Neutron Diffraction Study of the Low-Temperature Phases of Sodium Oxide Nitrite, Na3ONO2

Abstract

As a function of temperature the nitrite ions in sodium oxide nitrite, Na3ONO2, show unique and complex order/disorder phenomena. The crystal structures of the three low-temperature modifications of Na3ONO2 have been determined by neutron powder diffraction. In the cubic room-temperature modification (H-Na3ONO2, space group Pm3m) the nitrite ions exhibit a 12-fold orientational disorder. Upon cooling, the orientational disorder of the nitrite ions is reduced by two continuous order processes and one discontinuous phase transition. At T≈240 K, H-Na3ONO2 transforms by a continuous phase transition to tetragonal M-Na3ONO2 (space group I4/mcm, a= 6.49174(12) Å, c=9.19415(59) at T=195 K), and the orientational disorder of the nitrite ions is reduced via a continuous order process. The order parameter Q follows the power law Q≈(TC−T)β with TC=239.5(6) K and β=0.28(2). At T=178 K a discontinuous phase transition (ΔH=620 J/mol) takes place and M-Na3ONO2 transforms to T-Na3ONO2 (space group P4/mbm, a=6.46513(7) Å, c=4.62382(6) at T=125 K). In T-Na3ONO2 the nitrite ions exhibit a 2-fold orientational disorder. At T≈68 K a second continuous phase transition to TT-Na3ONO2 (space group P421m, a=6.45345(6) Å, c= 4.61973(6) at T=5 K) takes place and the orientational disorder is further reduced. This order process is characterized by a critical temperature of TC=67.4(2) K and a critical exponent of β=0.21(1).