Tuning crystal structure of potassium dihydrogen phosphate at ambient conditions

At ambient conditions, potassium dihydrogen phosphate (KDP) usually exists in tetragonal structure, and only transforms to monoclinic structure at a temperature around 503 K. In this study, stable monoclinic KDP is developed at room temperature using a mixture of graphene oxides and unsaturated KDP solution. Furthermore, a reversible phase transition between monoclinic and tetragonal structure is observed by tuning the distance between graphene oxide layers. The first-principles calculation suggests that the tetragonal structure is more energetically favorable when KDP is sandwiched by two graphitic surfaces with a large interlayer distance, whereas the monoclinic structure is more stable with a small interlayer distance. This is attributed to the interaction between ${\mathrm{K}}^{+}$ cations and π electrons in the graphitic surface, which distorts the local structural configurations. The increase of interaction strength with decreasing interlayer distance not only reduces the crystal-liquid interfacial free energy, which promotes the formation of the monoclinic structure in unsaturated solution, but also stimulates transformation of existing tetragonal KDP to monoclinic structure. The observations shed light on the nucleation mechanism for crystal structure that is usually infeasible and unstable at ambient conditions, and provide an avenue to tune crystal structures for a wide range of metal salts.

Figure 1

(a) The XRD patterns of as-prepared, moistened, and redesiccated KDP/GO samples are displaced along with the results of the GO membranes and KDP references. The inset is an enlargement of the region corresponding to the GO feature. Note that the patterns are shifted vertically for easier comparison. (b) The curve fit for the dominant features corresponding to monoclinic and tetragonal structures. (c) The evolution of area ratio of each fit component.

Figure 2

(a) The Raman spectra of as-prepared, moistened, and redesiccated KDP/GO samples as well as the GO and KDP references. (b) The curve fit for the dominant features corresponding to monoclinic and tetragonal KDP structures. (c) The integrated area of each fit component.

Figure 3

The XRD patterns of the two sets of KDP/GO samples: (1) prepared by 0.005, 0.01, and 0.05 mol/l KDP solution with a centrifuging speed of 7000 rpm; (2) prepared by a centrifuging speed of 2000, 4000, 5000, and 7000 rpm with 0.05 mol/l KDP solution. The results of GO membranes prepared by a centrifuging speed of 5000 and 700 rpm, as well as KDP references, are shown for comparison.

Figure 4

One layer of (a) monoclinic and (b) tetragonal KDP confined between two graphene sheets. The upper and lower layers of brown spheres represent graphene, while the purple, red, gray, and white spheres represent K, O, P, and H atoms, respectively. (c) Binding energies of monoclinic and tetragonal KDP change as a function of $d_{GG}$. The black square, red circles, and green circles represent monoclinic KDP, tetragonal KDP, and collapsed structure from tetragonal KDP.

Figure 5

The optimized structures of sandwiched monolayer KDP with representative values of $d_{GG}$. (a)–(c) monoclinic KDP and (d)–(f) tetragonal KDP. The upper and lower layers of brown spheres represent graphene, while the purple, red, gray, and white spheres represent K, O, P and H atoms, respectively.