Beschreibung:
<jats:title>Abstract</jats:title><jats:p>The thermal behaviour of a natural allanite-(Ce) has been investigated up to 1073 K (at room pressure) by means of in situ synchrotron powder X-ray diffraction and single-crystal neutron diffraction. Allanite preserves its crystallinity up to 1073 K. However, up to 700 K, the thermal behaviour along the three principal crystallographic axes, of the monoclinic <jats:italic>β</jats:italic> angle and of the unit-cell volume follow monotonically increasing trends, which are almost linear. At <jats:italic>T</jats:italic> > 700–800 K, a drastic change takes place: an inversion of the trend is observed along the <jats:italic>a</jats:italic> and <jats:italic>b</jats:italic> axes (more pronounced along <jats:italic>b</jats:italic>) and for the monoclinic <jats:italic>β</jats:italic> angle; in contrast, an anomalous increase of the expansion is observed along the <jats:italic>c</jats:italic> axis, which controls the positive trend experienced by the unit-cell volume at <jats:italic>T</jats:italic> > 700–800 K. Data collected back to room <jats:italic>T</jats:italic>, after the H<jats:italic>T</jats:italic> experiments, show unit-cell parameters significantly different with respect to those previously measured at 293 K: allanite responds with an ideal elastic behaviour up to 700 K, and at <jats:italic>T</jats:italic> > 700–800 K its behaviour deviates from the elasticity field. The thermo-elastic behaviour up to 700 K was modelled with a modified Holland–Powell EoS; for the unit-cell volume, we obtained the following parameters: <jats:italic>V</jats:italic><jats:sub>T0</jats:sub> = 467.33(6) Å<jats:sup>3</jats:sup> and α<jats:sub>T0</jats:sub>(<jats:italic>V</jats:italic>) = 2.8(3) × 10<jats:sup>–5</jats:sup> K<jats:sup>−1</jats:sup>. The thermal anisotropy, derived on the basis of the axial expansion along the three main crystallographic directions, is the following: α<jats:sub>T0</jats:sub>(<jats:italic>a</jats:italic>):α<jats:sub>T0</jats:sub>(<jats:italic>b</jats:italic>):α<jats:sub>T0</jats:sub>(<jats:italic>c</jats:italic>) = 1.08:1:1.36. The <jats:italic>T</jats:italic>-induced mechanisms, at the atomic scale, are described on the basis of the neutron structure refinements at different temperatures. Evidence of dehydroxylation effect at <jats:italic>T</jats:italic> ≥ 848 K are reported. A comparison between the thermal behaviour of allanite, epidote and clinozoisite is carried out.</jats:p>