A THEORETICAL TREATMENT OF “ORIENTATIONAL” DISORDER FOR ROUTINE USE

BY MICHAEL JEWESS

Link back to home page including e-mail contact details for Michael Jewess and related papers: SYMMETRYHOME

Link to full text as pdf: FULL TEXT

This work is an extension of work performed in the Inorganic Chemistry Laboratory, University of Oxford, UK, and was published in Acta Crystallographica B.

Abstract

Many crystals are known in which diatomic or polyatomic ions are 'orientationally' disordered. General principles are presented which must govern the precise positions among which the ions are disordered, if short-range ordering is ignored. These principles are applied, by way of example, to four crystals, namely the room-temperature forms of ammonium chloride, tetramethylammonium trichloromanganate(II) (NMe₄MnCl₃), scawtite [Ca₇(Si₆O₁₈)(CO₃).2H₂O], and bis-(adiponitrile)copper(I) nitrate {Cu[NC(CH₂)₄CN]₂NO₃}. In these crystals, the disordered ions are, respectively NH₄⁺, NMe₄⁺, CO₃^2-, and NO₃^-. It is shown that the structure analyses reported for the latter three crystals were based on physically unfeasible assumptions about the positions among which the ions are disordered. It is suggested that this explains the problems encountered in each case, and that routine use of the general principles here presented should improve the reliability of future refinements of structures of this type.

Keywords: thermodynamics, crystallography, disorder, positional disorder, orientational disorder, specific heat, heat capacity, anomaly, ammonium, ammonium chloride, tetramethylammonium, trichloromanganate(II), manganese, TMMC, adiponitrile, copper(I), nitrate, carbonate, scawtite.

CONTACT

Use this link symmetry to contact the author (Dr Michael Jewess).