Electrical and solid-state electrochemical studies of single crystals of cryolite-type ammonium hexafluorometallates

We present electrical transport studies of two ammonium hexafluorometallates of general formula (NH4)3GaxV1−xF6, highlighting the intriguing relationship between structure and electrical and electrochemical properties. Part of this study uses a method adapted to probe the solid-state electrochemical characteristics on individually contacted single crystals. For compound 1, x varies over 0.15 < x < 0.3, and for 2, x varies over 0.3 < x < 0.4, as determined by physical and chemical characterization. Electronically, compounds 1 and 2 are found to be wide bandgap electronic materials at room temperature. Variable temperature electrical measurements on single crystals of 1 and 2 show temperature-activated behavior for both. Within the temperature range probed, possible conduction pathways are discussed in relation to the structure and in comparison to other hexafluorometallates. We show that solid-state electrochemical studies on single crystals of 1 and 2 can probe redox properties of the metal centers in materials with low electrical conductivity, of interest in electrochemical energy storage applications.