We report on high-pressure powder synchrotron x-ray diffraction studies on MFe 2 As 2 (M = Ba, Ca) over a range of temperatures and pressures up to about 56 GPa using a membrane diamond-anvil cell. Our data indicate a phase transition to a collapsed tetragonal phase in both compounds upon compression. The data at 300 K are measured in both pressure-increasing and-decreasing cycles. Our measurements show that at 300 K in the Ba compound, the transition occurs at 27 GPa, which is much higher than the transition pressure of 1.7 GPa in the Ca compound. At low temperature, we could obtain data only in the pressure-increasing cycle, therefore a precise transition pressure is not identified. At a low temperature of 33 K, the transition to the tetragonal phase in the Ba compound starts, upon compression, at about 29 GPa, which is much higher than the transition pressure of 0.3 GPa at 40 K as known in the case of the Ca compound. The much higher transition pressure in the Ba compound may be due to its larger unit-cell volume at ambient pressure. It is important to note that the transition in both compounds occurs when they are compressed to almost the same value of the unit-cell volume and attain similar c t /a t ratios. We also show that the FeAs 4 tetrahedra are much less compressible and more distorted in the collapsed tetragonal phase than their nearly regular shape in the ambient pressure phase. We present a detailed analysis of the pressure dependence of the structures as well as equations of state in these important BaFe 2 As 2 and CaFe 2 As 2 compounds.

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Optical emission spectra of nanocrystalline zinc gallate (ZnGa2O4) and trivalent chromium ion doped zinc gallate (ZnGa2O4:Cr3+) are reported for different concentrations of the dopant ion. The measurements have been carried out over the temperature range between 77 and 296 K. The emission spectrum of nanocrystalline ZnGa2O4 shows two broad peaks. The intensity variation in these peaks, with temperature, is indicative of the effect of symmetry breaking in the electronic band structure of ZnGa2O4 in nanocrystalline samples. In addition, we find that the relative intensities of the sharp spectral lines of Cr3+ in nanocrystalline ZnGa2O4:Cr3+ are quite different from those reported for corresponding bulk samples. The spectral profiles of the so-called R1, R2, N1, and N2 lines have also been studied. The data are analyzed using crystal field theory, which includes an exchange interaction between the nearest neighbor Cr3+ pairs in ZnGa2O4. We estimate the exchange parameters for Cr3+ in nanocrystalline ZnGa2O4:Cr3+. Though, in the literature, there exist reports on optical properties of the corresponding bulk spinel, our approach and consequent results on nanocrystalline ZnGa2O4:Cr3+ are not only interesting from the physics point of view but also can be of use in nanotechnology.

Summary: ZnS:Dy and (ZnS:Mn,Dy) phosphors have been prepared in an inert atmosphere of pure nitrogen gas at about (950 AE 20)C. The photo luminescent (PL) and electroluminescent (EL) and spectra of these phosphors have been studied at room temperature. The purpose of this work is to know in what ways the spectraal characteristics of ZnS phosphors are affected by the introduction of Dy 3þ and Mn 2þ ions and also to determine unambiguously the electronic transitions involved in the energy transfer.

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