Thallous hypophosphate, Tl₄P₂O₆

Thallous hypophosphate, Tl₄P₂O₆, from thallous carbonate and hypo-phosphoric acid, is a sparingly soluble salt, which turns blue when exposed to light, and decomposes at 250° into thallium and thallium metaphosphate.

The acid hypophosphate, Tl₂H₂P₂O₆, prepared from the acid barium salt and thallous sulphate, forms monoclinic crystals, slightly soluble in cold, more readily in hot water. It melts at 200° and evolves phosphine at higher temperatures.