Hartman–Perdok analysis of crystal morphology and interface topology of β-LiNaSO4

Abstract

The trigonal b-LiNaSO4 low temperature polymorph belongs to the family of double sulphates with general formula LiMSO4 (M ¼ Na; NH4, Rb,y), which have very specific electrical properties. In this paper we present the b-LiNaSO4 theoretical growth morphology based on the Hartman–Perdok theory. Therefore, Periodic Bond Chains (PBCs) have been identified in order to determine the influence of the crystal structure on the crystal morphology. The shortest PBC is parallel to /1 00S and consists of a one-step proto-PBC with sulphate(I)–cation–sulphate(I, 1 0 0) strong bonds. All the other PBCs are built up from strong bonds in two or more consecutive steps, e.g., sulphate (I)–cation–sulphate(II)– cation–sulphate (I, u vw). The corresponding F forms are in order of decreasing dhkl : f1 0 %11 0g; f1 0 %11 1g; {0 0 0 2}, f1 0 %11 2g; f1 1 %22 0g ¼ f2 %11 %11 0g; f1 1 %22 2g ¼ f2 %11 %11 2g; y For many F forms several different slice configurations can be defined. Attachment energies have been calculated in electrostatic point charge models with formal charges. In addition, the effect of covalent S–O bonds on the growth forms has been taken into account by decreasing the effective charge on oxygen, qO: The theoretical growth form of b-LiNaSO4 based on attachment energies calculated in the LiNaS6+O42 point charge model shows the hexagonal prism f1 0 %11 0g; the hexagonal pyramid f1 0 %11 1g and the pedion (0 0 0 1). When the influence of the S–O bond decreases (LiNaS4+O4 1.5 model), the habit is slightly less elongated parallel to the c-axis due to the increased relative morphological importance of the pyramid form with respect to the prism. When we assume that the hexagonal prism face grows with halved slices d20%220 and thus using the attachment energies of E20%220 a instead of those of E10%110 a ; the growth forms changes drastically by the absence of the hexagonal prism form in both models. In addition, the trigonal prism f1 1 %22 0g is present as a minor form on this LiNaS4+O4 1.5 model with halved d20%220 slices. Experimentally grown LiNaSO4 crystals show habits that deviate from the theoretical growth forms. This must be due to external factors such as supersaturation and interaction of the crystal surface with the aqueous solutions during the growth. Growth experiments confirm that the growth morphology is strongly influenced by the degree of supersaturation.