Biomedical subjects

Radovan Cerný

Publications and source records attributed to Radovan Cerný.

5 recordsLinked to original sources

PubMed2004

Chemical heterogeneity of a crystal built of nanoscale coherently twinned Yb(2-x)(Fe,Ga)(17+2x) polytypes.

An X-ray study of single crystals extracted from an arc-melted Yb-Fe-Ga alloy showed that the diffraction pattern can be modeled by an intergrown crystal that has three sorts of domains: one hexagonal (1, LuFe(9.5) type) and two rhombohedral (2 a and 2 b, PrFe(7) type), the last two twinned by reticular merohedry. Crystals 1 and 2 are essentially polytypes with maximum degree of order (MDO polytypes), built up of nearly identical slabs that are stacked along [001] in ABAB em leader (1). and ABCABC em leader (2). sequences. Structure refinement was performed by a newly developed program that allowed us to refine several structures on a single data set. We found that the hexagonal and rhombohedral domains differ in chemical composition: while 1 shows a higher rate of Yb substitution by Fe(2) dumbbells, 2 shows partial substitution of Fe by Ga. Our observation of the nanoscale phase segregation is supported by latest finding of nonrandom distribution of stacking faults in a similar 2:17 alloy. An unequal distribution of chemical substitutions in 1 and 2 apparently compensates the inherent mismatch of basal plane dimensions of the individual MDO polytypes and thus constrains their cell parameters within the syntaxy. According to our knowledge this is the first example of two chemically distinct polytypes constituting a single crystal, refined on a single set of diffraction data.

Journal Article↗

PubMed2004

Mg(1 + x)Ir(1 - x) (x = 0, 0.037 and 0.054), a binary intermetallic compound with a new orthorhombic structure type determined from powder and single-crystal X-ray diffraction.

The new binary compound Mg(1 + x)Ir(1 - x) (x = 0-0.054) was prepared by melting the elements in the Mg:Ir ratio 2:3 in a sealed tantalum tube under an argon atmosphere in an induction furnace (single crystals) or by annealing cold-pressed pellets of the starting composition Mg:Ir 1:1 in an autoclave under an argon atmosphere (powder sample). The structure was independently solved from high-resolution synchrotron powder and single-crystal X-ray data: Pearson symbol oC304, space group Cmca, lattice parameters from synchrotron powder data a = 18.46948 (6), b = 16.17450 (5), c = 16.82131 (5) A. Mg(1 + x)Ir(1 - x) is a topologically close-packed phase, containing 13 Ir and 12 Mg atoms in the asymmetric unit, and has a narrow homogeneity range. Nearly all the atoms have Frank-Kasper-related coordination polyhedra, with the exception of two Ir atoms, and this compound contains the shortest Ir-Ir distances ever observed. The solution of a rather complex crystal structure from powder diffraction, which was fully confirmed by the single-crystal method, shows the power of powder diffraction in combination with the high-resolution data and the global optimization method.

Alloys↗

PubMed2002

The intermetallic compound Mg21Zn25.

The crystal structure of the intermetallic compound henicosamagnesium pentacosazinc, Mg(21)Zn(25), has been determined by single-crystal X-ray diffraction. It is isomorphous with Zr(21)Re(25) and deviates slightly from the rules that define the Frank-Kasper phases.

Journal Article↗

PubMed2002

Tl2Te and its relationship with Tl5Te3.

The crystal structure of Tl(2)Te, dithallium telluride, has been determined by single-crystal X-ray diffraction. The analysis of the structure shows that this compound is the first known representative of a new crystal structure type. The structural relationship with the related Tl(5)Te(3) phase is discussed.

Journal Article↗

PubMed2002

A tetragonal polymorph of caesium hydroxide monohydrate, CsOH x H2O, from X-ray powder data.

Tetragonal caesium hydroxide monohydrate, CsOH x H(2)O, a clathrate hydrate, is a polymorph of three known hexagonal or pseudo-hexagonal modifications. It was obtained as a by-product in a high-pressure experiment. Whether it is a high-pressure polymorph, however, remains to be verified. The Cs atoms are situated in cavities of the form of a bicapped pentagonal prism, within an infinite three-dimensional hydrogen-bonded oxygen framework that is locally identical to layers found in the hexagonal modifications. The Cs atom and one of the two H atoms are at sites with (macro)4m2 symmetry, the O atom has mm site symmetry and the second H atom has 2/m symmetry.

Journal Article↗