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

Abstract

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: V3+xSb1−x, ℓT-V3Sb2, hT-V2−xSb, V7.46Sb9, V1−xSb2. The V-Sb phase diagram is characterized by two degenerate eutectic reactions: L↔V3+xSb1−x+(V) (T > 1450 °C at 18.1 at.% Sb) and L↔V1−xSb2+(Sb) (T=(621 ± 5)°C at ∼99 at.% Sb), three peritectic reactions: L + V3+xSb1−x↔hT-V2−xSb (T=(1230 ± 10)°C at ∼42 at.% Sb), L + hT-V2−xSb↔V7.46Sb9 (T=(920 ± 10)°C at ∼87 at.% Sb), and L + V7.46Sb9↔V1−xSb2 (T=(869 ± 5)°C at ∼88 at.% Sb), a peritectoid reaction: V3+xSb1−x + hT-V2−xSb↔ℓT-V3Sb2 at (875 ± 25)°C, a eutectoid reaction: hT-V2−xSb↔ℓT-V3Sb2+V7.46Sb9 at (815 ± 15)°C and congruent melting of V3+xSb1−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 = 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: Ta3+xSb1−x, Ta5Sb4, TaSb2. Due to instrumental limits (≤1450 °C), only the peritectic reaction of TaSb2: L + Ta5Sb4 ↔ TaSb2 ((1080 ± 10)°C at ∼92 at.% Sb) and a degenerate Sb-rich eutectic reaction (L↔TaSb2+(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.