Band 122, Heft 5-6

Single crystals of cronstedtite, Fe 3 2+ (Fe 3+ Si)O 4 (OH) 5 , have been heated in air at temperatures up to 745°C and the structural transformations investigated using x-ray diffraction. One polymorph examined transformed at 715°C to a single crystal of a spinel, two others formed hexagonal ferrites, and a further polymorph produced no well-defined structure. The mechanisms of the transformations from cronstedtite to the spinel and to the ferrite are similar. First, at about 275°C, a ferric form of cronstedtite is formed with a structure very similar to the original polymorph. Between this temperature and 715°C, two mechanisms operate simultaneously in the crystal. The anion arrangement in the matrix of the crystal slowly adopts a close-packed structure appropriate to spinel or a hexagonal ferrite. At the same time, zones, resembling Guinier-Preston zones, are formed which are coherent with the matrix and which have a cation arrangement similar to, but not identical with, that of the spinel or the ferrite. The zones increase in number until, between 650°C and 715°C, the entire crystal (matrix and zones) adopts the spinel or the ferrite structure. Simple magnetic measurements show that the spinel and ferrite are ferrimagnetic but that the temporary zones show no trace of this effect. The mechanisms are contrasted with those which occur in the kaolinite-mullite series of transformations.

NaCl whiskers have been grown on NaCl single-crystal cleavages by heating them in a muffle furnace at 785°C. The growth of the whiskers has been studied at different temperatures. The whiskers have been cleaved along planes parallel to their axes and the matched faces so obtained have been chemically etched. Correspondence in the etch pattern produced on the matched faces has been established. Studies have been made on the structure of the pits, and the implications are discussed.

A condensed two-dimensional representation of inorganic close-packed structures is proposed, based on standard transparent sheets of film, which represent layers of the main types of close packings. The holes occupied by the small ions can easily be marked out on these sheets, and a structure is completely represented by a few sheets, just what is necessary to define the unit cell. An approximate three-dimensional representation of a structure can be obtained by placing the sheets at equal distances in an appropriate support. A good structural analogue can be built up, in a quick way, by stacking standard layers of transparent plastic spheres representing the large ions, and placing in the holes small opaque coloured balls representing the small ions.

A systematic derivation of inorganic close-packed structures is carried out for compounds of general formulae AX and AX 2 , formed by the sequence of equal layers of large ions (X) with small ions (A) in the holes. The derivation is based on the stability conditions for crystal structures, from which the types of close packing of the large ions were selected, and a general rule governing the distribution of the small ions was derived. A new notation is proposed for the structure types dealt with in this paper.

The LaF 3 structure has previously been described in the space group P 6 3 / mmc with two formula units per unit cell. Weak additional reflections on single-crystal photographs lead to a larger unit cell with a = 7.190 ± 0.002 Å, c = 7.367 ± 0.002 Å, space group P [unk] c 1, and six LaF 3 per unit cell. The structure has been refined by full-matrix least-squares computations. The deviation from the high symmetry is caused by displacements of the F ions. LaF 3 is shown to be isotypic with HoD 3 . The calculated ionic-lattice energy (−1151.6kcal/mole) agrees with the energy calculated by a Born-Haber cycle (−1144.4 kcal/mole). The probable critical radius ratio for the change from the LaF 3 to the orthorhombic YF 3 structure is 0.94. For smaller ratios the LaF 3 structure can be stabilized by anion vacancies.

In contradiction to former investigations Cu 3 P and Cu 3 As are anti-isostructural with LaF 3 (space group P [unk] c 1, 6 formula units in the unit cell). Phosphorus (arsenic) is surrounded by 11 copper atoms, Cu(1) has 4, Cu(2) and Cu(3) 3 phosphorus (arsenic) atoms as neighbours. It is very likely that α -Li 3 Sb, Li 3 As, Li 3 P, Na 3 Bi, Na 3 Sb, Na 3 As, Na 3 P, K 3 Bi, K 3 Sb, K 3 As also belong to the Cu 3 P type of structure. These compounds have been described earlier in the so-called Na 3 As type of structure (space group P 6 3 / mmc ) containing only two formula units in the unit cell.

X-ray photographs of (Ca, Na)CO 3 cancrinites show diffuse satellites with the indices ( h k l ± 3/7), The examined crystals from Litchfield, U.S.A., have the space group P 6 3 and the lattice dimensions a = 12.75 Å and c = 5.14 Å. The crystal structure has been determined–independently of all previous structure assumptions–from the relation of the transformation hauyn–cancrinite and refined on three-dimensional data by the least-squares method to R = 0.089. The structure is composed of an ordered Si, Al framework with channels around the threefold axis and the axis 6 3 ; the other cations and anions as Na, Ca, H 2 O, CO 3 etc. are enclosed in these channels.

A structure determination of YB 4 was carried out in order to determine the boron positions in metal tetraborides. The structure proposed by earlier workers based on ThB 4 was confirmed; however, departures in the occupancy of some boron positions were found. As a consequence, each octahedral boron group is, on the average, one atom short. This argues against the structure interpretation proposed earlier, which was based on a continuous framework of boron atoms. A new crystal-chemical interpretation of the tetraboride structure is presented in terms of tetrahedral units which demonstrates a structural framework common to MB 4 , M 3 B 2 , and M 2 B type compounds.

The crystal structure of rathite-I has been determined with the use of three-dimensional intensity data. Four chemical units of (Pb,Tl) 3 As 4 (As,Ag)S 10 are contained in the unit-cell of the symmetry P 2 1 / a with a = 25.16 Å, b = 7.94 Å, c = 8.47 Å, β = 100°28′. The true symmetry of rathite-I may be triclinic. The solution was obtained from the similarity of the crystal structure to that of rathite-III and from a peculiar feature of the x-ray diagrams. Among three independent Pb(Tl) atoms two are surrounded by nine S atoms and the other is surrounded by seven S atoms. As atoms have trigonal-pyramidal coordinations by S atoms. One As atom, however, is believed to occupy statistically two different trigonal-pyramidal S coordinations. Another As atom is partially replaced by Ag. The structure is composed of two kinds of layers parallel to (100). Layers of the first kind have the composition (Pb,Tl)S 3 , and consist of coordination polyhedra around the Pb(Tl) atoms which are coordinated by nine S atoms. The layers of the second kind are composed of Pb(Tl), As(Ag) and S atoms, having a deformed PbS-type structure. Trigonal As-S 3 pyramids are linked into strings of finite length.

X-ray studies on a phase transition in potassium chromium sulphate alum at 192.5°K have been carried out. From the results obtained it appears that the transition temperature is the ferroelectric Curie point of the alum, below which the structure becomes rhombohedral, with a ≃ 12.2 Å (which is almost the same as that of the cubic phase) and α ≃ 88°.

During microscopic studies of growth sequences of cadmium-iodide crystals in aqueous solution, it has been found that the particular crystal under observation sometimes suddenly rotates. The force needed for such a rotation is presumably provided by the convection currents in the solution, as otherwise the crystals are completely covered. The value of this force has been calculated and found to agree with the value of the critical stress required for causing dislocations in the crystals, thus establishing that convection currents are most likely responsible for the generation of dislocations during the growth of soft crystals from solution.

Experiments are made in order to verify the assumption put forward by B rasseur according to which H 2 O exists in a certain number of uranium minerals under the form 2 H 2 O = H 3 O + OH. Reactions with substances containing K, Rb and Cs confirm the validity of the oxonium hypothesis.

The unit-cell dimensions and space group of (1) Ni(C 2 H 7 N 5 ) 2 Cl 2 · 2H 2 O, (2) Cu(C 2 H 7 N 5 ) 2 (NO 3 ) 2 · 2H 2 O, (3) Pt(C 2 H 7 N 5 ) 3 (ClO 4 ) 2 , (4) Cu(C 2 H 7 N 5 ) 2 Cl 2 · 2H 2 O and (5) Ru(C 2 H 7 N 5 ) 3 (SO 4 ) 3 · 7H 2 O have been determined. The first four crystals belong to triclinic system with space group P 1 or P [unk] and fifth one belongs to monoclinic system with space group P 2 1 / c . The nickel and copper biguanide chlorides are isomorphous.