Making and Breaking Bonds in Superconducting SrAl_4–xSi_x(0 ≤x≤ 2)

Abstract

We explored the role of valence electron concentration in bond formation and superconductivity of mixed silicon−aluminum networks by using high-pressure synthesis to obtain the BaAl 4 -type structural pattern in solid solution samples SrAl 4−x Si x where 0 ≤ x ≤ 2. Local ordering of aluminum and silicon in SrAl 4−x Si x was evidenced by nuclear magnetic resonance experiments. Subsequent bonding analysis by quantum chemical techniques in real space demonstrated that the strong deviation of the lattice parameters in SrAl 4−x Si x from Vegard's law can be attributed to the strengthening of interatomic Al−Al and Al−Si bonds within the layers (perpendicular to [001]) for 0 ≤ x ≤ 1.5, followed by the breaking of the interlayer bonds (parallel to [001]) for 1.5 < x ≤ 2 and leading to the structural transition from the BaAl 4 structure type with three-dimensional anionic framework at lower x values to the two-dimensional anion of the BaZn 2 P 2 structure type with increasing x values. Low-temperature measurements of the resistivity and heat capacity reveal that SrAl 2.5 Si 1.5 and SrAl 2 Si 2 prepared at high pressures exhibit superconductivity with critical temperatures of 2.1 and 2.6 K, respectively.

Dimensionality Concept in Solid‐State Reactions: A Way to Control Synthesis of Functional Materials at the Nanoscale

“…It turns out that Al reacts with tetragonal EuSi 2 as readily as with trigonal EuSi 2 . Figure 3e images the film structure at some intermediate point of the reaction: a new, tetragonal polymorph of EuAl 2 Si 2 is synthesized, as evidenced by comparison with the recently discovered tetragonal SrAl 2 Si 2 [ 44 ] and electron energy loss spectroscopy (EELS) data (Figure S4, Supporting Information). What is remarkable is that the transformation from tetragonal EuSi 2 to tetragonal EuAl 2 Si 2 is still 1D, although both the reactant and the product are non‐layered.…”

“…Incidentally, the non‐ideal confinement results in a small amount of layered trigonal SrAl 2 Si 2 at the edges and surface defects. Occasionally, non‐layered tetragonal SrAl 2 Si 2 forms as tilted crystallites (Figure 5b); the tetragonal phase is identified by its lattice parameters [ 44 ] and EELS (Figure S5, Supporting Information). The fcc lattice of the crystalline Al is likely a driving force behind the non‐topotactic formation of tetragonal SrAl 2 Si 2 on the surface of confined SrSi 2 .…”

Structural metastability and Fermi surface Topology of SrAl2Si2

“…The first, so-called α phase, exhibits a stable trigonal (P 3m1) structure wherein slabs of three-edges-sharing tetrahedraly-coordinated AlSi 4 are intercalated, along the c-axis, with Sr planes. [7][8][9] The related electronic structure exhibits a series of hole and electron pockets contributing to a total density of states DOS at E F , N t (E F ) which is relatively lower than the one observed in isomorphous α CaAl 2 Si 2 . 8,9 The observed electrical transport properties were reported to be non-superconducting semimetallic with dominant contribution from electron charge carriers.…”

“…[7][8][9] The related electronic structure exhibits a series of hole and electron pockets contributing to a total density of states DOS at E F , N t (E F ) which is relatively lower than the one observed in isomorphous α CaAl 2 Si 2 . 8,9 The observed electrical transport properties were reported to be non-superconducting semimetallic with dominant contribution from electron charge carriers. [7][8][9] The second, so called β phase, is metastable at low pressures and exhibits BaZn 2 P 2 -type (I4/mmm) layered structure wherein slabs of four-edges-sharing AlSi 4 are intercalated, along the c-axis, with Sr planes 10 (see also Fig.…”

“…8,9 The observed electrical transport properties were reported to be non-superconducting semimetallic with dominant contribution from electron charge carriers. [7][8][9] The second, so called β phase, is metastable at low pressures and exhibits BaZn 2 P 2 -type (I4/mmm) layered structure wherein slabs of four-edges-sharing AlSi 4 are intercalated, along the c-axis, with Sr planes 10 (see also Fig. 1).…”