1. European Commission, Report on Critical Raw Materials for the EU, Report of the Ad hoc working group on defining critical raw materials (Brussels: European Commission, 2014), http://ec.europa.eu/enterprise/policies/raw-materials/files/docs/crm-report-on-critical-raw-materials_en.pdf .
2. B. Achzet, A. Reller, V. Zepf, C. Rennie, and M. Simmons, Materials Critical to the Energy Industry: An Introduction (Augsburg: University of Augsburg, 2011).
3. U.S. Department of Energy, Critical Materials Strategy (Washington, DC: U.S. Department of Energy, 2012), http://energy.gov/sites/prod/files/DOE_CMS2011_FINAL_Full.pdf .