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Equation of state and phase transition in KAlSi₃O₈ hollandite at high pressure

¹ Ecole Normale Superieure de Lyon, 69364 Lyon cedex 07, France
² Institute for Solid State Physics, University of Tokyo, Kashiwa, Chiba 277-8581, Japan
³ Geodynamics Research Center, Ehime University, Matsuyama 790-8577, Japan
⁴ Photon Factory, KEK, Tsukuba, Ibaraki 305-0801, Japan

Correspondence: ^* E-mail: yagi{at}issp.u-tokyo.ac.jp

The tetragonal hollandite structure (KAlSi₃O₈ hollandite) has been studied up to 32 GPa at room temperature using high-pressure in-situ X-ray diffraction techniques. A phase transformation from tetragonal I4/m phase to a new phase was found to occur at about 20 GPa. This transition is reversible on release of pressure without noticeable hysteresis and hence this new high-pressure phase is unquenchable to ambient conditions. The volume change associated with the transition is found to be small (not measurable), suggesting a second order transition. The diffraction pattern of the high-pressure phase can be indexed in a monoclinic unit cell (space group I2/m), which is isostructual with BaMn₈O₁₆ hollandite. The angle of the monoclinic unit cell increases continuously above the transition. A Birch-Murnaghan equation of state fit to pressure-volume data obtained for KAlSi₃O₈ hollandite yields a bulk modulus K₀ = 201.4 (7) GPa with K'₀ = 4.0.