Chromium Spinel Research Articles

Crystallographic Study of Chromium Spinels

Several chromium spinels have been prepared and regular x-ray diffraction intensities were measured on the powders. A Fortran program was developed in order to process the data. It appeared that the results, so obtained, were comparable to those from neutron diffraction techniques. A plot of the U parameter versus a0 for the thiochromites shows that metallic CuCr2S4 is exceptional. For its value of U, it has a much lower a0 than would be expected from a comparison of Mn, Fe, Co, and Zn thiochromites.

Diffuse Paramagnetic Neutron Scattering in Chromium Spinels

This paper describes an investigation of the diffuse neutron diffraction attributable to the paramagnetic state in normal, cubic spinels. The elastic neutron scattering is calculated from the high-temperature expansion of the correlation function 〈Si·Sj〉 as a power series in inverse temperature, the pertinent coefficients being evaluated elsewhere for a Heisenberg Hamiltonian with both nearest-neighbor A-B and B-B interactions. After normalization to the nuclear diffraction pattern, the computed diffuse scattering is compared with the experimental findings for both MnCr2O4 and MnCr2S4. Good agreement exists in both cases, despite the striking qualitative difference between these two patterns. This agreement supports the calculational model, and thereby answers earlier speculation concerning the origin of the liquid-type diffraction peaks observed in the room-temperature diffraction patterns of most chromium spinels. It is shown that these peaks are not directly related to any spin canting in the ground state. They arise from the basic antiferromagnetic nature of the exchange interactions, would persist in the absence of any B-B exchange, and can be suppressed by sufficiently strong ferromagnetic B-B interactions, as evidenced in MnCr2S4.

Gabbro-pegmatite ophiolite complex of Armenian Soviet Socialist Republic

The presence of ophiolites among the gabbro-pegmatites of the Armenian S.S.R. has been noted by the author. The geological position, structure, composition, origin, and textural properties imposed by emplacement processes are considered. Crystals are exhibited in both types of rocks and ll of the exposed gabbro-pegmatites show either schlieren (syngentic) or vein (epigenetic) structure, their only difference being in point of characteristic pyroxene-plagioclase ratios. Contact characteristics of certain types of pegmatite veins are noted, such as the mesocratic veins and the leucocratic veins. It is noted that gabbro-pegmatite composition is dominated by plagioclase feldspars, pyroxenes, and hornblende; the ores are represented by titanomagnetite, chromium spinel, and sulfides. Chemical composition is also given. Field laboratory data indicate that 1) gabbropegmatites with schlieren structure occur in gabbro while vein pegmatites cut gabbros and peridotites alike; 2) the geometry of the schlierin structure is dependent upon the time of the first magma split into the gabbro melt and the pegmatite melt; 3) the pegmatites with schlieren structure differ from gabbro which they invaded only because their textures are coarser; 4) faulting and jointing exercise strong controls over emplacement; 5) the vein pegmatites originated in the differentiation of primary magma at depth; 6) two generations of pegmatite veins have been identified; 7) endometasomatic processes played a major role in the evolution of both types of gabbro-pegmatites; 8) comparison shows that the texture and chemical properties bear a similarity with those at Mt. Sinaya, Central Ural, inferring that the gabbro-pegmatites had their origins in the same primary magma in which the gabbro and the ultrabasic rocks originated. A classification for the gabbro-pegmatites of Soviet Armenia is given. —A. W. Bellais

Jahn-Teller distortions induced by tetrahedral-site Fe 2+ ions

The Jahn-Teller effect for tetrahedral-site Fe 2+ ions is expressed to second-order in the interaction energy and to lowest-order anharmonic vibrational energies. The formal results are identical to those for Mn 3+ or Cu 2+ ions in octahedral sites. However, interpretation of the parameters that enter the formal expressions shows that the mode and transition temperature of any static, crystalline distortion is much more strongly influenced by long-range elastic coupling in the case of tetrahedral-site Fe 2+ ions. This result is used to interpret the absence of a static distortion from cubic symmetry in the spinel Fe[Al 2]O 4, for the occurrence of tetragonal ( c a < 1 and c a > 1 ) in chromium and vanadium spinels containing tetrahedral-site Fe 2+ ions, and for the complex crystallographic properties of the system Fe 3− x Cr xO 4.

Electron ordering transitions in several chromium spinel systems

Several systems of chromite spinels have been prepared and studied crystallographically from liquid nitrogen to room temperature in order to investigate the relative influence of Jahn-Teller and spin-orbit effects. In the system Co x Fe 1− x Cr 2O 4 there is a cubic to tetragonal ( c a < 1 ) transition for x ⩽ 0·3. For the system Ni x Fe 1− x Cr 2O 4 there is a cubic to tetragonal ( c a < 1 ) transition for x ⩽ 0.2 and a cubic to tetragonal ( c a > 1 ) transition for x ⩾ 0·3. It appears that A-site Co 2+ is more effective than A-site Ni 2+ in maintaining a tetragonal ( c a < 1 ) distortion at A-site Fe 2+. This implies that more A-site Fe 2+ is needed to change the sign of a tetrahedral Ni 2+ from c a > 1 to c a < 1 than is needed to cause a cooperative distortion of the structure with non-Jahn-Teller Co 2+ sharing the A-sites. The system NiFe x Cr 2− x O 4 shows a cubic to tetragonal ( c a > 1 ) distortion for x ⪅ 0.15 and a cubic to tetragonal distortion ( c a < 1 ) for x ⪆ 0.28. At 0.2 < x ⪅ 0.28 there is a cubic → tetragonal c a < 1 → orthorhombic series of transitions. This system is more complex than the others that were studied. In this system both spin-orbit and Jahn-Teller contributions are present; the sign of the distortion indicates which is dominant. In the orthorhombic region the two contributions are presumably of comparable magnitude. It has also been shown that a Jahn-Teller distortion of tetrahedral Fe 2+ may not only stabilize tetragonal ( c a > 1 or c a < 1 ) .distortions but also give rise to an orthorhombic distortion.

Electron ordering transitions in several chromium spinel systems : R.J. Arnott, A. Wold & D.B. Rogers (Lincoln Laboratory, Massachusetts Institute)

Electron ordering transitions in several chromium spinel systems : R.J. Arnott, A. Wold & D.B. Rogers (Lincoln Laboratory, Massachusetts Institute)

Crystallographic Transitions in Several Chromium Spinel Systems

The systems NixFe1−xCr2O4, CuxNi1−xCr2O4, and CuxCo1−xCr2O4 have been prepared and their crystallographic properties studied. From consideration of Jahn-Teller effects, it was shown that A-site Cu2+ is more effective than A-site Fe2+ in stabilizing a tetragonal (c/a&amp;lt;1) distortion at A-site Ni2+. In addition considerably more Cu2+ is required on the A site to distort CoCr2O4 from cubic symmetry to c/a&amp;lt;1 than is required to change the distortion of NiCr2O4 from c/a&amp;gt;1 to c/a&amp;lt;1.