Ba₅{V,Nb}₁₂Sb_19+x, novel variants of the Ba₅Ti₁₂Sb_19+x-type: crystal structure and physical properties

Abstract

The novel compounds Ba5{V,Nb}12Sb19+x, initially found in diffusion zone experiments between Ba-filled skutterudite Ba0.3Co4Sb12 and group V transition metals (V,Nb,Ta), were synthesized via solid state reaction and were characterized by means of X-ray (single crystal and powder) diffraction, electron probe microanalysis (EPMA), and physical (transport and mechanical) properties measurements. Ba5V12Sb19.41 (a = 1.21230(1) nm, space group P4[combining overline]3m; RF(2) = 0.0189) and Ba5Nb12Sb19.14 (a = 1.24979(2) nm, space group P4[combining overline]3m; RF(2) = 0.0219) are the first representatives of the Ba5Ti12Sb19+x-type, however, in contrast to the aristotype, the structure of Ba5V12Sb19.41 shows additional atom disorder. Temperature dependent ADPs and specific heat of Ba5V12Sb19.41 confirmed the rattling behaviour of Ba1,2 and Sb7 atoms within the framework built by V and Sb atoms. Electrical resistivity of both compounds show an upturn at low temperature, and a change from p- to n-type conductivity above 300 K in Ba4.9Nb12Sb19.4. As expected from the complex crystal structure and the presence of defects and disorder, the thermal conductivity is suppressed and lattice thermal conductivity of ∼0.43 W m(-1) K(-1) is near values typical for amorphous systems. Vicker's hardness of (3.8 ± 0.1) GPa (vanadium compound) and (3.5 ± 0.2) GPa (niobium compound) are comparable to Sb-based filled skutterudites. However, the Young's moduli measured by nanoindentation for these compounds EI(Ba4.9V12Sb19.0) = (85 ± 2) GPa and EI(Ba4.9Nb12Sb19.4) = (79 ± 5) GPa are significantly smaller than those of skutterudites, which range from about 130 to 145 GPa.

Synthesis, Crystal and Electronic Structure of the Titanium Bismuthides Sr5Ti12Bi19+x, Ba5Ti12Bi19+x, and Sr5–δEuδTi12Bi19+x (x ≈ 0.5–1.0; δ ≈ 2.4, 4.0)

“…The resistivity tends to saturate upon heating up to room temperature. A similar trend was previously observed for Ba 5 M 12 Sb 19+ x ( M = V, Nb) . In addition, in the case of the niobium antimonide Ba 5 Nb 12 Sb 19+ x , a semiconductor‐like behavior emerges at temperature above 400 K, accompanied by the Seebeck coefficient reaching a maximum in the same temperature range .…”

“…For both samples, the absolute values of the electrical resistivity lie in the range 0.3–1.2 10 –3 Ω cm. These numbers are comparable to the resistivity measured for Ba 5 Nb 12 Sb 19+ x and allow the classification of the phases under consideration as bad metals. The general appearance of the ρ ( T ) curve differs from that of a typical metal.…”

“…In addition, the occupation of the Sb site amounts to only about 5 % . In Ba 5 M 12 Sb 19+ x ( M = V, Nb), there are two partially occupied Sb positions: one with the coordinates ( x, x, x ), x ≈ 0.46; and one in the center of the unit cell . The Sb–Sb distances in the disordered part of the structure lie in the range 0.8–1.4 Å, suggesting that no more than one of the Sb atoms can be present in this volume at the same time, in accordance with the low occupancies of about 7 % and 50 % for the former and latter positions, respectively (for the V representative; in the Nb member, the latter Sb site remains unoccupied) .…”

“…Preliminary analysis of the powder X‐ray diffraction data (Figure S2) indicated that AE 5 Ti 12 Bi 19+ x crystallize isotypically to the antimonides Ba 5 M 12 Sb 19+ x ( M = Ti, V, Nb) , . The subsequent single‐crystal X‐ray diffraction study revealed notable differences between the studied bismuthides and the previously reported antimonides with respect to the details in the disordered part of the crystal structure.…”

“…Similarly to Ba 5 M 12 Sb 19+ x ( M = Ti, V),, AE 5 Ti 12 Bi 19+ x ( AE = Sr, Ba, Sr/Eu) crystallize in the non‐centrosymmetric space group P 43 m . The refined Flack parameter is near zero for all crystals, ensuring that the absolute structure has been established.…”