XPS studies and defect structure of pure and Li-doped SrBPO5

XPS studies on pure and Li-doped SrBPO5 containing network-linked BO4, PO4, and SrO9 polyhedra are reported. SrBPO5 belongs to the stillwellite family, which possesses interesting nonlinear optical (NLO) and ferroelectric properties. The measured binding energies (BEs) of the various elements bring out the finer aspects of site symmetry and the chemical environment. These are compared with the XPS BEs of pure and Li-doped BPO4. The two distinct XPS peaks in the Li spectra of doped SrBPO5 and BPO4 clearly indicate weakly and strongly bound states for lithium. A defect structure where the Li ions occupy both lattice and interstitial sites is proposed.