Energetics of cobalt phosphate frameworks: α, β, and red NaCoPO4
Graphical abstract
Relative stability of NaCoPO4 polymorphs compared to the most stable phase, α NaCoPO4.
Introduction
In the last decade, the discovery of a large variety of transition metal phosphates, especially with zeolitic or open framework structures, such as (H3NCH2CH2NH3)0.5CoPO4 with DAF and GIS frameworks [1], [2], or Zn3(PO4)2(PO3OH)(H2DACH), a large pore structure with 24 member ring channels [3], has motivated further research in this class of materials. Similar to aluminosilicate zeolites, many transition metal phosphates with zeolitic or open framework structures can be synthesized by hydrothermal methods in the presence of organic templates [1], [2], [3], [4], [5]. Although their structures can be identified, factors that control structures, synthesis processes, and phase stabilities are still not well understood. Therefore, the synthesis of new structures mainly relies on trials of different organic templates and conditions.
Calorimetry is a powerful method to study energetic relationships among different structures. Previous studies of energetics of a large number of aluminosilicate zeolites, pure silica zeolites, and aluminophosphate zeolites provided knowledge to rationalize the stability and synthesis conditions of these materials [6], [7], [8], [9], [10], [11]. However, trends in energetics of transition metal phosphates have not been investigated. Researchers believe that stabilities and synthesis conditions of transition metal phosphate frameworks should face some challenges that differ from aluminosilicate or aluminophosphate frameworks because transition metals have a greater tendency to form octahedral coordination [12]. Among zeolitic transition metal ions, cobalt (II) and zinc (II) are the most interesting because their electron configurations enable somewhat easier formation of tetrahedral coordination than for other transition metals. In order to study systematically the energetics of transition metal phosphate materials, we started with a study of the relationship between structures and stabilities of a simple inorganic transition metal phosphate, sodium cobalt (II) phosphate (NaCoPO4).
Sodium cobalt (II) phosphate is known in four different structures. The first, α NaCoPO4, determined by Hammond and Barbier [13], and the pink phase found by Feng et al. [14] have the same structure. α NaCoPO4 crystallizes in the space group Pnma, consists of edge-sharing chains of CoO6 octahedra cross-linked by PO4 tetrahedra, and sodium ions are located in 10-coordinate cavities (Fig. 1a). The second polymorph is β NaCoPO4 [13], also known as the blue phase [15]. Crystals of β NaCoPO4 are twinned and belong to the space group P61 or P65. In this structure, CoO4 and PO4 tetrahedra alternately share corners to form a 3D framework with one-dimensional six ring channels, and sodium ions reside in these channels (Fig. 1b). Six member rings in β NaCoPO4 are mixtures of UDUDUD and UUUDDD connections (U=up, D=down). The third polymorph crystallizes in the space group and has the same tetrahedral connections as ABW zeolite [16]. In the red phase, the fourth polymorph, cobalt is in trigonal bipyramids connected with PO4 tetrahedra to form a framework with one-dimensional channels (Fig. 1c) in the space group [17]. However, reports on thermal properties of these polymorphs are inconsistent. Engel [18], Kolsi [19], and Toda et al. [20] indicate that the low-temperature phase (α) reversibly transforms to the high-temperature phase (β) at 998 K, and NaCoPO4 melts at 1193 K. Feng et al. [14] reported that the red phase changes to the α phase at 983 K with an endothermic heat effect, and the α phase changes to β phase at 1193 K with an exothermic differential scanning calorimetry (DSC) peak.
By a combination of powder X-ray diffraction (XRD), DSC, and high-temperature oxide melt solution calorimetry, we clarify thermodynamic behavior of α, β, and red NaCoPO4 in this paper. Their relative stabilities and enthalpies of formation are reported. Results address the question of how cobalt is stabilized in tetrahedral coordination compared to octahedral and trigonal bipyramidal coordination.
Section snippets
Sample preparation
α and β NaCoPO4 samples for calorimetric studies were synthesized by solid-state reaction. Powder CoCO3 (Alfa Aesar, 99.5% metals basic) was mixed with a stoichiometric amount of solution of NaH2PO4 (Fisher, 98-102% NaH2PO4·H2O). The mixture was stirred continuously and heated at 353–363 K to evaporate water. Carbon dioxide was also liberated during heating. The reactant mixture was dried at 373 K, ground, and heated at 623 K overnight to remove all volatilities. The powder obtained after heating
Characterization of samples
Chemical compositions of samples from microprobe analysis presented in Table 1 show they have the ideal stoichiometry of NaCoPO4 within experimental errors. All samples have less than 0.09% of cobalt presented as Co3+. The TGA performed on the same day as calorimetric measurement showed no weight loss in solid-state samples and less than 0.1% weight loss for hydrothermal products. Although the color of our α samples is black or gray black, that is somewhat different from previous reports of
Conclusion
Thermal behavior, enthalpy of phase transition, and enthalpy of formation from oxides and from elements of three different polymorphs of NaCoPO4 have been determined. While red NaCoPO4 is a metastable phase with cobalt in trigonal bipyramid coordination, α NaCoPO4 with cobalt in octahedral coordination is the stable phase at low temperature, and β NaCoPO4 with cobalt in tetrahedral coordination is the stable phase at high temperature. Although cobalt is most enthalpically stable in octahedral
Acknowledgments
This work was supported by National Science Foundation under Grants DMR 01-01391 and DMR 06-01892. We thank Dr. Sarah Roeske for microprobe analysis, Dr. Lena Mazeina and Dr. Yosuke Moriya for translating German and Japanese papers.
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