Deviation from the stoichiometry and lattice flexibility

Abstract

Several compounds containing crystal water showed rather interesting behaviour during heat treatment. In temperature regions below phase transformations caused by partial or total loss of "structural" crystal water an anisotropic lattice deformation occured /T. Sato, T. Nakamura, Thermochimica Acta /1982/ 21, 281/. In the case of GaOOH this phenomenon was proved to be connected with the departure of water being in excess to the "structural" wa-, ter bounded within the GaOOH lattice, but nevertheless this excess water was fixed somehow inside the lattice, because the temperatur O region of its leaving was between 100-270 C even when using very low heating rates /M. Farkas-Jahnke, Eight European Crystallographic Meeting, Liege /1983/ Abstract 241/. The amount of water was very large, about 20 w% of the "structural" water. Similar phenomenon, i.e. the presence of excess "fixed" NH3 was observed in ammonium paramolybdate tetrahydrate samples prepared at different pH values. The "apparent" structure of the material in all cases was alike, i.e. the site and intensity distribution of the reflections in room temperature X-ray patterns made in focusing Guinier-cameras were the same. The ]ltt 3 content of samples, however, prepared at basic pH values was about 50 w% higher than the value corresponding to stoichiometry, while of those prepared at acidic pH values was approximately equal to the stoichiometric NH3 content. These two kinds of materials, looking thus alike according to the room temperature X-ray diffraction investigations differ significantly in the way of their thermal decomposition, investigated in a high temperature Guinier-Lenne type camera. When ageing the samples containing excess "fixed" NH3 for several days at room temperature solid state reaction was observed between the "mother" ammonium paramolybdate tetrahydrate lattice and the excess ~li3 fixed in the lattice, resulting in an other molybdate compound having higher NH3 content stoichiometrically. Not only the mentioned compounds, but also several other lattices may contain considerable amount of excess "fixed" species /water, ammonium etc./ which can influence the further chemical and physical behaviour of the material. The results of such investigations led us to the conclusion that it would be necessary to revise the present conception of the stoichiometry. Complex investigations would be required using beside the room temperature X-ray diffraction measurements high temperature X-ray techniques, DTA, DTG, mass spectrometry, JlTJlIIR and many other methods to reveal the real stoichiometry of the material and its relation to the lattice stoichiometry, i.e. to the ratio of the building elements of the crystal lattice. Due to this recognized flexibility of lattices, being able to incorporate excess water, or other species by expanding themselves anisotropically great care has to be taken when characterizing the structure of such a compound by its room temperature unit cell parameters.04.2-6 DISTORTED OCTAHEDRAL COPPERIII) COMPLEX OF CIMETIDINE. refle...

Scaling behaviour of pair correlation functions for randomly faulted hexagonal close-packed structures

“…In addition to the fault model approach, it is also possible to describe a faulted crystal without specifying the nature and geometry of stacking faults explicitly and carry out the calculation of diffracted intensity in the manner described by various workers (Zachariasen, 1947;Farkas-Jahnke, 1973;Estevez-Rams et al, 2001;Varn et al, 2002). By Fourier inversion of the observed intensity distribution along diffuse streaks, one can obtain the relevant pair correlation functions which carry information about the different types of stacking faults present in a particular crystal (Estevez-Rams et al, 2001;Estevez-Rams, Leoni et al, 2003;.…”

The diffraction of X-rays by close-packed polytypic crystals containing single stacking faults. I. General theory

“…Crystals exhibiting the phenomenon of polytypism also have a strong tendency for the formation of stacking faults, One-dimensionally disordered polytypic structures are often observed. This was found, among others, by Mitchell (1956), Jain & Trigunayat (1970), Lal & Trigunayat (1971), Minagawa (1977Minagawa ( , 1978 and Patosz (1981) in CdI2 crystals, Jagodzinski (1954Jagodzinski ( , 1971, Jagodzinski & Arnold (1960), Krishna & Marshall (1971a, b), and Pandey, Lele & Krishna (1977) in SiC crystals, Prasad & Srivastava (1973), Minagawa (1975Minagawa ( , 1979 and Chand & Trigunayat (1977) in PbI2 crystals, Prager (1977) in AgI crystals, Mehrotra (1978) in CdBr crystals, Rai & Krishna (1968), Steinberger, Kiflawi, Kal-0108-7673/88/050640-10503.00 man & Mardix (1973) and Farkas-Jahnke (1973b) in ZnS crystals, Iijima (1982) in mcGillite crystals, Kozielski (1975) and Patosz & Przedmojski (1976a, b) in Znl_~Cdx and ZnSI_,Se, mixed crystals, and , 1981a, b, 1982 in different doped ZnS and ZnSe crystals and in ZnS~_xSex mixed crystals.…”