Phase transitions in K₂Cr₂O₇ and structural redeterminations of phase II

Abstract

Crystals of phase II K2Cr2O7, potassium dichromate, space group P1 , grown from aqueous solution undergo a first-order transition to phase I, space group reportedly P21/n, at a phase-transition temperature, TPT, of 544 (2) K on first heating; the corresponding transition on cooling is at 502 (2) K. The endotherm on subsequent heatings occurs reproducibly at TPT = 531 (2) K. Mass loss between ca 531 and 544 K, identified as included water, is rapid and continues more slowly to higher temperatures for a total loss of ca 0.20%. The higher TPT on first heating is associated with the increasing pressure of superheated water occupying inclusion defects. The latent diagonal glide plane in phase II allows the structure of phase I to be inferred. The triclinic structure at 296 K has been independently redetermined. Normal probability analysis shows high consistency between the resulting and previous atomic coordinates, but with uncertainties reduced by a factor of ca 2. The earlier uncertainties are systematically underestimated by a comparable factor. The structure of phase IIb, space group A2/a on transposing axes, was determined at ca 300 K by Krivovichev et al. [Acta Cryst. (2000), C56, 629-630]. The first-order transition between phases I and II arises from the ca 60 degrees relative rotation of terminal O atoms in each tetrahedron as the n glide plane is gained or lost. A transition between phases IIb and I, also of first order, is likely but not between phases II and IIb. An intermediate phase may exist between phases IIb and I.

Synthesis and structural characterization oftris(ethylenediamine)nickel(II) dichromate

“…The bond lengths and angles for (en) in 1 are in the normal range (table 3). The cis N-Ni-N angles of the {NiN 6 } octahedron range from 81.22 (9) to 94.09(10) while the trans N-Ni-N angles scatter in a narrow range between 171.67(9) and 173.34 (10) , indicating a distorted octahedron. The distortion is also evident in terms of the Ni-N bond distances, which range from 2.112(2) to 2.167(2) Å , with a difference D of 0.055 Å between the longest and the shortest Ni-N bonds.…”

Bernauer′s Bands

“…PD crystallizes in the triclinic system, space group P " 1, a = 7.3837(8) , b = 7.4622(6) , c = 13.3949(12) , a = 96.204(7)8, b = 98.046(7)8, g = 90.943(8)8, Z = 4 at 296 K. [118] The optical properties were reported by Groth. [119,120] PD is biaxial (+) with the optic axial plane perpendicular to {100}.…”

Structural complexity of minerals: information storage and processing in the mineral world

“…For instance, the structure of lopezite ( Fig. 3a) is triclinic, space group P1 (Weakley et al, 2004). All atoms in the structure occupy 2i Wyckoff sites: there are four K, four Cr, and fourteen O sites, which correspond to the total number of 22 equivalence classes.…”