The intrinsic twinning and enigmatic twisting of aragonite crystals.

It is generally accepted that aragonite crystals of biogenic origin are characterized by significantly higher twin densities compared to samples formed during geological processes. Based on our single crystal X-ray diffraction (SCXRD) and transmission electron microscopy (TEM) study of aragonite crystals from various localities, we show that in geological aragonites, the twin densities are comparable to those of the samples from crossed lamellar zones of molluscs shells. The high twin density is consistent with performed calculations, according to which the Gibbs free energy of twin-free aragonite is close to that of periodically twinned aragonite structure. In some cases, high twin densities result in the appearance of diffuse scattering in SCXRD patterns. The obtained TEM and optical micrographs show that besides the twin boundaries (TBs) of growth origin, there are also TBs and especially stacking faults that were likely formed as the result of local strain compensation. SCXRD patterns of the samples from Tazouta, in addition to diffuse scattering lines, show Debye arcs in the [Formula: see text] plane. These Debye arcs are present only on one side of the Bragg reflections and have an azimuthal extent of nearly 30°, making the whole symmetry of the diffraction pattern distinctly chiral, which has not yet been reported for aragonite. By analogy with biogenic calcite crystals, we associate these arcs with the presence of misoriented subgrains formed as a result of crystal twisting during growth.

γ-BaB2O4: High-Pressure High-Temperature Polymorph of Barium Borate with Edge-Sharing BO4 Tetrahedra.

The α- and ß-modifications of barium metaborate are important functional materials used in optoelectronic devices. A new theoretically predicted modification of BaB2O4 has been synthesized under conditions of 3 GPa and 900 °C, using the DIA-type apparatus. The new high-pressure modification, γ-BaB2O4, crystallizes in a centrosymmetrical group of monoclinic syngony (P21/n (#14), a = 4.6392(4) Å, b = 10.2532(14) Å, c = 7.066(1) Å, ß = 91.363(10)°, Z = 4). A distinctive feature of the γ-BaB2O4 structure is the presence of edge-sharing tetrahedra [B2O6] which form infinite double chains ∞[B4O4O8/2] stretching along the a axis. The number of known structural types with the [B2O6] group is limited. Phase γ-BaB2O4 has the shortest distance between boron atoms of shared tetrahedra among all currently known compounds. The [B2O6] group angles are 95.5° and 105.5°. Thermodynamic stability and electronic properties of the γ-BaB2O4 modification were studied. The width of the band gap, calculated using the HSE06 functional, is 7.045 eV which implies transparency in the deep-UV region. Experimental and numerical methods which demonstrate a good match were used to the study the Raman spectra of γ-BaB2O4 and ß-BaB2O4 modifications. In the Raman spectra of γ-BaB2O4, the most intense band at a frequency of 853 cm-1 was found to correspond to the symmetric bending mode of the B-O-B-O ring in edge-sharing tetrahedra.

Towards high-quality nitrogen-doped diamond single crystals for X-ray optics.

In this manuscript, characterization of single-crystalline (111) plates prepared from type-Ib diamonds with a nitrogen content of 100-150 ppm by means of high-resolution rocking-curve imaging (RCI) is reported. Contrary to common opinion regarding the intrinsically poor diffraction quality of type-I diamonds, RCI showed the presence of nearly defect-free areas of several millimetres squared in the central part of the diamond plates. The observed broadening of the rocking curves is a result of the cutting and polishing processes, causing strains around the edges of the plates and rare defects. An improvement of the preparation technique will thus allow single-crystalline diamond plates to be made for Laue and Bragg monochromators and beam splitters from type-Ib material with areas large enough to be used as optical elements at fourth-generation synchrotron facilities.

Experimental and Ab Initio Studies of Intrinsic Defects in "Antizeolite" Borates with a Ba12(BO3)66+ Framework and Their Influence on Properties.

The porous Ba12(BO3)66+ framework of the so-called "antizeolite" borates with channels along the c axis is capable of accommodating various guest anionic groups, e.g. [BO3]3-, [F2]2-, [F4]4-, and [(Li,Na)F4]3-. Taking as an example the Ba12(BO3)6[BO3][LiF4] crystal, we put forward the argument that the optical properties of "antizeolite" borates are strongly influenced by the degree of channel packing with anionic groups and, correspondingly, by the conjugated intrinsic defects. With the use of optical, electron-spin resonance, Raman spectroscopy, and ab initio calculations, it was shown that intrinsic defects largely impact the absorption of light in the visible and UV regions (the color of the bulk crystals can change from colorless to dark brown), absorption-edge position, dichroism, and other optical properties. The change in the optical absorption in the visible range is caused by the appearance of new states in the electronic structure inside the band gap, which are associated mainly with the presence of single and double F centers-fluorine vacancies that capture electrons-in [□F4]4-, [F2]2-, and [LiF4]3- groups. The formation of F centers in the [F2]2- group is the most energetically favorable. It has been found that Ba12(BO3)6[BO3][LiF4] crystals are optically active gyrotropic with an isotropic point at 499 nm at 300 K and are of interest for practical application as narrow-band light filters.

Nature of the Color of Borates with "Anti-Zeolite" Structure.

Crystals of the Mn xBa12(BO3)8-2 xF8 x phase were grown from a high-temperature solution. This new fluoride borate is built of positively charged [Ba12(BO3)6]6+ blocks, the so-called "anti-zeolite" pattern. Using X-ray single-crystal diffraction, the bulk atomic arrangement in the centrosymmetric tetragonal unit cell in I4/ mcm could be elucidated. Crystals of the (MnF6)4- group-containing solid solution Mn xBa12(BO3)8-2 xF8 x are dark brown in color in contrast to the differently colored crystals of (LiF4)3- group-containing "anti-zeolite" LiBa12(BO3)7F4 ( P42 bc). According to the electron spin resonance and optical spectroscopic investigation, the absorption spectrum of LiBa12(BO3)7F4 crystals results from the absorption of light by both exciton and free charge carriers and can be tuned by varying the initial composition of the high-temperature solution.

Growth and Optical Properties of LixNa1-xBa12(BO3)7F4 Fluoride Borates with "Antizeolite" Structure.

Studied LixNa1-xBa12(BO3)7F4 (P42bc) solid solution belongs to the new class of "antizeolite" borates with [Ba12(BO3)6]6+ cation pattern, which contains channels filled by anionic clusters. Optical-quality crystals were grown from the compositions with different sodium-lithium ratio. The results of Rietveld refinement based on powder data demonstrate linear increase of parameter a and unit cell volume with Na/(Na + Li) ratio in cation site. Parameter c is less sensitive to the changes in stoichiometry, which is consistent with channel topology of LixNa1-xBa12(BO3)7F4 structure. Distinctive feature of LixNa1-xBa12(BO3)7F4 crystals is their deep purple color, which is due to both hole-type and electron-type centers. Crystals are characterized by linear dichroism effect.

Thermal equation of state of solid naphthalene to 13 GPa and 773 K: in situ X-ray diffraction study and first principles calculations.

In a wide range of P-T conditions, such fundamental characteristics as compressibility and thermoelastic properties remain unknown for most classes of organic compounds. Here we attempt to clarify this issue by the example of naphthalene as a model representative of polycyclic aromatic hydrocarbons (PAHs). The elastic behavior of solid naphthalene was studied by in situ synchrotron powder X-ray diffraction up to 13 GPa and 773 K and first principles computations to 20 GPa and 773 K. Fitting of the P-V experimental data to Vinet equation of state yielded T 0 = 8.4(3) GPa and T' = 7.2 (3) at V0 = 361 Å(3), whereas the thermal expansion coefficient was found to be extremely low at P > 3 GPa (about 10(-5) K(-1)), in agreement with theoretical estimation. Such a diminishing of thermal effects with the pressure increase clearly demonstrates a specific feature of the high-pressure behavior of molecular crystals like PAHs, associated with a low energy of intermolecular interactions.

LiBa12(BO3)7F4 (LBBF) crystals doped with Eu3+, Tb3+, Ce3+: structure and luminescence properties.

The luminescent properties of single crystals and polycrystalline samples of LiBa12(BO3)7F4 (LBBF) doped and co-doped with Eu3+, Tb3+, and Ce3+ have been studied in order to disclose their potential for application in white light-emitting diodes. Deciphering of LBBF:Eu3+ crystal structure (P42/mbc) makes it possible to determine the scheme of heterovalent isomorphic substitution 3Ba2+ ← 2Eu3+ + □, □ - vacancy in barium sites. Luminescent properties of LBBF crystals co-doped with Eu3+, Tb3+, Ce3+ and Eu3+, Tb3+ are compared. Both crystals demonstrate the luminescence close to day light with CIE coordinates and correlated color temperature (0.280; 0.305), 9232 K for LBBF:Eu3+,Tb3+,Ce3+ and (0.353; 0.390), 4849 K for LBBF:Eu3+,Tb3+ at 300 K at 370 nm excitation. The study of polycrystalline samples LBBF:Ce3+, LBBF:Tb3+, and LBBF:Eu3+, using X-ray diffraction, shows that the homogeneity regions of solid solutions differ significantly. In solid solutions LBBF:Tb3+ and LBBF:Eu3+, a gradual change in symmetry in the "tetragonal → orthorhombic → tetragonal" series takes place as the concentration of the dopant increases. When excited at a wavelength 395 nm, LBBF:Eu3+ samples with the content of Eu3+ 0.5 wt% and 2 wt% demonstrate intense red light with a high quantum yield of 63 and 60%, respectively, which allows them to be used as red components of composite phosphors for white LEDs.

A Photoluminescence Study of Eu3+, Tb3+, Ce3+ Emission in Doped Crystals of Strontium-Barium Fluoride Borate Solid Solution Ba4-xSr3+x(BO3)4-yF2+3y (BSBF).

The present study is aimed at unveiling the luminescence potential of Ba4-xSr3+x(BO3)4-yF2+3y (BSBF) crystals doped with Eu3+, Tb3+, and Ce3+. Owing to the incongruent melting character of the phase, the NaF compound was used as a solvent for BSBF crystal growth. The structure of BSBF: Eu3+ with Eu2O3 concentration of about 0.7(3) wt% was solved in the non-centrosymmetric point group P63mc. The presence of Eu2O3 in BSBF: Eu3+ leads to a shift of the absorption edge from 225 nm to 320 nm. The photoluminescence properties of the BSBF: Ce3+, BSBF: Tb3+, BSBF: Eu3+, and BSBF: Eu3+, Tb3+, Ce3+ crystals have been studied. The unusual feature of europium emission in BSBF is the intensively manifested 5D0→7F0 transition at about 574 nm, which is the strongest for BSBF: Eu3+ at 370 nm excitation and for BSBF: Eu3+, Tb3+, Ce3+ at 300 nm and 370 nm excitations. No evidence of Tb3+→Eu3+ energy transfer was found for BSBF: Eu3+, Tb3+, Ce3+. The PL spectra of BSBF: Eu3+ at 77 and 300 K are similar with CIE chromaticity coordinates of (0.617; 0.378) at 300 nm excitation and (0.634; 0.359) at 395 nm excitation and low correlated color temperature which implies application prospects in the field of lightning. Due to the high intensity of 5D0→7F0 Eu3+ transition at 370 nm excitation, the BSBF: Eu3+ emission is yellow-shifted.

A high-pressure single-crystal X-ray diffraction study of potassium guaninate hydrate, K+·C5H4N5O-·H2O.

The crystal structure of potassium guaninate hydrate, K+·C5H4N5O-·H2O, was studied in the pressure range of 1 atm to 7.3 GPa by single-crystal diffraction using synchrotron radiation and a laboratory X-ray diffraction source. Structural strain was compared to that of the same salt hydrate on cooling, and in 2Na+·C5H3N5O2-·7H2O under hydrostatic compression and on cooling. A polymorphic transition into a new, incommensurately modulated, phase was observed at ∼4-5 GPa. The transition was reversible with a hysteresis: the satellite reflections disappeared on decompression to ∼1.4 GPa.

Study of an EuBO3-ScBO3 system and EuSc3(BO3)4, EuSc(BO3)2 orthoborates.

The EuBO3-ScBO3 system was investigated by solid state synthesis and DSC methods. In this system, a new EuSc(BO3)2 compound was found. It crystallizes in the R3̄ space group with unit cell parameters of a = 4.8939(1) Å and c = 16.2663(5) Å. Whereas another compound in the system EuSc3(BO3)4 possesses two modifications: a low-temperature α-C2/c (a = 7.687(1) Å, b = 9.810(2) Å, c = 12.021(2) Å, and ß = 105.379(4)°) and a high-temperature ß-R32 (a = 9.7473(1) Å and c = 7.9205(2) Å). The α-EuSc3(BO3)4 crystal was grown with LiBO2-LiF flux, and ß-EuSc3(BO3)4 was obtained by the solid state synthesis. All of the obtained crystals exhibited typical Eu3+ luminescence spectra with peaks at 589 nm, 596 nm, 615 nm, 657 nm and 689 nm, which corresponded to the 5D0 → 7FJ (J = 0, 1, 2 and 4) electron transitions. The strongest peak of luminescence was located at 615 nm and corresponded to the 5D0 → 7F0 transition.

Incommensurately modulated crystal structure of flamite - natural analogue of \alpha _{\rm H}

Crystal structures of unquenchable high-temperature polymorphs of Ca2SiO4, important in cement chemistry, have eluded single-crystal X-ray analysis. However, the problem may be addressed by studying chemically stabilized Ca2SiO4 polymorphs at ambient temperature. Here an incommensurately modulated crystal structure of flamite [Pnma(0ß0)00s, q = 0.2728 (2)b*, a = 6.8588 (2) Å, b = 5.4301 (2) Å, c = 9.4052 (3) Å] is described. It is a mineral analogue of orthorhombic \alpha _{\rm H}^{\prime}-Ca2SiO4 (stable between 1160 and 1425°C), naturally stabilized by substitution with phosphorus. The incommensurate modulation results from wave-like displacement of cation sites accompanied by tilting of (Si,P)O4 tetrahedra and variation of the Na/(Ca + Na + K) ratio along the modulation period. The studied sample from Hatrurim Basin (Negev Desert, Israel) with composition (Ca1.75K0.12Na0.12)1.99(Si0.74P0.26)1.00O4 also demonstrates pseudomerohedral cyclic twinning around the a axis, which results from pseudohexagonal topology of the crystal structure and complicates the indexing of X-ray diffraction data.

In Situ Raman Study of Liquid Water at High Pressure.

A pressure shift of Raman band of liquid water (H2O) may be an important tool for measuring residual pressures in mineral inclusions, in situ barometry in high-pressure cells, and as an indicator of pressure-induced structural transitions in H2O. However, there was no consensus as to how the broad and asymmetric water Raman band should be quantitatively described, which has led to fundamental inconsistencies between reported data. In order to overcome this issue, we measured Raman spectra of H2O in situ up to 1.2 GPa using a diamond anvil cell, and use them to test different approaches proposed for the description of the water Raman band. We found that the most physically meaningful description of water Raman band is the decomposition into a linear background and three Gaussian components, associated with differently H-bonded H2O molecules. Two of these components demonstrate a pronounced anomaly in pressure shift near 0.4 GPa, supporting ideas of structural transition in H2O at this pressure. The most convenient approach for pressure calibration is the use of "a linear background + one Gaussian" decomposition (the pressure can be measured using the formula P (GPa) = -0.0317(3)·ΔνG (cm-1), where ΔνG represents the difference between the position of water Raman band, fitted as a single Gaussian, in measured spectrum and spectrum at ambient pressure).

High-Pressure-High Temperature (HP-HT) Stability of Polytetrafluoroethylene: Raman Spectroscopic Study Up to 10 GPa and 600 ℃.

The increasing demand for use of polymers at extreme conditions makes important the exploration of their behavior in a wide pressure and temperature range, which remains unknown for polytetrafluoroethylene (PTFE), one of the most common materials. An in situ Raman spectroscopic study of PTFE shows that it is stable within the range of 2-6 GPa at 500 ℃ and up to 12 GPa at 400 ℃. At T > 500 ℃ and P > 3.5 GPa, the graphitization of PTFE is observed, but judging from the preservation of liquid run products, PTFE can be used as a material for sample container up to 600 ℃ at this pressure. The obtained data allow the suggestion that the triple point between liquid, solid, and decomposed (carbonized) PTFE is located between 3 and 4 GPa at about 550 ℃, by analogy with the behavior of polycyclic aromatic hydrocarbons.

Incommensurately modulated twin structure of nyerereite Na1.64K0.36Ca(CO3)2.

The incommensurately modulated twin structure of nyerereite Na1.64K0.36Ca(CO3)2 has been first determined in the (3 + 1)-dimensional symmetry group Cmcm(α00)00s with modulation vector q = 0.383a*. Unit-cell values are a = 5.062 (1), b = 8.790 (1), c = 12.744 (1) Å. Three orthorhombic components are related by threefold rotation about [001]. Discontinuous crenel functions are used to describe the occupation modulation of Ca and some CO3 groups. The strong displacive modulation of the O atoms in vertexes of such CO3 groups is described using x-harmonics in crenel intervals. The Na, K atoms occupy mixed sites whose occupation modulation is described in two ways using either complementary harmonic functions or crenels. The nyerereite structure has been compared both with the commensurately modulated structure of K-free Na2Ca(CO3)2 and with the widely known incommensurately modulated structure of γ-Na2CO3.