XH defects in nonmetallic solids: Isotope effects and anharmonicities as probes of the defect environment

We have developed a technique to analyze the ir stretch modes of XH defects in nonmetallic solids, where X is an atom or ion heavier than hydrogen. This technique draws heavily upon molecular theory: The XH defect is treated by a Morse potential modified by coupling of X to the lattice with use of a one-dimensional model. The electrical anharmonicity of the dipole moment is included in obtaining transition strengths. The importance of overtone data as well as deuteration is demonstrated. A number of systems involving ${\mathrm{OH}}^{\mathrm{-}}$ in insulating solids are analyzed. These include (1) ${\mathrm{OH}}^{\mathrm{-}}$, ${\mathrm{OD}}^{\mathrm{-}}$, and ${\mathrm{OT}}^{\mathrm{-}}$ in ${\mathrm{TiO}}_2$ [J. B. Bates and R. A. Perkins, Phys. Rev. B 16, 3713 (1977)]; (2) ${\mathrm{OH}}^{\mathrm{-}}$ and ${\mathrm{OD}}^{\mathrm{-}}$ in Mg-doped LiF and NaF [R. Capelletti et al., Cryst. Latt. Defects Amorph. Mater. 16, 189 (1987)]; (3) ${\mathrm{OH}}^{\mathrm{-}}$ and ${\mathrm{OD}}^{\mathrm{-}}$ in ${\mathrm{LiNbO}}_3$ [A. Forster et al., Phys. Status Solidi B 143, 755 (1987)]; and (4) ${\mathrm{OH}}^{\mathrm{-}}$ and ${\mathrm{OD}}^{\mathrm{-}}$ in CsCl and CsBr [M. Krantz and F. Luty, Phys. Rev. B 37, 7038 (1988)]. The defects in ${\mathrm{TiO}}_2$ exhibit a surprisingly large mechanical anharmonicity. The data on Mg-doped fluorides indicate electrical anharmonicities which in some cases are considerably different from that of molecular ${\mathrm{OH}}^{\mathrm{-}}$.