Crystal chemical considerations are used to change the crystal field splitting of the 5d band of TmSe by alloying and to design new ternary materials with valence fluctuations like TmSe1-xTex and Tm1-xEuxSe. By varying x in the range 0 < x < 1 it is possible to change the valence of Tm, for the first time, all the way between +3 and +2. In this way the band structure changes from that of a metal to that of a semiconductor. The change of the free space in the NaCl-lattice cell as a function of composition appears to be a useful criterion for the design of such new ternary materials. This function is strongly dependent on the change of the ionic volume caused by the substitution and it seems to influence defect structure and valence instability of the material. Experimental evidence for the above is given from the cell dimensions and densities measured as a function of composition. The phase diagram study of TmSe1-xTex shows that like in TmSe a phase transition takes place at 1600 - 1700° C. Other materials which may possibly show valence instabilities are Tm1-xSmxSe, Tm1-xYbxSe, Tm1-xSrxSe, Tm1-xBaxSe and Tm1-xCaxSe, provided that they form mixed crystal series