Preparation and studies of new crystals in the K3H(SO4)2-(NH4)3H(SO4)2-H2O system

To elucidate the effect of isomorphic substitution on the kinetics of phase transitions, single crystals of (K _x (NH₄)_1?x ) _m H _n (SO₄)_{(m + n)/2} · yH₂O solid solutions are grown from the K₃H(SO₄)₂-(NH₄)₃H(SO₄)₂-H₂O system, whose end members are known to undergo superprotonic phase transitions of fundamentally different kinetics. The chemical composition of the single crystals grown is determined by energy dispersive X-ray microanalysis. The thermal and optical behavior of (K,NH₄)₉H₇(SO₄)₈ · H₂O single crystals is studied in the temperature range 295–420 K and the crystal structure at 295 K is determined. A comparison of the results of the studies with data for crystal K₉H₇(SO₄)₈ · H₂O published earlier shows that the substitution of ammonium for potassium atoms lowers the temperature of the structural phase transition by 8 K.