Like a metal, the conductivity of graphite in the direction of its planes decreases as the temperature is raised; it has the electronic band structure of a semimetal. The allotropes of carbon, including graphite, can accept foreign atoms or compounds into their structures via substitution, intercalation, or doping. The resulting materials are referred to as "carbon alloys". Carbon can form ionic salts, including a hydrogen sulfate, perchlorate, and nitrate (CX−.2HX, where X = HSO4, ClO4; and CNO.3HNO3). In organic chemistry, carbon can form complex cations—termed "carbocations"—in which the positive charge is on the carbon atom; examples are carbenium ion and carbonium ion, and their derivatives.