STRUCTURAL RELATIONSHIPS IN KH₂PO₄-TYPE COMPOUNDS. Jürgen Glinnemann, Matthias Becker, Theo Hahn, Gernot Heger. Institut für Kristallographie, RWTH Aachen, D-52056 Aachen, Germany.

The large number of seemingly unrelated crystal structures of compounds with formula type A(H,D)₂XO₄ with A = Li, Na, K, Rb, Cs, Tl, NH₄ and X = P, As can be grouped into five structure families in the sense of Megaw (1973). Each family can be derived from one of the following five basic structure types: NaCl, CsCl, NaTl, PtS, and a shear variant of CsCl with coordination number 7. These structure types form the head entries for each family tree. This derivation consists of the following procedure:

*Degradation of the point symmetry and increase of the unit cells of the ideal AB structures by stepwise application of group-subgroup relations;

*replacement of the B atoms by oriented XO₄ tetrahedra;

*insertion of positions for the H and D atoms.

This procedure permits to place all known crystal structures of the A(H,D)₂XO₄ compounds into a hierarchical order.

The distribution of the individual compounds and polymorphs over and within the families is rather heterogeneous. The behaviour of KD₂PO₄ and of Rb(H,D)₂PO₄ is exceptional as their polymorphs occur in two different families: KD₂PO₄ occurs in the families derived from NaTl and NaCl, the rubidium salts in the families derived from NaTl and the shear variant of CsCl.

The five family trees will be presented and their geometrical relations discussed.

Megaw, H. D. (1973). Crystal structures: a working approach, ch. 12, pp. 282-340. Philadelphia: Saunders.