Alkali metal
Two different types of anhydrous double indium sulfates have been made. One is from the family MMIII(XO4)3, with MI being a large singly positive ion such as K, Rb, Cs, Tl or NH3; MIII is triply charged and can be Al, Ga, In, Tl, V, Cr, Fe, Sc and other rare earths; and X is S or Se. Most of these have a rhombohedral crystal structure. However, triammonium indium trisulfate, (NH4)3In(SO4)3 converts from rhombohedral to monoclinic as the temperature drops below 80 °C, and converts back into a rhombohedral form with space group "R"3"c" as the temperature rises above 110 °C. The low temperature monoclinic form has space group "P"21/"c", a=8.96, b=15.64 c=9.13 β=108.28° Z=4 The high temperature form is termed "β-". An explanation for this transition is that ammonium (and also thallium) is a non-spherical ion and thus has lower symmetry. However, when it is heated enough, dynamical disorder causing random orientations makes the ions on average spherically symmetric. Alkali metal ions are spherical in shape at all temperatures and form rhombohedral structures. Double sulfates of this form exist of indium with the alkali metals sodium, potassium, rubidium, and cesium. These can be formed by heating a solid mixture of the individual sulfates to 350 °C.