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.

A high-pressure single-crystal X-ray diffraction study of potassium guaninate hydrate, K·CHNO·HO.

The crystal structure of potassium guaninate hydrate, K·CHNO·HO, 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·CHNO·7HO under hydrostatic compression and on cooling.

A Photoluminescence Study of Eu, Tb, Ce Emission in Doped Crystals of Strontium-Barium Fluoride Borate Solid Solution BaSr(BO)F (BSBF).

The present study is aimed at unveiling the luminescence potential of BaSr(BO)F (BSBF) crystals doped with Eu, Tb, and Ce. 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: Eu with EuO concentration of about 0.

LiBa(BO)F (LBBF) crystals doped with Eu, Tb, Ce: structure and luminescence properties.

The luminescent properties of single crystals and polycrystalline samples of LiBa(BO)F (LBBF) doped and co-doped with Eu, Tb, and Ce have been studied in order to disclose their potential for application in white light-emitting diodes. Deciphering of LBBF:Eu crystal structure (4/) makes it possible to determine the scheme of heterovalent isomorphic substitution 3Ba ← 2Eu + □, □ - vacancy in barium sites. Luminescent properties of LBBF crystals co-doped with Eu, Tb, Ce and Eu, Tb are compared.

γ-BaBO: High-Pressure High-Temperature Polymorph of Barium Borate with Edge-Sharing BO Tetrahedra.

The α- and β-modifications of barium metaborate are important functional materials used in optoelectronic devices. A new theoretically predicted modification of BaBO has been synthesized under conditions of 3 GPa and 900 °C, using the DIA-type apparatus. The new high-pressure modification, γ-BaBO, crystallizes in a centrosymmetrical group of monoclinic syngony (2/ (#14), = 4.

Study of an EuBO-ScBO system and EuSc(BO), EuSc(BO) orthoborates.

The EuBO-ScBO system was investigated by solid state synthesis and DSC methods. In this system, a new EuSc(BO) compound was found. It crystallizes in the 3̄ space group with unit cell parameters of = 4.

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.

Experimental and Ab Initio Studies of Intrinsic Defects in "Antizeolite" Borates with a Ba(BO) Framework and Their Influence on Properties.

The porous Ba(BO) framework of the so-called "antizeolite" borates with channels along the axis is capable of accommodating various guest anionic groups, e.g. [BO], [F], [F], and [(Li,Na)F].

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

Crystal structures of unquenchable high-temperature polymorphs of CaSiO, important in cement chemistry, have eluded single-crystal X-ray analysis. However, the problem may be addressed by studying chemically stabilized CaSiO polymorphs at ambient temperature. Here an incommensurately modulated crystal structure of flamite [Pnma(0β0)00s, q = 0.

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

Crystals of the Mn Ba(BO)F phase were grown from a high-temperature solution. This new fluoride borate is built of positively charged [Ba(BO)] 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.

In Situ Raman Study of Liquid Water at High Pressure.

A pressure shift of Raman band of liquid water (HO) 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 HO. 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 HO in situ up to 1.

Growth and Optical Properties of LiNaBa(BO)F Fluoride Borates with "Antizeolite" Structure.

Studied LiNaBa(BO)F (P4bc) solid solution belongs to the new class of "antizeolite" borates with [Ba(BO)] 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.

Incommensurately modulated twin structure of nyerereite NaKCa(CO).

The incommensurately modulated twin structure of nyerereite NaKCa(CO) 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.

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.

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.