TY - JOUR
T1 - A coupled electron diffraction and rigid unit mode (RUM) study of the crystal chemistry of some zeotypic AlPO4 compounds
AU - Withers, Ray L.
AU - Liu, Yun
PY - 2005/9
Y1 - 2005/9
N2 - Electron diffraction and lattice dynamical calculations are used to investigate the unit cells, space group symmetries and inherent displacive flexibility of the room-temperature average structures of AlPO4-8, AlPO4-16 and AlPO4-tridymite. The zero-frequency rigid unit modes (RUMs) of the idealized high-symmetry polymorphs thereof are also investigated along with their relationship to the lower-temperature polymorphism of these zeotypic aluminophosphates. The clear presence of G±12c* satellite reflections in addition to the Bragg reflections (G) of the underlying Cmc21 parent structure in the case of AlPO4-8 shows that the true unit cell of the room-temperature polymorph has a doubled c-axis due to a condensed RUM mode. Structured diffuse scattering is also observed which can be related to the thermal excitation of RUM modes. In the case of AlPO 4-tridymite, a complex F1 triclinic polymorph is observed and related to soft RUM modes while, in the case of AlPO4-16, a soft q=0 RUM mode is shown to be responsible for an observed phase transition in the case of the all SiO2 analogue of AlPO4-16. A large number of additional zero-frequency RUM modes also exist in the case of AlPO 4-16.
AB - Electron diffraction and lattice dynamical calculations are used to investigate the unit cells, space group symmetries and inherent displacive flexibility of the room-temperature average structures of AlPO4-8, AlPO4-16 and AlPO4-tridymite. The zero-frequency rigid unit modes (RUMs) of the idealized high-symmetry polymorphs thereof are also investigated along with their relationship to the lower-temperature polymorphism of these zeotypic aluminophosphates. The clear presence of G±12c* satellite reflections in addition to the Bragg reflections (G) of the underlying Cmc21 parent structure in the case of AlPO4-8 shows that the true unit cell of the room-temperature polymorph has a doubled c-axis due to a condensed RUM mode. Structured diffuse scattering is also observed which can be related to the thermal excitation of RUM modes. In the case of AlPO 4-tridymite, a complex F1 triclinic polymorph is observed and related to soft RUM modes while, in the case of AlPO4-16, a soft q=0 RUM mode is shown to be responsible for an observed phase transition in the case of the all SiO2 analogue of AlPO4-16. A large number of additional zero-frequency RUM modes also exist in the case of AlPO 4-16.
KW - Characteristic diffuse distributions
KW - Correlated tetrahedral rotations
KW - Dynamical disorder
KW - Microporous aluminophosphates
UR - http://www.scopus.com/inward/record.url?scp=24144455656&partnerID=8YFLogxK
U2 - 10.1016/j.jssc.2005.06.003
DO - 10.1016/j.jssc.2005.06.003
M3 - Article
SN - 0022-4596
VL - 178
SP - 2647
EP - 2657
JO - Journal of Solid State Chemistry
JF - Journal of Solid State Chemistry
IS - 9
ER -