Pure YH3-delta and Mg(0.1)Y(0.)9H(2.9-delta) exhibit large differences in their thermochromic properties. Whereas the transmittance of YH3-delta decreases by 9% to 36%, in Mg0.1Y0.9H2.9-delta it decreases by 64% to 83% between 30degreesC and 160degreesC. This thermochromic effect is caused by hydrogen desorption upon temperature increase. The main difference between the two systems is that the face-centered-cubic to hexagonal-close-packed phase transition between YH2+delta and YH3-delta is absent in Mg0.1Y0.9Hx, i.e., the YHx stays cubic for all 2less than or equal toxless than or equal to3. Because of this, there are no plateaus in the pressure-composition isotherms of Mg0.1Y0.9Hx, and the steepness of the optical transition upon hydrogen desorption increases drastically compared to pure YHx. (C) 2002 American Institute of Physics.