Structural aspects of the metal-insulator transition in V4O7

V₄O₇ has a transition with decreasing temperature at 250 K and the structure has been refined at 298 and 200 K. The triclinic structure (A$\text{1}$) consists of rutile-like layers of VO₆ octahedra extending indefinitely in the a-b plane and four octahedra thick along the c-axis. The average VO distances for the four independent V atoms are 1.967, 1.980, 1.969, and 1.984 Å at 298K and 1.948, 1.992. 1.961, and 2.009 Å at 200K. At 200K there is a clear separation into strings of V³⁺ or V⁴⁺ ions running parallel to the pseudorutile c-axis. In addition, all of the 3+ and half of the 4+ sites are paired to form short VV bonds. The remaining V⁴⁺ atom is displaced toward one oxygen so as to balance its electrostatic charge. The distortion at the metal-insulator transitions in V₄O₇, Ti₄O₇, VO₂ + Cr, and NbO₂ are compared.     

X-ray determination of the mean amplitudes of vibration and debye temperatures of some rare earth monochalcogenides

The x-ray diffraction intensities of Bragg reflections have been measured at room temperature for thulium selenide, samarium sulphide, samarium selenide and samarium telluride. On the basis of a common amplitude approximation, the Debye-Waller factor, the mean amplitude of vibration and the Debye temperature have been evaluated. The values of the Debye temperatures and mean amplitudes of vibration are 176±16°K, 0·185 ± 0·017 Å (TmSe), 155 ± 7°K, 0·244 ± 0·012 Å (SmS), 153 ± 14°K, 0·221 ± 0·020 Å (SmSe) and 151 ± 20°K, 0·204 ± 0·027 Å (SmTe).