Synthesis and properties of a high-pressure rutile-type AlAsO4

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Abstract
A rutile-type AlAsO4 polymorph with a = 4.359 Å and c = 2.815 Å has been synthesized at 90 kbar and 900 C. The density is 5.15 g/cm3, an increase of 54 per cent over the quartz modification. The indices of refraction of the rutile form of AlAsO4 are ε = 1.950, ω = 1.905, both ± 0.005. Molar refraction data for the quartz and rutile polymorphs of AlAsO4 show a percentage decrease of 10.4 relative to the quartz form and fulfill the Dachille-Roy prediction relating molar refraction and cation coordination. The main absorption band of the infra-red spectra of the quartz and rutile forms of AlAsO4 shows a shift of 23 per cent in the direction of increasing wavelength with increasing cation coordination. A corresponding shift of the same magnitude is shown by the quartz and rutile forms of SiO2 and GeO2.

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