth metals RE = La – Nd may serve as oxidic examples. Recently, LiLa 2 HO 3 and LiSr 2 H 3 O as well as solid solutions of these two compounds were studied concerning their crystal structures and their hydride‐anion conductivity .…”