CRYSTALLOGRAPHIC ANALYSIS OF PHASE TRANSFORMATIONS IN DANBURITE CaSi 2 B 2 O 8
- PDF / 512,934 Bytes
- 9 Pages / 612 x 792 pts (letter) Page_size
- 47 Downloads / 146 Views
CRYSTALLOGRAPHIC ANALYSIS OF PHASE TRANSFORMATIONS IN DANBURITE CaSi2B2O8 S. V. Borisov1*, S. A. Magarill1, and N.V. Pervukhina1,2
The crystallographic analysis of the structures of danburite CaSi2B2O8, which is orthorhombic (Pnma) at normal pressure, triclinic (P 1 ) at a pressure of 25.4 GPa, and monoclinic (P21/c) at a pressure of 32.3 GPa, shows that they are based on a joint packing of Ca cations and O anions whose regularity increases with pressure. A common structural fragment is the configuration of atoms related by the glide reflection plane retaining for Ca and a part of O, B positions with a possible Si migration according to the seesaw mechanism (N. V. Belov). DOI: 10.1134/S0022476620070070 Keywords: danburite phases under pressure, crystallographic analysis, cation and anion sublattices, pseudo symmetry, symmetry–stability, stable fragments.
INTRODUCTION The mechanisms of solid phase transitions with changes in temperatures and pressures are of interest from both practical and purely scientific points of view. In [1], the results of a careful examination of changes in the structure of naturally occurring danburite Ca-borosilicate at pressures up to ∼35 GPa are considered. Variations of the coordination environment of the key Si atom from tetrahedral to octahedral (through a trigonal bipyramid) and the related structure topologies are analyzed. In this process, the symmetry changes from orthorhombic (Pnma, danburite I [2]) to monoclinic (32.3 GPa, P21/c, danburite IV [1]) through triclinic (25.4 GPa, P 1 , danburite III [1]). The volume per formula unit V changes in the sequence VI = 136 Å3, VIII = 97.3 Å3, VIV = 98.2 Å3. The problems of symmetry compensation of an increase in volume with increasing pressure (VIII < VIV!) and of transformation of skeletal sublattices characterizing the types of joint atomic packing prompted us to carry out the crystallographic analysis of these phases.
CRYSTALLOGRAPHIC ANALYSIS The methodological basis of the crystallographic analysis is a postulate that the translational symmetry is formed by plane elastic standing vibrations in a medium of material particles (atoms). In the space, each such vibration creates a onedimensional translation equal to a half wavelength, for example, the distance between the neighboring nodal planes (Fig. 1). During crystallization (heat loss) this vibrational process arranges the material particles so that the nodal planes contain the most massive particles with the minimum amplitude, and hence, with the minimum kinetic energy. Thus, the order in the
1
Nikolaev Institute of Inorganic Chemistry, Siberian Branch, Russian Academy of Sciences, Novosibirsk, Russia; *[email protected]. 2Novosibirsk State University, Novosibirsk, Russia. Original article submitted October 25, 2019; revised January 29, 2020; accepted February 10, 2020. 0022-4766/20/6107-1059 © 2020 by Pleiades Publishing, Ltd.
1059
Fig. 1. Model of a one-dimensional standing wave (“string”). A standing wave is the sum of two identical vibrations spreading out in opposite di
Data Loading...
