I. The ditetragonal bipyramidal series, consisting of the salts (a) K₃TlCl₆ 2H₂0 (b) (NH₄)₃TlCl₆ 2H₂0 (c) K₃InCl₆ 2H₂0
(d) Rb₃TlBr₆ 2H₂0 (e) K₃INBr₆ 2H₂0 , was investigated,
and full crysteilogra.phical details are given for all the salts except K₃InBr₆ 2H₂0, which effloresces so readily that exact measurements are impossible. It had formerly been considered that K₃TlCl₆ 2H₂0, (NH₄)₃TlCl₆ 2H₂0, K₃InC1₆ I1/2H₂0, and Rb₃TlBr6 H₂0 were isomorphous. The
investigation has shown that all the salts of this isomorphous series have two molecules of water of crystallisation.
2. The rhombic bipyramidal series, consisting of the salts
(a) Rb₂₃TlCl₅ H₂0, (b) Cs₂TlCl₅ H₂0, (o) (NH₄)₂InCl₅ H₂₈ (d) Rb₂iInCl₅ H₂0 (e) Cs₂InCl₅ H₂0 (f) (NH₄)₂INBr₅ H₂O (g) Rb₂INBr₅ H₂0 (h) Cs₂INBr₅ H₂O,, Of these, the indium salts - with the possible exception of (NH₄)₂InCl₅ H₂0 - were prepared for the first time, and quantitative analyses are appended. The series is isomorphous, and crystallographical details are given in full, except in the case of (NH₄)₂INBr₅ H₂0, which is very deliquescent.
3. The following salts were found to crystallise in cubes in
the regular system.:- KTlBr₄ 2H₂0, (NH₄)TiBr₄ 2H₂0, RbT1Br4 H₂0, and CsTlBr4. Of these the first two had formerly been taken as rhombic.
Details of the crystallography of the silt
K₃TI₂Br₀ 3H₂0, which crystallises in the ditetragonal bipyramidel class, are also appended.
4. The results of the investigation on the isomorphous 'relationships in the above mentioned series and the additional
salts K₂FeCl₅ H₂O and (NH₄)₂FeCI₅ H₂O (which are isomorphous with the second series). may be summarised as follows :-
(a) Crystal Habit.
The alkalies .stand in the following order :- NH₄,
Rb, Cs. Cl and Br are very closely related. The salts
of Fe, In, Ti, differ widely from each other.
(b) Intdrfacial. Angles.
For the alkalies the order is : Rb, NH₄, Cs, with NH₄ very near Rb. The interval Cl - Br is of the same order of magnitude as the interval Rb - Cs, The greatest change in interfacial angles is obtained by replacement within the group Fe, In, Ti; the effect is roughly proportional to the change in atomic weight. The effect of the replacement In -> Tl is opposite in sense to that of NH₄ ->Cs, Cl -> Br, Fe -> In, or Kb-> NH₄..
(o) Molecular Volumes, Axial Constants, and Molecular
In regard to molecular volume and molecular distance
ratios, the alkalies stand in the following order :- K, NH₄,
Rb, Cs, with NH₄ near to Rb. Replacement affects mainly
the 4 values. Substitution of Br for Cl causes an especially large extension along the three axial directions, a fact which probably indicates symmetrical disposition of the halogen atoms in the molecule. Within the group Fe, In, Ti, replacement has only a small effect on the molecular volume and molecular distance ratios, the effect being seen mainly in the w value. In and Ti are more closely related than are Fe and In.
The axial constants afford no definite results in this
5. In the complex snits of indium and thallium under consideration, the greater the atomic weight of the alkali, the less is the amount of water in the salt. The greater the atomic weight of the alkali, the less also is the ratio RX : MX₃ in the chemical constitution. of the salt. The ratio RX : MX₃ is, speaking generally, lower in the indium chlorides than in the corresponding thallium chlorides, and in the
thallium bromides than in the. corresponding indium bromides.
The relative concentrations of complex ions in the thallium and
indium solutions are important factors in determining the stability of the various salts.