  In {\delta}-Ag2/3V2O5 with charge degrees of freedom in V, it is known that
the charge ordering state and physical properties of V that appear at low
temperatures depend strongly on the ordering state of Ag. In this study, we
focused on the Ag ions in the interlayer and studied the structure using
synchrotron radiation powder diffraction in dependence on temperature. We found
that when the sample is slowly cooled from room temperature and ordering occurs
at the Ag sites, V4+/V5+ charge ordering of V and subsequent V4+-V4+ structural
dimers are produced. Although quenching the sample from room temperature
suppresses the ordering of Ag, annealing at around 160 K promotes partial
ordering of Ag and allows a metastable phase to be realized. This metastable
phase is maintained even when the temperature is lowered again, producing a
remarkable change in low-temperature properties. These results indicate that
the ordered state of Ag, which is the key to control the charge-ordered state
and physical properties, can be controlled by low-temperature annealing. The
results of this study may provide a methodology for the realization of
metastable states in a wide range of material groups of vanadium compounds,
where competition among various charge ordered states underlies the physical
properties.
