Making and Breaking Bonds in Superconducting SrAl_4–<i>x</i>Si_<i>x</i> (0 ≤ <i>x</i> ≤ 2)

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 BaAl₄-type structural pattern in solid solution samples SrAl_4–<i>x</i>Si_<i>x</i> where 0 ≤ <i>x</i> ≤ 2. Local ordering of aluminum and silicon in SrAl_4–<i>x</i>Si_<i>x</i> 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–<i>x</i>Si_<i>x</i> 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 ≤ <i>x</i> ≤ 1.5, followed by the breaking of the interlayer bonds (parallel to [001]) for 1.5 < <i>x</i> ≤ 2 and leading to the structural transition from the BaAl₄ structure type with three-dimensional anionic framework at lower <i>x</i> values to the two-dimensional anion of the BaZn₂P₂ structure type with increasing <i>x</i> values. Low-temperature measurements of the resistivity and heat capacity reveal that SrAl_2.5Si_1.5 and SrAl₂Si₂ prepared at high pressures exhibit superconductivity with critical temperatures of 2.1 and 2.6 K, respectively

Synthesis, Structural Characterization, and Physical Properties of the Type‑I Clathrates <i>A</i>₈Zn₁₈As₂₈ (<i>A</i> = K, Rb, Cs) and Cs₈Cd₁₈As₂₈

The first arsenide clathrates <i>A</i>₈Zn₁₈As₂₈ (<i>A</i> = K, Rb, Cs) and Cs₈Cd₁₈As₂₈ have been synthesized in high yields via a two-step route. These compounds adopt the type-I structure and exhibit structural characteristics different from the recently reported antimonide clathrates Cs₈Zn₁₈Sb₂₈ and Cs₈Cd₁₈Sb₂₈. In arsenide clathrates, Zn (or Cd) and As atoms are statistically mixed at the three framework sites: 6<i>c</i>, 16<i>i</i>, and 24<i>k</i>; the alkali metals reside inside the cages at the 2<i>a</i> and 6<i>d</i> sites, with the 2<i>a</i> site being only partially filled. Single-crystal X-ray diffraction studies confirm that the Cd atoms preferably occupy the 6<i>c</i> and 24<i>k</i> sites over the 16<i>i</i> site, with more than 80% of Cd found at the former two positions. A unique structural feature is a framework disorder coupled with the partial occupancy of the cage’s 2<i>a</i> site. Optical absorption measurements and electronic property measurements reveal a semimetallic-like behavior for Cs₈Cd₁₈As₂₈ and semiconductor-like behavior for <i>A</i>₈Zn₁₈As₂₈ (<i>A</i> = Rb, Cs)

Synthesis, Structural Characterization, and Physical Properties of the Type‑I Clathrates <i>A</i>₈Zn₁₈As₂₈ (<i>A</i> = K, Rb, Cs) and Cs₈Cd₁₈As₂₈

The first arsenide clathrates <i>A</i>₈Zn₁₈As₂₈ (<i>A</i> = K, Rb, Cs) and Cs₈Cd₁₈As₂₈ have been synthesized in high yields via a two-step route. These compounds adopt the type-I structure and exhibit structural characteristics different from the recently reported antimonide clathrates Cs₈Zn₁₈Sb₂₈ and Cs₈Cd₁₈Sb₂₈. In arsenide clathrates, Zn (or Cd) and As atoms are statistically mixed at the three framework sites: 6<i>c</i>, 16<i>i</i>, and 24<i>k</i>; the alkali metals reside inside the cages at the 2<i>a</i> and 6<i>d</i> sites, with the 2<i>a</i> site being only partially filled. Single-crystal X-ray diffraction studies confirm that the Cd atoms preferably occupy the 6<i>c</i> and 24<i>k</i> sites over the 16<i>i</i> site, with more than 80% of Cd found at the former two positions. A unique structural feature is a framework disorder coupled with the partial occupancy of the cage’s 2<i>a</i> site. Optical absorption measurements and electronic property measurements reveal a semimetallic-like behavior for Cs₈Cd₁₈As₂₈ and semiconductor-like behavior for <i>A</i>₈Zn₁₈As₂₈ (<i>A</i> = Rb, Cs)

Phase Characterization, Thermal Stability, High-Temperature Transport Properties, and Electronic Structure of Rare-Earth Zintl Phosphides Eu₃M₂P₄ (M = Ga, In)

Two rare-earth-containing ternary phosphides, Eu₃Ga₂P₄ and Eu₃In₂P₄, were synthesized by a two-step solid-state method with stoichiometric amounts of the constitutional elements. Refinements of the powder X-ray diffraction are consistent with the reported single-crystal structure with space group <i>C</i>2<i>/c</i> for Eu₃Ga₂P₄ and <i>Pnnm</i> for Eu₃In₂P₄. Thermal gravimetry and differential scanning calorimetry (TG-DSC) measurements reveal high thermal stability up to 1273 K. Thermal diffusivity measurements from room temperature to 800 K demonstrate thermal conductivity as low as 0.6 W/m·K for both compounds. Seebeck coefficient measurements from room temperature to 800 K indicate that both compounds are small band gap semiconductors. Eu₃Ga₂P₄ shows p-type conductivity and Eu₃In₂P₄ p-type conductivity in the temperature range 300–700 K and n-type conductivity above 700 K. Electronic structure calculations result in band gaps of 0.60 and 0.29 eV for Eu₃Ga₂P₄ and Eu₃In₂P₄, respectively. As expected for a valence precise Zintl phase, electrical resistivity is large, approximately 2600 and 560 mΩ·cm for Eu₃Ga₂P₄ and Eu₃In₂P₄ at room temperature, respectively. Measurements of transport properties suggest that these Zintl phosphides have potential for being good high-temperature thermoelectric materials with optimization of the charge carrier concentration by appropriate extrinsic dopants