Different Types Of Order Research Articles

The nature of ordering and ordering defects in dolomite

The apparent lack of transformation-induced domains and stacking disorder defects in natural dolomites is considered in light of the different types of order in the dolomite structure. Experimentally produced twin domain boundaries and basal stacking defects are documented in a dolomite using high-resolution electron microscopy and electron diffraction techniques. Results reveal that upon cation ordering to form the dolomite structure, a twin domain is favored over an antiphase domain. The twin domain boundaries closely resemble antiphase boundaries (APB's) and are in contrast for superlattice reflections. However, background contrast within domains is shown to be different when imaged using certain fundamental reflections. The results allow speculation about the nature of ordering at low temperatures. Observations of twin domain boundaries in samples annealed at different temperatures allow estimation of the critical ordering temperature at 1,100°–1,150° C in stoichiometric dolomite.

Trace elements in diamonds of different types

STUDIES of optical and electrical properties have shown that natural diamond can be broadly categorised into four types, types IA, IB, IIA and IIB (refs 1–3). Type IA is the most common (98% abundance)3 and contains up to 0.34% N (ref.16), which has been argued5 to occur in atom pairs. Type IB diamond contains much less N, of which 1–2% occurs as single substitutional paramagnetic N. Type II diamonds contain even less N. The p-type semiconducting property of type IIB diamond is due to the presence of B (refs 6, 7), the concentration of which has been correlated with the acceptor concentration7. We have determined and report here the impurity contents of 23 diamonds of different types in order to investigate whether elements other than N and B contribute to the physical properties of diamond. Although there are few diamonds of each type and the statistics are therefore poor, two diamonds were found to contain an excess of Ni, while the three Type IIB diamonds possibly contained an excess of Al.

Study of phase transitions and of the possible existence of a fifth phase in NH4Br by Raman scattering under high pressure

We have investigated phases II, III, IV, and the previously suggested phase V in NH4Br under high pressure by Raman scattering experiments. The pressure and temperature dependences of band intensities and frequencies at the transitions are discussed. The possible existence of a fifth phase is examined by comparison with calculated Raman spectra for different types of ordering. These calculations show that the spectra in the range of phase V reported in the present and in all former Raman investigations are due to the same type of ordering as in phase IV. As there is no indication of a transition from phase IV, we conclude that phase IV is present where phase V was suggested. The same conclusion can be inferred from all other high pressure Raman scattering studies so far reported. The fifth phase, originally observed by ultrasonic measurements, might have been a property of the particular sample.

Anisotropie magnetique et ordre dans les alliages de fer-nickel contenant de 50á 75% de nickel

An analysis of the magnetic anisotropy symmetries corresponding to FeNi-, FeNi 3- and directional order shows that these different types of order can be distinguished experimentally by magnetic anisotropy measurements, if spherical single crystals are used as samples and if the order is formed by thermal annealing (or irradiation) in the presence of a magnetic field. This method was applied to Iron-Nickel alloys within the composition range 50–75% Nickel. The results obtained confirm the simultaneous existence of two critical temperatures corresponding to FeNi- and FeNi 3- type order respectively, and leads us to assume that in the composition range considered the alloys partiall segregate into ordered regions of FeNi and FeNi 3. The model employed would account for certain anomalies in our observations of the directional order.