  The addition of electron donors to the vacant A site of defect-perovskite
structure tungsten trioxide causes a series of structural and chemical phase
transitions; for instance, in the well-known case of the sodium tungsten
bronzes (NaxWO3). Here we calculate the effect of the addition of electronic
charge to WO3 without the complication of also including sodium ions. Our
density functional theory method enables isolation of electronic effects from
the additional size, chemical, and disorder effects present in experimental
samples. Our calculated low-temperature phase diagram between x=0 and x=1 moves
from the initial low-temperature monoclinic phase through a second (centered)
monoclinic phase, an orthorhombic phase, a tetragonal antiferroelectric, and an
aristotypic cubic phase, in broad agreement with the experimentally-observed
transformations in NaxWO3. Our work confirms that the observed structural
transformations are driven primarily by electronic factors. We find that the
dominant electronic effect is the covalent interaction between the tungsten 5d
and oxygen 2p orbitals.
