Negative thermal expansion in beta-quartz

Citation
Prl. Welche et al., Negative thermal expansion in beta-quartz, PHYS CHEM M, 26(1), 1998, pp. 63-77
Citations number
41
Categorie Soggetti
Earth Sciences
Journal title
PHYSICS AND CHEMISTRY OF MINERALS
ISSN journal
03421791 → ACNP
Volume
26
Issue
1
Year of publication
1998
Pages
63 - 77
Database
ISI
SICI code
0342-1791(199811)26:1<63:NTEIB>2.0.ZU;2-I
Abstract
Computer modelling and theoretical analysis are used to explain the nearly zero and slightly negative coefficients of thermal expansion in beta-quartz well above the alpha-beta phase transition temperature. Quartz was selecte d for study as an archetypal material with a framework structure of stiff u nits, namely SiO4 tetrahedra, linked through shared oxygen atoms as very fl exible hinges. The contributions of the soft mode, the Vallade mode, the TA (z) phonon branch and the phonon spectrum as a whole are discussed in detai l. The results fully support and illustrate a recent theory of the negative contribution to thermal expansion in framework structures. It is a geometr ical effect due to the rotation of the tetrahedral units, folding together as they vibrate. The very rapid increase in the lattice parameters for abou t 20 K above the transition temperature is well accounted for within quasih armonic theory, and is therefore not evidence for critical fluctuations or fluctuating patches of alpha(+), alpha(-) structure.