The investigation of the alkali metal intercalation and nonmetal interstitial derivatives of YCl has produced six new classes of compounds: 3R-Li(,0.09)YCl, 1T-Y(,2)Cl(,2)C, 2H-, and 3R-M(,0.1)('I)YClO, 1T-Cs(,0.1)YClO, and 1T-M(,0.4)('I)Y(,2)Cl(,2)C with M('I) = Li-Cs (except 2H-Li(,0.1)YClO). These are all synthesized as black plates by stoichiometric reaction of Y powder or strips, YCl(,3), and, as appropriate, M('I)Cl, Y(,2)O(,3), YOCl or C within sealed Nb tubing at 950(DEGREES)C for 1-3 weeks. The yields for all but 3R-Li(,0.09)YCl are above 90%;The structures of five new phases have been solved by single crystal methods: 3R-Li(,0.09)YCl, 3R-Na(,0.08)YClO(,1.0), 2H-K(,0.08)YClO(,0.82), 1T-K(,0.52)Y(,2)Cl(,2)C(,0.80), and the parent 3R-YCl. The oxygen or carbon atoms occupy essentially all tetrahedral or trigonal antiprismatic (tap) interstices, respectively, between double Y layers in the four-layered slabs sequenced Cl-Y-Y-Cl. High resolution Guinier powder patterns were also used to identify phases and structures.(,);3R-YCl is isostructural with ZrBr and this is retained in 3R-Li(,0.09)YCl with Li('+) randomly distributed over (TURN)40% of the tap sites between Cl layers. A decrease in the Y-Y distances accompanies the reduction of the slabs, though increasing Y-Cl and Cl-Cl separations cause an overall increase in the c-axis;In 1T-Y(,2)Cl(,2)C the size of the nonmetal requires substitution in the tap site between the Y layers while the second nearest neighbor interactions appear responsible for the reordering of the layers into a hexagonally-close-packed one-slab structure. The intercalated alkali metals in 1T-M(,0.4)('I)Y(,2)Cl(,2)C occupy the tap site between Cl layers within the 1T structure;The different M(,0.1)('I) YClO structures result from the different packing arrangements of slabs about M('I). In the YCl-like 3R structure the tap sites between Cl layers are randomly occupied with M('I) = Li-Cs. In the 2H- (M('I) = Na-Cs) and 1T-structures (M('I) = Cs) the alkali metal has trigonal prismatic coordination;The 3R-M(,0.1)('I)YClO is oxidized by I(,2)/CH(,3)CN to form 3R-YClO(,z) which is isostructural with YOF and ZrBrO(,x). In moist air the 3R phase, M('I) = K or Rb, undergoes reversible hydration to form a;crystalline hydrate, 1T-M(,.01)('I)(H(,2)O)(,a)YClO. In water the 3R phases (M('I) = Li-Cs) are oxidized and the crystals exfoliate; ('1)DOE Report IS-T-1063. This work was performed under contract No. W-7405-Eng-82 with the U.S. Department of Energy.