Thermo-elastic behaviour of Be₂BO₃OH (hambergite) up to 7 GPa and 1,100 K

The thermo-elastic behaviour of Be₂BO₃(OH)_0.96F_0.04 (i.e. natural hambergite, Z = 8, a = 9.7564(1), b = 12.1980(2), c = 4.4300(1) Aa, V = 527.21(1) Aa³, space group Pbca) has been investigated up to 7 GPa (at 298 K) and up to 1,100 K (at 0.0001 GPa) by means of in situ single-crystal X-ray diffraction and synchrotron powder diffraction, respectively. No phase transition or anomalous elastic behaviour has been observed within the pressure range investigated. P-V data fitted to a third-order Birch-Murnaghan equation of state give: V₀ = 528.89(4) Aa³, K_T0 = 67.0(4) GPa and K^' = 5.4(1). The evolution of the lattice parameters with pressure is significantly anisotropic, being: K_T0(a):K_T0(b):K_T0(c) = 1:1.13:3.67. The high-temperature experiment shows evidence of structure breakdown at T > 973 K, with a significant increase in the full-width-at-half-maximum of all the Bragg peaks and an anomalous increase in the background of the diffraction pattern. The diffraction pattern was indexable up to 1,098 K. No new crystalline phase was observed up to 1,270 K. The diffraction data collected at room-T after the high-temperature experiment showed that the crystallinity was irreversibly compromised. The evolution of axial and volume thermal expansion coefficient, α, with T was described by the polynomial function: α(T) = α₀ + α₁ T^-1/2. The refined parameters for Be₂BO₃(OH)_0.96F_0.04 are: α₀ = 7.1(1) x 10^-5 K^-1 and α₁ = -8.9(2) x 10^-4 K^-1/2 for the unit-cell volume, α₀(a) = 1.52(9) x 10^-5 K^-1 and α₁(a) = -1.4(2) x 10^-4 K^-1/2 for the a-axis, α₀(b) = 4.4(1) x 10^-5 K^-1 and α₁(b) = -5.9(3) x 10^-4 K^-1/2 for the b-axis, α₀(c) = 1.07(8) x 10^-5 K^-1 and α₁(c) = -1.5(2) x 10^-4 K^-1/2 for the c-axis. The thermo-elastic anisotropy can be described, at a first approximation, by α₀(a):α₀(b):α₀(c) = 1.42:4.11:1. The main deformation mechanisms in response to the applied temperature, based on Rietveld structure refinement, are discussed. (orig.)