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high-pressure high-temperature synthesis; powder x-ray diffraction; pressure-induced amorphization; negative thermal expansion; zirconium molybdate
Abstract:
High-pressure high-temperature syntheses using the gamma-
ZrMo2O8 (Pa (3) over bar, Z = 4) starting material with the
structure of the beta-ZrW2O8 type are carried out in a multi-
anvil apparatus. The recovered products are examined with
powder x-ray diffraction at ambient conditions. The sample
obtained at 1.3 GPa and 298 K is partially amorphous and
contains gamma-ZrMo2O8. Up to the onset of amorphization at 1.3
GPa, the recovered products from near room and high
temperatures are alpha-ZrMo2O8 (P (3) over bar 1c, Z = 6) and
beta-ZrMo2O8 (C2/c, Z = 4), respectively. In the structure of
alpha-ZrMo2O8, layers of ZrO6 Octahedra and MoO4 tetrahedra are
joined together by van der Waals forces. beta-ZrMo2O8 is built
of infinite ribbons of edge-sharing ZrO8 polyhedra joined by
MoO5 pyramids. The crystalline compounds synthesized at
pressures above 1.3 GPa and at high temperatures. originate
from the pressure-induced amorphous phase of gamma-ZrMo2O8. The
higher the pressure during the synthesis the higher the
temperature necessary to obtain a recoverable crystalline
product. Up to about 4 GPa, the synthesized material is beta-
ZrMo2O8. The recovered crystalline phases from above 4.0 GPa
are the ZrO2-MoO3 decomposition products. One of them is the
high-pressure high-temperature form Of MoO3 (P2(1)/m, Z = 2),
with all the Mo atoms in the octahedral coordination. (C) 2002
Editions scientifiques et medicales Elsevier SAS. All rights
reserved.
