Lithium alkaline earth tetrelides of the type Li₂AeTt (Ae=Ca, Ba, Tt=Si, Ge, Sn, Pb): synthesis, crystal structures and physical properties

3.1 Crystal structure description

The isotypes Li₂AeTt (Ae=Ca, Tt=Si, Ge, and Ae=Ba, Tt=Si, Ge, Sn, Pb) crystallize in the orthorhombic space group Pmmn (no. 59), while Li₂CaSn and Li₂CaPb realize a cubic Heusler-type arrangement in Fm3̅m (no. 225). A graphical representation of this situation in relation to the respective magnesium compounds can be found in Scheme 1. Upon changing the alkaline earth metal to strontium a different composition appears, which we will report in an independent contribution. Crystallographic data of the title compounds are summarized in Tables 1 and 2, atomic positions in Tables 3 and 4. The orthorhombic Li₂BaSi-type compounds can be obtained in single-phase as needle-like crystals with length of several mm (see Figs. 1 and 2). The crystal structures of these compounds are composed of slabs of a corrugated two-dimensional network formed by lithium and the group 4 element. Calcium or barium, respectively, are incorporated between these slabs (Fig. 3). Lithium is triangularly coordinated by the group 4 element and located slightly out of plane (Fig. 4, left). These triangles are connected via edges into two-dimensional slabs, in which the group 4 element occupies the extreme positions of the corrugation (Fig. 4, middle). The slabs are superimposed along the c-axis. The Ca and Ba atoms are located in between the slabs and coordinated by the group 4 element in form of a distorted tetrahedron. As a result, the group 4 element is surrounded by six lithium ions in one hemisphere and four barium ions in the other, resulting in a ten-fold coordination (Fig. 4, right).

Scheme 1:

Overview of the different crystal structures of compounds Li₂AeTt (Ae=Mg, Ca, Ba, Tt=Si, Ge, Sn, Pb).

Fig. 3:

View of the crystal structure of Li₂BaSi (left) and Li₂CaSi (right) along the b axis. Red: lithium, gray: group 4 element, orange: barium or calcium, respectively. The angle α indicates a measure for the degree of corrugation of the network of lithium and the tetrel, which is compressed/expanded along the a axis for the barium as compared to the calcium compounds. α equals to 89.74° and 73.79° for the barium and the calcium compound, respectively.

Fig. 4:

Crystal structure details of the Li₂BaSi-type orthorhombic compounds Li₂AeTt. Left: trigonal planar coordination of lithium (red) by the group four element (gray), with lithium located out of plane. Middle: Li₂Tt network forming the sinusoidal slabs that can be seen in Fig. 3 Right: 10-fold coordination of the tetrel by four alkaline earth metal atoms (orange) and six lithium atoms.

Remarkably, comparing the unit cell parameters of the two silicon or germanium compounds (Tables 1 and 2), the b and c axes remain almost constant and only the a axis is significantly altered. The two-dimensional lithium-tetrel network is rigid along the b axis but, due to the sinusoidal corrugation, flexible along the a axis. The shorter wavelength of the corrugated net leads to a larger amplitude, which should result in an increase of the c axis. However, this is compensated by the smaller ionic radius of calcium compared to barium. Fig. 3 illustrates the situation: the angle α shows that the corrugated network of lithium and the tetrel in the crystal structure is merely compressed/expanded along the a axis for the barium (α=89.74°) compared to the calcium (α=73.79°) compound.

Upon comparison of the unit cell parameters of the silicon with the germanium compound and the tin with the lead compound an only conspicuously minor change strikes the eye. This indicates that the elements that differ from one compound to the other possess very similar ionic radii. This observation is also supported by the distances between the tetrel and lithium or alkaline earth element, respectively (Table 5), and is a result of the reduction of atomic radii due to the incorporation of the d group block from silicon to germanium and f group elements from tin to lead in combination with the enhanced relativistic contraction of the latter. Similar observations were made earlier, for example, for compounds Ca₂Tt (Tt=Si, Ge, Sn, Pb) [11, 12].

The magnesium compounds Li₂MgTt (Tt=Si, Ge, Sn, Pb) crystallize in the cubic space group Fm3̅m (no. 225) in a Heusler-type (MnCu₂Al) structure. One Wyckoff position is reported to be equally occupied by lithium and magnesium in a disordered arrangement for the Ge to Pb [1, 2] compounds, whereas Li₂MgSi [4] was found ordered more recently and exhibits a 2×2×2 superstructure. The title compounds Li₂CaSn and Li₂CaPb, however, crystallize in an ordered Heusler-type arrangement (Fig. 5, left). Each element fully occupies one Wyckoff position (Table 4) such that the smaller lithium has a shorter distance to the tetrel compared to the relatively larger calcium (Table 5).

Fig. 5:

Left: heusler-type unit cell of compounds Li₂CaSn and Li₂CaPb. Right: coordination of the tetrel (gray) by eight lithium (red) and six calcium (orange) atoms.

The crystal structure and thus the atomic arrangement of compounds Li₂AeTt (Scheme 1) changes from a 6+4-fold coordination (Fig. 4, right) of the tetrel in the orthorhombic structures (Ae=Ba, Tt=Si, Ge, Sn, Pb and Li₂CaSi, Li₂CaGe) to a 8+6 fold coordination (Fig. 5, right) in the Heusler type phases (Ae=Mg, Tt=Si, Ge, Sn, Pb and Li₂CaSn, Li₂CaPb). This transition appears as a result of the different ratios r(Ae²⁺)/r(Tt⁴⁻) of the respective ionic radii. The tetrel atoms are coordinated by eight lithium and six magnesium atoms, while the coordination number drops to 10, six lithium and only four barium atoms, upon going from small Mg²⁺ to large Ba²⁺. In the calcium compounds the large tetrel atoms Sn and Pb can support the higher coordination number, whereas Si and Ge are too small for the coordination by 6 calcium and 8 lithium atoms and thus the compounds exhibit the 4+6 fold coordination.

3.2 Electrical and magnetic properties

The electrical resistivity of polycrystalline agglomerates of Li₂BaSn and Li₂BaPb is shown in Fig. 6 indicating semiconducting behavior for Li₂BaPb, whereas the Li₂BaSn sample may be considered as a heavily doped degenerated semiconductor with a small band-gap (~100 meV).

Fig. 6:

Electrical resistivity measurements of polycrystalline agglomerates of Li₂BaSn and Li₂BaPb.

The magnetic susceptibilities of Li₂BaGe, Li₂BaSn, and Li₂BaPb obtained at 7 T are shown in Fig. 7 The small upturns of the susceptibility at the lowest temperatures are probably due to minor paramagnetic impurities. No saturated magnetization indicating ferromagnetic impurities was observed. The extrapolated values of the susceptibilities of the diamagnetic samples at T=0 K are χ₀=−1.6×10⁻⁵, −6.7×10⁻⁵, and −0.9×10⁻⁵ emu mol⁻¹, respectively.

Fig. 7:

Magnetic susceptibilities of Li₂BaGe, Li₂BaSn, and Li₂BaPb obtained at an external magnetic field of 7 T.