  Materials with formula of A2B2O7 is a famous family with more than 300
compounds, and have abundant properties, like ferroelectric, multiferroic, and
photocatalyst properties, etc. Generally, two structures dominate this family,
which are pyrochlore and perovskite-like layered (PL) structure. Previously,
the structure and properties design of these materials are usually complex, and
solid solutions, which complicates the manufacture, as well as introducing
complexity in the study of the microscopic origins of the properties. Here, we
report that the pyrochlore-PL structure change happened in pure Eu2Ti2O7 under
high pressure and temperature, and the formed PL structure will transfer back
by heating. These results reveal that the PL structure formed in PL-pyrochlore
solid solutions, is due to tuning of the high-pressure formed PL structure in
pure pyrochlore compounds to ambient pressure. These results indicate the high
pressure and high temperature can be used to manipulate the crystal structures
from pyrochlore to PL structure, or vice versa. Furthermore, the PL Eu2Ti2O7
was confirmed as a lead free ferroelectric material for the first time.
