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The temperature–dependence of the lattice parameters, unit cell volume and the thermal expansion tensor of petalite (LiAlSi4O10) 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) Å3 at 15 K, reaching a minimum of 423.470(6) Å3 at 219 K, before slowly increasing to 425.004(7) Å3 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 Li2O–Al2O3–SiO2 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 (λii) and eigenvectors of the thermal expansion tensor have been calculated for the range 20 K to 600 K. The eigenvalue (λ22) 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, λ11 changes sign, negative to positive at 232 K, and λ33 is always found to be negative, but reducing in magnitude with increasing temperature. The orientation of λ11 is found to be approximately parallel to a, and λ33 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 λ22 (λ11: 3.2 × 10−6 K−1; λ33: −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.

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