ABSTRACT
MSb2O6 compounds (M = Mg, Co, Ni, Cu, Zn) are known in the tetragonal trirutile forms, slightly distorted monoclinically with M = Cu due to the Jahn-Teller effect. In this study, using a low-temperature exchange reaction between ilmenite-type NaSbO3 and molten MSO4-KCl (or MgCl2-KCl) mixtures, these five compositions were prepared for the first time as trigonal layered rosiaite (PbSb2O6)-type phases. Upon heating, they irreversibly transform to the known phases via amorphous intermediates, in contrast to previously studied isostructural MnSb2O6, where the stable phase is structurally related to the metastable phase. The same method was found to be applicable for preparing stable rosiaite-type CdSb2O6. The formula volumes of the new phases show an excellent correlation with the ionic radii (except for M = Cu, for which a Jahn-Teller distortion is suspected) and are 2-3% larger than those for the known forms although all coordination numbers are the same. The crystal structure of CoSb2O6 was refined via the Rietveld method: P3[combining macron]1m, a = 5.1318(3) Å, and c = 4.5520(3) Å. Compounds with M = Co and Ni antiferromagnetically order at 11 and 15 K, respectively, whereas the copper compound does not show long-range magnetic order down to 1.5 K. A comparison between the magnetic behavior of the metastable and stable polymorphs was carried out. FeSb2O6 could not be prepared because of the 2Fe2+ + Sb5+ = 2Fe3+ + Sb3+ redox reaction. This electron transfer produces an additional 5s2 shell for Sb and results in a volume increase. A comparison of the formula volume for the stable mixture FeSbO4 + 0.5Sb2O4 with that extrapolated for FeSb2O6 predicted that the trirutile-type FeSb2O6 can be stabilized at high pressures.
ABSTRACT
We report the revised crystal structure, static and dynamic magnetic properties of quasi-two dimensional honeycomb-lattice silver delafossite Ag3Co2SbO6. The magnetic susceptibility and specific heat data are consistent with the onset of antiferromagnetic long range order at low temperatures with Néel temperature TN â¼ 21.2 K. In addition, the magnetization curves revealed a field-induced (spin-flop type) transition below TN in moderate magnetic fields. The GGA+U calculations show the importance of the orbital degrees of freedom, which maintain a hierarchy of exchange interaction in the system. The strongest antiferromagnetic exchange coupling was found in the shortest Co-Co pairs and is due to direct and superexchange interaction between the half-filled xz + yz orbitals pointing directly to each other. The other four out of six nearest neighbor exchanges within the cobalt hexagon are suppressed, since for these bonds the active half-filled orbitals turned out to be parallel and do not overlap. The electron spin resonance (ESR) spectra reveal a broad absorption line attributed to the Co(2+) ion in an octahedral coordination with an average effective g-factor g = 2.40 ± 0.05 at room temperature and show strong divergence of the ESR parameters below â¼150 K, which implies an extended region of short-range correlations. Based on the results of magnetic and thermodynamic studies in applied fields, we propose a magnetic phase diagram for the new honeycomb-lattice delafossite.
ABSTRACT
It is known that recurrent nephroliths form in about half of the operated patients within 5 years after operation while 60% of all recurrences occur 3 years after removal of the primary concrement. To prevent recurrent nephroliths, it is important not only to detect metabolic disturbances but also to investigate chemical composition of uroliths. Mineral composition of 112 stag-horn concrements from patients living in the south of Russia was studied with x-ray phase analysis which showed that 62.9% concrements had mixed composition. Compared to patients with primary stag-horn nephrolithiasis, patients with recurrent one more often had calcium-phosphate and calcium-oxalate concrements than urate concrements. Chemical composition of the concrements depended on features of the relief of the region where the patients live. Newly established mineral characteristics of stag-horn concrements in the citizens of south Russia should be taken into consideration both in surgical treatment and follow-up of such patients.
