A density-functional study of charge doping in WO3

TL;DR

This study uses density functional theory to analyze how adding electrons to WO3 induces structural phase transitions, highlighting the primary role of electronic effects, especially covalent interactions, in these transformations.

Contribution

It provides a detailed theoretical analysis of charge doping effects in WO3, isolating electronic influences from ionic effects, and maps the phase transitions driven mainly by electronic factors.

Findings

Charge doping causes a sequence of structural phase transitions in WO3.

Electronic effects, especially W 5d and O 2p covalent interactions, drive these transformations.

Theoretical phase diagram aligns with experimental observations in NaxWO3.

Abstract

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.