Remarkable features in lattice parameter ratios of crystals

Abstract

A rich phase chemistry has been predicted for pyridine, with a computational study by Price et al. [1] revealing 12 energetically similar structures. This, coupled with the simple nature of the molecule makes pyridine a good candidate for detailed structural studies. As well as being interesting in itself, it is hoped that the structural behavior of pyridine may shed light on more fundamental trends in structure and bonding in molecular materials. Previously only two crystal phases of pyridine have been identified by experiment. The Z´=4 phase (pyridine-I) first found by Mootz et al. [2] is unexpectedly complicated and has been found to arise from a range of crystallisation conditions. The simpler Z´=1 phase (pyridine-II) has previously only been grown for pyridine at high pressure [3], or for perdeuteropyridine from a pentane solution [4]. Using the HRPD instrument at the ISIS facility, we have performed a study of the crystallographic behaviour from 2K to melting of a pure perdeuteropyridine powder sample. Pyridine-I was found at lower temperatures, until a phase transition to pyridine-II was observed with increasing temperature. On further increase in temperature, a re-entry to the pyridine-II phase was seen.

Remarkable features in lattice-parameter ratios of crystals. I. Orthorhombic, tetragonal and hexagonal crystals

“…The present one (denoted as part I) is restricted to the hexagonal, tetragonal and orthorhombic lattices, as presented in a poster at the ECM22 conference in Budapest (de Gelder & Janner, 2004). The discussion of the monoclinic and the triclinic lattice distributions is postponed to part II ( de Gelder & Janner, 2005).…”

Assessing free lattice and structure parameters from the atomic level

“…The main references exploited to obtain lattice and structure parameters were the ICSD along with some printed resources . Consequently, the results refer mainly to inorganic structures as distinguished from organic, metal‐organic and bio‐macromolecular compounds addressed also by .…”

“…The aim of the present paper is to pay attention to the bond configuration between atoms in inorganic structures, possessing frequency maxima of free parameters. In previous contributions apparently unknown to the authors [2][3][4][5], the present author and his coworkers [6][7][8] have analyzed a number of inorganic structure types with regard to the abundance of free parameters (cf. Table 1) and found a peaked behavior of the frequency distribution similar to that reported by Constant Table 1 Structure types analysed with respect to interatomic distances as function of free lattice (c/a) and structure parameters (u,z).…”

“…, Janner proposed that exceptional values of lattice parameter ratios apparently correspond to integral lattices, characterized by an integral metric tensor of their basis vectors. Subsequently, browsing the inorganic, organic and biological databases, de Gelder & Janner considered experimental data of free lattice parameters of non‐cubic crystals and found peaks of abundance as a function of those parameters. Plotting frequency versus lattice parameter ratios, an exponential decay from a maximum was observed, being more pronounced for organic than for inorganic compounds.…”

“…A peaked component superimposed the monotonous curve particularly for inorganic structures. De Gelder & Janner attributed the corresponding distinguished values of lattice parameters to integral or to pseudo‐symmetric lattices. They felt that a structural interpretation of these peaks was missing .…”