AIPO4-25 - FRAMEWORK TOPOLOGY, TOPOTACTIC TRANSFORMATION FROM AIPO4-21, AND HIGH-LOW DISPLACIVE TRANSITION

Upon calcination, as-synthesized AlPO4-21 transforms to AlPO4-25, perhaps topotactically. Room-temperature AlPO4-25 has unit cell parameters a = 15.085 (2) Å, b = 18.761 (2) Å, c = 8.363 (1) Å; Ibam (72). The four-connected tetrahedral net of AlPO4-25 appears to retain the (468)2(682)1 three-connected 2D net of AlPO4-21, but the double-crankshaft chains disappear. Instead, up and down connections alternate to yield dbs chains. AlPO4-25 inverts to the geometrically regular high variety near 530 K, with the accompanying halving of one axis. Time-of-flight neutron powder diffraction data at 593 K yielded a good Rietveld refinement for the high-temperature structure with artificially disordered Al0.5P0.5 in an orthorhombic cell with a = 9.4489 (4) Å, b = 15.2028 (5) Å, c = 8.4084 (3) Å, Acmm (67). © 1990 American Chemical Society.