Spryite, $${\text{Ag}}_{8}\left({{\text{As}}_{0.5}^{3 +} {\text{As}}_{0.5}^{5 +}}\right){\text{S}}_{6}$$ Ag 8 As 0.5 3 + As 0.5 5 + S 6 : structure determination and inferred absence of superionic conduction of the first As3+-bearing argyrodite