Prepn of high purity crystals, resistance measurements, activation energies: Justi, Lautz, Abh. Braunschw. Wiss. Ges. 5, 36 (1953), C.A. 48, 1756i (1954). Electrical properties: Willardson et al., J. Electrochem. Soc. 101, 354 (1954); and prepn: Kover, Compt. Rend. 243, 648 (1956). Zone melting to remove impurities and to improve electr properties: Scheel, Z. Metallkd. 46, 58 (1955), C.A. 49, 4486f (1955). Oxidation by water: Rudorff, Kohlmeyer, Z. Metallkd. 45, 608 (1954), C.A. 49, 7432c (1955).