DOI: 10.1021/cg8013255
Experimental Evidence for the Influence of Mn3þ Concentration on the Impurity Incorporation and Habit Modification Mechanism of Potassium Dihydrogen Phosphate
2010, Vol. 10 1053–1058
C. M. R. Remedios,† A. O. dos Santos,‡ X. Lai,§ K. J. Roberts,*,§ S. G. C. Moreira,† M. A. R. Miranda,# A. S. de Menezes,‡ F. P. Rouxinol,‡ and L. P. Cardoso‡,§ †
Departamento de Fı´sica, Universidade Federal do Par a, Belem-Pa, 60740-000, Brazil, ‡IFGW, UNICAMP, CP 6165, 13083-970 Campinas, S~ ao Paulo, Brazil, §Institute of Particle Science and Engineering, School of Process, Environmental and Materials Engineering, University of Leeds, Leeds LS2 9JT, U.K., and #Department of Physics, University of Guelph, Guelph, Ontario, N1G2W1, Canada Received December 5, 2008; Revised Manuscript Received January 5, 2010
ABSTRACT: High resolution X-ray diffraction is applied to study Mn3þ doped potassium dihydrogen phosphate (KDP) single crystals as a function of dopant concentration with quantitative dopant composition within the crystals being assessed using Rutherford backscattering spectroscopy (RBS). Synchrotron radiation high resolution X-ray multiple diffraction studies using both the Renninger scanning and two-dimensional angular mapping techniques are consistent with the Mn3þ ionic complex entering the crystal lattice via a substitutional incorporation mechanism for Mn3þ at a concentration of 0.1 wt %. For a higher Mn concentration of 0.9 wt %, the data indicate incorporation via an interstitial mechanism associated with a decrease in lattice perfection (an increased mosaic spread). This analysis is supported through X-ray powder diffraction studies which reveal lattice contraction for lower dopant levels with lattice dilation for the higher dopant levels. The potential impact of this observation on the associated crystal growth habit modification process is discussed.
*To whom correspondence should be addressed. E-mail: K.J.Roberts@ leeds.ac.uk.
coloration, and this, supported by UV-visible spectroscopy, is strongly indicative of the incorporation of the impurity as a hydrated complex with the metal ion octahedral coordinated.13 A structural model allowing for both charge compensation and cation hydration was provided in the latter paper on Cr3þ doped ADP suggesting Cr3þ incorporation as a CrCl2(H2O)4þ complex which substitutes within the bulk lattice for a (PO4)3ionic group displacing, in doing so, one (PO4)3-, two protons, and two (NH4)þ ions. However, other studies on Mn3þ doped KDP using Raman spectroscopy,14 X-ray standing-waves,4 and X-ray multiple diffraction studies3 have provided clear evidence for an interstitial incorporation model which was proposed8 to be associated with the incorporation of the impurity as an Mn(H2O)43þ complex which displaces one hydrogen bonding proton together with two Kþ cations. This latter model is attractive mindful of the fact that such a structure for the incorporated complex is well-matched to detailed studies of the known habit modification effects,15 that is, through binding to two phosphate groups on the prismatic {100} habit faces. In contrast, binding of the complex to the pyramidic {101} surface is precluded, due to steric hindrances associated with this latter surface’s characteristic mixed cation/ anion surface chemistry. Despite this previous body of work, a fully integrated approach and model which would be consistent with both substitutional and interstitial incorporation has not yet been provided. This generic issue is addressed in this paper through further studies using X-ray multiple diffraction as carried out at the Brazilian National Synchrotron Laboratory (LNLS) aimed at probing how the nature of Mn3þ dopant incorporation within KDP changes with different impurity levels within the crystal growth solution. The latter is, in turn, cross-correlated with dopant levels measured in the as-grown crystals as assessed using Rutherford back scattering spectroscopy (RBS). X-ray multiple diffraction is particularly well-suited
r 2010 American Chemical Society
Published on Web 01/29/2010
I. Introduction Understanding the structural origin of the morphology of crystallized materials is not only of long-standing fundamental interest but is also important for a wide range of material systems such as the processing of crystalline products produced in chemicals manufacture, the design and assembly of natural mineral structures produced through biomineralisation, and deleterious scale formation in hydrocarbon flow systems. Potassium and ammonium dihydrogen phosphate (KH2PO4 - KDP, NH4H2PO4 - ADP) crystallize in the tetragonal space group I42d1 and are well-studied systems with applications in electro-optics, ferroelectrics, etc., reflecting their demand, for example, for large defect-free crystals for laseroptic applications.2 These materials have also been extensively researched as model systems for crystallization research, notably associated with the role played by trivalent habit modifying species3,4 on their crystal morphology. Such species are well-known to transform the crystal morphology (Figure 1) of KDP and ADP from a compact habit dominated by prismatic {100} and pyramidal {101} crystal faces (a) to a modified morphology elongated along the c-axis (b), which is characterized by formation of noncrystallographic tapering {h0l} faces.5-7 The exact mechanism for the observed habit modification has been the subject of numerous studies, including several by this group. A significant challenge has been to understand the structural nature of the incorporating species and extended X-ray absorption fine structure8 and Mossbauer9,10 studies have been used to show that the dopant’s valence state is not changed with incorporation. Several studies have considered the impurity incorporation as an isolated nonhydrated species within the crystal lattice.8,9,11 However, visual observation of the doped crystals reveals strong sector-zoned12
pubs.acs.org/crystal
1054
Crystal Growth & Design, Vol. 10, No. 3, 2010
Figure 1. The crystal morphology of KDP: (a) schematic of habit grown from pure solution; (b) schematic grown from solution with trivalent cations; (c) micrograph of KDP crystal grown in the presence of Mn3þ ions (note brown coloration indicating where the impurity is incorporated within the crystal).
in this work reflecting the technique’s high sensitivity to the kind of subtle changes within the crystal structure which might be expected to be related to dopant binding and incorporation processes likely to be associated with crystal habit modification. II. Multiple Diffraction X-ray multiple diffraction16 (XRMD) occurs when two or more sets of crystallographic planes inside a crystal simultaneously satisfy Bragg’s law for a certain incident X-ray beam. A straightforward approach to obtain this phenomenon is by means of the azimuthal angle Renninger scanning (RS)17 technique. In this, first a set of planes is set by angular rotation to the Bragg condition (ω-rotation) [primary plane (h0, k0, l0)], and then the crystal is rotated around the normal to the primary plane [φ-rotation]. If the intensity at the detector is monitored continuously, it remains constant at the Bragg peak value of the primary beam since the φ rotation does not change the incident angle of the X-ray beam. For a certain specific φ angle, a different plane [(hS, kS, lS) secondary plane] also diffracts the incident X-ray beam but, in this case, in a different direction. However, there is another plane (hS-h0, kS-k0, lS-l0) [called the coupling plane] that also satisfies Bragg’s law with respect to the secondary diffracted beam to enable the diffraction of this beam back toward the primary direction. As a result, one obtains a scan comprising a background of the primary Bragg diffracted intensity versus φ upon which is superimposed the results of multiple diffraction. The latter is evidenced through the appearance, at specific azimuthal angles, of positive (Umweganregung) and negative peaks (Aufhellung) peaks, which are associated with the interference between the waves diffracted by the primary and the “secondary þ coupling pair”. This RS pattern can provide useful structural information such as lattice parameters, crystallographic symmetry distortion, and crystalline quality (mosaic spread). A special case of the RS method is when the secondary diffracted beam lies parallel to the primary diffraction plane. This Bragg surface diffraction (BSD)16 case is particularly
Remedios et al.
useful for studying surface impurity incorporation effects as the secondary beam’s diffraction path is, of course, highly sensitive to the surface structure. Besides the conventional RS, another X-ray multiple diffraction scanning methodology that can give information on the crystalline quality can be obtained from an analysis of the ω:φ mapping scans.18 In this method, the φ rotation is performed for a range of ω angles each targeting an exact angular position of the multiple-beam Bragg condition. This approach results in a three-dimensional plot of the primary intensity versus ω and φ in a coupled way from which a through analysis of the isointensity contours of such plots one can obtain information on the lattice coherence along the beam path and hence upon the crystalline quality. It has been shown18 that when the fwhm of the peak in the φ scan is larger than in the ω scan then there is almost no loss of threedimensional (3-D) lattice coherence, that is, confirming that the crystal is perfect or nearly perfect. In this paper, these attributes are applied to probe the structure and perfection of single crystals of KDP, in particular how this is directly affected by the incorporation of crystal habit modifying Mn3þ dopant ion species within the crystal lattice; specifically, the work addresses how the defect incorporation mechanism might change with impurity concentration, notably related to the previously postulated interstitial3,4,14,15 or substitutional13 incorporation states. III. Experimental Section Four single crystal samples of KDP were prepared (one pure and three Mn3þ doped). These were grown by slow evaporation from saturated aqueous solutions at controlled temperature (313 K). Mn3þ doped samples were prepared for 1, 3, and 5 mol % solution concentrations by adding KMnO4 and MnCl3 at a 1:1 molar ratio to the growth solutions with a pH between 3.8 and 4. The as-grown crystals, see example provided in Figure 1c, displayed high-index faces and showed characteristic sector-zoning effects with the latter associated with a slight “browning” coloration with the {100} growth sectors. Elemental composition analyses of the resultant crystals were made using RBS at the Laboratory for Analysis of Materials by Ion Beams of the Institute of Physics, University of S~ ao Paulo, Brazil. RBS was used to detect O, P, K, and Mn using a beam of singly ionized 2.4 MeV helium atoms aligned (less than 3° off generally to avoid channeling) normal to the [100] larger crystal face with detection at 10° off-normal. From the RBS data, the real densities of O, P, K, and Mn atoms of the crystal surface were obtained using the RUMP computational program.19 X-ray powder diffraction (XRPD) data was collected using a Philips X0 Pert MRD diffractometer operating at 40 kV/40 mA, using CuKR radiation and pyrolytic graphite diffracted-beam monochromator. Powder samples were prepared from single crystals via grinding to a mesh size
