No description available.

No description available.

No description available.

No description available.

No description available.

No description available.

No description available.

The rare earth polysulfides and polyselenides of Tm, Yb, and Lu are unknown. The ionic radius of the rare earth has become too small to allow the structure common to the known members of the series, and with the polysulfides the stable temperature zone is too low to give the high sulfur pressure necessary for synthesis of RS_2. It was felt that high pressure, high temperature techniques could be used to overcome both problems and to allow synthesis of compounds not possible by ordinary methods. Synthesis studies were carried out on mixtures of the rare earth element plus sulfur in the 1:2 mole ratio for Nd, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, and Y. The known polysulfide series was extended three members, TmS_2, YbS_2, and LuS_2. A high pressure pseudo-cubic polymorph was found for nine members, GdS_2, TbS_2, DyS_2, HoS_2, ErS_2, TmS_2, YbS_2, LuS_2, and YS_2. The minimum pressure of formation for the cubic polymorphs was found to be a smooth function of the ionic radius of the rare earth elements except for Y. YS_2 required a higher pressure to form the cubic polymorph than expected from the usual value of the ionic radius of Y, but the compressibility of Y is somewhat higher than Dy, which Y otherwise closely resembles. The reaction product diagrams of the rare earth polysulfides were all very similar. Regions were found where Ho_2S_3 and Yb_2S_3 were found on their respective diagrams. These compounds had the recently reported cubic Th_3P_4 type structure. Synthesis studies were carried out on 1:2 molar mixtures of rare earth element plus selenium for Er, Tm, Yb, and Lu. The known polyselenide series was extended three members to include TmSe_2, YbSe_2, and LuSe_2. Cubic Er_2Se_3 with the Th_3P_4 type structure was found at the highest pressure and temperature tried for the Er + 2 Se system. Prior to this work Er_2Se_3 was known only with the orthorhombic Sc_2S_3 type structure. The results of this investigation suggest that high pressure, high temperature techniques can be used to extend other series of rare earth compounds and several possibilities are suggested.