Neutron and X-ray diffraction studies of TeCl4 and TeBr4 liquids
Résumé
High-energy X-ray and neutron diffraction measurements have been carried out on TeCl4 and TeBr4 liquids just above their respective melting points. Both neutron SN(Q) and X-ray SX(Q ) structure factors exhibit a pronounced first sharp diffraction peak (FSDP) at Q1(TeCl4)≈1.2Å-1 and Q1(TeBr4)≈1.1Å-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 L1 ≈ 2π/Q1 ≈ 5.3–5.7 Å. The total correlation functions TN(r) and TX(r ) show first nearest-neighbor distances at r1TeCl4)≈2.36Å-1 and r1(TeBr4)≈2.54Å-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 ⩽ NTe–X ⩽ 4.0, where X = Cl or Br. A distinct correlation at r2(TeCl4)≈2.75Å and r2(TeBr4)≈2.85Å was attributed to short nearest-neighbor Te–Te contacts. The respective coordination number, NTe–Te ≈ 1, suggests a Te2X8 dimer to be the main structural unit in tellurium tetrahalide liquids, i.e., intermediate between tetrameric Te4X16 solid and monomeric TeX4 gas. This proposed short range order is consistent with intermediate range structure since the average molecular size of elliptic Te2Cl8 (≈5.4 Å) and Te2Br8 (≈5.8 Å) agrees with derived intermolecular separation and periodicity L1.