Abstract
A new ternary compound, the F-type phase, was synthesized from LaAs and silicon in a fused silica ampoule at 900 °C under controlled arsenic vapour pressures using a temperature-gradient furnace. This compound was orthorhombic with a = 6.084(1) A ̊ , b = 5.986(1) A ̊ and c = 26.235(3) A ̊ . Conditions for the formation of this ternary system were examined by X-ray techniques and the stoichiometric composition of the phase was found to be LaSiAs 3. No reliable evidence was obtained to prove the existence of significant non-stoichiometry. The F-type phase was formed at 900 °C under an arsenic pressure in the range 0.2–15 atm. Above this range LaAs 2 and SiAs 2 were formed, whilst below 0.08 atm LaAs and silicon remained unreacted. Under a constant arsenic pressure of 1 atm this phase was formed at temperatures between 750 and 940 °C. Above 950 °C it melted, whilst below 750 °C it decomposed to LaAs 2 and SiAs 2.
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