Constitution of the systems {V,Nb,Ta}-Sb and physical properties of di-antimonides {V,Nb,Ta}Sb2

The binary phase diagrams {V,Nb,Ta}-Sb below 1450 °C were studied by means of XRPD, EPMA, and DTA measurements. In the V-Sb system, five stable binary phases were observed in this investigation: V3þxSb1-x, [T-V3Sb2, hT-V2-xSb, V7.46Sb9, V1-xSb2. The V-Sb phase diagram is characterized by two degenerate eutectic reactions: L4V3þxSb1-xþ(V) (T > 1450 °C at 18.1 at.% Sb) and L4V1-xSb2þ(Sb) (T¼(621 ± 5)°C at ~99 at.% Sb), three peritectic reactions: L þ V3þxSb1-x4hT-V2-xSb (T¼(1230 ± 10)°C at ~42 at.% Sb), L þ hT-V2-xSb4V7.46Sb9 (T¼(920 ± 10)°C at ~87 at.% Sb), and L þ V7.46Sb94V1-xSb2 (T¼(869 ± 5)°C at ~88 at.% Sb), a peritectoid reaction: V3þxSb1-x þ hT-V2-xSb4[T-V3Sb2 at (875 ± 25)°C, a eutectoid reaction: hT-V2-xSb4[T-V3Sb2þV7.46Sb9 at (815 ± 15)°C and congruent melting of V3þxSb1-x (T > 1450 °C). An X-ray single crystal study of V5Sb4C1-x proved the existence of interstitial elements in the octahedral voids of a partially filled Ti5Te4-type structure (x~0.5; RF2 ¼ 0.0101), therefore this phase (earlier labeled “V5Sb4”) was excluded from the binary equilibrium phase diagram. V5Sb4C1-x is the first representative of a filled Ti5Te4-type structure. A re-investigation of the Nb-Sb system removed the contradiction between the hitherto reported phase diagrams and confirmed the version derived by Melnyk et al. (see ref. [1]). Three binary phases exist in the Ta-Sb system: Ta3þxSb1-x, Ta5Sb4, TaSb2. Due to instrumental limits (<1450 °C), only the peritectic reaction of TaSb2: L þ Ta5Sb44TaSb2 ((1080 ± 10)°C at ~92 at.% Sb) and a degenerate Sb-rich eutectic reaction (L4TaSb2þ(Sb); (622 ± 5)°C; ~99 at.% Sb) have been determined. Physical properties (mechanical and transport properties) of binary di-antimonides were investigated with respect to a potential use of these metals either as diffusion barriers or electrodes for thermoelectric devices based on skutterudites. All group-V metal di-antimonides have low metallic-type resistivity and relatively high thermal conductivity. Magnetic field has little influence on the resistivity of V1-xSb2 at low temperature, while on {Nb,Ta}Sb2 it increases the resistivity, especially on NbSb2. The coefficient of thermal expansion (CTE) decreases from V1-xSb2 to TaSb2, particularly the CTE value of NbSb2 is in the range of average n-type filled skutterudites. In contrast to the CTE value, elastic moduli increase from V1-xSb2 to TaSb2. The value for V1-xSb2 is in the range of Sb-based skutterudites, whereas the values for {Nb,Ta}Sb2 are significantly higher.