Abstract. Vl+xSe 2 is trigonal, space group P3ml, with a = 3.356 (1), c = 6.104 (2)A. The structure was refined to R(F) = 0.022 for 1441 observed reflections. The structure (CdI 2 type) consists of selenium layers perpendicular to the c axis, defining two octahedral sites (000,00,}). Vanadium atoms V(1) fill 000 sites and the small amount x of vanadium atoms V(2) occupies 00~ sites.
HgCr2Se4 crystals have been grown from the vapor. Se annealing gives p-type samples whereas Hg annealing gives n-type materials. The low temperature mobility of n-type HgCr2Se4 is about 2000 cm2/V·s. A jump of the resistivity appears around T
c. Under high applied field this jump completely disappears (Δρ/ρ=-0.99 at T
c under 150 kOe).
A defect model is proposed in which Se vacancies act as doubly charged donors and Hg as an acceptor. The carrier density can be controlled by heat treatment. A band scheme consistent with the available data is proposed.
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