Phase incommensurable dans le composé mixte (NH4) 2(BeF4)0,82(SO4)0,18

Phase transitions in the solid solution (NH 4 ) 2 (BeF 4 ) 0.82 (SO 4 ) 0.18 have been studied by means of X-ray scattering. The transition sequence is particularly complicated. The incommensurate phase appears between −106°C and −120.5°C, but in the −106 to −113°C temperature range, the incommensurate superstructure is double. Moreover the so-called «lock in» phase is already present at the normal-incommensurate transition: it appears from −102.5°C. These results are briefly discussed. The possibility of inhomogeneities in the crystal is considered Etude par diffraction RX de la sequence de transition. La phase incommensurable qui va de −106°C a −120,5°C presente une surstructure incommensurable double de −106°C a −113°C. La phase de blocage apparait des −102,5°C