Polymorphism, crystal growth, crystal morphology and solid-state miscibility of alkali nitrates

Abstract

In this review, we recollect and discuss the most relevant advancement in crystallographic aspects of alkali nitrates compounds published since 1975, the year of publication of the last review by Rao et al. about some topics discussed here. This review is focused in five nitrate compounds of the first periodic table column: LiNO3, NaNO3, KNO3, RbNO3 and CsNO3. For each one, we have compiled the information about crystal structure, polymorphism and phase transition, crystal growth and crystal morphology, and the phase diagram and solid-state miscibility results. Considerable numbers of papers have been published in the last 40 years, in particular in relation to binary phase diagrams and solidstate miscibility of alkali nitrates. A variety of phase diagram descriptions appears in the 10 possible binary combinations for the five compounds. To finish, we discuss and propose a geometric model in order to explain the different binary phase diagrams observed between these compounds. 4 2. Lithium nitrate 2.1. Crystal structure, polymorphism and phase transitions As Rao et al. [6] have shown, lithium nitrate crystallizes in rhombohedral (calcite type), R3 � c group with Z = 6, a = 4.692Å and c = 15.22Å at 298 K. There is no evidence for a phase transition in the thermodynamic properties from 298K to the melting point.[10] However, discontinuities in the absorption coefficient [11] and changes in electric properties [12] may indicate an orientational disorder transition on nitrate groups just like NaNO3 or NH4NO3.After Rao et al., onlyWu et al. [13] determined the LiNO3 crystal structure. The authors give a significant variation in the c-parameter as can be seen in Table 1. The unit cell determined by Wu et al. [13] is presented in Figure 1, where blue spheres are lithium ions and red spheres are oxygen atoms. These authors grew the crystal from the melt and despite not having a good single crystal they were able to determine its structure. According to them, LiNO3 is isostructural to both calcite (CaCO3) and nitratine (NaNO3). No evidence has been found for a high-temperature phase. Moreover, Stromme [33] modelled a positional ordered equilibrium structure of LiNO3 at all temperatures.Accordingly, LiNO3 has only one structure as a function of temperature, as is recollected in Table 1. Crystal growthWe have not found any paper related to anhydrous lithium nitrate growth because it is deliquescent. Nonetheless, Chernov and Sipyagin [7] gave some kinetic data for LiNO3 • 3H2O compound, which is not of interest for this review. MorphologyAs far as we know, neither theoretical nor experimental morphology is reported for lithium nitrate crystals. However, we expect that the theoretical growth morphology determined by the Bravais-Friedel-Donnay-Harker (BFDH) method will be equal to that of calcite [34] and nitratine [35] because they are isostructural crystals. Therefore, {012}, {012 � } and {001} forms appear in the theoretical growth morphology (Figure 2).

Phase equilibria in the reciprocal NaCl–KCl–NaNO3–KNO3 system

“…[31][32][33]. The compound NaNO 3 crystallizes at room temperature in the trigonal R 3̅ c space group.…”

Experimental investigation and thermodynamic evaluation of the CsNO3-LiNO3-NaNO3 ternary system

“…Most of the published values are acceptable within the admitted experimental uncertainty of ±2 K. The data of Poon and Salje[114] (552 K), Schmahl and Salje[116] ((552.3±0.5) K) and Antao et al[120] ((552.7±0.5) K) are very close to the upper limit of acceptable values. While, the values of Roget et al[22] (544.95 K), Rao and Rao[77] (545 K), Beneš et al[162] (545.5 K) and Benages-Vilau[164,165] (544 K), are just below the lower limit of acceptable values. Only the data of Mohammad et al…”

Measurement of Crystal Face Specific Growth Kinetics

“…With the reconstructed 3D coordinates, the crystal shape, size and their evolution, hence face specific growth rates, can be calculated. Figure 4 is the equilibrium morphology of NaNO3 predicted by Benegas-Vilau, 39 which is also obtained in this study. The six faces of a rhombohedral NaNO3 crystal can be classified to three groups of crystallographically equivalent faces, i.e.…”

“…39 Take point B(X, Y, Z) and E(X0, Y0, Z0) as an example, the distance between two points in 3D space can be expressed as…”