High-resolution vibrational spectroscopy of Pb-OH defects in KMgF 3 fluoroperovskite single crystals

High-resolution (0.04 ${\mathrm{cm}}^{\ensuremath{-}1})$ Fourier transform infrared spectroscopy in the temperature range 9\char21{}300 K is applied to detect and analyze the ${\mathrm{OH}}^{\ensuremath{-}}$ stretching modes in air grown ${\mathrm{KMgF}}_{3}$ single crystals, doped with different Pb amounts. In addition to the 3733.7 ${\mathrm{cm}}^{\ensuremath{-}1}$ line attributed to the stretching mode of isolated ${\mathrm{OH}}^{\ensuremath{-}},$ two main lines peaking at 3550.9 and 3567.7 ${\mathrm{cm}}^{\ensuremath{-}1}$ are due to the ${\mathrm{OH}}^{\ensuremath{-}}$ stretching modes perturbed by neighboring Pb defects. Suitable thermal treatments and isotopic substitutions provide models of the complexes in which OH and Pb are embedded. Lead is recognized as favoring the ${\mathrm{OH}}^{\ensuremath{-}}$ inclusion into the lattice and causing an inhomogeneous broadening of the IR lines related to the stretching modes of ${\mathrm{OH}}^{\ensuremath{-}}$ interacting with other cation impurities. Anharmonicity effects are monitored by the weak overtones of the OH-related lines and discussed in the framework of the Morse model for the anharmonic oscillator. The anharmonicity and the Morse parameters, which show a very weak temperature dependence in the 9\char21{}300 K range, are very close to those displayed by alkali fluorides. The temperature dependence of the line position and linewidth of the narrow (0.4\char21{}0.9 ${\mathrm{cm}}^{\ensuremath{-}1})$ Lorentzian-shaped IR lines and of the related overtones is successfully analyzed by means of the single phonon coupling model. The coupled phonon frequencies, evaluated from the fitting, for the Pb-perturbed ${\mathrm{OH}}^{\ensuremath{-}}$ stretching modes fall in the frequency range of the highest phonon state density of the host matrix.