Polytypic Modifications Research Articles

Phase Relations in the Ln2O3-Cr2O3-B2O3 (Ln = Gd-Lu) Ternary Oxide Systems.

In this work, isothermal sections of the Ln2O3-Cr2O3-B2O3 (Ln = Gd-Lu) ternary oxide systems at 900, 1000, and 1100 °C were constructed by determining the phase relations by using a powder X-ray diffraction technique. As a result, these systems were divided into subsidiary subsystems. Two types of double borates, LnCr3(BO3)4 (Ln = Gd-Er) and LnCr(BO3)2 (Ln = Ho-Lu), were observed in the investigated systems. Regions of phase stability for LnCr3(BO3)4 and LnCr(BO3)2 were determined. It was shown that the LnCr3(BO3)4 compounds crystallized in rhombohedral and monoclinic polytype modifications up to 1100 °C; above this temperature and up to the melting points, the monoclinic modification was predominantly formed. The LnCr3(BO3)4 (Ln = Gd-Er) and LnCr(BO3)2 (Ln = Ho-Lu) compounds were characterized by using a powder X-ray diffraction method and thermal analysis.

Zirconolite Polytypes and Murataite Polysomes in Matrices for the REE-Actinide Fraction of HLW.

Electron backscatter diffraction (EBSD) has been used for more than 30 years for analyzing the structure of minerals and artificial substances. In recent times, EBSD has been widely applied for investigation of irradiated nuclear fuel and matrices for the immobilization of radioactive waste. The combination of EBSD and scanning electron microscopy (SEM/EDS) methods allows researchers to obtain simultaneously data on a specimen’s local composition and structure. The article discusses the abilities of SEM/EDS and EBSD techniques to identify zirconolite polytype modifications and members of the polysomatic murataite–pyrochlore series in polyphase ceramic matrices, with simulations of Pu (Th) and the REE-actinide fraction (Nd) of high-level radioactive waste.

Two New Rb3Sc(IO3)6 Polytypes in Proposed Nonlinear Optical Family A3M(IO3)6 (A = K, Rb; M = Sc, In): Topology–Symmetry Analysis, Order–Disorder and Structure–Properties Relation

Two polytypic modifications of new non-centrosymmetric iodate Rb3Sc(IO3)6 are synthesized hydrothermally. Their structures demonstrate different degrees of ordering and are solved in the same space group, Pc, with differing choice of b-axis: either 20 or 40 Å is selected from the same diffraction data, measured on crystals with better ordering. The difference in NLO properties of two polytypes is disclosed. Topology–symmetry analysis of separated blocks and layers allowed determination of local symmetry corresponding to mirror plane m, which defines strong optical nonlinearity. Layer alternation indicates a polytypic nature of Rb, Sc-iodate and its similarity to K, In- and K, Sc-nonlinear-optical iodate crystals, space group Fdd2, belonging to the proposed common family A3M(IO3)6 (A = K, Rb; M = Sc, In), whose properties are explained on a symmetry basis for every known member. Hypothetical structural variant is predicted; structural disorder and crystal growth conditions are discussed as factors affecting properties.

Структурные особенности твердых растворов Nd-=SUB=-x-=/SUB=-Gd-=SUB=-1-x-=/SUB=-Cr-=SUB=-3-=/SUB=-(BO-=SUB=-3-=/SUB=-)-=SUB=-4-=/SUB=-

Optical studies of NdxGd1-xCr3(BO3)4 0.01≤x≤1 crystals have been carried out. A significant concentration effect of the Nd3+ ion on the crystal structure of these solid solutions was identified. It was found from the absorption spectra of neodymium ions in the 4I9/2→11F3/2 transition region that at a concentration x>0.6 two nonequivalent Nd3+ centers appear, which is explained by the formation of two polytype modifications with space groups R32 and C2/c. At a concentration x<0.6, only one modification with the space group R32 is observed.

Minerals of the System Stichtite–Pyroaurite–Iowaite–Woodallite from Serpentinites of the Terekta Ridge (Gorny Altai, Russia)

Abstract —Hydrotalcite supergroup minerals stichtite, pyroaurite, iowaite, and woodallite form a complex solid-solution system at the Kyzyl-Uyuk locality (Terekta Ridge, Gorny Altai, Russia). The diversity of these minerals is due to: (1) subdivision by anionic interlayer composition into carbonate (stichtite and pyroaurite) and chloride (iowaite and woodallite) species and (2) isomorphism of M3+ cations, mainly between Cr- (stichtite and woodallite) and Fe3+-dominant species (pyroaurite and iowaite), with a quantitative predominance of stichtite and iowaite. Most of the studied samples correspond to stichtite and woodallite with high Fe3+ contents or pyroaurite and iowaite with high Cr3+ contents. According to vibrational (IR and Raman) spectroscopy data, the interlayer Cl– is partially substituted by OH– rather than CO32- groups. We suppose that the presence/absence of a band in the region 1400–1350 cm–1 in the Raman spectra of stichtite can be explained by the local distortion of triangular CO3 groups. Stichtite and iowaite occur here in polytypic modifications 3R and 2H that are the most widespread for the hydrotalcite supergroup iowaite and stichtite is 2:1. For both minerals, the polytype 3R strongly dominates over 2H. The lowest 3R/2H ratio determined for the Terekta iowaite and stichtite is 2:1. The Altai stichtite is close in 3R/2H to the stichtite from Tasmania (Australia) and differs significantly from that in Transvaal samples (South Africa).

Chemically Induced Polytypic Phase Transitions in the Mg[(UO2)(TO4)2(H2O)](H2O)4 (T = S, Se) System.

Chemically induced polytypic phase transitions have been observed during experimental investigations of crystallization in the mixed uranyl sulfate-selenate Mg[(UO2)(TO4)2(H2O)](H2O)4 (T = S, Se) system. Three different structure types form in the system, depending upon the Se:S ratio in the initial aqueous solution. The phases with the Se/(Se + S) ratios (in mol %) in the ranges 0-9, 16-47, and 58-100 crystallize in the space groups P21, Pmn21, and P21/c, respectively. The structures of the phases are based upon the same type of uranyl-based sulfate/selenate chains that, through hydrogen bonds, are linked into pseudolayers of the same topological type. The layers are linked into three-dimensional structures via interlayer Mg-centered octahedra. The three structure types contain the same layers but with different stacking sequences that can be conveniently described as belonging to the 1M, 2O, and 2M polytypic modifications. The Se-for-S substitution demonstrates a strong selectivity with preferential incorporation of Se into less tightly bonded T1 site. The larger ionic radius of Se6+ relative to S6+ induces rotation of (T1O4) tetrahedra in the adjacent layers and reconstruction of the structure types. From the information-theoretic viewpoint, the intermediate Pmn21 structure type is more complex than the monoclinic end-member structure types.

Synthesis of new polytype modifications of Fe7C3 AT 5,5 GPa

The paper presents the results of experimental studies on the synthesis of carbide Fe7C3 at 5,5 GPa. It was found that carbide Fe3C and several polytypes of carbide Fe7C3 are formed together with diamond when the system is cooled. It is believed that Fe7C3 carbide may be a component of the Earth’s inner core. The obtained results indicate that carbide Fe7C3 in the form of polytypic modifications under natural conditions could be formed at relatively low pressures at the stage of differentiation of the Earth.

Synthesis of New Polytypic Modifications of Fe7C3 at 5.5 GPa

The results of experimental synthesis of carbide Fe7C3 at 5.5 GPa are presented. It is found that carbide Fe3C and several polytypes of carbide Fe7C3 are formed along with diamond during cooling of the system. It is considered that the Fe7C3 phase is a component of the Earth’s inner core. Our results indicate that polytypic carbide modifications could have been formed under natural conditions at relatively low pressures during differentiation of the Earth.

COMPOSITION AND STRUCTURE OF THE HUNTITE-FAMILY COMPOUNDS

The literature data on the composition and structure of rare-earth borate compounds of the huntite family with the general composition LnM3(BO3)4, where Ln3+ = Y, La = Lu and M3+ = Al, Fe, Cr, Ga, Sc as well as a number of solid solutions with М3+ = Sc are systematized. The difference between the real compositions of crystals and the compositions of the initial mixture, the most characteristic of rare-earth scandium borates, is shown. The significant role of the composition in the manifestation of the compounds symmetry is established. The necessity of determining the crystals symmetry only on single-crystals with detailed analysis of diffraction reflections is proved. Morphotropic series were selected depending on changes in the ionic radii of Ln and M. Attention is paid to the peculiarity of the structural behavior of Cr3+ ions. It was revealed that the formation of solid solutions and internal solid solutions is most likely for rare-earth scandium borates. The implementation of polytypic modifications for LnM3(BO3)4, where M3+ = Al, Cr, and polymorphs for a number of Ln ions with M3+ = Fe, the existence of which is not excluded for M3+ = Sc, is demonstrated. Crystal-chemical effects observed for huntite-like crystals (morphotropy, isomorphism, polymorphism, polytypy; internal solid solutions; phase order-disorder phase transitions of different nature) with specific features of scandium borates are presented. The realization of polymorphism and polytypism for compounds of the huntite family confirms the crystal-chemical situation, according to which hightemperature polymorphic modifications should form more symmetrical compounds, but it is not typical of polytypic modifications.

Crystal chemistry of natural layered double hydroxides: 4. Crystal structures and evolution of structural complexity of quintinite polytypes from the Kovdor alkaline-ultrabasic massif, Kola peninsula, Russia

AbstractTwo quintinite polytypes, 3Rand 2T, which are new for the Kovdor alkaline-ultrabasic complex, have been structurally characterized. The crystal structure of quintinite-2Twas solved by direct methods and refined toR1= 0.048 on the basis of 330 unique reflections. The structure is trigonal,P$\bar 3$c1,a= 5.2720(6),c= 15.113(3) Å andV= 363.76(8) Å3. The crystal structure consists of [Mg2Al(OH)6]+brucite-type layers with an ordered distribution of Mg2+and Al3+cations according to the$\sqrt 3 $×$\sqrt 3 $superstructure with the layers stacked according to a hexagonal type. The complete layer stacking sequence can be described as …=Ab1C = Cb1A=…. The crystal structure of quintinite-3Rwas solved by direct methods and refined toR1= 0.022 on the basis of 140 unique reflections. It is trigonal,R$\bar 3$m,a= 3.063(1),c= 22.674(9) Å andV= 184.2(1) Å3. The crystal structure is based upon double hydroxide layers [M2+,3+(OH)2] with disordered distribution of Mg, Al and Fe and with the layers stacked according to a rhombohedral type. The stacking sequence of layers can be expressed as …=АB = BC = CA=… The study of morphologically different quintinite generations grown on one another detected the following natural sequence of polytype formation: 2H→ 2T→ 1Mthat can be attributed to a decrease of temperature during crystallization. According to the information-based approach to structural complexity, this sequence corresponds to the increasing structural information per atom (IG): 1.522 → 1.706 → 2.440 bits, respectively. As theIGvalue contributes negatively to the configurational entropy of crystalline solids, the evolution of polytypic modifications during crystallization corresponds to the decreasing configurational entropy. This is in agreement with the general principle that decreasing temperature corresponds to the appearance of more complex structures.

Structural varieties of polytypes

We present the results of theoretical analysis of structural varieties of compounds for which polytypism is important. As a result, the possibility of isomeric polytype varieties existing has been established, as well as regularities in their formation, depending on the period of repeatability of layers in packaging. In addition, the methods of density functional theory are used to calculate the structure and properties of various polytypic modifications of diamond. Dependences of the interlayer distances and the difference total energy on the hexagonality of diamond polytypes and their isomeric varieties have been found. The theoretical powder X-ray diffraction patterns of diamond polytypes are calculated and their comparative analysis is performed.

Synthesis and crystal structure of 3R and 1T′ polytypes of NH4Sc(SO4)2

Intermediate compound in a technologically affordable production of scandium oxide – anhydrous complex scandium-ammonium sulfate NH4Sc(SO4)2 – has been prepared using a hydrothermal route. The resultant samples were characterized by X-ray powder diffraction, infrared and optical spectroscopy, scanning electron microscopy, thermal and elemental analysis. The first ever decoding of the crystal structure of NH4Sc(SO4)2 unveils two polytypic modifications – 3R and 1T′. Both polytypes are the wide band-gap semiconductors. They possess a similar thermal stability, while irreversible 1T′→3R phase transition can be achieved using a mechanical action. In addition, relative thermodynamic stability and electronic properties of a large set of NH4Sc(SO4)2 polymorphs were studied by means of DFT calculations. The study confirms the stability of 3R and metastability of 1T′ polytypes as well as suggests a minor content of substitutional hydronium within the ammonium sublattice.

Long-term evolution of luminescent properties in CdI2 crystals

Fresh and aged melt-grown or gas-phase grown CdI2 crystals are studied by means of low-temperature photoluminescence spectroscopy. Noticeable transformations of emission spectra are observed after long-term aging. The formation of nanostructures containing cadmium oxide and cadmium hydroxide as well as the changes in local surrounding of iodine atoms and the possible growth of polytypic modifications of CdI2 are taken into account when considering the diversity of optical spectra.

The mineralogical, isotope (K–Ar), structural, and textural features of the Jurassic siliciclastic complex in various tectonic environments (Greater Caucasus, Chechnya, and Georgia)

The variations in the structural, textural, mineralogical, and geochemical (isotope) features of Lower to Middle Jurassic siliciclastic sediments along the Chanty-Argun River in Mountainous Chechnya and Georgia are discussed. This profile transects areas with various types of deformed sediments, from the northern comparatively weakly deformed and altered zone, to the southern zone of intense deformation and cleavage. Southward along the profile, these alterations are accompanied by the evolution of clay mineral assemblages, as well as polytypic modifications of micas and their crystallinity index. Increasing intensity of rock alteration and cleavage leads to a change of the K–Ar system, which results in a substantially rejuvenated isotope age of the sediments with a simultaneous increase of their stratigraphic age.

Crystal structure and energy bands of (Ga/In)Se and Cu(In,Ga)Se2 semiconductors in comparison

Crystallographic and electronic structures of binary compounds GaSe and InSe, known to exist in several polytype modifications, are discussed in comparison with those of chalcopyrite-type CuInSe2, CuGaSe2, and their solid solutions. The results are obtained within the density-functional theory (DFT), using two calculation codes, WIEN2k and VASP. The van der Waals interaction, important in stabilizing the structure of the layer compounds GaSe and InSe, was taken into account in the VASP calculation along the scheme of Grimme. For obtaining more realistic electronic band structures than is possible with the conventional generalized gradient approximation (GGA), the modified Becke–Johnson (mBJ) exchange–correlation (XC) potential was used, as implemented in the WIEN2k code. The results of two computer codes were compared in what concerns the optimization of the ground-state structures. The similarities in electronic structures of the systems studied are discussed under an angle of general trends proper to the compounds of Cu–(In,Ga)–Se composition.

Effect of symmetry reduction on the electronic transitions in polytypic GdAl3(BO3)4:Eu:Tb crystals

The existence of a recently described monoclinic phase (C2/c, Z=8) (Beregi et al., 2012) in addition to the well-known Huntite type rhombohedral (R32) polytypic modification of the GdAl3(BO3)4 (GAB) crystal at room temperature provides a unique possibility to investigate the incorporation of rare earth dopants into slightly modified crystal lattice by spectroscopic methods. In these characteristic GAB structures the dopant ions, e.g. Tb3+ or Eu3+, possess slightly different neighbor geometries and local symmetries. The Tb3+: 7F6→5D4 and Eu3+: 7F0,1,2→5D0,1,2 electronic transitions were successfully identified in the absorption spectra using polarization, concentration and temperature dependent measurements in both polytypic modifications. The positions of the investigated Tb lines are shifted by up to 10cm−1 due to symmetry changes. In addition, some of the Eu lines show splittings of about 4–30cm−1 as a consequence of the change of the local environment. From the room temperature absorption measurements some of the low energy crystal field levels of 7F and 5D states of the Eu3+ ions were successfully determined for both modifications.

Structure of ordered coaxial and scroll nanotubes: general approach.

The explicit formulas for atomic coordinates of multiwalled coaxial and cylindrical scroll nanotubes with ordered structure are developed on the basis of a common oblique lattice. According to this approach, a nanotube is formed by transfer of its bulk analogue structure onto a cylindrical surface (with a circular or spiral cross section) and the chirality indexes of the tube are expressed in the number of unit cells. The monoclinic polytypic modifications of ordered coaxial and scroll nanotubes are also discussed and geometrical conditions of their formation are analysed. It is shown that tube radii of ordered multiwalled coaxial nanotubes are multiples of the layer thickness, and the initial turn radius of the orthogonal scroll nanotube is a multiple of the same parameter or its half.

Cayalsite-(Y), a new rare-earth calcium aluminium fluorosilicate with OD character

The crystal structure of cayalsite-(Y), with idealized composition CaY6Al2Si4O18F6, has been determined on the basis of single-crystal X-ray diffraction data. The mineral occurs in several polytypic modifications arising from the combination of three non-equivalent structural layers. Two maximum degree of order (MDO) polytypes have been identified in cayalsite-(Y). The orthorhombic MDO polytype, cayalsite-(Y)-1 O , has lattice parameters a = 15.993(1), b = 5.5306(3) and c = 9.6590(7) A. It is described in space group symmetry Pban and refines to R obs = 2.2%. The monoclinic MDO polytype, cayalsite-(Y)-1 M , a = 11.0602(7), b = 5.5280(2), c = 16.0195(9) A, β = 118.925(3)° is described in space group symmetry P 2/ c and refines to R obs = 3.5% in a two-phase mixture with cayalsite-(Y)-1 O . Predominantly heavy lanthanide elements substitute for Y and Ca in cayalsite-(Y), with some Ca also substituting for the rare-earth elements (REE). A typical empirical composition, normalized to 24 anions, is ![Formula][1] Cayalsite-(Y) forms colourless to faintly pink, prismatic crystals in cavities of yttrian fluorite in two granitic pegmatites located in Tysfjord, Nordland, Norway. Cayalsite-(Y)-1 O crystals are optically biaxial (+), with indices of refraction α = 1.730(5), β = 1.740(5), γ = 1.760(5) and 2 V meas = 56.5(5). The crystal structure of cayalsite-(Y) is composed of linear chains of edge sharing [AlO6] octahedra with isolated [SiO4] tetrahedra. The REE and Ca-cations occur in eight-fold coordination by O and F anions. Apart from substitutional disorder of REE and Ca atoms, the cayalsite structure is characterized by substitutional and positional disorder affecting the local position of [SiO4] tetrahedra in one of its layers. Avoidance of close Si-F contacts causes the partial splitting of O and F positions. The possible interplay of layer stacking and cation ordering is discussed. [1]: /embed/mml-math-1.gif

IR spectroscopy of rare-earth aluminum borates RAl3(BO3)4 (R = Y, Pr-Yb)

Rare-earth aluminum borates RAl3(BO3)4 (R = Y, Nd-Yb) obtained by spontaneous high-temperature flux crystallization form two polytype modifications described by the space groups R32 (D 3 7 ) and C2/c (C 2 6 ). They differ in the symmetry versions in mutual arrangement of layers. These borates have been investigated by mid- and far-IR spectroscopy in combination with factor-group analysis of the vibrations of BO 3 3− ions; translational motions of Al3+, R3+ (Y, Nd-Yb), and BO 3 3− ions; and BO 3 3− rotations. Rare-earth aluminum borates are assigned to different space groups according to their IR spectra. Borates with large rare-earth Nd and Pr cations are crystallized into the space group C2/c (C 2 6 ), while borates with smaller (Y, Sm-Yb) ions are crystallized into the space group R32 (D 3 7 ). The rhombohedral structure of the latter compounds includes monoclinically ordered domains, as is evidenced by the presence of monoclinic-phase bands in the IR spectrum. NdAl3(BO3)4 and SmAl3(BO3)4 can form both monoclinic and rhombohedral polytypes. The IR spectrum of the monoclinic SmAl3(BO3)4 phase contains bands due to the rhombohedral phase, while the IR spectrum of NdAl3(BO3)4 (which is described by the space group R32) contains bands due to the monoclinic polytype. The presence of domains with another arrangement of layers in the polytype structure is a characteristic sign that corresponds to the order-disorder theory; this theory explains the structure of polytypes.

Allotwinning in a molecular crystal: (1R,3S)-dimethyl 2-oxocyclohexane-1,3-dicarboxylate

Two polytypic modifications of (1R,3S)-dimethyl 2-oxocyclohexane-1,3-dicarboxylate intergrow to form allotwins. One phase shows monoclinic symmetry, Cc, while the other is orthorhombic Pmc21. The structures may be considered as two different modes of superstructure ordering of an underlying, disordered structure. Considered in the same metrically orthorhombic unit cell a = 37.8883, b = 4.7233, c = 11.6835 Å, the two can be conveniently distinguished by their non-standard centering, Cc being represented as Xc with the centering vectors (0 0 0); (½ 0 ½); (¼ ½ ¾); (¾ ½ ¼); and Pmc21 as Xmc21 with the centering vectors (0 0 0); (½ 0 0). The difference between the allotwin domains lies only in the relationship between next-nearest neighbors along the stacking direction 100 and hence the conformations and packing efficiencies are identical for both phases and all three domains. The stacking sequences for the two phases correspond to ABAB for the orthorhombic stacking and ABA'B'/AB'A'B for the two (equivalent) monoclinic stackings. The monoclinic phase dominates comprising ca 80% of the total volume. Within the monoclinic phase, the ratio between the two possible orientations is highly unbalanced (6:1) indicating relatively large domains. The allotwinning detected in this sample may appear exotic in molecular compounds, but we suggest that the rarity of such examples in the literature is a reflection of the paucity of software that can identify and handle such cases. It is easy to overlook the possibility that a complex diffraction pattern originates from allotwinning and assume that normal mono-component twinning is the cause. The underlying mechanism in this case is the formation of two-dimensional layers of molecules that allow for two equivalent, but in terms of absolute geometry, different, ways of stacking.