The temperature–dependence of the volume expansivity and the thermal expansion tensor of petalite between 4.2 K and 600 K

摘要

The temperature–dependence of the lattice parameters, unit cell volume and the thermal expansion tensor of petalite (LiAlSi_(4)O_(10)) have been determined from high resolution, time–of–flight powder neutron diffraction data collected at ninety temperatures between 4.2 K and 600 K. At low temperatures, after a short saturation interval, the unit cell volume decreases from 424.114(6) ?~(3) at 15 K, reaching a minimum of 423.470(6) ?~(3) at 219 K, before slowly increasing to 425.004(7) ?~(3) by 600 K. Petalite may be considered to be a further example of a low expansion material (defined as having a coefficient of linear thermal expansion of less than 2 × 10~(?6) K~(?1)) in the well–studied Li_(2)O–Al_(2)O_(3)–SiO_(2) system, however in this case, this technologically useful property is found to occur at low temperatures, in the interval 157 K to 298 K. From the temperature variation of the unit cell parameters, the eigenvalues (λ_(ii)) and eigenvectors of the thermal expansion tensor have been calculated for the range 20 K to 600 K. The eigenvalue (λ_(22)) associated with the unique monoclinic axis (b ) is positive for all temperatures above saturation, ~ 50 K, and reaches a maximum value of 1.42 × 10~(?5) K~(?1) at 600 K. In the a –c plane, above the 20 K saturation temperatures, λ_(11) changes sign, negative to positive at 232 K, and λ_(33) is always found to be negative, but reducing in magnitude with increasing temperature. The orientation of λ_(11) is found to be approximately parallel to a , and λ_(33) is approximately parallel to c* at all temperatures. The 600 K thermal expansion coefficients associated with these two principal axes is of the order of four times smaller than that associated with λ_(22) (λ_(11): 3.2 × 10~(?6) K~(?1); λ_(33): ?3.4 × 10~(?6) K~(?1)). Between 20 K and 232 K, the thermal expansion tensor is therefore represented by a hyperboloid of two sheets, and above this temperature, the representation quadric changes to a hyperboloid of one sheet. The orientation of the principal axes in (010) is continuous through the change in the representation quadric, and only shows a small variation throughout the temperature interval 20 K to 600 K.