A. G. Shtukenberg

Combined molecular beam epitaxy and diffractometer system for in situ x-ray studies of crystal growth

A combination of a molecular beam epitaxy (MBE) machine and a six circle diffractometer has been constructed at a dedicated wiggler beamline at the storage ring BESSY II for in situ investigations of III–V compound crystal growth. The growth conditions in our system reach a high MBE standard with a noncooled base pressure of 2×10−10 mbar. A fast entry load lock is available for sample exchange. Large-area Be windows in the ultrahigh vacuum chamber allow us to measure reflections at entrance and exit angles up to 45°, i.e., large perpendicular momentum transfers are possible. In situ reflection high energy electron diffraction and x-ray fluorescence measurements can be performed simultaneously with x-ray scattering. A GaAs(001) surface prepared and examined in our system reveals terrace widths of 450 nm and β(2×4) reconstruction domain sizes of 210 nm. The possibility of time-resolved x-ray diffraction studies is demonstrated by observation of intensity oscillations during layer-by-layer homoepitaxial grow...

Growth ordering and anomalous birefringence in ugrandite garnets

Abstract Intermediate members of the grossular−andradite and grossular−uvarovite series are known to display anomalous birefringence, which is inconsistent with the ideal cubic space group of garnet Ia3̅d. To determine the reason for such birefringence, the crystal structures of at least 15 samples were refined by different authors including the present ones. The crystals with the value of anomalous birefringence of ∆n > 0.001 are normally characterized by partial ordering of the octahedral cations (Al/Fe and Al/Cr for grossular−andradite and grossular−uvarovite series, respectively). This reduces the symmetry to the orthorhombic space group Fddd or even to the triclinic space group I1̅ . As is seen from the distribution of occupancies over octahedral sites in the triclinic space group, eight occupancies are grouped into two quartets of similar occupancies, leading to pseudo-orthorhombic crystal structures. The variety of structures with different degrees of pseudo-orthorhombicity is due to the action of the growth dissymmetrization mechanism. Simulations of the optical indicatrix in the point-dipole approximation confirm cation ordering as the main cause of the anomalous birefringence.

Layer-by-layer growth of GaAs(0 0 1) studied by in situ synchrotron X-ray diffraction

We investigate the time-dependent surface evolution during molecular beam epitaxy of GaAs(0 0 1) using synchrotron X-ray diffraction at a newly-built dedicated beamline at the synchrotron BESSY II. The crystal truncation rods analyzed at growth temperature agree with the room-temperature bð2 � 4Þ reconstruction published in the literature. The layer coverage evolution during growth is analyzed by fitting the oscillating intensity along crystal truncation rods. Our results show that the structure of the reconstructed surface unit cell does not change during growth. Using numerical simulations,we determine the terrace size distribution on the surface at growth temperature and verify the validity of our analysis for multi-level initial surfaces,as long as the mean terrace size is larger than the nucleation distance during growth. 2002 Elsevier Science B.V. All rights reserved.

Metamorphic vanadian-chromian silicate mineralization in carbon-rich amphibole schists from the Malé Karpaty Mountains, Western Carpathians, Slovakia

Abstract Mineralization, involving vanadian-chromian silicates, has been studied in Lower Paleozoic, carbon-rich amphibole schists with pyrite and pyrrhotite near Pezinok, southwest Slovakia. A detailed electron microprobe study has revealed the presence of V,Cr-rich garnet, clinozoisite, and muscovite, associated with amphiboles (magnesiohornblende, tremolite, actinolite, and edenite), diopside, and albite. The garnet contains 5-19 wt% V2O3, 5-11 wt% Cr2O3, and 2-13 wt% Al2O3 (16-64 mol% goldmanite, 19-36 mol% uvarovite, and 9-59 mol% grossular end-members). The garnet is unzoned or shows V-rich cores and Al-rich rims, or irregular coarse oscillatory zoning with V, Cr, and Al, locally involving Ca and Mn as well. The V,Cr-rich clinozoisite to mukhinite and “chromian clinozoisite” contains 2-9.5 wt% V2O3 and 1.5-11 wt% Cr2O3; the muscovite contains 2.5-8 wt% V2O3 and 0-7 wt% Cr2O3. The mineralization originated from primarily V-, Cr-, and C-rich mafic pyroclastic rocks, affected by volcano-exhalative processes. These rocks were weakly metamorphosed during early Hercynian regional metamorphism (M1), followed by late-Hercynian contact metamorphism (M2) with crystallization of V,Cr-rich silicates, diopside, amphiboles, phlogopite, titanite, albite, quartz, carbonate, pyrite, and pyrrhotite. The youngest Alpine(?) retrograde metamorphic event (M3) is connected with production of V,Cr-poor muscovite, clinochlore, clinozoisite, pumpellyite-(Mg), prehnite, quartz, and carbonates, under prehnite-pumpellite facies conditions.

Effect of crystal composition and growth rate on sector zoning in solid solutions grown from aqueous solutions

Abstract Sector zoning in solid solutions of Tutton′s salts (NH4)2M(SO4)2−6H2O (M = Co, Ni, Zn, Mg) and nitrates of divalent metals Me(NO3)2 (Me = Sr, Pb, Ba) grown from low-T aqueous solutions under controlled conditions is studied. The effect of crystal composition and growth rate (supersaturation) on the intersector difference in concentrations of isomorphic components is examined experimentally and modelled theoretically based on the phenomenological approach to crystallization in a ‘solid solution−aqueous solution’ system. Calculations show a predominating contribution of the selective adsorption phenomena in the formation of a sector-zoned crystal with usually minor, but sometimes a major contribution of growth kinetics.

An application of the point-dipole model to the problem of optical anomalies in grandite garnets

Abstract The origin of anomalous birefringence in grossular-andradite (grandite) garnets is considered. Using the point-dipole model we have calculated the anomalous optical indicatrix for four crystals (three from the paper by Takéuchi et al. (1982) and one studied previously by the present authors). The calculated indicatrix is in good agreement with the observed one. This suggests that the origin of anomalous birefringence in grandite garnets is due to Al/Fe ordering. Interrelations between the observed optical indicatrix, the structure of the growing face, the site occupancy in the crystal structure and the calculated optical indicatrix suggest a growth origin of Fe/Al ordering in grandite garnets and confirm the growth dissymmetrization hypothesis.

On the complex zonality in grandite garnets and implications

Abstract The available data on compositional zoning in grossular-andradite (grandite) garnets occurring at different scales, mainly due to the variations of their Fe3+/Al ratios, and the hypotheses on the origin of this zoning are reviewed. Four zoned crystals of grandites showing different optical properties have been studied by means of X-ray diffraction. Optical and structural studies suggest three superimposed scales of Fe3+/Al zonality along [110]. The drastic change of the structural characteristics from sample to sample correlates with the change of their optical patterns. The superfine oscillatory zoning has been described in terms of irregular interstratified structures with a high degree of segregation. The self-affinity of zoning at different scales testify to the possible origin of the zonality due to the non-linear dynamics at the growth from. The wide variation of the layer compositions revealed by the modelling of the X-ray diffraction profiles seems to contradict the hypothesis of unmixing in grandite garnets.

Refinement of the crystal structures of synthetic nickel- and cobalt-bearing tourmalines

The crystal structures of synthetic tourmalines with a unique composition containing 3d elements (Ni, Fe, and Co) have been refined: (Ca0.12▭0.88)(Al1.69Ni0.812+Fe0.502+)(Al5.40Fe0.603+)(Si5.82Al0.18O18)(BO3)3(OH)3.25O0.75 I, a = 15.897(5), c = 7.145(2) Å, V = 1564(1) Å; Na0.91(Ni1.202+Cr0.963+Al0.63Fe0.182+Mg0.03)(Al4.26Ni1.202+Cr0.483+Ti0.06)(Si5.82Al0.18)O18(BO3)3(OH)3.73O0.27 II, a = 15.945(5), c = 7.208(2) Å, V = 1587(1) Å3 and Na0.35(Al1.80Co1.202+)(Al5.28Co0.662+Ti0.06)(Si5.64B0.36)O18(BO3)3(OH)3.81O0.19 III, a = 15.753(8), c = 7.053(3) Å, V = 1516(2) Å3. The reliability factors are R1 = 0.038−0.057 and wR2 = 0.041–0.060. It is found that 3d elements occupy both Y- and Z positions in all structures. The excess positive charge is compensated for due to the incorporation of divalent oxygen anions into the O3(V)+O1(W) positions.

Symmetry and crystal structure of biaxial elbaite-liddicoatite tourmaline from the Transbaikalia region, Russia

Abstract Optical anomalies and crystal structures have been studied for an elbaite-liddicoatite tourmaline specimen from the Malkhan pegmatite field (Transbaikalia region, Russia). The specimen is characterized by a complicated zoning and sector zoning distribution of anomalous biaxiality, with the axial angle 2V increasing from 3 up to 23° going from the first to the last growth zones of the most developed growth sector, o{022̅1}. The crystal structures of three samples cut out from different growth zones of that sector and characterized by the axial angles 11, 16, and 23° were refined in the trigonal space group R3m as well as in its monoclinic and triclinic subgroups Cm, R1, and P1, respectively (R = 0.019-0.039). The final choice of the low symmetry space group R1 is based on the analysis of the diffraction patterns and on the results of crystal structures refinements, which revealed ordered Al/Li distributions over the Y octahedral sites. The degree of order at Y sites correlates with the axial angle 2V, which implies a causal relationship. The Al/Li ordering reflects the geometrical differences of the octahedral sites with respect to the growth front orientation. The results obtained suggest that a growth dissymmetrization mechanism is the main reason for the observed cation ordering and optical anomalies.

Symmetry reduction and cation ordering in solid solutions of strontium-lead and barium-lead nitrates

Abstract Kinetic ordering of the cations in (Sr,Pb)(NO3)2 and (Ba,Pb)(NO3)2 solid solutions with anomalous birefringence was studied by means of single crystal X-ray diffraction. The crystal structures of two samples of (Sr0.62Pb0.38)(NO3)2 and (Ba0.20Pb0.80)(NO3)2, respectively, taken from the {111} growth sectors as well as of two samples of (Sr0.62Pb0.38) (NO3)2 taken from the {100} growth sector were refined in the ideal structure space group Pa-3 and in two of its subgroups – trigonal R-3 and triclinic P-1 with R = 0.009 — 0.027. The final choice of trigonal symmetry for the samples from the {111} growth sectors is based on the analysis of the diffraction patterns and of the cation distributions over the crystallographic sites, which reflects the different orientations of the cation coordination polyhedra with respect to the (111) growth face (cubic setting). The observed optical indicatrices were successfully reproduced in the point-dipole approximation.