Temperature-dependent diffraction studies on the phase evolution of tetraindium heptabromide

摘要

The mixed-valent compound In4Br7 undergoes a higher-order phase transition below 250 K which leads to a decrease in symmetry from the trigonal (R (3) over barm) to the monoclinic (C2/c) system via R (3) over barc. The phase transition has been monitored by X-ray powder diffraction using a linear position-sensitive detector between 15 and 300 K, and the crystal structures at room temperature and at 90 K have been refined by means of time-of-flight neutron powder-diffraction data; at 90 K, the lattice parameters are a = 13.066(l) Angstrom, b = 7.520(l) Angstrom, c = 31.105(l) Angstrom, and beta = 98.20(l)degrees; the new unit cell contains 88 atoms (Z = 8) of which 12 are symmetry-independent. Due to their electronic instability because of a second-order Jahn-Teller effect, two of the three crystallographically independent monovalent indium cations are severely affected by the phase transition with respect to their coordination spheres; bond-valence calculations reveal significant strengthening of In+-Br- bonding upon symmetry reduction. Structural changes and group-subgroup relationships as well as possible intermediate phases are discussed. (C) 2003 Elsevier Inc. All rights reserved. [References: 16]