Nature of the Color of Borates with “Anti-Zeolite” Structure

Crystals of the Mn_<i>x</i>Ba₁₂(BO₃)_{8–2<i>x</i>}F_8<i>x</i> 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 <i>I</i>4/<i>mcm</i> could be elucidated. Crystals of the (MnF₆)^4– group-containing solid solution Mn_<i>x</i>Ba₁₂(BO₃)_{8–2<i>x</i>}F_8<i>x</i> are dark brown in color in contrast to the differently colored crystals of (LiF₄)^3– group-containing “anti-zeolite” LiBa₁₂(BO₃)₇F₄ (<i>P</i>4₂<i>bc</i>). 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 Li_<i>x</i>Na_1–<i>x</i>Ba₁₂(BO₃)₇F₄ Fluoride Borates with “Antizeolite” Structure

Studied Li_<i>x</i>Na_1–<i>x</i>Ba₁₂(BO₃)₇F₄ (<i>P</i>4₂<i>bc</i>) 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 <i>a</i> and unit cell volume with Na/(Na + Li) ratio in cation site. Parameter <i>c</i> is less sensitive to the changes in stoichiometry, which is consistent with channel topology of Li_<i>x</i>Na_1–<i>x</i>Ba₁₂­(BO₃)₇F₄ structure. Distinctive feature of Li_<i>x</i>Na_1–<i>x</i>Ba₁₂­(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

Aragonite-II and CaCO₃‑VII: New High-Pressure, High-Temperature Polymorphs of CaCO₃

The importance for the global carbon cycle, the <i>P</i>–<i>T</i> phase diagram of CaCO₃ has been under extensive investigation since the invention of the high-pressure techniques. However, this study is far from being completed. In the present work, we show the existence of two new high-pressure polymorphs of CaCO₃. The crystal structure prediction performed here reveals a new polymorph corresponding to distorted aragonite structure and named aragonite-II. In situ diamond anvil cell experiments confirm the presence of aragonite-II at 35 GPa and allow identification of another high-pressure polymorph at 50 GPa, named CaCO₃-VII. CaCO₃-VII is a structural analogue of CaCO₃-<i>P</i>2₁/<i>c</i>-l, predicted theoretically earlier. The <i>P</i>–<i>T</i> phase diagram obtained based on a quasi-harmonic approximation shows the stability field of CaCO₃-VII and aragonite-II at 30–50 GPa and 0–1200 K. Synthesized earlier in experiments on cold compression of calcite, CaCO₃-VI was found to be metastable in the whole pressure–temperature range

Growth and Optical Properties of Li_<i>x</i>Na_1–<i>x</i>Ba₁₂(BO₃)₇F₄ Fluoride Borates with “Antizeolite” Structure

Studied Li_<i>x</i>Na_1–<i>x</i>Ba₁₂(BO₃)₇F₄ (<i>P</i>4₂<i>bc</i>) 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 <i>a</i> and unit cell volume with Na/(Na + Li) ratio in cation site. Parameter <i>c</i> is less sensitive to the changes in stoichiometry, which is consistent with channel topology of Li_<i>x</i>Na_1–<i>x</i>Ba₁₂­(BO₃)₇F₄ structure. Distinctive feature of Li_<i>x</i>Na_1–<i>x</i>Ba₁₂­(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

High-Pressure–High-Temperature Study of Benzene: Refined Crystal Structure and New Phase Diagram up to 8 GPa and 923 K

The high-temperature structural properties of solid benzene were studied at 1.5–8.2 GPa up to melting or decomposition using multianvil apparatus and <i>in situ</i> neutron and X-ray diffraction. The crystal structure of deuterated benzene phase II (<i>P</i>2₁/<i>c</i> unit cell) was refined at 3.6–8.2 GPa and 473–873 K. Our data show a minor temperature effect on the change in the unit cell parameters of deuterated benzene at 7.8–8.2 GPa. At 3.6–4.0 GPa, we observed the deviation of deuterium atoms from the benzene ring plane and minor zigzag deformation of the benzene ring, enhancing with the temperature increase caused by the displacement of benzene molecules and decrease of van der Waals bond length between the π-conjuncted carbon skeleton and the deuterium atom of adjacent molecule. Deformation of benzene molecule at 723–773 K and 3.9–4.0 GPa could be related to the benzene oligomerization at the same conditions. In the pressure range of 1.5–8.2 GPa, benzene decomposition was defined between 773–923 K. Melting was identified at 2.2 GPa and 573 K. Quenched products analyzed by Raman spectroscopy consist of carbonaceous material. The defined benzene phase diagram appears to be consistent with those of naphthalene, pyrene, and coronene at 1.5–8 GPa