“…Compounds with mixed tetrahedral-octahedral (T-O) framework structures feature a broad range of different atomic arrangements, resulting in topologies with various interesting properties, such as ion exchange (Masquelier et al, 1996) and ion conductivity (Chouchene et al, 2017), as well as unusual piezoelectric (Ren et al, 2015), magnetic (Ouerfelli et al, 2007) or nonlinear optical features (frequency doubling) (Sun et al, 2017). In order to further increase the insufficient knowledge about the crystal chemistry and structure types of arsenates, a comprehensive study of the system M + -M 3+ -O-(H)-As 5+ (M + = Li, Na, K, Rb, Cs, Ag, Tl, NH 4 ; M 3+ = Al, Ga, In, Sc, Fe, Cr, Tl) was undertaken, which led to a large number of new compounds, most of which have been published (Schwendtner & Kolitsch, 2007, 2018a, and references therein).…”