X-ray absorption spectroscopy study of local dynamics and thermal expansion inReO3

Abstract

The thermal expansion of polycrystalline ReO 3 is studied in vacuum by x-ray diffraction from 300 to 600 K and by extended x-ray absorption fine structure (EXAFS) at the Re L 3 edge from 30 to 600 K. A detailed EXAFS analysis is presented up to the sixth coordination shell around Re. The crystal thermal expansion is weak throughout the explored range, negative below 100 K, positive from 150 to 500 K, and negative again above 500 K. The analysis of mean square relative displacements obtained by EXAFS and of mean square displacements available from x-ray and neutron diffraction measurements confirms the presence of significant static disorder in the investigated samples. EXAFS results suggest that below 500 K in ReO 3 at least some of the octahedra are slightly and disorderly rotated by an average angle θ , whose value decreases when the temperature increases. This explains why ReO 3 , in spite of the possibility of supporting rigid unit modes, shows a weak positive expansion at increasing temperatures, from 150 up to 500 K. Results are discussed and compared with recent experimental and theoretical work on materials characterized by negative thermal expansion.

Thermal disorder and correlation effects in anti-perovskite-type copper nitride

“…These findings are in agree-ment with expected anisotropy of copper motion, as observed for Cu 3 N in diffraction studies [20,21]: the vibrations of copper atoms in the direction orthogonal to -N-Cu-N-chains are significantly larger than their vibrations along the chains, and they also grow faster upon increasing temperature. Note that similar trends have been observed by us previously in perovskite-type ReO 3 [48,49,14] and ScF 3 [38].…”

“…crease [20]. In this sense the behaviour of Cu 3 N is similar to the one, observed for such perovskite-type materials as rhenium oxide ReO 3 and scandium fluoride ScF 3 [14,15,38]. The character of chemical bonding in Cu 3 N, in turn, has been a subject of the discussion in the literature.…”

“…The obtained value of γ for Cu 0 -N 1 atom pair is 2.2(1), which indicates that at high-temperatures the relative amplitude of Cu vibrations is twice larger in the direction perpendicular to the -N-Cu-N-chain. This value is larger than the corresponding value for the first coordination shell in ReO 3 (γ ∼ 1.6 [52,14]). Note that similarly large value (γ = 1.8(2)) for anisotropy parameter was obtained also for Cu 0 -Cu 3a atom pair in Cu 3 N.…”

“…Many of the unique properties of perovskite-type materials are a result of local distortions and peculiar atomic dynamics in such materials. Importance of such effects as anisotropy and anharmonicity of atomic vibrations and the presence of strong correlations in atomic displacements is acknowledged, for instance, in many studies of phase-transitions [7,8,9], ferroelectric properties [7,10,11] and negative thermal expansion effect [12,13,14,15], observed in perovskites.…”

Nearest-neighbour distribution of distances in crystals from extended X-ray absorption fine structure

“…The bond expansion has been measured by total scattering in framework structures [46][47][48][49][50] as well as by EXAFS in diamond-zincblende structures, 14,51,52 in framework structures 53,54 and in delafossite structures. 55 In those works, average values of the coefficient of bond thermal expansion have been estimated from linear fits to the temperature dependence of the bond length; by this procedure, however, the non-linear low-temperature behaviour is not evidenced, as is instead commonly done for the lattice expansion measured by Bragg diffraction or dilatometric techniques.…”