Elastic behavior and pressure-induced structure evolution of topaz up to 45 GPa

The behavior of a natural topaz, Al 2.00 Si 1.05 O 4.00 (OH 0.26 F 1.75 ), has been investigated by means of in situ single-crystal synchrotron X-ray diffraction up to 45 GPa. No phase transition or change in the compressional regime has been observed within the pressure-range investigated. The compressional behavior was described with a third-order Birch–Murnaghan equation of state (III-BM-EoS). The III-BM-EoS parameters, simultaneously refined using the data weighted by the uncertainties in P and V, are as follows: K V = 158(4) GPa and K V ′ = 3.3(3). The confidence ellipse at 68.3 % (Δχ 2 = 2.30, 1σ) was calculated starting from the variance–covariance matrix of K V and K′ obtained from the III-BM-EoS least-square procedure. The ellipse is elongated with a negative slope, indicating a negative correlation of the parameters K V and K V ′ , with K V = 158 ± 6 GPa and K V ′ = 3.3 ± 4. A linearized III-BM-EoS was used to obtain the axial-EoS parameters (at room- P ), yielding: K ( a ) = 146(5) GPa [ β a = 1/(3 K ( a )) = 0.00228(6) GPa −1 ] and K′ ( a ) = 4.6(3) for the a -axis; K ( b ) = 220(4) GPa [ β b = 0.00152(4) GPa −1 ] and K′ ( b ) = 2.6(3) for the b -axis; K ( c ) = 132(4) GPa [ β c = 0.00252(7) GPa −1 ] and K′ ( c ) = 3.3(3) for the c -axis. The elastic anisotropy of topaz at room- P can be expressed as: K ( a ): K ( b ): K ( c ) = 1.10:1.67:1.00 ( β a : β b : β c = 1.50:1.00:1.66). A series of structure refinements have been performed based on the intensity data collected at high pressure, showing that the P -induced structure evolution at the atomic scale is mainly represented by polyhedral compression along with inter-polyhedral tilting. A comparative analysis of the elastic behavior and P / T -stability of topaz polymorphs and “phase egg” (i.e., AlSiO 3 OH) is carried out.