Researchers from Vrije University have created an optical method of monitoring the hydrogen navigation that makes an yttrium film switch from reflective to transparent. The method should simplify work for other researchers seeking materials for "switchable mirrors." Research into hydrogen diffusion in rare-earth metals goes back to the 1970s, when scientists were exploring alternative fuel sources. They discovered that hydrogen could be safely stored in metal. In 1995, Hans Huiberts, a PhD student working in Ronald Griessen's group, noticed that as he introduced hydrogen into a thin film of yttrium, the normally reflective material slowly turned transparent (see Photonics Spectra, July 1996, p. 32). This observation contradicted theoretical models of how hydrogen should behave, leaving him and other scientists mystified. Transition of state Two years later, researchers are still unsure exactly why the transition of state occurs. In a recent letter to Nature, a second team from the university that included Sense Jan van der Molen and researchers at Philips Research Lab, performed experiments to pinpoint precisely when the phase transitions occur. Van der Molen's team re-created Huiberts' experiments with yttrium hydride, placing a small amount of palladium on the substrate's surface. Next they introduced hydrogen gas into a vacuum chamber, causing the palladium to act as a hydrogen "sponge," quickly absorbing the gas and transferring it to the substrate. As this occurred, they used an optical microscope to observe the hydrogen's commute through the film. By tracking the element's movement, the researchers could determine when phase transitions occurred. That data could prove useful as they work with other materials, namely yttrium gadolinium and gadolinium magnesium alloys. Researchers are looking to these materials because yttrium trihydride is not perfectly transparent, displaying a yellowish tinge. Meanwhile, researchers at Philips, based in Eindhoven, the Netherlands, are working to develop commercial products. Besides smart windows, the mirrors could be used to control sunlight coming through windows.