Neutron diffraction studies of the P[unk] → I[unk] transition in anorthite, CaAl2Si2O8, and the crystal structure of the body-centered phase at 514 K

Abstract

Abstract The crystal structure of anorthite, CaAl2Si2O8 (Val di Fassa, Monzoni, Italy, An, 100%) has been refined by full-matrix least-squares methods using single-crystal neutron diffraction data (sinθ/λ ≤ 0.78 Å−1) measured at 514 K. Crystal data: triclinic, space group I[unk] (assumed), Z = 8, a = 8.194(1), b = 12.878(4), c = 14.171(6) Å; α = 93.07(2), β = 115.07(2), γ = 91.47(2)°, V = 1343.6 Å3, μn = 4.2 × 10−3 cm−1 F(000) = 532.16 fm, λ = 1.0462(1) Å. R(F 2) = 0.049, wR(F 2) = 0.068, S = 1.27 for 5098 reflections and 293 parameters, including 40 third-order thermal tensor coefficients for four oxygen atoms. The aluminosilicate framework is virtually body-centered, whereas the Ca atoms occur in split-positions with unequal site occupancies. The eight tetrahedral sites show complete Al/Si ordering based on refined scattering-lengths values. The 16 distinct Si–O and [Al–O] bonds (e.s.d.'s, 0.001 Å) have average values of 1.617 ± 0.031 Å and [1.741 ± 0.029 Å]. The O–Si–O and [O–Al–O] angles are in the ranges 101.44(6) – 117.60(7)° and [97.87(7)–119.01(7)°] and have averages of 109.42 and [109.31°]. The unique Si–O–Al angles are between extremes 124.16(6) and 154.7(2)°. The two non-equivalent Ca atoms occupy statistically two sites with respective separations, 0.834(5), 0.494(5) Å, and weights equal ⅔, ⅓ within limits of the analysis. These results are interpreted in terms of a dynamical model in which the I[unk] phase is a statistical dynamical average of small mobile P[unk] domains. The unequal Ca occupancy in the split positions results from the persistence of considerable short-range order near Tc . Significant anharmonic thermal vibrations of some of the oxygen atoms are attributed to critical fluctuations associated with the phase transition near Tc .