The TmSe-TmS System

Single crystals TmSe Si.x have been grown by directional freezing of the melts in closed tungsten ampules. According to a figure in the paper, the lattice constants of samples with 

A compound described by the formula Tm5Se6(=TmSei 20) orTm4Se5( TmSei 25) has been obtained in the composition range TmSei 20 to TmSei 25. It has an NaCl type structure with a = 5.629 A, M. Guittard, J. Flahaut, M. Lepeltier (Bull. Soc. Chim. France 1968 4759/65). 

Studies of the phase diagram (see p. 316) leave the question unresolved if the composition TmSei 20 falls into the homogeneity range of the monoselenide phase, which may be the case at high temperatures, or characterizes an individual phase, which possibly appears only at lower temperatures see also the discussion of the monoselenide phase group on p. 318. 

A recent phase diagram (see p. 316) does not indicate a compound TmaSe4 which was reported in earlier publications. 

Tm3Se4 is rhombohedral of the CaH02Se4 structure type with a = 6.91 A [2]. A parameter a = 5.64 A is given for an assumed pseudocubic lattice. Tm3Se4 is isomorphous with Y3Se4 [1]. 

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