A study of matrix and admixture elements in fluorine-rich ionic conductors by pulsed glow discharge mass spectrometry

RATIONALE

Dopants in ionic conductors play a crucial role in achieving the required electrochemical properties. A slight variation in their concentration considerably affects the conductivity of crystals and their applicability as ionic conductors and laser materials. To ensure the growth of high-quality fluoride crystals, adequate approaches for the quantification of matrix and admixture/dopant components are required.

METHODS

A panel of SrF₂ - and GdF₃ -doped LaF₃ single crystals was investigated. The electrical conductivity of the crystals was measured using impedance spectroscopy in the frequency range 100 Hz-1 MHz to control for crystal quality. Pulsed glow discharge mass spectrometry (GDMS) was used to simultaneously quantify fluorine, strontium, lanthanum, and gadolinium in the crystals. X-ray fluorescence, scanning electron microscopy-energy dispersive X-ray spectroscopy, and arc optical emission spectrometry were used for validation.

RESULTS

Quasiperiodic intensity drifts under sputtering of the ionic conductors were observed and attributed to F^- redistribution on the sample surface, affecting surface conductivity and sputtering rate. Several sample preparation protocols were tested to address that effect. Full coating of the sample with a layer of silver several micrometers thick provided stable and effective sputtering. The parameters for the GDMS determination of F, Sr, La, and Gd were optimized. The elements' distribution was studied in different parts of the crystals.

CONCLUSIONS

An analytical approach to the direct multi-element analysis of fluoride-containing ionic conductors using pulsed GDMS with La_1-x-y Sr_x Gd_y F_3-x as an example was designed and tested. Instability effects of ionic conductivity were explained and coped with, providing effective and stable sputtering.