Thermal Behavior and Phase Transition of Uric Acid and Its Dihydrate Form, the Common Biominerals Uricite and Tinnunculite

Single crystals and powder samples of uric acid and uric acid dihydrate, known as uricite and tinnunculite biominerals, were extracted from renal stones and studied using single-crystal and powder X-ray diffraction (SC and PXRD) at various temperatures, as well as IR spectroscopy. The results of high-temperature PXRD experiments revealed that the structure of uricite is stable up to 380 °C, and then it loses crystallinity. The crystal structure of tinnunculite is relatively stable up to 40 °C, whereas above this temperature, rapid release of H2O molecules occurs followed by the direct transition to uricite phase without intermediate hydration states. SCXRD studies and IR spectroscopy data confirmed the similarity of uricite and tinnunculite crystal structures. SCXRD at low temperatures allowed us to determine the dynamics of the unit cells induced by temperature variations. The thermal behavior of uricite and tinnunculite is essentially anisotropic; the structures not only expand, but also contract with temperature increase. The maximal expansion occurs along the unit cell parameter of 7 Å (b in uricite and a in tinnunculite) as a result of the shifts of chains of H-bonded uric acid molecules and relaxation of the π-stacking forces, the weakest intermolecular interactions in these structures. The strongest contraction in the structure of uricite occurs perpendicular to the (101) plane, which is due to the orthogonalization of the monoclinic angle. The structure of tinnunculite also contracts along the [010] direction, which is mostly due to the stretching mechanism of the uric acid chains. These phase transitions that occur within the range of physiological temperatures emphasize the particular importance of the structural studies within the urate system, due to their importance in terms of human health. The removal of supersaturation in uric acid in urine at the initial stages of stone formation can occur due to the formation of metastable uric acid dihydrate in accordance with the Ostwald rule, which would serve as a nucleus for the subsequent growth of the stone at further formation stages; afterward, it irreversibly dehydrates into anhydrous uric acid

Effect of Fe-Doping on Thermal Expansion and Stability of Bismuth Magnesium Tantalate Pyrochlorere

A continuous series of solid solutions (Bi1.5Mg0.75−xFexTa1.5O7±Δ (x = 0–0.75)) with the pyrochlore structure were synthesized with the solid-phase method. It was shown that iron, like magnesium, is concentrated in the structure in the octahedral position of tantalum. Doping with iron atoms led to an increase in the upper limit of the thermal stability interval of magnesium-containing pyrochlore from 1050 °C (x = 0) up to a temperature of 1140 °C (x = 1). The unit cell constant a and thermal expansion coefficient (TEC) increase uniformly slightly from 10.5018 Å up to 10.5761 Å and from 3.6 up to 9.3 × 10−6 °C−1 in the temperature range 30–1100 °C. The effect of iron(III) ions on the thermal stability and thermal expansion of solid solutions was revealed. It has been established that the thermal stability of iron-containing solid solutions correlates with the unit cell parameter, and the lower the parameter, the more stable the compound. The TEC value, on the contrary, is inversely proportional to the cell constant

Pressure-Induced Phase Transitions in Danburite-Type Borosilicates

The high-pressure behaviors of two isotypic borosilicates (maleevite, BaB2Si2O8, and pekovite, SrB2Si2O8) have been studied using in situ single-crystal X-ray diffraction and Raman spectroscopy. Maleevite undergoes one reconstructive phase transition between 36 and 38 GPa with the formation of a triclinic phase, maleevite-II, featuring octahedrally coordinated silicon. In contrast, pekovite undergoes two phase transitions: first, an isosymmetric order–disorder phase transition to pekovite-II (between 18 and 23 GPa) and then a reconstructive phase transition with the formation of triclinic pekovite-III (between 29 and 33 GPa). The structure of pekovite-II is characterized by the splitting of the Si site into two sites. The results have been confirmed by Raman spectroscopy and density functional theory (DFT) calculations. Raman spectra indicate that the reconstructive phase transitions of both borosilicates are irreversible. Upon decompression, the triclinic phases persist metastably at least down to 12 and 17 GPa, for pekovite and maleevite, respectively. The comparison of the high-pressure behavior of danburite-group minerals with the general formula MB2Si2O8 (M = Ca, Sr, Ba) reveals that increasing size of an extraframework cation for M = Sr and Ba governs the stability of the danburite-type structure and prevents the formation of pentacoordinate silicon species observed in danburite (M = Ca)

Structure Refinement and Thermal Stability Studies of the Uranyl Carbonate Mineral Andersonite, Na₂Ca[(UO₂)(CO₃)₃]·(5+<i>x</i>)H₂O

A sample of uranyl carbonate mineral andersonite, Na2Ca[(UO2)(CO3)3]&#183;5&#8722;6H2O, originating from the Cane Springs Canyon, San Juan Co., UT, USA was studied using single-crystal and powder X-ray diffraction at various temperatures. Andersonite is trigonal, R&#8722;3m, a = 17.8448(4), c = 23.6688(6) &#197;, V = 6527.3(3) &#197;3, Z = 18, R1 = 0.018. Low-temperature SCXRD determined the positions of H atoms and disordered H2O molecules, arranged within the zeolite-like channels. The results of high-temperature PXRD experiments revealed that the structure of andersonite is stable up to 100 &#176;C; afterwards, it loses crystallinity due to release of H2O molecules. Taking into account the well-defined presence of H2O molecules forming channels&#8217; walls that to the total of five molecules p.f.u., we suggest that the formula of andersonite is Na2Ca[(UO2)(CO3)3]&#183;(5+x)H2O, where x &#8804; 1. The thermal behavior of andersonite is essentially anisotropic with the lowest values of the main thermal expansion coefficients in the direction perpendicular to the channels (plane (001)), while the maximal expansion is observed along the c axis&#8212;in the direction of channels. The thermal expansion around 80 &#176;C within the (001) plane becomes negative due to the total release of &#8220;zeolitic&#8222; H2O molecules. The information-based structural complexity parameters of andersonite were calculated after the removal of all the disordered atoms, leaving only the predominantly occupied sites, and show that the crystal structure of the mineral should be described as complex, possessing 4.535 bits/atom and 961.477 bits/cell, which is comparative to the values for another very common natural uranyl carbonate, liebigite

Features of the Phase Formation of Cr/Mn/Fe/Co/Ni/Cu Codoped Bismuth Niobate Pyrochlore

The phase formation process of Bi2Cr1/6Mn1/6Fe1/6Co1/6Ni1/6Cu1/6Nb2O9+Δ containing 3d-ions of transition elements in equimolar quantities was studied in a wide temperature range (400–1050 °C). The complex oxide crystallizes in the structural type of pyrochlore (sp. gr. Fd-3m:2, a = 10.4937(2) Å). The investigation of the multi-element pyrochlore phase formation process showed that the synthesis goes through a series of successive stages, during which the transition from Bi-rich to Bi-depleted compounds takes place. The predecessor of the pyrochlore phase is bismuth orthorhombic modification orthoniobate (α-BiNbO4) with an equimolar ratio of Bi(III)/Nb(V) ions. The pyrochlore phase is formed as a result of bismuth orthoniobate doping with transition element ions. The complex oxides Bi14CrO24, Bi25FeO40, BiNbO4, and Bi5Nb3O15 appeared as intermediate phases during the synthesis. The interaction between the initial oxide precursors is fixed at temperatures above 500 °C. The phase transition of α-Bi2O3 into β-Bi2O3 near 500 °C is observed. Varying the heat treatment duration at each synthesis step did not qualitatively change the phase composition of the sample but had an effect on the quantitative phase ratio. Phase-pure pyrochlore of the given composition by solid-phase synthesis method can be obtained at a temperature no lower than 1050 °C. Ceramics are characterized by low-porous dense microstructure with blurred outlines of grain boundaries

Crystal Structure Evolution of Slawsonite SrAl2_{2}Si2_{2}O8_{8} and Paracelsian BaAl2_{2}Si2 _{2}O8_{8} upon Compression and Decompression

The high-pressure behavior of slawsonite, SrAl$_{2}$Si$_{2}$O$_{8}$ , has been studied using in situ single-crystal X-ray diffraction (SCXRD) and Raman spectroscopy up to 31 GPa. Slawsonite undergoes displacive phase transition between 6 and 8 GPa with the formation of slawsonite-II, featuring fivefold coordinated silicon and aluminum. The results have been confirmed by the changes in vibrational modes using Raman spectroscopy. High-pressure evolution of the Raman spectra of isotypic paracelsian, BaAl$_{2}$Si$_{2}$O$_{8}$ , was studied upon compression and decompression up to 37.5 GPa. Raman data for paracelsian upon compression are in good agreement with previously obtained SCXRD data, which demonstrated three phase transitions at ∼6, 28, and 32 GPa with the formation of AlO$_5$, SiO$_5$, AlO$_6$, and SiO$_6$ polyhedra. Raman data upon decompression show the possibility to quench the high-pressure modification, containing AlO5 polyhedra. The comparison of the high-pressure behavior of slawsonite with paracelsian reveals that the increasing size of extra framework cation from Sr$^{2+}$ to Ba$^{2+}$ reduces the phase transition pressure but does not change the transformation pathway

One of Nature&rsquo;s Puzzles Is Assembled: Analog of the Earth&rsquo;s Most Complex Mineral, Ewingite, Synthesized in a Laboratory

Through the combination of low-temperature hydrothermal synthesis and room-temperature evaporation, a synthetic phase similar in composition and crystal structure to the Earth&rsquo;s most complex mineral, ewingite, was obtained. The crystal structures of both natural and synthetic compounds are based on supertetrahedral uranyl-carbonate nanoclusters that are arranged according to the cubic body-centered lattice principle. The structure and composition of the uranyl carbonate nanocluster were refined using the data on synthetic material. Although the stability of natural ewingite is higher (according to visual observation and experimental studies), the synthetic phase can be regarded as a primary and/or metastable reaction product which further re-crystallizes into a more stable form under environmental conditions

Crystal Chemistry of the Copper Oxalate Biomineral Moolooite: The First Single-Crystal X-ray Diffraction Studies and Thermal Behavior

Moolooite, Cu(C2O4)·nH2O, is a typical biomineral which forms due to Cu-bearing minerals coming into contact with oxalic acid sources such as bird guano deposits or lichens, and no single crystals of moolooite of either natural or synthetic origin have been found yet. This paper reports, for the first time, on the preparation of single crystals of a synthetic analog of the copper-oxalate biomineral moolooite, and on the refinement of its crystal structure from the single-crystal X-ray diffraction (SCXRD) data. Along with the structural model, the SCXRD experiment showed the significant contribution of diffuse scattering to the overall diffraction data, which comes from the nanostructural disorder caused by stacking faults of Cu oxalate chains as they lengthen. This type of disorder should result in the chains breaking, at which point the H2O molecules may be arranged. The amount of water in the studied samples did not exceed 0.15 H2O molecules per formula unit. Apparently, the mechanism of incorporation of H2O molecules governs the absence of good-quality single crystals in nature and a lack of them in synthetic experiments: the more H2O content in the structure, the stronger the disorder will be. A description of the crystal structure indicates that the ideal structure of the Cu oxalate biomineral moolooite should not contain H2O molecules and should be described by the Cu(C2O4) formula. However, it was shown that natural and synthetic moolooite crystals contain a significant portion of “structural” water, which cannot be ignored. Considering the substantially variable amount of water, which can be incorporated into the crystal structure, the formula Cu(C2O4)·nH2O for moolooite is justified