A calorimetric study of polymorphism in rubidium nitrite

Article abstract of DOI:10.1016/0021-9614(87)90072-3

The heat capacity of rubidium nitrite has been measured from 83 to 483 K.The measurements confirmed the existence of a major essentially first-order transition at (263.7+/-0.2) K, at which the molar enthalpy change Δ_trsH_m was found to be 8.494 kJ * mol^-1, giving 32.3 J * K^-1 * mol^-1 for the molar entropy change Δ_trsS_m.The value of Δ_trsS_m is consistent with the view that the transition is one from a low-temperature (monoclinic) modification in which the nitrite ions are orientationally ordered to a high-temperature (cubic) form in which each such ion has 32 possible orientations. (R ln 32 = 28.8 J * K^-1 * mol^-1).The heat capacity of rubidium nitrite showed an unusual dependence on its thermal history, especially below about 250 K.Moreover, in the early stages of the investigation two minor transitions or thermal anomalies appeared, the first beginning at about 225 K and reaching its peak at 248.6 K, the second starting at about 400 K and giving a maximum in the heat capacity at about 415 K.The former was observed twice, and the latter once only.There was no sign of either of them in the latter stages of the study, i.e. after further thermal cycling of the sample.For the lower of these transitions, the measurements gave values for Δ_trsH_m⁰ and Δ_trsS_m of 71 J * mol^-1 and 0.29 J * K^-1 * mol^-1, respectively.For the upper transition, the corresponding values of these quantities were 310 J * mol^-1 and 0.75 J * K^-1 * mol^-1.It is possible, however, that in the measurements in which these anomalies were observed, the sample was not completely in the modification or condition responsible for them.