Neutron diffraction structural study of pyroelectric Ba(NO2)2⋅H2O at 298, 102, and 20 K

Abstract

Neutron diffraction studies of barium nitrite monohydrate [Ba(NO2)2⋅H2O] have been made at 20, 102, and 298 K on the same crystal. Intensities of 954 (20 K), 2179 (102 K), and 2393 (298 K) reflections were measured at the Brookhaven National Laboratory high flux beam reactor. The structures were refined on the basis of the hexagonal space group P65 to yield agreement factors R(F2) = 0.028 (20 K), 0.034 (102 K), and 0.061 (298 K). The thermal expansion is nonlinear along both a and c axes, with α1 = 3.8×10−6 K−1, β1 = 2.8×10−8 K−2, α3 = 21.3×10−6 K−1, β3 = 10.9×10−8 K−2. The cell dimensions are a = 7.0524(24) Å, c = 17.6372(80) Å at 20 K, a = 7.0559(23) Å, c = 17.6810(77) Å at 102 K, and a = 7.074 90(3) Å, c = 17.890 87(12) Å at 298 K. The mean-square amplitude of thermal motion of Ba is proportional to temperature in the range 298 to 20 K, with small departures from linearity for the O and N amplitudes and larger departures for the H nuclear amplitudes. The Ba2+ ion is coordinated by nine oxygen atoms and one nitrogen atom at an average distance of 2.881 Å at 20 K, 2.884 Å at 102 K, and 2.902 Å at 298 K. The increase in length corresponds completely to the lattice expansion. A maximum difference within a nitrite ion of 0.015 Å in N–O bond lengths, observed at 298 K, is reduced to 0.008 Å at 20 K for a mean N–O distance of 1.254 Å. One O–H bond of 0.958 Å, which forms a bifurcated hydrogen bond with two oxygen atoms, is identical in length to that in the free water molecule; the other is slightly elongated by formation of a nearly linear hydrogen bond to a nitrogen atom. The largest nuclear displacement between 298 and 20 K in this pyroelectric crystal is only 0.038 Å along the polar axis.