FIRST-ORDER VALANCE CHARGE IN CENI1-XCOXSN LESS-THAN-OR-EQUAL-TO-X-LESS-THAN-OR-EQUAL-TO-0.4)

The first-order Ce valence change in CeNi1-xCoxSn (0.35 less than or e qual to x less than or equal to 0.4) has been studied using neutron di ffraction. A dramatic change in the lattice parameters takes place at a valence transition temperature, T-v, which is strongly x dependent. The observed diffraction patterns are consistent with the orthorhombic Pnma space group above and below T-v. The transition occurs with coex istence of the high- and low-temperature phases across a certain range of temperature. The very narrow range of concentrations (0.35 less th an or equal to x less than or equal to 0.4) which makes this first-ord er valence change possible seems to indicate that a very specific Ce l ocal environment is neccesary for this transition. However, a careful Rietveld refinement has not clarified this point.