Direct evidence for a low-frequency phonon mode mechanism in the negative thermal expansion compound

¹ ISIS Facility, Rutherford Appleton Laboratory - Chilton, Didcot OX11 0QX, UK
² Inorganic Chemistry, South Parks Road - Oxford, OX1 3QR, UK
³ Department of Chemistry and Center for Advanced Materials Research Oregon State University, Corvallis, OR 97331, USA

Received: 26 October 1998
Accepted: 23 March 1999

Abstract

Accurate lattice parameters have been determined between 2 K and 520 K for the negative thermal expansion compound , by high-resolution neutron powder diffraction measurements. Two different analyses of the temperature dependence of the lattice constant i) in terms of Debye and Einstein contributions and ii) using MaxEnt techniques to reconstruct a Gruneisen-parameter–weighted phonon density of states both provide direct evidence that the negative thermal expansion behaviour is driven by low-frequency modes with an energy scale of 3–8 meV. Comparison with earlier specific heat measurements indicates that these rigid-unit modes are associated with very large Gruneisen parameters.