Whenever a compound crystal is cut normal to a randomly chosen direction, there is an overwhelming probability that the resulting surface corresponds to a polar termination and is highly unstable. Indeed, polar oxide surfaces are subject to complex stabilization processes that ultimately determine their physical and chemical properties. However, owing to recent advances in their preparation under controlled conditions and to improvements in the experimental techniques for their characterization, an impressive variety of structures have been investigated in the last few years. Recent progress in the fabrication of oxide nano-objects, which have been largely stimulated by a growing demand for new materials for applications ranging from micro-electronics to heterogeneous catalysis, also offer interesting examples of exotic polar structures. At odds with polar orientations of macroscopic samples, some smaller size polar nano-structures turn out to be perfectly stable. Others are subject to unusual processes of stabilization, which are absent or not effective in their extended counterparts. In this context, a thorough and comprehensive reflexion on the role that polarity plays at oxide surfaces, interfaces and in nano-objects seems timely.This review includes a first section which presents the theoretical concepts at the root of the polar electrostatic instability and its compensation and introduces a rigorous definition of polar terminations that encompasses previous theoretical treatments; a second section devoted to a summary of all experimental and theoretical results obtained since the first review paper by Noguera (2000 J. Phys.: Condens. Matter 12 R367); and finally a discussion section focusing on the relative strength of the stabilization mechanisms, with special emphasis on ternary compound surfaces and on polarity effects in ultra-thin films.

https://iopscience.iop.org/article/10.1088/0034-4885/71/1/016501/pdf

Polarity of oxide surfaces and nanostructures

-- Part C: 1. The continuum approach to crystal interfaces: basics. I: Theoretical framework. 2. Operators for orientation-dependent parameters. 3. Mechanics and thermodynamics of interfaces. 4. Equilibrium structures of junctions, edges and vertices. 5. Growth: kinematic wave theory revisited. 6. Kinetic behaviour of junctions, edges and vertices. 7. Structural phase transitions of a crystal surface as a branch of soliton physics. 8. Miscellaneous topics. II: Growth rate functions. 9. Analytic representations of R(n,A) functions. 10. Nonlinear networks and the 'assembled function' notation. 11. Physical aspects of assembling operations. 12. The systematic construction of R(n,A) functions by a sequence of assembling steps. 13. Notes on sphere-growth experiments. 14. Summary: Conclusions and outstanding questions. References. List of symbols, abbreviations and notations.

Morphology of crystals

Crystal morphologies are predicted from data stored in files in the CIF format (crystallographic information file standard of the International Union of Crystallography) on the basis of the Bravais–Friedel, Donnay–Harker model. Several simple improvements to the calculation are introduced with Win­XMorph, version 1.4.9, in conjunction with illustrations of the morphologies of quartz, sucrose, lactose, pyrite and lysozyme. The morphologies of the recently discovered pentamorphs of 1,8-dihydroxyanthraquinone are predicted. Win­XMorph is available free-of-charge for educational use.

/pdf/from-cif-to-virtual-morphology-using-the-winxmorph-program-uuo71klp3j.pdf

From CIF to virtual morphology using the WinXMorph program

Spinel-containing alumina-based refractory castables

Unidirectional seeded single crystal growth from solution of benzophenone

Habit changes of lead chloride, PbCl2, caused by growth from pure aqueous solution and the effect of KCl, NH4Cl, CdCl2 and HCl as additives

RbTiOPO4 crystals doped with Nb show an important flattening of the morphology that makes their use difficult in nonlinear optics applications. Here, we identify the possible causes of this morphological change:  Nb acts as an impurity that brakes the velocity of growth of the {100} faces and modifies the growth mechanism of {201} faces of the crystals. We also propose a way of obtaining isometric crystals by forcing crystal growth in the a crystallographic direction. This causes a new change in morphology with benefits for later nonlinear optics applications due to the larger useful area obtained in the x−y plane.

Change in the Morphology of RbTiOPO4 Introduced by the Presence of Nb

https://dspace.library.uu.nl/bitstream/handle/1874/17219/aguilo_87_theoretical.pdf?sequence=1

Theoretical growth and equilibrium forms of ADP (NH4H2PO4)

The α phase of SiO2 is known as a technologically important material. According to the Hartman–Perdok theory, so-called F forms are present: the hexagonal prism m {10\bar{1}0}, the major rhombohedron r {10\bar{1}1} and the minor rhombohedron z {01\bar{1}1}. F faces grow according to a two-dimensional growth mechanism and are the only forms to be expected on the growth form. Computation of attachment energies, which are considered to be directly related to the growth rates of the corresponding F faces, has been performed using an electrostatic point-charge model, taking into account Born repulsion and van der Waals contributions computed by means of the Gilbert equation or the van Beest and van Santen potential. All the theoretical growth forms are prismatic with the two aforementioned rhombohedra, which are almost equally important and independent of the corrections for the van der Waals and Born interactions. The equilibrium form is prismatic and shorter as a result of the hexagonal prism form being less important. On the atomic scale, the differences in surface topologies between the F forms depend on the variation in depth below the surface boundary of those central Si atoms that are partially unshielded because of the incomplete tetrahedral coordination.

C.F. Woensdregt

Papers

Habit changes of lead chloride, PbCl2, caused by growth from pure aqueous solution and the effect of KCl, NH4Cl, CdCl2 and HCl as additives

Change in the Morphology of RbTiOPO4 Introduced by the Presence of Nb

Theoretical growth and equilibrium forms of ADP (NH4H2PO4)

Modeling growth and equilibrium form of α-SiO2

Melt growth of spessartine (Mn3Al2Si3O12)