K2WO4 crystallizes in space group C2/m with the axes a = 12.39, b = 6.105, c= 7"560/~ and/~= 115.96 °.The four formula units per unit cell form layers parallel to (001), in which each WO4 tetrahedron is surrounded by an almost regular hexagon of potassium ions. The W-O bond length is 1.79/~, the K-O and O-O distances are in general agreement with the accepted values. This structure type occurs also in other A2BX4 compounds, among which are K2MoO4, Rb2WO4, Rb2MoO4, (NH4)2MoO4, (NH4)2SeO4 and (NH4)2CrO4.
Introduction ExperimentalIn recent years, whilst much work has been expended on the structures of the tungstates and molybdates of alkaline earth and other di-and trivalent metals, less attention has been given to those of the alkali metals. During investigations in this laboratory on compounds of the A2BX4 type, it was found that in the group of alkali tungstates and molybdates the crystal structures of the potassium, rubidium and caesium salts were still unknown. Lithium tungstate has the hexagonal phenacite structure (Zachariasen & Plettinger, 1961), and sodium tungstate the spinel structure (Sadikov & Shishakov, 1965). The corresponding molybdates are isomorphous with the tungstates (Zachariasen, 1926; Lindqvist, 1950). As it is interesting to note the conversion of one structure type into another when a larger but chemically similar cation is substituted in a compound, it seemed appropriate to determine the still unknown structures of K, Rb and Cs molybdates and tungstates. Together with this determination, work was carried out, and is still in progress, on the polymorphism which many of these alkali salts exhibit and extensive results of these investigations will be published later. They have proved useful in ascertaining that only one unknown type of structure was involved, namely the monoclinic structure in which potassium and rubidium tungstate and molybdate crystallize. The caesium compounds proved to be isomorphous with orthorhombic/?-potassium sulphate. The potassium tungstate structure has been found to occur in a number of other substances, among which are ammonium selenate, chromate and molybdate. Some evidence for this had already been given by Groth (1908), since the morphological constants resemble each other. Of all these compounds, Wyckoff (1965) in his description of the A2BX4 crystal structures only mentions ammonium chromate, the data for which were taken from Bujor (1944). The proposed structure was rather unlikely, however, and a new model was suggested by Hasegawa, Fujishige & Ogawa (1963). Potassium tungstate has some properties which make preparation of a suitable single crystal rather difficult. It is highly hygroscopic, reacts with carbon dioxide, reacts with hot water to form a paratungstate and gives a hydrate when crystallized at room temperature (Pascal, 1959). Single crystals cannot be prepared from the melt since there exists a transition point at about 370 °C (Schmidt-Dumont & Weeg, 1951), at which the product decrepitates vigorously into very small fragments. The only way of...
