Neutron and X-ray diffraction studies of TeCl4 and TeBr4 liquids

High-energy X-ray and neutron diffraction measurements have been carried out on TeCl ₄ and TeBr ₄ liquids just above their respective melting points. Both neutron S _N( Q) and X-ray S _X( Q) structure factors exhibit a pronounced first sharp diffraction peak (FSDP) at Q 1 ( TeCl 4 ) ≈ 1.2 Å - 1 and Q 1 ( TeBr 4 ) ≈ 1.1 Å - 1 indicating a significant intermediate range order in the two liquids. The complementarity between neutrons and X-rays allows the FSDP to be attributed to Te–Te correlations at a characteristic length scale L ₁ ≈ 2π/ Q ₁ ≈ 5.3–5.7 Å. The total correlation functions T _N( r) and T _X( r) show first nearest-neighbor distances at r 1 ( TeCl 4 ) ≈ 2.36 Å - 1 and r 1 ( TeBr 4 ) ≈ 2.54 Å - 1 in good agreement with a sum of covalent radii for Te, Cl or Br. The Te coordination was found to be nearly tetrahedral, 3.8 ⩽ N _Te–X ⩽ 4.0, where X = Cl or Br. A distinct correlation at r 2 ( TeCl 4 ) ≈ 2.75 Å and r 2 ( TeBr 4 ) ≈ 2.85 Å was attributed to short nearest-neighbor Te–Te contacts. The respective coordination number, N _Te–Te ≈ 1, suggests a Te ₂X ₈ dimer to be the main structural unit in tellurium tetrahalide liquids, i.e., intermediate between tetrameric Te ₄X ₁₆ solid and monomeric TeX ₄ gas. This proposed short range order is consistent with intermediate range structure since the average molecular size of elliptic Te ₂Cl ₈ (≈5.4 Å) and Te ₂Br ₈ (≈5.8 Å) agrees with derived intermolecular separation and periodicity L ₁.